Quantification of the Origin of Conventional Natural Gases on the Global Scale

Milkov, Alexei V.¹
(1)BP, Moscow, Russian Federation.

The current view is that primary microbial gas (predominantly methane with δ13C <-55‰) formed from decomposition of sedimentary organic matter accounts for ~20% of the world’s natural gas resources and the rest is catagenetic oil-associated (30%) and deep metagenetic non-associated (50%) thermogenic gas. Recently, experiments demonstrated that anaerobic biodegradation of oil results in formation of secondary microbial methane and such gas was documented in subsurface accumulations worldwide. Here, I quantify the amount of primary microbial, secondary microbial and thermogenic gas in the global endowment of conventional recoverable free and oil-dissolved gas, which is estimated at 15,401 tcf (Ahlbrandt et al., 2005). Free primary microbial gas was calculated based on published data for provinces and individual fields with 12C-enriched methane. Oil-dissolved primary microbial gas was estimated based on global statistics describing the distribution of such gas. Total recoverable primary microbial gas (407-589 tcf) accounts for ~3-4% of worldwide conventional gas endowment. Evidence of secondary microbial gas occurs in around 40 basins with biodegraded oils. Free secondary microbial gas was estimated based on the assumption that 40-80% of all gas in free gas and gas cap accumulations within the global biodegradation zone is secondary microbial. The amount of secondary microbial methane dissolved in biodegraded oil pools was estimated using global statistics about gas-oil ratio in pools with various biodegradation and reasonable assumptions about fraction of secondary microbial methane in total dissolved gas. The total global endowment of recoverable secondary microbial gas may be 845-1644 tcf, which accounts for ~5-11% of the global conventional gas endowment. The vast majority of the conventional gas endowment (85-92%) has thermogenic origin.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.