四川盆地川东地区高陡构造形成机制与天然气聚集

Formation mechanism and natural gas accumulation of the high and steep structure in the eastern Sichuan Basin

川东地区属高陡构造发育区,伴随四川盆地经历了华南纪—早古生代、晚古生代—三叠纪、侏罗纪—第四纪3大伸展聚敛构造旋回;具有明显的顺层挤压滑脱与垂向分层特点.围绕下寒武统、下志留统、下三叠统膏泥岩滑脱层可划分为上、中、下3套构造层;在各滑脱层之下,围绕各自所发育的有效烃源岩层,形成各自独立的含油气系统.川东高陡构造主要为印支期形成,燕山期发展,喜马拉雅期改造定型,其构造变形机制总体表现为"断层转折、楔入反冲与双重构造"模式特征.下构造层沿下寒武统滑脱层,形成叠瓦构造、双重构造和楔入反冲构造;中构造层沿下志留统滑脱层,形成断层转折褶皱、对冲构造和反冲构造;上构造层沿下三叠统滑脱层,形成断层传播褶皱、对冲构造和反冲构造.区内天然气资源丰富,主要集中于中构造层,具有下构造层继承性低隆起构造高部位的寒武系—震旦系白云岩与礁滩发育区聚集,中部构造层沿不整合面的潜伏构造围斜部石炭系白云岩与志留系砂体发育区聚集,中部构造层的潜伏构造背斜与向斜区二叠系与下三叠统礁滩与白云岩发育区聚集成藏特点,对应其有利勘探方向.

The eastern Sichuan area is a high and steep tectonic zone, with the Sichuan Basin experiencing three extensional and convergent tectonic cycles of the Nanhua to Early Paleozoic, the Late Palaeozoic to Triassic, and the Jurassic to Quaternary. It has obvious characteristics of bedding slipping and vertical stratification. Three structural layers(lower structural layer, middle structural layer, and upper structural layer)are identified based on the three detachment layers(Lower Cambrian, Lower Silurian, and Lower Triassic gypsum mudstone). Under each detachment layer, independent hydrocarbon systems are formed around the respective effective hydrocarbon source rocks. The high and steep structures in eastern Sichuan are mainly formed in Indosinian period, developed in Yanshan Period, and reformed in Himalaya Period. Tectonic deformations of the eastern Sichuan high and steep structures are mainly characterized by “fault bending, wedge back-thrusting, and duplex”. The lower structural layer includes imbricate structure, duplex structure, and wedge back-thrust structure along the Lower Cambrian detachment layer;the middle structural layer includes fault bend fold, ramp structure, and back-thrust structure along the Lower Silurian detachment layer;the upper structural layer includes fault propagation fold, ramp structure, and back-thrust structure along the Lower Triassic detachment layer. The eastern Sichuan Basin is rich in natural gas and mainly concentrated in the middle structural layer. The characteristics of hydrocarbon accumulation include the lower structural layer Cambrian to Sinian dolomite and reef shoal zone in the high structural position of the inheriting low uplift;the middle structural layer Carboniferous dolomite and Silurian sandbody development zone in anticlinal area along unconformity surface;the middle structural layer Permian and Lower Triassic reef beaches and dolomite developed areas in the latent structural anticline and syncline. These potential oil and gas accumulation areas correspond to their favorable exploration directions.