Abstract

Approximately 1700 tcf (∼48 trillion m3) of dry gas (>99% methane) reserves and resources occur in western Siberia, mostly in shallow (<1500 m [<4921 ft]) Cenomanian pools in the northern part of the basin. This dry gas constitutes about 11% of the world's conventional gas endowment and about 17% of the annual gas production. The origin of the dry gas has been debated extensively over the last 45 yr but remains controversial. Widely discussed hypotheses on the origin include early-mature thermogenic gas from coal, primary microbial gas from dispersed organic matter or coal, and thermogenic gas from deep source rocks. However, all these hypotheses are in some ways inconsistent with the molecular or isotopic composition of the gases or the results of basin and petroleum systems modeling. Here, I present geochemical and geological evidence that a significant (although yet not quantified) part of the shallow dry gas in the northern West Siberian Basin originated from methanogenic biodegradation of petroleum. Circumstantial evidence includes the occurrence of heavily biodegraded oil legs and residual oil in many Cenomanian gas pools, as well as geochemical evidence of heavy to slight biodegradation in Jurassic–Albian reservoirs commonly underlying the Cenomanian pools. Direct evidence includes, most importantly, 13C-enriched CO2 in pools with biodegraded oil (although data are limited), which indicates 40–70 wt.% conversion of oil-derived CO2 to secondary microbial methane. Distinctive hydrocarbon molecular and isotopic compositions of most gases in Cenomanian pools (average dryness C1/(sum C1-C5) is 0.9976; average δ13C of methane is −51.8‰) suggest that they represent mixtures of biodegraded thermogenic gases from deep, mainly Jurassic, source rocks and secondary microbial methane with an occasional small addition of primary microbial methane. Contribution of early-mature coal-derived gas is possible in areas with the most significant thermal stress of Hauterivian–Aptian sediments but remains speculative. Review of petroleum habitats of five representative oil-gas-condensate fields in western Siberia (including the world's second largest gas field, Urengoyskoe) suggests that methanogenic biodegradation may best explain the observed distribution and properties of fluids in the shallow reservoirs of those fields.

Recognition of secondary microbial gas in western Siberia helps explain the observed dominance of gas in the shallow, cool northern part of the basin, where conditions were more favorable for prolonged petroleum biodegradation than in the central and southern parts of the basin. Secondary microbial gas has been recognized worldwide and may (1) represent a volumetrically significant exploration target in shallow reservoirs (perhaps more significant than primary microbial gas) and (2) indicate effective thermogenic petroleum systems in the deeper sections. Large volumes (up to ∼66,500 tcf [∼1884 trillion m3]) of secondary microbial methane could have been generated from biodegraded petroleum accumulations worldwide. Although a part of that gas accumulated as oil-dissolved, free, and hydrate-bound gas, most gas apparently escaped into the overburden, atmosphere, and ocean and could have affected global climate in the geologic past.

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