Depositional environment of shale in Wufeng and Longmaxi Formations, Sichuan Basin

Abstract

Formation of organic-rich shale was controlled by depositional processes and environment condition. According to petrology, mineralogy, fossil composition and sedimentary structure, seven lithofacies of shale were identified in the Wufeng Formation to Longmaxi Formation, e.g., siliceous shale, silty shale, argillaceous shale, calcareous shale, shell-bearing argillaceous limestone/calcareous mudstone, siltstone-fine sandstone and bentonite. For shale in the Wufeng and Longmaxi Formations, the depositional mode was not only the suspension deposition under the low-energy hydrodynamic condition, but also the biogenic deposition, the storm deposition and the bottom current deposition indicated by large amount of siliceous biological fossils, biological detritus with size graded structure, silt laminations, lenticular bedding, ripple cross-bedding and other sedimentary structures. Trace element analysis suggested that the siliceous shale in the Wufeng Formation and the lower part of Longmaxi Formation was developed in the oxygen-dysoxic/anoxic depositional environment, which was rich in organic matters; then due to influence of the sea level gradually decline and bottom current, the silty and argillaceous shales were formed, and the oxygen-dysoxic/anoxic depositional environment was destroyed, gradually leading to the worse preservation condition of organic matter.