南海神狐海域含水合物地层测井响应特征

分析了南海北部神狐海域含天然气水合物沉积层声波速度及密度的分布特征和变化规律,并通过对比DSDP 84航次570号钻孔含天然气水合物层段测井资料,总结出神狐海域含水合物地层的测井响应规律特征:神狐海域含水合物地层存在着明显的高声波速度、低密度特征,地层密度随声波速度的变化并不是单一的反比例关系,总体趋势上随声波速度的升高而降低;含水合物地层高声波速度值主要集中在197~220 m段,饱和度值在15%~47%之间,低密度值集中在200~212 m段,分布在水合物饱和度大于20%的地层内;含水合物地层声波速度平均值为2 076 m/s,其上覆和下伏地层的声波速度平均值为1 903 m/s和1 892 m/s,所对应的地层密度值分别为1.89 g/cm3、1.98 g/cm3和2.03 g/cm3,声波速度受孔隙度和饱和度的共同影响,地层密度受水合物饱和度影响较大;从水合物上覆地层到声波速度最高值段,声波速度值增加了9.1%,相对应的地层密度值减少了4.55%,从水合物声波速度最高值段到下伏地层,声波速度值减少了8.86%,相对应的地层密度值增加了7.41%。这些测井响应特征,可用来识别地层中天然气水合物,并可以用来计算水合物的饱和度,同时结合其他地质和地球物理资料,确定水合物层的厚度、分布范围,计算天然气水合物的资源量。

Logging Response Characteristics of Gas Hydrate Formation in Shenhu Area of the South China Sea

With analysis of the distributing feature and the changing law of the sonic velocity and density of sediments with gas hydrate in Shenhu Area of the South China Sea,by comparing with logging data of gas hydrate formation at Site 570 of DSDP 84,the logging response characteristics of gas hydrate formation in Shenhu Area were conducted.The results show that:(1)Sediments with gas hydrate in Shenhu Area have the clear features that the sonic velocity is high and density is low,and the density is generally decreased with the increasing of the sonic velocity.(2)The high sonic velocity section in sediments with gas hydrate is in the depth of 197 to 220 m,and the low density section is in the depth of 200 to 212 m;the value of saturation is in the range of 15%to 47%.(3)The average sonic velocities of sediments with gas hydrates and the overlying strata and underlying strata are 2,076 m/s,1,903 m/s and 1,892 m/s,respectively,and the corresponding densities are 1.89 g/cm3,1.98 g/cm3 and 2.03 g/cm3,respectively;sonic velocity is effected by porosity and saturation,and density is greatly influenced by saturation;(4) The sonic velocity increases by 9.1% from the overlying strata of hydrates to the maximum-value section,and the corresponding density decreased by 4.55%;the sonic velocity decreases by 8.86% from the maximum-value section to the underlying strata of hydrates,and the corresponding density increases by 7.41%.The logging response characteristics can be used to identify the existence of gas hydrates and to calculate the saturation of gas hydrate,furthermore,to provide accurate area and thickness of gas hydrate zone for calculating gas hydrates reserves with combination of other geological and geophysical data.