Well Log Anomalies Caused by Gas Hydrate Dissociation in the Shenhu Area, Northern South China Sea

Abstract

AbstractThe drilling GMGS‐1 reveals that gas hydrates are present above a stable zone with a thickness of 10 m at site SH3 in the Shenhu area, northern South China Sea. At this place, the gas hydrate saturation estimated from observed pore‐water chloride values is up to 26% in the pore space. Higher resistivity, lower P‐wave velocity and slightly increasing density characterize the gas hydrate‐bearing sediments. To analyze this low P‐wave velocity, the simplified three‐phase Biot‐type equation (STPBE) is used to calculate the water‐saturated P‐wave velocity. The results show that at the depth of 195 m, the P‐wave velocity from wireline logging is lower than that of the water‐saturated velocity of the normal sediments. The Archie's parameters used to calculate gas hydrate saturation from resistivity were defined from the cross plot between density porosity and formation factor. Gas hydrate saturation estimated from resistivity using the Archie equation with a=1.1 and m=2.3 is about 5%∼20% of the pore space, with a maximum value 26.8%, which shows that the distribution of gas hydrates is inhomogeneous in vertical direction. The dissociation of gas hydrates (the mixture of free gas and water) may be caused by drilling. Either the in‐situ free gas or gas released from gas hydrate dissociation can cause the low well‐log P‐wave velocity. Due to surface seismic data were acquired before drilling, synthetic seismograms generated using different P‐wave velocities are compared with the real seismic data, which can be used to identify the cause of the low P‐wave velocity observed in the well log.