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Biogenic methane is produced in rapidly deposited Mississippi delta sediments in concentrations sufficient to create excess pore pressures. These excess pressures, interacting with underconsolidated clays, can induce submarine mudslides and other phenomena which are hazardous to offshore platforms and pipelines. By utilizing a geochemical model for methane production, an estimate can be made of the total amount of gas that could be generated. Calculations of theoretical in-situ CH4 were made on the basis of the concentrations of pressure-independent species, that is, dissolved SO42- and dissolved inorganic carbon, in the pore waters of modern Mississippi delta sediments. The maximum theoretical CH4 value was 4.65 × 105 /SUP> ppm. Depth profiles of observed and theoretical CH4 values were similar. From theoretical CH4 concentrations and the pressure-solubility relationship, a maximum gas-pressure expression was developed. Gas pressures, Po, attained a maximum value of 57.8 × 104 dynes cm-2 (8.5 psi) at the depth of 20.4 m below the sediment-water interface. Because of surface tension, in-situ Po decreases with bubble size. However, near-maximum gas pressures may be released during storm waves, mudslides, or other changes in hydrostatic pressure, where bubble combination can occur. Gas pressures are important in decreasing the effective stress, especially in regions of rapid sediment deposition, and should be considered when implantin bottom-mounted structures.
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