北部湾盆地乌石凹陷断裂系统演化特征及其对油气成藏的控制作用

Evolutionary characteristics of fault system and its control effect on hydrocarbon accumulation in Wushi Sag,Beibuwan Basin

为明确北部湾盆地乌石凹陷断裂系统的演化特征,根据乌石凹陷三维地震资料,结合区域应力旋转变化,分析断裂系统特征及演化规律,建立乌石凹陷斜向伸展块体旋转控制的伸展—滑脱断裂发育模式,研究断裂系统对油气成藏的控制作用。结果表明:古新世至早始新世,伸展应力场与先存的NE向断裂接近垂直,断裂主要表现为拉张演化特征;中—晚始新世至渐新世,伸展应力场发生顺时针旋转,与先存的NE向断裂呈锐角相交,部分断裂活化,在流沙港组二段顶、底部厚层塑性油页岩沉积背景下,断裂呈斜向伸展与塑性地层共同控制下的伸展—滑脱演化特征;随应力方向继续旋转,滑脱断裂上盘发育调节断裂,形成“似花状”构造样式。乌石凹陷发育的伸展断裂控制始新世烃源岩与大型储集体的展布;伸展—滑脱断裂控制凹陷构造带差异及圈闭群形成与定型,其中滑脱带下降盘地层整体呈“背形隆起”的特征,油气汇聚条件优越,是乌石凹陷油气最有利富集区。该结果对乌石凹陷油气勘探及有利区带优选具有指导意义。

In order to clarify the evolution characteristics of the fault system in Wushi Sag,Beibuwan Basin,the author basing on 3D seismic date in Wushi Sag,combining with the changes of the regional stress field,analyzed the characteristics and evolution law of fault system,established the rotation of the extension-detachment fault development model controlled by the oblique extensional block in Wushi Sag,and studied its control effect on hydrocarbon accumulation.The study shows that:from the Paleocene to the Early Eocene,the extensional stress field is nearly perpendicular to the existing north-east trending faults.During this period,the faults are mainly characterized by extensional evolution;from the Mid-Late Eocene to the Oligocene,the extensional stress field rotates clockwise and intersects the pre-existing north-east fault at an acute angle,which mankes the early fault was activated,and due to the thick plastic oil shale at the top and bottom of the second member of the Liushagang,the faults shows the extension-slip evolution characteristics under the joint control of oblique tension and plastic formation;as the stress continues to rotate,the up fault of the detachment fault develops a tension-torsional adjustment fault,forming a floweriness structural.The developing extensional faults in the sag control the spread of Eocene hydrocarbon source rock and large reservoir collectives;extension-slip faults control the difference of the structural zone and the formation of the trap groups,among them,the throw side of slip zone shows the "anticlinal rise",the gas-oil convergence background is superior,and it is the most favorable enrichment area in the Wushi Sag.This result has guiding significance for gas-oil exploration and zone optimization in Wushi Sag.