川东南构造变形特征及其对页岩气保存的影响

Structure deformation of southeast Sichuan and its influences on preservation of shale gas

川东南地区构造变形复杂,二叠系和志留系含有丰富的页岩气资源。依据岩性和地震资料的解释,寒武系膏盐层与中–下三叠统膏盐层对构造变形有重要控制作用,多数断层沿膏盐层滑脱。研究区包括盆内和盆缘两部分,盆内发育形态对称的盖层滑脱式褶皱;盆缘发育基底卷入式褶皱冲断构造,分为山前推覆带和山前转换带,前者发育高陡的三角楔构造,后者由冲断带、褶皱带和斜坡带组成。构造演化分析表明:晚侏罗世齐岳山断层开始发育,盆内地层发生挠曲变形;白垩纪盆缘形成三角楔构造,盆内主要构造和断裂已经发育;新生代齐岳山褶皱隆升,盆内寒武系之上沉积盖层褶皱形成多个背斜和向斜。山前推覆带构造高陡、变形强烈,页岩气保存条件差;山前转换带构造变形程度适中,其褶皱带背斜完整,页岩气保存条件好;盆内中–下三叠统膏盐层封盖性好,埋深适中的背斜为页岩气有利勘探目标区。

The structure in southeast Sichuan is complicated, and there are abundant shale gas resources in Permian and Silurian strata. Based on the analysis of lithology and interpretation of seismic data, Cambrian gypsum-salt layer and Middle-Lower Triassic gypsum-salt layer had important control action to structural deformation,most faults sliped along gypsium-salt layer. The study area includes two parts: inner basin and basin margin.In inner basin symmetric folds with decollement of overlying strata were developed, basin margin was featured by structure of base-involved fold and thrust fault, and can be distinguished as piedmont nappe zone and piedmont conversion zone. High-steep triangle wedge was developed in piedmont nappe zone. Conversion zone can be divided into thrust belt, fold belt and slope belt. Analysis of structure evolution showed that in Late Jurassic Qiyueshan fault started developing, the strata in inner basin were bended and deformed. In Cretaceous triangle wedge was formed in basin margin, and main folds and faults had been already developed in inner basin. In Cenozoic Qiyueshan mountain was folded and uplifted, and several anticlines and synclines were formed in sedimentary cover above Cambrian in inner basin. In piedmont nappe zone, high-steep structure and intense deformation led to the poor preservation condition of shale gas. In piedmont conversion zone, moderate deformation led to the good preservation condition of shale gas, especially in fold belt. Due to the good sealing-capping property of gypsum-salt bed of Middle-Lower Triassic, anticlines with suitable burial depth are the favorable exploration targets.