Beschreibung: Elemental sulfur is commonly regarded as the product of oxidative sulfur cycling in the sediment. However, reports on the occurrence of elemental sulfur in seepage areas are few and thus its origin and mechanisms controlling its distribution are insufficiently understood. Here, we analyzed the multiple sulfur isotopic compositions for elemental sulfur and pyrite from an iron-dominated gas hydrate-bearing sedimentary environment of the South China Sea to unravel the impact of sulfate-driven anaerobic oxidation of methane (SO4-AOM) on the formation of elemental sulfur. The multiple sulfur isotopes reveal variable ranges for both elemental sulfur and pyrite (δ34S: between −15.7 and +23.3‰ for elemental sulfur and between −35.3 and +34.4‰ for pyrite; Δ33S: between −0.08 and +0.06‰ for elemental sulfur and between −0.03 and +0.15‰ for pyrite). The enrichment of 34S in pyrite throughout the sediment core suggests pronounced SO4-AOM in paleo-sulfate-methane transition zones (SMTZ). In addition, the occurrence of seep carbonates with very negative δ13C values (as low as −57‰, V-PDB) coincides with the inferred paleo-SMTZs and agrees with formerly locally pronounced SO4-AOM. Interestingly, the multiple sulfur isotopic composition of elemental sulfur reveals a different pattern from that of pyrite derived from organoclastic sulfate reduction (i.e., with low δ34S and high Δ33S values for the latter). In comparison to coexisting pyrite, most of the elemental sulfur reveals higher δ34S values (as much as +28.9‰), which is best explained by an enrichment of 34S in the residual pool of dissolved sulfide generated by SO4-AOM. As an intermediate sulfur phase, elemental sulfur can form via sulfide oxidation coupled to iron reduction, but it can only persist in the absence of free sulfide. Therefore, the occurrence of 34S enriched elemental sulfur is likely to represent an oxidative product after hydrogen sulfide had vanished due to vertical displacement of the SMTZ. Our observations suggest that elemental sulfur may serve as a useful recorder for reconstructing the dynamics of sulfur cycling in modern and possibly ancient seepage areas.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Research on the formation and distribution of submarine channel systems and associated gas-bearing fluids is of great significance for gas hydrate exploration. Disseminated gas hydrates with high saturation up to 65% were recovered from a submarine ridge, equivalent to the levee of the channel–levee system in the Shenhu area, northern South China Sea. Sedimentary deposits in the submarine ridge were dominated by fine-grained silt and clay-rich silt; gas hydrates with relatively high saturation preferentially accumulated in coarser sediments with less clay content. Although abundant foraminifera fossils may have increased reservoir pore space, their presence was not a necessary condition for high-saturation hydrates. Higher levels of pyrite appeared in the reservoirs corresponding to high-saturation hydrates, which suggests that the reducing environment caused by sufficient methane provided adequate gas to form higher-saturation hydrates. Because of the migration of the channel–levee system, different channels formed their respective depositional systems composed of channel-filling, buried channel-filling, erosion grooves, and slumped turbidities. Relatively coarse-grained deposits were identified in the channel fillings and levees, and the accumulation of hydrates was affected by the lithological features of the sediments and their spatial coupling with the gas hydrate stability zone (GHSZ). GHSZ modeling based on in situ measurements indicated that erosion and sedimentation, as well as variations of the geothermal gradient, resulted in the upward/downward migration of bottom simulating reflectors (BSRs). On the erosion flank of the channel, the strata thinned, and rapid erosion was likely to destroy the shallower BSR, causing gas hydrate decomposition and methane release, and may have caused turbidite slumping and seepage, whereas the strata thickened on the deposition flank of the channel. The BSR in the channel–levee system would gradually move toward the new GHSZ, eventually forming a new BSR; parts of the BSR that formed under the original P–T conditions have remained, and double BSRs occurred in the seismic profile. The thermal fluid that moved upward through a gas chimney may also have caused the migration of the GHSZ, resulting in the emergence of double BSRs. During the lateral migration of the channel and the vertical migration of the gas-bearing fluid, there was a dynamic adjustment relationship between the GHSZ and the erosion–deposition process of the channel, resulting in the dynamic accumulation of hydrates in the Shenhu area. A model to demonstrate the relationship between channel migration and variation of the BSR was established, which is of great significance for understanding the formation and accumulation mechanisms of gas hydrates.

Beschreibung: Fluid flow patterns at cold seeps provide insights into the mechanism and influence of methane emission into the ocean, which is critical in its environmental impact assessment. Here, we report pore fluid compositions of three ~8 m long piston cores (QDN-14A, QDN-14B and R1) collected from the newly-discovered active Haima cold seeps on the northwestern slope of the South China Sea. Reaction-transport models were further applied to quantify related biogeochemical processes and to reveal the patterns of fluid flow. Extremely low δ13C values (〈 -52‰) of dissolved inorganic carbon (DIC) near the sulfate-methane transition in the three cores suggest that anaerobic oxidation of methane is the predominant biogeochemical process. The presence of small pieces of gas hydrates along with negative anomalies of porewater chloride and sodium concentrations reflects gas hydrate dissociation. Nearly invariable concentrations of sulfate, DIC, and calcium on a meter-scale were observed in the uppermost part of the sediment cores QDN-14A and QDN-14B. This irrigation-like feature is inferred to result from enhanced methane flux in QDN-14A and QDN-14B. We infer that lateral migration of methane-rich fluids from R1 site to QDN-14A and QDN-14B sites together with upward migrated methane is responsible for the enhanced methane flux. This speculation is supported by the occurrence of gas hydrates which might have clogged the fluid channel in the seepage center (R1) and driven the transportation of methane-bearing fluid along a coarser sediment layer in surrounding sediments (QDN-14A and QDN-14B). The proposed scenario is further demonstrated using a non-steady-state modeling that reconstructed the porewater irrigation-like feature assuming an increased methane flux. The modeling result predicts that gas-hydrate formation in core R1 started at least 150 yr B.P. The proposed fluid flow pattern within a localized seep site may have a great implication for understanding the heterogeneity of sedimentary records.