Factors Controlling Organic Matter Accumulation in the Wufeng–Longmaxi Formations in Northwestern Hunan Province: Insights from Major/Trace Elements and Shale Composition

Organic matter (OM) is the material basis of shale gas accumulation, so understanding the mechanisms of OM accumulation plays a critical role in shale gas exploration. The marine shale from the Ordovician Wufeng–Silurian Longmaxi formations is thought to be a good shale gas source in China's northwestern Hunan Province in the Upper Yangtze platform. A total of 37 samples were collected from a well SY5, and OM accumulation mechanisms were analyzed based on geochemical proxies associated with shale composition. Total organic carbon (TOC) decreased upward from Wufeng to the lower and upper members of the Longmaxi. Average TOC contents were 2.64, 1.24, and 0.29%, respectively. Wufeng shale developed from migrated OM and biogenic quartz, whereas depositional OM and terrigenous quartz were widespread in the lower Longmaxi, and barely any OM was observed in the upper Longmaxi. The Fe–S–C systems and Mo–U covariation suggested that the oxygen level increased from Wufeng to the upper Longmaxi. The organic-rich Wufeng was deposited under anoxic conditions, and redox conditions during the lower Longmaxi deposition fluctuated greatly (suboxic to euxinic) and were mainly suboxic. The upper Longmaxi was deposited under oxic conditions. High excess silica concentrations and the C/P ratio of Wufeng shale implied high paleoproductivity, which may have contributed to phosphorus recycling under a strong reducing environment. P/Ti ratios showed that the lower Longmaxi was deposited under moderate paleoproductivity and the upper Longmaxi was deposited under low paleoproductivity. The Al and Ti concentrations increased and Zr/Al decreased from Wufeng to the upper Longmaxi, reflecting increased terrigenous flux. The high terrigenous flux occurred as the diluent decreased OM concentrations during the Longmaxi deposition. The strong reducing conditions, high paleoproductivity, and low terrigenous flux, which were favorable to OM production and preservation during Wufeng deposition, further determined the content and types of OM and quartz. The high OM and quartz contents were associated with favorable types (migrated OM and biogenic quartz), creating Wufeng shale high-quality reservoirs, which were conducive to shale gas accumulation. Ba and P did not act as paleoproductivity proxies under strong reducing conditions due to their redistribution, and the V-related ratios were unreliable due to high thermal maturity of marine shale. The element Si was not a proxy of terrigenous flux when the silica was biogenic in origin, but the excess silica concentration was a reliable proxy for paleoproductivity.