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

Abstract

A suite of oils and associated gases from the Liaohe Basin NE China, which have similar maturity and source rock origin but with varying degrees of biodegradation, have been analyzed for the molecular compositions of oil and gas as well as for the isotopic values. Isotopic compositions of the gas samples show wide variation (δ13C1 from −55.4‰ to −36.8‰, δ13C2 from −39.4‰ to −28.5‰, δ13C3 from −31.9‰ to −16.4‰ and δ13n-C4 from −26.6‰ to −20.4‰). Most gas samples display clear geochemical evidence of alteration by biodegradation, with relatively high dryness (C1/C1–4>0.97), high ratios of isobutane/n-butane (0.4–1.8) and C2/C3 (0.3–42.3). The carbon isotopic compositions of wet gas components (C3 and C4) in the biodegraded gases are heavier than these in non-biodegraded gases with Δδ13C (C3−C2) of up to 13.4‰, but methane and ethane isotope signatures are variably heavier or lighter than those in non-biodegraded gases. Carbon dioxide contents and isotopic values provide evidence for gas origins. Gases with isotopically heavy carbon dioxide (δ13C>+2‰), and isotopically light methane (δ13C<−50‰) indicate that secondary microbial methane was generated from CO2 reduction under anaerobic conditions. Gases with isotopically heavy methane (>−45‰), together with heavy carbon dioxide (>+8‰), probably indicate a high degree of CO2 conversion to methane under later stages of secondary microbial methane generation. We speculate that the presence of isotopically heavy methane (>−45‰) and isotopically light carbon dioxide (<−8‰) may be indicative of anaerobic methane oxidation but much additional work is needed to evaluate that.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.