The fault system is one of the structural carrier systems of gas hydrate accumulation, which plays a vital role in controlling the distribution of natural gas hydrate (NGH) accumulation. The previous studies mainly focus on summarizing the vertical migration mode of high flux fluid along the fault with obvious geophysical response characteristics on the seismic profile, such as “fault with gas chimney,” “fault with mud diapir,” and “fault with submarine collapse”, but lack of evaluation methods for the fault carrier system. We use the X sag in the deep-water continental margin slope area of the northern South China Sea as an example to study the fault systems closely related to NGH. This paper puts to use attribute technologies, such as coherence, curvature, and fusion, to analyze the characteristics and combination of the fault systems. We discussed migration patterns and evaluation methods of dominant fault carrier systems. This research proves that the strike-slip fault system in the platform area can directly connect the gas source bed with high-quality hydrocarbon generation to the gas hydrate stability zone (GHSZ). The activity of this fault system is more conducive to the accumulation of hydrocarbon in the GHSZ. This area has a good site for pore-filling gas hydrate prospecting and a preferential favorable fault carrier system. The composite fault system, consisting of a normal dip-slip fault system and a polygonal fault system, in the slope area can jointly communicate the biogenic gas-rich reservoir. Its activity and well-migration performance are the main reasons for the submarine gas leakage and collapse. It is a secondary favorable fault carrier system in the study area. There may be massive and vein natural gas hydrate formation in fractures in the leakage passage, and pore-filled gas hydrate may exist in the submarine nonleakage area. In this work, a three-factor evaluation method of the fault carrier system is proposed for the first time. This method is of great significance for the evaluation and exploration of NGH reservoirs in the continental margin slope area of the northern South China Sea.

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.

This study investigates the formation mechanism of natural gas hydrates in the deep-water Qiongdongnan Basin, with particular emphasis on the controlling role of sandy reservoirs in the hydrate accumulation system. On the basis of geological observations of sand bodies inside and outside the gas hydrate stability zone (GHSZ), including locally continuous hydrate-bearing sand bodies within the stability zone, the sealing effect of overlying mudstones, and sandy reservoirs near the base of the stability zone that provide pathways for the migration of underlying free gas, a new conceptual model for hydrate accumulation is proposed. A Transformer-encoder-based artificial intelligence algorithm was applied to three-dimensional seismic data and associated well-log data to predict the spatial distribution of sandy reservoirs and gas hydrate enrichment zones in the study area. The prediction results indicate high-saturation hydrate enrichment within high-quality sandy reservoirs inside the stability zone, and also suggest the sealing efficiency of overlying mudstones and the possible connectivity between sand bodies beneath the stability zone and deeper gas sources. This study supports the proposed accumulation mechanism and illustrates the application of artificial intelligence methods for hydrate reservoir prediction under complex geological conditions, providing an integrated interpretation and technical reference for selecting natural gas hydrate exploration targets in the deep-water northern South China Sea.

The bottom-simulating reflector (BSR) is an important seismic marker of the base of the gas hydrate stability zone and underlying free gas. Its amplitude variation with offset (AVO), expressed here as angle-dependent P–P reflection amplitude, depends on fluid composition, pore structure, clay content, and hydrate occurrence mode. Using a horizontal two-layer model, we compare the effects of hydrate saturation, free-gas saturation, hydrate reservoir porosity, clay content, and the hydrate frame participation fraction f_ms—the fraction of the total hydrate volume incorporated into the load-bearing solid phase—on BSR AVO responses. Elastic properties are calculated by solid averaging, a reference-constrained frame relation, fluid mixing, and Gassmann substitution. Response trends are checked using the exact Zoeppritz solution and two-dimensional staggered-grid finite-difference time-domain (FDTD) modeling, and the f_ms responses are compared between two dry-frame models. Increasing hydrate saturation from 0 to 80% changes the numerical intercept from −0.0971 to −0.2361; under uniform Wood mixing, the nonlinear free-gas response is concentrated at low saturation; porosity primarily changes the impedance contrast, whereas clay content primarily changes the amplitude variation with incidence angle. Increasing f_ms raises velocity, makes the intercept more negative, and increases the gradient, but whether the gradient changes sign depends on the frame model.

Capillary pressure and relative permeability are two key factors governing gas–water two-phase flow during hydrate depressurization; however, their relative contributions to gas production and the coupling mechanisms remain poorly understood. In this study, a unified and consistent framework has been developed to examine the coupled effects of capillary pressure and relative permeability for gas–water flow during hydrate depressurization. More specifically, an empirical relationship based on the van Genuchten (VG) model has been obtained for the entry pressure P₀ and the shape parameter m, together with their representative ranges, by regressing the capillary pressure data from hydrate-bearing sediments collected from the public domain. Then, a numerical model was constructed to quantify the individual and coupled effects of capillary pressure and relative permeability on gas production in a target hydrate formation. The simulations employed fixed P₀ values and prescribed relative permeability curves, without dynamically updating during hydrate dissociation. Across the compiled datasets, P₀ is found to decrease inversely with pore radius and increase as a power-law function with hydrate saturation, whereas m exhibits considerable scatter in both sand-dominated (0.62–0.82) and silt-dominated (0.52–0.85) sediments. Within the tested VG model, parameter ranges, and Shenhu reservoir conditions, capillary pressure primarily suppresses local gas migration by enhancing water retention, with its limited impact on gas production, while cumulative gas production is overestimated without considering capillary pressure. In contrast, relative permeability plays a dominant role in hydrate production by directly controlling gas and water flow capacity and influencing pressure propagation. This study clarifies the relative roles and coupling mechanisms of capillary pressure and relative permeability during hydrate depressurization and provides a practical basis for selecting van Genuchten parameters and conducting flow-function sensitivity analyses in hydrate-bearing reservoirs.