西沙海域中新世碳酸盐台地的时空分布及其油气成藏模式

摘要

本文基于南海北部西沙海域地震数据及钻井资料,结合碳酸盐台地的边缘特征及其地震反射特征,首次建立了研究区中新世碳酸盐台地边界的三个识别标志:岩性突变界面、断层界面和潮汐水道.在对地震数据系统的解释和钻井资料分析基础之上,厘定了该区生物礁及碳酸盐台地的时空分布,总结其分布规律,西沙海域中新世碳酸盐台地可以分为六期,进一步划分三个演化阶段,分别为中中新世早期的繁盛阶段、中中新世晚期的衰退阶段、晚中新世以来的淹没阶段,各阶段碳酸盐台地的沉积相带发育完整,其中生物礁主要分布台地的西—西南缘.统计结果显示,生物礁及碳酸盐台地全面发育时期的面积达到了80000km2,后期随着台地的衰退而规模有所减小.西沙海域中新世碳酸盐岩分布广泛、规模巨大,而且在发育过程中经历了多期暴露而遭受淋滤,进一步增大了其孔隙度,具备形成优质储层的前提.结合区域油气地质条件分析与探讨,建立生物礁及碳酸盐台地的成藏模式,提出西沙海域碳酸盐台地具有典型“下生上储上盖”的成藏过程,指出西沙海域邻近华光凹陷和中建南盆地北部的碳酸盐台地是油气勘探的两大有利区带.%Based on the seismic and drilling data in the Xisha area in north South Sea,associated with marginal characteristics of carbonate platform,three identification marks have been set up for identifying the boundary of the middle Miocene carbonatic rock platform:lithologic shifts,fault escarpments and tidal channel.The spatial-temporal distribution and its rule of Xisha carbonate platform has been defined using the integrated explanation of seismic and drilling data.The Xisha carbonate platform can be divided into three evolution stages consisting of six periods:middle Miocene peak stage,mid-middle Miocene recession stage,and later middle Miocene submersed sage,each of which developed its own complete sedimentary facies.Reef is mainly distributed in the western and southwestern margins of the platform.Statistics show that reef and carbonatic rock platform covered an area of up to 80000km2 at the peak stage and decreased in size with recession of the platform.The Miocene carbonatic rocks widely distributed in the Xisha waters experienced long-term exposure and leaching,which enlarged the porosity and provided the basis for the formation of high quality reservoirs for gas-oil exploration.According to the analysis of petroleum geological conditions,this study established an accumulation model for reef and carbonate platform,proposed a hydrocarbon accumulation process characterized by "lower-generation,upper-accumulation and upper-capping",and finally outlined the two favorable zones for oil and gas exploration:the Guanghua sag round the Xisha Sea area and carbonate platform in the north of Zhongjiannan Basin.