Block characteristics and controlling factors of oligocene clastic reservoirs in the BD21 deep-water deep-layer giant gas field, Qiongdongnan Basin

In recent years, milestone oil and gas exploration breakthroughs have been achieved on the northern slope of the Baodao Sag in the eastern deep-water area of the Qiongdongnan Basin, northern South China Sea. The BD21 high-temperature, normal-pressure large gas field was discovered for the first time in the Paleogene Oligocene Lingshui Formation of deep-water and deep-buried strata. To accurately verify the reserves of the BD21 high-temperature, normal-pressure large gas field in clastic reservoirs of the deep-water deep-buried Lingshui Formation, and to support the later development layout, dynamic adjustment and recovery factor improvement of the gas field, this study adopts thin-section identification, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), conventional reservoir physical property tests, together with drilling and logging data. It focuses on the compartmentalized reservoir characteristics, heterogeneity of reservoir physical properties and their controlling factors of the Lingshui Formation clastic reservoirs in the western, central, eastern and northern blocks of the BD21 large gas field, with special emphasis on the physical properties, distribution and controlling factors of relatively high-quality (sweet spot) reservoirs. The results show that, under generally consistent sedimentary and diagenetic conditions with minor inter-block variations, siliceous cementation derived from formation water during diagenetic evolution is the primary factor controlling the reservoir physical property heterogeneity among different blocks. For instance, the main Lingshui Formation reservoirs in the central block are buried deeper than 4100 m (including water depth), yet some intervals still exhibit moderate porosity and high permeability. By contrast, reservoirs in the western block are characterized by low porosity and low permeability. The main reason is that the central block experienced weak dissolution, and the associated formation water contained low silica-oxygen components, resulting in markedly weaker siliceous cementation than other blocks. In the western block, silicate minerals such as feldspar and glauconite underwent much stronger dissolution compared with the central, eastern and northern blocks. This raised the concentrations of silicon and oxygen in formation water, promoting extensive quartz overgrowths adjacent to dissolution pores and forming tight siliceous cement. Such cementation severely blocked pore throats and degraded pore connectivity. This study provides guidance and references for reserve verification, gas field development and production adjustment in the study area.