四川盆地万源—达州克拉通内裂陷的发现及勘探意义

Discovery of Wanyuan-Dazhou Intracratonic Rift and its exploration significance in the Sichuan Basin,SW China

随四川盆地震旦系—寒武系天然气勘探的深入,地质认识不断创新,在川东北地区万源—达州发现了一近北东向展布的克拉通内裂陷。利用地震、野外露头资料,结合盆地区域构造背景分析,勾绘了万源—达州内裂陷的边界与空间展布,梳理了裂陷演化及其理论与勘探意义。研究揭示:(1)震旦系灯一、二段在地震剖面上有明显的"陡坎"带,显示台地边缘相带的存在,总体呈近北东向展布;(2)裂陷周缘灯影组一、二段较厚,灯影组三、四段较薄,裂陷内部灯影组一、二段较薄,灯影组三、四段较厚;(3)裂陷演化分为震旦纪灯一段—灯二段沉积期的形成阶段、灯三段—灯四段沉积期的充填与沉降阶段以及早寒武世麦地坪组—筇竹寺组沉积期的萎缩—消亡3个阶段,其形成受早期南华纪裂谷及区域性隆升联合控制。万源—达州克拉通内裂陷的发现改变了四川盆地震旦纪—早寒武世构造沉积格局的传统认识,该裂陷周缘震旦系天然气成藏条件有利,是今后的重要勘探方向。

With the development of gas exploration of the Sinian-Cambrian strata in the Sichuan Basin, creative understandings of geology were achieved. An N-E trending intracratonic rift was found in the Wanyuan-Dazhou area, northeastern Sichuan Basin. Based on seismic interpretation data, outcrop data and analysis of regional structural geological background, studies on the boundary, distribution, formation and evolution history of Wanyuan-Dazhou rift were carried out, and the significance on exploration was discussed. The following findings were obtained. （1） The seismic section indicated that a steep-slope belt was developed in the first and second members （Z2dnl-Z2dn2） of the Dengying Formation of the Sinian System, showing platform edge facies. The rift generally strikes in NE direction. （2） The thicknesses of the first and second members of the Dengying Formation are thicker than the third and fourth members （Z2dn3-Z2dn4） at the rift periphery, while it came to the opposite conclusion at the inside of the rift. （3） The rift formed in Z2dn1-Z2dn2 of Sinian deposition period. Filling and subsidence occurred in Z2dn3-Z2dn4 of Sinian deposition period. The shrinkage stage was the deposition period of the Early Cambrian Maidiping Formation - Qiongzhusi Formation. The formation of the rift was controlled by the Nanhua rift and regional uplift. The finding of the Wanyuan-Dazhou rift changed the traditional understanding of the Sinian - Early Cambrian tectonic sedimentary framework, and the rift will be the significant exploration direction in the future due to its superior natural gas accumulation conditions in the Sinian.