Geochemical and petrographic characteristics of Wufeng-Longmaxi shales, Jiaoshiba area, southwest China: Implications for organic matter differential accumulation

The organic matter-rich shales in Wufeng-Longmaxi Formation have been considered to be a high-quality source rock and a main shale-gas producer in southern China. As the material basis of shale gas, the organic matter content in shales exhibits a notable vertical difference in the Jiaoshiba area of Fulin shale gas field. This study integrated detailed geochemical analysis and sedimentary observations to investigate organic matter enrichment and depletion mechanism. Three intervals are identified by organic matter contents and their according variations. Six major lithofacies are identified based on mineral composition X-ray diffraction (XRD) analyses, microscopic observations and Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) data. Geochemical redox proxies (Mo-EF, U-EF, V/Cr and Corg/P) indicate that organic matter-rich shales of Wufeng and bottom Longmaxi formation are deposited in anoxic conditions, whereas organic matter-lean shales of the middle to top Longmaxi formation are deposited in oxic environment. This pattern is influenced by both worldwide marine transgression and regional tectonic movement. The productivity proxies (P/Al, Cu/Al, Ni/Al, SiO2-bio) indicate that the organic matter-rich Wufeng-Longmaxi shales were deposited under moderate to high surficial primary productivity. Increasing terrigenous influxes dominated organic matter dilution and may have been promoted by regional tectonic uplift. The organic matter enrichment or depletion mechanisms in each unit are controlled by different geological events. Enrichment was controlled by multiple reasons, including both global and regional factors, whereas depletion was mainly dominated by regional tectonic uplift. Additionally, the sequence stratigraphic boundaries coincide with organic matter shifting interfaces, indicating that high-resolution sequence stratigraphy of fine-grained sediments may have a certain effect on optimal reservoir predication. Conversely, boundaries of TOC and geochemical proxies may also help to subdivide sequence boundaries.