Palaeogeography and its controls on organic carbon accumulation in the Wufeng–Longmaxi formations of Chongqing, southeastern Sichuan Basin, southwestern China

The Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation are important target strata for shale gas exploration in the Sichuan Basin, China. In this paper, the sedimentary facies and palaeogeography of the two formations in Chongqing area have been analyzed, and total organic carbon (TOC) contents are correlated with the sedimentary facies to prospect the favorable areas for shale gas exploration. According to the lithological characteristics and sedimentary structures, five sedimentary facies are identified, i.e., siliceous deep-water shelf, muddy deep-water shelf, calcareous shallow-water shelf, muddy shallow-water shelf, and silty-muddy shallow-water shelf facies, respectively. The palaeogeography reconstruction is based on the lithological distribution in the study area. The lower part of Wufeng Formation was developed with the muddy deep-water shelf, siliceous deep-water shelf, and calcareous shallow-water shelf facies. The Guanyinqiao Member, representing the upper part of Wufeng Formation, was developed with the transition from siliceous deep-water shelf and muddy deep-water shelf facies to a domination of muddy shallow-water shelf and calcareous shallow-water shelf facies, which was deposited during the glacial-induced global regression. The Lower Member of Longmaxi Formation was characterized by the expansion of siliceous deep-water shelf facies and contraction of muddy shallow-water shelf and calcareous shallow-water shelf facies, signifying a rapid sea level rise associated with global warming. The Upper Member of Longmaxi Formation was developed with muddy shallow-water shelf facies, with the expansion of calcareous shallow-water shelf and silty-muddy shallow-water shelf facies. The shales with higher TOC content tend to be developed in the siliceous deep-water shelf and muddy deep-water shelf facies that were distributed in the Nanchuan, Shizhu, Yongchuan, and Wuxi areas. The results of lithofacies palaeogeography reconstruction have a certain significance to predict shale-gas target areas in Wufeng and Longmaxi formations of Chongqing.