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.