Calculation of Methane Hydrate Solubility in Marine Environment and Its Constraints on Gas Hydrate Occurrence

Abstract Besides appropriate temperature and pressure ranges, sufficient gas supply is necessary for gas hydrate crystallization within gas hydrate stability zone. In this article we formulate the calculation of methane solubility assuming hydrate plus‐aqueous solution plus free gas, hydrate plus‐aqueous solution, or aqueous solution plus free gas coexist at equilibrium. There are four fields of methane phase (free gas, dissolved gas, hydrate, and hydrate + free gas) in the diagram of methane solubility versus depth. The contents of hydrate and free gas at Sites 1249, 1250 and 997 are computed. Calculated hydrate contents are 10%~61% (13.5~72.4 mbsf) and 0.7%~1.9% (35~106.5 mbsf) in sediment pores at Site 1249 and at Site 1250, respectively. The layer of free gas, 22 m in thickness and ~2.4% in sediment pores, immediately occurs below the gas hydrate zone at these two sites. At Site 997, hydrate occurs from 202.4 to 433.3 mbsf occupying about 5%~7% of porosity. The free gas layer is ~80 m thick below the hydrate layer methane gas occupying about 0.2%~28% of sediment pore space.