Steroid acids with extended side chains and fumarate addition in reservoirs

Abstract

Molecular markers that record microbial activity in oil reservoirs are still sparse. Here we report a series of novel steroid acids with extended side chains in in-reservoir biodegraded oils. Due to the lack of authentic standards, these steroid acids are tentatively identified based on ToF-MS spectra, chromatographic retention behavior, and comparisons with previously reported regular steroid acids and hopanoic acids. The correlation between significant consumption of steranes and generation of the steroid acids in oils suggests that these unique steroid acids originated from the biodegradation of regular steranes. The side chains of these steroid acids extended by up to three CH₂ units are consistent with the fumarate addition mechanism. Some co-occurring alkylsuccinates were also tentatively identified, supporting the occurrence of anaerobic biodegradation by fumarate addition. Furthermore, regular steroid acids with the same carbon numbers and configurations as those detected in the aerobically degraded samples from our previous laboratory study (Xie et al., 2024) were not detected in the oil samples in this study, indicating that the biodegradation pathway of steranes in oil reservoirs may differ from the pathway under aerobic conditions. If confirmed, these findings not only substantiate the existence of anaerobic biodegradation, but also provide the first direct evidence for the presence of hydrocarbon activation by fumarate addition in oil reservoirs.