A Cloudina-rich Bed from the Dengying Formation,Northeastern Sichuan Basin,and Its Reservoir Characteristics
[Objective]The discovery of the Anyue gas field and the Mianyang-Changning intratonic sag has attract-ed considerable attention to the strata near the Ediacaran-Cambrian boundary in the Sichuan Basin.The reservoirs of these oil and gas fields occur mainly in the microbial and karst porous rich dolomite of the second and the fourth mem-ber of Dengying Formation.This study discusses the impact of Cloudina skeletal remains,one of the earliest metazo-an mineral skeletons,on the terminal Ediacaran carbonate reservoirs,in order to expand regional oil and gas explora-tion strategies.[Methods]Drilling core material from the topmost Dengying Formation of the well Luye 1,northeast-ern Sichuan Basin,was examined for sedimentology,diagenesis,pore types,and porosity evolution,using a combi-nation of methods including optical microscopy,cathodoluminescence,scanning electron microscopy,and micro-CT analysis.[Results and Discussions]Abundant tubular animal fossils,diagnosed as Cloudina,were aggregated in a layer of bioclastic micrite dolomite.These fossils are preserved in a micritic to microsparitic matrix,with their body axes oriented along the bedding plane.Micritic clots and filaments of probably microbial origin were found all around.The recognized pore types include biogenic pores formed by tubular fossils,intergranular pores,inter-and intracrys-talline dissolution pores,and bitumen pores.The main diagenetic processes include dissolution,cementation,and filling at different periods.The high-resolution micro-CT scan shows that the porosity of the studied material is con-spicuously affected by the presence/absence of the Cloudina skeleton.Only 0.1%to 0.4%of the porosity can be attributed to the dolomitic matrix,while the contribution of the tubular fossils is more than 0.7%.[Conclusion]Cloudina aggregates can contribute a considerable amount of porosity to the terminal Ediacaran dolomite,in addition to the microbial fabrics.The presence of mineralized animal skeletons at the end of the Ediacaran not only marks a big step in biological evolution but has also profoundly changed the types of oil and gas reservoirs.
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