The Cloudina at the Top of the Dengying Formation, Northeastern Sichuan Basin, and its Sedimentary Significance
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摘要: 【目的】近年来,继安岳气田的发现和绵阳-长宁裂陷的提出,四川盆地埃迪卡拉系-下寒武统地层得到了重点关注。这些油气藏的储集层主要发育灯影组二段和四段的微生物岩和富岩溶孔隙白云岩中。本研究旨在讨论埃迪卡拉纪末期首次出现的动物骨骼化石对当时白云岩储层的潜在影响,以拓展区域油气勘探开发思路。【方法】初步调查了四川盆地东部的鹿页1井灯影组四段顶部展开了岩石学、古生物学及孔隙类型,利用阴极发光方法得到了各种孔隙类型与成岩期次的演化序列,使用micro-CT技术揭示研究材料中的孔隙分布。【结果】研究发现川东巫溪地区灯影组顶存在较丰富的管状动物化石,依据其形态特征可鉴定为Cloudina。Cloudina富集层中富含生物骨架结构形成的生物孔,此外在基质中还存在粒间孔、晶间溶孔、沥青孔、晶内溶孔等多种孔隙类型。研究材料中的孔隙形态与化石分布呈密切相关。其中,化石富集层具有0.7%的孔隙度,远高于基质中0.1%-0.4%的孔隙度。【结论】富集成层的Cloudina的矿化骨骼可显著提升碳酸盐岩储层的孔隙度。这说明埃迪卡拉纪末期动物矿化骨骼的出现不仅是生物演化上的创新,也为油气储层的类型带来了深刻的改变。Abstract: [Objective] The discovery of the Anyue gas field and in the Mianyang-Changning Sag have directed considerable attention to the Ediacaran-Cambrian boundary strata in the Sichuan Basin. The reservoirs of these oil and gas fields occur mainly in the microbial and dolomite-rich bioclastic limestones of Dengying Formation II and IV. This study discusses the biomineralized (skeletal) fossils of Cloudina that lived at the end of the Ediacaran Period, and their potential impact on the carbonate reservoirs of that age. [Methods] A preliminary investigation of the lithology, paleontology and pore types was conducted on material from Well Luye 1 at the top of Dengying Formation IV, eastern Sichuan Basin. A range of pore types and their diagenetic sequences was recorded using cathodoluminescence methods. The pore distribution was determined by micro computed tomography (micro-CT). [Results and Discussions] In the Wuxi area of eastern Sichuan the abundant tubular fossils at the top of the Dengying Formation were identified as Cloudina, and is rich in biopores formed by the skeletal structures. The matrix also contains various pore types (intergranular pores, inter- and intracrystalline dissolution pores and bitumen pores). The morphology of the pores in the research material is closely related to the fossil distribution. The porosity of the fossil-rich layer is 0.7%, much higher than the matrix porosity of 0.1-0.4%. [Conclusion] Cloudina occur as a type of biomineralized tubular fossil which greatly increases the porosity of carbonate reservoirs. This indicates that the presence of mineralized animal skeletons at the end of the Ediacaran Period not only marks a step in biological evolution, but it has also resulted in profound changes to the types of oil and gas reservoirs.
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Key words:
- Northeastern Sichuan region /
- Ediacaran Period /
- Cloudina /
- pores /
- hydrocarbon reservoirs
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