Extensive accumulation of organic matter in the Late Permian Dalong Formation, Western Hubei Trough, Southern China

Abstract

The black shale in the Upper Permian Dalong Formation in the Western Hubei (WH) Trough is characterized by its extreme organic matter accumulation (OMA), and its vertical heterogeneity necessitates more nuanced research. The characteristics of the lithology, conodont, total organic carbon (TOC), mineral composition, silica origin, palaeoenvironmental evolution, and the OMA mechanisms of the Dalong Formation were systematically studied. The results indicate that the Dalong Formation can be subdivided into six distinct members, primarily composed of argillaceous siliceous rocks, siliceous dolomite, siliceous shales, calcareous siliceous rocks, and siliceous limestone, respectively. The silica in the organic-rich shale is predominantly of mixed biogenic and terrigenous origin, with localized contributions from hydrothermal sources. The organic-rich shale (ORS) is primarily distributing below the Wuchiapingian and Changhsingian boundary (WCB), which is deposited under moderate terrigenous input, semi-humid to semi-arid climate, euxinic conditions in a semi-open to the semi-restricted basin, and high palaeoproductivity caused by the upwelling setting and high-frequency volcanic ash layers. The WCB is characterized by the arid climate, oxic conditions in a strongly restricted basin, and low palaeoproductivity. Palaeoproductivity is identified as the principal controlling factor for the OMA. Additionally, the hydrothermal silica is commonly deposited under euxinic conditions, which are conducive to the preservation of organic matter. Evidence suggesting that thicker volcanic ash layer during the Changhsingian Stage may be one of the reasons for the mass extinction in the Late Permian. The north-central of the WH Trough exhibits a higher TOC content (> 4 %), overmature source rock with the vitrinite reflectance (R_o) ranging from 2 % to 3 %, and an euxinic conditions, suggesting more beneficial to shale gas enrichment.