Reservoir Body Development Characteristics in Deep Carbonate Gas Reservoirs: A Case Study of the Fourth Member of the Dengying Formation, Anyue Gas Field

Abstract

Deep carbonate rocks are characterized by strong heterogeneity and fracture and cavity development, which have important influence on the storage and seepage capacity of reservoirs. To comprehensively characterize the developmental characteristics of the reservoir body in the intra–platform reservoir of the fourth member of the Dengying Formation in the Anyue gas field, this study employed a multiscale pore–throat structure characterization method that combines physical property analysis, core surface observation, cast thin section observation, a nuclear magnetic resonance (NMR) test, and CT scanning analysis. The results reveal that the primary storage spaces in the intra–platform reservoirs consist of inter–crystalline pores and small cavities (<2 mm), with thick throats and fractures serving as the primary flow channels. The rock density is lower in areas where solution fractures and cavities are developed, and the fractures and cavities are generally distributed in clusters. Notably, the intra–platform reservoir of the fourth member of the Dengying Formation is characterized by low asphaltene content. The presence of fractures in fracture–cavity type cores can reduce seepage resistance in the near–fracture area and enhance the drainage efficiency of small pores, as observed in the NMR test combined with centrifugation. In the centrifugal experiments, the increase in centrifugal force had the most significant impact on drainage efficiency, with the highest efficiency being 25.82% for cavity–type cores and the lowest being 6.39% for pore–type cores. Furthermore, by integrating the results of cast thin section and NMR test, the cavity–type reservoirs were further classified into two categories: dissolved cavity storage type and dissolved pore storage type. This study clarifies the storage and seepage characteristics of dissolved–pore storage reservoirs, which are challenging to develop but have high development potential. With reasonable surface operation measures, these reservoirs can provide important support for stable production in the middle and late stages of intra–platform reservoir development.