Black Shale Lithofacies of the Wufeng–Longmaxi Formation in the Southeast Margin of Chongqing, China

Abstract

China’s shale gas has undergone nearly 20 years of exploration; unconventional oil and gas geological evaluation theories and research methods have been greatly enriched, but how to quickly, conveniently, and accurately identify the sweet spots of shale gas is still puzzling many researchers. This study focuses on the black shale of the Wufeng–Longmaxi Formation in the southeastern edge of the Sichuan Basin; lithofacies classification, the relationship between lithofacies and depositional environments, and the correlation between lithofacies and shale gas–bearing capacity are discussed. At last, we have established the lithofacies classification criteria; the Wufeng–Longmaxi Formation deposited eight types of lithofacies, which the paleoenvironment during deposition evolved gradually from anaerobic environment to oxygen-poor and oxygen-rich environment. The black high-carbon and high-silicon shale lithofacies and the black carbon-rich and silicon-rich shale lithofacies are rich in organic matter, and they were deposited in high primary productivity, low terrigenous detritus input, and euxinic environment. The black medium-carbon medium-silica shale lithofacies and the black medium-carbon and high-silica shale lithofacies contain organic matter, which are deposited in medium primary productivity, middle terrigenous detritus input, and oxygen-poor and low hydrodynamic environment. The gray–black low-carbon low-silicon clay-rich shale lithofacies, the gray low-carbon and high-silicon shale lithofacies, and the gray–white low-carbon and silicon-rich shale lithofacies are poor in organic matter, which are deposited in a transitional environment of low primary productivity and oxygen poor–oxygen rich. In the analysis of the relationship between organic matter–rich black shale facies and sedimentary environment, it is shown that the enrichment of organic matter is positively correlated with the oxidation–reduction discrimination indicators Ni/Co, U/Th ratio of ancient oceans, and the evaluation indicators Ba_bio and Ba/Al ratios of primary productivity. Only under the favorable sedimentary geochemical conditions and good preservation conditions can deposit lithofacies sections (zones). Based on the optimization of shale gas dessert section and the drilling of horizontal wells, the optimization of favorable black shale lithofacies types and the classification of shale gas dessert section are the key to shale gas exploration. The shale gas–bearing capacity is closely related to lithofacies. Black carbon-rich silicon-rich shale lithofacies and black high-carbon high-silicon shale lithofacies have the best gas-bearing capacity and are favorable lithofacies.