A study on the enrichment of organic materials in black shales of the Wufeng to Longmaxi Formations in eastern Sichuan Basin

Black shales of the Wufeng to Longmaxi Formations, the main target strata for exploration of shale-type gas in South China, were formed from the Late Ordovician to the Early Silurian, a crucial transitional period in the Earth's history. In this paper, based on geochemical data of back shales and bentonites from the Huadi No.1 well and the Huangying section in Wulong, the enrichment mechanism of organic materials of black shales of the Wufeng to Longmaxi Formations in the eastern Sichuan Basin is studied in detail. The results show that the TOC varies in a high-low-high mode from the Wufeng Formation to Longmaxi Formation, and the redox indices (V/V+Ni, V/Cr, Ni/Co) imply that the water experienced an anoxic-oxic-anoxic process. The productivity (Cu, Ba_bio, Si_bio) and terrigenous inputs indices indicate that black shales of the Wufeng and Longmaxi Formations are with higher productivity and terrigenous inputs, compared with those of Guanyinqiao Formation. The correlations between the TOC in the sediments and redox indexes, productivity and terrigenous inputs show that the enrichment process of organic materials in black shales is controlled by the redox indexes, the productivity and terrigenous inputs. The redox indexes played the most important role, the productivity was in the second place and terrigenous inputs were in the third. It is found that frequent volcanic activities maybe have dual effects on organic-rich shale:on one hand, volcanic ash provided a sufficient supply of nutrients, which triggered high marine productivity; on the other hand, volcanic ash prevented more sunshine coming into the ocean. The extremely anoxic environment led to more death of living things, and enhanced the amount of organic materials buried and preserved.