Molecular markers recording microbial activity in oil reservoirs are still sparse. Here we present a series of new steroid acids with extended side chains in biodegraded oils. The distributions of the steroid acids and a comparison with sterane homologs in oils with different biodegradation grades suggest that these unique steroid acids originated from the biodegradation of regular steranes via side chain extension following the fumarate addition pathway. This finding not only confirms the existence of anaerobic biodegradation, but also provides the first direct evidence for the presence of hydrocarbon activation by fumarate addition in deep reservoirs.

Carbon chain extension, Steroid acids, Anaerobic biodegradation, Fumarate addition