Influence of maceral composition on geochemical characteristics of immature shale kerogen: Insight from density fraction analysis

International Journal of Coal Geology 103 (2012) 60–69

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Influence of maceral composition on geochemical characteristics of immature shale kerogen: Insight from density fraction analysis M. Mastalerz a,⁎, A. Schimmelmann b, G.P. Lis c, A. Drobniak a, A. Stankiewicz d a

Indiana University, Indiana Geological Survey, 611 North Walnut Grove Ave, Bloomington, IN 47401‐2205, USA Indiana University, Department of Geological Sciences, 1001 E 10th St., Bloomington, IN 47405‐1405, USA School of Geographical Sciences, University of Bristol, University Road, Bristol, BS81SS, UK d Schlumberger Testing Services, 1, rue Henri Becquerel, 92140 Clamart Cedex, France b c

a r t i c l e

i n f o

Article history: Received 1 December 2011 Received in revised form 23 July 2012 Accepted 27 July 2012 Available online 3 August 2012 Keywords: Shale Macerals Density fractions Alginite Amorphinite

a b s t r a c t Variations in the relative proportions of individual macerals in shales can significantly influence the geochemical characteristics of bulk organic matter. Density fractions of kerogen from the thermally immature New Albany Shale (Devonian and Mississippian) with contrasting maceral compositions exhibit strong geochemical differences. The parental shale is characterized by a vitrinite reflectance (Ro) of 0.45%, a total organic carbon content of 13 wt.%, and a sulfur content of 6.2 wt.%. Organic matter is dominated by amorphinite and alginite, with vitrinite and inertinite accounting only for 1% by volume. Alginite-dominated density fractions (density ca. 1.0–1.15 g/cm3) contain significantly more aliphatic hydrogen, a stronger carboxyl/carbonyl contribution, and reduced Fourier transform infrared spectroscopy absorbance in the 1000 to 1100 cm−1 region assigned to ether bonds (C\O\C), as compared to the amorphinite-dominated density fraction (density ca. 1.2–1.6 g/cm 3). Aromaticity generally increases from alginite-dominated to amorphinite-dominated fractions. Density fractions dominated by amorphinite are more deuterium-depleted (δDn values of nonexchangeable hydrogen up to −105‰) than alginite-rich density fraction (δDn values reach −90‰). In contrast, changes in relative proportions of alginite and amorphinite in the New Albany Shale do not significantly affect the amount of isotopically exchangeable hydrogen in total hydrogen (i.e., hydrogen exchangeability). Alginite is relatively 13 C-enriched, whereas density fractions having a high content of amorphinite are relatively 13C-depleted. Our results suggest that even small bulk geochemical and isotopic differences can gain relevance after deconvolution from maceral-related variability. The masking influence of maceral abundance patterns must be considered when interpreting bulk geochemical data as paleoenvironmental proxies. The findings of this study on shale and Type II kerogen are relevant for all types of kerogens in sediments and rocks. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Growing interest in shale sequences and shale gas requires a better understanding of the influences of shale composition, porosity, and permeability on shale gas producibility. Unlike coals, shales consist predominantly of fine-grained minerals, with organic matter (OM) ranging typically from below 1 wt.% to more than 20 wt.%. Although OM is a minor component in shales, it is not only responsible for in situ gas generation, but also provides storage sites for shale gas in the OM micropore structure (Chalmers and Bustin, 2006; Ross and Bustin, 2009; Strąpoć et al., 2010). Amorphous OM (also called amorphinite or bituminite macerals; Teichműller and Ottenjann, 1977; Stach et al., 1982) is the dominant OM type in marine shales. Alginite may be present in various amounts in immature and mature shales but is usually less prominent than amorphinite. Contributions of terrestrially derived macerals (vitrinite ⁎ Corresponding author. Tel.: +1 18128559416; fax: +1 18128552862. E-mail address: [email protected] (M. Mastalerz). 0166-5162/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coal.2012.07.011

and inertinite) may be locally common, but are usually much smaller than those of amorphinite or alginite. Amorphinite, however, is not a homogenous material and varies significantly in properties, depending on origin, degree of degradation, and degradation pathways (e.g., Stankiewicz et al., 1996; Stasiuk, 1993; Stasiuk and Goodarzi, 1988). From the perspective of shale gas potential, amorphinite seems to have the best methane-holding capacity of all macerals in shales (Mastalerz et al., 2010; Ross and Bustin, 2009). Variations in the relative proportions of individual macerals in shales significantly influence the geochemical characteristics of bulk OM. Improved understanding of this influence can strengthen our interpretation of the geochemical and stable isotopic signals not only for exploration, but also for constraining the climate of the paleoenvironment (e.g., Hasegawa et al., 2003; van Bergen and Poole, 2002). Even small bulk geochemical and isotopic differences can gain relevance after deconvolution from maceral-related variability. Rimmer et al. (2006) addressed this issue for terrestrial OM by isotopically analyzing density fractions of high volatile A bituminous coal with a vitrinite reflectance Ro of 0.88%. This coal's bulk stable isotope ratios of δ13C=−23.5‰ and δ15N=+2.1‰ mask a

M. Mastalerz et al. / International Journal of Coal Geology 103 (2012) 60–69

surprisingly large isotopic variance among density fractions where δ13C ranges from −22.91 to −25.78‰, and δ15N ranges from 0.72 to 4.00‰. The most negative δ13C values belong to the lightest (liptinitic) density fractions, whereas increasing 13C-enrichment is observed toward denser, vitrinitic, and ultimately inertinitic materials. Similar carbon isotopic shifts from liptinite to inertinite were observed by Schwartzkopf (1984) and Whiticar (1996), who additionally documented hydrogen isotopic differentiation between liptinite (δ 2H or δD values from − 120 to −105‰), vitrinite (−107 to −90‰), and inertinite (−97 to −78‰) in bituminous coals from Germany. Nitrogen isotopes ratios (δ 15N) become lighter with increasing density of coal macerals (Rimmer et al., 2006). In marine sediments, variations in geochemical properties of OM have been studied in isolated density fractions (e.g., Dickens et al., 2006; Goodnight, 2004; Kruge et al., 1989; Robl et al., 1987; Senftle et al., 1987; Stankiewicz et al., 1996; Suzuki, 1984; Taulbee et al., 1990) and with in-situ techniques (e.g., Carmo et al., 1997; Mastalerz et al., 1998; Stasiuk et al., 1993). Most of these studies concentrated on chemical and molecular differences among macerals. In comparison to chemical variation, significantly less, though, is known about the isotopic differentiation of OM types in marine shales (Goodnight, 2004; Goodnight et al., 2002; Lis et al., 2002; Rimmer et al., 2003). The chemical composition and stable isotope ratios of particulate sedimentary organic matter (SOM) carry signals from their original biochemical precursor materials that had been biosynthesized in specific paleoenvironments. Bulk SOM or operationally defined fractions of SOM like kerogen (i.e. the insoluble fraction of SOM) are typically used to reconstruct aspects of the paleoenvironment (e.g., Krishnamurthy et al., 1995; Lovan and Krishnamurthy, 2011; Schimmelmann et al., 2001a,b, 2004), although the pooled character of SOM isolates averages signals and provides less diagnostic power than individual types (macerals) of SOM that can be traced to individual species or even organisms. Particulate SOM in coal and shale is the geochemical source of most fluid fossil fuels via microbial or thermal transformation to oil and gas. The paleoenvironmentally controlled chemical and stable isotopic compositions of SOM directly relate to those of oil and gas, and in this regard are highly relevant for source rock evaluation and effective exploration (e.g., Schimmelmann et al., 2004). This study is a continuation of our geochemical characterization of kerogen Type II sequences. Changes in kerogen with increasing thermal maturity were documented in Devonian and early Mississippian shales of the New Albany and Exshaw formations (Lis et al., 2005, 2006, 2008; Werner-Zwanziger et al., 2005). In the present study, we focus on the immature New Albany Shale to document geochemical differences that result from differences in maceral composition. 2. Materials and methods

61

detailed sedimentologic and sequence stratigraphic studies of this formation identified a range of facies from relatively shallow to deep-water environments (Schieber, 2004). The New Albany Shale is a significant source of gas within the Illinois Basin (Partin, 2004). The rock material selected for this study represents an immature shale from a core drilled in Boone County, Indiana, that is archived at the Indiana Geological Survey in Bloomington. The shale is characterized by a Ro of 0.45%, a TOC value of 13 wt.%, and a sulfur content of 6.2 wt.%. OM is dominated by amorphinite and alginite (Table 1). Vitrinite and inertinite are rare and do not exceed 1% by volume on a mineral matter-free basis.

2.2. Sample preparation This study uses kerogen concentrates that were prepared from shales using wet-chemical acid demineralization according to Robl and Davis (1993) and Schimmelmann et al. (1999). Kerogen is operationally defined as the insoluble fraction of OM. Two sets of density fractions from New Albany Shale kerogen were analyzed in this study. Both sets were prepared using density gradient centrifugation at the Department of Geological Sciences of Southern Illinois University in Carbondale, Illinois. Brij-35, a nonionic polyoxyethylene surfactant, was used to disaggregate small grains for effective separation by density. In our control experiments on bulk kerogen, treatment with Brij-35 and subsequent extensive water washing had no effect on FTIR characteristics discussed in this study. The first sample set represents six density fractions (IN F8–IN S28; Table 1) selected from more than fifty fractions ranging in density from 1.01 to 1.60 g/cm3. These six so-called selected range density fractions represent snapshots from the entire density spectrum and were characterized petrographically, isotopically, and with Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). The second set of three density fractions (NAS9101a–NAS9101c; Table 1) comes from the same location and corresponds in maturity and composition to the first set, however, the second set has the advantage that all kerogen density fractions are included with known weights of the three individual density fractions for mass balance calculations (Fig. 1). The bulk kerogen was initially divided into five density fractions (b 1.1089; 1.1089–1.177; 1.177–1.2306; 1.2306–1.3006; and >1.3006 g/cm 3), which were screened petrographically. Based on this petrographic screening and the resulting maceral proportions in these five fractions, the entire material from all these fractions was recombined and subsequently was divided into three so-called whole range density fractions (b 1.12; 1.12–1.25; and 1.25–1.60 g/cm 3) that jointly covered the entire density range of kerogen while maintaining distinct petrographic compositions. These three fractions were analyzed petrographically and with FTIR.

2.1. Materials Shale samples were obtained from the New Albany Shale in Indiana. The New Albany Shale reaches a thickness of up to 140 m and is present throughout the Illinois Basin, covering southern and central Illinois, southwestern Indiana, and western Kentucky. This organic-rich shale formed from the Middle Devonian to the Early Mississippian, with main deposition in the Late Devonian (Cluff et al., 1981). OM within the shale is predominantly Type II kerogen (Chou et al., 1991; Lis et al., 2006; Strąpoć et al., 2010) that spans a relatively wide range of maturity equivalent to Ro from 0.29% to 1.5% (Hasenmueller and Comer, 1994). The total organic carbon (TOC) content ranges from 0.1 to 20 wt.% (Frost and Shaffer, 1994). The interpretation of the sedimentary record originally suggested that the New Albany Shale was deposited in a marine, stratified, oxygen-depleted depositional environment (Cluff et al., 1981). Across the Frasnian–Famennian boundary, de la Rue et al. (2007) suggested an abrupt change in water column redox conditions from oxic/dysoxic to anoxic, and possibly euxinic. Additional

Table 1 Petrographic composition (in volume %) of six selected range density fractions, three whole range density fractions, and parental bulk kerogen. OM = organic matter. Sample name

Density range (g/cm3)

Alginite (%)

Amorphous OM (%)

Vitrinite + inertinite (%)

Minerals (%)

Selected range density fractions IN F8 1.0353–1.0446 70.0 IN F15 1.0757–1.1031 80.0 IN S7 1.1991–1.2299 4.0 IN S19 1.4221–1.4369 2.0 IN S22 1.4659–1.4800 2.0 IN S28 1.5429–1.5541 8.0

23.5 13.5 87.0 86.0 88.0 84.0

0.5 0.5 1.0 2.0 2.0 2.0

6.0 6.0 8.0 10.0 8.0 6.0

Whole range density fractions NAS9101-a 1.00–1.12 NAS9101-b 1.12–1.25 NAS9101-c 1.25–1.60 Bulk kerogen 1.00–1.60

5.0 18.0 94.0 82.0

0.0 0.0 0.5 1.0

0.5 2.0 2.5 5.0

94.5 80.0 3.0 12.0

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M. Mastalerz et al. / International Journal of Coal Geology 103 (2012) 60–69

alginite

alginite+amorphinite

Weight (mg)

amorphinite

amorphinite+ vitrinite+ inertinite

1.0

Fig. 1. Density distribution of New Albany Shale kerogen. Approximate density ranges for the macerals alginite and amorphinite are indicated.

2.3. Petrographic analysis Polished shale fragments and kerogen fractions embedded in Lucite® were characterized petrographically using a Leitz MPV-II reflected light microscope, in both reflected and fluorescent modes. Each maceral analysis covered 500 points. Sample preparation and analysis followed standard procedures used in organic petrography (Taylor et al., 1998). 2.4. Nuclear magnetic resonance (NMR) spectroscopy 13 C NMR spectroscopy is a nondestructive technique to qualitatively and semi-quantitatively assess the abundance of aliphatic, aromatic, and other distinctly chemically bonded types of organic carbon in kerogen (Hu et al., 2001; Mann et al., 1991; Mao et al., 2010). 13C NMR experiments were performed on a Bruker AVANCE DSX spectrometer using a 9.4 T magnet (13C Larmor frequency of 100 MHz) under Magic Angle Spinning (MAS), using 4-mm-diameter rotors in the Chemistry Department at Indiana University. The spinning rate of 13.5 kHz was adjusted to move the first-order spinning side bands from the aromatic resonances beyond the aliphatic resonance region. All samples were measured using cross polarization (CP) and the two pulse phase modulation (TPPM) proton decoupling scheme with contact times of 2.0 ms and relaxation delays of 0.5 s. The NMR aromaticity factor fa represents the ratio of the integrated signal of the aromatic region to the total integrated signal (Maciel and Dennis, 1982; Mann et al., 1991).

2.5. Fourier transmission infrared spectroscopy Samples for FTIR spectroscopy were prepared as potassium bromide (KBr) pellets (Painter et al., 1981) and were analyzed on a Nicolet 20SXC spectrometer equipped with a DTGS detector, collecting 300 scans per sample at a resolution of 4 cm−1. Spectra were normalized to 1 mg of material. Absorption bands were identified by comparing spectra with published studies (e.g., Painter et al., 1981; Sobkowiak and Painter, 1992; Wang and Griffith, 1985). The CH2/CH3 ratio was determined using Fourier deconvolution applied to the aliphatic stretching region 2800–3000 cm−1 (Lin and Ritz, 1993; Painter et al., 1981). 2.6. Hydrogen isotopic exchangeability Hex, δDn, and δ 13C values Many studies report that some organic hydrogen (e.g., linked to heteroatoms N, O, S; some aromatic H; some H in α-position to carbonyl groups) is isotopically exchangeable with water and some other organic hydrogen (Hoering, 1984; Mastalerz and Schimmelmann, 2002; Qian et

al., 1997; Schimmelmann et al., 1993, 1999, 2001a,b). The exchangeable organic hydrogen (Hex, expressed in % of total organic hydrogen) is able to isotopically equilibrate with water hydrogen and retains no isotopic memory of original biomass. In contrast to Hex, most hydrogen linked to aliphatic carbon is isotopically nonexchangeable (Schimmelmann et al., 1999, and references therein) and its δD value potentially reflects the isotopic characteristics of biogenic source materials, as well as paleo- and depositional environments. The maturation process, however, alters the bulk isotopic composition of nonexchangeable hydrogen in kerogen, and only δD values of kerogens having low thermal maturity are considered reliable for paleoenvironmental reconstruction (Kamaleldin and Spalding, 2001; Krishnamurthy et al., 1995; Schimmelmann et al., 2006). In this study, two aliquots of each kerogen were isotopically equilibrated in water vapors with hydrogen isotopic δDwater values of either + 1196‰ or − 137‰ (vs. VSMOW) for 10 h at 115 °C. The resulting isotopic difference ΔδDbulk kerogen between two equilibrated aliquots of kerogen is used in a mass-balance calculation to arrive at the abundance of isotopically exchangeable organic hydrogen Hex (Schimmelmann et al., 1999). The complex sterical structure of large kerogen molecules does not permit physical access of water molecules to some chemical sites containing potentially exchangeable organic hydrogen. Sterical hindrance renders Hex values an operational, yet reproducible parameter. Kerogen does not contain a significant amount of inorganic hydrogen. Further isotopic massbalance calculations yield δDn values for the organic hydrogen in kerogen that does not isotopically exchange with water vapor over 10 h at 115 °C. Isotopic analytical procedures, the reporting of Hex in % of total hydrogen, and mass-balance calculations to arrive at δDn values in customary δ-notation followed the description given in Schimmelmann et al. (1999). The analytical precision is ±3‰ for δDn, ±0.05‰ for δ 13CVPDB, and ±0.5% for Hex. We report our hydrogen isotopic data according to Coplen's (1996) guidelines relative to VSMOW (0‰) and normalized to SLAP (−428‰). 3. Results and discussion 3.1. Petrographic composition Within the six selected range density fractions, alginite is the dominant maceral in low-density fractions below 1.1 g/cm3 (Table 1). For example, samples IN F8 and IN F15 having densities below 1.1031 g/cm3 contain 70 and 80 vol.% alginite, respectively. The alginite content decreases significantly with increasing density up to 1.1991 g/cm3 as demonstrated by only 4.0 vol.% alginite in IN S7 (Table 1). In the whole range density fractions, the 1.00 to 1.12 g/cm3 density fraction (Fig. 2A) has the highest content of alginite (94.5 vol.%), and the 1.12 to 1.25 g/cm3 density fraction (Fig. 2B) still contains 80 vol.% alginite (Table 1). Fractions having densities larger than 1.25 g/cm3 (Fig. 2 C, D, E) are dominated by amorphinite reaching almost 90 vol.%. Terrestrial OM is represented by rare vitrinite and inertinite (Fig. 2E), with the highest concentrations of up to 2 vol.% found in heavier fractions. Inorganic matter is present mainly as pyrite in concentrations between 0.5 and 10%. Even though we were careful to remove mineral matter, some very fine pyrite can be shielded by OM and remain in kerogen. Maceral-related density distributions in kerogens obtained in this study are similar to those obtained earlier on Type IIS kerogens from the Monterey (Miocene) and Duwi (Campanian/Maastrichtian) formations (Stankiewicz et al., 1996) and from Permian torbanite (Han et al., 1995). The combined evidence suggests that the alginite density is below 1.15 g/cm 3, the amorphinite density is between 1.20 and 1.4 g/cm 3, and densities between 1.15 and 1.20 g/cm 3 refer to mixtures of alginite and amorphinite (Fig. 1), regardless of the age or the depositional environment of OM. However, these density ranges may be shifted for kerogens of higher maturity (Okiongbo et al., 2005) and possibly as a result of oxidation as well as biochemical

M. Mastalerz et al. / International Journal of Coal Geology 103 (2012) 60–69

63

B

A

100µm

100µm

C

100µm

D

100µm

E

F

vitrinite amorphinite

100µm

100µm 50µm

H

G

100µm 50µm

100µm 50µm

Fig. 2. Photomicrographs of density fractions of kerogen in oil immersion. A—density fraction 1.00–1.12 g/cm3, fluorescent light, greenish yellow and brownish yellow fluorescent alginite; B—density fraction 1.12–1.25 g/cm3, fluorescent light, yellow fluorescent alginite and weakly fluorescent amorphinite; C—density fraction 1.25–1.30 cm3, fluorescent light; rare yellow alginite and weakly to non-fluorescent amorphinite; D—density fraction 1.30–1.35 g/cm3, E—density fraction > 1.4296 g/cm3, reflected light, non-fluorescent amorphinite with sporadic small particles of vitrinite, F—parent New Albany Shale sample, reflected light, amorphinite occurs concentrated in the matrix, G—parent New Albany Shale sample with abundant Tasmanites alginite (yellow), fluorescent light, H—Leiosphaeridia alginite (yellow) in the parent New Albany Shale sample, fluorescent light.

and/or microbial degradation of amorphinite. Increasing maturity within the oil and gas windows chemically disproportionates SOM and yields hydrogen-depleted solid residues that differ greatly in physical and chemical characteristics from their immature precursor materials. At very high maturity, the differences among macerals diminish and organic material eventually converges toward graphite (Boudou et al., 2008). A study by Okiongbo et al. (2005) documented an increase in density of organic matter of Type II kerogen through the peak and late phase of petroleum generation. To the best of our knowledge, no study has traced systematic density changes of macerals in response to organic matter degradation.

3.2. FTIR spectra FTIR spectra of the six selected density fractions show an abundance of aliphatic compounds, as indicated by high absorbance in aliphatic stretching bands in the 2800 to 3000 cm−1 region (Fig. 3). Aromatic bands (3000–3100 and 700–900 cm−1) show very low to undetectable absorbance. Main differences between density fractions occur in (1) the 1600 to 1800 cm−1 region, where trends of decreasing carboxyl/carbonyl absorbance (1710 cm−1) and increasing phenolic hydroxyl/aromatic C_C absorbance (1608 cm−1) are associated with amorphinite-rich, higher-density fractions; and (2) in the 1000 to 1100 cm−1 region,

M. Mastalerz et al. / International Journal of Coal Geology 103 (2012) 60–69

481

2852

1092 1031

2926 1446

3

Density range (g/cm ) Sample name

1710 1608

64

Alginite 70% Amorphinite 23.5%

Alginite 80% Amorphinite 13.5%

Alginite 4% Amorphinite 87%

Alginite 2% Amorphinite 86%

Alginite 2% Amorphinite 88%

Alginite 8% Amorphinite 84%

Fig. 3. FTIR spectra of six selected range density fractions of kerogen. Densities and the contents of alginite and amorphinite (in volume %) are listed above sample names.

The two lightest, alginite-dominated density fractions contain (1) significantly more aliphatic hydrogen with absorbance peaks at 2852 and 1926 cm−1, (2) a stronger carboxyl/carbonyl contribution compared to phenolic hydroxyl/aromatic carbon, and (3) reduced absorbance in

1.00-1.12 (Alginite 94.5%, Amorphinite 5%) 1.12-1.25 (Alginite 80%, Amorphinite 18%)

1446

1710 1608

481

Absorbance

2852

1031

1.25-1.60 (Alginite 3%, Amorphinite 94%)

1092

A

2926

where bands assigned to ether bonds (C\O\C) have significantly lower absorbance in less dense, alginite-rich fractions. A comparison among density fractions having different maceral compositions is shown for whole range density fractions in Fig. 4A.

Wavenumbers (cm-1)

Absorbance

B

Wavenumbers (cm-1)

Fig. 4. FTIR spectra of three whole range density fractions of kerogen (A) and parental bulk kerogen (B). Data along the Y-axis are in relative absorbance units. Alginite and amorphinite contents are expressed in volume %.

M. Mastalerz et al. / International Journal of Coal Geology 103 (2012) 60–69

the 1000 to 1100 cm−1 region assigned to ether bonds (C\O\C) compared to the amorphinite-dominated density fraction 1.26 to 1.60 g/cm3. The FTIR signature of parental bulk kerogen (Fig. 4B) is a combination of the characteristics of the three whole range density fractions. Differences in abundances of chemical functional groups are semiquantitatively expressed by FTIR ratios (Table 2). FTIR data from the three whole range density fractions are especially informative. With increasing density, aromaticity (Ar700–900/2800–2300) increases progressively, and the ratio CH2/CH3 decreases from 4.4 for almost pure alginite in the 1.00 to 1.12 g/cm 3 density fraction to 1.7 for amorphinitedominated kerogen in the 1.25 to 1.60 g/cm3 density range. The latter indicates shortening of aliphatic chain length and/or branching of aliphatic chains (Lin and Ritz, 1993). The ratio of carboxyl/carbonyl (peak at 1703 cm−1) to the phenolic hydroxyl/aromatic carbon (peak at 1624 cm−1) decreases progressively from 1.29 to 0.54 from alginite-rich to amorphinite-rich fractions. For the six selected range density fractions, the listed trends are not always clear because some intermittent density fractions were not included in the analyses. 3.3. NMR spectra The general trend of elevated aromaticity with increasing density is well documented by nuclear magnetic resonance (NMR) spectra of selected range density fractions (Fig. 5). With increasing density, absorbance in the aromatic region (110–170 ppm) becomes more prominent relative to the aliphatic region (0–60 ppm). Spectra of higherdensity fractions show resonances at 73 ppm owing to the presence of carbon attached to heteroatoms like N, O, and S (Breitmaier and Voelter, 1987). The aromaticity factor fa represents the ratio of the integrated signal of the aromatic region to the total integrated signal. Values of the aromaticity factor fa that represent the ratio of the integrated signal of the aromatic region to the total integrated signal tend to increase with higher density and enhanced presence of amorphinite (Table 3). The documented variations in fa from 0.24 in alginite-rich density fractions to more than 0.38 in amorphinite-rich density fractions indicate an important influence of maceral composition on bulk aromaticity. NMR is a valuable approach to assess thermal maturity (e.g., Carr and Williamson, 1990; Witte et al., 1988), but the newly documented maceral-related variance in aromaticity mandates that rocks or kerogens having comparable OM compositions must be used when maturity assessment is made based on aromaticity. This is especially important for rocks of low maturity, such as in this study. Maceral-related differences in coal progressively diminish with increasing maturity (e.g., Mastalerz and Bustin, 1993). It is currently unknown to what extent the thermal evolution of macerals in shales may cause similar convergence. A direct NMR-based comparison between the relative influences of maceral composition (this study) and thermal maturity on aromaticity is possible by using data from Werner-Zwanziger et al. (2005) and Lis et al. (2006). These two studies measured bulk kerogens from the New

65

Albany Shale across a maturity range equivalent to a vitrinite reflectance Ro range of 0.29% to 1.40%. The observed range of maturity-related variance of aromaticity fa in bulk kerogen from 0.35 to 0.75 was larger than this study's range of maceral-related aromaticity fa of 0.24 to 0.38. 3.4. Hydrogen and carbon isotope ratios and hydrogen exchangeability Hydrogen isotopic analysis documents a depletion of deuterium in density fractions dominated by amorphinite (Table 3, Fig. 6A), with the result that δDn values of the alginite-rich density fraction reach −90‰ whereas amorphinite-rich density fractions express δDn values of approximately −105‰. Similarly, alginite is relatively 13C-enriched and density fractions having a high content of amorphous organic matter are relatively 13C-depleted (Fig. 6B). The hydrogen and carbon isotopic differences among macerals in shales are significant and, therefore, environmental or maturityrelated interpretations of isotopic data from bulk OM in rocks must take this intrinsic variance into consideration. Hydrogen isotope ratios, in particular, show large differences among macerals from New Albany Shale kerogen spanning δDn values from −87‰ to −105‰ (Table 3, Fig. 6A). The ca. 18‰ isotopic range of macerals equals about one-third of the total δDn range (i.e., −69‰ to −118‰) of bulk New Albany Shale kerogens spanning the maturity range from an Ro of 0.29 to 1.40% (Fig. 6D). In turn, the carbon isotopic ranges of 0.4‰ (from −29.6 to −30.0‰) covered by selected density fractions and 1.5‰ (from −28.5 to −30.0‰) in whole range density fractions are comparable to the entire δ13C range spanned by bulk kerogens of the New Albany Shale rank series (Fig. 6B and E), demonstrating that maceral composition significantly affects the carbon isotopic composition of bulk kerogen. There is also a significant difference in weight percent of C and N contents among the density fractions, especially for the whole range density fractions (Table 3), with carbon content decreasing and nitrogen content increasing toward denser fractions. In contrast, the observed percentages of organic exchangeable hydrogen in total hydrogen of kerogen density fractions range only from 1.9 to 2.9% (Table 3) and follow no systematic petrographic trend (Fig. 6C). This suggests that variance in relative proportions of alginite and amorphinite in New Albany Shale does not significantly affect the bulk hydrogen exchangeability. 3.5. Comments of the origin of alginite and amorphinite The New Albany Shale studied is a marine sediment with amorphinite and alginite as dominant macerals (Table 1, Fig. 2F, G, H). Abundance of alginite, with distinct Tasmanites (Fig. 2G) and Leiosphaeridia (Fig. 2H), suggests a limited supply of dissolved oxygen in the water column and dysoxic or anoxic conditions (Strąpoć et al., 2010). Our earlier studies using a py-GC/MS technique (Lis et al., 2008) documented the dominance of n-alk-1-ene/n-alkane doublets having a chain length with up to 23 carbon atoms in the alginite-dominated whole range

Table 2 FTIR-derived characteristics of six selected range density fractions, three whole range density fractions, and parental bulk kerogen. Values indicate integrated areas under the bands of specific functional groups. Wavenumber ranges (cm−1) of spectral regions are indicated in column headers. Sample name

Density range (g/cm3)

3000–3100

2800–3000

700–900

Ar700–900/

CH2/CH3

1703/1624

Selected range density fractions IN F8 1.0353–1.0446 IN F15 1.0757–1.1031 IN S7 1.1991–1.2299 IN S19 1.4221–1.4369 IN S22 1.4659–1.4800 IN S28 1.5429–1.5541

0.011 0.044 0.150 0.061 0.043 0.019

147.9 156.4 83.2 103.0 122.1 61.3

1.17 0.98 1.68 5.31 4.32 0.42

0.01 0.01 0.02 0.05 0.04 0.01

4.04 7.34 2.45 2.39 2.63 2.48

1.27 1.51 0.66 0.60 0.61 0.39

Whole range density fractions NAS9101-a 1.00–1.12 NAS9101-b 1.12–1.25 NAS9101-c 1.25–1.60 Bulk kerogen b1.60

0.010 0.068 0.420 0.040

139.8 154.7 62.0 67.3

0.09 0.85 4.92 2.92

0.00 0.01 0.08 0.04

4.40 3.90 1.70 2.20

1.29 0.89 0.54 0.34

2800–2300

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0-60

Density range (g/cm3 ) Sample name

IN kerogen

73

Chemical shift (ppm) Fig. 5. 13C NMR spectra of six selected range density fractions of kerogen and their parental bulk kerogen (IN kerogen). The marked ranges (0–60 and 110–170 ppm) represent aliphatic and aromatic carbon regions, respectively.

density fractions, supporting a highly aliphatic alginite content observed by both FTIR and NMR spectroscopy in this study. Alkylbenzenes were present in smaller quantities than n-alk-1ene/n-alkane doublets with toluene and 1,2,4-trimethylbenzene being the most prominent. Hartgers et al. (1994c) suggested that toluene might be formed from linear precursor carbon skeletons by cyclization and aromatization. The n-alk-1-ene/n-alkane doublets, thus, may have served as precursor material that evolved through thermal maturation into toluene. In contrast to alginite-dominated fractions, amorphinite-dominated fractions were characterized by smaller contributions of n-alk-1-ene/nalkane doublets (Lis et al., 2008). In turn, the alkylbenzenes gained importance with increasing density, and 1,2,3,4-tetramethylbenzene became dominant alkylbenzene in the heavy density amorphinite-dominated fraction. The suggested sources for the 1,2,3,4-tetramethylbenzene are aromatic carotenoids derived from either Chlorobiaceae photosynthetic sulfur bacteria (Clegg et al., 1997; Douglas et al., 1991; Hartgers et al., 1994a,b; Koopmans et al., 1996; Pedentchouk et al., 2004; Summons and Powell, 1987) or algae (Carmo et al., 1997; Hoefs et al., 1995;

Pedentchouk et al., 2004). The dominance of 1,2,3,4-tetramethylbenzene in the pyrolysates of the amorphinite-rich fractions as compared to the smaller amounts found in the alginite-rich fraction pyrolysates suggests a higher contribution of photosynthetic sulfur bacteria to the amorphinite precursor OM. 3.6. Paleoenvironmental signals in bulk kerogen versus density fractions The use of geochemical proxies of bulk OM for paleoenvironmental reconstruction is averaging the signals from a variety of macerals having different origins. Maceral-specific analyses can potentially yield more detailed information than bulk OM. Vitrinite reflectance data, for example, owe their exceptional diagnostic value to the fact that they are maceral-specific. Other macerals having distinct chemistries can be expected to have enhanced paleoenvironmental value, for example, liptinite is aliphatic-rich having a high abundance of carbon-bound, nonexchangeable hydrogen. The use of liptinite for hydrogen stable isotope measurements may reduce the isotopic noise from varying contributions of non-aliphatic organic hydrogen pools present in other macerals in

Table 3 Carbon, nitrogen, 13C NMR-derived aromaticity fa, % of isotopically exchangeable hydrogen, and isotopic composition of six selected range density fractions of kerogen, three whole range density fractions, and of parental bulk kerogen (IN kerogen). nd—not determined. C (weight %)

N (weight %)

fa

δ13C (‰)

δDn (‰)

Hydrogen exch. (%)

Selected range density fractions IN F8 1.0353–1.0446 IN F15 1.0757–1.1031 IN S7 1.1991–1.2299 IN S19 1.4221–1.4369 IN S22 1.4659–1.4800 IN S28 1.5429–1.5541 IN kerogen b1.6

70.1 69.9 64.7 54.3 59.1 68.7 nd

1.04 1.63 2.35 1.85 1.97 2.10 nd

0.289 0.242 0.377 0.366 0.385 0.371 0.389

−29.6 −29.6 −29.8 −29.7 −29.8 −30.0 −29.7

−95 −87 −104 −102 −105 −104 −93

2.5 2.0 2.9 1.9 2.9 2.6 2.5

Whole range density fractions NAS9101-a 1.00–1.12 NAS9101-b 1.12–1.25 NAS9101-c 1.25–1.60

72.3 68.3 51.5

0.93 1.25 1.62

nd nd nd

−28.5 −29.5 −30.0

−86 −93 −96

1.4 1.4 1.9

Sample name

Density range (g/cm3)

M. Mastalerz et al. / International Journal of Coal Geology 103 (2012) 60–69

D

selected range density fractions

δ

δ

A

67

bulk kerogen whole range density fractions

E

δ

δ

B

selected range density fractions bulk kerogen whole range density fractions

C

F selected range density fractions bulk kerogen whole range density fractions

Fig. 6. Hydrogen isotope ratios (A), carbon isotope ratios (B), and hydrogen exchangeability (C) in relation to the ratio of alginite to amorphinite in New Albany Shale kerogens. Changes in hydrogen isotope ratios (D), carbon isotope ratios (E), and exchangeable hydrogen in % (F) in New Albany Shale kerogens with increasing rank. Data from Lis et al. (2005, 2006). Gray areas in D–F represent the ranges of values from this study (based on all density fractions of kerogen).

bulk kerogen. However, the value of the maceral-specific geochemical approaches remains limited for practical reasons. The technical effort to produce maceral isolates from bulk kerogen in sufficient amounts (e.g., at least 0.5 mg for current on-line mass-spectrometric D/H determination by high-temperature pyrolysis via a glassy carbon interface) is prohibitive. Maceral isolates in the form of density fractions rarely consist of only one type of maceral but typically contain aggregates of organic and some inorganic components that cannot be separated. The preparation of OM density fractions with high enrichment of individual macerals requires experience and patience. Currently no method is available to prepare pure macerals, except under unusual circumstances, for example, in large fragments of fossil wood and from fossil resin/amber. Paleoenvironmental and depositional changes within a sedimentary sequence synchronously influence not only chemical, physical, and isotopic compositions of all fractions of OM, but also play an important role on the deposition and preservation of assemblages of particulate OM that forms bulk kerogen. The maceral composition of OM assemblages in itself represents a valuable paleoenvironmental archive. In the absence of analytical methods that can reproducibly and economically measure maceral-specific paleoenvironmental proxies, we should continue to rely on bulk kerogen as analytes. At the same time, we must be cognizant of the underlying heterogeneity at the maceral level and be careful with environmental interpretations. The demineralization of rock with acids and the purification of kerogen involve multiple steps of washing with water and organic solvents. Each washing runs a risk that lightweight and hydrophobic density fractions of OM are preferentially lost during decanting, which can introduce significant analytical bias when interpreting resulting nonrepresentative, bulk kerogenbased data in terms of paleoenvironmental changes.

4. Conclusions 1) The density of kerogen in shales depends on maceral composition. In immature kerogen, the alginite density is below 1.15 g/cm 3, the amorphinite density is between 1.20 and 1.4 g/cm 3, and densities between 1.15 and 1.20 g/cm 3 correspond to mixtures of alginite and amorphinite, likely regardless of the age or the depositional environment. However, these density ranges may be shifted for kerogens of higher maturity or those more reworked. 2) The maceral composition of kerogen in immature shale greatly influences the chemistry of bulk kerogen, especially such properties as aromaticity, functional group abundances, and elemental composition, as well as carbon and hydrogen isotopic compositions. This influence mandates that rocks or kerogens having comparable OM compositions must be used when maturity or paleoenvironmental assessments are made based on organic geochemical parameters. If rocks of similar maceral compositions are not available, the differences in maceral composition must be considered during interpretations. 3) The isolation procedure of bulk kerogen from rock must avoid bias against any organic density fractions to ensure that the kerogen is representative of the insoluble organic matter in the source rock. Acknowledgments The authors acknowledge support from the U.S. Department of Energy (DOE), Basic Energy Sciences, Grant No. DE-FG02-11ER16246. Ulrike Werner-Zwanziger (now at Dalhousie University) assisted with NMR measurements, and Peter E. Sauer at Indiana University performed stable isotopic measurements. We thank the reviewers for their

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