井震联合分析预测神狐海域天然气水合物可能的垂向分布

Integrated analysis of well logs and seismic data to deduce the possible distribution in depth of gas hydrate in Shenhu Area,South China Sea

已有的地质与地球物理信息表明南中国海北部陆坡神狐海域存在天然气水合物,2007年钻井和取样结果证实其以细颗粒均匀存在于水合物稳定带内,但区域分布范围极不规则.由于测井记录长度和取样深度的范围有限,天然气水合物垂向分布特征难以确定.为了深入了解此区域天然气水合物深度上的分布规律,对SH7井的多种测井数据进行综合分析并与地震反射特征建立联系,得到三个主要的沉积物性变化层位并以此为基础进行叠后数据的伪井约束反演,结果表明在已证实的含水合物层之下还存在高速层.而AVO叠前正演模拟分析结果与实际地震数据对比表明,研究区的天然气水合物稳定带可能由含水合物沉积物与饱水沉积物的薄互层组成,这就意味着在取样获得天然气水合物样品的深度之下,很可能存在另一个含水合物层.

The available geological and geophysical information indicates the presence of gas hydrate at Shenhu Area of Northern South China Sea.The drilling cores recovered in 2007 testify the expectation and reveal that the gas hydrate disseminates evenly within the hydrate stable zone（HSZ）,but its regional distribution is inhomogeneous.However,the logging record length and coring depth coverage are limited,which causes the depth distribution of gas hydrate to be indeterminate.Logs at well site SH7 were used to link the information of physical properties ofsediments to the coincident seismic data acquired in this area.According to the relation between physical properties of sediments and seismic reflection characteristics,three primary formations were calibrated and the pseudo-well constrained impedance inversion was then applied on poststack seismic data.The results showed a high velocity zone below the interval bearing gas hydrate.To determine whether this high velocity zone is related to the occurrence of gas hydrate or not,the technique of AVO forward modeling was further used to analyze the reflecting characteristic of pre-stack seismic data.The comparison between synthetic seismic models and real seismic data indicates that the HSZ was composed of thin interbeds of gas hydrate-bearing sediments and water-saturated sediments,which suggests that another gas hydrate deposit exists most likely below the depth where gas hydrate-bearing sediments were cored in the hole.