Stratigraphic division and sedimentary environment of the Permian Wujiaping Formation in the Longmen-Wushankan area of the Eastern Sichuan Basin: insights for shale gas exploration

Abstract

The Longmen–Wushankan area, a newly emerging region in the Sichuan Basin, has seen significant exploration activities across the Permian marine shale gas following a breakthrough in the Hongxing area. Recently, well DY1H achieved a high test output from the Wujiaping Formation, highlighting the promising exploration potential for shale gas. To better understand the lithofacies types and their sedimentological significance in the Wujiaping Formation shale of the Longmen-Wushankan area, an integrated approach of conodont analysis, total organic carbon (TOC), X-ray diffraction, argon ion polishing scanning electron microscopy, and major and trace element data is implemented. This study defines the stratigraphy of the Wujiaping Formation, clarifying its top–bottom boundaries and the boundaries between its three members. The lithofacies are classified, and the sedimentary paleoenvironment is analyzed to explore the relationship between lithofacies, environment, and organic matter enrichment, thereby establishing a sedimentary model. Two conodonts identified in the upper part of the section correspond to the conodont belt at the base of the Permian Dalong Formation, dated to approximately 253–254 Ma. The lower boundary is marked by an unconformity, indicated by the appearance of Wangpo Shale. Four lithofacies types are identified in the study area, which are further categorized into 36 lithofacies types based on TOC content, including high-carbon, medium-carbon, low-carbon, and ultra-low-carbon, with high-carbon siliceous shale being the most favorable. The First and Second members of the Wujiaping Formation were deposited during a phase of sea-level transgression with significant terrigenous input compared to the Third Member that was deposited during a sea-level regression. Enrichment factor values of redox-sensitive elements indicated a change from oxic to anoxic water column settings, which were predominated during deposition of the studied succession. This was consistent with a change from shallow to deep sea-level, with siliceous material primarily derived from terrigenous to biogenic sources. The deposition of the Third Member was controlled by the Kaijiang-Liangping Trough, resulting in a deep-water shelf with anoxic conditions and enhanced productivity of labile organic matter. This indicates predominant organic matter production and preservation conditions at this time compared to decomposition and dilution of organic matter during deposition of the First and Second members. As high-quality lithofacies in the study area, environmental conditions significantly influenced the deposition of high-carbon siliceous shale, positioning siliceous shale as a key target for future exploration.

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