Advances in Marine Gas Hydrate Exploration and Discovery

Natural gas hydrates are extensively distributed across terrestrial permafrost zones and continental margins worldwide [1]. Recognized as a clean and efficient energy resource, they have attracted significant exploration interest from major global economies [2,3,4,5]. In recent decades, substantial progress has been achieved in hydrate exploration and discovery, driven by advancements in multidisciplinary approaches, including geological, geophysical, and geochemical investigations, as well as numerical simulation techniques [6]. Current research on gas hydrate focuses on several critical aspects: (1) the origin of hydrate gas and its genetic relationship with deep conventional oil and gas reservoirs [7,8,9]; (2) the mechanisms governing hydrate accumulation and reservoir formation [10]; (3) the spatial heterogeneity in hydrate deposits, including their characteristics, evolution, and controlling factors [11]); and (4) innovations in drilling and production testing technologies [12,13,14]. This research domain represents a highly interdisciplinary field, integrating knowledge from tectonics, sedimentology, geophysics, geochemistry, petrology, mineralogy, and numerical modeling [15]. The expansion of this field has been facilitated by advancements in field exploration methodologies, laboratory analytical techniques, and computational simulation capabilities. Substantial progress in gas hydrate research has been driven by scientific and technological breakthroughs, including field exploration and discoveries in the South China Sea (SCS) [16] and other continental margins worldwide [17], as well as laboratory-based numerical simulations [18,19]. This Special Issue aims to advance research on gas hydrate exploration and discovery in continental margins as well as gas hydrate test mining simulation, with an emphasis on the mechanism of accumulation and the assessment of marine hydrate’s environmental and resource benefits. By addressing these key aspects, we seek to provide theoretical insights and methodological frameworks to support future exploration, drilling, and test production of marine hydrates.

Understanding the controlling factors of submarine gas seepage and bottom current activity on pockmark formation and development is crucial for interpreting seafloor geomorphological evolution. Li et al. (contribution 1) revealed that in the northwestern Xisha Uplift region, deep-sourced gas migrates along fault systems, fractures, and gas chimneys before escaping at the seafloor, where it forms characteristic pockmark features. Their study further demonstrates that established pockmarks undergo significant modification under the influence of bottom current activity.

Precise identification of mass transport deposits (MTDs) in gas hydrate exploration significantly improves bottom simulating reflector (BSR) detection accuracy and facilitates refined resource assessment. Gong et al. (contribution 2) demonstrated that MTDs with low-permeability, high-density properties effectively cap shallow deep-sea fluids. These deposits inhibit upward free gas migration from underlying strata, promoting gas accumulation for hydrate formation below.

Identifying hydrate gas sources is crucial for predicting their distribution. Zhong et al. (contribution 3) employed acid extraction in deepwater depressions to detect hydrocarbons, proposing a “lower-generation, upper-accumulation, and micro-fracture leakage” model to explain hydrocarbon anomalies in the Chaoshan Depression, SCS. Here, underlying reservoirs control hydrocarbon migration via micro-fractures to seabed surfaces. Su et al. (contribution 4) suggested thermogenic gas, associated with mud diapirism and fault migration, as the primary hydrate source.

Shallow gas, with substantial resource potential, has been verified through 3D seismic data and drilling in deepwater areas. Yang et al. (contribution 5) employed seismic data and geophysical attributes to examine how sedimentary facies interfaces control shallow gas distribution. Their study revealed that vertically stacked channelized submarine fans, which onlap these facies interfaces, serve as primary shallow gas reservoirs. The researchers found sand-rich sediments aligned along the southwest-northeast trending facies boundary, effectively constraining shallow gas accumulation within the basin.

The three-phase coexistence zone (hydrate-gas-water) represents a critical research focus internationally due to its high exploitation potential. Yu et al. (contribution 6) established an advanced characterization model by applying deterministic complex geological modeling techniques combined with seismic and logging data. Their model accurately captures geological structure interactions while using them as auxiliary constraints, significantly reducing accuracy uncertainties caused by complex geological conditions.

Understanding how mineral composition, grain size, and sedimentary processes govern gas hydrate accumulation is crucial for predicting reservoir distribution and guiding exploration. Bai et al. (contribution 7) demonstrated that while grain size shows limited influence, clay mineral content (particularly smectite) in fine-grained sediments significantly inhibits hydrate formation, highlighting mineralogy’s dominant role in accumulation efficiency.

Understanding the accumulation and evolutionary processes of gas hydrates is crucial for elucidating their formation mechanisms and assessing resource potential. Guan et al. (contribution 8) developed a novel flow-reaction model to investigate the spatiotemporal evolution of gas hydrate systems in the SCS. Their study systematically compared six distinct environmental scenarios and three cases of paleo-hydrate occurrence. Remarkably, the simulation results demonstrate strong concordance with observed field distributions of massive hydrate reservoirs dating back to 30 kyr BP (before present), providing valuable validation of the model’s predictive capabilities.

Understanding the mechanical properties of gas hydrate-bearing sediments (GHBS) is essential for safe and sustainable hydrate production. Yuan et al. (contribution 9) revealed that elevated hydrate saturation and confining pressure substantially improve GHBS strength and stiffness, while inducing more significant dilatancy during shear. Their study further demonstrates hydrate’s dual role in pore-filling and sediment-cementation, with both effects being strongly dependent on saturation and confinement conditions.

Wan et al. (contribution 10) developed a gas hydrate reservoir model using China’s offshore hydrate test data, numerically analyzing production behavior and reservoir evolution during stepwise depressurization via vertical wells. Their results demonstrate that this approach sustains favorable gas-to-water ratios while enhancing production efficiency—reducing water output mitigates sanding issues and improves economic viability.

Sand production significantly challenges marine gas hydrate extraction. Liu et al. (contribution 11) investigated gas-water-sand inflow patterns in both well orientations. For screen-only completions, sand control precision should be tiered based on screen plugging heterogeneity. Gravel-packed completions require (1) increased gravel density without reservoir destabilization, (2) cement-coated gravel for enhanced stability, and (3) screen precision tailored to particle size distribution for sustained production.

Shi et al. (contribution 12) conducted physical and numerical simulations of gravel packing to evaluate reduced-density materials and their packing/sand control performance. Their numerical results demonstrated that lightweight ceramsite performs poorly in horizontal and highly deviated wells due to inadequate compaction and insufficient sand retention, particularly with viscous slurries. In contrast, high-density particles promote better gravitational settling, yielding superior packing density and sand control effectiveness.

Assessing seafloor subsidence during hydrate extraction is crucial for safe gas hydrate production. Song and Zou (contribution 13) employed a modified Mohr–Coulomb model to characterize hydrate-bearing sediments, tracking strength degradation during depressurization. Using shear strength analysis to evaluate slope stability, they demonstrated that hydrate dissociation progressively slows during extraction. Their results indicate that large-scale submarine landslides triggered by hydrate decomposition are unlikely to occur.