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Origin of bitumen fractions in the Jurassic-early Cretaceous Vaca Muerta Formation in Argentina: insights from organic petrography and geochemical techniques
Accepted Manuscript Origin of bitumen fractions in the Jurassic-early Cretaceous Vaca Muerta Formation in Argentina: insights from organic petrography and geochemical techniques
Aleksandra Małachowska, Maria Mastalerz, LaBraun Hampton, Jan Hupka, Agnieszka Drobniak PII: DOI: Reference:
S0166-5162(18)30216-7 https://doi.org/10.1016/j.coal.2018.11.013 COGEL 3122
To appear in:
International Journal of Coal Geology
Received date: Revised date: Accepted date:
15 March 2018 14 November 2018 15 November 2018
Please cite this article as: Aleksandra Małachowska, Maria Mastalerz, LaBraun Hampton, Jan Hupka, Agnieszka Drobniak , Origin of bitumen fractions in the Jurassic-early Cretaceous Vaca Muerta Formation in Argentina: insights from organic petrography and geochemical techniques. Cogel (2018), https://doi.org/10.1016/j.coal.2018.11.013
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ACCEPTED MANUSCRIPT Origin of bitumen fractions in the Jurassic-early Cretaceous Vaca Muerta Formation in Argentina: Insights from organic petrography and geochemical techniques Aleksandra Małachowskaa, Maria Mastalerzb, LaBraun Hamptonb, Jan Hupkaa, Agnieszka Drobniakb
b
Indiana Geological and Water Survey, Indiana
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University, Bloomington, Indiana 47405-2208, USA
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and Chemical Technology, Poland
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Gdansk University of Technology, Faculty of Chemistry, Department of Process Engineering
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a
Abstract
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This paper investigates chemical functional groups of the two extracted bitumen fractions
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in shales of the Jurassic to early Cretaceous Vaca Muerta Formation of the Neuquén Basin in Argentina, South America. The results indicate that Bitumen I is strongly aliphatic and
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appears to be genetically related to fluorescent amorphous organic matter. In contrast,
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Bitumen II consists of highly condensed, aromatic hydrocarbons, and has some correspondence to nonfluorescent amorphous organic material. Comparison of Rock-Eval VI
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pyrolysis data (S1 and S2) with the bitumen yields suggests that Bitumen I relates to S1 but also to S2. In addition, Bitumen I has a positive correlation with light liquid hydrocarbons (C5–C29), but also partially with heavier hydrocarbons (above C30). This suggests that Bitumen I corresponds to the majority of lighter hydrocarbons up to C29 and some portion to heavier hydrocarbons. These results have implications for the assessment of the mobility of generated hydrocarbons and their availability for production.
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Keywords: kerogen, macerals, bitumen, mature shale, Vaca Muerta Formation, FTIR 1. Introduction Studies of bitumen and its variability in chemical composition, generation pathways
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or physical state have been carried out since the early stages of petroleum exploration (Rogers
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et al., 1974; Hunt, 1979; Jones, 1980; Powell, 1984; Curiale, 1986; Jacob, 1989; George et al.,
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1993; Stasiuk, 1997; Hwank et al., 1998; Mastalerz and Glikson, 2000; Curiale and Curtis, 2016). The occurrence of multiple populations of bitumens in the same rock can be related to
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different sources, formation mechanisms, various thermal events and/or different temporal
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pulses of hydrocarbon migration (Evans et al., 1971; Rogers et al., 1974; Curiale, 1986; Gentzis and Goodarzi, 1990; George et al., 1994; Stasiuk, 1997; Fink et al., 2016, Wang et al.,
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2017; Chen et al., 2017; Mastalerz et al., 2018).
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Several studies have indicated the presence of more than one generation of bitumen that could be isolated from rocks through sequential extraction or after demineralization or both.
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Nabbefeld et al. (2010a) and Holman et al. (2012 and 2014) termed bitumen obtained after
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solvent extraction from the undigested rock as “Bitumen I” and that obtained by extraction after demineralization of rock as “Bitumen II." It was suggested that Bitumen II is less likely
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overprinted by migration processes because of its strong association with organic matter (OM) and its effective shielding by the kerogen-mineral matrix (Sherman et al., 2007). Nabbefeld et al. (2010a, b) in their study of marine Permian/Triassic sediments demonstrated that Bitumen II generally had lower aromaticity that Bitumen I and that this difference in aromaticity increased with increasing clay to total organic carbon (TOC) ratio.
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ACCEPTED MANUSCRIPT Sequential extractions on various non-demineralized rocks also demonstrated the presence of different fractions of bitumen. Price and Clayton (1992) observed a reduction in aromaticity and increase in the proportion of high-molecular-weight aromatics with successive extraction steps after sequential Soxhlet extraction of Devonian immature source rocks. Mueller and
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Philp (1998) suggested that high-molecular-weight petroleum was resistant to conventional
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Soxhlet extraction but could be extracted ultrasonically with p-xylene. Schwark et al. (1997)
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used successive extraction to obtain oils from reservoir sandstones and documented lower maturity for the final extracts compared to the earlier ones.
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In comparison, sequential extraction of Pennsylvanian coals from the Illinois Basin using
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methanol and dichloromethane (DCM) (Furmann et al., 2013) showed that DCM extraction resulted in a wider range of n-alkanes (C15–C33) than from methanol extraction (C15–C27).
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DCM extracts also yielded relatively higher concentrations of hopanes and selected aromatic
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hydrocarbons. That study also showed that extraction using solvents modified the pore structure of coals. In a paper by Wei et al. (2014), Upper Devonian to Early Mississippian
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New Albany Shale samples having high organic matter content were treated with DCM and
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later with toluene, yielding extracts of lower aromaticity after initial extraction with DCM and
changes
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more aromatic bitumen after extraction with toluene. Extraction of these bitumens resulted in
in the pore characteristics of these shale samples. The occurrence of different bitumen fractions in rocks poses the question of whether subsequent bitumens are of the same origin as those of the first generation, remaining trapped in the rock matrix and unable to be easily extracted, or if they originated from different source. Based on the distinct composition of Bitumen I and Bitumen II in Paleoproterozoic
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ACCEPTED MANUSCRIPT marine rocks in Australia, Williford et al. (2011) and Holman et al. (2012 and 2014) suggested that Bitumen II was closely related to the in-situ organic matter, whereas Bitumen I could be overprinted by migrated hydrocarbon phases. This paper discusses differences in chemical functional group characteristics of Bitumen I
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and Bitumen II in relation to the kerogen in mature shales from the Jurassic to early
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Cretaceous Vaca Muerta Formation in Argentina. The primary definitions of Bitumen I
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and Bitumen II of this study follow those used in Nabbefeld et al. (2010 a, b) and Holman et al. (2012, & 2014), with Bitumen I obtained by extraction of the pulverized rock using DCM,
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and Bitumen II obtained by DCM extraction of kerogen after rock demineralization using
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hydrofluoric acid.
The main objectives of the paper were to 1) study the differences in chemical functional
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groups of the two bitumen fractions in relation to corresponding kerogen macerals present in
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the Vaca Muerta Formation, and 2) investigate the kerogen transformation into Bitumen I and
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Bitumen II and its implications for hydrocarbons mobilization. 2. Geological setting of the Vaca Muerta Formation
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The Neuquén Basin is located in central Argentina and Chile, in the southern section
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of the Andean mountains, along with their eastern side, and between 32S and 40N latitude (Howell et al., 2005) (Fig. 1). The basin covers an area of approximately 120,000 km2 (Yrigoyen, 1991; Romero-Sarmiento et al., 2017) and contains one of the most complete records of the Jurassic-Cretaceous marine invertebrates in the world (Howell et al., 2005). Vaca Muerta is a mature Jurassic to Early Cretaceous heterogeneous and organic-rich shale formation, the main source rock for hydrocarbons in the Neuquén Basin, and most promising in terms of production potential in South America (Urien and Zambrano, 1994; Cruz et al., 4
ACCEPTED MANUSCRIPT 1996; Villar et al., 2006; Ukar et al., 2017). The formation exhibits extensive variability in lithology, organic, and mineral content (Fantín et al., 2014; Ptaszyńska, 2015; Williams et al., 2015; Suarez-Rivera et al., 2016; Małachowska et al., 2016 and 2017, Romero-Sarmiento et al., 2017; Ukar et al., 2017). Geochemical properties of this formation are not well
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3. Methodology
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al., 2009; Romero-Sarmiento et al., 2017; Ukar et al., 2017).
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documented because there has been only a limited number of studies on this topic (Monreal et
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3.1 Rock sampling and core analysis
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Twenty-three rock samples were collected from the 137m long core of the Vaca Muerta Formation. For this study, the core was divided into three sections (upper, middle, and lower)
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marked as A, B and C respectively (Fig. 2). These sections are closely related to three
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dominant lithofacies: Upper Vaca Muerta, Middle Vaca Muerta and Lower Vaca Muerta (also
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called La Cocina). Samples were collected every 2-3 m from the three sections of interest. The choice was guided by previously documented wireline-log based heterogeneous rock analysis
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and the distribution of petrophysical properties along a core spanning the entire section of Vaca Muerta Formation (Williams et al., 2015). Taking into account rock heterogeneity, each
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section was chosen to represent a thicker interval than just the three meters of the core. Megascopic heterogeneity of the samples was reflected dominantly by the color variabilities related to mineralogy and organic matter content of the rocks. 3.2 Mineralogy and geochemical analysis For mineralogy analysis, the weight of each sample was about 100 g. Larger particles were crushed
and
milled,
whereas
smaller
ones 5
milled
to
reduce
particles
size
ACCEPTED MANUSCRIPT and homogenize the sample. After milling, the samples were sieved through 850-micron sieve (20 mesh). X-ray diffraction analysis and semi-quantitative mineralogy were performed on a Bruker D8 analyzer. The detailed methodology is described by Bhargava et al. (2005). Total organic carbon (TOC) analyses were conducted on a LECO CS-444 carbon analyzer
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at about 1000° C. Approximately 0.15 g per sample were pre-treated with HCl and vacuum
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filtered on glass fiber paper. The residual sample and the paper were placed in a ceramic
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crucible, dried and combusted with pure oxygen. Rock-Eval VI pyrolysis measurements were performed on 0.1 grams of the same ground sample that was used for LECO TOC on a Rock-
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Eval VI instrument following the procedure as described by Grassmann et al. (2005).
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3.3 Gas chromatography and thermogravimetry analysis
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To measure the total hydrocarbon composition (C1–C30 and above), a quantitative
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extraction and gas chromatography method and thermogravimetric analysis were used. About 2 mg of the sample material was crushed to powder in a cryogenically chilled and sealed
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crushing cell. The crushed material was loaded into a solvent extraction cell.
The
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hydrocarbons extracted with a mixture of dichloromethane (DCM) and at elevated pressure and temperature (Williams et al., 2014). The resulting solvent/hydrocarbon mixture
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(hydrocarbons above C5) was characterized by gas chromatography (GC) using an Agilent 7000D and 7010B GC/MS Triple Quadrupole systems. More details on the extraction methodology can be found in a similar study of the Duvernay shale (Davis et al., 2013), and Vaca Muerta shales (Williams et al., 2014). In case of inability to detect heavy hydrocarbon (above C30) directly via GC due to detection limit issues, a thermogravimetry analysis (TGA) following procedures described by Williams et al (2014) was used.
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ACCEPTED MANUSCRIPT 3.4 Porosity and permeability measurements A crushed sample pressure-decay system was used to measure permeabilities and porosities of the samples based on pressure pulse decay measurement, following the procedure
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described in Akkutlu and Fathi (2011).
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3.5 Isolation of Bitumen I, Bitumen II, and kerogen
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Kerogen isolation from the rock matrix followed procedures used in previous studies (Forsman and Hunt, 1958; Forsman, 1963; Saxby, 1970; Durand and Nicaise, 1980; Robl
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and Davis, 1993; Ibrahimov and Bissada, 2010; Holman et al., 2012 and 2014). Bitumen was
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extracted, and kerogen was isolated following the scheme of Figure 3. Pulverized rock material was placed in test tubes, and about 10 ml of DCM was added. Samples were mixed,
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ultrasonicated for about 10 minutes and centrifuged. The resultant brown to dark brown color
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of the supernatant fluid indicated whether the sample needed to be further extracted to ensure that all the Bitumen I was extracted. The Bitumen I yield was determined gravimetrically.
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DCM was evaporated from the remaining rock material by blowing dry N2 gas into the
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headspace.
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After Bitumen I was extracted, sediments were diluted with hydrochloric acid (HCl, ca. 10 %), and warmed to ca. 60 °C for 30 minutes to ensure complete reaction of carbonates. The mixture was cooled, centrifuged, and the supernatant was decanted. Deionized water was used to transfer the decalcified sediment into a polyethylene centrifuge tube. The decarbonized rock sediment was then demineralized using pre-cooled hydrofluoric acid (HF, ca. 24 %). The samples stored in plastic tubes were treated twice with HF (centrifuged each time). Finally, they were rinsed twice with deionized water. After HF demineralization, 7
ACCEPTED MANUSCRIPT samples were frozen and then freeze-dried. The residual dry powders were weighed and placed in a glass centrifuge tube. The extraction process for Bitumen II was repeated in the same way as for Bitumen I. In summary, considering the extraction procedure in this study, Bitumen I was defined as
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easily extractable OM from the rock, and Bitumen II as an organic matter fraction that is
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entrapped within the kerogen/mineral matrix, and that can only be extracted from kerogen after demineralization. Sequential steps of the methodology for extraction of Bitumen I,
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Bitumen II, and isolated kerogen are shown in Fig. 3.
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3.5 FTIR technique
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For each sample, four fractions: Bitumen I, Bitumen II, kerogen, and original rock were analyzed using the FTIR technique. All fractions were prepared as KBr pellets, and a Nicolet
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6700 spectrometer equipped with a DTGS detector was used. Each spectrum was generated
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by collecting 300 scans per sample at a resolution of 4 cm-1. Assignments of peaks for
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functional groups were based on Painter et al. (1981, 1985) and Wang and Griffiths (1985). The following four FTIR indices have been used in this study to evaluate the chemistry of
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organic matter (Kister et al., 1990; Mastalerz and Bustin, 1996; Lis et al., 2005, Chen et al., 2012, Liu et al., 2017): (1) aromaticity (AR1); (2) aliphatic chain length (ACL); (3) oxidation index (Ox1); and (4) sulfur index (SI). The calculations of these indices utilized integrated areas of the selected functional groups (Table 1). 3.6 Organic petrography
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ACCEPTED MANUSCRIPT Shale samples were prepared as polished blocks using the standard organic petrography sample preparation techniques (ICCP, 1963). A Leica DM 2500P microscope linked to a TIDAS PMT IV photometric system was used to measure vitrinite reflectance and carry out organic matter characterization of the bulk samples. Maceral composition was assessed based
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on reflected white light and fluorescent light microscopy. The volume percent of each maceral
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on the mineral-matter-free basis was converted to mineral-matter-containing basis, using
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the measured total organic carbon content (TOC, wt. %). All macerals were then recalculated
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from volume percent to the weight percent composition. 4. Results and Discussion
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4.1 Core and geochemical rock analysis
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The 137 meters of continuous core-scale observations show that rock composition progressively change from a silicate/clay-rich and organic-rich at its base, to a calcareous, low
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organic content rocks at the top of the core. Selected intervals of Vaca Muerta core indicating
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high variability in the lithology and rock components are presented in Figure 4. The mineral composition (by wt. %), total organic carbon (TOC, wt. %) content, vitrinite reflectance (Ro
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%), total porosity (TP, %), Hydrogen index (HI), and Oxygen index (OI) are presented in
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Figure 5. These analyses indicated high average TOC content, from 1-3 wt. % at the top of the core, 7-13 wt. % in the middle section of the core, and 9-12 % in the bottom part. Total porosity is around 3% in the top section of the core, ranges 3 to 13 % in the middle section, and 2-10 % for the lower part of the core (Fig.5).
4.2 Insights from Organic Petrography
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ACCEPTED MANUSCRIPT Kerogen in the Vaca Muerta core consists of marine-derived macerals (kerogen Type II) (Fig. 6). The terrestrial organic matter, including small vitrinite and inertinite particles, is scarce. The measured Ro ranges from 0.5 to 0.7 %, with most samples having average vitrinite reflectance of 0.7 %, which corresponds well to the maturity suggested by Tmax
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values from Rock-Eval pyrolysis using the equation VRo = (0.0180*Tmax)-7.16 (Jarvie,
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2001). The significant contribution to kerogen comes from amorphous organic matter (AOM)
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that is either fluorescent (FOM) or non-fluorescent (NON-FOM). On mineral matter free basis, FOM ranges from 10-40 % of the total organic material by volume, and NON-FOM
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constitutes 5-30 vol. %.
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Alginite accounts for 5-10 vol. %, liptodetrinite likely of algal origin (classified as “other liptinite” in Fig. 6) ranges from 5-25 vol. %, whereas vitrinite, and inertinite account for only
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about 0-3 vol. %. In addition to primary kerogen macerals, solid bitumen (secondary product)
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is prevalent in most samples, forming the dominant organic matter component in most
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samples.
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4.3 Functional group characteristics In this study, 23 FTIR spectra of the original rock, 23 spectra of Bitumen I, 19 spectra of
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Bitumen II, and 21 spectra of kerogen were generated and, subsequently, the average spectrum of each category (Fig. 7) was used for comparative purpose (Fig. 8). Although similar functional group bands characterized all the average spectra (Fig. 7), their adsorbences differ between these four categories, and this difference was particularly notable between Bitumen I and Bitumen II. The main differences between Bitumen I and Bitumen II are as follows: 10
ACCEPTED MANUSCRIPT a) In Bitumen I there is more substantial contribution of aliphatic C-Hx bands in the aliphatic stretching region (2800 – 3000 cm-1) and C-Hx bands in the aliphatic bending region (1350 – 1500 cm-1) (Fig. 7B). b) In Bitumen II, there is a larger contribution of oxygenated groups (1500 – 1700
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cm-1) compared to Bitumen I (Fig. 7B).
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c) Functional group distribution of Bitumen II is more similar to kerogen (Fig. 7C) than
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to that of Bitumen I. In comparison to kerogen, Bitumen II seems to have an almost identical contribution of oxygenated groups and only a slightly higher
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intensity of aliphatic C-Hx bands in the aliphatic stretching region.
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Semi-quantitative analysis of all spectra expressed by various ratios reveals further differences between bitumens and kerogen (Fig. 8). Concerning aromaticity (Fig. 8A),
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on average Bitumen I is less aromatic (AR1 0.09) compared to Bitumen II (AR1 0.33).
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Kerogen has the same aromaticity as Bitumen II. Bitumen II samples have broader range of aromaticity than Bitumen I, as indicated by higher standard deviation.
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The CH2/CH3 ratio is largest in Bitumen I (2.42) followed by Bitumen II (2.35) and
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kerogen (2.12) (Fig. 8B). The CH2/CH3 ratio indicates length of aliphatic chains (ACL), with higher ratios suggesting longer and less-branched aliphatic chains (Lin and Ritz, 1993; Lis et
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al., 2005). Therefore, these results indicate that Bitumen I has the longest and straightest aliphatic chains, followed by Bitumen II and kerogen. The oxidation index (Ox1, Fig. 8C), which expresses the contribution of oxygenated groups relative to the aliphatic stretching bands, is lowest (0.13) in Bitumen I. This is a result of high intensity of aliphatic bands, and the minimal contribution of oxygenated groups (fig. 8D). This ratio is significantly higher in Bitumen II (1.32) and the highest in kerogen (2.33).
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ACCEPTED MANUSCRIPT This is due to the higher contribution of oxygenated groups and low contribution of aliphatic stretching bands (Fig. 8E). Sulfur index (SI, Fig. 8F) is lowest in Bitumen I (0.49) and notably higher in Bitumen II (1.06) and kerogen (1.16). The above results demonstrate that shales of the Vaca Muerta Formation contain two types
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of bitumen (Bitumen I and Bitumen II) with different chemistry shown by aromaticity, the
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abundance of oxygenated groups, and sulfur content. Most previous research implies that the
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two bitumen populations originated from different organic matter (Nabbefeld et al., 2010b; Williford et al., 2011; Holman et al., 2012). However, there was no consistency in the
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previous research about the relationship between the chemistry of Bitumen I and Bitumen II.
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In some cases, early (easier extractable) bitumen was more aromatic (Nabbefeld et al., 2010b), but in other cases, more aliphatic (Holman et al., 2012). Our results are similar to those of
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Holman et al. (2012). These differences in chemistry between Bitumen I and Bitumen II
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documented in different studies suggest that bitumen fractions can have different origins.
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4.3.1 Origin of bitumen fractions
Comparing the amounts of extracted bitumens with maceral content (wt. %), samples
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containing a high quantity of FOM also contain a higher amount of Bitumen I, whereas
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samples containing a significant amount of NON-FOM contain a substantial amount of Bitumen II (Fig. 9 and 10 ). Based on these observations we suggest that Bitumen I is related to FOM, whereas Bitumen II has some correspondence to NON-FOM. We speculate that fluorescent oil-prone organic matter, more abundant in hydrogen than non-fluorescent amorphous organic material, may constitute a direct source of Bitumen I, consisting mostly of hydrogen-rich aliphatic hydrocarbons. In turn, a NON-FOM, more deficient in hydrogen, may have some genetic connection to Bitumen II which is a complex mixture of condensed
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ACCEPTED MANUSCRIPT aromatic and low number of aliphatic hydrocarbons. However, more studied are needed to determine if this relationship is causation or just association. Interestingly, the results of the reflected light microscopy show the presence of two types of solid bitumen, one lower reflecting but dominant with Ro ~ 0.46 %, and higher reflecting
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but rare with Ro ~ 0.6 % (Fig.11). Considering significant variabilities in hydrocarbons
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chemistry between Bitumen I and Bitumen II, the differences in their response to organic
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solvents, and positive correlations between Bitumen I and solid bitumen (Fig. 12), we suggest that there could be a genetic connection between extracted bitumens, and Bitumen I in
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particular, and solid bitumen. However, because proportions between two types of solid
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bitumen have not been determined, it is difficult to assess how strong is the correspondence
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between two extracted bitumens and two solid bitumens.
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4.3.3 The implications for hydrocarbon mobilization Based on the molecular weight of hydrocarbons and properties such as viscosity,
determining
mobile
hydrocarbons
were
established.
Among
some
of
them
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for
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as well as the porosity and permeability of the reservoir, several criteria and methods
are gas-filled porosity, effective oil saturation, oil saturation index (Jarvie, 2012; Kausik,
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2015), carbon saturation index, and reservoir producibility index (Kausik et al., 2014 and 2015). However, the boundary between the easy-to-produce and difficult-to-produce liquid hydrocarbons is not precisely established. In some cases, only very light hydrocarbons are produced, but in other cases, even heavy compounds may be mobilized and produced (Fan et al., 2005; Williams et al., 2014, Suarez-Rivera et al., 2016).
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ACCEPTED MANUSCRIPT Rock-Eval VI pyrolysis enables classifying producible hydrocarbons as free oil and gas released as the S1 peak. However, some hydrocarbons released at the maximum temperature Tmax, known as the S2 peak, may also be mobile. Indeed, in this study the comparison between yields of the two bitumens and RE data (S1 and S2) suggests that Bitumen I relates to S1 but
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also to S2 (Fig. 13 and 14). Similar to its comparison with RE data, Bitumen I correlates well
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with light liquid (C5–C29), hydrocarbons but also partially with heavier hydrocarbons (above
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C30) (Fig. 15 and 16). This suggests that Bitumen I corresponds to the majority of lighter hydrocarbons up to C29 and some portion to heavier hydrocarbons.
data
suggest
that
Bitumen
I
is
the
source
of
the
mobile
fraction
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TGA
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These relationships between the yields of Bitumen I and Bitumen II versus RE, GC, and
of hydrocarbons in rock, whereas Bitumen II is much less mobile. Therefore, understanding
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chemical differences between different proportions of Bitumen I and Bitumen II in relation
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to the origin in kerogen macerals leads to a better assessment of mobile hydrocarbon
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5. Conclusions
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generation and their accessibility to production.
This study investigated a continuous core of approximately 137 m length in the Jurassic
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to Early Cretaceous Vaca Muerta shale from the Neuquén Basin in Argentina. The overall objective of the paper was to relate the differences in chemistry of the extracted bitumen fractions to kerogen macerals, and discuss the implications of bitumen chemistry for hydrocarbons mobility. The main conclusions of this paper are: 1. Kerogen in the Vaca Muerta Formation is composed dominantly of oil-prone marinederived macerals (kerogen Type II). The terrestrial organic matter, including small
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ACCEPTED MANUSCRIPT vitrinite and inertinite particles, is scarce. 2. In the shales of the Vaca Muerta Formation, we document two types of bitumen: Bitumen I and Bitumen II using solvent extraction. Bitumen I is significantly more aliphatic and
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has fewer sulfur compounds than Bitumen II. Bitumen II is composed of highly
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condensed, aromatic hydrocarbons.
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3. The distinct chemical differences between bitumens indicate that Bitumen II is not just
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a residual remain of Bitumen I but that these are genetically different bitumens. There is a strong genetic connection between oil-prone fluorescent kerogen and Bitumen I.
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4. The comparison between yields of the two bitumens and RE data (S1 and S2) suggest that
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during Rock-Eval pyrolysis Bitumen I would be released dominantly at S1 but also partly at S2. This suggests that Bitumen I represents both free hydrocarbons of S1 and some
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mobile hydrocarbons of S2.
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5. Bitumen I correlates with light liquid (C5–C29), hydrocarbons but also partially with heavier hydrocarbons (above C30). This suggests that Bitumen I composes the majority
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of lighter hydrocarbons up to C29 and some portion (mobile) of heavier hydrocarbons.
Acknowledgments
The authors wish to thank Schlumberger Company who financially supported substantial steps of this work as a part of the doctoral work of the first author. Special thanks to Dr. Roberto Suarez-Rivera and Professor Sidney Green for mentoring support during the investigation, and to Dr. Arndt Schimmelmann for assistance with the extraction of bitumen fractions and kerogen isolation. 15
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ACCEPTED MANUSCRIPT Wei, L., Mastalerz, M., Schimmelmann, A., Chen, Y., 2014. Influence of Soxhlet-extractable Bitumen and oil on porosity in thermally maturing organic-rich shales. International Journal of Coal Geology 132, 38–50. Williams R., Willberg D., Handwerger D., Ekart D., Petriello J., Suarez-Rivera R., 2014.
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Fig. 1. Location of the Neuquén Basin and other perspective shale basins in Argentina
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(modified from Stratas Advisors, < https://www.epmag.com/argentina-holds-vast-shale-
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resources-833866>).
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Fig. 2. Section A, B and C where core samples were selected for this study based on difference in properties. (A) Calcareous mudstone; (B) Calcareous-argillaceous mudstone; (C) Argillaceous mudstone with horizontal mineralized veins and volcanic ash layers.
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Fig. 3. Methodology for the sequential extraction of Bitumen I, Bitumen II, and isolated
Table 1
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kerogen (modified from Robl and Davis, 1993, and Nabbefeld et al., 2010a, b).
FTIR-derived indices used in this study.
Wavenumber range Parameter
Functional groups -1
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Aromaticity (3000-3100)/(2800-3000) (AR1)
stretching C-H
Aliphatic chain
CH2/CH3 in the aliphatic stretching region 2800-3000
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Oxidation index
Oxygenated groups + aromatic carbon/
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S-S stretching / Aliphatic stretching C-H
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Sulfur index (SI)
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Fig. 4. Selected intervals of Vaca Muerta core; high variability in the lithology and rock components
is associated with rock heterogeneity (mineralogy, chemistry, pore structure, strength).
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Fig. 5. Total organic carbon (TOC, wt. %), random vitrinite reflectance (Ro, %) total porosity (TP, %), Hydrogen index (HI), Oxygen index (OI), and mineralogy (wt. %) of the Vaca
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Fig. 6. Maceral composition (vol. % on mineral matter free basis) for the most representative
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macerals.
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Bitumen II and kerogen.
Fig. 8. Semiquantitative FTIR indices for Bitumen I, Bitumen II, kerogen, and bulk original rock in the samples from Vaca Muerta Shale (see Table 2): (A) Aromaticity index AR1; (B) CH2/CH3 ratio (ACL); (C) Oxidation index Ox1; (D) Relative abundance of the bands in the 34
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Fig. 9. The relationship between Bitumen I & Bitumen II, and oil-prone fluorescent macerals (FOM). There is a strong correspondence of FOM to Bitumen I. Bitumen II shows a much weaker association with fluorescent organic matter.
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Fig. 10. The relationship between Bitumen I, Bitumen II, and nonfluorescent organic matter
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Fig. 11. Photomicrographs of organic matter. A and C – reflected light, B and D –
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Fig. 12. Correlation between solid bitumen content versus Bitumen I. Solid bitumen shows no
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hydrocarbons.
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Fig. 14 Bitumen I and Bitumen II contents based on extraction and gravimetric quantification and S2 peak from the Rock-Eval analysis. This graph shows the correspondence between
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more and less mobile hydrocarbons defined in each analysis; the amount of Bitumen I correlates well with hydrocarbons of the S2 peak.
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Fig. 15. Bitumen I and Bitumen II contents based on extraction and gravimetric
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ACCEPTED MANUSCRIPT Highlights
Vaca Muerta shale is highly heterogeneous and exhibits strong geochemical variations.
Extracted Bitumen I & Bitumen II fractions are chemically different.
Bitumen I and Bitumen II show relationship with fluorescent and non-fluorescent
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Bitumen I has better mobility than Bitumen II, facilitating oil production
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Aleksandra Małachowska, Maria Mastalerz, LaBraun Hampton, Jan Hupka, Agnieszka Drobniak PII: DOI: Reference:
S0166-5162(18)30216-7 https://doi.org/10.1016/j.coal.2018.11.013 COGEL 3122
To appear in:
International Journal of Coal Geology
Received date: Revised date: Accepted date:
15 March 2018 14 November 2018 15 November 2018
Please cite this article as: Aleksandra Małachowska, Maria Mastalerz, LaBraun Hampton, Jan Hupka, Agnieszka Drobniak , Origin of bitumen fractions in the Jurassic-early Cretaceous Vaca Muerta Formation in Argentina: insights from organic petrography and geochemical techniques. Cogel (2018), https://doi.org/10.1016/j.coal.2018.11.013
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ACCEPTED MANUSCRIPT Origin of bitumen fractions in the Jurassic-early Cretaceous Vaca Muerta Formation in Argentina: Insights from organic petrography and geochemical techniques Aleksandra Małachowskaa, Maria Mastalerzb, LaBraun Hamptonb, Jan Hupkaa, Agnieszka Drobniakb
b
Indiana Geological and Water Survey, Indiana
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University, Bloomington, Indiana 47405-2208, USA
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and Chemical Technology, Poland
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Gdansk University of Technology, Faculty of Chemistry, Department of Process Engineering
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a
Abstract
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This paper investigates chemical functional groups of the two extracted bitumen fractions
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in shales of the Jurassic to early Cretaceous Vaca Muerta Formation of the Neuquén Basin in Argentina, South America. The results indicate that Bitumen I is strongly aliphatic and
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appears to be genetically related to fluorescent amorphous organic matter. In contrast,
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Bitumen II consists of highly condensed, aromatic hydrocarbons, and has some correspondence to nonfluorescent amorphous organic material. Comparison of Rock-Eval VI
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pyrolysis data (S1 and S2) with the bitumen yields suggests that Bitumen I relates to S1 but also to S2. In addition, Bitumen I has a positive correlation with light liquid hydrocarbons (C5–C29), but also partially with heavier hydrocarbons (above C30). This suggests that Bitumen I corresponds to the majority of lighter hydrocarbons up to C29 and some portion to heavier hydrocarbons. These results have implications for the assessment of the mobility of generated hydrocarbons and their availability for production.
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Keywords: kerogen, macerals, bitumen, mature shale, Vaca Muerta Formation, FTIR 1. Introduction Studies of bitumen and its variability in chemical composition, generation pathways
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or physical state have been carried out since the early stages of petroleum exploration (Rogers
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et al., 1974; Hunt, 1979; Jones, 1980; Powell, 1984; Curiale, 1986; Jacob, 1989; George et al.,
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1993; Stasiuk, 1997; Hwank et al., 1998; Mastalerz and Glikson, 2000; Curiale and Curtis, 2016). The occurrence of multiple populations of bitumens in the same rock can be related to
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different sources, formation mechanisms, various thermal events and/or different temporal
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pulses of hydrocarbon migration (Evans et al., 1971; Rogers et al., 1974; Curiale, 1986; Gentzis and Goodarzi, 1990; George et al., 1994; Stasiuk, 1997; Fink et al., 2016, Wang et al.,
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2017; Chen et al., 2017; Mastalerz et al., 2018).
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Several studies have indicated the presence of more than one generation of bitumen that could be isolated from rocks through sequential extraction or after demineralization or both.
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Nabbefeld et al. (2010a) and Holman et al. (2012 and 2014) termed bitumen obtained after
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solvent extraction from the undigested rock as “Bitumen I” and that obtained by extraction after demineralization of rock as “Bitumen II." It was suggested that Bitumen II is less likely
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overprinted by migration processes because of its strong association with organic matter (OM) and its effective shielding by the kerogen-mineral matrix (Sherman et al., 2007). Nabbefeld et al. (2010a, b) in their study of marine Permian/Triassic sediments demonstrated that Bitumen II generally had lower aromaticity that Bitumen I and that this difference in aromaticity increased with increasing clay to total organic carbon (TOC) ratio.
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ACCEPTED MANUSCRIPT Sequential extractions on various non-demineralized rocks also demonstrated the presence of different fractions of bitumen. Price and Clayton (1992) observed a reduction in aromaticity and increase in the proportion of high-molecular-weight aromatics with successive extraction steps after sequential Soxhlet extraction of Devonian immature source rocks. Mueller and
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Philp (1998) suggested that high-molecular-weight petroleum was resistant to conventional
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Soxhlet extraction but could be extracted ultrasonically with p-xylene. Schwark et al. (1997)
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used successive extraction to obtain oils from reservoir sandstones and documented lower maturity for the final extracts compared to the earlier ones.
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In comparison, sequential extraction of Pennsylvanian coals from the Illinois Basin using
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methanol and dichloromethane (DCM) (Furmann et al., 2013) showed that DCM extraction resulted in a wider range of n-alkanes (C15–C33) than from methanol extraction (C15–C27).
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DCM extracts also yielded relatively higher concentrations of hopanes and selected aromatic
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hydrocarbons. That study also showed that extraction using solvents modified the pore structure of coals. In a paper by Wei et al. (2014), Upper Devonian to Early Mississippian
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New Albany Shale samples having high organic matter content were treated with DCM and
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later with toluene, yielding extracts of lower aromaticity after initial extraction with DCM and
changes
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more aromatic bitumen after extraction with toluene. Extraction of these bitumens resulted in
in the pore characteristics of these shale samples. The occurrence of different bitumen fractions in rocks poses the question of whether subsequent bitumens are of the same origin as those of the first generation, remaining trapped in the rock matrix and unable to be easily extracted, or if they originated from different source. Based on the distinct composition of Bitumen I and Bitumen II in Paleoproterozoic
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ACCEPTED MANUSCRIPT marine rocks in Australia, Williford et al. (2011) and Holman et al. (2012 and 2014) suggested that Bitumen II was closely related to the in-situ organic matter, whereas Bitumen I could be overprinted by migrated hydrocarbon phases. This paper discusses differences in chemical functional group characteristics of Bitumen I
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and Bitumen II in relation to the kerogen in mature shales from the Jurassic to early
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Cretaceous Vaca Muerta Formation in Argentina. The primary definitions of Bitumen I
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and Bitumen II of this study follow those used in Nabbefeld et al. (2010 a, b) and Holman et al. (2012, & 2014), with Bitumen I obtained by extraction of the pulverized rock using DCM,
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and Bitumen II obtained by DCM extraction of kerogen after rock demineralization using
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hydrofluoric acid.
The main objectives of the paper were to 1) study the differences in chemical functional
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groups of the two bitumen fractions in relation to corresponding kerogen macerals present in
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the Vaca Muerta Formation, and 2) investigate the kerogen transformation into Bitumen I and
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Bitumen II and its implications for hydrocarbons mobilization. 2. Geological setting of the Vaca Muerta Formation
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The Neuquén Basin is located in central Argentina and Chile, in the southern section
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of the Andean mountains, along with their eastern side, and between 32S and 40N latitude (Howell et al., 2005) (Fig. 1). The basin covers an area of approximately 120,000 km2 (Yrigoyen, 1991; Romero-Sarmiento et al., 2017) and contains one of the most complete records of the Jurassic-Cretaceous marine invertebrates in the world (Howell et al., 2005). Vaca Muerta is a mature Jurassic to Early Cretaceous heterogeneous and organic-rich shale formation, the main source rock for hydrocarbons in the Neuquén Basin, and most promising in terms of production potential in South America (Urien and Zambrano, 1994; Cruz et al., 4
ACCEPTED MANUSCRIPT 1996; Villar et al., 2006; Ukar et al., 2017). The formation exhibits extensive variability in lithology, organic, and mineral content (Fantín et al., 2014; Ptaszyńska, 2015; Williams et al., 2015; Suarez-Rivera et al., 2016; Małachowska et al., 2016 and 2017, Romero-Sarmiento et al., 2017; Ukar et al., 2017). Geochemical properties of this formation are not well
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3. Methodology
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al., 2009; Romero-Sarmiento et al., 2017; Ukar et al., 2017).
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documented because there has been only a limited number of studies on this topic (Monreal et
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3.1 Rock sampling and core analysis
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Twenty-three rock samples were collected from the 137m long core of the Vaca Muerta Formation. For this study, the core was divided into three sections (upper, middle, and lower)
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marked as A, B and C respectively (Fig. 2). These sections are closely related to three
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dominant lithofacies: Upper Vaca Muerta, Middle Vaca Muerta and Lower Vaca Muerta (also
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called La Cocina). Samples were collected every 2-3 m from the three sections of interest. The choice was guided by previously documented wireline-log based heterogeneous rock analysis
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and the distribution of petrophysical properties along a core spanning the entire section of Vaca Muerta Formation (Williams et al., 2015). Taking into account rock heterogeneity, each
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section was chosen to represent a thicker interval than just the three meters of the core. Megascopic heterogeneity of the samples was reflected dominantly by the color variabilities related to mineralogy and organic matter content of the rocks. 3.2 Mineralogy and geochemical analysis For mineralogy analysis, the weight of each sample was about 100 g. Larger particles were crushed
and
milled,
whereas
smaller
ones 5
milled
to
reduce
particles
size
ACCEPTED MANUSCRIPT and homogenize the sample. After milling, the samples were sieved through 850-micron sieve (20 mesh). X-ray diffraction analysis and semi-quantitative mineralogy were performed on a Bruker D8 analyzer. The detailed methodology is described by Bhargava et al. (2005). Total organic carbon (TOC) analyses were conducted on a LECO CS-444 carbon analyzer
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at about 1000° C. Approximately 0.15 g per sample were pre-treated with HCl and vacuum
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filtered on glass fiber paper. The residual sample and the paper were placed in a ceramic
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crucible, dried and combusted with pure oxygen. Rock-Eval VI pyrolysis measurements were performed on 0.1 grams of the same ground sample that was used for LECO TOC on a Rock-
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Eval VI instrument following the procedure as described by Grassmann et al. (2005).
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3.3 Gas chromatography and thermogravimetry analysis
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To measure the total hydrocarbon composition (C1–C30 and above), a quantitative
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extraction and gas chromatography method and thermogravimetric analysis were used. About 2 mg of the sample material was crushed to powder in a cryogenically chilled and sealed
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crushing cell. The crushed material was loaded into a solvent extraction cell.
The
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hydrocarbons extracted with a mixture of dichloromethane (DCM) and at elevated pressure and temperature (Williams et al., 2014). The resulting solvent/hydrocarbon mixture
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(hydrocarbons above C5) was characterized by gas chromatography (GC) using an Agilent 7000D and 7010B GC/MS Triple Quadrupole systems. More details on the extraction methodology can be found in a similar study of the Duvernay shale (Davis et al., 2013), and Vaca Muerta shales (Williams et al., 2014). In case of inability to detect heavy hydrocarbon (above C30) directly via GC due to detection limit issues, a thermogravimetry analysis (TGA) following procedures described by Williams et al (2014) was used.
6
ACCEPTED MANUSCRIPT 3.4 Porosity and permeability measurements A crushed sample pressure-decay system was used to measure permeabilities and porosities of the samples based on pressure pulse decay measurement, following the procedure
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described in Akkutlu and Fathi (2011).
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3.5 Isolation of Bitumen I, Bitumen II, and kerogen
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Kerogen isolation from the rock matrix followed procedures used in previous studies (Forsman and Hunt, 1958; Forsman, 1963; Saxby, 1970; Durand and Nicaise, 1980; Robl
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and Davis, 1993; Ibrahimov and Bissada, 2010; Holman et al., 2012 and 2014). Bitumen was
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extracted, and kerogen was isolated following the scheme of Figure 3. Pulverized rock material was placed in test tubes, and about 10 ml of DCM was added. Samples were mixed,
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ultrasonicated for about 10 minutes and centrifuged. The resultant brown to dark brown color
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of the supernatant fluid indicated whether the sample needed to be further extracted to ensure that all the Bitumen I was extracted. The Bitumen I yield was determined gravimetrically.
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DCM was evaporated from the remaining rock material by blowing dry N2 gas into the
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headspace.
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After Bitumen I was extracted, sediments were diluted with hydrochloric acid (HCl, ca. 10 %), and warmed to ca. 60 °C for 30 minutes to ensure complete reaction of carbonates. The mixture was cooled, centrifuged, and the supernatant was decanted. Deionized water was used to transfer the decalcified sediment into a polyethylene centrifuge tube. The decarbonized rock sediment was then demineralized using pre-cooled hydrofluoric acid (HF, ca. 24 %). The samples stored in plastic tubes were treated twice with HF (centrifuged each time). Finally, they were rinsed twice with deionized water. After HF demineralization, 7
ACCEPTED MANUSCRIPT samples were frozen and then freeze-dried. The residual dry powders were weighed and placed in a glass centrifuge tube. The extraction process for Bitumen II was repeated in the same way as for Bitumen I. In summary, considering the extraction procedure in this study, Bitumen I was defined as
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easily extractable OM from the rock, and Bitumen II as an organic matter fraction that is
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entrapped within the kerogen/mineral matrix, and that can only be extracted from kerogen after demineralization. Sequential steps of the methodology for extraction of Bitumen I,
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Bitumen II, and isolated kerogen are shown in Fig. 3.
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3.5 FTIR technique
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For each sample, four fractions: Bitumen I, Bitumen II, kerogen, and original rock were analyzed using the FTIR technique. All fractions were prepared as KBr pellets, and a Nicolet
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6700 spectrometer equipped with a DTGS detector was used. Each spectrum was generated
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by collecting 300 scans per sample at a resolution of 4 cm-1. Assignments of peaks for
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functional groups were based on Painter et al. (1981, 1985) and Wang and Griffiths (1985). The following four FTIR indices have been used in this study to evaluate the chemistry of
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organic matter (Kister et al., 1990; Mastalerz and Bustin, 1996; Lis et al., 2005, Chen et al., 2012, Liu et al., 2017): (1) aromaticity (AR1); (2) aliphatic chain length (ACL); (3) oxidation index (Ox1); and (4) sulfur index (SI). The calculations of these indices utilized integrated areas of the selected functional groups (Table 1). 3.6 Organic petrography
8
ACCEPTED MANUSCRIPT Shale samples were prepared as polished blocks using the standard organic petrography sample preparation techniques (ICCP, 1963). A Leica DM 2500P microscope linked to a TIDAS PMT IV photometric system was used to measure vitrinite reflectance and carry out organic matter characterization of the bulk samples. Maceral composition was assessed based
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on reflected white light and fluorescent light microscopy. The volume percent of each maceral
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on the mineral-matter-free basis was converted to mineral-matter-containing basis, using
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the measured total organic carbon content (TOC, wt. %). All macerals were then recalculated
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from volume percent to the weight percent composition. 4. Results and Discussion
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4.1 Core and geochemical rock analysis
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The 137 meters of continuous core-scale observations show that rock composition progressively change from a silicate/clay-rich and organic-rich at its base, to a calcareous, low
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organic content rocks at the top of the core. Selected intervals of Vaca Muerta core indicating
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high variability in the lithology and rock components are presented in Figure 4. The mineral composition (by wt. %), total organic carbon (TOC, wt. %) content, vitrinite reflectance (Ro
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%), total porosity (TP, %), Hydrogen index (HI), and Oxygen index (OI) are presented in
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Figure 5. These analyses indicated high average TOC content, from 1-3 wt. % at the top of the core, 7-13 wt. % in the middle section of the core, and 9-12 % in the bottom part. Total porosity is around 3% in the top section of the core, ranges 3 to 13 % in the middle section, and 2-10 % for the lower part of the core (Fig.5).
4.2 Insights from Organic Petrography
9
ACCEPTED MANUSCRIPT Kerogen in the Vaca Muerta core consists of marine-derived macerals (kerogen Type II) (Fig. 6). The terrestrial organic matter, including small vitrinite and inertinite particles, is scarce. The measured Ro ranges from 0.5 to 0.7 %, with most samples having average vitrinite reflectance of 0.7 %, which corresponds well to the maturity suggested by Tmax
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values from Rock-Eval pyrolysis using the equation VRo = (0.0180*Tmax)-7.16 (Jarvie,
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2001). The significant contribution to kerogen comes from amorphous organic matter (AOM)
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that is either fluorescent (FOM) or non-fluorescent (NON-FOM). On mineral matter free basis, FOM ranges from 10-40 % of the total organic material by volume, and NON-FOM
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constitutes 5-30 vol. %.
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Alginite accounts for 5-10 vol. %, liptodetrinite likely of algal origin (classified as “other liptinite” in Fig. 6) ranges from 5-25 vol. %, whereas vitrinite, and inertinite account for only
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about 0-3 vol. %. In addition to primary kerogen macerals, solid bitumen (secondary product)
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is prevalent in most samples, forming the dominant organic matter component in most
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samples.
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4.3 Functional group characteristics In this study, 23 FTIR spectra of the original rock, 23 spectra of Bitumen I, 19 spectra of
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Bitumen II, and 21 spectra of kerogen were generated and, subsequently, the average spectrum of each category (Fig. 7) was used for comparative purpose (Fig. 8). Although similar functional group bands characterized all the average spectra (Fig. 7), their adsorbences differ between these four categories, and this difference was particularly notable between Bitumen I and Bitumen II. The main differences between Bitumen I and Bitumen II are as follows: 10
ACCEPTED MANUSCRIPT a) In Bitumen I there is more substantial contribution of aliphatic C-Hx bands in the aliphatic stretching region (2800 – 3000 cm-1) and C-Hx bands in the aliphatic bending region (1350 – 1500 cm-1) (Fig. 7B). b) In Bitumen II, there is a larger contribution of oxygenated groups (1500 – 1700
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cm-1) compared to Bitumen I (Fig. 7B).
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c) Functional group distribution of Bitumen II is more similar to kerogen (Fig. 7C) than
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to that of Bitumen I. In comparison to kerogen, Bitumen II seems to have an almost identical contribution of oxygenated groups and only a slightly higher
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intensity of aliphatic C-Hx bands in the aliphatic stretching region.
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Semi-quantitative analysis of all spectra expressed by various ratios reveals further differences between bitumens and kerogen (Fig. 8). Concerning aromaticity (Fig. 8A),
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on average Bitumen I is less aromatic (AR1 0.09) compared to Bitumen II (AR1 0.33).
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Kerogen has the same aromaticity as Bitumen II. Bitumen II samples have broader range of aromaticity than Bitumen I, as indicated by higher standard deviation.
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The CH2/CH3 ratio is largest in Bitumen I (2.42) followed by Bitumen II (2.35) and
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kerogen (2.12) (Fig. 8B). The CH2/CH3 ratio indicates length of aliphatic chains (ACL), with higher ratios suggesting longer and less-branched aliphatic chains (Lin and Ritz, 1993; Lis et
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al., 2005). Therefore, these results indicate that Bitumen I has the longest and straightest aliphatic chains, followed by Bitumen II and kerogen. The oxidation index (Ox1, Fig. 8C), which expresses the contribution of oxygenated groups relative to the aliphatic stretching bands, is lowest (0.13) in Bitumen I. This is a result of high intensity of aliphatic bands, and the minimal contribution of oxygenated groups (fig. 8D). This ratio is significantly higher in Bitumen II (1.32) and the highest in kerogen (2.33).
11
ACCEPTED MANUSCRIPT This is due to the higher contribution of oxygenated groups and low contribution of aliphatic stretching bands (Fig. 8E). Sulfur index (SI, Fig. 8F) is lowest in Bitumen I (0.49) and notably higher in Bitumen II (1.06) and kerogen (1.16). The above results demonstrate that shales of the Vaca Muerta Formation contain two types
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of bitumen (Bitumen I and Bitumen II) with different chemistry shown by aromaticity, the
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abundance of oxygenated groups, and sulfur content. Most previous research implies that the
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two bitumen populations originated from different organic matter (Nabbefeld et al., 2010b; Williford et al., 2011; Holman et al., 2012). However, there was no consistency in the
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previous research about the relationship between the chemistry of Bitumen I and Bitumen II.
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In some cases, early (easier extractable) bitumen was more aromatic (Nabbefeld et al., 2010b), but in other cases, more aliphatic (Holman et al., 2012). Our results are similar to those of
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Holman et al. (2012). These differences in chemistry between Bitumen I and Bitumen II
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documented in different studies suggest that bitumen fractions can have different origins.
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4.3.1 Origin of bitumen fractions
Comparing the amounts of extracted bitumens with maceral content (wt. %), samples
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containing a high quantity of FOM also contain a higher amount of Bitumen I, whereas
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samples containing a significant amount of NON-FOM contain a substantial amount of Bitumen II (Fig. 9 and 10 ). Based on these observations we suggest that Bitumen I is related to FOM, whereas Bitumen II has some correspondence to NON-FOM. We speculate that fluorescent oil-prone organic matter, more abundant in hydrogen than non-fluorescent amorphous organic material, may constitute a direct source of Bitumen I, consisting mostly of hydrogen-rich aliphatic hydrocarbons. In turn, a NON-FOM, more deficient in hydrogen, may have some genetic connection to Bitumen II which is a complex mixture of condensed
12
ACCEPTED MANUSCRIPT aromatic and low number of aliphatic hydrocarbons. However, more studied are needed to determine if this relationship is causation or just association. Interestingly, the results of the reflected light microscopy show the presence of two types of solid bitumen, one lower reflecting but dominant with Ro ~ 0.46 %, and higher reflecting
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but rare with Ro ~ 0.6 % (Fig.11). Considering significant variabilities in hydrocarbons
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chemistry between Bitumen I and Bitumen II, the differences in their response to organic
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solvents, and positive correlations between Bitumen I and solid bitumen (Fig. 12), we suggest that there could be a genetic connection between extracted bitumens, and Bitumen I in
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particular, and solid bitumen. However, because proportions between two types of solid
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bitumen have not been determined, it is difficult to assess how strong is the correspondence
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between two extracted bitumens and two solid bitumens.
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4.3.3 The implications for hydrocarbon mobilization Based on the molecular weight of hydrocarbons and properties such as viscosity,
determining
mobile
hydrocarbons
were
established.
Among
some
of
them
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for
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as well as the porosity and permeability of the reservoir, several criteria and methods
are gas-filled porosity, effective oil saturation, oil saturation index (Jarvie, 2012; Kausik,
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2015), carbon saturation index, and reservoir producibility index (Kausik et al., 2014 and 2015). However, the boundary between the easy-to-produce and difficult-to-produce liquid hydrocarbons is not precisely established. In some cases, only very light hydrocarbons are produced, but in other cases, even heavy compounds may be mobilized and produced (Fan et al., 2005; Williams et al., 2014, Suarez-Rivera et al., 2016).
13
ACCEPTED MANUSCRIPT Rock-Eval VI pyrolysis enables classifying producible hydrocarbons as free oil and gas released as the S1 peak. However, some hydrocarbons released at the maximum temperature Tmax, known as the S2 peak, may also be mobile. Indeed, in this study the comparison between yields of the two bitumens and RE data (S1 and S2) suggests that Bitumen I relates to S1 but
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also to S2 (Fig. 13 and 14). Similar to its comparison with RE data, Bitumen I correlates well
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with light liquid (C5–C29), hydrocarbons but also partially with heavier hydrocarbons (above
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C30) (Fig. 15 and 16). This suggests that Bitumen I corresponds to the majority of lighter hydrocarbons up to C29 and some portion to heavier hydrocarbons.
data
suggest
that
Bitumen
I
is
the
source
of
the
mobile
fraction
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TGA
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These relationships between the yields of Bitumen I and Bitumen II versus RE, GC, and
of hydrocarbons in rock, whereas Bitumen II is much less mobile. Therefore, understanding
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chemical differences between different proportions of Bitumen I and Bitumen II in relation
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to the origin in kerogen macerals leads to a better assessment of mobile hydrocarbon
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5. Conclusions
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generation and their accessibility to production.
This study investigated a continuous core of approximately 137 m length in the Jurassic
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to Early Cretaceous Vaca Muerta shale from the Neuquén Basin in Argentina. The overall objective of the paper was to relate the differences in chemistry of the extracted bitumen fractions to kerogen macerals, and discuss the implications of bitumen chemistry for hydrocarbons mobility. The main conclusions of this paper are: 1. Kerogen in the Vaca Muerta Formation is composed dominantly of oil-prone marinederived macerals (kerogen Type II). The terrestrial organic matter, including small
14
ACCEPTED MANUSCRIPT vitrinite and inertinite particles, is scarce. 2. In the shales of the Vaca Muerta Formation, we document two types of bitumen: Bitumen I and Bitumen II using solvent extraction. Bitumen I is significantly more aliphatic and
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has fewer sulfur compounds than Bitumen II. Bitumen II is composed of highly
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condensed, aromatic hydrocarbons.
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3. The distinct chemical differences between bitumens indicate that Bitumen II is not just
US
a residual remain of Bitumen I but that these are genetically different bitumens. There is a strong genetic connection between oil-prone fluorescent kerogen and Bitumen I.
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4. The comparison between yields of the two bitumens and RE data (S1 and S2) suggest that
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during Rock-Eval pyrolysis Bitumen I would be released dominantly at S1 but also partly at S2. This suggests that Bitumen I represents both free hydrocarbons of S1 and some
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mobile hydrocarbons of S2.
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5. Bitumen I correlates with light liquid (C5–C29), hydrocarbons but also partially with heavier hydrocarbons (above C30). This suggests that Bitumen I composes the majority
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of lighter hydrocarbons up to C29 and some portion (mobile) of heavier hydrocarbons.
Acknowledgments
The authors wish to thank Schlumberger Company who financially supported substantial steps of this work as a part of the doctoral work of the first author. Special thanks to Dr. Roberto Suarez-Rivera and Professor Sidney Green for mentoring support during the investigation, and to Dr. Arndt Schimmelmann for assistance with the extraction of bitumen fractions and kerogen isolation. 15
ACCEPTED MANUSCRIPT
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Fig. 1. Location of the Neuquén Basin and other perspective shale basins in Argentina
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(modified from Stratas Advisors, < https://www.epmag.com/argentina-holds-vast-shale-
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resources-833866>).
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Fig. 2. Section A, B and C where core samples were selected for this study based on difference in properties. (A) Calcareous mudstone; (B) Calcareous-argillaceous mudstone; (C) Argillaceous mudstone with horizontal mineralized veins and volcanic ash layers.
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Fig. 3. Methodology for the sequential extraction of Bitumen I, Bitumen II, and isolated
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kerogen (modified from Robl and Davis, 1993, and Nabbefeld et al., 2010a, b).
FTIR-derived indices used in this study.
Wavenumber range Parameter
Functional groups -1
(cm )
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Aromaticity (3000-3100)/(2800-3000) (AR1)
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Oxidation index
Oxygenated groups + aromatic carbon/
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S-S stretching / Aliphatic stretching C-H
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Sulfur index (SI)
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Fig. 4. Selected intervals of Vaca Muerta core; high variability in the lithology and rock components
is associated with rock heterogeneity (mineralogy, chemistry, pore structure, strength).
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Fig. 5. Total organic carbon (TOC, wt. %), random vitrinite reflectance (Ro, %) total porosity (TP, %), Hydrogen index (HI), Oxygen index (OI), and mineralogy (wt. %) of the Vaca
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Fig. 6. Maceral composition (vol. % on mineral matter free basis) for the most representative
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Fig. 8. Semiquantitative FTIR indices for Bitumen I, Bitumen II, kerogen, and bulk original rock in the samples from Vaca Muerta Shale (see Table 2): (A) Aromaticity index AR1; (B) CH2/CH3 ratio (ACL); (C) Oxidation index Ox1; (D) Relative abundance of the bands in the 34
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Fig. 9. The relationship between Bitumen I & Bitumen II, and oil-prone fluorescent macerals (FOM). There is a strong correspondence of FOM to Bitumen I. Bitumen II shows a much weaker association with fluorescent organic matter.
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Fig. 10. The relationship between Bitumen I, Bitumen II, and nonfluorescent organic matter
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Fig. 11. Photomicrographs of organic matter. A and C – reflected light, B and D –
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Fig. 12. Correlation between solid bitumen content versus Bitumen I. Solid bitumen shows no
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hydrocarbons.
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Fig. 14 Bitumen I and Bitumen II contents based on extraction and gravimetric quantification and S2 peak from the Rock-Eval analysis. This graph shows the correspondence between
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more and less mobile hydrocarbons defined in each analysis; the amount of Bitumen I correlates well with hydrocarbons of the S2 peak.
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Fig. 15. Bitumen I and Bitumen II contents based on extraction and gravimetric
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ACCEPTED MANUSCRIPT Fig. 16. Bitumen I and Bitumen II contents based on extraction and gravimetric quantification and heavier hydrocarbons (above C30) identified through gravimetric analysis. Bitumen I
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Vaca Muerta shale is highly heterogeneous and exhibits strong geochemical variations.
Extracted Bitumen I & Bitumen II fractions are chemically different.
Bitumen I and Bitumen II show relationship with fluorescent and non-fluorescent
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