Secondary microbial gas formation associated with biodegraded oils from the Liaohe Basin, NE China

It seems likely that most of the variations in isotopic signatures of gases related to biodegraded oil can be described by various sequential biodegradation, carbon dioxide reduction and gas mixing processes. Methanogenesis through carbon dioxide reduction does not necessarily produce isotopically depleted methane. The methane becomes richer in 13C when a large fraction of carbon dioxide is reduced to methane, resulting in newly generated methane that is sometimes heavier than the original thermogenic methane. Anaerobic methane oxidation may exist in the present case study. Mixing of newly generated methane with thermogenic methane already in place in the reservoir, coupled with anaerobic methane oxidation, can result in very complicated isotopic signatures. Care should be taken when gas isotopic values were applied for source and maturity assessment.