The deep-water sedimentary basins of the Bering Sea may hold a significant quantity of natural gas, both within natural gas hydrate and as free gas below the gas hydrate stability zone (Scholl and Hart, 1993). To facilitate estimation of gas quantities in this environment, a suite of volume expansion ratios and ideal gas deviation factors, or z-factors, for 100% methane gas have been calculated using the Peng-Robinson equation of state (Peng and Robinson, 1976). Volume expansion ratios relate free gas volume under in-situ pressure (P) and temperature (T) conditions to free gas volume at standard conditions. Z-factors relate the real gas behavior to that predicted by the Ideal Gas Law.

Constant-pressure and constant-temperature graphs with a table of calculated values are presented for pressures in the range 30 to 52 MPa and temperatures from 4° to 80°C. These ranges are relevant to the sub-bottom conditions to ~1.4 km below seafloor in the Aleutian and Bowers Basins. A data table and graphs are included representing volume ratio and z-factor as a function of sub-bottom depth along the estimated P-T path for the upper 1.2 km of the sedimentary section of the central Aleutian Basin. In addition, a contour plot of volume expansion ratio is presented for general reference for 100% methane gas in the pressure range 1 to 60 MPa and temperature range 0° to 80°C.

For free methane gas near the base of the hydrate stability zone at 360 m below seafloor in the Bering Sea, under conditions of 3,600 m water depth and 60°C/km geothermal gradient, the ratio of gas volume at standard conditions to gas volume in-situ is ~365. By comparison, natural gas hydrate has an expected free gas yield of ~164 volumes at standard conditions per volume of solid hydrate dissociated (Collett, 2000). This means that a given volume of free gas immediately below the base of hydrate stability contains more than twice the number of molecules of methane as an equal volume of the overlying natural gas hydrate. In the temperature range 0° to 80°C, free gas and solid gas hydrate will contain approximately the same number of molecules of methane per unit volume at pressures between 12 and 20 MPa, as an increasing function of temperature. This is equivalent to the hydrostatic load ~1.2 to 2 km depth below sea surface. Throughout the range of conditions relevant to the deep-water Bering Sea, the z-factor for the Peng-Robinson equation of state is between 0.9 and 1.1. The Ideal Gas Law, represented by z=1, is therefore a useful first approximation to real gas volumes in this environment.