BASELINE CONDITIONS

In order to estimate the amount of gas hydrate or free gas in sediments, elastic parameters for sediments without gas hydrate or free gas are required; these parameters serve as baseline conditions (no gas or gas hydrate in pore space) for estimating hydrocarbon saturations. One interesting aspect of CWL velocity logs is that the measured V_P is much lower than the velocities anticipated for gas-hydrate-bearing sediments, particularly for Sites 1245 and 1247. It is speculated that the low V_P is indicative of free gas in the sediment, which is either the free gas naturally coexisting with gas hydrate in the GHSZ or gas that has been dissociated from gas hydrate during drilling. If the amount of free gas is small, the effect on V_S is negligible. Therefore, in the case that a small amount of free gas exists in sediments, V_S is more appropriate to derive the baseline conditions for non-gas-hydrate-bearing sediments.

Figure F4 shows the velocities measured below the GHSZ at each site with modeled velocities using the BGTL. S-wave velocity predicted from the BGTL with m = 2, p = 2 MPa, and C_v = 0.2 (solid line) follows the average trend of the measured S-wave velocities at Site 1245 (open circles). For velocities at Site 1244, the BGTL with m = 1.3 (dashed line) matches well with the measured S-wave velocities (green dots). The BGTL with m = 1.8 (not shown) is optimal for velocities at Site 1247. However, corresponding P-wave velocities predicted from the BGTL indicate that predicted P-wave velocities are higher than observed P-wave velocities, implying the presence of free gas in sediments.

Therefore, m = 1.3, m = 2, and m = 1.8 are used for the BGTL parameters at Sites 1244, 1245, and 1247, respectively.

Figure F5 shows electrical resistivity vs. LWD density-derived porosities for all measured depths for the three boreholes. The resistivity of formation (R_t) is given by the Archie relation

R_t = aR_w ^–j, (12)

where

a, j = Archie parameters, and

R_w = the resistivity of connate water.

The baseline resistivities for all three boreholes are interpreted based on the trend of resistivity increase with respect to porosity decrease and are given by

R_t = 0.55 ^–1.3 (13)

The exponent 1.3 is less than the Humble relationship (where the exponent = 2.15), but it is in the low end of the range of cementation factor shown by Jackson et al. (1978). The Archie parameter a and the resistivity of connate water (R_w) is absorbed in the apparent resistivity

R_a (R_a = aR_w), (14)

and R_a = 0.55 ·m is used for all the analysis of resistivity logs.