Shale facies from the Wufeng-Lower Longmaxi Formations in the Huangying section of Wulong County, southeastern Sichuan Basin, China

Abstract

With the increasing interest on shale oil and shale gas around the world, it is essential to discover alternative economic shale gas fields outside the Fuling gas field, which is China’s first large-scale shale gas field with 100 billion cubic meters of reserves in the Sichuan Basin (China). Based on comprehensive analysis of dozens of black shale samples, the Wufeng Formation and the Lower Longmaxi Formation in the Huangying section of Wulong County showed good prospects for shale gas. An integrated study of petrology, organic geochemistry, trace element analysis, and sedimentology reveals that the sea level during the Early Silurian period was relatively high. Consequently, carbon-rich and high-silica lithofacies were deposited, corresponding to argillaceous-siliceous deepwater shelf microfacies. During deposition of the Guanyinqiao bed (latest Ordovician), the sea level was relatively low, resulting in the formation of moderately high carbon and high-silica shale facies, corresponding to argillaceous shallow shelf microfacies. A complete transgressive-regressive third-order sequence was recorded in the Longmaxi Formation. Carbon-rich and high-silica lithofacies, which are associated with carbonaceous deepwater shelf microfacies and siliceous-argillaceous deepwater shelf microfacies, respectively, dominated the transgressive system tract. Compared with the Fuling area with a record of high shale gas productivity, the Wufeng Formation and the transgressive system tract of the Longmaxi Formation in the study area demonstrate great potential for shale gas.