Geochemical evidence for secondary microbial gas in deep hot reservoirs of the Tarim Basin

Abstract

It is hard to determine secondary microbial gas from deep burial reservoirs with multiple generations of petroleum charge. Herein, oils at depths from 5000 m to 6500 m in the Ordovician carbonate reservoirs, Tarim basin, are found to have been biodegraded to secondary microbial gas. These oils experienced severe biodegradation and thus contain abundant 25-norhopanes and 17-nortricyclic terpanes. The associated gases have methane δ13C1 from −51.9‰ to −47.3‰ and δ2H1 from −327.8‰ to −192.4‰, and CO2 δ13CCO2 from −0.7‰ to +15.3‰. These features suggest that the gases are secondary microbial gas, generated predominantly via CO2 reduction with preferential reduction of 13C-depleted CO2 and contribution of methane hydrogen from formation water in closed environments. The secondary microbial gas may have generated from biodegradation of oils at reservoir temperatures of about <75°C during the Late Permian, and has subsequently mixed with a later charge of non-biodegraded oils and wet gas during the Late Cretaceous. Consequently, the present gas shows relatively low dryness (C1/∑C1–4 < 0.87) and has varied δ13C1 and δ2H1 values in methane. The study implies that the signatures of secondary microbial gas can easily be masked by thermogenic gas and thus more secondary microbial gas has yet to be identified.