Secondary Microbial Origin of Gas in Giant Cenomanian Pools of Western Siberia

Milkov, Alexei V.^1
1 BP, Moscow, Russia.

Cenomanian pools of Western Siberia contain ~1700 tcf (~33% of world proved gas reserves) of dry (predominantly methane) gas. However, the origin of the gas is controversial. Main hypotheses include (1) migration of thermogenic gas from deeply (>4 km) buried Jurassic-Triassic source rocks, (2) thermogenic origin from early mature coals in Aptian-Albian-Cenomanian sediments and
³ primary microbial origin from reduction of CO2 liberated from dispersed organic matter or coals. Release from groundwater due to Neogene uplift has been proposed to explain enormous volumes of gas in place. Here I suggest (following Goncharov et al., 1983) that Cenomanian gas originated, at least partially, from microbial conversion of thermogenic hydrocarbons to methane during biodegradation of oil and gas in Cenomanian and deeper Albian-Neocomian reservoirs. This hypothesis is supported by both geological and geochemical evidences. For example, several gas pools represent gas caps above accumulations of heavy biodegraded oil (e.g., Russkoe field). Many gas pools contain oil smears visible in cores or identifiable in well-logs. Cenomanian pools are characterized by optimal geological conditions for rapid microbial degradation of petroleum (depths 100-1500m, temperatures 5-50°C, large aquifers of low-salinity water). In many fields with giant Cenomanian gas pools (e.g., Urengoy), biodegraded oils and condensates occur in deeper Albian-Neocomian reservoirs. Seals above these Albian-Neocomian reservoirs are relatively poor and likely allowed gas and oil migration into Cenomanian traps. Gases in Cenomanian pools are too dry (average C₁/C₂₊=0.998, n=238) to have early mature thermogenic origin and are more enriched in ¹³C (average δ¹³C₁=-51‰, n=115) than primary microbial gases. At least some gases have CO₂ significantly enriched in ¹³C. Gases in Cenomanian pools apparently contain more nitrogen than gases in Neocomian pools, and that nitrogen is relatively enriched in ¹⁵N. These geological and geochemical observations support secondary microbial origin (perhaps mainly microbial generation of methane by reduction of CO₂ from biodegraded oil in Cenomanian-Neocomian reservoirs) of giant Cenomanian gas pools of Western Siberia. The study highlights the importance of this yet poorly recognized pathway of natural gas generation.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009