Sedimentary environment evolution of Wufeng Formation-Longmaxi Formation and its control on shale deposition in the southern Sichuan Basin
Based on core observation,thin section analysis,electron microscope examination,XRF scanning,whole rock X-ray diffraction,ICP-MS analysis,and total organic carbon content measurement,we analyzed the shale facies types and sedi-mentary environment evolution of Wufeng-Longmaxi Formation in southern Sichuan area,and discussed the controlling effect of high-frequency sedimentary environment evolution on lithofacies.The Wufeng-Longmaxi shale in southern Sichuan area can be divided into six lithofacies types:organic-rich layered biosiliceous shale,organic-rich laminated(felsic-calcareous)silty shale,organic-rich(calcareous)silty shale,organic-rich laminated calcareous shale,medium-organic laminated(felsic-cal-careous)silty shale,and medium-organic massive mudstone.The paleo-sedimentary environment is reconstructed utilizing characteristic element ratios Sr/Ba,U/Th,V/Cr,V/Ni,Ni/Co,Sr/Cu,as well as the content of Mo,Cu,Sr,and seven evolutionary stages are delineated:the lower Wufeng Formation deposited under warm,humid conditions,with relatively oxi-dized,low salinity water;the upper Wufeng Formation witnessed increased water reduction and enhanced paleoproductivity.The top of the Wufeng Formation experienced mass extinction during the Hirnantian glaciation,characterizedby arid and cold paleoclimate.From bottom to top,the paleoclimates of Longmaxi Formation experienced the evolution of"warm/wet"-"rela-tively arid/hot"-"warm/wet"-"relatively arid/hot"and the ancient water body also experienced the evolution from reduction to relative oxidation.The sedimentary environment significantly influences shale lithofacies and organic matter enrichment.The content of organic matter in different lithofacies is positively correlated with marine paleoproductivity and water reducibili-ty.Organic-rich layered biosiliceous shale and organic-rich laminated(felsic-calcareous)silty shale,deposited during stages of high paleoproductivity and strong water reduction,display abundant organic matter and high quartz content,making them favorable targets for shale gas exploration and development.This study provides a theoretical foundation for further exploration and development of shale gas and the investigation of fine-grained sedimentary rocks.
国家科技期刊平台
NSTL
万方数据
维普
