Calculations of theoretical in situ CH4 were made based upon the concentrations of pressure independent species, i.e., dissolved SO42- and dissolved inorganic carbon, in the pore waters of modern Mississippi delta sediments. The maximum theoretical CH4 value was 4.65 × 105 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) 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 implanting bottom mounted structures.
The occurrence of methane gas in marine sediments may have a pronounced effect upon their acoustical properties,1,2,3 geotechnical parameters,4 and certain seafloor deformational features associated with submarine mudslides5. High concentrations of methane gas are generally found in sediments which accumulate rapidly and contain sufficient organic matter to support anaerobic bacterial decomposition processes. Once the concentration of gas in the sedimentary environment reaches its maximum solubility, gas bubbles appear. It is the existence of the sedimentary gas bubble which perturb otherwise predictable acoustical and geotechnical properties of marine sediments.
Methane accumulation in anaerobic marine sediments appear at depths below which the dissolved sulfate ion has been removed by microbial sulfate reduction.4,6,7,8 Biogenic methane production in the absence of sulfate is a dynamic process which continues with burial throughout the sedimentary column until temperatures exceed about 50 °C. Above this temperature methane and other light-hydrocarbon gases are produced by non-biological, thermocatalytic reactions.
During the early stages of methane production, it is dissolved in the undersaturated interstitial waters. Once the saturation concentration is reached, gas bubbles form. The presence of gas bubbles can increase the pore water pressure which in turn decreases the effective stress resulting in zones of sediment instability. It is important that in regions of rapid sediment deposition, both the accumulation of sedimentary gas and under consolidation are likely to occur simultaneously. This is a common occurrence associated with deltaic sediments world-wide. These phenomenon acting together can significantly increase the pore water pressure and should be considered in design of seafloor mounted structures.
The solubility of methane in aqueous media, within a range of temperatures and salinities which may be encountered in sea water, has been reported by Yamamoto.9 methane solubilities in water and saline solutions at pressures above 100 atmospheres have also been reported.10,11 No data, however, is available on the solubility of methane in a heterogeneous medium of water-sediment-gas. The presence of sediment, especially clay minerals may alter the solubility observed in aqueous media.
