Shallow, methane-rich gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms in accumulating marine sediments, and is referred to as biogenic gas. The most efficient biologic mechanism of methane generation is anaerobic respiration and CO₂ reduction. The factors that control methane production after sediment burial are: anoxic and sulfate-deficient environments, low temperature, availability of organic matter, and sufficient pore space. The timing of these factors is such that most biogenic gas is generated prior to burial depths of 1,000 m. Biogenic gas is characterized by enrichment of the light isotope ¹²C in the methane (^dgr¹³C₁ lighter than -55 ppt) and by large proportions of m thane (C₁/C_1-5 > 0.98).

In marine sediments most of the biogenic gas formed is retained in solution in the interstitial (pore) waters. This can be attributed to higher methane solubility at the higher hydrostatic pressures and lower temperatures due to the weight of the overlying column of cool seawater. This dissolution capacity permits accumulation of gas in nonindurated sediments while they are being compacted, and while traps and seals are being formed. Free gas occurs either when the solubility minimum is exceeded, or by exsolution brought about by a reduction in hydrostatic pressure or an increase in temperature. Possible trapping and sealing mechanisms are as follows: early carbonate cementation in zones of high CO₃²-activity resulting from reduction of CO₂, low-permea ility reservoirs, bentonites, gas hydrates, and subnormal pressures.

More than 20% of the world's discovered gas reserves are of biogenic origin and commonly are associated with marine sequences. Accumulations of ancient biogenic gas have been discovered in Canada, Germany, Italy, Japan, Trinidad, U.S.A., and U.S.S.R. The accumulations occur in Cretaceous and younger rocks and at depths of burial less than 3,350 m.

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