丽水凹陷基岩岩性分布预测——以丽水A洼为例

Prediction of lithologic distribution of basement in the Lishui Sag:a case study of the Lishui Sub-sag A

东海陆架盆地丽水凹陷基岩潜山十分发育,钻井已证实其含油气性,勘探潜力巨大。综合现有基岩岩芯、铸体薄片、测井、测年、三维地震资料,在构建基岩岩相-地震相模板及岩石物理分析的基础上,对丽水凹陷基岩岩性开展定性和半定量预测,进而预测岩性平面分布特征。丽水凹陷基岩主要由片麻岩、花岗片麻岩、花岗岩组成,片麻岩为古元古界变质岩陆核,花岗岩来自中生代岩浆,沿深大断裂上涌侵入。地震剖面上片麻岩表现为中−高频,中−强振幅,同相轴连续性、成层性好;花岗岩表现为中−低频,弱−中振幅,呈杂乱反射;花岗片麻岩地震相介于前二者之间。岩石物理分析结果显示,密度参数对岩性区分效果最好,叠前密度反演能较好地半定量预测基岩分布特征。研究表明,丽水A洼主体基岩纵向呈片麻岩上覆花岗岩的二元结构特征;平面上不同构造单元的基岩顶面岩性各异,凸起区花岗岩剥蚀程度高,基岩为片麻岩或花岗片麻岩,洼槽区缺乏深大断裂,为原始区域片麻岩基底,其他区域为花岗岩。

Basement is well developed in the Lishui sag,East China Sea Shelf Basin.Previous wells showed good oil and gas properties and great potential for further exploration.Using well cores,thin sections,zircon U-Pb dating,well log data,and 3D seismic data,we combined the facies-seismic model and petrophysical analysis to predict the basement facies qualitatively and semi-quantitatively and their lateral distributions.Results show that the basement of the Lishui Sag is composed of mainly gneiss,granitic gneiss,and granite.The gneiss is the Proterozoic metamorphic continental core,and the granite was the intrusive of the Mesozoic magma via deep and large faults.Seismic characteristics of gneiss show continuous bedding planes with medium-high frequency and medium-high amplitude,while granite shows chaotic reflectors with low-medium frequency and low-medium amplitude.Granitic gneiss displays seismic characteristics between these two.The petrophysical analysis indicate that the density is most effective parameter in differentiating lithologies,and pre-stack density inversion could predict better the distribution of basement rocks semi-quantitatively.It is suggested that the granite in the Lishui Sub-sag A is overlain by metamorphic rocks,and the facies in the basement top varies laterally in different tectonic units.The granite in the uplift regions has been severely eroded,and left with only gneiss and granitic gneiss basement.The places lacking deep and large faults in the sag are made up of metamorphic basement,while other areas are characterized by granitic basement.