琼东南盆地长昌凹陷中部伸展-走滑复合断裂带及其油气勘探意义

The extension-strike-slip composite fault belts in central Changchang Sag of Qiongdongnan Basin and their significance for oil and gas exploration

长昌凹陷位于琼东南盆地深水区东部,是重要的油气勘探潜力区。基于高精度三维地震资料,开展了长昌凹陷中部复杂断裂的精细解释。根据断裂发育位置、组合形态、构造样式,结合区域应力特点的综合分析,认为凹陷中部发育2条NWW向的伸展-走滑复合断裂带,其形成与演化过程可分为3个阶段:早渐新世,受NW-SE向区域拉张应力的作用,发育主干伸展断裂;晚渐新世早期,受南海西南次海盆海底扩张的影响,区域张应力发生顺时针旋转,由NW-SE向转变为SN向,开始产生右旋走滑作用,导致形成一系列次级断裂;晚渐新世晚期-早中新世,长昌凹陷内基底断裂持续活动,同时受凹陷北部、南部2个刚性隆起阻碍作用增强的影响,走滑作用更加强烈,产生了大量的负花状构造。北部断裂带具有右行左阶的走滑性质,断裂带内发育挤压应力环境,断裂封闭性较好,有利于油气聚集;南部断裂带具有右行右阶的走滑性质,断裂带内发育拉张应力环境,有利于油气运移。

Changchang Sag,located in the east of the deep water area of Qiongdongnan Basin,is an important potential area for oil and gas exploration.Based on high-precision three-dimensional seismic data,fine interpretation of complex faults in central Changchang Sag is carried out.According to the development position,combination form and structural style of faults,combined with the characteristics of regional stress,it is considered that there are two NWW trending extension-strike-slip composite faults in the study area,whose evolution can be divided into three stages:(1)in the Early Oligocene,under the action of NW-SE regional tensile stress,the main extension faults are developed;(2)in the early period of Late Oligocene,under the sea floor expansion of southwest sub-basin of South China Sea,the regional tensile stress rotates clockwise,and changes from NW-SE to SN,which produces dextral strike slip,resulting in a series of secondary faults;(3)from the late period of Late Oligocene to Early Miocene,the basement faults in Changchang Sag are continuously active,and the strike slip becomes more intense,resulting in the development of a large number of negative flower structure due to the enhanced obstructing of two rigid uplifts in the north and south of the depression.The north fault belt is characterized by right-slip faults with left step,which has good sealing due to the extrusion stress.The south fault belt is characterized by right-slip faults with right step,which is conducive to the hydrocarbon migration due to the tensile stress.