[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(represen-ted by radiolarian)are also important for identifying the genesis of this siliceous-argillaceous source rock.[Meth-ods]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 palaeoproduc-tivity.In this study,radiolarians were separated by the acid-etching method to identify different morphological char-acteristics,and the correlation between radiolarian abundance and total organic carbon(TOC)content was also an-alysed through thin-section observation.[Results]A large number of radiolarians were found in the Wujiaping For-mation in the Well HY3 in the Hongxing area of eastern Sichuan Province.These fossils were identified as Spumel-laria 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.
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