Influence of Water Solubility, Phase Equilibria, and Capillary Pressure on Methane Occurence in Sediments
George E. Claypool
Microbial methane is generated in rapidly accumulating marine sediments (>40 m/my) where pore waters are deficient in dissolved oxygen and sulfate. Based on indirect geochemical evidence, microbial methane generation is largely confined to depths of between 10 and 1000 meters beneath the sea floor. Under shelf conditions (water depth <200 m), methane concentrations can exceed solubility in pore water and accumulate as free gas, or escape the sediment as bubbles, or be oxidized in surface sediments. Under some deeper- water conditions of continental slope and rise sediments, more of the methane can be retained and buried because of increased solubility, and because methane in excess of solubility can be stabilized as methane hydrate.
Few direct measurements of methane concentration in subsurface pore waters have been made. However, methane-water phase transitions (gas-water contacts, base of gas hydrate reflector) on seismic records can be used with methane solubility relationships to estimate gas contents of sediments. Comparison of various environments shows a relatively narrow range of dissolved methane contents. In marine sediments, free gas (and methane hydrate) is stable only in contact with methane-saturated pore water. Finergrained sediments can be supersaturated with respect to a gas (and gas hydrate?) phase because of capillary pressure inhibition of bubble (or hydrate?) formation. The amount of methane dissolved in marine sediment pore water is necessarily larger than that present as gas hydrate.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California