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

Highlights

• Compound-specific sulfur isotopes distinguish TSR and non-TSR altered oils.

• Thermal maturation of source rocks may lead to ³⁴S-enriched sulfur compounds.

• δ³⁴S results suggest charge of TSR-derived H₂S in a reservoir in the Bighorn Basin.

• δ³⁴S of sulfur compounds can constrain sulfate sources in TSR reservoirs.

Abstract

Compound-specific sulfur isotope analysis was applied to a suite of 18 crude oils generated from the Permian Phosphoria Formation in the Bighorn Basin, western USA. These oils were generated at various levels of thermal maturity and some experienced thermochemical sulfate reduction (TSR). This is the first study to examine the effects of thermal maturation on stable sulfur isotopic compositions of individual organosulfur compounds (OSCs) in crude oil. A general trend of ³⁴S enrichment in all of the studied compounds with increasing thermal maturity was observed, with the δ³⁴S values of alkyl-benzothiophenes (BTs) tending to be enriched in ³⁴S relative to those of the alkyl-dibenzothiophenes (DBTs) in lower-maturity oils. As thermal maturity increases, δ³⁴S values of both BTs and DBTs become progressively heavier, but the difference in the average δ³⁴S value of the BTs and DBTs (Δ³⁴S BT-DBT) decreases. Differences in the isotopic response to thermal stress exhibited by these two compound classes are considered to be the result of relative differences in their thermal stabilities. TSR-altered Bighorn Basin oils have OSCs that are generally enriched in ³⁴S relative to non-TSR-altered oils, with the BTs being enriched in ³⁴S relative to the DBTs, similar to the findings of previous studies. However, several oils that were previously interpreted to have been exposed to minor TSR have Δ³⁴S BT-DBT values that do not support this interpretation. The δ³⁴S values of the BTs and DBTs in some of these oils suggest that they did not experience TSR, but were derived from a more thermally mature source. The heaviest δ³⁴S values observed in the OSCs are enriched in ³⁴S by up to 10‰ relative to that of Permian anhydrite in the Bighorn Basin, suggesting that there may be an alternate or additional source of sulfate in some parts of the basin. These results indicate that the sulfur isotopic composition of OSCs in oil provides a sensitive indicator for the extent of TSR, which cannot be determined from other bulk geochemical parameters. Moreover, when combined with additional geochemical and geologic evidence, the sulfur isotopic composition of OSCs in oils can help to identify the source of sulfate for TSR alteration in petroleum reservoirs.

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.