DOWNHOLE LOGS

During Leg 204, conventional downhole wireline logs (CWL) and logging while drilling (LWD) logs were acquired (Shipboard Scientific Party, 2003). The logs are not necessary obtained from the same hole; however, the holes are always very close to each other and the best quality logs from the various holes were used for the analysis. The logs used in this analysis include density, P-wave velocity, S-wave velocity, natural gamma ray, electrical resistivity, and NMR logs acquired at Sites 1244, 1245, and 1247 (Holes 1244E, 1245E, and 1247B are CWL holes and Holes 1244D, 1245A, and 1247A are LWD holes).

Because the density-derived porosity from CWL was degraded more than that from the LWD logs owing to borehole rugosity, LWD density–derived porosity logs are used in all analyses, assuming a two-component system with a matrix density of 2.65 g/cm3 and a water density of 1.03 g/cm3. Porosity correction due to the presence of gas hydrate is accomplished simultaneously with the estimation of gas hydrate saturations (Lee and Collett, 1999).

Clay volume content is calculated from the gamma log using the formula pertinent to tertiary clastic with Gcn = 20 (API units) and Gsh = 120 (API units), where Gcn and Gsh are gamma log response in a zone considered clean and log response in a shale bed (Western Atlas, 1995), respectively. The average clay volume contents for Site 1244 (depth range = 85–226 mbsf), Site 1245 (depth range = 85–294 mbsf), and Site 1247 (depth range = 74–202 mbsf) are 13%, 18%, and 15%, respectively.

Figure F2 shows a relationship between LWD density–derived porosity and depth with several empirical compaction curves. The trends in the measured porosities vs. depth at three sites are similar for depths shallower than 250 mbsf. Abnormal porosities at Site 1244 for depths deeper than 250 mbsf are due to the degraded quality of the density logging data below 250 mbsf. Observed compaction trends lie between the compaction curve for shale, derived by Baldwin and Butler (1985), and the compaction curve for terrigenous marine sediments, derived by Hamilton (1976). Compaction curves by Sclater and Christie (1980) for sand and shale are approximately lower and upper limits of observed compaction trends.

The velocity ratio, VP/VS, with respect to the LWD density–derived porosity is shown in Figure F3. Figure F3A shows the measured velocity ratios for entire logged intervals and Figure F3B shows measured ratios only for gas hydrate intervals, as interpreted by Collett et al. (this volume). At each well, two distinct modes of velocity ratios with respect to porosity are indicated. For a given density-derived porosity, velocity ratios within the gas hydrate interval (Fig. F3B) are higher than those outside the interval for all wells, indicating either the effect of free gas in sediments on elastic velocities or the difference in the degree of consolidation. Also at a given porosity, velocity ratios at Site 1244 are higher than those at Sites 1245 and 1247, indicating that the degree of consolidation is less at Site 1244. This implies that the BGTL parameter n or m for Site 1244 is different from those for Sites 1245 and 1247. As indicated in Figure F3B, the exponent n appropriate for gas hydrated sediments at Site 1244 is between n = 1.5 and n = 2.1, whereas it is between n = 1 and n = 1.5 for Sites 1245 and 1247.

In summary, the following observations can be made for density and velocity logs (only CWL logs are used in this analysis):

  1. LWD density–derived porosity generally follows the compaction curve and shows limited scattering in the LWD-derived porosities in the gas-hydrate-bearing interval (shallower than ~130 mbsf), suggesting that the density log–derived porosities are of high quality.
  2. Each well log shows two distinct groups of Poisson's (and VP/VS) ratios: one for the upper sedimentary section, affected by gas hydrate, and the other for the lower sediment section, affected only by free gas. The decrease of Poisson's ratio at each well from the upper to the lower section is caused partially by compaction and partially by the effect of free gas on velocities.
  3. For a given porosity, the VP/VS ratio at Site 1244 is highest, whereas the lowest ratio occurs at Site 1245; this implies that the BGTL parameter, m, is lowest at Site 1244 and highest at Site 1245, which in turn implies that sediments at Site 1244 are less consolidated than those at Site 1245.

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