SUMMARY AND CONCLUSIONS

Based on the physical properties of sediments, such as elastic velocities and electrical resistivity, the amounts of gas hydrate and free gas are estimated from downhole logs at Sites 1244, 1245, and 1247 of ODP Leg 204. In order to accurately estimate the amount of gas hydrate, S-wave velocities, not P-wave velocities, are used with the BGTL. This is done because P-wave velocities are more strongly affected by the gas-hydrate-bearing sediments that are commingled with free gas within the gas hydrate stability zone, possibly owing to the dissociation of gas hydrate during drilling or the migration of free gas through the GHSZ. The amounts of free gas are estimated from P-wave velocities using the BGT with parameters derived from the BGTL and are comparable to those estimated from elastic moduli of sediments. However, the amounts of estimated free gas have a high degree of uncertainty, because the calibration constant e is not well constrained and there is no available independent measurements of free gas amounts. This study shows that:

  1. At Site 1244, the average gas hydrate saturation within the GHSZ is 10% from S-wave velocities, whereas it is 6.5% from electrical resistivities. If the calibration constant of e = 8 is appropriate, average saturations of free gas between 127 and 226 mbsf are negligible (0.4% and 0.3%, based on VP and moduli methods, respectively). However, the free gas estimates are 1.6% and 1.2% if e = 2 is used, which is comparable to that estimated from the NMR log.
  2. At Site 1245, the average gas hydrate saturation within the GHSZ is 10% from S-wave velocities, whereas it is 7.9% from electrical resistivity logs. Average saturations of free gas between 129 and 294 mbsf are 1.6% and 1.7%, based on VP and the moduli methods with e = 8, respectively. However, the free gas estimates are ~6.4% and 6.8% if e = 2 is used.
  3. At Site 1247, the average gas hydrate saturation within the GHSZ is 6% from S-wave velocities, whereas it is 4.5% from electrical resistivity logs. Average saturations of free gas between 129 and 197 mbsf are 1.7% and 2%, based on VP and moduli methods with e = 8, respectively. However, the free gas estimates are ~6.8% and 8% if e = 2 is used.

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