Factors controlling organic matter accumulation in the Longmaxi Formation shale, Changning area, South Sichuan Basin

Abstract

Based on total organic content (TOC) tests and major-trace element test data, this study examined organic matter accumulation and controlling factors of the Longmaxi Formation shale in the Changning area, southern Sichuan Basin. The results showed that (1) TOC content of the Long11 submember (S1l11) shale was between 0.46% and 8.35%. Vertically, TOC revealed a ‘sandwich cake’ pattern with a high TOC content concentrated in the 1st layers of Long11 submember (S1l11–1) and 3rd layers of Long11 submember (S1l11–3) and a lower TOC content in the 2nd layers of Long11 submember (S1l11–2) and 4th layers of Long11 submember (S1l11–4). Spatial distribution of TOC showed variations and migration in S1l11. (2) S1l11 shale TOC showed a logarithmic correlation with sedimentary environment index, negative correlation with terrigenous influx, and complex correlation with paleoproductivity. (3) Factors controlling organic matter accumulation were mainly redox conditions, followed by paleoproductivity and terrigenous input in Changning area. When the paleoproductivity index (Babio) was less than 2200 μg/g, the redox conditions was the controlling factor of organic matter accumulation. On the contrary, it is ultrahigh paleoproductivity. Vertically, organic matter accumulation mode evolved from “preservation conditions” to “productivity–preservation conditions synergy mode” to “productivity mode”. (4) An anoxic condition was the basis of organic matter accumulation. Medium–high paleoproductivity was the key to organic matter accumulation. These study results are expected to contribute to shale gas exploration and development.