【摘要】 南海北部神狐海域GMGS-1钻探揭示SH3井天然气水合物位于稳定带上部,厚度约为10m.氯离子异常计算的水合物饱和度最高达26%,高水合物饱和度层出现高电阻率和低纵波速度.为分析该低纵波速度异常,本文基于简化的三相介质理论计算了饱和水纵波速度,在深度195m附近,测量的纵波速度小于饱和水纵波速度.利用阿尔奇公式,基于原位温度、盐度、密度孔隙度和测量的电阻率,利用交会分析确定了该井的阿尔奇常数为a=1.1和m=2.3.基于该参数,利用阿尔奇方程计算的水合物饱和度占孔隙空间5%～20%,局部地层水合物饱和度达26.8%,在垂向上分布不均匀.由于钻探可能导致水合物发生分解而产生游离气,原位游离气和水合物分解产生的气体都能造成低纵波速度异常.由于地震资料采集在测井之前完成,利用不同速度制作合成地震记录并与地震资料进行对比,能够确定水合物稳定带上部的低速异常形成原因.更多还原

【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.更多还原