Generation of Gases for Hydrate Formation

FIGURE 7.3 Stages of organic matter oxidation in anoxic sediments. (Reproduced from Hesse, R., Geosci. Can., 14, 165 (1986). With permission from the Geological Association of Canada.) 

Below the stage of methane production is the fermentation (fifth) stage, followed by the sixth, thermocatalytic stage at still greater depths. The initial five stages require the presence of bacteria below stage five, bacterial activity ceases 

Carbonates are generated in all of the zones, but they are consumed only below the third stage. After the depletion of more than 80% of the sulfate, the components of XCO2 become oxidants, leading to the production of methane. It is possible to distinguish between the CO2 produced and the CO2 consumed, determining the extent of carbon isotopic fractionation, particularly associated with the methane production in stage four. 

Methane formed by biogenic processes ranges in 13C from about — 60%c to —85%c, while methane from thermogenic processes ranges from —25%c to —55%c (Hunt 1979, p. 25). 

Gas Characteristics of Gulf of Mexico and Caspian Sea Hydrate Samples 

Biogenic methane is produced as an end product of the metabolism of a diverse group of 

Deeper in the sediment, thermal alteration of organic matter generates methane and higher order 

Biogenic and thermogenic gases can usually be distinguished on the basis of chemical and isotopic composition. Biological gas is dominantly composed of methane, which is depleted in relative to thermogenic methane (Whiticar 

Isotopic discrimination, nevertheless, should be used with caution. Various environmental factors such as substrate limitation and temperature may obscure the 5 C distinction between thermogenic and biogenic sources. In addition. 

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