Effects of thermal maturation and thermochemical sulfate reduction on compound-specific sulfur isotopic compositions of organosulfur compounds in Phosphoria oils from the Bighorn Basin, USA
Introduction
The stable sulfur isotopic composition of crude oil is controlled by the incorporation of sulfur into organic matter during diagenesis (Monster, 1972, Gransch and Posthuma, 1974, Orr, 1974, Orr, 1986, Dinur et al., 1980, Anderson and Pratt, 1995, Aizenshtat and Amrani, 2004b, Werne et al., 2004) and the decomposition and rearrangement of sulfur-containing compounds during catagenesis (Amrani, 2014). Low thermal stability of sulfur-containing organic compounds leads to early generation of petroleum and the release of H2S (Tannenbaum and Aizenshtat, 1984, Tannenbaum and Aizenshtat, 1985, Lewan, 1985, Lewan, 1998, Orr, 1986, Baskin and Peters, 1992, Aizenshtat et al., 1995, Nelson et al., 1995, Koopmans et al., 1998). Pyrolysis experiments on sulfur-rich source rocks have shown that 34S enrichment occurs in the generated bitumen, oil, and residual kerogen relative to the original sulfur isotopic composition of the source rock, and that this enrichment varies from ⩽2‰ for closed-system experiments (Idiz et al., 1990, Amrani et al., 2005a) to 4–8‰ for open systems (Aizenshtat and Amrani, 2004a). Similar observations of 34S enrichment in petroleum relative to the source kerogen have also been made in natural systems (Orr, 1986, Aizenshtat and Amrani, 2004a). The H2S generated in pyrolysis experiments has been found to be 34S depleted, which has been invoked as the mechanism for 34S enrichment in the generated petroleum (Amrani, 2014).
Secondary processes, such as biodegradation, water washing and hydrothermal alteration, can affect the sulfur isotopic composition of crude oil (Thode, 1981). In particular, thermochemical sulfate reduction (TSR) has a significant impact on whole oil δ34S values (Orr, 1974, Machel et al., 1995, Manzano et al., 1997). Inorganic sulfur in sulfate minerals is typically enriched in 34S relative to organic sulfur in kerogen and oil, and the progressive incorporation of this 34S-enriched sulfur into organosulfur compounds (OSCs) leads to relatively 34S-enriched values in TSR-altered oils (Orr, 1974, Powell and Macqueen, 1984, Cai et al., 2003, Cai et al., 2009, Amrani et al., 2012). The first documented report of TSR occurrence in nature was a study of oils from the Bighorn Basin in the western United States by Orr (1974). That work showed that the thermal maturity of the source rock at the time of oil generation and TSR can both affect the δ34S values of whole crude oils; however, the effect of TSR is more significant (Orr, 1974). Moreover, Orr (1974) showed that individual boiling point fractions of Bighorn oils had distinct sulfur isotopic compositions as a result of thermal maturation and TSR (Fig. 1).
Gas chromatography linked to multi-collector inductively coupled plasma mass spectrometry (GC-MC-ICP-MS) is a robust analytical technique for determining the stable sulfur isotopic composition of individual OSCs (Amrani et al., 2009). This method has been shown to yield accurate and precise δ34S values of individual aromatic and sulfidic sulfur-containing compounds in crude oil samples (Amrani et al., 2012, Greenwood et al., 2014, Gvirtzman et al., 2015, Li et al., 2015, Cai et al., 2016). Studies of oils from the Smackover Formation, Gulf Coast, U.S. (Amrani et al., 2012, Gvirtzman et al., 2015) and the Tarim Basin, western China (Li et al., 2015, Cai et al., 2016) have shown that the δ34S values of benzothiophene and dibenzothiophene and their alkylated forms (BTs and DBTs) are quite sensitive to the effects of TSR, especially in the early stages. The present study examines the application of compound-specific sulfur-isotope analysis (CSSIA) to a suite of 18 oil samples generated from the Permian Phosphoria Formation source rock in the Bighorn Basin, western USA. The objectives are threefold: (1) evaluate the effect of source rock thermal maturity on the sulfur isotopic composition of individual OSCs in crude oil; (2) assess the applicability of the compound-specific sulfur isotopic results determined on Smackover Formation and Tarim Basin oils for understanding the occurrence of TSR in the Bighorn Basin; and (3) determine if the δ34S values of individual aromatic and sulfidic compounds in oils can confirm, or improve upon, the findings of previous studies (Orr, 1974, Bjorøy et al., 1996, Lillis and Selby, 2013) on the generation and alteration of petroleum accumulations in the Bighorn Basin.
Section snippets
Geologic setting
The Bighorn Basin is a large sedimentary and structural basin located in the states of Wyoming and Montana in the western US (Fig. 2). The boundaries of the basin are defined by Laramide (Late Cretaceous through Eocene) faulted and folded uplifts (Beartooth, Absaroka, Owl Creek, and Bighorn Mountains), and the northernmost boundary is constrained by the Nye-Bowler lineament (Roberts et al., 2008). The Phosphoria Formation is the dominant source of oil in the basin although some Cretaceous oil
Sample information
A total of 18 oil samples were acquired for this study, including 17 from producing oil fields and one drill stem test (DST) sample from a wildcat well. The well locations are shown in Fig. 2 and additional geographic data (e.g., depth of producing interval, producing formation, field name) are provided in Table 1. The samples were selected to cover a range of thermal maturities of generation and extents of alteration by TSR, without significant interference from alteration by biodegradation or
Bulk oil geochemistry
Selected geochemical parameters for the whole oils and oil fractions are shown in Table 2. The data in Table 2 are from Lillis and Selby (2013), with the exception of four oil samples (Ebet 181, Golden Eagle 1, Wycol 9, Tensleep 55) that were analyzed for this study. All of the oils have API gravity values in the medium-weight oil range of 21–31° API, pristane/phytane (pr/ph) in the range of 0.5–0.8, and low levels (rank ⩽2) of biodegradation as defined by Peters and Moldowan (1993), with the
Oil source
All but two of the studied Bighorn Basin oils were produced from Paleozoic reservoirs (Table 1), which were typically charged by the Permian Phosphoria Formation. The two exceptions (Cruse 1-A and Stateland 50) were derived from the Lower Cretaceous Cloverly Formation. Viable Cretaceous oil-prone source rocks exist in the region (e.g., Mowry and Thermopolis shales) and are potential candidates for the source of these oils. Phosphoria-sourced oils have been shown to have a distinctive
Conclusions
This study examined 18 oils from the Bighorn Basin, western USA to assess the effects of thermal maturation and TSR on the sulfur isotopic composition of individual OSCs in crude oil. Non-TSR-altered Bighorn Basin oils have progressive 34S enrichment of the BTs and DBTs that correlates with the increasing thermal maturity of the source rock. At low thermal maturity, BTs are 34S enriched (∼5‰) relative to DBTs, whereas at higher maturity homogenization leads to BTs and DBTs with increasingly
Acknowledgements
We thank the U.S. Geological Survey, Energy Resources Program, organic geochemistry laboratory personnel, Zach Lowry, Augusta Warden, Tammy Hanna, and Mark Drier for providing biomarker and bulk oil geochemical analyses. Alon Amrani thanks the Israeli Science Foundation (ISF) grant number 1269/12 for partial support of this study. Lubna Shawar thanks the Ministry of Infrastructure of Israel for PhD study grant. We appreciate the detailed comments and suggestions by Mike Lewan, Richard Worden,
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