Pore Evolution of Dolomite Reservoirs in Different Sedimentary Facies Zones: A Case Study of the Cambrian Xiaoerbulake Formation in Bamai Area, Tarim Basin

The Cambrian Xiaoerbulake Formation in the Tarim Basin is a key strategic layer for deep oil and gas exploration in the basin. However, the current research on the dolomite reservoirs of the Xiaoerbulake Formation in the Bamai area focuses on reservoir characterization, and there are still some controversies in its multi-stage diagenetic evolution sequence and the genetic mechanism of deep-seated pores. This study is based on the identification of core cuttings from multiple wells in the Bamai area, combined with carbon and oxygen isotopes, rare earth elements, fluid inclusions and other test methods to carry out systematic analysis: The main reservoir rocks of the Xiaoerbulake Formation in the study area are microbialite and granular dolomite: the two types of rocks have high original porosity, which are expanded by penecontemporaneous atmospheric fresh water dissolution, and the primary pores are effectively preserved by penecontemporaneous dolomitization; after the burial period, the degree of dolomitization increased, and some of the original rocks were transformed into fine-grained dolomite with intercrystalline pores, accompanied by the filling of primary pores by dolomite and calcite in the formation water. After the hydrothermal fluid enters the reservoir along the fault, although the hydrothermal process is usually accompanied by TSR reaction to generate acidic fluid, which can dissolve the early filling minerals and form small-scale secondary pores, it is still dominated by hydrothermal carbonate, sulfate and other mineral precipitation filling. Gypsum dolomite can also be used as an important reservoir. Gypsum minerals in such rocks are easy to form gypsum dissolution pores through atmospheric water dissolution in the penecontemporaneous period, and the connectivity of reservoir rocks is improved and further dissolved under the transformation of structural fractures. In the area, the sealing conditions formed by the underlying Yuertusi Formation and the overlying Wusonggeer Formation are conducive to the preservation of the original pores of the reservoir; the role of structural faults is dual: it can not only communicate different reservoir spaces and enhance reservoir percolation capacity, but also serve as a dominant migration channel for hydrothermal fluids, leading to hydrothermal mineral precipitation and destruction of reservoirs. In general, the pore evolution of different sedimentary facies belts in the study area is the result of the multi-stage coupling of the early Cambrian sedimentary pattern and multi-stage tectonic-diagenetic fluid. The research results can provide new theoretical support for the study of the evolution of Cambrian carbonate reservoir space in the northern margin of the Tethys.