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.

Estimating organic carbon fluxes in data-limited tropical island catchments is constrained by sparse discharge records and concentration measurements. For the Changhua River, Hainan Island, we compared three calendar-month runoff-coefficient models based on the mean, median, and geomet-ric mean with a linear model incorporating current- and previous-month precipitation. Models were fitted to monthly records from 2011–2023 and evaluated against all 12 months of 2024. Holdout root mean square errors were 42.669, 39.544, 38.382, and 31.801 mm, respectively. The dual-precipitation model performed best, with a Nash–Sutcliffe efficiency of 0.763 and a percentage bias of −10.220%. Its fixed parameters were applied to September–November 2025 precipitation. The resulting monthly equivalent runoff coefficients were transferred to Ningyuan and Lingshui using their precipitation and drainage areas. Estimated runoff volumes were multiplied by equally weighted mean concentrations from three October–November surveys. Conditional dissolved or-ganic carbon exports were 2763.373, 730.024, and 655.773 t C for Changhua, Ningyuan, and Lingshui, respectively; corresponding particulate organic carbon exports were 571.135, 201.645, and 441.045 t C. These estimates depend on unverified catchment transferability and concentration rep-resentativeness, particularly for unsampled September. They apply to a limited observation period and do not establish annual carbon budgets or carbon sequestration.

Submarine natural gas hydrate accumulation requires deep methane to enter the gas hydrate stability zone, become immobilized within a reservoir, and remain confined beneath a caprock. The long-term coupling among reservoir supply, caprock retention, and hydrate self-sealing remains poorly resolved. We developed a mass-conservative synthetic two-dimensional Darcy flow and reactive-transport model that couples a quasi-steady potential field, mobile methane transport, supply- and pore-capacity-limited hydrate formation, and saturation-dependent permeability reduction. Single-factor, full-factorial, global-sensitivity, numerical-convergence, and long-term experiments showed that reservoir and caprock permeability both controlled methane supply; hydrate mass was more sensitive to caprock permeability (PRCC = 0.919) than to reservoir permeability (PRCC = 0.738), and the reservoir response plateaued above approximately 100 mD. Caprock permeability and thickness jointly regulate flux and leakage, with maximum conversion occurring at intermediate caprock permeability rather than at maximum flux. In the 30 kyr reference case, self-sealing reduced source-zone flow by 7.14%. In the 500 kyr extension, hydrate-mass growth and source-flow change first remained simultaneously below 1% over the preceding 10 kyr at approximately 338 kyr. The parameter combinations maximizing hydrate mass, conversion efficiency, and sustained accumulation differed, and, at 30 kyr under the stated thresholds, only 8 of 81 combinations jointly achieved high conversion, elevated reservoir saturation, and low leakage. Favorable accumulation therefore reflects dynamic matching among supply, retention, and self-sealing under specified boundary conditions, rather than an isolated extreme in any single property.

Clay-surface conduction in fine-grained, weakly consolidated marine sediments, together with the scale mismatch between core and logging observations, often causes resistivity- and acoustic-based estimates of gas hydrate saturation to diverge. For Well GMGS5-W08, we develop a core-constrained, scale-matched electroacoustic fusion method (CC-SMEF) for gas hydrate saturation estimation. The method derives a low-frequency clay background from gamma-ray and core clay data. An optimizable vertical window then summarizes the logs around each core reference depth as robust neighborhood features. A low-dimensional, regularized kernel map integrates deep resistivity, P-wave velocity, gamma ray, and the clay background on this common representation scale. Chlorinity-derived saturation provides the geochemical reference, and the model was compared with the Archie and Simandoux methods at all reference points and with the isotropic three-phase acoustic method at available matched depths. Pointwise leave-one-out evaluation indicated lower observed error than Archie, although the uncertainty interval included no improvement and explanatory capability remained moderate. The fixed-parameter ablation is exploratory. Under the stricter four-block protocol, saturation-model tuning and fitting were restricted to training blocks while the clay background remained calibrated from all 22 core clay measurements; CC-SMEF RMSE was 17.89 percentage points versus 15.87 for Archie, so superiority in held-out-depth prediction was not established. CC-SMEF is therefore interpreted as a local interpolation framework for the represented W08 intervals; application elsewhere requires independent calibration and validation.