Sedimentary tectonic pattern of Wufeng and Longmaxi Formations in the northern margin of Sichuan Basin, South China

Organic-rich black shales in the Sichuan Basin from the Upper Ordovician and Lower Silurian are considered excellent source rocks and unconventional reservoirs of hydrocarbons in South China. The Wufeng and Longmaxi Formations in the Micangshan – Dabashan areas of the northern margin of Sichuan Basin were taken as the research objects. The purpose of this study is to reconstruct the sedimentary tectonic pattern of the Wufeng and Longmaxi Formations in the study area. First, the stratigraphic framework correlation and sedimentary facies analysis of Wufeng and Longmaxi Formations in the study area were carried out. Then, Mo/TOC and Al-Co[EF]×Mn[EF] geochemical indicators were used to identify the degree of water restricted during the deposition period. It was found that the thickness of black shale was relatively small near the uplift area. However, far away from the uplift area, from west to east, the black shale thickened and the TOC content increased gradually, which were controlled by the upwelling of continental margin and intracontinental depression. The results show that: (1) At the front edge of Micangshan, during the deposition period of Wufeng Formation, Fucheng and Qiaoting profiles were located in the intersection area of upwelling formed by two kinds of monsoon, which led to the long-term influence of upwelling in this area. The sedimentation period of Longmaxi Formation was mainly affected by ‘Xixiang Rise’, which led to the uplift of Wangcang and Nanjiang areas into islands. (2) At the front edge of Dabashan and Western Hubei – Eastern Chongqing areas, the Wufeng Formation presented a tectonic pattern of alternating uplift and depression. Due to the continuous compression of the Cathaysian Block from southeast to northwest, the Shizhu – Wuxi areas continued to subside during the deposition period of Longmaxi Formation, and the black shale was quite thick.四川盆地晚奥陶世-早志留世富有机质黑色页岩，被认定为中国南方优质烃源岩与非常规油气储层。本次研究以四川盆地北缘米仓山-大巴山地区的五峰组与龙马溪组为研究对象，旨在重建研究区五峰组及龙马溪组的沉积构造格局。首先对研究区五峰组、龙马溪组开展了地层格架对比与沉积相分析；随后利用Mo/TOC比值及Al-Co[EF]×Mn[EF]地球化学指标，识别沉积时期的水体局限程度。研究结果显示，隆起区附近黑色页岩厚度相对较薄；而远离隆起区时，自西向东黑色页岩厚度逐渐增厚、总有机碳（Total Organic Carbon，TOC）含量逐步升高，该分布特征受大陆边缘上升流与陆内坳陷共同调控。研究结果表明：（1）在米仓山前缘带，五峰组沉积期，涪城（Fucheng）与桥亭（Qiaoting）剖面位于两类季风形成的上升流交汇区域，使得该区域长期受上升流影响；龙马溪组沉积期主要受“西乡隆起”控制，旺苍与南江地区抬升为岛屿。（2）在大巴山前缘带及鄂西-渝东地区，五峰组呈现隆凹相间的构造格局；受华夏地块自东南向西北持续挤压作用影响，龙马溪组沉积期石柱-巫溪一带持续沉降，黑色页岩厚度可观。