Abstract: Shallow Gas Accumulations--Their Origin and Significance for Future Exploration

Dudley D. Rice

Large resources of natural gas are present at shallow depths in the northern Rocky Mountains and elsewhere. This gas, predominantly methane, is the product of the immature stage of hydrocarbon generation. The methane is generated by the breakdown of organic matter by anaerobic bacteria and consequently is called biogenic gas. Reduction of CO₂ is the principal mechanism of methane generation under anoxic conditions. The accumulation of biogenic gas is favored by rapid deposition of organic-rich sediments and by the presence of a reservoir or seal either during peak generation or later when uplift and erosion cause exsolution of gases from formation waters. In deep-marine or permafrost environments, the loss of gas is prevented by stabilization of the solid metha e hydrate. Ancient gas of predominantly biogenic origin is characterized by the enrichment of the light isotope C¹² in the methane (^dgr C¹³ < -58^pmil) and by large amounts of methane relative to ethane and higher hydrocarbons (C₁/C₁ - _N > 0.98). Future exploration for gases of biogenic origin is important for several reasons. Generation of biogenic gas is widespread because it is controlled by biologic activity at shallow depths and is not dependent on factors of temperature and geologic time. Biogenic gas is generated in both marine and nonmarine sediments. The economics of drilling and the increasing wellhead price of natural gas make shallow accumulations an attractive exploration target. Finally, present worldwide reserve figures indicate that similar large resources are yet to be discovered.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA