Relationship between organic matter characteristics and depositional environment in the Silurian Longmaxi Formation in Sichuan Basin.

The carbon and sulfur content, Rock-Eval pyrolysis and the carbon isotopes of organic matter were analyzed using 124 fresh outcrop samples collected from the Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation in Shuanghe Town, Changning County in Sichuan Basin. The results show that TOC content is high (3.04% to 7.28%) in the Wufeng Formation and the lower part of the Longmaxi Formation, and the thickness of high TOC interval is about 20 m. The TOC content sharply decreases from 0.81% to 1.83% in the upper part of the Longmaxi Formation and then becomes stable with its content values less than 1%. The sulfur content ranges from 0.02% to 4.69%,with an average of 0.95%. The Δ13Corg values of the Longmaxi shale are concentrated in the range of -31.2‰ to -29.4‰, with an average of -30. 1%‰, indicating that organic matter is dominated by Type I and Type II. Combined with petrology and fossils characteristics of the Longmaxi Formation, the authors suggest that the main controls to the organic matter enrichment are sea level rise caused by global warming and the deep water reducing environment favorable for organic matter high productivity. In addition, a relatively slow deposition rate and reducing conditions lead to the better preservation of the organic matter. Therefore, the Wufeng Formation and the lower part of Longmaxi Formation marine shales with abundant TOC content in Sichuan basin is the main target for shale-gas exploration. [ABSTRACT FROM AUTHOR]