ANALYSIS OF GEOTHERMAL DATA

Temperature，pressure and gas composition control the occurrence and distribution of gas hydrates.The existence of gas hydrates is related to water depth，seabed temperature and geothermal gradient from the phase diagram of gas hydrates stability（Fig.1of reference［13］）And there is difference between the pure water phase and seawater phase curves［13～15］.Based on the gas hydrates phase curves of pure and sea water，the thickness of the gas hydrates stability zone at site 1144and 1148can be calculated from measured water depth，seabed temperature and geothermal gradient，converted to TWT through time-depth transform，and then compared with the seismic BSR depth.

At site 1144，the water depth is 2037m；the seabed temperature is 3.1℃and the geothermal gradient is 24℃/km.The thickness of GHSZ is 800m and 0.89s of TWT calculated from the pure water phase curve.The thickness of GHSZ is 720m and 0.78s of TWT calculated from the seawater phase curve.The BSR depth on the seismic profile is 0.72s so that the thickness of the GHSZ is consistent with BSR depth.(https://www.daowen.com)

At site 1148，the water depth is 3294m；the seabed temperature is 3.5℃and the geothermal gradient is 83℃/km.The thickness of GHSZ is 250m and 0.29s of TWT calculated from the pure water phase curve.With respect to the seawater phase curve，the thickness of GHSZ is 230m and 0.27s of TWT.BSR depth on seismic profile is 0.48s and 475m based on the average logging velocity 1985m/s.The base limit of GHSZ is 475m from the sonic logging data.Therefore，the thickness of GHSZ based on sonic logging data is consistent with the BSR depth from the seismic profile.However，it is different from the result from the geothermal data.The BSR geothermal gradient is 43.8℃/km from the pure water phase curve and 40.8℃/km from the seawater phase diagram and is close to the average geothermal gradient 39.35℃/km in Yingqiong and 42.07℃/km in Yinggehai basin.

In general，the thickness of GHSZ from geothermal data at site 1144is consistent with seismic BSR depth.There is difference between the thickness of GHSZ from geothermal data and the seismic BSR depth at site 1148.Nevertheless，the analysis of sonic logging data is consistent with the seismic BSR depth and the thickness of GHSZ from average geothermal gradient on the north margin of the SCS.