ESTIMATION OF GAS HYDRATE AND FREE GAS SATURATIONS

The amount of gas hydrate is estimated from S-wave velocities because P-wave velocities are more affected by free gas. In order to accurately predict S-wave velocities and to estimate saturations of gas hydrate, the BGTL with a depth-dependent exponent n (using depth-dependent differential pressure under normal hydraulic pressure), a depth-dependent G (using clay volume content and estimated C_h), and parameters shown in Table T1 are used. Saturations of gas hydrate are also estimated from electrical resistivity logs using the Archie relationship.

Figure F6 shows the estimated gas hydrate saturations from S-wave velocities (solid curve) and from the resistivity logs (dotted curve) within the GHSZ. The general trend and the magnitude of gas hydrate amount estimated from both S-wave velocity and resistivity logs are comparable. The average saturations of gas hydrate estimated from S-wave velocities are 10.2%, 10.4%, and 6.1% at Sites 1244, 1245, and 1247, respectively; those estimated form resistivities are 6.5%, 7.9%, and 4.5%, respectively (Table T2). Estimates from velocities are higher than those from resistivities. The largest discrepancy occurs at Site 1244, where the average gas hydrate estimated from S-wave velocity is >50% higher than that estimated from the resistivity logging data.

The amount of free gas can be estimated from two different methods using elastic velocities. The first method applies the BGT with parameters estimated from the BGTL to the P-wave velocity, as indicated in Lee (2004). The second method estimates free gas directly from the elastic moduli of sediments.

Because the shallower sedimentary sections on Hydrate Ridge contain gas hydrates, the effect of gas hydrate on P-wave velocity needs to be accounted for in the estimation of free gas. This is accomplished by incorporating the amount of gas hydrate estimated from S-wave velocity into the P-wave velocity model. Figure F7 shows the estimated free gas as solid curves, whereas Table T2 contains the statistics of the estimated free gas saturations. The average saturations of free gas, assuming a patchy distribution with e = 8 (equation 11), are estimated to be 0.6%, 1.4%, and 1.5% for Sites 1244, 1245, and 1247, respectively. The above free gas estimations are based on the P-wave velocity only. However, if both V_P and V_S are available, the amount of free gas can be estimated directly from the moduli of the sediments (Brie et al., 1995; Murphy et al., 1993). From equations 1 and 8, the Biot coefficient for partially gas-saturated sediments can be written as follows:

_bgt = 1 – (µ_bgtl/µ_ma) = 1 – (V_S²/µ_ma), (15)

where

V_S = observed S-wave velocity, and

= observed density.

From equation 3, the bulk modulus of formation can be written as

k = (V_P² – 4V_S²/3) = k_ma(1 – _bgt) + _bgt²M. (16)

From equations 15 and 16, M can be calculated; gas saturations can be calculated from M and equation 9.

Figure F7 also shows free gas saturations estimated from elastic moduli; these are displayed as dotted curves. Because the effect of gas hydrate is not accounted for in this analysis, we can only compare the estimated free gas saturations below the GHSZ; within this interval the two methods yield similar estimates of gas saturations.