Microscopic Differences and Connectivity Evaluation for Typical Unconsolidated Sandstone Gas Reservoirs in the South China Sea: A Case Study from Lingshui A Area, Qiongdongnan Basin

With the discovery of ultra-deepwater and ultra-shallow large natural gas reservoirs in the South China Sea, unconsolidated sandstone reservoirs have once again become a focal point of research. In response to the unclear controlling factors and the need for connectivity evaluation of unconsolidated sandstone reservoirs in the Ledong formation of the Lingshui A area in the Qiongdongnan Basin, this study employs a range of experimental techniques, namely, cast thin sections, scanning electron microscopy, X-ray diffraction mineral analysis, laser granulometry, and nuclear magnetic resonance (NMR), to investigate the microstructural characteristics of these reservoirs. The primary objective is to elucidate the controlling mechanisms behind pore-throat variability and to identify how sedimentary factors, mineral composition, and gas hydrate occurrence modes collectively influence pore-throat structural characteristics. Guided by fractal theory, a multidimensional analysis of pore-throat modal characteristics is conducted using a combination of image analysis, mercury intrusion capillary pressure, and NMR techniques. Furthermore, connectivity evaluation factors (CEFs) are constructed based on reservoir parameters to quantitatively assess reservoir connectivity and to establish connectivity evaluation standards. Results indicate that sedimentary factors are the principal determinants of pore-throat differences in the study area. Coarse grains, low clay content, and enhanced reservoir porosity contribute to improved permeability. At the microscopic level, variations in mineral composition also play a critical role. Higher quartz content and reduced clay mineral content, particularly illite and chlorite, are associated with excellent reservoir properties. By contrast, diagenesis exhibits a limited effect on reservoir quality. The gas hydrate occurrence state is characterized as a pore suspension type, exhibiting a relatively uniform pore-throat distribution. This load-bearing hydrate type may enhance pore-throat heterogeneity. Pore-throat modes are classified into three types: bimodal, multimodal I, and multimodal II. The bimodal hydrate occurrence state is dominated by the pore suspension type and characterized by low sorting coefficients, reduced fractal dimensions, increased uniform pore-throat structures, and improved reservoir connectivity. Five key reservoir parameters are selected, forming the basis of the CEFs, which comprehensively characterize reservoir connectivity. Ultimately, a connectivity evaluation standard and a microscopic connectivity model for typical unconsolidated sandstones in the South China Sea are established, providing critical guidance for future reservoir development strategies.