珠江口盆地文昌A凹陷中深层原油来源及成藏特征

摘要

The first potential commercial oil discovery in middle-deep strata of Wenchang A sag in Pearl Riv-er Mouth basin is obtained in Zhuhai and Enping Formations of the Oligocene in W10-8 structure in the downthrown block of Zhu-Ⅲ south fault,and high productive oil flow is tested,which changes the gas prone exploration situation in Paleogene Wenchang A sag.W10-8 structure is systematically analyzed with organic geochemistry,fluid inclusion stratigraphy and reservoir sedimentology.Results show that:①oil geochemical characteristics vary vertically;the oil density is low and ranges from 0.776 to 0.817g/cm3 at 20℃;the oil is mainly from Eocene Wenchang Formation and Lower Oligocene Enping Formation source rocks.②the middle-deep traps in downthrown block of Zhu-Ⅲ south fault are adjacent to the hydrocarbon generation center of high quality lacustrine source rocks of Wenchang and Enping Formations,and multi-stage charging processes and mixed accumulations exist.③the downthrown block of Zhu-Ⅲ south fault develops multi-cycle fan deltas with wide scale and large thickness in the Oligocene,causing good reservoir properties even at great depth(>3900m),which increases the lower depth limit of effective reservoir in Wenchang A sag.④Zhu-Ⅲ south fault and is secondary faults provide effective hydrocarbon migration pathways from source rocks to traps vertically,leading to multiple oil-gas accumulations in different stra-ta.The research results show that the Paleogene in Wenchang A sag and its adjacent areas have the basic geological conditions for hydrocarbon migration and accumulation,and the Oligocene is an important target for oil exploration,which extends a new domain for oil exploration in middle-deep strata in this area.%珠江口盆地文昌A凹陷近年钻探的W10-8构造首次在珠三南断裂下降盘中深层渐新统恩平组和珠海组获得潜在商业原油发现并测试获高产油流,改变了该地区长期以来以找古近系气藏为主的勘探局面.运用有机地球化学、流体包裹体地层学、储层沉积学等分析技术对该油藏进行系统剖析,结果表明:①钻获原油的地球化学特征在纵向上存在差异,油质总体较轻,密度(20℃)为0.776~0.8 1 7 g/cm3 ,为来自始新统文昌组和下渐新统恩平组烃源岩的双重混源;②珠三南断裂下降盘中深层圈闭紧邻文昌组和恩平组湖相优质烃源岩生烃中心,烃源供给充足,油气存在多期充注混合成藏;③渐新世珠三南断裂下降盘发育多期规模广、厚度大的扇三角洲储集体,且在埋深较大(>3 900 m)的情况下仍具有较好的储集物性,加大了文昌A凹陷有效储层埋深下限;④珠三南断裂及其派生断层有效沟通圈闭和烃源岩,在不同层系形成复式聚集成藏特征.这些研究成果证实了文昌A凹陷及周缘古近系具备油气运聚成藏的基本地质条件,揭示了渐新统可作为重要的原油勘探层系,从而开拓了该地区中深层找油新领域.