Abstract:

In recent years, with the breakthrough of well Eyiye 1, the Cambrian Shuijingtuo Formation (Series 2, Stage 3) has become a new target for shale gas exploration and development in the Middle Yangtze area. Based on previous studies, this paper presents a brief summary of the main fossil types and palaeobiological assemblages and their temporospatial distribution in the Shuijingtuo Formation to provide a reference for shale gas exploration in western Hubei. Well-preserved fossil assemblages occur abundantly in the Early Cambrian sedimentary succession in the Yichang, Zigui, Changyang and Xingshan areas, which provides an excellent geological record of the Cambrian explosion, with rapid metazoan diversification events spanning the Ediacaran-Cambrian transition, making this region an ideal place for studying Cambrian strata and paleontology. Many fossil Lagerstätten, namely the Yanjiahe biota, Shuijingtuo biota (including the Qingjiang biota), Shipai biota and Jingshan Burgess Shale-type biota, were recovered from the bottom part of the Cambrian Terreneuvian upward to Series 2. Among them, the Shuijingtuo biota is comparable with the Niutitang fauna in Guizhou and the Xihaoping biota in southern Shaanxi, and represented by fossils of more than 10 phyla, such as trilobites, radiolarians, brachiopods, bradoriids, hyoliths, archaeocyatha, monoplacophora, lobopodium, protoconodonts, sponges, gastropods, chancelloriids and possibly cnidarians, as well as some unidentified taxa and microflora. Whilst the Qingjiang biota found in Changyang is of typical Burgess Shale-type fossil Lagerstätten and distinguished by soft-bodied taxa, which opens up a new window on the Cambrian explosion in the deep water habitat far away from the coast. The Cambrian Shuijingtuo Formation in the Yichang area developed a black rock system with distinct lithology, palaeobiological assemblages and tectonic movement, which is dominated by black shale or calcareous shale mixed with carbon limestone and contains high abundance of diverse sponge fossils, indicating a deep-water, low-oxygen sedimentary environment at that time. Meanwhile, the flourishing of microbial organisms indicates the ocean at that time possessed high primary productivity that laid a good material foundation for the burial and enrichment of organic matter and formation of shale gas; and the hydrocarbon reservoir did not experience deformation and destruction owing to the Huangling anticline rigid substrate. Therefore, the black rock system of the Shuijingtuo Formation in the Yichang area has high exploration and development potential for shale gas.

Key words: Yichang area, Cambrian Series 2, Stage 3, Shuijingtuo Formation, fossil assemblages