Organic-rich siliceous rocks in the upper Permian Dalong Formation (NW middle Yangtze): Provenance, paleoclimate and paleoenvironment

The organic-rich shales of the upper Permian Dalong Formation in the middle Yangtze area are considered an important shale gas exploration horizon. However, the origin of siliceous matter and the mechanism of organic matter enrichment are still controversial. Based on geochemical data, we studied the provenance, paleoclimate, paleoenvironment, primary productivity and hydrological conditions of the Dalong Formation in western Hubei Province and discussed the mechanism of organic matter enrichment. The shales of the Dalong Formation can be divided into two units, organic-rich and organic-lean shales, with average TOC contents of 5.81 wt% and 0.45 wt %, respectively. The western Hubei Basin was a strongly restricted basin during deposition, and the provenance of the Dalong Formation had a continental margin sedimentary background, mainly from mixed felsic and basic sources. Terrigenous clastic quartz, biological quartz and hydrothermal quartz are recognized in the shales of the Dalong Formation. Based on U, V, and Mo concentrations and V/(V + Ni) ratios, the organic-rich shales were deposited in anoxic-sulfurized conditions. In contrast, the organic-lean shales were deposited under oxic-suboxic conditions. The Dalong Formation had moderate to high primary productivity and controls on the redox conditions in the basin. The decrease in primary productivity in organic-lean shales may be related to the transition from a warm and humid climate to a hot and dry climate. The sedimentary model shows that the formation of organic-rich siliceous rocks in the Dalong Formation was controlled by primary productivity. The decrease in primary productivity driven by the hot and dry climate in the latest Permian is the main factor controlling the enrichment of organic matter and the formation of bioquartz.