西湖凹陷天台构造走滑-转换体系特征及控藏作用

Characteristics of strike-transition system of Tiantai structure of Xihu Sag and its reservoir-controlling effect

西湖凹陷天台构造发育的复杂弧形断裂体系及其控藏作用十分复杂,制约了该区油气勘探。基于三维地震资料及多口探井实钻成果,深入分析天台构造内多套断裂系统构造特征及其对油气成藏的影响作用。结果表明:①天台构造受NE向发育的基底卷入型走滑断裂和NW向展布的薄层滑脱型转换断裂共同控制,是走滑-转换断裂过渡体系控制下的多期差异反转背斜,其构造内部具有明显的差异构造特征。构造北段断裂体系以走滑作用为主,受平湖断裂体系影响大,南段断裂体系以转换断裂为主,中段为2套断裂体系交会区,形成弧形硬连接断裂段。②天台构造的构造演化过程主要可分为断陷早期、断陷晚期及挤压期3个构造演化阶段。在断陷早期NE向断裂大量发育,局部NW向断裂调节差异位移,断裂体系形成,天台构造北部伸展量明显大于南部;断陷晚期断裂活动终止,天台构造整体活动减弱,仅局部大断裂持续活动;龙井运动挤压期天台构造经受近EW向强挤压,中部挤压强度大于南部和北部,形成整一大背斜,部分断裂复活,局部火成岩发育。③走滑-转换体系控制圈闭发育及油气运聚、保存。受多期构造运动叠加影响天台构造内呈现多种类型圈闭差异立体叠合的特征,中段构造圈闭叠合性好,受火成岩侵入影响,构造圈闭有效性差,南、北段深部独立发育构造高点,深浅圈闭叠合性较差;南段薄层滑脱型转换断裂输导能力强,但保存条件较差,北段局部增压背斜保存条件更好,具有更大的勘探潜力。该研究成果可为西湖凹陷天台构造深层油气勘探提供参考。

The arc-shaped fault system developed in the Tiantai structure of the Xihu Sag and its reservoir-controlling effect are very intricate,restricting the further exploration of oil and gas in this area.Based on3D seismic data and the drilling results of exploration wells,the structural characteristics of multiple fault systems within the Tiantai structure and their impact on oil and gas accumulation were analyzed.The results show that①The Tiantai structure is controlled by the NE-trending basement-involved strike-slip faults and the NW-trending thin-layer detachment transition faults and formed a multi-period differential inversion anticline under the control of strike-transition fault system.Distinct differential structural characteristics can be observed within this structure.The fault system in the northern part of the structure is dominated by strike-slip action and is greatly influenced by the Pinghu fault system.The southern fault system is dominated by transition faults,while the central part is the intersection area of the two fault systems,forming an arc-shaped hard-linked fault segment.②The evolution of the Tiantai structure can be mainly divided into three stages:early rift period,late rift period,and compressional period.During the early rift period,NE-trending faults developed extensively,and local NW-trending faults regulated differential displacement.The fault system was formed in this period,and the northern part of the Tiantai structure experienced significantly more extension than the southern part.During the late rift period,fault activity ceased,and the overall activity of the Tiantai structure weakened.Only some major faults remained active.During the compressional period of the Longjing movement,the Tiantai structure was subjected to strong near EW-trending compression.The central part experienced greater compression than the southern and northern parts and formed a large anticline.Some faults were reactivated,and local igneous rocks developed.③The strike-transition structural system controls the development of traps and the accumulation and storage of oil and gas.Influenced by multiple periods of tectonic movements,the Tiantai structure exhibits a characteristic of various types of traps with differential three-dimensional superposition.The central part has good structural trap superposition,but its effectiveness is influenced by the igneous rocks.The southern and northern parts have independent deep structural traps,but the superposition of upper and lower traps is poor;the southern part has a strong conductive capacity of thin-layer detachment transition faults,but the preservation conditions are poor,while the northern part has better preservation conditions in pressure-increased anticline,offering greater exploration potential.The research can provide a reference for deep oil and gas exploration in the Tiantai structure of the Xihu Sag.