Geochemical characteristics and organic matter enrichment of the Late Ordovician-Early Silurian black shale in Nanchuan area, Sichuan Basin

Abstract

The enrichment of organic matter in black shale is a controversial issue among the researchers and critical problem in the exploration of shale gas. In order to understand the relationship between sedimentary environment and organic matter accumulation in black shale of Wufeng Formation. Longmaxi Formation and limestone of Guanyinqiao Bed, multiple geochemical method, such as organic carbon, major and trace elements, are tested from the Nanchuan section. Research results show that the organic carbon is enriched in the middle to upper part of Wufeng Formation and the bottom of Longmaxi Formation. Ba and P contents exhibit a high primary productivity during Late Ordovician-Early Silurian in Sichuan basin. Redox proxies (V/Cr, Ni/Co) indicate that Wufeng Formation deposited in an anoxic environment and Longmaxi Formation deposited in euxinic condition, but Guanyinqiao Bed was predominated by oxic-dysoxic environment during the sedimentation. The poor relation between TOC and Ba, suggests primary productivity are not the major factors controlling the enrichment of organic matter, but an anoxic environment is the predominant condition causing the accumulation and preservation of organic matter suggested by a positive correlation between TOC and redox proxies.