Lithofacies palaeogeography, depositional model and shale gas potential evaluation in the O3-S1 Wufeng-Longmaxi Formation in the Sichuan Basin, China

Abstract

The black shales of Wufeng and Longmaxi Formation (Late Ordovician-Early Silurian period) in Sichuan Basin are the main strata for marine shale gas exploration, which have a yearly shale gas production of 228×10⁸ m³ and cumulative shale gas production of 919×10⁸ m³. According to the lithological and biological features, filling sequences, sedimentary structures and lab analysis, the authors divided the Wufeng /Guanyinqiao and Longmaxi Formations into shore, tidal flat, shoal, shallow water shelf and deep water shelf facies, and confirmed that a shallow water deposition between the two sets of shales. Although both Formations contain similar shales, their formation mechanisms differ. During the deposition of Wufeng shale, influenced by the Caledonian Movement, the Central Sichuan and Guizhou Uplifts led to the transformation of the Sichuan Basin into a back-bulge basin. Coinstantaneous volcanic activity provided significant nutrients, contributing to the deposition of Wufeng Formation black shales. In contrast, during the deposition of Longmaxi shale, collisions caused basement subsidence, melting glaciers raised sea levels, and renewed volcanic activity provided additional nutrients, leading to Longmaxi Formation black shale accumulation. Considering the basic sedimentary geology and shale gas characteristics, areas such as Suijiang-Leibo-Daguan, Luzhou-Zigong, Weirong-Yongchuan, and Nanchuan-Dingshan are identified as key prospects for future shale gas exploration in the Wufeng-Longmaxi Formations.