Lithofacies Palaeogeographic Evolution of the Middle Permian Sequence Stratigraphy and Its Implications for Shale Gas Exploration in the Northern Sichuan and Western Hubei Provinces

In the last two years, significant breakthroughs have been achieved in the shale gas exploration within the Upper Permian Wuchiaping Formation to Talung Formation in the northern Sichuan and western Hubei provicnes, South China. Furthermore, a promising exploration perspective has also emerged in the Middle Permian Kuhfeng Formation. However, the distribution patterns of the organic⁃rich strata and the potential shale gas resources in these formations are not yet fully understood. By identifying sedimentary facies and correlating sequence stratigraphy in the Middle Permian of northern Sichuan and western Hubei area, the characteristics of lithofacies paleogeography in different sequences and their evolution were analyzed. Following this, it explores the distribution patterns of organic⁃rich layers and the shale gas resource potential in the Kuhfeng Formation. The research reveals that three third ⁃ order sequences, namely SQ1, SQ2, and SQ3, are identified from the Middle Permian formations in the northern Sichuan and western Hubei area. Specifically, sequence SQ3 corresponds to the Kuhfeng Formation and was deposited during the late Capitanian. During the deposition of sequence SQ3, three northwest ⁃ trending deep ⁃ water basins developed on a carbonate platform (corresponding to sequences SQ1 and SQ2). These basins are the western Hubei Basin, the Kaijiang⁃Liangping Platform Basin (trough), and the Guang⁃Wang Platform Basin (trough), respectively.The Kuhfeng Formation exhibits a relatively continuous and stable distribution of organic ⁃ rich layers, mainly including bedded chert and siliceous shale layers with high organic carbon content (average TOC content ranging from 3.0% to 10%). The Emeishan mantle plume contributed to the formation of the deep ⁃ water slope ⁃ basin environment during the late Middle Permian in this area, further controlling the large ⁃ scale regional deposition of organic ⁃ rich bedded cherts and siliceous shales within the Kuhfeng Formation. Based on these above results and further comprehensive analyses of geological evaluation factors of shale gas, such as the effective thickness of bedded chert and siliceous shale, TOC content, and tectonic preservation conditions, five favorable areas for shale gas reservoirs covering approximately 12,000 km2 are identified in the northeastern Sichuan to the western Hunan⁃Hubei region. The estimated total shale gas resource is approximately 1.75 trillion cubic meters, demonstrating significant potential of shale gas exploration and development in the region. © 2024 China University of Geosciences. All rights reserved.