Preservation of organic matter in shale linked to bacterial sulfate reduction (BSR) and volcanic activity under marine and lacustrine depositional environments

Abstract

Black organic-rich shale is a sedimentary product of strongly reducing environmental water bodies, and is the main source of shale oil and gas. During its formation, the presence of sulfate promotes bacterial sulfate reduction (BSR), but little attention has been paid to how BSR affects the preservation of organic matter (OM) in different sedimentary environments. In this study, the Wufeng-Longmaxi Formations from the Sichuan Basin, the Chang-7 Member (the seventh member of the Triassic Yanchang Formation) from the Ordos Basin, the Qingshankou and Nenjiang Formations in the Songliao Basin, as well as the New Albany shale from the Appalachian Basin are selected for investigation. Results show that there is a positive correlation between total sulfur (TS) and total organic carbon (TOC) contents, and the sulfate reduction index (SRI) and TOC show two-stage exponential changes. When the SRI is higher than 1.375, the overall TOC content is low, indicating that a large amount of organic matter is consumed through intense BSR. When the SRI is lower than 1.375, the TOC content is significantly higher, indicating that weak BSR has relatively little effect on organic matter degradation.In the normal marine sedimentary environment for the New Albany shale in North America, the high concentration of sulfate was conducive to BSR, but the increased input of terrestrial OM compensated for the marine OM consumption by BSR, resulting in a high TOC value in the shale. Volcanic activity provides additional sulfates into the water column and triggers BSR occurrence, but the high concentration of nutrients brought by volcanic activity in the shales of the Wufeng-Longmaxi Formations and the Chang-7 Member promoted paleoproductivity, offset the effect of OM consumption by BSR, and resulted in the high TOC values seen in these shales. When a large amount of sulfate was input into terrestrial lakes through volcanic activity or marine transgression, there were significantly different effects on the preservation of organic matter in lacustrine shales. Nutrients from the volcanic activity increased the paleoproductivity of lakes during the Chang-7 period, while decreasing the impact of BSR on OM, resulting in a high TOC value in the Chang-7 Member shale. However, when the lacustrine basin was subjected to marine transgression, paleoproductivity weakly increased and BSR consumed a larger proportion of the OM, reducing the TOC values and forming the relatively low TOC shale of the Qingshankou and Nenjiang Formations in the Songliao Basin.