Occurrence-Type-Constrained Well-Logging Evaluation of Natural Gas Hydrates in the South China Sea: Methods, Applicability

Natural gas hydrate reservoirs in the South China Sea contain pore-filling, fracture-filling, and pore–fracture composite occurrences in fine-grained, clay-rich, and locally sandy sediments. Existing logging studies provide numerous resistivity, acoustic, nuclear magnetic resonance, image-log, and multimineral methods, but model assumptions are not always matched to occurrence type. This review organizes evidence from the Shenhu, Qiongdongnan, and Dongsha areas along an occurrence type–response mechanism–model selection–independent validation–uncertainty chain. Pore-filling reservoirs require mineral and clay constraints before integrated volumetric inversion using resistivity, acoustic, density/neutron, and Nuclear Magnetic Resonance (NMR) logs. Fracture-filling reservoirs should be evaluated primarily from electrical images, azimuthal resistivity, and electrical/acoustic anisotropy, with fracture and matrix porosities treated separately. Composite reservoirs require staged matrix inversion and fracture-component estimation, whereas hydrate–free-gas coexistence intervals require multiphase rock physics. We propose a workflow that classifies occurrence type before inversion, imposes model entry and exit criteria, validates results with independent evidence, and reports ranges rather than unsupported point estimates. This framework clarifies why no universal high-resistivity–high-velocity template or saturation equation is valid for all South China Sea hydrate reservoirs.