四川盆地东部走滑断裂识别与特征分析及形成演化:以涪陵地区为例

Formation and evolution of strike-slip fault zones in the eastern Sichuan Basin and identification and characterization of the fault zones:A case study of the Fuling area

近年来的国内外油气勘探实践表明含油气盆地内的基底走滑断裂带为一种新的高产油气富集带,油气勘探开发前景广阔。对基底走滑断裂的识别、几何学和运动学特征分析及“控储、控藏”作用方面的研究越来越引起高度关注。本文利用四川盆地东部涪陵地区的高精度三维地震剖面的断裂构造精细解析、相干体属性和水平时间切片分析、野外露头断裂特征观测等多种资料,基于走滑构造理论,在研究区新发现了规模较大的北西向和北东向基底走滑断裂带的分布,并建立了川东地区走滑断裂在剖面上直立断层和花状构造、平面上线性延伸和地质界线错开、空间上的“丝带效应”和“海豚效应”及主干断裂产状和力学性质“明显分段性”的识别标准。并提出了基于走滑断裂垂直位移量变化和两侧构造活动强度差异性对比分析厘定走滑方向的新方法;依据地震水平时间切片地层界线错开、走滑断裂两侧厚度差异估算走滑距离的方法,实际应用结果表明,NW向基底走滑断裂带现今整体呈现左行走滑特征,但不同时期走滑性质多变,自形成期以来至少经历了5次左行滑动与3次右行滑动,总体表现出“深部左行,浅部右行,北部活动强度大,南部活动强度小”的特点,左行走滑距累积可达到1400~3400 m。北东向基底走滑断裂带现今呈现右行走滑特征,累积走滑距约1930 m。本文提出了NW向与NE向基底走滑断裂带形成演化模式存在着明显的差异性,其主要受控于区域构造运动和不同时期应力场环境演化及区域滑脱层的应力释放作用,NW向走滑作用受下三叠统膏盐滑脱层的影响,NE向走滑活动则受中下志留统页岩滑脱层的影响。

Basement strike-slip fault zones of petroliferous basins are newly recognized high-yield hydrocarbon enrichment zones with broad prospects for oil/gas exploration and development.Increasingly more attention has been paid to the identification and geometrical/kinematical characterization of basement strike-slip faults as well as the role of“reservoir/pool controls”in this type of fault systems.This paper reports the new discovery of large-scale NW and NE-trending basement strike-slip fault zones in Fuling area,eastern Sichuan Basin,and establishes the identification criteria for strike-slip faults in this area based on characteristic fault patterns.The characteristic fault patterns include vertical faults and flower-like structures on the profile;planar linear extension of fault with staggered geological boundaries;and obvious segmentation of master faults in“ribbon-like”or“dolphin-like”strike-slip fault patterns and with changing mechanical properties.A new method is proposed for determining the strike-slip directions based on seismic analysis to measure the change of vertical displacements and the difference in tectonic-activity intensities between the two sides of the fault.It is found that the NW-trending basement strike-slip fault zone generally presents left-lateral slip but with variable strike-slip deformations in different evolutionary stages.Since its formation,the NW-trending faults zone has experienced at least 5 left-lateral and 3 right-lateral slip events which can be characterized by left-lateral slip in the deep crust,right-lateral slip in the shallow crust,high activity-intensity in the north and low activity-intensity in the south,with a cumulative left-lateral strike-slip offset reaching up to 14003400 m.In comparison,the NE-trending basement strike-slip fault zone is characterized by right-lateral strike-slip,and the cumulative strike-slip offset is about 1930 m.It suggests that there are significant differences in the formation and evolution of the two fault zones controlled mainly by regional tectonic movements,evolution of stress field environment in different evolutionary stages as well as stress release of regional detachment layers where the NW-trending strike-slip fault zone is affected by Lower-Triassic gypsum-salt detachment layer and the NE-trending strike-slip zone is affected by Middle-Lower-Silurian shale detachment layer.