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Geochemical Features of Organic Matter and Trace Elements in Shales and Indication to Depositional Environments of Wufeng—Longmaxi Formations, Sichuan Basin, China


Marine shales in Wufeng Formation of the upper Ordovician to Longmaxi Formation of lower Silurian are rich in organic matter and are favorable targets for shale gas resources in the south of China. Geochemical characterization including the carbon and sulfur content, carbon isotopes of organic matter, major and trace elements analyses for fresh outcrop samples collected from Shuanghe town, Changning County in Sichuan basin was conducted in this study to define main geological controls to organic matter accumulation in Sichuan basin during the Wufeng—Longmaxi period. The results showed that TOC content was high (2.2%-7.76%) in the Wufeng Formation and lower part of the Longmaxi Formation, and the thickness of the high TOC interval was about 20m. TOC content sharply decreased from 1.83% to 0.81% in the upper part of the Longmaxi Formation and then became stable values of less than 1%. The dCorg values of the Longmaxi shale was ranging from −31.2‰ to −29.4‰, with an average of −30.1‰, indicating that organic matter was dominated by type-I and type-II. The major elements in the shales were dominated by SiO2 (53.9 wt.%), Al2O3 (10.1 wt.%) and CaO (9.5 wt.%). A positive correlation between SiO2 and TOC contents showed that organic-rich intervals contained high quartz contents and were favorable for the hydraulic fracturing. Excess Ba contents of 429.7-1433.6ppm revealed moderate productivity of organic matter, and the primary productivity wasn't the main factor to organic richness. Instead the redox conditions might play a critical role to the preservation of organic matter. Ni/Co, V/Cr and DOPT are good indicators to redox condition of bottom sea water column. Intermediate to high DOPT, Ni/Co and V/Cr indicated that the Wufeng formation was mainly deposited under suboxic to anoxic conditions, and high DOPT, Ni/Co and V/Cr suggested organic-rich lower part of overlying Longmaxi Formation was deposited under strong euxinic conditions. However, low DOPT, Ni/Co and V/Cr showed organic-poor upper part of Longmaxi Formation was formed in oxic bottom-water environment. Mo/TOC and Mo-U co-variation further showed that sedimentation during the Longmaxi period occurred under moderately restricted conditions, but during Wufeng period was strongly restricted conditions. This is the possible reason why the distribution of organic-matter shales in Longmaxi formation is more extensive than in Wufeng Formation.