南海北部神狐海域天然气水合物分解的测井异常

doi:10.6038/cjg20130828

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Abstract

During the gas hydrate drilling expedition, GMGS-1 indicated that gas hydrate was discovered above the gas hydrate stability zone with a thickness of 10 meters at site SH3 in the Shenhu area, the northern slope of South China Sea. Gas hydrate saturations derived from observed pore-water chloride values in core samples reach the highest value of 26% in the pore space. Higher resistivity, lower P-wave velocity and slightly increasing density were identified. To explain the lower P-wave velocity, the Simplified Three-Phase Biot-type Equation (STPBE) was used to calculate the water-saturated P-wave velocity. 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 Archie equation with a=1.1 and m=2.3 is about 5%~20% of the pore space, with a maximum value of 26.8%, which shows that the distribution of gas hydrate is heterogeneous in vertical. The dissociation of gas hydrate (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. Because surface seismic data were acquired before drilling, synthetic seismograms generated using different P-wave velocities were compared with the seismic data, which can be used to identify the cause of the low P-wave velocity observed in the well log.

Key words： Gas hydrate Saturation Dissociation P-wave velocity Shenhu area

Received: 12 July 2012

PACS:

P738

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	WANG Xiu-Juan
	WU Shi-Guo
	WANG Ji-Liang
	YANG Sheng-Xiong
	WANG Zhen-Zhen

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http://www.geophy.cn/EN/10.6038/cjg20130828 OR http://www.geophy.cn/EN/Y2013/V56/I8/2799

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