Geochemical characteristics and organic matter enrichment model of Wufeng-Longmaxi formations shale in northeast Sichuan China

The Sichuan Basin, located in southwestern China, is considered the primary producing area for shale gas development in the country. Its rich shale gas reserves have made it a significant contributor to China’s energy production. Within the Sichuan Basin, northeast Sichuan has emerged as a region of particular interest for shale gas development. In order to investigate the controlling factors and enrichment patterns of organic matter-rich shales from the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in northeast Sichuan, inductively coupled plasma mass spectrometry (ICP–MS) was used to analyze 14 shale samples from the Wufeng Formation-Longmaxi Formation in northeast Sichuan. Combined with the characteristics of logging curves, the sedimentary system, redox conditions, terrestrial debris input, primary productivity, and paleoclimate relationship of the Wufeng Formation to the Longmaxi Formation were explored. As a result, three third-order sequences (SSQ1, SSQ2, and SSQ3) were identified, and the sea level experienced a process from high-low-high-low, which, in combination with the shale elemental parameters of w(V)/w(Cr), w(V)/w(V+Ni), w(Ni)/w(Co), w(Sr)/w(Cu), w(Sr)/w(Ba), and T (℃), indicated that the Wufeng Formation-Longmaxi Formation underwent an oxidized-poorly oxygenated-anoxic reduced-poorly oxygenated-oxidized freshwater environment; the climate has undergone a warm-humid, dry-cold, warm-humid, dry-cold, warm-humid evolution, and the temperature has gone through the process of decreasing – increasing – decreasing – increasing. The organic matter enrichment pattern of the Wufeng Formation-Longmaxi Formation in northeast Sichuan can be divided into a Ⅲ stage pattern. Stage Ⅰ, the depositional period from the Wufeng Formation to the bottom of the Longmaxi Formation, is a stage of gradual increase in organic carbon; stage Ⅱ, the depositional period of the middle part of the Longmaxi Formation, is a stage of enrichment of organic carbon (sweet-spot member); and stage Ⅲ, the depositional period of the upper part of the Longmaxi Formation, is a stage of gradual decrease in organic carbon. It is expected to provide a basis for the exploration of shale gas in northeast Sichuan, and the next step will be to carry out research on the resource response of major geological events to organic matter enrichment in the study area and deepen the shale gas enrichment model.