Evolution and migration of shale facies and their control on shale gas: A case study from the Wufeng-Longmaxi Formations in the Sichuan Basin and its surroundings

The Fuling gas field, the largest commercial discovery of shale gas in China, has been producing from the organic-rich shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the Sichuan Basin, China. A refined shale gas E&P and sweet spots identification require studies to be carried out not only at the sedimentary facies scale but also at the petrographic scale. This paper focuses on the characteristics of organic-rich shales. Based on data from outcrops, drilling cores, X-ray diffraction, thin sections, and scanning electric microscope, a systematic analysis of wells and well-correlation sections indicates that shales in the study area underwent two progressive and regressive cycles. In addition, differences in lithofacies and their organic or inorganic components are significant either in the vertical or in the horizontal direction, which generally controls the material basis of shale gas. The early-stage siliceous shale distributes in the east, whereas the late-stage carbonate-bearing shale extends to the west. In the southeastern Sichuan Basin, the siliceous composition decreases, whereas the clay minerals increase upward. In the central Sichuan uplift, there is a higher carbonate content And with our new lithofacies maps, distribution of the sweet spots was identified. The northeastern Sichuan Basin has the siliceous shale in the Wufeng Formation and the silicon-bearing shale in the Longmaxi Formation, making it a play area with the greatest gas potential. The southern Sichuan Basin occurs as a favorable lithofacies area with the stacked siliceous shale in the Wufeng Formation and the calcareous shale in the Longmaxi Formation. The relatively favorable lithofacies area covers the regions extending from the periphery of southern Sichuan Basin toward the central Sichuan uplift and the northern Guizhou uplift, featuring stacked carbonate-bearing shale and clay mineral-bearing shale in the Longmaxi Formation. The shallow-marine areas close to the uplifts are dominated by mudstone and silty mudstone, thereby possessing poor material basis.