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Graptolites as fossil geo-thermometers and source material of hydrocarbons: An overview of four decades of progress
Journal Pre-proof Graptolites as fossil geo-thermometers and source material of hydrocarbons: an overview of four decades of progress Qingyong Luo, Goodarzi Fariborz, Ningning Zhong, Ye Wang, ´ Nansheng Qiu, Christian B. Skovsted, Vaclav Such´y, Niels Hemmingsen Schovsbo, Rafał Morga, Yaohui Xu, Jingyue Hao, Anji Liu, Jin Wu, Weixun Cao, Xu Min, Jia Wu
PII:
S0012-8252(19)30494-5
DOI:
https://doi.org/10.1016/j.earscirev.2019.103000
Reference:
EARTH 103000
To appear in: Received Date:
25 July 2019
Revised Date:
28 October 2019
Accepted Date:
30 October 2019
Please cite this article as: Luo Q, Fariborz G, Zhong N, Wang Y, Qiu N, Skovsted CB, Such´y V, Hemmingsen Schovsbo N, Morga R, Xu Y, Hao J, Liu A, Wu J, Cao W, Min X, Wu J, Graptolites as fossil geo-thermometers and source material of hydrocarbons: an overview of four decades of progress, Earth-Science Reviews (2019), doi: https://doi.org/10.1016/j.earscirev.2019.103000
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Graptolites as fossil geo-thermometers and source material of hydrocarbons: an overview of four decades of progress Qingyong Luoa, b, Goodarzi Fariborzc, Ningning Zhonga, b*, Ye Wanga, b, Nansheng Qiua, b, Christian B. Skovstedd, Václav Suchýe, Niels Hemmingsen Schovsbof, Rafał Morgag, Yaohui Xuh, Jingyue Haoa, b, Anji Liua, b, Jin Wua, b, Weixun Caoa, b, Xu Mina, b, Jia Wua, b State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing 102249, China; b
c
d
e
College of Geoscience, China University of Petroleum, Beijing 102249, China;
FG &Partner Ltd, Research Group, 29 Hawkside Mews NW., Calgary, Alberta, Canada;
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a
Department of Palaeobiology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden;
Nuclear Physics Institute, v. v. i.,Academy of Sciences of the Czech Republic, Na Truhlářce 39/64, 180 86 Prague 8, Czech Republic;
g
Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark;
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f
Silesian University of Technology, Faculty of Mining, Safety Engineering and Industrial Automation, Institute of Applied
Hubei Cooperative Innovation Center of Unconventional Oil and Gas, Yangtze University, Wuhan, 430100, China.
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h
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Geology, Akademicka 2, 44-100 Gliwice, Poland;
*Corresponding author at: State Key Laboratory of Petroleum Resources and Prospecting, China
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University of Petroleum, Changping, Beijing, 102249, China. Tel.: +86 10 89734548. E-mail address:
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[email protected].
Abstract
The thermal maturity of lower Paleozoic graptolite-bearing marine sediments, which host many
hydrocarbon deposits worldwide, has long been difficult to determine due to the absence of wood-derived vitrinite particles for conventional vitrinite reflectance. In 1976, graptolite reflectance was introduced as a new indicator for organic maturity of these deposits and has been used since in many regional studies. The majority of these studies, however, were done on a limited sample set and a limited range of thermal
maturity, which resulted in a number of controversial views concerning the usefulness of graptolite reflectance as an alternative paleothermal indicator and its correlation with vitrinite reflectance through various proxies. In this paper, we review previous studies and combine those analyses with new data to assess the physical and chemical characteristics of graptolite periderm with increasing thermal maturity. We conclude that graptolite random reflectance (GRor) is a better parameter for the thermal maturity assessment than graptolite maximum reflectance (GRomax) due to the better quality of available data. Combining published data with results of our study of both natural and heat-treated graptolites and vitrinite, we present a new correlation between GRor and equivalent vitrinite reflectance (EqVRo), as
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EqVRo = 0.99GRor + 0.08. Chemical composition of graptolite periderm is similar to vitrinite; graptolites are mainly kerogen Type II-III, are gas prone and have a substantial hydrocarbon potential. Lower Paleozoic graptolite-bearing organic-rich sediments are important shale gas source rocks and reservoirs globally and make a significant contribution to worldwide petroleum reserves.
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Keywords: Graptolite reflectance; Optical characteristics; Chemical composition; Microstructure;
1. Introduction and previous studies
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Wufeng–Longmaxi Formations; Alum Shale; Hot shale; Shale gas.
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Lower Paleozoic graptolite-bearing rocks were mainly deposited in marine environments, and are important source rocks globally, especially shale gas deposits in the Wufeng–Longmaxi (also known as
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Wufeng–Lungmachi) sediments from China (Zou et al., 2010; Zou et al., 2012; Dai et al., 2014; Dai et al., 2016; Luo et al., 2016; Zou et al., 2016; Luo et al., 2017; Luo et al., 2018). These organic matter (OM)-rich facies were also identified as true or potential hydrocarbon source rocks in the Anadarko basin
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in the USA (Wang and Philp, 1997), North Africa and Arabian Peninsula (Jones and Stump, 1999; Lüning et al., 2000), Taurus region of Turkey (Varol et al., 2006), the Czech Republic (Suchý et al., 2002), and
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the Siberian platform (Makarov and Bazhenova, 1981). More recently, such shales have been recognized as potential targets for unconventional shale gas deposits in the Norwegian-Danish Basin (Schovsbo et al. 2011, 2014; Pool et al. 2012), and Baltic Basin of Central Europe (Littke et al., 2011; Karcz et al., 2013; Yang et al. 2017). Vitrinite reflectance is a most commonly used indicator for the thermal maturity (Stach et al., 1982; Taylor et al., 1998; Suárez-Ruiz et al., 2012; Hackley and Cardott, 2016). However, due to the lack of vitrinite (coalified wood) in pre-Devonian rocks, the determination of thermal maturity of lower Paleozoic
graptolite-bearing rocks is always a difficult topic and a hot debate for petroleum industry (Goodarzi, 1984; Goodarzi, 1985a; Goodarzi and Norford, 1985, 1987; Bertrand and Heroux, 1987; Bustin et al., 1989; Bertrand, 1990; Goodarzi et al., 1992a; Bertrand, 1993; Petersen et al., 2013; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018). Thus, the surrogate proxies, such as the reflectance of zooclasts, vitrinitelike particles and solid bitumen, Tmax, and biomarkers, have been proposed to assess organic maturity in lower Paleozoic graptolite-bearing sediments (Teichmüller, 1978; Goodarzi and Norford, 1987; Jacob, 1989; Schoenherr et al., 2007; Suárez-Ruiz et al., 2012; Schmidt et al., 2019). Tmax may be unreliable due to low S2 in overmature sediments (Peters, 1986; Peters and Cassa, 1994).
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The maturity-related biomarker ratios may be also influenced by depositional environments and biological sources, which will increase the difficulty of data interpretation (Radke and Welte, 1983; Radke et al., 1986; Radke, 1988; George and Ahmed, 2002; Peters et al., 2005). In addition, the biomarker ratios may be invalid to assess thermal maturity of overmature sediments (Peters et al., 2005). Graptolite-bearing
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rocks often contain bitumen and other zooclasts (Teichmüller, 1978; Goodarzi and Norford, 1987; Jacob, 1989; Schoenherr et al., 2007; Suárez-Ruiz et al., 2012; Schmidt et al., 2019). The difficulty with using
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of solid bitumen reflectance is due to large variation in most samples, and their origin, e.g., by thermal cracking, biodegradation and deasphalting (George et al., 1994; Hwang et al., 1998; Mastalerz et al., 2018),
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all of which increase difficulty in data interpretation (e.g., Gonçalves et al., 2014; Fink et al., 2016). The origin of discrete vitrinite-like particles remains controversial, and possible explanations include:
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migrated bitumen, either indigenous or exogenous to the host rock (Bertrand and Heroux, 1987); gelification of polysaccharides (Buchardt and Lewan, 1990); residues of algae after maturation (Wang et al., 1994); biodegraded zooclasts that are the product of a reducing to strongly reducing environment
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(Xiao et al., 1997); marine humification of planktonic and benthic organisms (Romankevich, 1984), the so-called “marine vitrinite group” (Zhong and Qin, 1995); and fragments of graptolites (Petersen et al.,
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2013).
Zooclasts have clear advantage in reflectance studies due to their specific biological sources
compared to that of solid bitumen and vitrinite-like particles, and thus their reflectance was naturally regarded as having a superior potential as a thermal maturity proxy. In general, graptolites are more common than other zooclasts (e.g., chitinozoans and scolecodonts) in lower Paleozoic marine rocks, and as a result, the nature of graptolite reflectance has been a “hot topic” for organic petrologists over several decades (Kurylowicz et al., 1976; Teichmüller, 1978; Goodarzi, 1984, 1985a; Bertrand and Heroux, 1987;
Bertrand, 1990; Link et al., 1990; Cardott and Kidwai, 1991; Hoffknecht, 1991; Goodarzi et al., 1992b; Malinconico, 1992; Tricker et al., 1992; Malinconico, 1993; Wang et al., 1993; Cole, 1994; Gentzis et al., 1996; Liu et al., 2001; Bertrand et al., 2003; Petersen et al., 2013; İnan et al., 2016; Lavoie et al., 2016; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018; Synnott et al., 2018; Wang et al., 2019a). Graptolite reflectance is a useful proxy of thermal maturity (Goodarzi, 1984; Goodarzi and Norford, 1985), and has been used to assess eroded thicknesses when used in conjunction with reflectance of bitumen and sedimentological and tectonic evidence (Goodarzi et al., 1992b; Gentzis et al., 1996). The graptolite maximum reflectance (GRomax) or graptolite random reflectance (GRor) was adopted to estimate
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thermal maturity in worldwide sediments (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi et al., 1985; Goodarzi and Norford, 1989; Goodarzi et al., 1992b; Malinconico, 1992, 1993; Rantitsch, 1995; Gentzis et al., 1996; Bertrand et al., 2003; Petersen et al., 2013; İnan et al., 2016; Lavoie et al., 2016; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018). Some researchers have established correlations among
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reflectances of graptolite and other zooclasts, pyrobitumen and vitrinite (Bertrand and Heroux, 1987; Bertrand, 1990; Bertrand, 1993; Yang and Hesse, 1993; Bertrand and Malo, 2001; Bertrand et al., 2003),
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and Conodont Alteration Index (CAI; Goodarzi and Norford, 1985; Hoffknecht, 1991; Gentzis et al., 1996). Moreover, the graptolite reflectance has also been successfully correlated with some inorganic
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paleothermal indices, including the illite “crystallinity” index (e.g. Kemp et al., 1985; Oliver, 1988; Hoffknecht, 1991; Rantitsch, 1995, 1997; Suchý et al., 2015), and metamorphic index minerals in pre-
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greenschist meta-sedimentary rocks (e.g. Malinconico, 1992). Graptolite reflectance is commonly related to the lithology, and it will be higher in shales than in limestones (Link et al., 1990). Redox conditions may have an impact on graptolite reflectance, and higher reflectance was observed in graptolites deposited
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under oxic environments in comparison with those from anoxic environments (Cole, 1994). Weathering may affect graptolite reflectance (Goodarzi and Norford, 1985; Hoffknecht, 1991; Goodarzi et al., 1992a).
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Raman spectroscopy of graptolites was widely studied and used to assess organic maturity (Suchý et al., 2004; Liu et al., 2013; İnan et al., 2016; Mumm and İnan, 2016; Morga and Pawlyta, 2018; Wang et al., 2019a). However, these studies were mostly focused on a small number of samples representing a very limited range of thermal maturity. Thus, the physical and chemical characteristics of graptolites in wider thermal maturity settings is reviewed herein for lower Paleozoic sediments from a number of localities based on the literature, and new unpublished data is presented. In reviewing the optical characteristics, chemical composition, and microstructure of the graptolites, their implications for the global petroleum
industry were also discussed.
2. The biological structure and composition of graptolites Graptolites are extinct colonial planktonic hemichordate invertebrates that lived mainly in the early Paleozoic ocean (Clarkson, 1981). Their structure has been widely studied and is illustrated in Fig. 1 (Moore, 1955; Clarkson, 1981; Crowther, 1981). The rhabdosome (colony) may have more than one branch (stipe) (Fig. 1a). Along a stipe, thecae house the individual zooids (Fig. 1b, 1c), which were connected by a stolon to other individuals through the common canal (Fig. 1c). The periderm material of the rhabdosome is generally composed of two layers, fusellar tissue and cortical tissue (Fig. 1e)
reflected light under the optical microscope as discussed below.
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(Crowther, 1981). The periderm with fusellar layers, thecae and common canal may be observed in
Initially, studies of the chemical structure of graptolites were related to the periderm based on the textures and phylogeny, and these studies assumed that graptolite periderm is chitinous (Eisenack, 1932;
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Kozlowski, 1949). Later, a collagen-like structure for the cortical and fusellar tissues was proposed for graptolites (Fig. 1) (Towe and Urbanek, 1972; Crowther, 1981; Bates and Kirk, 1986; Urbanek and
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Mierzejewski, 1986; Liu et al., 1996). Both collagen and chitin are supportive tissues, which mainly serve to support cellular structures (Tasch, 1980). Collagen is a glycoprotein, and chitin with a molecular
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formula of C32H54O21N4 is a nitrogenous carbohydrate forming a N-acetyl glucosamine groups polymer (Leninger, 1975; Bustin et al., 1989). More recent studies indicate, however, that the residual graptolite
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periderms are composed of complex aliphatic polymers that are immune to base hydrolysis rather than collagen (e.g. Briggs et al., 1995; Gupta et al., 2006). Aliphatic components of graptolite periderms are
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probably derived from the graptolite itself via in situ polymerization (e.g. Gupta et al., 2006).
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Fig. 1. The biological structure of graptolites showing their theca, aperture, common canal, virgella and periderm (Modified after Moore,
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1955; Clarkson, 1981). A) Rhabdosome (colony) with two stipes (branches); B) close-up of rhabdosome structures; C) close-up of two thecae which house individual zooids; D) transverse cross-section of rhabdosome; E) periderm structure.
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3. Sampling and methods
The graptolite-bearing sediments examined for this study included the Wufeng–Longmaxi
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Formations, Pingliang Formation, China (Luo et al., 2017; Luo et al., 2018; Wang et al., 2019a); Liteň Formation from Czech (Suchý et al., 2002); and Alum Shale from Estonia and Sweden (Petersen et al.,
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2013; Sanei et al., 2014; Luo et al., 2018). Detailed geological information can be found in these above references. Two graptolite-bearing Wufeng–Longmaxi samples (sample ID: CKMB-Y1 (GRor = 1.32%) and CKMB-Y2 CKMB-Y1 (GRor = 1.28%)) (Wang et al., 2019a) and an upper Carboniferous coal (VRor = 1.07%) were selected to conduct artificial maturation at 350℃, 400℃, 450℃ and 500℃ for three days, respectively. The natural and heated samples were cut parallel or perpendicular to bedding in order to make polished sections on EcoMet 250 with AutoMet 250 for the maceral observation and reflectance measurement. Organic petrography and reflectance were done on a Leica 4500P microscope with
CRAIC/MPS 200 microscope photometer. The maximum-minimum and random reflectances were measured under polarized and non-polarized light, respectively, as described in Luo et al. (2016); Luo et al. (2017); Luo et al. (2018). For scanning electron microscopy (SEM), sediment was cut, polished using emery paper, and milled using Ar ion milling. For study of pores, graptolites were found and then marked using a diamond lens under an optical microscope. The SEM study was on a Zeiss Crossbeam 540 Focused Ion Beam-SEM (FIB-SEM).
4. Optical characteristics of graptolite periderm
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Kurylowicz et al. (1976) reported the reflectance characteristics of graptolites and proposed that their optical properties are similar to vitrinite. Clausen and Teichmüller (1982) described the morphology and reflectance of graptolites from Germany and Sweden, with maximum reflectances ranging from 0.8% to 10.0%. In 1980–1990s, Goodarzi and colleagues extensively studied the optical properties of Middle
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Ordovician to upper Silurian graptolites from Poland, Sweden, Turkey and Canada (Goodarzi, 1984, 1985a; Goodarzi and Norford, 1985; Goodarzi et al., 1985; Goodarzi and Norford, 1987; Bustin et al.,
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1989; Goodarzi and Norford, 1989; Riediger et al., 1989; Link et al., 1990; Goodarzi et al., 1992a; Goodarzi et al., 1992b; Gentzis et al., 1996), which advanced the understanding of the petrographic
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characteristics of graptolites. In recent years, graptolite-bearing sediments have become significant targets of shale gas exploration, and a resurgence of work has been conducted on the organic petrology of
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graptolites (Petersen et al., 2013; Sanei et al., 2014; Haeri-Ardakani et al., 2015; Caricchi et al., 2016; İnan et al., 2016; Lavoie et al., 2016; Luo et al., 2016; Ma et al., 2016; Mumm and İnan, 2016; Cheshire et al., 2017; Luo et al., 2017; Cardott and Curtis, 2018; Luo et al., 2018; Morga and Kamińska, 2018;
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Morga and Pawlyta, 2018; Reyes et al., 2018; Synnott et al., 2018; Wang et al., 2019a). Microscopically, graptolites can be identified by their morphology, e.g., fusellar layer, granularity,
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other visible structures as well as anisotropy (Kurylowicz et al., 1976; Teichmüller, 1978; Clausen and Teichmüller, 1982; Goodarzi, 1984, 1985a; Bertrand and Heroux, 1987; Goodarzi and Norford, 1987, 1989; Bustin et al., 1989; Riediger et al., 1989; Bertrand, 1990; Petersen et al., 2013; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018). Two textures are apparent in graptolites, i.e., non-granular (NGG) and granular (GG), in Paleozoic sediments worldwide, e.g., Poland, Sweden, Turkey and Canada (Goodarzi, 1984, 1985a; Goodarzi and Norford, 1985; Goodarzi et al., 1985; Goodarzi and Norford, 1987, 1989; Goodarzi et al., 1992a; Goodarzi et al., 1992b), the Alum Shale from Europe (Petersen et al., 2013; Sanei
et al., 2014; Luo et al., 2018), and the Wufeng–Longmaxi Formations and Pingliang Formation from China (Fig. 2) (Wang et al., 1993; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018; Wang et al., 2019a). In general, GG and NGG are found in carbonates and mudstones, respectively (Goodarzi and Norford, 1985; Goodarzi and Norford, 1987; Petersen et al., 2013). NGG and GG were thought to be derived from different sections of the rhabdosome. While the former may be part of the wall with fusellar layers, the latter may be derived from the common canal (Goodarzi, 1984). However, according to recent artificial maturation studies on Estonian Alum Shale with GG, this texture is completely altered to NGG when GRor reached up to 1.24% after heating for 3 days at 400 ℃. This indicates that pristine GG may be altered
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to NGG due to thermal stress (Luo et al., 2018). GG has weaker anisotropy and lower reflectance than NGG (Figs. 2b and 3) (Goodarzi, 1984; Goodarzi and Norford, 1987; Petersen et al., 2013; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018). Suchý et al. (2002) found that NGG random reflectance has a positive correlation with GG random
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reflectance in lower Silurian sediments from the Barrandian Basin, Czech Republic. In rotation of the objective stage under polarized light, NGG will display extinction twice, and thus two GRomax and two
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show such clear patterns (Fig. 3c and d).
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GRomin can be measured (Figs. 3a, b and 4) (Luo et al., 2016; Luo et al., 2018), however, GG does not
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Fig. 2 Granular-textured graptolites (GG) and non-granular-textured graptolites (NGG) from Chinese and Estonian sediments. (a) GG in
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low-maturity Alum Shale from Estonia (GRor = 0.65%), non-polarized light; (b) GG and NGG in high-maturity shale from Chongqing, China (GRomax = 2.57%), non-polarized light (Luo et al., 2018); (c) grey black NGG in low-maturity shale from Gansu, China (GRor =
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0.48%), non-polarized light; (d) NGG in overmature shale from Chongqing, China (GRor = 3.85%), showing strong anisotropy, polarized light; (e) NGG in overmature shale from Chongqing, China (GRor = 3.77%), showing strong anisotropy, polarized light; (f) NGG in
overmature shale from Chongqing, China (GRomax = 4.47%), showing fusellar layers, polarized light (modified from Luo et al., 2018). (a-e) sections perpendicular to bedding; (f) sections parallel to bedding.
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Fig. 3. Reflectance measured on NGG (a and b) and GG (c and d) under polarized light in a circular rotation of the microscope stage.
Fig. 4. Photomicrographs displaying extinction characteristics of the same NGG from the Wufeng–Longmaxi shales in sections perpendicular to bedding with 360° rotation of the microscope stage, oil immersion, polarized light. The angle between each picture is
90 degrees. The graptolite reflectance (%) is given beside the measurement site (red square).
In general, NGG is more frequently found than GG in most Paleozoic sediments (Goodarzi and Norford, 1987; Petersen et al., 2013; Luo et al., 2016), thus, only NGG will be discussed in the main discussion of this paper. In polished blocks, low-maturity graptolites are often similar to natural bitumen stringers, but they are more angular than bitumen and often show typical graptolitic features (Figs. 2, 4 and 5), and they display a plate-like appearance at high maturity (Fig. 2). NGG is blocky in sections parallel to bedding, but may display a long stipe-like morphology in sections normal to bedding (Figs. 2
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and 5) (Link et al., 1990). The fusellar layers are observed in sections parallel to bedding (Fig. 2). NGG was divided into two classes, lath- and blocky-shaped (Riediger et al., 1989). Blocky-shaped graptolites show lower maximum reflectance and bireflectance (BRo) than lath-shaped graptolites. Some NGG of the Liteň Formation from Czech Republic displays weak brown fluorescence (Fig. 5), and Hoffknecht (1991)
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and Wang et al. (2019a) also found similar phenomena in low-maturity graptolites.
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Fig. 5. NGG from the Czech Republic (Liteň Formation, Barrandian area) with weak brown fluorescence of the Liteň Formation in sections perpendicular to bedding. (a, c, e) under reflected light; (b, d, f) the same field as (a, c, e), under fluorescence light. (a-b) GRor =
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0.85%; (c-f) GRor = 0.83%.
5. Physical and chemical properties of graptolites as thermal maturity indicators 5.1 Chemical structure changes of graptolites with maturation OM is sensitive to pressure and temperature (Khavari-Khorasani, 1975; Teichmüller, 1982; Goodarzi and Norford, 1985). Optical properties of OM include reflectance, refractive and absorptive index (van Krevelen, 1961). Reflectance of OM is related to the aromatic structure of organic molecules; reflectance
and aromaticity increase continuously with increase in maturity. The trends of optical properties of OM over the visible spectrum (400–700nm) are used to determine molecular structural changes occurring during maturation (Khavari-Khorasani, 1975; Goodarzi, 1985a; Goodarzi and Macqueen, 1990). Structurally, OM is composed of ordered and amorphous carbons (Cartz and Hirsch, 1960; van Krevelen, 1961; Goodarzi, 1985a). The ratio of ordered (aromatic) to amorphous (alicyclic side chains, aliphatic) carbon atoms increases with maturation/heat treatment (Cartz and Hirsch, 1960; van Krevelen, 1961) in two phases: ① Devolatilization of amorphous carbon increase in aromaticity and size of the polynuclear systems
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during early stage of maturation and low temperature heat treatment; ② Transformation of 2D (turbostratic) to 3D (graphitic) ordering, which occurs at high maturity and at temperature >600 ℃, results in increase and better ordering of the aromatic structure (Cartz and Hirsch, 1960; van Krevelen, 1961; Goodarzi and Murchison, 1972; Goodarzi, 1985a).
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Dispersion of optical properties of graptolite periderm is similar to that of bitumen and vitrinite in the visible spectrum and three spectral patterns are documented:
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① Low-maturity (CAI = 1) graptolite behaves similarly to low rank vitrinite and bitumen with curves of all optical parameters decreasing with increasing wavelength from blue to red (Figs. 6 and
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7), indicating low content of aromatic carbon (Marshall and Murchison, 1971; Khavari-Khorasani, 1975);
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② Moderately-mature (CAI = 4) graptolite behaves similarly to semi-anthracite and impsonite bitumen. The optical parameters are nearly flat, from blue to red (Figs. 6 and 7), indicating gradual molecular variations, e.g., greater condensed aromatic carbon (Marshall and Murchison, 1971;
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Khavari-Khorasani, 1975);
③ Highly mature graptolite (CAI = 3.5–5) behaves similarly to anthracite and pyrobitumens. The
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trends of optical indices rise continuously from blue to red (Figs. 6 and 7), similar to the pattern from a highly aromatic, condensed molecular structure (Marshall and Murchison, 1971; Cook et al., 1972; Khavari-Khorasani, 1975; Goodarzi, 1985a). Graptolites at this stage develop macro-properties similar to graphite (Teichmüller et al., 1979).
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Fig. 6. Dispersion of maximum reflectance (a) in air (%Ramax) and (b) in oil (%Romax) of graptolite fragments with increased maturity based on their CAI (conodont alteration index), shown as number beside each spectrum for graptolite in limestone (L), argillaceous
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limestone (A) and shale (S) matrices (after Goodarzi, 1985a).
Fig. 7. Dispersion of refractive (a) and (b) absorptive indices of graptolite fragments with increased maturity based on their CAI, shown as number beside each spectrum for graptolites in limestone (L), argillaceous limestone (A) and shale (S) matrices (after Goodarzi, 1985a).
5.2 Graptolite reflectance 5.2.1 Graptolite maximum, minimum and random reflectance Similar to vitrinite (Davis, 1978), GRomax and graptolite minimum reflectance (GRomin) are observed in sections parallel and normal to bedding, respectively (Goodarzi, 1984; Link et al., 1990; Goodarzi et al., 1992a). However, in some samples, GRomax in sections parallel to bedding is lower than that in sections normal to bedding (Table 1 and Fig. 8), which may be because the graptolites in these samples are not truly parallel to bedding or normal to bedding due to an unusual burial process and/or sample preparation
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(Malinconico, 1992, 1993; Luo et al., 2017). In sections parallel to bedding, GRomax correlates positively with GRomin (Fig. 9), whereas in sections perpendicular to bedding, such relationship cannot be observed (Hoffknecht, 1991; Luo et al., 2017). This is because GRomin in sections parallel to bedding is generally
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equal to the graptolite intermediate reflectance (GRoint) (Luo et al., 2017).
Table 1. Comparison of graptolites reflectance (including maximum, minimum and bireflectance) in sections parallel and normal to
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bedding. Part of the data for the Wufeng–Longmaxi graptolites are derived from Luo et al. (2017), and the data of the upper Silurian graptolites of Turkey are from Goodarzi (1984).
Sample no.
Sections parallel to bedding
Sections normal to bedding
GRomax
GRomin
BRo
GRomax
GRomin
BRo
(%)
(%)
(%)
(%)
(%)
(%)
Shale
3.41
1.92
1.49
3.68
0.99
2.69
Shale
5.48
2.61
2.87
5.61
0.64
4.98
Shale
4.76
3.14
1.62
5.64
0.75
4.90
Shale
4.61
4.17
0.44
4.42
1.08
3.34
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Depth Location
Formation
Lithology
(m) DTB-6-9
Datianba, China
Outcrop
Wufeng-
Jinxi, China
Outcrop
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JX-4
Longmaxi
QQ1-3
Qianjiang, China
794.00
No. 2
Tufanbeyli, Turkey
Outcrop
upper
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Silurian
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Fig. 10 displays a comparison of GRor in sections parallel and perpendicular to bedding. In contrast to GRomax, GRor in sections perpendicular to bedding, especially in samples with higher thermal maturity, is much lower than that in sections parallel to bedding, similar to the results in the Qusaiba Hot Shales from Saudi Arabia (İnan et al., 2016), and is much more concentrated than that in sections parallel to bedding as indicated by their lower SD values (Fig. 10). In general, unimodal histograms of GRomax and GRor were found in investigated samples, especially in sections perpendicular to bedding (Figs. 8 and 10) (Cole, 1994; Haeri-Ardakani et al., 2015; Lavoie et al., 2016; Luo et al., 2018; Reyes et al., 2018). In
addition, NGG reflectance in carbonate rocks is lower than that of shales at similar depths, which may be due to oxidation caused by carbonate dissolution (Goodarzi and Norford, 1985; Link et al., 1990). This is
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similar to that of VRor in different lithologies (Goodarzi et al., 1988; Goodarzi et al., 1993).
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Fig. 8. Comparison of GRomax of the Wufeng–Longmaxi sediments in (a, c and e) sections parallel to bedding and (b, d and f) sections
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perpendicular to bedding from Chongqing.
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Fig. 9. The linear coalification trend for graptolites based on the plot of GRomax VS. GRomin (after Goodarzi, 1984).
Fig. 10. Comparison of GRor of the Wufeng–Longmaxi sediments in (a, c, e) sections parallel to bedding and (b, d, f) sections perpendicular to bedding from Chongqing (cited from Luo et al., 2019).
From the methodological point of view, the main difference between the measurement technique of vitrinite and graptolite reflectance has to do with the sample preparation. Graptolite maturity determination was generally made on the basis of whole-rock polished blocks in sections parallel to bedding and using GRomax (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi and Norford, 1987, 1989; Gentzis et al., 1996). GRomax will increase with burial depth and is a useful tool to unravel the thermal maturity of lower Paleozoic deposits (Clausen and Teichmüller, 1982; Goodarzi, 1984, 1985a; Goodarzi and Norford, 1987, 1989; Riediger et al., 1989; Link et al., 1990; Gentzis et al., 1996; Luo et
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al., 2016; Luo et al., 2017). However, other researchers have used GRor to assess maturity levels (Bertrand and Heroux, 1987; Bertrand, 1990; Bertrand, 1993; Cole, 1994; Bertrand and Malo, 2001; Bertrand et al., 2003; Petersen et al., 2013; Sanei et al., 2014; İnan et al., 2016; Yang, 2016; Luo et al., 2018; Reyes et al., 2018; Synnott et al., 2018; Wang et al., 2019a).
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Based on data from published literature, GRomax shows a strong positive correlation with GRor in sections perpendicular to bedding, and the former is around twice as large as the latter (Fig. 11). The
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comparison of SD for GRomax and GRor is shown in Fig. 12. In general, SD is greater for GRomax than that of GRor. Therefore, for routine maturation study, measurements of GRor is as accurate and as useful as
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those of GRomax.
Wang et al., 2019a
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Mean random reflectance of graptolite (%)
6
y = 0.52x + 0.43 r = 0.95
Luo et al., 2018 Malinconico, 1993
4
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Link et al., 1990
2
0 0
3 6 Mean maximum reflectance of graptolite (%)
9
Fig. 11. Mean GRomax vs. mean GRor in sections perpendicular to bedding (Link et al., 1990; Malinconico, 1993; Luo et al., 2018; Wang et al., 2019a).
0.9 (a)
0.9 Malinconico, 1992 Luo et al., 2018 Wang et al., 2019a
(b)
0.6
SD (%)
SD (%)
0.6 y = 0.03x + 0.07 r = 0.51
Luo et al., 2018 Petersen et al., 2013 Bertrand et al., 2003 Lavoie et al., 2016 Williams et al. 1998 Wang et al., 2019a
0.3
0.3
y = 0.07x + 0.07 r = 0.75 0
0 9
0
2 4 Mean random reflectance of graptolite (%)
6
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0 3 6 Mean maximum reflectance of graptolite (%)
Fig. 12. The relationship between (a) mean GRomax in sections perpendicular to bedding, (b) mean GRor and SD (Malinconico, 1992; Williams et al., 1998; Bertrand et al., 2003; Petersen et al., 2013; Lavoie et al., 2016; Luo et al., 2018; Wang et al., 2019a).
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5.2.2 Graptolite bireflectance (BRo)
BRo in sections parallel to bedding is much lower than that in sections perpendicular to bedding,
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especially in overmature sediments (Table 1) (Goodarzi, 1984). The anisotropy of graptolites will increase with increasing thermal maturity as suggested by positive correlation between GRomax and BRo (Luo et
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al., 2017). Vitrinite displays lower bireflectance than that of graptolite at similar maturation levels because of the biaxial negative property of graptolites (Bustin et al., 1989; Hoffknecht, 1991; Luo et al., 2017). 5.2.3 The coalification path of the graptolites
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The plot of GRomax vs. GRomin can be used to assess the coalification path of the graptolites (Fig. 9). However, it should be noted that GRomin in Fig. 9 was measured in sections parallel to bedding, and is
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nearly equal to GRoint (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi and Norford, 1987; Goodarzi et al., 1992a; Malinconico, 1993; Luo et al., 2017; Luo et al., 2018). When all the graptolite
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reflectance data from the published literature are plotted in Fig. 9, a linear coalification trend can be determined for the graptolites measured parallel to bedding (Fig. 9) (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi and Norford, 1987; Goodarzi et al., 1992a; Malinconico, 1993; Luo et al., 2017; Luo et al., 2018). 5.2.4 The relationship between graptolite reflectance and other paleothermal indices 5.2.4.1 Comparison with other organoclasts reflectance Similar to vitrinite and solid bitumen (see also Ferreiro Mählmann and Le Bayon, 2016 for a recent
review on vitrinite and solid bitumen thermal maturity studies), graptolite reflectance displays an increasing trend with increasing burial depth, and can determine organic maturity in lower Paleozoic sediments without vitrinite, although no consensus of the exact relationship between these indices has yet been reached (Table 2 and Fig. 13) (Clausen and Teichmüller, 1982; Goodarzi, 1984, 1985a, b; Bertrand and Heroux, 1987; Goodarzi and Norford, 1987; Teichmüller, 1987; Bustin et al., 1989; Goodarzi and Norford, 1989; Bertrand, 1990; Goodarzi et al., 1992a; Bertrand, 1993; Cole, 1994; Gentzis et al., 1996; Bertrand and Malo, 2001; Bertrand et al., 2003; Petersen et al., 2013; Luo et al., 2018; Synnott et al., 2018). Goodarzi (1985a) found that the optical dispersion of graptolites is similar to vitrinite and bitumen,
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indicating that their physical and chemical nature is sensitive to temperature and can be used as a proxy for organic maturity in lower Paleozoic deposits. The graptolites show higher reflectance and stronger anisotropy than that of solid bitumen, chitinozoans and scolecodonts (Goodarzi, 1984, 1985b; Goodarzi and Norford, 1987, Bertrand et al., 2003; Petersen et al., 2013). According to Yang and Hesse (1993), the
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bireflectance of graptolites displays a similar trend to that of pyrobitumen when Ro < 4.5–5.0 %. Other researchers suggested that graptolite reflectance is similar to that of chitinozoan for both natural and
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artificially matured samples (Bertrand and Heroux, 1987; Bertrand, 1990; Cole, 1994; Reyes et al., 2018). Zooclast reflectance (including chitinozoan, graptolite, and scolecodonts) was also compared with
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vitrinite and solid bitumen reflectances in lower Paleozoic deposits from Canada (Bertrand and Heroux, 1987; Bertrand, 1990; Bertrand, 1993; Bertrand and Malo, 2001; Bertrand et al., 2003). In these Canadian
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studies, graptolite reflectance was slightly lower than, or similar to, vitrinite reflectance (Bertrand, 1990) (Table 2 and Fig. 13). In a similar way, Yang and Hesse (1993) established that GRor is lower than VRor when GRor < 2.6%, but graptolite has higher reflectance than vitrinite when GRor > 2.6%. The Silurian
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Qusaiba Shale from Saudi Arabia deposited under anoxic environments has lower graptolite random reflectance than that deposited in oxic environments (Cole, 1994). Zhong and Qin (1995) proposed a
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formula between EqVRo and GRor based on vitrinite-like particle reflectance (Rvl) and GRor in the sediments from Tarim Basin and South China. In recent years, various new equations have been proposed to illustrate the relationship between EqVRo and GRor or GRomax (Petersen et al., 2013; Colţoi et al., 2016; Synnott et al., 2018; Luo et al., 2018; Wang et al., 2019a) (Table 2 and Fig. 13). The equations proposed by Cole (1994) (anoxic); Petersen el al., (2013) and Coltoi et al., (2016) are very close to each other; whereas the formulas presented by Bertrand (1990) and Luo et al. (2018) are essentially coincident and close to the 1:1 line (Fig. 13).
Table 2. Various equations describing the relationship between EqVRo and GRor or GRomax. GRor data Equations
Source
Formation range
Log10 GRor = -0.04+1.10×Log10 EqVRo
(Bertrand, 1990)
0.5–3.0%
upper Gaspe Limestone Group and Chaleurs Group, Canada
EqVRo = 0.8 ×GRor (anoxic)
(Cole, 1994)
0.62–2.05%
Silurian Qusaiba Shale, Saudi Arabia
(Zhong and Qin,
1.8–4.9%
Cambrian-Ordovician, China
0.47–2.14%
Alum Shale, Scandinavia
EqVRo = 0.65 ×GRor (oxic) EqVRo = 0.882 ×GRor－0.366
1995) EqVRo = 0.73 ×GRor + 0.16
(Petersen et al., 2013) (Colţoi et al., 2016)
No data
Silurian intervals, Romania
EqVRo = 0.232+ 0.499×GRor
(Synnott et al.,
0.59–1.02%
Cape Phillips Formation, Canada
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EqVRo = 0.785×GRor + 0.05
2018) EqVRo = 1.055×GRor－0.053
(Luo et al., 2018)
0.65–4.03%
(Wang et al., 2019a)
1.21–4.91%
Wufeng–Longmaxi Formation
EqVRo = 0.546×GRomax +0.35 EqVRo = 0.97×GRor－0.2 (2.19% <
Wufeng–Longmaxi Formation
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GRor < 3.5%); EqVRo = 0.22×GRor +2.55(GRor > 3.5%) EqVRo = 0.99×GRor + 0.08;
This study
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EqVRo = 0.515×GRomax + 0.506
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6
Cole (1994) (anoxic) Cole (1994) (oxic)
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Zhong and Qin (1995) Petersen et al. (2013) Colţoi et al. (2016) Synnott et al. (2018) Luo et al. (2018) (GRo)
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EqvRo (%)
4
Luo et al. (2018) (GRomax) Wang et al. (2019a) (2.19% < GRor < 3.5%)
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2
Bertrand (1990)
Wang et al. (2019a) (3.5% < GRor < 4.91%) This study (GRo) This study (GRomax) 1:1 line
0 0
2 4 GRo (%) (except where noted)
6
Fig. 13. The various relationships between GRo or GRomax and EqvRo.
In China, graptolites were usually interpreted as solid bitumen or vitrinite-like particles in the Wufeng–Longmaxi Formations, due to the misidentification on OM, leading to a long-term debate on their thermal maturity. Luo et al. (2018) have described how to discriminate between graptolite and bitumen in those formations. NGG displays a smoother surface, stronger anisotropy, and higher random and maximum reflectance than solid bitumen based on observation and measurement under polarized and non-polarized light (Luo et al., 2018; Wang et al., 2019a). In the Wufeng–Longmaxi Formations, the solid bitumen random reflectance (SBor) has strong positive correlations with GRor and GRomax in sections perpendicular to bedding (Fig. 14a and b) (Luo et al., 2018; Wang et al., 2019a). Data from other
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graptolite-bearing sediments (Yang and Hesse, 1993; Bertrand et al., 2003; Yang, 2016; Reyes et al., 2018) are plotted with the Wufeng–Longmaxi data in Fig. 14c and d, which indicates that SBor and GRor are nearly equivalent, very different from their relationship in Fig. 14a. This is because the reflectances from Yang and Hesse (1993), Bertrand et al. (2003), Yang, (2016), and Reyes et al. (2018) were measured in
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random orientation, whereas the data in the two Wufeng–Longmaxi studies were measured on sections
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perpendicular to bedding (Luo et al., 2018; Wang et al., 2019a). 9
12
y = 2.12x - 0.57 r = 0.92
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Luo et al., 2018
Mean maximum reflectance of graptolite (%)
(b)
Wang et al., 2019a
na
6
3
y = 1.19x - 0.09 r = 0.95
0 0
ur
Mean random reflectance of graptolite (%)
(a)
2
4 Wang et al., 2019a Luo et al., 2018 0
4
Mean random reflectance of solid bitumen (%)
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8
6
0
2
4
Mean random reflectance of solid bitumen (%)
6
9
9
6
(d)
Wang et al., 2019a Luo et al., 2018 Reyes et al., 2018 Yang et al., 2016 Bertrand et al., 2003 Yang and Hesse, 1993
Mean random reflectance of graptolite (%)
Mean random reflectance of graptolite (%)
(c)
outlier
3 y = 1.05x + 0.20 r = 0.94 0
Reyes et al., 2018 Yang et al., 2016
outlier
Bertrand et al., 2003 Yang and Hesse, 1993
6
3
y = 0.98x + 0.32 r = 0.95 0
0
2
4
6
0
2
4
6
Mean random reflectance of solid bitumen (%)
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Mean random reflectance of solid bitumen (%)
Fig. 14. The relationship between mean SBor and (a, c and d) mean GRor, and (b) mean GRomax (Yang and Hesse, 1993; Bertrand et al.,
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2003; Yang, 2016; Luo et al., 2018; Reyes et al., 2018; Wang et al., 2019a).
5.2.4.2 Comparison with CAI
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Some studies have correlated the relationship between GRomax and VRor through CAI. When sediments have GRomax of 0.6–1.2%, it means that they are immature (~0.2–0.5% VRor, CAI 1.5); when
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GRomax is 1.2–2.2%, sediments are in the oil window (~0.5–1.30% VRor, CAI: 1.5–2.5) (Fig. 15) (Goodarzi and Norford, 1989; Goodarzi et al., 1992a; Gentzis et al., 1996). However, these results should be used very carefully, because a CAI generally implies a range of thermal maturity (Epstein et al., 1977).
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Graptolite and vitrinite reflectances are more sensitive to thermal maturity than CAI, especially in overmature sediments (Goodarzi and Norford, 1985). While graptolite reflectance displays an excellent
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correlation with CAI at low thermal maturity (CAI: 1–4), their correlation is not clear in sediments with
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higher CAI (Goodarzi and Norford, 1985; Goodarzi et al., 1992a).
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Fig. 15. The relationship between CAI and GRomax (Goodarzi and Norford, 1989; Goodarzi et al., 1992a; Gentzis et al., 1996).
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5.2.4.3 Comparison with Tmax
Graptolite reflectance has also been compared to Tmax for the same samples (Cole, 1994; Petersen et
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al., 2013; İnan et al., 2016; Synnott et al., 2018). Cole (1994) studied organic maturity of Silurian Qusaiba deposits, Saudi Arabia, using zooclasts reflectance and conventional maturity indicators (e.g., Tmax), and proposed that GRor will increase with the increasing oxygen contents of the depositional environments.
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He found that EqVRo of the sediments deposited under anoxic environments is equal to 80% of GRor, whereas EqVRo of the sediments deposited under oxic environments is equal to 65% of GRor (Table 2)
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(Cole, 1994). Petersen et al. (2013) have determined the relationship between GRor and EqVRo on the basis of Tmax, which can be expressed as: EqVRo = 0.73×GRor + 0.16 (Table 2). It should be noted that
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GRor displays a bimodal distribution due to the random orientation of the polished blocks in their study. İnan et al. (2016) determined the thermal maturity of the hot shales from Saudi Arabia based on graptolite reflectance, organic geochemical and spectroscopic methods. Other studies on Tmax and graptolite reflectance are conflicting and inconclusive (Table 2) (e.g., Petersen et al., 2013; Synnott et al., 2018). The difficulty in using Tmax is mostly due to unreliable values of Tmax, the influence of kerogen type, and low S2 values (e.g., Petersen et al., 2013; Synnott et al., 2018).
5.2.4.4 Comparison of artificial thermal-treatment of graptolites and coals/vitrinite Laboratory experiments where graptolites and vitrinite with similar maturity were heat treated (220– 600 ℃) under similar conditions have shown that graptolites develop high reflectance at 600 ℃ (Bustin et al., 1989). Reyes et al. (2018) used hydrous pyrolysis on immature graptolite-bearing sediments (GRor=0.55%) of the Boas River Formation treated to temperatures of 310 to 350℃, and proposed the relationship between GRor and Rvl as the following equation: Rvl = 0.79×GRor. In contrast, Luo et al. (2018) heated graptolite-bearing sediment, with wider maturity (GRor = 0.65 to 4.03%), through a temperature range of 350 ℃ and 550 ℃ and found that their reflectance is similar to that of the vitrinite.
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Comparison of all published data and data from this study on heat–treated and natural graptolites and vitrinite (or vitrinite-like particles) (Bustin et al., 1989; Bertrand, 1990; Luo et al., 2018; Reyes et al., 2018), regardless of the difference of the experimental methods, indicates a strong and positive relationship (Fig. 16). This relationship can be expressed as:
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EqVRo = 0.99×GRor + 0.08 (1)
This equation is essentially coincident with Bertrand (1990) and Luo et al. (2018), and close to the
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1:1 line (Fig. 9).
Equation (1) combined with Fig. 11 can correlate GRomax to that of EqVRo and it can be expressed
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as:
(2)
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EqVRo = 0.515×GRomax + 0.506
Jo
ur
Vitrinite random reflectance (%)
6
y = 0.99x + 0.08 r = 0.99
4
Luo et al., 2018 2
Bertrand, 1990 Bustin et al., 1989 Reyes et al., 2018 This study
0 0
2
4
6
Graptolite random reflectance (%)
Fig. 16. The relationship between VRor and GRor. The Rvl (reflectance of vitrinite-like particles) values adopted from Reyes et al. (2018)
were converted to EqVRo according to the equation proposed by Xiao et al. (2000).
5.3. The chemical composition of graptolite periderm The electron microprobe, FTIR and Raman spectrum have been widely used to study the chemical composition of various organic macerals including graptolite fragments (Zerda et al., 1981; Green et al., 1983; Jehlička and Bény, 1992; Wopenka and Pasteris, 1993; Bustin et al., 1993; Mastalerz and Bustin, 1993b, a, 1995, 1996, 1997; Ward and Gurba, 1999; Kelemen and Fang, 2001; Ward et al., 2005, 2007, 2008; Chen et al., 2012; Chen et al., 2014; Wang et al., 2014; Wilkins et al., 2014; Wilkins et al., 2015;
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Chen et al., 2015b; Lünsdorf, 2016; Cheshire et al., 2017; Henry et al., 2018; Hao et al., 2019). 5.3.1 Electron microprobe
Recently, the electron microprobe has been used to identify elemental compositions of graptolites in the Silurian Llandovery–Ludlow shales from northern Poland (Morga and Kamińska, 2018). GRor of these
-p
shales ranges between 1.30% and 1.83%. Carbon is the predominant element in graptolite periderm; carbon, oxygen, nitrogen and sulfur contents range from 84.92 to 91.45 %, 2.56 to 8.43 %, 1.43 to 2.89 %
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and 0.02 to 0.90 %, respectively (Morga and Kamińska, 2018). In order to compare the elemental composition of graptolite and vitrinite (or telocollinite), data for vitrinite were collected from the literature
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(Mastalerz and Bustin, 1997; Ward et al., 2005, 2008). O content in the graptolites negatively correlates with C content (Fig. 17a), similar to vitrinite (Mastalerz and Bustin, 1997; Ward et al., 2005, 2008). For
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elemental comparison with reflectance in Fig. 17, the vitrinite maximum reflectance reported by Ward et al. (2005, 2008) was converted to VRor based on the equation proposed by Komorek and Morga (2002), and GRor in Morga and Kamińska (2018) was calculated to EqVRo based on equation (1). C and O content
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of graptolites is similar to that of vitrinite with similar thermal maturity (Fig. 17b and c) (Mastalerz and Bustin, 1997; Ward et al., 2005, 2008). Similar to the vitrinite, C content of graptolites in shales from
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Poland increases as organic maturity increases, while O content decreases as organic maturity increases (Fig. 17b and c). The O/C ratio in the graptolites displays a negative correlation with EqVRo (Morga and Kamińska, 2018), comparable to that of vitrinite (Mastalerz and Bustin, 1997; Ward et al., 2005, 2008) (Fig. 17d). These variations of the elemental composition in graptolites are consistent with the maturation process and/or the loss of the O-bearing functional groups from organic macromolecules during thermal evolution (Taylor et al., 1998; Bustin and Guo, 1999).
30
4
(a)
(b)
Vitrinite (Ward et al., 2008) Vitrinite (Ward et al., 2005) Vitrinite (Mastalerz and Bustin, 1997)
3
Graptolite (Morga and Kamińska, 2018)
O (%)
EqVRo (%)
20
2
10 1
0
0 70
80
90
100
60
70
C (%)
4
4
(c)
(d)
90
100
2
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2
-p
3
EqVRo (%)
3
EqVRo (%)
80 C (%)
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60
1
0 0
10
lP
1
20
30
0 0
O (%)
0.1
0.2
0.3
0.4
O/C (%)
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Fig. 17. Comparison of the chemical composition and reflectance of graptolites and vitrinite. The data for the graptolites are from Morga and Kamińska (2018); data on vitrinite from Mastalerz and Bustin (1997) and Ward et al. (2005, 2008). (a) The negative correlation
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between O and C; (b) the positive correlation between EqVRo and C; (c) the negative correlation between EqVRo and O; (d) the negative
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correlation between EqVRo and O/C. The legends in b–d are the same as in a.
5.3.2 Fourier Transform Infrared Spectroscopy (FTIR) FTIR can provide fundamental information not only about the molecular structure of macerals but
also about the thermal maturity of the hydrocarbon source rocks, and has been widely used to study the functional groups of the macerals in coals with variable rank (Mastalerz and Bustin, 1993a, b, 1995, 1996; Chen et al., 2012; Wang et al., 2014; Chen et al., 2015b; Wang et al., 2017b). However, several studies have also been conducted on graptolites using FTIR (Bustin et al., 1989; Liu et al., 1996; Suchý et al.,
2002; Suchý et al., 2004; Caricchi et al., 2016; İnan et al., 2016; Morga and Kamińska, 2018). Bustin et al. (1989) used FTIR to study evolution of graptolites during artificial thermal-treatment, and found that graptolites display a decrease of aliphatic chains and a depletion of aromatic C–H with increase in thermal maturity/heat treatment due to dealkylation and aromatization. Bustin et al. (1989) also found that graptolites are highly aromatic and have fewer aliphatic structures than vitrinite, whereas Hoffknecht (1991) proposed that the percent of aliphatic compounds in graptolites is greater than that in vitrinite. The parameter CH2/CH3 band intensity ratio can provide information about the length of aliphatic chains (Lin and Ritz, 1993), and Suchý et al. (2002) found that the CH3/CH2 ratio increases with
ro of
increasing GRor. The CH3/CH2 ratio reported by Suchý et al. (2002) was converted to the CH2/CH3 ratio, and plotted in Fig. 18 where it is compared it with the results of Morga and Kamińska (2018). The data from these two studies displays opposite correlations of CH2/CH3 ratio with GRor (Fig. 18). The data of the Silurian deposits in the Polish Baltic Basin from Caricchi et al. (2016), however, did not show any
-p
obvious correlation (Fig. 18), which may be because in that study, FTIR was conducted on the kerogen concentrate rather than pure graptolites, and the reflectance data was very scattered in a short interval of
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the same well. In low rank coals (Ro < 1.5%), the CH2/CH3 ratio shows a strong negative correlation with vitrinite reflectance, similar to the results of Morga and Kamińska (2018). This indicates that the aliphatic
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chains become shorter with maturation. The CHar/(CH2+CH3) ratio increases with burial depth and GRor, suggesting an increase in the aromatization of graptolite periderm (Morga and Kamińska, 2018).
na
Graptolites have been thought to be primarily type II kerogen and less commonly type III according to FTIR parameters of ‘A2’ and ‘C2’ factors (Morga and Kamińska, 2018 and references therein).
GRor (%)
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ur
3
2.5
2
1.5
1 Suchý et al., 2002 Caricchi et al., 2016 Morga and Kamińska, 2018
0.5
0 0.4
0.8
1.2 CH2/CH3
1.6
2
Fig. 18. The relationship between CH2/CH3 and GRor.
5.3.3 Raman spectroscopy Raman spectrum is a non-destructive and rapid microstructure analysis technique, and has been used widely as an indicator for thermal maturity of sediments in the last several decades (Zerda et al., 1981; Green et al., 1983; Jehlička and Bény, 1992; Wopenka and Pasteris, 1993; Kelemen and Fang, 2001; Romero-Sarmiento et al., 2014; Wilkins et al., 2014; Wilkins et al., 2015; Lünsdorf, 2016; Cheshire et al., 2017; Henry et al., 2018; Jubb et al., 2018; Wilkins et al., 2018; Khatibi et al., 2018a, b). The Raman
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spectrum of OM is generally composed of two broad peaks, the graphite band (G band) and the disordered band (D band). In the recent decade, this technique has also been used to study graptolites (Suchý et al., 2004; Liu et al., 2013; İnan et al., 2016; Mumm and İnan, 2016; Cheshire et al., 2017; Morga and Pawlyta, 2018; Wang et al., 2019a). Suchý et al. (2004) used micro-Raman spectroscopy to determine the
-p
microstructure of the graptolite materials located around an igneous sill, and proposed that the parameter AD/AG (1350/1600 cm–1) peak area can be used to infer organic maturity. Liu et al. (2013) analysed the
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Raman spectrum from a Silurian graptolite-bearing sediment from Qiaokou, Sichuan, China. Mumm and İnan (2016) and İnan et al. (2016) have conducted Raman spectroscopy on Silurian Qusaiba samples with
lP
GRor ranging from 0.76% to 2.20%, and found that both G band position and Raman band separation (RBS; between G peak shift and D peak shift) display strong positive correlations with GRor. Cheshire et
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al. (2017) have also used RBS to estimate the thermal maturity of the Silurian Qusaiba samples with EqVRo ranging from 0.9% to 2.1% according to the relationship between RBS and VRor proposed by Sauerer et al. (2017), which is consistent with the results from other proxies, (e.g., kerogen elemental
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composition, kerogen skeletal density and BET specific surface area). Morga and Pawlyta (2018) found that Raman band intensity ratio (ID1/IG) shows a robust correlation with GRor in the Polish Silurian shales,
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and can be used to calculate EqVRo values. The relationship between ID1/IG and EqVRo has been determined by Morga and Pawlyta (2018) as equation (3): EqVRo = 1.7319 (ID1/IG) – 0.3295
(3)
Wang et al. (2019a) studied Raman spectra of graptolites in Chinese sediments from a wider thermal maturity range (GRor ranging from 1.21 to 4.91%). They found that D1 peak position decreases with increasing GRor when GRor < 4.0–4.5%, whereas D1 peak position increases with increasing GRor when GRor > 4.0–4.5%. Both G peak position and RBS display a reverse trend with increasing GRor in
comparison with D1 peak position (Wang et al., 2019a). However, it is worth noting that samples with GRor > 4.5% are very limited and the maximum of GRor in this sample set is only 4.91%, thus, more data are needed to illustrate their variation tendency at the high-maturity end. Wang et al. (2019a) established the relationship between GRor and EqVRo as equations (4) and (5), respectively, through the intermediate conversion of RBS (2.1% < GRor < 3.5%) and ID1/IG (GRor > 3.5%) based on the relationship between VRor and RBS (2.1% < GRor < 3.5%), ID1/IG (GRor > 3.5%) proposed by Liu et al. (2013). EqVRo = 0.97 GRor – 0.2 (2.1% ＜ GRor ＜ 3.5%) EqVRo = 0.22 GRor ＋ 2.55 (GRor ＞ 3.5%)
(4) (5)
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Recently, Hao et al. (2019) compared the Raman spectral parameters of the naturally and artificially matured samples, and the latter has a lower degree of coalification than the former due to insufficient structural transformation of artificially matured samples. They found that GRor displays positive correlations with the full width at half maximum of the D1 band and the G band (FWHM-D/FWHM-G),
EqVRo = 0.089 RBS – 19.937
(6)
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relationship between EqVRo and RBS as follows:
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AD/AG and RBS, and negative correlations with D1 peak position and FWHM-G, and established the
In order to compare the relationship between RBS and reflectance among various types of OM, the
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data for vitrinite, inertinite, bitumen and graptolite were collected from several publications noted in Fig. 19 (Kelemen and Fang, 2001; Suchý et al., 2004; Wilkins et al., 2014; Mumm and İnan, 2016; Sauerer et
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al., 2017; Morga and Pawlyta, 2018; Wang et al., 2019a; Hao et al., 2019). The RBS of vitrinite, inertinite, bitumen and graptolites increases as thermal maturity increases when the reflectance is lower than 4%, with the exception of the data from Morga and Pawlyta (2018). However, these data become significantly
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scattered when the reflectance is higher than 4%, especially for the coal/vitrinite data from Liu et al. (2013) and Kelemen and Fang (2001). In addition, G peak position is negatively correlated with GRor in samples
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from Poland (Morga and Pawlyta, 2018), which is opposite to the results from the Saudi Arabian Qusaiba samples and the Chinese Wufeng–Longmaxi shales (İnan et al., 2016; Mumm and İnan, 2016; Wang et al., 2019a). These inconsistencies may be due to differences in conditions as follows: (1) sample preparation; (2) experimental conditions (e.g., laser wavelength); (3) data processing; (4) analysis of various OM, and intra-particle chemical heterogeneity (Lünsdorf, 2016; Sauerer et al., 2017; Henry et al., 2018; Jubb et al., 2018; Hao et al., 2019). Thus, uniform conditions are required to better determine the correlation between Raman spectral parameters and graptolite reflectance in future studies.
290
RBS (cm-1)
260
Vitrinite & inertinite (Wilkins et al., 2014)
Vitrinite (Liu et al., 2013)
Type III kerogen (Kelemen and Fang, 2001)
Type II kerogen (Sauerer et al., 2017)
Type II kerogen (Kelemen and Fang, 2001)
Graptolite (Suchý et al., 2004)
Graptolite (Mumm and İnan, 2016)
Graptolite (Morga and Pawlyta, 2018)
Graptolite (Wang et al., 2019a)
Graptolite (Hao et al., 2019)
Bitumen (Liu et al., 2013)
1
2
3
4 Ro (%)
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Coal (Kelemen and Fang, 2001)
200 0
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230
5
6
7
8
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Fig. 19. The relationship between RBS and reflectance for the vitrinite, inertinite, bitumen and graptolite. The minimum and maximum values of vitrinite and bitumen random reflectance have been given by Liu et al. (2013) rather than the average, thus, half of the total
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minimum and maximum values were used as Ro in this figure.
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6. The microstructure of graptolites
6.1 Scanning Electron Microscopy (SEM) Loucks et al. (2012) classified three types of shale pore systems based on SEM: OM, interparticle,
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and non-OM intraparticle pores. Although OM pores are within OM, they can form an organic interconnected pore network due to OM connectivity, thus making a significant contribution to the overall
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porosity (Passey et al., 2010; Loucks et al., 2012; Mastalerz et al., 2013; Milliken et al., 2013). OM pores in the size range from 5 to 750 nm can adsorb and store methane simultaneously (Loucks et al., 2009), and shales with higher OM content display stronger capacity of methane adsorption because of larger surface area supported by OM micropores (Hickey and Henk, 2007; Ross and Bustin, 2007). OM pores are widely identified in North American and Chinese gas-bearing shales, e.g., the Barnett, Ohio, Antrim, Marcellus, Woodford, Wufeng–Longmaxi and Longtan shales (Hill and Nelson, 2000; Curtis, 2002; Jarvie et al., 2007; Chalmers et al., 2012; Curtis et al., 2012; Loucks et al., 2012; Mastalerz et al., 2013; Milliken
et al., 2013; Tian et al., 2013; Cardott et al., 2015; Hackley and Cardott, 2016; Luo et al., 2016; Ma et al., 2016; Gentzis et al., 2017; Zhang et al., 2017). Pores within graptolites have been studied by SEM (Luo et al., 2016; Ma et al., 2016; Cardott and Curtis, 2018). While many micro-nanopores are widely found in the overmature Wufeng–Longmaxi Formations (Fig. 20) (Luo et al., 2016; Ma et al., 2016), scarce nanopores are present in immature graptolites (GRor=0.66%) from the Lower Ordovician Polk Creek Shale (Cardott and Curtis, 2018). Micro-nano pores in Chinese graptolites generally display irregular or elliptical or nearly spherical shapes, and can be more easily observed in sections parallel to bedding than in sections perpendicular to bedding (Luo et al., 2016; Ma et al., 2016).
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Graptolites fragments with pores were recently thought to be solid bitumen due to their presence within interparticle pores (İnan et al., 2018). However, Luo et al. (2018) found some graptolites within the interparticle pore space (Fig. 5 in Luo et al., 2018), which look like solid bitumen, but morphologically display graptolitic fusellar layers under polarized light. In order to resolve this conflict, graptolites were
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marked using diamond lenses during observation under reflected light and then observed under SEM. Many pores were found in the graptolites, although they were smaller and less numerous than those within
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solid bitumen (Fig. 20).
The development and distribution of OM pores in some graptolites are controlled by their fine
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biological structures (Luo et al., 2016; Ma et al., 2016). Graptolites are generally thought to have low porosity, ranging from 1.62% to 4.23% and averaging 2.51% (Ma et al., 2016). Pores in graptolites can
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further make up an interconnected system due to their alignment along the fusellar layers, and may connect with discrete porous OM detritus and/or other pore systems (Luo et al., 2016; Ma et al., 2016), which is beneficial to the storage and the exploitation of shale gas in graptolite-bearing sediments. OM
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pores are also regarded as an important pore system in other gas bearing shales, e.g., Barnett shale (Loucks
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et al., 2009; Chalmers et al., 2012).
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Fig. 20. OM pores in the Wufeng–Longmaxi Formations. (a) Solid bitumen and graptolite under dry objective; (b) same graptolite under SEM as (a); (c) SEM image showing of location of images d-f; (d-f) images show the pores in solid bitumen are slightly larger and more
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numerous than that in the graptolite (marked by arrows).
6.2 High resolution transmission electron microscopy (HRTEM) HRTEM can study OM microstructure at nm scale (Krzesińska et al., 2009; Chalmers et al., 2012; Pawlyta, 2013; Romero-Sarmiento et al., 2014; Morga and Pawlyta, 2018). Morga and Pawlyta (2018) studied the nanostructure of graptolites with GRor of 1.83% using HRTEM. The parallel alignment of carbon layers in the graptolite can be observed in their HRTEM photos, and the dimension of the basic
structural units is around 1–2 nm, indicating high molecular ordering, in agreement with the study of Goodarzi (1984).
7. The regional maturation and hydrocarbon generation potential of the graptolitebearing sediments 7.1 The hydrocarbon generation potential of graptolites Wang et al. (2017a) concentrated mature (EqVRo around 1.10%) graptolite fragments from two Wufeng–Longmaxi shales. The TOC in the two isolated graptolites is 42.93% and 71.34%, S1 is 6.55 and 15.18 mg/g, S2 is 31.71 and 60.36 mg/g, and HI is 74 mg/g TOC and 85 mg/g TOC, respectively, which
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is much higher than that in the whole rocks (Table 3). İnan et al. (2016) found that an immature isolated graptolite had HI of 200 mg/g TOC and OI of 30 mg/g TOC, much higher than that of mature graptolites (Wang et al., 2017a). These data indicate that the immature graptolites are relatively hydrogen-rich, similar to kerogen type II-III, and have good hydrocarbon potential which is consistent with finding of
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Hoffknecht (1991), Bustin et al. (1989) and Liu et al. (1996) based on FTIR. Pyrolysis chromatography of the two Chinese graptolies found that the CH4 yields are 12.35 mg/g and 23.61 mg/g, respectively
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(Wang et al., 2017a). The weak brown fluorescence in the graptolites of the Liteň Formation from the Czech Republic also indicates that the graptolites have some hydrocarbon generation potential (Fig. 5),
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which means that the graptolite can not be classified as “inertinite-like” macerals (Hoffknecht, 1991). Further, the formation of the OM pores is related with hydrocarbon generation (Loucks et al., 2009),
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which can explain the rare nanopores in low-maturity graptolites from the Lower Ordovician Polk Creek Shale (Cardott and Curtis, 2018). On the other hand, the abundant pores in the graptolites with high maturity also supports their gas generation and storage potential (Fig. 20). These factors suggest that these
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graptolites are mainly gas prone, which is consistent with the results of Hoffknecht (1991), and that graptolites possibly make a significant contribution to the contents of shale gas in graptolite-bearing
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sediments.
Table 3. Comparison of the Rock-Eval data of whole rock versus graptolites (MB-2 and MB-3 from Wang et al., 2017a; K1 from İnan et
Sample ID
al., 2016).
Lithology
Formation
S1 (mg/g)
S2 (mg/g)
Tmax (℃)
HI
OI
(mg/g TOC)
(mg/g TOC)
TOC (%)
MB-2 (whole rock)
Shale
O3w
0.54
1.77
455
3.04
58
8
MB-3 (whole rock)
Shale
S1l
0.69
2.88
458
3.88
74
5
MB-2 (graptolite)
Graptolite
O3w
6.55
31.71
464
42.93
74
43
MB-3 (graptolite)
Graptolite
S1l
K1 (graptolite)
Graptolite
S1l
15.18
60.36
456
71.34
85
2
200
30
Following are examples of worldwide regional maturation and hydrocarbon generation potential studies on graptolite-bearing sedimentary strata.
7.2 Canada Graptolite reflectance was used in several regional thermal maturation studies in Canada (Riediger et al., 1989; Link et al., 1990; Goodarzi et al., 1992c; Gentzis et al.,1996). Most of these regional studies were carried out in Canadian Arctic (Goodarzi et al.,1992c; Gentzis et al.,1996) on graptolitic shale and
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equivalent Upper Ordovician-lower Middle Devonian strata over a distance of over 600 km to determine horizontal variation of thermal maturity and depositional history from Melville Island in southwest to Ellesmere Island in northeast (Fig. 21). The Middle Devonian is up to 3000 m thick, and extends from central Ellesmere Island to Melville Island (Fig. 21) (Trettin, 1989, 1990).
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Based on graptolite reflectance, the sedimentary succession is overmature (3.5–4.2% GRomax) in western Melville Island, mature in central Melville island (1.75–2.1% GRomax), and immature (0.60–0.80%
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GRomax) in the Prince of Wales Islands, eastern Bathurst, Cornwallis, Baillie Hamilton, Dundas, and
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Devon Island (Fig. 21). Therefore, the maturity of graptolitic shale increases from mature (1.32% GRomax) in south of Ellesmere Island to overmature (4.7% GRomax) in north (Fig. 21). Variations in thermal maturity may be due to the areal difference in geothermal gradient and
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overburden thicknesses. Since regional geothermal gradient has been determined to be ~25 °C/Km in Melville Island (Gentzis, 1991), and this value is considered within conventional scope of geothermal
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gradient from other North American basins (Gretener, 1982), the difference in thermal maturation is
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probably due to the thicknesses of the original overburden (Fig. 22).
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Fig 21. Map of the Canadian Arctic Islands showing graptolite maximum reflectance (%GRomax) and maturity of the graptolitic-bearing strata (modified after Gentzis et al., 1996). The green area on the map are designated national park and wilderness.
0
GRomax (%) 2 3
4
5
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1000
1
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0
3000
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Eroded section (m)
2000
4000 5000 6000 7000 8000
Fig. 22. Variation of graptolite maximum reflectance (%GRomax) and overburden strata (eroded sedimentary loading) in Arctic Canada (after Gentzis et al., 1996).
7.3 China The Wufeng–Longmaxi graptolite-bearing sediments in China have various thermal maturities (Luo et al., 2016; Luo et al., 2017; Luo et al., 2018; Wang et al., 2019a), and have been the focus of interest due to both the increased importance of shale gas in the global petroleum industry (Curtis, 2002; Bowker,
2007; Jarvie et al., 2007; Ross and Bustin, 2009; Loucks et al., 2012; Dai et al., 2014; Hackley and Cardott, 2016), and the successful exploration and exploitation of shale gas in the area (Dai et al., 2014; Dai et al., 2016). The Wufeng–Longmaxi sediments are characterized by abundant OM (TOC: 0.51–25.73%, average 2.59%), overmature levels (mostly EqVRo > 2%), thick beds (30–130 m), abundant OM pores and strong gas generation intensity (Table 4) (Zou et al., 2011; Dai et al., 2014; Dai et al., 2016; Luo et al., 2016; Ma et al., 2016; Luo et al., 2017; Luo et al., 2018), important for the generation and accumulation of shale gas. The first large shale gas field was discovered in the Wufeng–Longmaxi Formations, Fuling, Chongqing, with gas reserves over 100×109 m3 (Dai et al., 2014; Dai et al., 2016).
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PetroChina developed the Changning-Weiyuan shale gas field in the Wufeng–Longmaxi sediments in 2014, and the gas production reached 28×108 m3 in 2016 (Ma and Xie, 2018).
In the Wufeng–Longmaxi Formations, graptolite-derived OM accounts for 20–93 vol.% of total OM (Luo et al., 2016). The abundance of graptolites displays a positive correlation with TOC in both Wufeng–
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Longmaxi Formation from Sichuan Basin and Pingliang Formation from Erdos Basin (Zhu et al., 2015; Deng et al., 2016; Borjigin et al., 2017). Based on the comparison of natural and artificial maturation
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samples of Wufeng–Longmaxi Formations and Alum Shale (Sweden) (Luo et al., 2018), OM in the highmaturity Wufeng–Longmaxi shales is very similar to the artificially-matured Alum Shale (Fig. 23). This
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implies that OM in the immature Wufeng–Longmaxi shales is similar to OM in the immature Alum Shale. The Wufeng–Longmaxi Formations have been subdivided and correlated on the basis of lithology
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or logging curves, however, it has not been accepted widely due to the lower resolution and precision of these methods (Zou et al., 2015; Chen et al., 2017). As important biostratigraphic indicator, graptolite species have been widely used for the stratigraphic correlation (Chen, 1984; Bergström and Mitchell,
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1986; Chen et al., 1987; Cooper, 1999; Chen et al., 2000; Chen et al., 2004; Chen et al., 2005; Loydell, 2011; Chen et al., 2017). Chen et al. (2015a) proposed a new subdivision and correlation for the Wufeng–
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Longmaxi Formations based on the graptolite biostratigraphy. According to this revised stratigraphy, intervals (WF2–WF3 and LM2–LM6) were regarded as the best targets for shale gas (Chen et al., 2015a; Chen et al., 2017), and they generally contain abundant graptolites (> 30%) (Qiu et al., 2018). The pores in these graptolites also act as a reservoir and/or migration pathway of the shale gas (Ma et al., 2016; Luo et al., 2018; Qiu et al., 2018).
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Fig. 23. Comparison of OM in natural Wufeng–Longmaxi shales (a) and heat-treated Alum Shale heated at 550 ℃ for 3 days (b).
A regional maturation pattern based on GRor converted to EqVRo (Yang, 2016; Luo et al., 2018; Wang et al., 2019a) for the Wufeng–Longmaxi Formations from Chongqing and surrounding areas indicates that graptolite-bearing strata are mostly overmature, with maturity increasing from southwest
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towards northeast with an EqVRo of 1.17–4.93%, the maximum in sample TT from Pingshan, Sichuan
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province, and minimum in well YC1 from Chengkou, Chongqing (Fig. 24).
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Table 4. Geological and geochemical data of selected global graptolite-bearing sediments. Area/Basin
Formation
Age
TOC (%)
China
Chongqing
Wufeng–Longmaxi
late Ordovician–early Silurian
0.51–25.73
Basin
Alum Shale
Miaolingian–Tremadocian
Sweden
Colonus Trough
Alum Shale
Miaolingian–Tremadocian
Swedena
Central Sweden
Alum Shale
Poland
BPLBb
Poland Poland
Norwegian– Denmark
Danish
EqVRo (%)
Depth (m)
Thickness (m)
Porosity (%)
Gas content (m3/t)
1.17–4.93
900–4500
30–130
3–10
1.7–4.5
pr
Country/Continent
3.00–9.00
1.8–2.5
2000–4000
30–180
3–6
avg. 0.85
3.00–16.00
1.7–2.0
700–1000
70–90
avg. 6.5%
avg. 0.85
Miaolingian–Tremadocian
1.00–21.00
0.5–1.1
20–100
20–35
/
max. 7.6
Piaśnica
Furongian–Tremadocian
3–12
0.55–2.40c
/d
10–34
3.17–6.89
/
BPLB
Sasino
Caradoc
1–7
0.5–4.2c
/
1.5–70
2.24–10.94
/
BPLB
Pasłęk
Llandovery
1–6
0.7–4.0c
/
20–80
3.95–5.56 (Jantar Member)
/
c
/
30–1000
5.25–7.88
/
2000–5000
20–100
/
2.4–8.5
Pr
e-
(onshore)
BPLB
Pelplin–lower part
Wenlock
0.5–1.7
0.6–2.5
North Africa
Ghadames Basin
Tanezzuft
Llandovery–Wenlock
2–16
0.7–2.0
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Poland
Biogenic gas;
b
BPLB = Baltic–Podlasie–Lublin Basin;
c
Mean random reflectance of the vitrinite–like macerals (Rvl);
d
“/” means no data.
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a
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Fig. 24. The EqVRo distribution of the Wufeng–Longmaxi shales in Chongqing and surrounding regions. Data at each sample location is
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well/outcrop name/EqVRo. Data are from Yang (2016); Luo et al. (2018); Wang et al. (2019a).
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7.4 Denmark and Sweden
Lower Palaeozoic strata is widely distributed in Denmark, Sweden, Norway, Poland and the Baltic
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States (Fig. 25). In this area, terrestrial to shallow marine sandy deposition commenced in lower Cambrian (Serie 2) following an overall transgression of the Baltic continent (Nielsen and Schovsbo, 2011, 2015). From the middle Cambrian (Miaolingian) to the early Ordovician (Tremadocian), organic rich mud deposition known as the Alum Shale Formation in Denmark, Sweden and Norway, the Türisalu Formation in Estonia, the Kaporye Formation in the St. Petersburg region and the Słowińska Formation as well as Piaśnica Formation in NE Poland completely dominated (Nielsen and Schovsbo, 2007). The shale is now present in the deeply buried strata towards the Polish-German and Swedish-Norwegian Caledonian fronts,
in the Baltic area and as erosional outliers in Sweden (Fig. 25), representing only a minor part of the original huge deposition area that its maximum at the early Ordovician may have extended for more than
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800 000 km2 (Schovsbo et al., 2018).
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Fig. 25. Outline of approximate original distribution of the Alum Shale Formation and present-day occurrence of the lower Paleozoic strata in southern Scandinavia (modified from Schovsbo et al., 2018).
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Thermogenic gas trapped in the Alum Shale has been explored in Denmark (Schovsbo and Jakobsen, 2019) and Sweden (Pool et al., 2012) (Table 4). In Denmark and Sweden, a considerable resource has
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been estimated by the U.S. Geological Survey (mean = 67 × 109 m3 gas; Gautier et al., 2013) and by the European Geological Surveys (Zijp et al., 2017) to be present mostly in the Cambrian (Miaolingian) to
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Ordovician (Tremadocian) Alum Shale Formation. Typical Alum Shale has TOC values from 5–10% (even up to 25% TOC in some samples) (Fig. 26), and their thermal maturity ranges from immature (EqVRo<0.5%) in Central Sweden and Estonia, dry gas mature (EqVRo: 1.6–2.5%) in most of Denmark and southern Sweden, and to post-mature to low-grade metamorphic in Norway (EqVRo >3%; Buchardt et al., 1997; Petersen et al., 2013) (Table 4). The shale is graptolitic with macroscopic remains of typical pelagic Rhapdinopora occurring abundantly in the Tremadocian shale. In the Cambrian, microscopic remains of graptolites also occur presumably from benthic species (Petersen et al., 2013) - an
interpretation that has been supported by the rediscovery of Miaolingian benthic graptolites in the Alum Shale from Norway (Wolvers and Maletz, 2016). At this time, no quantification of the contribution of graptolite carbon to the total TOC content has been made, and the only published study of the Alum Shale nanopore system by Henningsen et al., 2018 did not specifically investigate the graptolite contribution to the total porosity. The lower Paleozoic shale gas prospective areas in Denmark largely follow the margins of the Norwegian–Danish Basin (Schovsbo et al., 2014). Exploration of this play is still limited, and representative well data and production test data for the shales are lacking. The only exploration well in
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Denmark revealed gas in the Alum Shale Formation, however, no test production was carried out, so the commercial potential is still unknown (Schovsbo and Jacobsen, 2019). The well was drilled within a socalled ‘sweet spot’, defined as an area with expected highest gas content (c.f. Schovsbo et al., 2014). Results from this well revealed the Alum Shale to be 40 m thick, compared to up to 180 m in the
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depocenter offshore Denmark. In Sweden, thermogenic gas has been explored in Scania, within the Colonus Trough, which is a fault bounded graben system in southern Sweden. Exploration indicated that
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the Alum Shale Formation was gas mature (EqVRo: 1.7–2%), located at 700–1000 m depth (Table 4), but did not contain gas in economically producible quantities, possibly due to gas leakage as a result of uplift
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(Pool et al., 2012). In south-central Sweden, gas-bearing immature to low-maturity Alum Shale with depths < 150 m have been known for several decades and been under exploration for commercial shale
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gas production (Table 4); they are thought to be composed of mixtures of thermogenic and bacterially-
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derived gas (Schultz et al., 2015; Schovsbo and Nielsen, 2017).
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Fig. 26. Composite profile of TOC content in the Alum Shale in Scania, southern Sweden (modified from Schovsbo, 2003).
7.5 Poland
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The highest shale gas exploration potential in Poland is related to the Cambrian–Lower Ordovician (upper Furongian–Tremadocian), Upper Ordovician (Caradoc) and Silurian (Llandovery–Wenlock) black
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and dark grey shales with abundant graptolite remains within the Baltic-Podlasie-Lublin Basin (BPLB) at the slope of the East European Craton (EEC) (Table 4, Figs. 27 and 28) (Poprawa, 2010; Karcz et al.,
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2013; Podhalańska, 2013). Due to lateral facies variability, hydrocarbon potential of these rocks varies throughout the area. The lower Paleozoic deposits were buried to significantly different depths in the
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individual parts of the BPLB. Tectonic processes resulted in a break-up of the Basin into segments (Karcz et al., 2013). The current burial depth within the significant part of BPLB is shallow enough for
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commercial shale gas exploration, and geological structure is simple, especially within the Baltic Basin and Podlasie Basin (Poprawa, 2010).
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Fig. 27 The distribution of Upper Ordovician–lower Silurian sediments in the Baltic-Podlasie-Lublin Basin (BPLB) at the slope of the East European Craton (EEC) (modified after Poprawa, 2010). Abbreviations: SPW =Płock-Warsaw zone; SBN = Biłgoraj-Narol zone;
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TESZ = Trans-European Suture zone.
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The Furongian–Tremadocian Alum-Shale-equivalent black bituminous shales belong to the Piaśnica
Formation, which occurs only within the Baltic Basin (Fig. 28). Their thickness is from 10 m (in the onshore part) to 34 m (in the offshore part) (Szymański, 2008; Więcław et al., 2010; Podhalańska et al., 2016a), thinner than the Alum Shale in Denmark (30–180 m) and southern Sweden (70–90 m); and average TOC content ranges between 3% and 12.0% (Table 4). Thermal maturity of OM increases with the increasing depth from NE to SW from the main phase of oil generation through the condensate and wet gas phase to the dry gas generation phase. The Rock-Eval Tmax temperature and Rvl vary from 430 °C
to 505 °C, and from 0.55% to 2.4%, respectively (Poprawa, 2010; Więcław et al., 2010; Karcz and Janas, 2016; Podhalańska et al., 2016a) (Table 4). The effective porosity ranges between 3.17% and 6.89% (Table 4) (Dyrka, 2016). S2 is up to 72 mg HC/g rock, while HI varies from 6 to 484 mg HC/ TOC (Więcław et al., 2010). The gas content in the Piaśnica Formation reaches 7.6 m3/t in the Lębork S-1 well (Lehr and Keeley, 2016) (Table 4), comparable to that in North American gas-bearing sediments (e.g.,
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Jarvie, 2012).
ro of -p re lP na ur Jo Fig. 28 Generalized lithostratigraphy of the upper Cambrian-Silurian deposits in the Polish part of the Baltic-Podlasie-Lublin Basin (after Podhalańska 2016b).
The Caradoc (Sandbian–lower Katian) rocks (i.e., Sasino Formation) are most fully developed in the northern and western parts of the Baltic Basin, mainly composed of dark grey and black shales with rich graptolite fauna (Fig. 28) (Modliński and Szymański, 1997; Podhalańska, 2013; Podhalańska et al., 2016a). Similar rocks but with lower thickness and stratigraphic extent stretch to the southeast in the Podlasie and Lublin areas (different regional lithostratigraphic equivalents). Thickness of the Sasino Formation and its equivalents in other parts of the BPLB varies from 1.5 m in the eastern onshore part of the Basin to 70 m in its offshore part (Table 4) (Modliński and Szymański, 1997, 2008; Poprawa, 2010; Więcław et al., 2010; Podhalańska et al., 2016a; Podhalańska et al., 2016b). The average TOC content in
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the Sasino Formation ranges from 1% to 4%, and even up to 7%, in the offshore part (Table 4) (Poprawa, 2010; Więcław et al., 2010; Karcz and Janas 2016; Podhalańska et al., 2016a). Thermal maturity increases from NE to SW with increasing depth from the main phase of oil generation (0.6–1.1% Rvl) through condensate and wet gas generation phase (1.1–1.4% Rvl) to the dry gas generation phase (>1.4% Rvl),
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reaching 4.2% within the Baltic Basin (Table 4) (Więcław et al., 2010; Podhalańska et al., 2016a). Tmax values reported for the BPLB are 425–474oC (Więcław et al., 2010; Karcz and Janas 2016). The effective
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porosity ranges between 2.24% and 10.94% (Table 4) (Dyrka, 2016). S2 reported for the Baltic Basin varies from 0.05 to 10.6 mg HC/g rock, and HI ranges from 11 to 359 HC/g TOC.
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Llandovery (Rhuddanian-Telychian) shales make up the Pasłęk Formation, whereas the Wenlock (Sheinwoodian-Homerian) rocks belong to the lower part of the Pelplin Formation (Sheinwoodian-
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Ludfordian as a whole) (Fig. 28) (Podhalańska et al., 2016b). They contain rich and variable graptolite fauna, which made it possible to determine biostratigraphic levels and limits of chronostratigraphic units (Podhalańska, 2013). The Llandovery shales of the Pasłęk Formation occur throughout vast parts of the
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western slope of EEC. Their thickness generally increases from the east to the west, reaching a maximum of 80 m. However, in the major part of the BPLB, it ranges from 20 to 40 m (Table 4). Average TOC
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values are usually 1% to 2.5%, except for the Podlasie Basin, where they reach 6% (and even > 15% in individual layers) (Table 4). High TOC content was particularly documented in the lower part of the Pasłęk Formation (the Jantar member) (Modliński et al., 2006; Poprawa 2010; Więcław et al., 2010; Karcz et al., 2013). Rvl in the Baltic Basin varies from 0.7% to 2.3%, and rises to 4.0% in the Podlasie Basin, showing the NE-SW trend (Table 4) (Poprawa, 2010; Więcław et al., 2010; Karcz et al., 2013). Tmax temperature reported for the Baltic Basin falls within the range of 435–534oC. S2 varies from 0.12 to 40.6 mg HC/g rock, while HI varies from 8 to 436 mg HC/g TOC (Więcław et al., 2010).
The Jantar Member (Rhuddanian–lower Aeronian), composed of bituminous shales, is the most perspective unit of the Pasłęk Formation within the onshore and offshore areas of the Baltic Basin (Fig. 28) (Podhalańska et al., 2016a). Its maximum thickness is 18 m (Modliński et al., 2006; Podhalańska et al., 2016a). The average TOC content varies from 2% to 5%, and the thermal maturity increases from NE to SW, starting with the oil generation window and passing through condensate and wet gas phase to dry gas generation phase (Poprawa 2010; Karcz and Janas, 2016; Podhalańska et al., 2016a; Podhalańska et al., 2016b). Rock-Eval Tmax values range between 438 and 485 °C (Karcz and Janas, 2016). Average effective porosity is 3.95–5.56% (Table 4) (Dyrka, 2016).
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The Wenlock shales have the largest extent of all stratigraphic units. They have been documented within the whole BPLB (Fig. 28) (Podhalańska et al., 2016a). Their thickness is highly variable, from 30 m in SE part of the Lublin Basin to over 1000 m in western part of the Baltic Basin (Table 4) (Poprawa, 2010; PIG, 2012; Karcz et al., 2013; Podhalańska et al., 2016a). Average content of OM in individual
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Wenlock sections in central and western parts of the Baltic Basin and the Podlasie Basin usually ranges from 0.5% to 1.4% TOC. In eastern part of the Baltic Basin and in the Lublin Basin, it is higher, rising to
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about 1–1.7% TOC (Poprawa, 2010, Więcław et al., 2010; Karcz and Janas, 2016; Podhalańska et al., 2016a). Thermal maturity of OM increases from NE to SW, from the oil generation phase through the
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condensate and wet gas phases to dry gas generation phase (Podhalańska et al., 2016a). Rvl equals 0.6– 2.5% (Table 4), and Tmax values vary from 425oC to 509oC (Więcław et al., 2010; Karcz and Janas 2016;
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Podhalańska et al., 2016a). Average effective porosity is 5.25–7.88% (Table 4) (Dyrka, 2016). S2 in the Baltic Basin ranges between 0.07 and 3.5 mg HC/g rock. HI is 7–335 mg HC/g TOC (Więcław et al., 2010).
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7.6 Arabia and North Africa
The lower Silurian organic-rich shales (hot shales) were widely deposited in the Arabia and North
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Africa due to a strong rise of sea level during the latest Ordovician to early Silurian, including e.g., Tanezzuft Formation in Libya, Tunisia and Algeria, Tanf Formation in Syria, Akkas Formation in Iraq, Qusaiba Formation in Saudi Arabia, and Batra or Mudawwara Formation in Jordan (Loydell, 1998; Lüning et al., 2003; Soua, 2014; İnan et al., 2016). They are characterized by type II kerogen, TOC up to 20%, and thickness is generally less than 100 m (Lüning et al., 2000; Belaid et al., 2010; Soua, 2014; Wang et al., 2019b). Their thermal maturity varies significantly due to variable burial history, ranging from immature to post mature (Lüning et al., 2005; Belaid et al., 2010; Soua, 2014; İnan et al., 2016;
Wang et al., 2019b). In North Africa, 80–90% of Paleozoic-sourced petroleum are derived from these Silurian hot shales, and in Arabia, some oil and sweet gas in Paleozoic-Mesozoic reservoirs are regarded to be sourced from these organic-rich sediments (Lüning et al., 2000; Al-Juboury and Al-Hadidy, 2009; İnan et al., 2016). In recent years, the hot shales were thought to contain shale oil/gas potential due to their wide distribution, thickness, abundant OM and variable thermal maturity (Soua, 2014; İnan et al., 2016; Wang et al., 2019b). For example, the gas content of the hot shales falls between 2.4 and 8.5 m3/t in the Ghadames Basin, and their geochemical characteristics and petroleum accumulation conditions are comparable to the Wufeng–Longmaxi Formations (Table 4) (Wang et al., 2019b).
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8. Conclusions Based on a review on the optical characteristics, chemical composition, and microstructure of graptolites, the following conclusions can be reached:
(1) There are two types of graptolites: granular (GG) and non-granular (NGG). GG have lower
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reflectance and weaker anisotropy than NGG. GG can be altered to NGG with increasing thermal maturity. (2) The aromaticity and ordering of the aromatic structure of graptolite increasing with increasing
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maturity is similar to that of vitrinite and bitumen, illustrating that their physical and chemical parameters are reliable geo-thermometers. There is a positive correlation between GRor and GRomax, allowing GRor
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to be used as a measure of thermal maturation for the lower Paleozoic sediments. The relationship between GRor and EqVRo can be expressed as: EqVRo = 0.99GRor + 0.08.
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(3) Electron microprobe and FTIR analyses are very useful tools to study chemical compositions of graptolites, indicating that their compositions are similar to vitrinite. Standard measurement and data processing are required for the improved analysis of Raman spectrum for graptolites. Graptolites contain
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abundant pores, which are less abundant and smaller than those in solid bitumen. (4) Based on the chemical composition and organic geochemical characteristics of the graptolites,
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they are mainly gas prone, similar to vitrinite, and have a significant hydrocarbon generation potential. (5) OM in the Wufeng–Longmaxi sediments is dominated by graptolites and solid bitumen. Their
thermal maturity was determined by GRor, in the range of 1.17–4.93%, indicating marginally mature to overmature. (6) The graptolite-bearing shales are worldwide hydrocarbon source rocks and contribute significantly to global petroleum reserves. Graptolite serve both as source material and as reservoir, due to nano-microporosity, and are important to the accumulation of shale gas in these sediments.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 41503028, 41773031 and 41830424). Field sampling in the Czech Republic was partly supported by OP RDE, MEYS, under the project “Ultra-trace isotope research in social and environmental studies using accelerator mass spectrometry” (Reg. No. CZ.02.1.01/0.0/0.0/16_019/0000728). We thank Dr. Roger MacQueen of the Geological Survey of Canada for reviewing an early version of this manuscript and for his valuable suggestion. We also thank Zhongliang Ma at the Wuxi Institute of Petroleum Geology, Sinopec Petroleum Exploration and Development Research Institute for conducting the artificial maturation experiments in
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this study. We are very grateful to Dr. MaryAnn Love Malinconico and one anonymous reviewer for their critical but constructive and valuable comments that significantly improved the quality and language of this paper. The authors also appreciate Managing Editor Dr. Shuhab Khan for his precious time and energy
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to handle this paper.
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Exploration and Development 43, 182–196.
PII:
S0012-8252(19)30494-5
DOI:
https://doi.org/10.1016/j.earscirev.2019.103000
Reference:
EARTH 103000
To appear in: Received Date:
25 July 2019
Revised Date:
28 October 2019
Accepted Date:
30 October 2019
Please cite this article as: Luo Q, Fariborz G, Zhong N, Wang Y, Qiu N, Skovsted CB, Such´y V, Hemmingsen Schovsbo N, Morga R, Xu Y, Hao J, Liu A, Wu J, Cao W, Min X, Wu J, Graptolites as fossil geo-thermometers and source material of hydrocarbons: an overview of four decades of progress, Earth-Science Reviews (2019), doi: https://doi.org/10.1016/j.earscirev.2019.103000
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Graptolites as fossil geo-thermometers and source material of hydrocarbons: an overview of four decades of progress Qingyong Luoa, b, Goodarzi Fariborzc, Ningning Zhonga, b*, Ye Wanga, b, Nansheng Qiua, b, Christian B. Skovstedd, Václav Suchýe, Niels Hemmingsen Schovsbof, Rafał Morgag, Yaohui Xuh, Jingyue Haoa, b, Anji Liua, b, Jin Wua, b, Weixun Caoa, b, Xu Mina, b, Jia Wua, b State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing 102249, China; b
c
d
e
College of Geoscience, China University of Petroleum, Beijing 102249, China;
FG &Partner Ltd, Research Group, 29 Hawkside Mews NW., Calgary, Alberta, Canada;
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a
Department of Palaeobiology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden;
Nuclear Physics Institute, v. v. i.,Academy of Sciences of the Czech Republic, Na Truhlářce 39/64, 180 86 Prague 8, Czech Republic;
g
Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark;
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f
Silesian University of Technology, Faculty of Mining, Safety Engineering and Industrial Automation, Institute of Applied
Hubei Cooperative Innovation Center of Unconventional Oil and Gas, Yangtze University, Wuhan, 430100, China.
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h
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Geology, Akademicka 2, 44-100 Gliwice, Poland;
*Corresponding author at: State Key Laboratory of Petroleum Resources and Prospecting, China
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University of Petroleum, Changping, Beijing, 102249, China. Tel.: +86 10 89734548. E-mail address:
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[email protected].
Abstract
The thermal maturity of lower Paleozoic graptolite-bearing marine sediments, which host many
hydrocarbon deposits worldwide, has long been difficult to determine due to the absence of wood-derived vitrinite particles for conventional vitrinite reflectance. In 1976, graptolite reflectance was introduced as a new indicator for organic maturity of these deposits and has been used since in many regional studies. The majority of these studies, however, were done on a limited sample set and a limited range of thermal
maturity, which resulted in a number of controversial views concerning the usefulness of graptolite reflectance as an alternative paleothermal indicator and its correlation with vitrinite reflectance through various proxies. In this paper, we review previous studies and combine those analyses with new data to assess the physical and chemical characteristics of graptolite periderm with increasing thermal maturity. We conclude that graptolite random reflectance (GRor) is a better parameter for the thermal maturity assessment than graptolite maximum reflectance (GRomax) due to the better quality of available data. Combining published data with results of our study of both natural and heat-treated graptolites and vitrinite, we present a new correlation between GRor and equivalent vitrinite reflectance (EqVRo), as
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EqVRo = 0.99GRor + 0.08. Chemical composition of graptolite periderm is similar to vitrinite; graptolites are mainly kerogen Type II-III, are gas prone and have a substantial hydrocarbon potential. Lower Paleozoic graptolite-bearing organic-rich sediments are important shale gas source rocks and reservoirs globally and make a significant contribution to worldwide petroleum reserves.
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Keywords: Graptolite reflectance; Optical characteristics; Chemical composition; Microstructure;
1. Introduction and previous studies
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Wufeng–Longmaxi Formations; Alum Shale; Hot shale; Shale gas.
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Lower Paleozoic graptolite-bearing rocks were mainly deposited in marine environments, and are important source rocks globally, especially shale gas deposits in the Wufeng–Longmaxi (also known as
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Wufeng–Lungmachi) sediments from China (Zou et al., 2010; Zou et al., 2012; Dai et al., 2014; Dai et al., 2016; Luo et al., 2016; Zou et al., 2016; Luo et al., 2017; Luo et al., 2018). These organic matter (OM)-rich facies were also identified as true or potential hydrocarbon source rocks in the Anadarko basin
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in the USA (Wang and Philp, 1997), North Africa and Arabian Peninsula (Jones and Stump, 1999; Lüning et al., 2000), Taurus region of Turkey (Varol et al., 2006), the Czech Republic (Suchý et al., 2002), and
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the Siberian platform (Makarov and Bazhenova, 1981). More recently, such shales have been recognized as potential targets for unconventional shale gas deposits in the Norwegian-Danish Basin (Schovsbo et al. 2011, 2014; Pool et al. 2012), and Baltic Basin of Central Europe (Littke et al., 2011; Karcz et al., 2013; Yang et al. 2017). Vitrinite reflectance is a most commonly used indicator for the thermal maturity (Stach et al., 1982; Taylor et al., 1998; Suárez-Ruiz et al., 2012; Hackley and Cardott, 2016). However, due to the lack of vitrinite (coalified wood) in pre-Devonian rocks, the determination of thermal maturity of lower Paleozoic
graptolite-bearing rocks is always a difficult topic and a hot debate for petroleum industry (Goodarzi, 1984; Goodarzi, 1985a; Goodarzi and Norford, 1985, 1987; Bertrand and Heroux, 1987; Bustin et al., 1989; Bertrand, 1990; Goodarzi et al., 1992a; Bertrand, 1993; Petersen et al., 2013; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018). Thus, the surrogate proxies, such as the reflectance of zooclasts, vitrinitelike particles and solid bitumen, Tmax, and biomarkers, have been proposed to assess organic maturity in lower Paleozoic graptolite-bearing sediments (Teichmüller, 1978; Goodarzi and Norford, 1987; Jacob, 1989; Schoenherr et al., 2007; Suárez-Ruiz et al., 2012; Schmidt et al., 2019). Tmax may be unreliable due to low S2 in overmature sediments (Peters, 1986; Peters and Cassa, 1994).
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The maturity-related biomarker ratios may be also influenced by depositional environments and biological sources, which will increase the difficulty of data interpretation (Radke and Welte, 1983; Radke et al., 1986; Radke, 1988; George and Ahmed, 2002; Peters et al., 2005). In addition, the biomarker ratios may be invalid to assess thermal maturity of overmature sediments (Peters et al., 2005). Graptolite-bearing
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rocks often contain bitumen and other zooclasts (Teichmüller, 1978; Goodarzi and Norford, 1987; Jacob, 1989; Schoenherr et al., 2007; Suárez-Ruiz et al., 2012; Schmidt et al., 2019). The difficulty with using
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of solid bitumen reflectance is due to large variation in most samples, and their origin, e.g., by thermal cracking, biodegradation and deasphalting (George et al., 1994; Hwang et al., 1998; Mastalerz et al., 2018),
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all of which increase difficulty in data interpretation (e.g., Gonçalves et al., 2014; Fink et al., 2016). The origin of discrete vitrinite-like particles remains controversial, and possible explanations include:
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migrated bitumen, either indigenous or exogenous to the host rock (Bertrand and Heroux, 1987); gelification of polysaccharides (Buchardt and Lewan, 1990); residues of algae after maturation (Wang et al., 1994); biodegraded zooclasts that are the product of a reducing to strongly reducing environment
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(Xiao et al., 1997); marine humification of planktonic and benthic organisms (Romankevich, 1984), the so-called “marine vitrinite group” (Zhong and Qin, 1995); and fragments of graptolites (Petersen et al.,
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2013).
Zooclasts have clear advantage in reflectance studies due to their specific biological sources
compared to that of solid bitumen and vitrinite-like particles, and thus their reflectance was naturally regarded as having a superior potential as a thermal maturity proxy. In general, graptolites are more common than other zooclasts (e.g., chitinozoans and scolecodonts) in lower Paleozoic marine rocks, and as a result, the nature of graptolite reflectance has been a “hot topic” for organic petrologists over several decades (Kurylowicz et al., 1976; Teichmüller, 1978; Goodarzi, 1984, 1985a; Bertrand and Heroux, 1987;
Bertrand, 1990; Link et al., 1990; Cardott and Kidwai, 1991; Hoffknecht, 1991; Goodarzi et al., 1992b; Malinconico, 1992; Tricker et al., 1992; Malinconico, 1993; Wang et al., 1993; Cole, 1994; Gentzis et al., 1996; Liu et al., 2001; Bertrand et al., 2003; Petersen et al., 2013; İnan et al., 2016; Lavoie et al., 2016; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018; Synnott et al., 2018; Wang et al., 2019a). Graptolite reflectance is a useful proxy of thermal maturity (Goodarzi, 1984; Goodarzi and Norford, 1985), and has been used to assess eroded thicknesses when used in conjunction with reflectance of bitumen and sedimentological and tectonic evidence (Goodarzi et al., 1992b; Gentzis et al., 1996). The graptolite maximum reflectance (GRomax) or graptolite random reflectance (GRor) was adopted to estimate
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thermal maturity in worldwide sediments (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi et al., 1985; Goodarzi and Norford, 1989; Goodarzi et al., 1992b; Malinconico, 1992, 1993; Rantitsch, 1995; Gentzis et al., 1996; Bertrand et al., 2003; Petersen et al., 2013; İnan et al., 2016; Lavoie et al., 2016; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018). Some researchers have established correlations among
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reflectances of graptolite and other zooclasts, pyrobitumen and vitrinite (Bertrand and Heroux, 1987; Bertrand, 1990; Bertrand, 1993; Yang and Hesse, 1993; Bertrand and Malo, 2001; Bertrand et al., 2003),
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and Conodont Alteration Index (CAI; Goodarzi and Norford, 1985; Hoffknecht, 1991; Gentzis et al., 1996). Moreover, the graptolite reflectance has also been successfully correlated with some inorganic
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paleothermal indices, including the illite “crystallinity” index (e.g. Kemp et al., 1985; Oliver, 1988; Hoffknecht, 1991; Rantitsch, 1995, 1997; Suchý et al., 2015), and metamorphic index minerals in pre-
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greenschist meta-sedimentary rocks (e.g. Malinconico, 1992). Graptolite reflectance is commonly related to the lithology, and it will be higher in shales than in limestones (Link et al., 1990). Redox conditions may have an impact on graptolite reflectance, and higher reflectance was observed in graptolites deposited
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under oxic environments in comparison with those from anoxic environments (Cole, 1994). Weathering may affect graptolite reflectance (Goodarzi and Norford, 1985; Hoffknecht, 1991; Goodarzi et al., 1992a).
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Raman spectroscopy of graptolites was widely studied and used to assess organic maturity (Suchý et al., 2004; Liu et al., 2013; İnan et al., 2016; Mumm and İnan, 2016; Morga and Pawlyta, 2018; Wang et al., 2019a). However, these studies were mostly focused on a small number of samples representing a very limited range of thermal maturity. Thus, the physical and chemical characteristics of graptolites in wider thermal maturity settings is reviewed herein for lower Paleozoic sediments from a number of localities based on the literature, and new unpublished data is presented. In reviewing the optical characteristics, chemical composition, and microstructure of the graptolites, their implications for the global petroleum
industry were also discussed.
2. The biological structure and composition of graptolites Graptolites are extinct colonial planktonic hemichordate invertebrates that lived mainly in the early Paleozoic ocean (Clarkson, 1981). Their structure has been widely studied and is illustrated in Fig. 1 (Moore, 1955; Clarkson, 1981; Crowther, 1981). The rhabdosome (colony) may have more than one branch (stipe) (Fig. 1a). Along a stipe, thecae house the individual zooids (Fig. 1b, 1c), which were connected by a stolon to other individuals through the common canal (Fig. 1c). The periderm material of the rhabdosome is generally composed of two layers, fusellar tissue and cortical tissue (Fig. 1e)
reflected light under the optical microscope as discussed below.
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(Crowther, 1981). The periderm with fusellar layers, thecae and common canal may be observed in
Initially, studies of the chemical structure of graptolites were related to the periderm based on the textures and phylogeny, and these studies assumed that graptolite periderm is chitinous (Eisenack, 1932;
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Kozlowski, 1949). Later, a collagen-like structure for the cortical and fusellar tissues was proposed for graptolites (Fig. 1) (Towe and Urbanek, 1972; Crowther, 1981; Bates and Kirk, 1986; Urbanek and
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Mierzejewski, 1986; Liu et al., 1996). Both collagen and chitin are supportive tissues, which mainly serve to support cellular structures (Tasch, 1980). Collagen is a glycoprotein, and chitin with a molecular
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formula of C32H54O21N4 is a nitrogenous carbohydrate forming a N-acetyl glucosamine groups polymer (Leninger, 1975; Bustin et al., 1989). More recent studies indicate, however, that the residual graptolite
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periderms are composed of complex aliphatic polymers that are immune to base hydrolysis rather than collagen (e.g. Briggs et al., 1995; Gupta et al., 2006). Aliphatic components of graptolite periderms are
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probably derived from the graptolite itself via in situ polymerization (e.g. Gupta et al., 2006).
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Fig. 1. The biological structure of graptolites showing their theca, aperture, common canal, virgella and periderm (Modified after Moore,
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1955; Clarkson, 1981). A) Rhabdosome (colony) with two stipes (branches); B) close-up of rhabdosome structures; C) close-up of two thecae which house individual zooids; D) transverse cross-section of rhabdosome; E) periderm structure.
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3. Sampling and methods
The graptolite-bearing sediments examined for this study included the Wufeng–Longmaxi
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Formations, Pingliang Formation, China (Luo et al., 2017; Luo et al., 2018; Wang et al., 2019a); Liteň Formation from Czech (Suchý et al., 2002); and Alum Shale from Estonia and Sweden (Petersen et al.,
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2013; Sanei et al., 2014; Luo et al., 2018). Detailed geological information can be found in these above references. Two graptolite-bearing Wufeng–Longmaxi samples (sample ID: CKMB-Y1 (GRor = 1.32%) and CKMB-Y2 CKMB-Y1 (GRor = 1.28%)) (Wang et al., 2019a) and an upper Carboniferous coal (VRor = 1.07%) were selected to conduct artificial maturation at 350℃, 400℃, 450℃ and 500℃ for three days, respectively. The natural and heated samples were cut parallel or perpendicular to bedding in order to make polished sections on EcoMet 250 with AutoMet 250 for the maceral observation and reflectance measurement. Organic petrography and reflectance were done on a Leica 4500P microscope with
CRAIC/MPS 200 microscope photometer. The maximum-minimum and random reflectances were measured under polarized and non-polarized light, respectively, as described in Luo et al. (2016); Luo et al. (2017); Luo et al. (2018). For scanning electron microscopy (SEM), sediment was cut, polished using emery paper, and milled using Ar ion milling. For study of pores, graptolites were found and then marked using a diamond lens under an optical microscope. The SEM study was on a Zeiss Crossbeam 540 Focused Ion Beam-SEM (FIB-SEM).
4. Optical characteristics of graptolite periderm
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Kurylowicz et al. (1976) reported the reflectance characteristics of graptolites and proposed that their optical properties are similar to vitrinite. Clausen and Teichmüller (1982) described the morphology and reflectance of graptolites from Germany and Sweden, with maximum reflectances ranging from 0.8% to 10.0%. In 1980–1990s, Goodarzi and colleagues extensively studied the optical properties of Middle
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Ordovician to upper Silurian graptolites from Poland, Sweden, Turkey and Canada (Goodarzi, 1984, 1985a; Goodarzi and Norford, 1985; Goodarzi et al., 1985; Goodarzi and Norford, 1987; Bustin et al.,
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1989; Goodarzi and Norford, 1989; Riediger et al., 1989; Link et al., 1990; Goodarzi et al., 1992a; Goodarzi et al., 1992b; Gentzis et al., 1996), which advanced the understanding of the petrographic
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characteristics of graptolites. In recent years, graptolite-bearing sediments have become significant targets of shale gas exploration, and a resurgence of work has been conducted on the organic petrology of
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graptolites (Petersen et al., 2013; Sanei et al., 2014; Haeri-Ardakani et al., 2015; Caricchi et al., 2016; İnan et al., 2016; Lavoie et al., 2016; Luo et al., 2016; Ma et al., 2016; Mumm and İnan, 2016; Cheshire et al., 2017; Luo et al., 2017; Cardott and Curtis, 2018; Luo et al., 2018; Morga and Kamińska, 2018;
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Morga and Pawlyta, 2018; Reyes et al., 2018; Synnott et al., 2018; Wang et al., 2019a). Microscopically, graptolites can be identified by their morphology, e.g., fusellar layer, granularity,
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other visible structures as well as anisotropy (Kurylowicz et al., 1976; Teichmüller, 1978; Clausen and Teichmüller, 1982; Goodarzi, 1984, 1985a; Bertrand and Heroux, 1987; Goodarzi and Norford, 1987, 1989; Bustin et al., 1989; Riediger et al., 1989; Bertrand, 1990; Petersen et al., 2013; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018). Two textures are apparent in graptolites, i.e., non-granular (NGG) and granular (GG), in Paleozoic sediments worldwide, e.g., Poland, Sweden, Turkey and Canada (Goodarzi, 1984, 1985a; Goodarzi and Norford, 1985; Goodarzi et al., 1985; Goodarzi and Norford, 1987, 1989; Goodarzi et al., 1992a; Goodarzi et al., 1992b), the Alum Shale from Europe (Petersen et al., 2013; Sanei
et al., 2014; Luo et al., 2018), and the Wufeng–Longmaxi Formations and Pingliang Formation from China (Fig. 2) (Wang et al., 1993; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018; Wang et al., 2019a). In general, GG and NGG are found in carbonates and mudstones, respectively (Goodarzi and Norford, 1985; Goodarzi and Norford, 1987; Petersen et al., 2013). NGG and GG were thought to be derived from different sections of the rhabdosome. While the former may be part of the wall with fusellar layers, the latter may be derived from the common canal (Goodarzi, 1984). However, according to recent artificial maturation studies on Estonian Alum Shale with GG, this texture is completely altered to NGG when GRor reached up to 1.24% after heating for 3 days at 400 ℃. This indicates that pristine GG may be altered
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to NGG due to thermal stress (Luo et al., 2018). GG has weaker anisotropy and lower reflectance than NGG (Figs. 2b and 3) (Goodarzi, 1984; Goodarzi and Norford, 1987; Petersen et al., 2013; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018). Suchý et al. (2002) found that NGG random reflectance has a positive correlation with GG random
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reflectance in lower Silurian sediments from the Barrandian Basin, Czech Republic. In rotation of the objective stage under polarized light, NGG will display extinction twice, and thus two GRomax and two
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show such clear patterns (Fig. 3c and d).
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GRomin can be measured (Figs. 3a, b and 4) (Luo et al., 2016; Luo et al., 2018), however, GG does not
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Fig. 2 Granular-textured graptolites (GG) and non-granular-textured graptolites (NGG) from Chinese and Estonian sediments. (a) GG in
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low-maturity Alum Shale from Estonia (GRor = 0.65%), non-polarized light; (b) GG and NGG in high-maturity shale from Chongqing, China (GRomax = 2.57%), non-polarized light (Luo et al., 2018); (c) grey black NGG in low-maturity shale from Gansu, China (GRor =
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0.48%), non-polarized light; (d) NGG in overmature shale from Chongqing, China (GRor = 3.85%), showing strong anisotropy, polarized light; (e) NGG in overmature shale from Chongqing, China (GRor = 3.77%), showing strong anisotropy, polarized light; (f) NGG in
overmature shale from Chongqing, China (GRomax = 4.47%), showing fusellar layers, polarized light (modified from Luo et al., 2018). (a-e) sections perpendicular to bedding; (f) sections parallel to bedding.
5
6
(a)
(b)
5
4
Ro(%)
Ro(%)
4 3 2
3 2
1
1
0
0 0
40
80
120 160 200 240 280 320 360
0
40
80
Degree
120 160 200 240 280 320 360 Degree
2.5
2.2 (d)
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(c) 2.1
Ro(%)
Ro(%)
2.2 2
1.9
1.6
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1.9
1.8 0
40
80
120 160 200 240 280 320 360
0
40
80
120 160 200 240 280 320 360 Degree
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Degree
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Fig. 3. Reflectance measured on NGG (a and b) and GG (c and d) under polarized light in a circular rotation of the microscope stage.
Fig. 4. Photomicrographs displaying extinction characteristics of the same NGG from the Wufeng–Longmaxi shales in sections perpendicular to bedding with 360° rotation of the microscope stage, oil immersion, polarized light. The angle between each picture is
90 degrees. The graptolite reflectance (%) is given beside the measurement site (red square).
In general, NGG is more frequently found than GG in most Paleozoic sediments (Goodarzi and Norford, 1987; Petersen et al., 2013; Luo et al., 2016), thus, only NGG will be discussed in the main discussion of this paper. In polished blocks, low-maturity graptolites are often similar to natural bitumen stringers, but they are more angular than bitumen and often show typical graptolitic features (Figs. 2, 4 and 5), and they display a plate-like appearance at high maturity (Fig. 2). NGG is blocky in sections parallel to bedding, but may display a long stipe-like morphology in sections normal to bedding (Figs. 2
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and 5) (Link et al., 1990). The fusellar layers are observed in sections parallel to bedding (Fig. 2). NGG was divided into two classes, lath- and blocky-shaped (Riediger et al., 1989). Blocky-shaped graptolites show lower maximum reflectance and bireflectance (BRo) than lath-shaped graptolites. Some NGG of the Liteň Formation from Czech Republic displays weak brown fluorescence (Fig. 5), and Hoffknecht (1991)
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na
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and Wang et al. (2019a) also found similar phenomena in low-maturity graptolites.
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Fig. 5. NGG from the Czech Republic (Liteň Formation, Barrandian area) with weak brown fluorescence of the Liteň Formation in sections perpendicular to bedding. (a, c, e) under reflected light; (b, d, f) the same field as (a, c, e), under fluorescence light. (a-b) GRor =
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0.85%; (c-f) GRor = 0.83%.
5. Physical and chemical properties of graptolites as thermal maturity indicators 5.1 Chemical structure changes of graptolites with maturation OM is sensitive to pressure and temperature (Khavari-Khorasani, 1975; Teichmüller, 1982; Goodarzi and Norford, 1985). Optical properties of OM include reflectance, refractive and absorptive index (van Krevelen, 1961). Reflectance of OM is related to the aromatic structure of organic molecules; reflectance
and aromaticity increase continuously with increase in maturity. The trends of optical properties of OM over the visible spectrum (400–700nm) are used to determine molecular structural changes occurring during maturation (Khavari-Khorasani, 1975; Goodarzi, 1985a; Goodarzi and Macqueen, 1990). Structurally, OM is composed of ordered and amorphous carbons (Cartz and Hirsch, 1960; van Krevelen, 1961; Goodarzi, 1985a). The ratio of ordered (aromatic) to amorphous (alicyclic side chains, aliphatic) carbon atoms increases with maturation/heat treatment (Cartz and Hirsch, 1960; van Krevelen, 1961) in two phases: ① Devolatilization of amorphous carbon increase in aromaticity and size of the polynuclear systems
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during early stage of maturation and low temperature heat treatment; ② Transformation of 2D (turbostratic) to 3D (graphitic) ordering, which occurs at high maturity and at temperature >600 ℃, results in increase and better ordering of the aromatic structure (Cartz and Hirsch, 1960; van Krevelen, 1961; Goodarzi and Murchison, 1972; Goodarzi, 1985a).
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Dispersion of optical properties of graptolite periderm is similar to that of bitumen and vitrinite in the visible spectrum and three spectral patterns are documented:
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① Low-maturity (CAI = 1) graptolite behaves similarly to low rank vitrinite and bitumen with curves of all optical parameters decreasing with increasing wavelength from blue to red (Figs. 6 and
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7), indicating low content of aromatic carbon (Marshall and Murchison, 1971; Khavari-Khorasani, 1975);
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② Moderately-mature (CAI = 4) graptolite behaves similarly to semi-anthracite and impsonite bitumen. The optical parameters are nearly flat, from blue to red (Figs. 6 and 7), indicating gradual molecular variations, e.g., greater condensed aromatic carbon (Marshall and Murchison, 1971;
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Khavari-Khorasani, 1975);
③ Highly mature graptolite (CAI = 3.5–5) behaves similarly to anthracite and pyrobitumens. The
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trends of optical indices rise continuously from blue to red (Figs. 6 and 7), similar to the pattern from a highly aromatic, condensed molecular structure (Marshall and Murchison, 1971; Cook et al., 1972; Khavari-Khorasani, 1975; Goodarzi, 1985a). Graptolites at this stage develop macro-properties similar to graphite (Teichmüller et al., 1979).
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Fig. 6. Dispersion of maximum reflectance (a) in air (%Ramax) and (b) in oil (%Romax) of graptolite fragments with increased maturity based on their CAI (conodont alteration index), shown as number beside each spectrum for graptolite in limestone (L), argillaceous
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limestone (A) and shale (S) matrices (after Goodarzi, 1985a).
Fig. 7. Dispersion of refractive (a) and (b) absorptive indices of graptolite fragments with increased maturity based on their CAI, shown as number beside each spectrum for graptolites in limestone (L), argillaceous limestone (A) and shale (S) matrices (after Goodarzi, 1985a).
5.2 Graptolite reflectance 5.2.1 Graptolite maximum, minimum and random reflectance Similar to vitrinite (Davis, 1978), GRomax and graptolite minimum reflectance (GRomin) are observed in sections parallel and normal to bedding, respectively (Goodarzi, 1984; Link et al., 1990; Goodarzi et al., 1992a). However, in some samples, GRomax in sections parallel to bedding is lower than that in sections normal to bedding (Table 1 and Fig. 8), which may be because the graptolites in these samples are not truly parallel to bedding or normal to bedding due to an unusual burial process and/or sample preparation
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(Malinconico, 1992, 1993; Luo et al., 2017). In sections parallel to bedding, GRomax correlates positively with GRomin (Fig. 9), whereas in sections perpendicular to bedding, such relationship cannot be observed (Hoffknecht, 1991; Luo et al., 2017). This is because GRomin in sections parallel to bedding is generally
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equal to the graptolite intermediate reflectance (GRoint) (Luo et al., 2017).
Table 1. Comparison of graptolites reflectance (including maximum, minimum and bireflectance) in sections parallel and normal to
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bedding. Part of the data for the Wufeng–Longmaxi graptolites are derived from Luo et al. (2017), and the data of the upper Silurian graptolites of Turkey are from Goodarzi (1984).
Sample no.
Sections parallel to bedding
Sections normal to bedding
GRomax
GRomin
BRo
GRomax
GRomin
BRo
(%)
(%)
(%)
(%)
(%)
(%)
Shale
3.41
1.92
1.49
3.68
0.99
2.69
Shale
5.48
2.61
2.87
5.61
0.64
4.98
Shale
4.76
3.14
1.62
5.64
0.75
4.90
Shale
4.61
4.17
0.44
4.42
1.08
3.34
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Depth Location
Formation
Lithology
(m) DTB-6-9
Datianba, China
Outcrop
Wufeng-
Jinxi, China
Outcrop
na
JX-4
Longmaxi
QQ1-3
Qianjiang, China
794.00
No. 2
Tufanbeyli, Turkey
Outcrop
upper
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Silurian
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Fig. 10 displays a comparison of GRor in sections parallel and perpendicular to bedding. In contrast to GRomax, GRor in sections perpendicular to bedding, especially in samples with higher thermal maturity, is much lower than that in sections parallel to bedding, similar to the results in the Qusaiba Hot Shales from Saudi Arabia (İnan et al., 2016), and is much more concentrated than that in sections parallel to bedding as indicated by their lower SD values (Fig. 10). In general, unimodal histograms of GRomax and GRor were found in investigated samples, especially in sections perpendicular to bedding (Figs. 8 and 10) (Cole, 1994; Haeri-Ardakani et al., 2015; Lavoie et al., 2016; Luo et al., 2018; Reyes et al., 2018). In
addition, NGG reflectance in carbonate rocks is lower than that of shales at similar depths, which may be due to oxidation caused by carbonate dissolution (Goodarzi and Norford, 1985; Link et al., 1990). This is
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similar to that of VRor in different lithologies (Goodarzi et al., 1988; Goodarzi et al., 1993).
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Fig. 8. Comparison of GRomax of the Wufeng–Longmaxi sediments in (a, c and e) sections parallel to bedding and (b, d and f) sections
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perpendicular to bedding from Chongqing.
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Fig. 9. The linear coalification trend for graptolites based on the plot of GRomax VS. GRomin (after Goodarzi, 1984).
Fig. 10. Comparison of GRor of the Wufeng–Longmaxi sediments in (a, c, e) sections parallel to bedding and (b, d, f) sections perpendicular to bedding from Chongqing (cited from Luo et al., 2019).
From the methodological point of view, the main difference between the measurement technique of vitrinite and graptolite reflectance has to do with the sample preparation. Graptolite maturity determination was generally made on the basis of whole-rock polished blocks in sections parallel to bedding and using GRomax (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi and Norford, 1987, 1989; Gentzis et al., 1996). GRomax will increase with burial depth and is a useful tool to unravel the thermal maturity of lower Paleozoic deposits (Clausen and Teichmüller, 1982; Goodarzi, 1984, 1985a; Goodarzi and Norford, 1987, 1989; Riediger et al., 1989; Link et al., 1990; Gentzis et al., 1996; Luo et
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al., 2016; Luo et al., 2017). However, other researchers have used GRor to assess maturity levels (Bertrand and Heroux, 1987; Bertrand, 1990; Bertrand, 1993; Cole, 1994; Bertrand and Malo, 2001; Bertrand et al., 2003; Petersen et al., 2013; Sanei et al., 2014; İnan et al., 2016; Yang, 2016; Luo et al., 2018; Reyes et al., 2018; Synnott et al., 2018; Wang et al., 2019a).
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Based on data from published literature, GRomax shows a strong positive correlation with GRor in sections perpendicular to bedding, and the former is around twice as large as the latter (Fig. 11). The
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comparison of SD for GRomax and GRor is shown in Fig. 12. In general, SD is greater for GRomax than that of GRor. Therefore, for routine maturation study, measurements of GRor is as accurate and as useful as
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those of GRomax.
Wang et al., 2019a
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Mean random reflectance of graptolite (%)
6
y = 0.52x + 0.43 r = 0.95
Luo et al., 2018 Malinconico, 1993
4
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Link et al., 1990
2
0 0
3 6 Mean maximum reflectance of graptolite (%)
9
Fig. 11. Mean GRomax vs. mean GRor in sections perpendicular to bedding (Link et al., 1990; Malinconico, 1993; Luo et al., 2018; Wang et al., 2019a).
0.9 (a)
0.9 Malinconico, 1992 Luo et al., 2018 Wang et al., 2019a
(b)
0.6
SD (%)
SD (%)
0.6 y = 0.03x + 0.07 r = 0.51
Luo et al., 2018 Petersen et al., 2013 Bertrand et al., 2003 Lavoie et al., 2016 Williams et al. 1998 Wang et al., 2019a
0.3
0.3
y = 0.07x + 0.07 r = 0.75 0
0 9
0
2 4 Mean random reflectance of graptolite (%)
6
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0 3 6 Mean maximum reflectance of graptolite (%)
Fig. 12. The relationship between (a) mean GRomax in sections perpendicular to bedding, (b) mean GRor and SD (Malinconico, 1992; Williams et al., 1998; Bertrand et al., 2003; Petersen et al., 2013; Lavoie et al., 2016; Luo et al., 2018; Wang et al., 2019a).
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5.2.2 Graptolite bireflectance (BRo)
BRo in sections parallel to bedding is much lower than that in sections perpendicular to bedding,
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especially in overmature sediments (Table 1) (Goodarzi, 1984). The anisotropy of graptolites will increase with increasing thermal maturity as suggested by positive correlation between GRomax and BRo (Luo et
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al., 2017). Vitrinite displays lower bireflectance than that of graptolite at similar maturation levels because of the biaxial negative property of graptolites (Bustin et al., 1989; Hoffknecht, 1991; Luo et al., 2017). 5.2.3 The coalification path of the graptolites
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The plot of GRomax vs. GRomin can be used to assess the coalification path of the graptolites (Fig. 9). However, it should be noted that GRomin in Fig. 9 was measured in sections parallel to bedding, and is
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nearly equal to GRoint (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi and Norford, 1987; Goodarzi et al., 1992a; Malinconico, 1993; Luo et al., 2017; Luo et al., 2018). When all the graptolite
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reflectance data from the published literature are plotted in Fig. 9, a linear coalification trend can be determined for the graptolites measured parallel to bedding (Fig. 9) (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi and Norford, 1987; Goodarzi et al., 1992a; Malinconico, 1993; Luo et al., 2017; Luo et al., 2018). 5.2.4 The relationship between graptolite reflectance and other paleothermal indices 5.2.4.1 Comparison with other organoclasts reflectance Similar to vitrinite and solid bitumen (see also Ferreiro Mählmann and Le Bayon, 2016 for a recent
review on vitrinite and solid bitumen thermal maturity studies), graptolite reflectance displays an increasing trend with increasing burial depth, and can determine organic maturity in lower Paleozoic sediments without vitrinite, although no consensus of the exact relationship between these indices has yet been reached (Table 2 and Fig. 13) (Clausen and Teichmüller, 1982; Goodarzi, 1984, 1985a, b; Bertrand and Heroux, 1987; Goodarzi and Norford, 1987; Teichmüller, 1987; Bustin et al., 1989; Goodarzi and Norford, 1989; Bertrand, 1990; Goodarzi et al., 1992a; Bertrand, 1993; Cole, 1994; Gentzis et al., 1996; Bertrand and Malo, 2001; Bertrand et al., 2003; Petersen et al., 2013; Luo et al., 2018; Synnott et al., 2018). Goodarzi (1985a) found that the optical dispersion of graptolites is similar to vitrinite and bitumen,
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indicating that their physical and chemical nature is sensitive to temperature and can be used as a proxy for organic maturity in lower Paleozoic deposits. The graptolites show higher reflectance and stronger anisotropy than that of solid bitumen, chitinozoans and scolecodonts (Goodarzi, 1984, 1985b; Goodarzi and Norford, 1987, Bertrand et al., 2003; Petersen et al., 2013). According to Yang and Hesse (1993), the
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bireflectance of graptolites displays a similar trend to that of pyrobitumen when Ro < 4.5–5.0 %. Other researchers suggested that graptolite reflectance is similar to that of chitinozoan for both natural and
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artificially matured samples (Bertrand and Heroux, 1987; Bertrand, 1990; Cole, 1994; Reyes et al., 2018). Zooclast reflectance (including chitinozoan, graptolite, and scolecodonts) was also compared with
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vitrinite and solid bitumen reflectances in lower Paleozoic deposits from Canada (Bertrand and Heroux, 1987; Bertrand, 1990; Bertrand, 1993; Bertrand and Malo, 2001; Bertrand et al., 2003). In these Canadian
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studies, graptolite reflectance was slightly lower than, or similar to, vitrinite reflectance (Bertrand, 1990) (Table 2 and Fig. 13). In a similar way, Yang and Hesse (1993) established that GRor is lower than VRor when GRor < 2.6%, but graptolite has higher reflectance than vitrinite when GRor > 2.6%. The Silurian
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Qusaiba Shale from Saudi Arabia deposited under anoxic environments has lower graptolite random reflectance than that deposited in oxic environments (Cole, 1994). Zhong and Qin (1995) proposed a
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formula between EqVRo and GRor based on vitrinite-like particle reflectance (Rvl) and GRor in the sediments from Tarim Basin and South China. In recent years, various new equations have been proposed to illustrate the relationship between EqVRo and GRor or GRomax (Petersen et al., 2013; Colţoi et al., 2016; Synnott et al., 2018; Luo et al., 2018; Wang et al., 2019a) (Table 2 and Fig. 13). The equations proposed by Cole (1994) (anoxic); Petersen el al., (2013) and Coltoi et al., (2016) are very close to each other; whereas the formulas presented by Bertrand (1990) and Luo et al. (2018) are essentially coincident and close to the 1:1 line (Fig. 13).
Table 2. Various equations describing the relationship between EqVRo and GRor or GRomax. GRor data Equations
Source
Formation range
Log10 GRor = -0.04+1.10×Log10 EqVRo
(Bertrand, 1990)
0.5–3.0%
upper Gaspe Limestone Group and Chaleurs Group, Canada
EqVRo = 0.8 ×GRor (anoxic)
(Cole, 1994)
0.62–2.05%
Silurian Qusaiba Shale, Saudi Arabia
(Zhong and Qin,
1.8–4.9%
Cambrian-Ordovician, China
0.47–2.14%
Alum Shale, Scandinavia
EqVRo = 0.65 ×GRor (oxic) EqVRo = 0.882 ×GRor－0.366
1995) EqVRo = 0.73 ×GRor + 0.16
(Petersen et al., 2013) (Colţoi et al., 2016)
No data
Silurian intervals, Romania
EqVRo = 0.232+ 0.499×GRor
(Synnott et al.,
0.59–1.02%
Cape Phillips Formation, Canada
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EqVRo = 0.785×GRor + 0.05
2018) EqVRo = 1.055×GRor－0.053
(Luo et al., 2018)
0.65–4.03%
(Wang et al., 2019a)
1.21–4.91%
Wufeng–Longmaxi Formation
EqVRo = 0.546×GRomax +0.35 EqVRo = 0.97×GRor－0.2 (2.19% <
Wufeng–Longmaxi Formation
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GRor < 3.5%); EqVRo = 0.22×GRor +2.55(GRor > 3.5%) EqVRo = 0.99×GRor + 0.08;
This study
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EqVRo = 0.515×GRomax + 0.506
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6
Cole (1994) (anoxic) Cole (1994) (oxic)
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Zhong and Qin (1995) Petersen et al. (2013) Colţoi et al. (2016) Synnott et al. (2018) Luo et al. (2018) (GRo)
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EqvRo (%)
4
Luo et al. (2018) (GRomax) Wang et al. (2019a) (2.19% < GRor < 3.5%)
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2
Bertrand (1990)
Wang et al. (2019a) (3.5% < GRor < 4.91%) This study (GRo) This study (GRomax) 1:1 line
0 0
2 4 GRo (%) (except where noted)
6
Fig. 13. The various relationships between GRo or GRomax and EqvRo.
In China, graptolites were usually interpreted as solid bitumen or vitrinite-like particles in the Wufeng–Longmaxi Formations, due to the misidentification on OM, leading to a long-term debate on their thermal maturity. Luo et al. (2018) have described how to discriminate between graptolite and bitumen in those formations. NGG displays a smoother surface, stronger anisotropy, and higher random and maximum reflectance than solid bitumen based on observation and measurement under polarized and non-polarized light (Luo et al., 2018; Wang et al., 2019a). In the Wufeng–Longmaxi Formations, the solid bitumen random reflectance (SBor) has strong positive correlations with GRor and GRomax in sections perpendicular to bedding (Fig. 14a and b) (Luo et al., 2018; Wang et al., 2019a). Data from other
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graptolite-bearing sediments (Yang and Hesse, 1993; Bertrand et al., 2003; Yang, 2016; Reyes et al., 2018) are plotted with the Wufeng–Longmaxi data in Fig. 14c and d, which indicates that SBor and GRor are nearly equivalent, very different from their relationship in Fig. 14a. This is because the reflectances from Yang and Hesse (1993), Bertrand et al. (2003), Yang, (2016), and Reyes et al. (2018) were measured in
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random orientation, whereas the data in the two Wufeng–Longmaxi studies were measured on sections
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perpendicular to bedding (Luo et al., 2018; Wang et al., 2019a). 9
12
y = 2.12x - 0.57 r = 0.92
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Luo et al., 2018
Mean maximum reflectance of graptolite (%)
(b)
Wang et al., 2019a
na
6
3
y = 1.19x - 0.09 r = 0.95
0 0
ur
Mean random reflectance of graptolite (%)
(a)
2
4 Wang et al., 2019a Luo et al., 2018 0
4
Mean random reflectance of solid bitumen (%)
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8
6
0
2
4
Mean random reflectance of solid bitumen (%)
6
9
9
6
(d)
Wang et al., 2019a Luo et al., 2018 Reyes et al., 2018 Yang et al., 2016 Bertrand et al., 2003 Yang and Hesse, 1993
Mean random reflectance of graptolite (%)
Mean random reflectance of graptolite (%)
(c)
outlier
3 y = 1.05x + 0.20 r = 0.94 0
Reyes et al., 2018 Yang et al., 2016
outlier
Bertrand et al., 2003 Yang and Hesse, 1993
6
3
y = 0.98x + 0.32 r = 0.95 0
0
2
4
6
0
2
4
6
Mean random reflectance of solid bitumen (%)
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Mean random reflectance of solid bitumen (%)
Fig. 14. The relationship between mean SBor and (a, c and d) mean GRor, and (b) mean GRomax (Yang and Hesse, 1993; Bertrand et al.,
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2003; Yang, 2016; Luo et al., 2018; Reyes et al., 2018; Wang et al., 2019a).
5.2.4.2 Comparison with CAI
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Some studies have correlated the relationship between GRomax and VRor through CAI. When sediments have GRomax of 0.6–1.2%, it means that they are immature (~0.2–0.5% VRor, CAI 1.5); when
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GRomax is 1.2–2.2%, sediments are in the oil window (~0.5–1.30% VRor, CAI: 1.5–2.5) (Fig. 15) (Goodarzi and Norford, 1989; Goodarzi et al., 1992a; Gentzis et al., 1996). However, these results should be used very carefully, because a CAI generally implies a range of thermal maturity (Epstein et al., 1977).
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Graptolite and vitrinite reflectances are more sensitive to thermal maturity than CAI, especially in overmature sediments (Goodarzi and Norford, 1985). While graptolite reflectance displays an excellent
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correlation with CAI at low thermal maturity (CAI: 1–4), their correlation is not clear in sediments with
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higher CAI (Goodarzi and Norford, 1985; Goodarzi et al., 1992a).
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Fig. 15. The relationship between CAI and GRomax (Goodarzi and Norford, 1989; Goodarzi et al., 1992a; Gentzis et al., 1996).
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5.2.4.3 Comparison with Tmax
Graptolite reflectance has also been compared to Tmax for the same samples (Cole, 1994; Petersen et
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al., 2013; İnan et al., 2016; Synnott et al., 2018). Cole (1994) studied organic maturity of Silurian Qusaiba deposits, Saudi Arabia, using zooclasts reflectance and conventional maturity indicators (e.g., Tmax), and proposed that GRor will increase with the increasing oxygen contents of the depositional environments.
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He found that EqVRo of the sediments deposited under anoxic environments is equal to 80% of GRor, whereas EqVRo of the sediments deposited under oxic environments is equal to 65% of GRor (Table 2)
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(Cole, 1994). Petersen et al. (2013) have determined the relationship between GRor and EqVRo on the basis of Tmax, which can be expressed as: EqVRo = 0.73×GRor + 0.16 (Table 2). It should be noted that
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GRor displays a bimodal distribution due to the random orientation of the polished blocks in their study. İnan et al. (2016) determined the thermal maturity of the hot shales from Saudi Arabia based on graptolite reflectance, organic geochemical and spectroscopic methods. Other studies on Tmax and graptolite reflectance are conflicting and inconclusive (Table 2) (e.g., Petersen et al., 2013; Synnott et al., 2018). The difficulty in using Tmax is mostly due to unreliable values of Tmax, the influence of kerogen type, and low S2 values (e.g., Petersen et al., 2013; Synnott et al., 2018).
5.2.4.4 Comparison of artificial thermal-treatment of graptolites and coals/vitrinite Laboratory experiments where graptolites and vitrinite with similar maturity were heat treated (220– 600 ℃) under similar conditions have shown that graptolites develop high reflectance at 600 ℃ (Bustin et al., 1989). Reyes et al. (2018) used hydrous pyrolysis on immature graptolite-bearing sediments (GRor=0.55%) of the Boas River Formation treated to temperatures of 310 to 350℃, and proposed the relationship between GRor and Rvl as the following equation: Rvl = 0.79×GRor. In contrast, Luo et al. (2018) heated graptolite-bearing sediment, with wider maturity (GRor = 0.65 to 4.03%), through a temperature range of 350 ℃ and 550 ℃ and found that their reflectance is similar to that of the vitrinite.
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Comparison of all published data and data from this study on heat–treated and natural graptolites and vitrinite (or vitrinite-like particles) (Bustin et al., 1989; Bertrand, 1990; Luo et al., 2018; Reyes et al., 2018), regardless of the difference of the experimental methods, indicates a strong and positive relationship (Fig. 16). This relationship can be expressed as:
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EqVRo = 0.99×GRor + 0.08 (1)
This equation is essentially coincident with Bertrand (1990) and Luo et al. (2018), and close to the
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1:1 line (Fig. 9).
Equation (1) combined with Fig. 11 can correlate GRomax to that of EqVRo and it can be expressed
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as:
(2)
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EqVRo = 0.515×GRomax + 0.506
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Vitrinite random reflectance (%)
6
y = 0.99x + 0.08 r = 0.99
4
Luo et al., 2018 2
Bertrand, 1990 Bustin et al., 1989 Reyes et al., 2018 This study
0 0
2
4
6
Graptolite random reflectance (%)
Fig. 16. The relationship between VRor and GRor. The Rvl (reflectance of vitrinite-like particles) values adopted from Reyes et al. (2018)
were converted to EqVRo according to the equation proposed by Xiao et al. (2000).
5.3. The chemical composition of graptolite periderm The electron microprobe, FTIR and Raman spectrum have been widely used to study the chemical composition of various organic macerals including graptolite fragments (Zerda et al., 1981; Green et al., 1983; Jehlička and Bény, 1992; Wopenka and Pasteris, 1993; Bustin et al., 1993; Mastalerz and Bustin, 1993b, a, 1995, 1996, 1997; Ward and Gurba, 1999; Kelemen and Fang, 2001; Ward et al., 2005, 2007, 2008; Chen et al., 2012; Chen et al., 2014; Wang et al., 2014; Wilkins et al., 2014; Wilkins et al., 2015;
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Chen et al., 2015b; Lünsdorf, 2016; Cheshire et al., 2017; Henry et al., 2018; Hao et al., 2019). 5.3.1 Electron microprobe
Recently, the electron microprobe has been used to identify elemental compositions of graptolites in the Silurian Llandovery–Ludlow shales from northern Poland (Morga and Kamińska, 2018). GRor of these
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shales ranges between 1.30% and 1.83%. Carbon is the predominant element in graptolite periderm; carbon, oxygen, nitrogen and sulfur contents range from 84.92 to 91.45 %, 2.56 to 8.43 %, 1.43 to 2.89 %
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and 0.02 to 0.90 %, respectively (Morga and Kamińska, 2018). In order to compare the elemental composition of graptolite and vitrinite (or telocollinite), data for vitrinite were collected from the literature
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(Mastalerz and Bustin, 1997; Ward et al., 2005, 2008). O content in the graptolites negatively correlates with C content (Fig. 17a), similar to vitrinite (Mastalerz and Bustin, 1997; Ward et al., 2005, 2008). For
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elemental comparison with reflectance in Fig. 17, the vitrinite maximum reflectance reported by Ward et al. (2005, 2008) was converted to VRor based on the equation proposed by Komorek and Morga (2002), and GRor in Morga and Kamińska (2018) was calculated to EqVRo based on equation (1). C and O content
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of graptolites is similar to that of vitrinite with similar thermal maturity (Fig. 17b and c) (Mastalerz and Bustin, 1997; Ward et al., 2005, 2008). Similar to the vitrinite, C content of graptolites in shales from
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Poland increases as organic maturity increases, while O content decreases as organic maturity increases (Fig. 17b and c). The O/C ratio in the graptolites displays a negative correlation with EqVRo (Morga and Kamińska, 2018), comparable to that of vitrinite (Mastalerz and Bustin, 1997; Ward et al., 2005, 2008) (Fig. 17d). These variations of the elemental composition in graptolites are consistent with the maturation process and/or the loss of the O-bearing functional groups from organic macromolecules during thermal evolution (Taylor et al., 1998; Bustin and Guo, 1999).
30
4
(a)
(b)
Vitrinite (Ward et al., 2008) Vitrinite (Ward et al., 2005) Vitrinite (Mastalerz and Bustin, 1997)
3
Graptolite (Morga and Kamińska, 2018)
O (%)
EqVRo (%)
20
2
10 1
0
0 70
80
90
100
60
70
C (%)
4
4
(c)
(d)
90
100
2
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2
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3
EqVRo (%)
3
EqVRo (%)
80 C (%)
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60
1
0 0
10
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1
20
30
0 0
O (%)
0.1
0.2
0.3
0.4
O/C (%)
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Fig. 17. Comparison of the chemical composition and reflectance of graptolites and vitrinite. The data for the graptolites are from Morga and Kamińska (2018); data on vitrinite from Mastalerz and Bustin (1997) and Ward et al. (2005, 2008). (a) The negative correlation
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between O and C; (b) the positive correlation between EqVRo and C; (c) the negative correlation between EqVRo and O; (d) the negative
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correlation between EqVRo and O/C. The legends in b–d are the same as in a.
5.3.2 Fourier Transform Infrared Spectroscopy (FTIR) FTIR can provide fundamental information not only about the molecular structure of macerals but
also about the thermal maturity of the hydrocarbon source rocks, and has been widely used to study the functional groups of the macerals in coals with variable rank (Mastalerz and Bustin, 1993a, b, 1995, 1996; Chen et al., 2012; Wang et al., 2014; Chen et al., 2015b; Wang et al., 2017b). However, several studies have also been conducted on graptolites using FTIR (Bustin et al., 1989; Liu et al., 1996; Suchý et al.,
2002; Suchý et al., 2004; Caricchi et al., 2016; İnan et al., 2016; Morga and Kamińska, 2018). Bustin et al. (1989) used FTIR to study evolution of graptolites during artificial thermal-treatment, and found that graptolites display a decrease of aliphatic chains and a depletion of aromatic C–H with increase in thermal maturity/heat treatment due to dealkylation and aromatization. Bustin et al. (1989) also found that graptolites are highly aromatic and have fewer aliphatic structures than vitrinite, whereas Hoffknecht (1991) proposed that the percent of aliphatic compounds in graptolites is greater than that in vitrinite. The parameter CH2/CH3 band intensity ratio can provide information about the length of aliphatic chains (Lin and Ritz, 1993), and Suchý et al. (2002) found that the CH3/CH2 ratio increases with
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increasing GRor. The CH3/CH2 ratio reported by Suchý et al. (2002) was converted to the CH2/CH3 ratio, and plotted in Fig. 18 where it is compared it with the results of Morga and Kamińska (2018). The data from these two studies displays opposite correlations of CH2/CH3 ratio with GRor (Fig. 18). The data of the Silurian deposits in the Polish Baltic Basin from Caricchi et al. (2016), however, did not show any
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obvious correlation (Fig. 18), which may be because in that study, FTIR was conducted on the kerogen concentrate rather than pure graptolites, and the reflectance data was very scattered in a short interval of
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the same well. In low rank coals (Ro < 1.5%), the CH2/CH3 ratio shows a strong negative correlation with vitrinite reflectance, similar to the results of Morga and Kamińska (2018). This indicates that the aliphatic
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chains become shorter with maturation. The CHar/(CH2+CH3) ratio increases with burial depth and GRor, suggesting an increase in the aromatization of graptolite periderm (Morga and Kamińska, 2018).
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Graptolites have been thought to be primarily type II kerogen and less commonly type III according to FTIR parameters of ‘A2’ and ‘C2’ factors (Morga and Kamińska, 2018 and references therein).
GRor (%)
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3
2.5
2
1.5
1 Suchý et al., 2002 Caricchi et al., 2016 Morga and Kamińska, 2018
0.5
0 0.4
0.8
1.2 CH2/CH3
1.6
2
Fig. 18. The relationship between CH2/CH3 and GRor.
5.3.3 Raman spectroscopy Raman spectrum is a non-destructive and rapid microstructure analysis technique, and has been used widely as an indicator for thermal maturity of sediments in the last several decades (Zerda et al., 1981; Green et al., 1983; Jehlička and Bény, 1992; Wopenka and Pasteris, 1993; Kelemen and Fang, 2001; Romero-Sarmiento et al., 2014; Wilkins et al., 2014; Wilkins et al., 2015; Lünsdorf, 2016; Cheshire et al., 2017; Henry et al., 2018; Jubb et al., 2018; Wilkins et al., 2018; Khatibi et al., 2018a, b). The Raman
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spectrum of OM is generally composed of two broad peaks, the graphite band (G band) and the disordered band (D band). In the recent decade, this technique has also been used to study graptolites (Suchý et al., 2004; Liu et al., 2013; İnan et al., 2016; Mumm and İnan, 2016; Cheshire et al., 2017; Morga and Pawlyta, 2018; Wang et al., 2019a). Suchý et al. (2004) used micro-Raman spectroscopy to determine the
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microstructure of the graptolite materials located around an igneous sill, and proposed that the parameter AD/AG (1350/1600 cm–1) peak area can be used to infer organic maturity. Liu et al. (2013) analysed the
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Raman spectrum from a Silurian graptolite-bearing sediment from Qiaokou, Sichuan, China. Mumm and İnan (2016) and İnan et al. (2016) have conducted Raman spectroscopy on Silurian Qusaiba samples with
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GRor ranging from 0.76% to 2.20%, and found that both G band position and Raman band separation (RBS; between G peak shift and D peak shift) display strong positive correlations with GRor. Cheshire et
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al. (2017) have also used RBS to estimate the thermal maturity of the Silurian Qusaiba samples with EqVRo ranging from 0.9% to 2.1% according to the relationship between RBS and VRor proposed by Sauerer et al. (2017), which is consistent with the results from other proxies, (e.g., kerogen elemental
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composition, kerogen skeletal density and BET specific surface area). Morga and Pawlyta (2018) found that Raman band intensity ratio (ID1/IG) shows a robust correlation with GRor in the Polish Silurian shales,
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and can be used to calculate EqVRo values. The relationship between ID1/IG and EqVRo has been determined by Morga and Pawlyta (2018) as equation (3): EqVRo = 1.7319 (ID1/IG) – 0.3295
(3)
Wang et al. (2019a) studied Raman spectra of graptolites in Chinese sediments from a wider thermal maturity range (GRor ranging from 1.21 to 4.91%). They found that D1 peak position decreases with increasing GRor when GRor < 4.0–4.5%, whereas D1 peak position increases with increasing GRor when GRor > 4.0–4.5%. Both G peak position and RBS display a reverse trend with increasing GRor in
comparison with D1 peak position (Wang et al., 2019a). However, it is worth noting that samples with GRor > 4.5% are very limited and the maximum of GRor in this sample set is only 4.91%, thus, more data are needed to illustrate their variation tendency at the high-maturity end. Wang et al. (2019a) established the relationship between GRor and EqVRo as equations (4) and (5), respectively, through the intermediate conversion of RBS (2.1% < GRor < 3.5%) and ID1/IG (GRor > 3.5%) based on the relationship between VRor and RBS (2.1% < GRor < 3.5%), ID1/IG (GRor > 3.5%) proposed by Liu et al. (2013). EqVRo = 0.97 GRor – 0.2 (2.1% ＜ GRor ＜ 3.5%) EqVRo = 0.22 GRor ＋ 2.55 (GRor ＞ 3.5%)
(4) (5)
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Recently, Hao et al. (2019) compared the Raman spectral parameters of the naturally and artificially matured samples, and the latter has a lower degree of coalification than the former due to insufficient structural transformation of artificially matured samples. They found that GRor displays positive correlations with the full width at half maximum of the D1 band and the G band (FWHM-D/FWHM-G),
EqVRo = 0.089 RBS – 19.937
(6)
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relationship between EqVRo and RBS as follows:
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AD/AG and RBS, and negative correlations with D1 peak position and FWHM-G, and established the
In order to compare the relationship between RBS and reflectance among various types of OM, the
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data for vitrinite, inertinite, bitumen and graptolite were collected from several publications noted in Fig. 19 (Kelemen and Fang, 2001; Suchý et al., 2004; Wilkins et al., 2014; Mumm and İnan, 2016; Sauerer et
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al., 2017; Morga and Pawlyta, 2018; Wang et al., 2019a; Hao et al., 2019). The RBS of vitrinite, inertinite, bitumen and graptolites increases as thermal maturity increases when the reflectance is lower than 4%, with the exception of the data from Morga and Pawlyta (2018). However, these data become significantly
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scattered when the reflectance is higher than 4%, especially for the coal/vitrinite data from Liu et al. (2013) and Kelemen and Fang (2001). In addition, G peak position is negatively correlated with GRor in samples
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from Poland (Morga and Pawlyta, 2018), which is opposite to the results from the Saudi Arabian Qusaiba samples and the Chinese Wufeng–Longmaxi shales (İnan et al., 2016; Mumm and İnan, 2016; Wang et al., 2019a). These inconsistencies may be due to differences in conditions as follows: (1) sample preparation; (2) experimental conditions (e.g., laser wavelength); (3) data processing; (4) analysis of various OM, and intra-particle chemical heterogeneity (Lünsdorf, 2016; Sauerer et al., 2017; Henry et al., 2018; Jubb et al., 2018; Hao et al., 2019). Thus, uniform conditions are required to better determine the correlation between Raman spectral parameters and graptolite reflectance in future studies.
290
RBS (cm-1)
260
Vitrinite & inertinite (Wilkins et al., 2014)
Vitrinite (Liu et al., 2013)
Type III kerogen (Kelemen and Fang, 2001)
Type II kerogen (Sauerer et al., 2017)
Type II kerogen (Kelemen and Fang, 2001)
Graptolite (Suchý et al., 2004)
Graptolite (Mumm and İnan, 2016)
Graptolite (Morga and Pawlyta, 2018)
Graptolite (Wang et al., 2019a)
Graptolite (Hao et al., 2019)
Bitumen (Liu et al., 2013)
1
2
3
4 Ro (%)
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Coal (Kelemen and Fang, 2001)
200 0
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230
5
6
7
8
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Fig. 19. The relationship between RBS and reflectance for the vitrinite, inertinite, bitumen and graptolite. The minimum and maximum values of vitrinite and bitumen random reflectance have been given by Liu et al. (2013) rather than the average, thus, half of the total
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minimum and maximum values were used as Ro in this figure.
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6. The microstructure of graptolites
6.1 Scanning Electron Microscopy (SEM) Loucks et al. (2012) classified three types of shale pore systems based on SEM: OM, interparticle,
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and non-OM intraparticle pores. Although OM pores are within OM, they can form an organic interconnected pore network due to OM connectivity, thus making a significant contribution to the overall
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porosity (Passey et al., 2010; Loucks et al., 2012; Mastalerz et al., 2013; Milliken et al., 2013). OM pores in the size range from 5 to 750 nm can adsorb and store methane simultaneously (Loucks et al., 2009), and shales with higher OM content display stronger capacity of methane adsorption because of larger surface area supported by OM micropores (Hickey and Henk, 2007; Ross and Bustin, 2007). OM pores are widely identified in North American and Chinese gas-bearing shales, e.g., the Barnett, Ohio, Antrim, Marcellus, Woodford, Wufeng–Longmaxi and Longtan shales (Hill and Nelson, 2000; Curtis, 2002; Jarvie et al., 2007; Chalmers et al., 2012; Curtis et al., 2012; Loucks et al., 2012; Mastalerz et al., 2013; Milliken
et al., 2013; Tian et al., 2013; Cardott et al., 2015; Hackley and Cardott, 2016; Luo et al., 2016; Ma et al., 2016; Gentzis et al., 2017; Zhang et al., 2017). Pores within graptolites have been studied by SEM (Luo et al., 2016; Ma et al., 2016; Cardott and Curtis, 2018). While many micro-nanopores are widely found in the overmature Wufeng–Longmaxi Formations (Fig. 20) (Luo et al., 2016; Ma et al., 2016), scarce nanopores are present in immature graptolites (GRor=0.66%) from the Lower Ordovician Polk Creek Shale (Cardott and Curtis, 2018). Micro-nano pores in Chinese graptolites generally display irregular or elliptical or nearly spherical shapes, and can be more easily observed in sections parallel to bedding than in sections perpendicular to bedding (Luo et al., 2016; Ma et al., 2016).
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Graptolites fragments with pores were recently thought to be solid bitumen due to their presence within interparticle pores (İnan et al., 2018). However, Luo et al. (2018) found some graptolites within the interparticle pore space (Fig. 5 in Luo et al., 2018), which look like solid bitumen, but morphologically display graptolitic fusellar layers under polarized light. In order to resolve this conflict, graptolites were
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marked using diamond lenses during observation under reflected light and then observed under SEM. Many pores were found in the graptolites, although they were smaller and less numerous than those within
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solid bitumen (Fig. 20).
The development and distribution of OM pores in some graptolites are controlled by their fine
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biological structures (Luo et al., 2016; Ma et al., 2016). Graptolites are generally thought to have low porosity, ranging from 1.62% to 4.23% and averaging 2.51% (Ma et al., 2016). Pores in graptolites can
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further make up an interconnected system due to their alignment along the fusellar layers, and may connect with discrete porous OM detritus and/or other pore systems (Luo et al., 2016; Ma et al., 2016), which is beneficial to the storage and the exploitation of shale gas in graptolite-bearing sediments. OM
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pores are also regarded as an important pore system in other gas bearing shales, e.g., Barnett shale (Loucks
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et al., 2009; Chalmers et al., 2012).
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Fig. 20. OM pores in the Wufeng–Longmaxi Formations. (a) Solid bitumen and graptolite under dry objective; (b) same graptolite under SEM as (a); (c) SEM image showing of location of images d-f; (d-f) images show the pores in solid bitumen are slightly larger and more
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numerous than that in the graptolite (marked by arrows).
6.2 High resolution transmission electron microscopy (HRTEM) HRTEM can study OM microstructure at nm scale (Krzesińska et al., 2009; Chalmers et al., 2012; Pawlyta, 2013; Romero-Sarmiento et al., 2014; Morga and Pawlyta, 2018). Morga and Pawlyta (2018) studied the nanostructure of graptolites with GRor of 1.83% using HRTEM. The parallel alignment of carbon layers in the graptolite can be observed in their HRTEM photos, and the dimension of the basic
structural units is around 1–2 nm, indicating high molecular ordering, in agreement with the study of Goodarzi (1984).
7. The regional maturation and hydrocarbon generation potential of the graptolitebearing sediments 7.1 The hydrocarbon generation potential of graptolites Wang et al. (2017a) concentrated mature (EqVRo around 1.10%) graptolite fragments from two Wufeng–Longmaxi shales. The TOC in the two isolated graptolites is 42.93% and 71.34%, S1 is 6.55 and 15.18 mg/g, S2 is 31.71 and 60.36 mg/g, and HI is 74 mg/g TOC and 85 mg/g TOC, respectively, which
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is much higher than that in the whole rocks (Table 3). İnan et al. (2016) found that an immature isolated graptolite had HI of 200 mg/g TOC and OI of 30 mg/g TOC, much higher than that of mature graptolites (Wang et al., 2017a). These data indicate that the immature graptolites are relatively hydrogen-rich, similar to kerogen type II-III, and have good hydrocarbon potential which is consistent with finding of
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Hoffknecht (1991), Bustin et al. (1989) and Liu et al. (1996) based on FTIR. Pyrolysis chromatography of the two Chinese graptolies found that the CH4 yields are 12.35 mg/g and 23.61 mg/g, respectively
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(Wang et al., 2017a). The weak brown fluorescence in the graptolites of the Liteň Formation from the Czech Republic also indicates that the graptolites have some hydrocarbon generation potential (Fig. 5),
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which means that the graptolite can not be classified as “inertinite-like” macerals (Hoffknecht, 1991). Further, the formation of the OM pores is related with hydrocarbon generation (Loucks et al., 2009),
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which can explain the rare nanopores in low-maturity graptolites from the Lower Ordovician Polk Creek Shale (Cardott and Curtis, 2018). On the other hand, the abundant pores in the graptolites with high maturity also supports their gas generation and storage potential (Fig. 20). These factors suggest that these
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graptolites are mainly gas prone, which is consistent with the results of Hoffknecht (1991), and that graptolites possibly make a significant contribution to the contents of shale gas in graptolite-bearing
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sediments.
Table 3. Comparison of the Rock-Eval data of whole rock versus graptolites (MB-2 and MB-3 from Wang et al., 2017a; K1 from İnan et
Sample ID
al., 2016).
Lithology
Formation
S1 (mg/g)
S2 (mg/g)
Tmax (℃)
HI
OI
(mg/g TOC)
(mg/g TOC)
TOC (%)
MB-2 (whole rock)
Shale
O3w
0.54
1.77
455
3.04
58
8
MB-3 (whole rock)
Shale
S1l
0.69
2.88
458
3.88
74
5
MB-2 (graptolite)
Graptolite
O3w
6.55
31.71
464
42.93
74
43
MB-3 (graptolite)
Graptolite
S1l
K1 (graptolite)
Graptolite
S1l
15.18
60.36
456
71.34
85
2
200
30
Following are examples of worldwide regional maturation and hydrocarbon generation potential studies on graptolite-bearing sedimentary strata.
7.2 Canada Graptolite reflectance was used in several regional thermal maturation studies in Canada (Riediger et al., 1989; Link et al., 1990; Goodarzi et al., 1992c; Gentzis et al.,1996). Most of these regional studies were carried out in Canadian Arctic (Goodarzi et al.,1992c; Gentzis et al.,1996) on graptolitic shale and
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equivalent Upper Ordovician-lower Middle Devonian strata over a distance of over 600 km to determine horizontal variation of thermal maturity and depositional history from Melville Island in southwest to Ellesmere Island in northeast (Fig. 21). The Middle Devonian is up to 3000 m thick, and extends from central Ellesmere Island to Melville Island (Fig. 21) (Trettin, 1989, 1990).
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Based on graptolite reflectance, the sedimentary succession is overmature (3.5–4.2% GRomax) in western Melville Island, mature in central Melville island (1.75–2.1% GRomax), and immature (0.60–0.80%
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GRomax) in the Prince of Wales Islands, eastern Bathurst, Cornwallis, Baillie Hamilton, Dundas, and
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Devon Island (Fig. 21). Therefore, the maturity of graptolitic shale increases from mature (1.32% GRomax) in south of Ellesmere Island to overmature (4.7% GRomax) in north (Fig. 21). Variations in thermal maturity may be due to the areal difference in geothermal gradient and
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overburden thicknesses. Since regional geothermal gradient has been determined to be ~25 °C/Km in Melville Island (Gentzis, 1991), and this value is considered within conventional scope of geothermal
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gradient from other North American basins (Gretener, 1982), the difference in thermal maturation is
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probably due to the thicknesses of the original overburden (Fig. 22).
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Fig 21. Map of the Canadian Arctic Islands showing graptolite maximum reflectance (%GRomax) and maturity of the graptolitic-bearing strata (modified after Gentzis et al., 1996). The green area on the map are designated national park and wilderness.
0
GRomax (%) 2 3
4
5
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1000
1
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0
3000
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Eroded section (m)
2000
4000 5000 6000 7000 8000
Fig. 22. Variation of graptolite maximum reflectance (%GRomax) and overburden strata (eroded sedimentary loading) in Arctic Canada (after Gentzis et al., 1996).
7.3 China The Wufeng–Longmaxi graptolite-bearing sediments in China have various thermal maturities (Luo et al., 2016; Luo et al., 2017; Luo et al., 2018; Wang et al., 2019a), and have been the focus of interest due to both the increased importance of shale gas in the global petroleum industry (Curtis, 2002; Bowker,
2007; Jarvie et al., 2007; Ross and Bustin, 2009; Loucks et al., 2012; Dai et al., 2014; Hackley and Cardott, 2016), and the successful exploration and exploitation of shale gas in the area (Dai et al., 2014; Dai et al., 2016). The Wufeng–Longmaxi sediments are characterized by abundant OM (TOC: 0.51–25.73%, average 2.59%), overmature levels (mostly EqVRo > 2%), thick beds (30–130 m), abundant OM pores and strong gas generation intensity (Table 4) (Zou et al., 2011; Dai et al., 2014; Dai et al., 2016; Luo et al., 2016; Ma et al., 2016; Luo et al., 2017; Luo et al., 2018), important for the generation and accumulation of shale gas. The first large shale gas field was discovered in the Wufeng–Longmaxi Formations, Fuling, Chongqing, with gas reserves over 100×109 m3 (Dai et al., 2014; Dai et al., 2016).
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PetroChina developed the Changning-Weiyuan shale gas field in the Wufeng–Longmaxi sediments in 2014, and the gas production reached 28×108 m3 in 2016 (Ma and Xie, 2018).
In the Wufeng–Longmaxi Formations, graptolite-derived OM accounts for 20–93 vol.% of total OM (Luo et al., 2016). The abundance of graptolites displays a positive correlation with TOC in both Wufeng–
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Longmaxi Formation from Sichuan Basin and Pingliang Formation from Erdos Basin (Zhu et al., 2015; Deng et al., 2016; Borjigin et al., 2017). Based on the comparison of natural and artificial maturation
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samples of Wufeng–Longmaxi Formations and Alum Shale (Sweden) (Luo et al., 2018), OM in the highmaturity Wufeng–Longmaxi shales is very similar to the artificially-matured Alum Shale (Fig. 23). This
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implies that OM in the immature Wufeng–Longmaxi shales is similar to OM in the immature Alum Shale. The Wufeng–Longmaxi Formations have been subdivided and correlated on the basis of lithology
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or logging curves, however, it has not been accepted widely due to the lower resolution and precision of these methods (Zou et al., 2015; Chen et al., 2017). As important biostratigraphic indicator, graptolite species have been widely used for the stratigraphic correlation (Chen, 1984; Bergström and Mitchell,
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1986; Chen et al., 1987; Cooper, 1999; Chen et al., 2000; Chen et al., 2004; Chen et al., 2005; Loydell, 2011; Chen et al., 2017). Chen et al. (2015a) proposed a new subdivision and correlation for the Wufeng–
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Longmaxi Formations based on the graptolite biostratigraphy. According to this revised stratigraphy, intervals (WF2–WF3 and LM2–LM6) were regarded as the best targets for shale gas (Chen et al., 2015a; Chen et al., 2017), and they generally contain abundant graptolites (> 30%) (Qiu et al., 2018). The pores in these graptolites also act as a reservoir and/or migration pathway of the shale gas (Ma et al., 2016; Luo et al., 2018; Qiu et al., 2018).
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Fig. 23. Comparison of OM in natural Wufeng–Longmaxi shales (a) and heat-treated Alum Shale heated at 550 ℃ for 3 days (b).
A regional maturation pattern based on GRor converted to EqVRo (Yang, 2016; Luo et al., 2018; Wang et al., 2019a) for the Wufeng–Longmaxi Formations from Chongqing and surrounding areas indicates that graptolite-bearing strata are mostly overmature, with maturity increasing from southwest
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towards northeast with an EqVRo of 1.17–4.93%, the maximum in sample TT from Pingshan, Sichuan
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province, and minimum in well YC1 from Chengkou, Chongqing (Fig. 24).
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Table 4. Geological and geochemical data of selected global graptolite-bearing sediments. Area/Basin
Formation
Age
TOC (%)
China
Chongqing
Wufeng–Longmaxi
late Ordovician–early Silurian
0.51–25.73
Basin
Alum Shale
Miaolingian–Tremadocian
Sweden
Colonus Trough
Alum Shale
Miaolingian–Tremadocian
Swedena
Central Sweden
Alum Shale
Poland
BPLBb
Poland Poland
Norwegian– Denmark
Danish
EqVRo (%)
Depth (m)
Thickness (m)
Porosity (%)
Gas content (m3/t)
1.17–4.93
900–4500
30–130
3–10
1.7–4.5
pr
Country/Continent
3.00–9.00
1.8–2.5
2000–4000
30–180
3–6
avg. 0.85
3.00–16.00
1.7–2.0
700–1000
70–90
avg. 6.5%
avg. 0.85
Miaolingian–Tremadocian
1.00–21.00
0.5–1.1
20–100
20–35
/
max. 7.6
Piaśnica
Furongian–Tremadocian
3–12
0.55–2.40c
/d
10–34
3.17–6.89
/
BPLB
Sasino
Caradoc
1–7
0.5–4.2c
/
1.5–70
2.24–10.94
/
BPLB
Pasłęk
Llandovery
1–6
0.7–4.0c
/
20–80
3.95–5.56 (Jantar Member)
/
c
/
30–1000
5.25–7.88
/
2000–5000
20–100
/
2.4–8.5
Pr
e-
(onshore)
BPLB
Pelplin–lower part
Wenlock
0.5–1.7
0.6–2.5
North Africa
Ghadames Basin
Tanezzuft
Llandovery–Wenlock
2–16
0.7–2.0
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Poland
Biogenic gas;
b
BPLB = Baltic–Podlasie–Lublin Basin;
c
Mean random reflectance of the vitrinite–like macerals (Rvl);
d
“/” means no data.
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a
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Fig. 24. The EqVRo distribution of the Wufeng–Longmaxi shales in Chongqing and surrounding regions. Data at each sample location is
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well/outcrop name/EqVRo. Data are from Yang (2016); Luo et al. (2018); Wang et al. (2019a).
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7.4 Denmark and Sweden
Lower Palaeozoic strata is widely distributed in Denmark, Sweden, Norway, Poland and the Baltic
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States (Fig. 25). In this area, terrestrial to shallow marine sandy deposition commenced in lower Cambrian (Serie 2) following an overall transgression of the Baltic continent (Nielsen and Schovsbo, 2011, 2015). From the middle Cambrian (Miaolingian) to the early Ordovician (Tremadocian), organic rich mud deposition known as the Alum Shale Formation in Denmark, Sweden and Norway, the Türisalu Formation in Estonia, the Kaporye Formation in the St. Petersburg region and the Słowińska Formation as well as Piaśnica Formation in NE Poland completely dominated (Nielsen and Schovsbo, 2007). The shale is now present in the deeply buried strata towards the Polish-German and Swedish-Norwegian Caledonian fronts,
in the Baltic area and as erosional outliers in Sweden (Fig. 25), representing only a minor part of the original huge deposition area that its maximum at the early Ordovician may have extended for more than
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800 000 km2 (Schovsbo et al., 2018).
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Fig. 25. Outline of approximate original distribution of the Alum Shale Formation and present-day occurrence of the lower Paleozoic strata in southern Scandinavia (modified from Schovsbo et al., 2018).
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Thermogenic gas trapped in the Alum Shale has been explored in Denmark (Schovsbo and Jakobsen, 2019) and Sweden (Pool et al., 2012) (Table 4). In Denmark and Sweden, a considerable resource has
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been estimated by the U.S. Geological Survey (mean = 67 × 109 m3 gas; Gautier et al., 2013) and by the European Geological Surveys (Zijp et al., 2017) to be present mostly in the Cambrian (Miaolingian) to
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Ordovician (Tremadocian) Alum Shale Formation. Typical Alum Shale has TOC values from 5–10% (even up to 25% TOC in some samples) (Fig. 26), and their thermal maturity ranges from immature (EqVRo<0.5%) in Central Sweden and Estonia, dry gas mature (EqVRo: 1.6–2.5%) in most of Denmark and southern Sweden, and to post-mature to low-grade metamorphic in Norway (EqVRo >3%; Buchardt et al., 1997; Petersen et al., 2013) (Table 4). The shale is graptolitic with macroscopic remains of typical pelagic Rhapdinopora occurring abundantly in the Tremadocian shale. In the Cambrian, microscopic remains of graptolites also occur presumably from benthic species (Petersen et al., 2013) - an
interpretation that has been supported by the rediscovery of Miaolingian benthic graptolites in the Alum Shale from Norway (Wolvers and Maletz, 2016). At this time, no quantification of the contribution of graptolite carbon to the total TOC content has been made, and the only published study of the Alum Shale nanopore system by Henningsen et al., 2018 did not specifically investigate the graptolite contribution to the total porosity. The lower Paleozoic shale gas prospective areas in Denmark largely follow the margins of the Norwegian–Danish Basin (Schovsbo et al., 2014). Exploration of this play is still limited, and representative well data and production test data for the shales are lacking. The only exploration well in
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Denmark revealed gas in the Alum Shale Formation, however, no test production was carried out, so the commercial potential is still unknown (Schovsbo and Jacobsen, 2019). The well was drilled within a socalled ‘sweet spot’, defined as an area with expected highest gas content (c.f. Schovsbo et al., 2014). Results from this well revealed the Alum Shale to be 40 m thick, compared to up to 180 m in the
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depocenter offshore Denmark. In Sweden, thermogenic gas has been explored in Scania, within the Colonus Trough, which is a fault bounded graben system in southern Sweden. Exploration indicated that
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the Alum Shale Formation was gas mature (EqVRo: 1.7–2%), located at 700–1000 m depth (Table 4), but did not contain gas in economically producible quantities, possibly due to gas leakage as a result of uplift
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(Pool et al., 2012). In south-central Sweden, gas-bearing immature to low-maturity Alum Shale with depths < 150 m have been known for several decades and been under exploration for commercial shale
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gas production (Table 4); they are thought to be composed of mixtures of thermogenic and bacterially-
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derived gas (Schultz et al., 2015; Schovsbo and Nielsen, 2017).
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Fig. 26. Composite profile of TOC content in the Alum Shale in Scania, southern Sweden (modified from Schovsbo, 2003).
7.5 Poland
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The highest shale gas exploration potential in Poland is related to the Cambrian–Lower Ordovician (upper Furongian–Tremadocian), Upper Ordovician (Caradoc) and Silurian (Llandovery–Wenlock) black
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and dark grey shales with abundant graptolite remains within the Baltic-Podlasie-Lublin Basin (BPLB) at the slope of the East European Craton (EEC) (Table 4, Figs. 27 and 28) (Poprawa, 2010; Karcz et al.,
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2013; Podhalańska, 2013). Due to lateral facies variability, hydrocarbon potential of these rocks varies throughout the area. The lower Paleozoic deposits were buried to significantly different depths in the
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individual parts of the BPLB. Tectonic processes resulted in a break-up of the Basin into segments (Karcz et al., 2013). The current burial depth within the significant part of BPLB is shallow enough for
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commercial shale gas exploration, and geological structure is simple, especially within the Baltic Basin and Podlasie Basin (Poprawa, 2010).
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Fig. 27 The distribution of Upper Ordovician–lower Silurian sediments in the Baltic-Podlasie-Lublin Basin (BPLB) at the slope of the East European Craton (EEC) (modified after Poprawa, 2010). Abbreviations: SPW =Płock-Warsaw zone; SBN = Biłgoraj-Narol zone;
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TESZ = Trans-European Suture zone.
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The Furongian–Tremadocian Alum-Shale-equivalent black bituminous shales belong to the Piaśnica
Formation, which occurs only within the Baltic Basin (Fig. 28). Their thickness is from 10 m (in the onshore part) to 34 m (in the offshore part) (Szymański, 2008; Więcław et al., 2010; Podhalańska et al., 2016a), thinner than the Alum Shale in Denmark (30–180 m) and southern Sweden (70–90 m); and average TOC content ranges between 3% and 12.0% (Table 4). Thermal maturity of OM increases with the increasing depth from NE to SW from the main phase of oil generation through the condensate and wet gas phase to the dry gas generation phase. The Rock-Eval Tmax temperature and Rvl vary from 430 °C
to 505 °C, and from 0.55% to 2.4%, respectively (Poprawa, 2010; Więcław et al., 2010; Karcz and Janas, 2016; Podhalańska et al., 2016a) (Table 4). The effective porosity ranges between 3.17% and 6.89% (Table 4) (Dyrka, 2016). S2 is up to 72 mg HC/g rock, while HI varies from 6 to 484 mg HC/ TOC (Więcław et al., 2010). The gas content in the Piaśnica Formation reaches 7.6 m3/t in the Lębork S-1 well (Lehr and Keeley, 2016) (Table 4), comparable to that in North American gas-bearing sediments (e.g.,
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na
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Jarvie, 2012).
ro of -p re lP na ur Jo Fig. 28 Generalized lithostratigraphy of the upper Cambrian-Silurian deposits in the Polish part of the Baltic-Podlasie-Lublin Basin (after Podhalańska 2016b).
The Caradoc (Sandbian–lower Katian) rocks (i.e., Sasino Formation) are most fully developed in the northern and western parts of the Baltic Basin, mainly composed of dark grey and black shales with rich graptolite fauna (Fig. 28) (Modliński and Szymański, 1997; Podhalańska, 2013; Podhalańska et al., 2016a). Similar rocks but with lower thickness and stratigraphic extent stretch to the southeast in the Podlasie and Lublin areas (different regional lithostratigraphic equivalents). Thickness of the Sasino Formation and its equivalents in other parts of the BPLB varies from 1.5 m in the eastern onshore part of the Basin to 70 m in its offshore part (Table 4) (Modliński and Szymański, 1997, 2008; Poprawa, 2010; Więcław et al., 2010; Podhalańska et al., 2016a; Podhalańska et al., 2016b). The average TOC content in
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the Sasino Formation ranges from 1% to 4%, and even up to 7%, in the offshore part (Table 4) (Poprawa, 2010; Więcław et al., 2010; Karcz and Janas 2016; Podhalańska et al., 2016a). Thermal maturity increases from NE to SW with increasing depth from the main phase of oil generation (0.6–1.1% Rvl) through condensate and wet gas generation phase (1.1–1.4% Rvl) to the dry gas generation phase (>1.4% Rvl),
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reaching 4.2% within the Baltic Basin (Table 4) (Więcław et al., 2010; Podhalańska et al., 2016a). Tmax values reported for the BPLB are 425–474oC (Więcław et al., 2010; Karcz and Janas 2016). The effective
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porosity ranges between 2.24% and 10.94% (Table 4) (Dyrka, 2016). S2 reported for the Baltic Basin varies from 0.05 to 10.6 mg HC/g rock, and HI ranges from 11 to 359 HC/g TOC.
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Llandovery (Rhuddanian-Telychian) shales make up the Pasłęk Formation, whereas the Wenlock (Sheinwoodian-Homerian) rocks belong to the lower part of the Pelplin Formation (Sheinwoodian-
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Ludfordian as a whole) (Fig. 28) (Podhalańska et al., 2016b). They contain rich and variable graptolite fauna, which made it possible to determine biostratigraphic levels and limits of chronostratigraphic units (Podhalańska, 2013). The Llandovery shales of the Pasłęk Formation occur throughout vast parts of the
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western slope of EEC. Their thickness generally increases from the east to the west, reaching a maximum of 80 m. However, in the major part of the BPLB, it ranges from 20 to 40 m (Table 4). Average TOC
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values are usually 1% to 2.5%, except for the Podlasie Basin, where they reach 6% (and even > 15% in individual layers) (Table 4). High TOC content was particularly documented in the lower part of the Pasłęk Formation (the Jantar member) (Modliński et al., 2006; Poprawa 2010; Więcław et al., 2010; Karcz et al., 2013). Rvl in the Baltic Basin varies from 0.7% to 2.3%, and rises to 4.0% in the Podlasie Basin, showing the NE-SW trend (Table 4) (Poprawa, 2010; Więcław et al., 2010; Karcz et al., 2013). Tmax temperature reported for the Baltic Basin falls within the range of 435–534oC. S2 varies from 0.12 to 40.6 mg HC/g rock, while HI varies from 8 to 436 mg HC/g TOC (Więcław et al., 2010).
The Jantar Member (Rhuddanian–lower Aeronian), composed of bituminous shales, is the most perspective unit of the Pasłęk Formation within the onshore and offshore areas of the Baltic Basin (Fig. 28) (Podhalańska et al., 2016a). Its maximum thickness is 18 m (Modliński et al., 2006; Podhalańska et al., 2016a). The average TOC content varies from 2% to 5%, and the thermal maturity increases from NE to SW, starting with the oil generation window and passing through condensate and wet gas phase to dry gas generation phase (Poprawa 2010; Karcz and Janas, 2016; Podhalańska et al., 2016a; Podhalańska et al., 2016b). Rock-Eval Tmax values range between 438 and 485 °C (Karcz and Janas, 2016). Average effective porosity is 3.95–5.56% (Table 4) (Dyrka, 2016).
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The Wenlock shales have the largest extent of all stratigraphic units. They have been documented within the whole BPLB (Fig. 28) (Podhalańska et al., 2016a). Their thickness is highly variable, from 30 m in SE part of the Lublin Basin to over 1000 m in western part of the Baltic Basin (Table 4) (Poprawa, 2010; PIG, 2012; Karcz et al., 2013; Podhalańska et al., 2016a). Average content of OM in individual
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Wenlock sections in central and western parts of the Baltic Basin and the Podlasie Basin usually ranges from 0.5% to 1.4% TOC. In eastern part of the Baltic Basin and in the Lublin Basin, it is higher, rising to
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about 1–1.7% TOC (Poprawa, 2010, Więcław et al., 2010; Karcz and Janas, 2016; Podhalańska et al., 2016a). Thermal maturity of OM increases from NE to SW, from the oil generation phase through the
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condensate and wet gas phases to dry gas generation phase (Podhalańska et al., 2016a). Rvl equals 0.6– 2.5% (Table 4), and Tmax values vary from 425oC to 509oC (Więcław et al., 2010; Karcz and Janas 2016;
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Podhalańska et al., 2016a). Average effective porosity is 5.25–7.88% (Table 4) (Dyrka, 2016). S2 in the Baltic Basin ranges between 0.07 and 3.5 mg HC/g rock. HI is 7–335 mg HC/g TOC (Więcław et al., 2010).
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7.6 Arabia and North Africa
The lower Silurian organic-rich shales (hot shales) were widely deposited in the Arabia and North
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Africa due to a strong rise of sea level during the latest Ordovician to early Silurian, including e.g., Tanezzuft Formation in Libya, Tunisia and Algeria, Tanf Formation in Syria, Akkas Formation in Iraq, Qusaiba Formation in Saudi Arabia, and Batra or Mudawwara Formation in Jordan (Loydell, 1998; Lüning et al., 2003; Soua, 2014; İnan et al., 2016). They are characterized by type II kerogen, TOC up to 20%, and thickness is generally less than 100 m (Lüning et al., 2000; Belaid et al., 2010; Soua, 2014; Wang et al., 2019b). Their thermal maturity varies significantly due to variable burial history, ranging from immature to post mature (Lüning et al., 2005; Belaid et al., 2010; Soua, 2014; İnan et al., 2016;
Wang et al., 2019b). In North Africa, 80–90% of Paleozoic-sourced petroleum are derived from these Silurian hot shales, and in Arabia, some oil and sweet gas in Paleozoic-Mesozoic reservoirs are regarded to be sourced from these organic-rich sediments (Lüning et al., 2000; Al-Juboury and Al-Hadidy, 2009; İnan et al., 2016). In recent years, the hot shales were thought to contain shale oil/gas potential due to their wide distribution, thickness, abundant OM and variable thermal maturity (Soua, 2014; İnan et al., 2016; Wang et al., 2019b). For example, the gas content of the hot shales falls between 2.4 and 8.5 m3/t in the Ghadames Basin, and their geochemical characteristics and petroleum accumulation conditions are comparable to the Wufeng–Longmaxi Formations (Table 4) (Wang et al., 2019b).
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8. Conclusions Based on a review on the optical characteristics, chemical composition, and microstructure of graptolites, the following conclusions can be reached:
(1) There are two types of graptolites: granular (GG) and non-granular (NGG). GG have lower
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reflectance and weaker anisotropy than NGG. GG can be altered to NGG with increasing thermal maturity. (2) The aromaticity and ordering of the aromatic structure of graptolite increasing with increasing
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maturity is similar to that of vitrinite and bitumen, illustrating that their physical and chemical parameters are reliable geo-thermometers. There is a positive correlation between GRor and GRomax, allowing GRor
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to be used as a measure of thermal maturation for the lower Paleozoic sediments. The relationship between GRor and EqVRo can be expressed as: EqVRo = 0.99GRor + 0.08.
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(3) Electron microprobe and FTIR analyses are very useful tools to study chemical compositions of graptolites, indicating that their compositions are similar to vitrinite. Standard measurement and data processing are required for the improved analysis of Raman spectrum for graptolites. Graptolites contain
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abundant pores, which are less abundant and smaller than those in solid bitumen. (4) Based on the chemical composition and organic geochemical characteristics of the graptolites,
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they are mainly gas prone, similar to vitrinite, and have a significant hydrocarbon generation potential. (5) OM in the Wufeng–Longmaxi sediments is dominated by graptolites and solid bitumen. Their
thermal maturity was determined by GRor, in the range of 1.17–4.93%, indicating marginally mature to overmature. (6) The graptolite-bearing shales are worldwide hydrocarbon source rocks and contribute significantly to global petroleum reserves. Graptolite serve both as source material and as reservoir, due to nano-microporosity, and are important to the accumulation of shale gas in these sediments.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 41503028, 41773031 and 41830424). Field sampling in the Czech Republic was partly supported by OP RDE, MEYS, under the project “Ultra-trace isotope research in social and environmental studies using accelerator mass spectrometry” (Reg. No. CZ.02.1.01/0.0/0.0/16_019/0000728). We thank Dr. Roger MacQueen of the Geological Survey of Canada for reviewing an early version of this manuscript and for his valuable suggestion. We also thank Zhongliang Ma at the Wuxi Institute of Petroleum Geology, Sinopec Petroleum Exploration and Development Research Institute for conducting the artificial maturation experiments in
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this study. We are very grateful to Dr. MaryAnn Love Malinconico and one anonymous reviewer for their critical but constructive and valuable comments that significantly improved the quality and language of this paper. The authors also appreciate Managing Editor Dr. Shuhab Khan for his precious time and energy
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to handle this paper.
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