Radiolarian assemblage from the Upper Permian Wujiaping Formation in the eastern Sichuan Basin and its hydrocarbon source significance

Abstract

Objective The upper Permian Wujiaping Formation in the Hongxing area of eastern Sichuan Province has good shale gas accumulation conditions and is also an important successor for Permian shale gas exploration in the Sichuan Basin. However, the formation mechanism of these siliceous-argillaceous source rocks is still unclear. Apart from their petromineralogical characteristics, geochemical composition and macrofossil assemblage (represented by radiolarian) are also important for identifying the genesis of this siliceous-argillaceous source rock. Methods Radiolarian is a significant planktonic component of the Phanerozoic marine ecosystem. Various radiolarian assemblages indicate different sedimentary environments, and their abundance is a good indicator of palaeoproductivity. In this study, radiolarians were separated by the acid-etching method to identify different morphological characteristics, and the correlation between radiolarian abundance and total organic carbon (TOC) content was also analysed through thin-section observation. Results A large number of radiolarians were found in the Wujiaping Formation in the Well HY3 in the Hongxing area of eastern Sichuan Province. These fossils were identified as Spumellaria and Entactinaria, belonging to a typical shallow-water radiolarian assemblage, namely, the Se assemblage. In the Second Member of the Wujiaping Formation, the radiolarian abundance exhibited a relatively positive correlation with the TOC content. Conclusion Based on the radiolarian assemblage characteristics, sedimentary sequence and palaeogeographic background of the Wujiaping Formation, it is inferred that these source rocks from the Second Member of the Wujiaping Formation formed in a shallow-water environment less than 60 m deep. The accumulation of organic matter in the Second Member of the Wujiaping Formation was probably affected by primary productivity, redox conditions, terrigenous debris input and upwelling, among which high palaeoproductivity was one of the main controlling factors.

Keywords

• wujiaping formation
• hongxing area
• radiolarian assemblage
• organic matter accumulation
• paleoproductivity
• total organic carbon (toc)