Description: Logging-while-drilling (LWD) and wireline log (CWL) data were acquired during China's second gas hydrate drilling expedition (GMGS-2) in the east of Pearl River Mouth Basin, South China Sea. Disseminated and massive gas hydrates deposits were found at different sites. Gas hydrate-bearing lithologies identified from the sample coring included the fine-grained sediments and coarse-grained sediments. LWD logs from Site GMGS2-08 indicate significant gas hydrate in clay-bearing sediments including two layers with massive gas hydrate with a bulk density near to 1.08 g/cm3. High electrical resistivities with a range of 2.5–2000.0 Ω m and high P-wave velocities are simultaneously observed in the hydrate-bearing sediments. The average gas hydrate saturation estimated from the pore water freshing analysis ranges from 45 to 55% of the pore space. Buried carbonate layers above the massive gas hydrate deposit discovered at Sites GMGS2-08 indicate that the formations are likely to have formed initially at the surface and then were buried. Significant high amplitude seismic anomalies, discontinuous bottom simulating reflection (BSR) and blanking zone are detected in the drilling zone. The hydrate-bearing sediments predominantly consist of silty clay and limestone grains in which the gas hydrates are deposited primarily in the form of laminated, massive, veins or nodule. The gas hydrates occurrences are subjected to the sediment lithology, new tectonic activities, migration of fluid and gas and also the factors such as heat flow, salinity and time which affect the nucleation of gas hydrates. Its natural morphologies present massive, laminated, nodular, nugget and disseminated, of which the former four often formed in shallow fault, inter-layer's weak cementation zone and on the seabed. The “buried” gas hydrates with high saturation are good zones for gas hydrate exploitation.

Description: Integrated three-dimensional seismic, logging, sediment cores, and geochemical testing data were collected from Guangzhou Marine Geological Survey 3 and 4 hydrate drilling expeditions and used in this study for a comprehensive investigation of the geological and geophysical features and accumulation mechanism of hydrates in the first offshore gas-hydrate production test region (GHPTR) in the Shenhu area of the South China Sea. Seismic signatures indicative of disseminated hydrates and free gas include the bottom simulating reflector (BSR), gas chimney, and mud diapir associated with enhanced seismic reflections, acoustic blanking, masking, and chaotic appearance have been observed. The acoustic travel-time responses, density, and compensated neutron three porosity log analysis, high-precision grid tomography inversion analysis, and constrained sparse spike inversion confirm the presence of free gas below the gas-hydrate-bearing zone (GHBZ). Free-gas-bearing zones have significantly different p-wave impedances and low-velocity anomalies than the overlying GHBZ and surrounding strata. These anomalous zones are controlled by the structural attitude of the reservoir strata, which are characterized as inter-bedded stratigraphic units. Variations in the type and geological characteristics of the hydrocarbon migration pathways were observed between sites W18 and W19 on the western ridge and sites W11 and W17 on the eastern ridge in the GMGS study area. The efficiency of gas migration in the western ridge may be higher than that in the eastern ridge, resulting in variations in hydrate gas types, thickness of the GHBZ, and gas migration flux and accumulation. Except for site W11, hydrates were recovered below the structure I inferred BSR at sites W17, W18, and W19. The gas-hydrate stability zone calculations reveal that the structure I hydrate stability zone differs from the BSR depth and is generally shallower than the base of the logging anomaly, indicating the coexistence of structure I and II hydrates. The BSR is not indicative of the BGHSZ; it is rather regarded as a transitional indicator of structure I and II gas hydrates in the GHPTR. The appearance of free gas and hydrates below the structure I inferred BSR indicates that the Shenhu area is characterized by a complex hydrate formation and accumulation system resulting from the supply of biogenic and thermogenic gases. Despite fine-grained host sediments predominating the GHPTR, the coupling of favorable conditions including efficient hydrocarbon generation, sufficient gas supply, multiple pathways for gas migration, and relatively high reservoir porosity have led to the development of highly saturated gas-hydrate accumulations within relatively thick sedimentary sections, which demonstrates a significant resource potential.