A relationship between elemental geochemical characteristics and organic matter enrichment in marine shale of Wufeng Formation-Longmaxi Formation, Sichuan Basin

Abstract

In this study, a test was carried out on organic carbon, sulfur content, major and trace elements of 24 shale samples from fresh outcrop sections in Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation, Shuanghe Town, Changning County, Sichuan Basin. Moreover, this study analyzed the vertical variation characteristics of major and trace element contents and their relationship with sea-basin paleo-sedimentary environment and explored the major controlling factors for organic matter enrichment in Ordovician-Silurian shale of Sichuan Basin. Research results show that the organic carbon content is higher (2.2%-7.76%)in the reservoir intervals about 20 m above Wufeng Formation to the bottom of Longmaxi Formation, then decreased upwards to stable value of 0.81%-1.83%. Major components of shale include SiO2(55.67%), Al2O3(10.13%)and CaO (9.51%). The contents of TiO2and SiO2are lower at the bottom of profile and then increased upwards, indicating the increased input of terrigenous clastics. The SiO2and CaO contents of organic-rich shale are apparently higher than those of organic-poor shale, but the contents of Al2O3, Fe2O3and TiO2are lower. Meanwhile, redox sensitive elements (Mo, U, V, Ni, Co, Cr)and nutritional elements Ba are significantly enriched in the organic-rich interval. Ni/Co, V/Cr, U/Th and the degree of pyritization(DOPT) indicate that Wufeng Formation presents large changes in water mass redox conditions, dominated by suboxic and anoxic conditions. The water mass at the bottom of Longmaxi Formation has a higher reducibility than Wufeng Formation, indicating a euxinic environment with certain content of H2S. However, normal oxygen enrichment environment exists in the middle and upper part. The contents of P and excess Ba (Baxs) in shale demonstrate a higher productivity in the sedimentary stage of Wufeng Formation-Longmaxi Formation, while TOC content has a positive correlation with redox proxies, suggesting that the changes in organic matter content is predominantly controlled by redox environment. The cross plots of Mo/TOC and DOPTshow that redox condition and water retention is the major controlling factor for trace element enrichment in Longmaxi Formation shale and Wufeng Formation, respectively. Organic matter enrichment in Longmaxi Formation is mainly controlled by the degree of seawater anoxic resulting from the rise and fall of sea level. However, anoxic environment in Wufeng Formation caused by water retention leads to favorable preservation of organic matter. © 2015, Editorial Office of ACTA PETROLEI SINICA. All right reserved.