The Cloudina at the Top of the Dengying Formation, Northeastern Sichuan Basin, and its Sedimentary Significance

Key words: 
• Northeastern Sichuan region /
• Ediacaran Period /
• Cloudina /
• pores /
• hydrocarbon reservoirs

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.