--> Thermochemical Sulfate Reduction in Upper Devonian Nisku Formation Reservoirs, Alberta: Insights From Molecular Characterization With FTICR-MS and GC×GC Approaches

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Thermochemical Sulfate Reduction in Upper Devonian Nisku Formation Reservoirs, Alberta: Insights From Molecular Characterization With FTICR-MS and GC×GC Approaches

Abstract

Among the natural processes that could generate H2S in petroleum systems, thermochemical sulfate reduction (TSR) is considered to be dominant in several higher temperature plays. However, due to the number of external factors affecting crude oil molecular signatures, together with TSR, most of the geochemical proxies developed from carbon and sulfur isotopic composition of selected chemical compounds fail to generate a complete understanding of TSR processes in a broad range of cases. Recently, Walter et al. (2015)1 tackled the study of petroleum alteration from TSR processes in the Smackover Formation, using a combination of advanced analytical methods, observing an increase in thiadiamondoid and multiply oxygenated species (O>3) with increasing TSR severity together with the generation of precursors to solid bitumen compounds. We extend that approach and in this study, 17 samples (including black oils, sweet and sour gas condensates) from the Upper Devonian Nisku Formation reservoirs, Brazeau River area, Alberta, Canada, were analyzed by ultra-high resolution mass spectrometry (FTICR-MS) and comprehensive two-dimensional gas chromatography (GC×GC). This area has been extensively studied and shows several cases of TSR occurring in some of the condensate accumulations. FTICR-MS data, using atmospheric pressure photoionization (APPI), can be used to make high resolution molecular characterizations of the sulfur species in the evolving petroleum. By comparing isolated sweet condensate pools to sour ones, distributions of diamondoid and thiadiamondoid species can be used to distinguish the differential impacts of thermal maturity and TSR on petroleum composition. The approach may also provide clues to the interaction of petroleum compounds with migrated H2S from deeper accumulations and examination of mixed nitrogen, sulfur, and oxygen containing species in the sample suite, indicates other processes than direct TSR are also active. We also describe the molecular characterization of the solid bitumen present in the Nisku reservoirs. [1] Walters, C. C.; Wang, F. C.; Qian, K.; Wu, C.; Mennito, A. S.; Wei, Z. Geochim. Cosmochim. Acta 2015, 153, 37–71.