Influence of sedimentary environment on organic matter enrichment in shale: A case study of the Wufeng and Longmaxi Formations of the Sichuan Basin, China

Abstract

Abstract As two essential controls on the formation of organic matter (OM)-enriched shales, both paleoproductivity and preservation models remain controversial for the Wufeng and Longmaxi shales in the Sichuan Basin, southwest China. This study provides geochemical parameters as proxies for the paleoproductivity characteristics (Ba, P/Al, and (Ni+Cu)/Al), bottom water redox status (U/Th, V/(V+Ni), V/Cr, and Mo), and terrigenous clastic flux (Th and Al) of the Wufeng–Longmaxi Formations in the Sichuan Basin, a back-deep basin in a foreland basin system in southwest China. Combining these data with total organic carbon (TOC) content and sedimentation rate data obtained from graptolite zones allowed us to discuss the validity of these popular indices and to identify the main factors that controlled OM enrichment in the formations. TOC contents exceed 4 wt% in the upper Wufeng Formation to the bottom part of the Longmaxi Formation (LM1-3), which are the most OM-rich intervals. The average P/Al ratio for well JY2 in the Jiaoshiba area is 0.009, which is lower than those for well YY1 (0.028) in the Yongchuan area and well DY1 (0.02) in the Dingshan area, indicating the lowest productivity levels were in the Jiaoshiba area. Based on the redox proxies, the samples from well DY1 reflect less reducing bottom water conditions than those from wells JY2 and YY1. The Th and Al indices increase upwards, indicating that the terrigenous clastic input increased, and that those in the Yongchuan area were lower than those in the Jiaoshiba and Dingshan areas. The sedimentation rate was too low to dilute the OM in the Sichuan Basin, especially during the depositional time of the OM-rich intervals, such that the redox status of the water became more critical. Given the more strongly reducing conditions of the sedimentary environment and the high thermal maturity of the OM in this formation, the Ba concentrations appeared anomalously low and became inaccurate as a paleoproductivity index, as well as the validity of V, V/Cr, and V/(V+Ni) anomalously decreased as redox indices.