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Authigenic carbonates recovered from two newly discovered active cold seeps on the northwestern slope of the South China Sea have been studied using petrography, mineralogy, stable carbon and oxygen isotopic, as well as trace element compositions, together with AMS 14 C ages of shells of seep-dwelling bivalves to unravel fluid sources, formation conditions, and seepage dynamics. The two seeps (ROV1 and ROV2), referred to as ‘Haima seeps’ herein, are approximately 7 kilometers apart, and are typified by abundant carbonate rocks represented bycrusts and nodules. Aragonite and high-Mg calcite are the main carbonate minerals. Based on low δ 13 C carbonate values ranging from −43.0‰ to −27.5‰ (V-PDB) methane is apparently the predominant carbon source of seep carbonates. The corresponding δ 18 O values, varying from 2.5‰ to 5.8‰ (V-PDB), mostly are higher than calculated values representing precipitation in equilibrium with seawater (2.5‰ to 3.8‰), which probably reflects past destabilization of locally abundant gas hydrates. In addition, we found that carbonates with bivalve shells are generally aragonite-dominated, and bear no barium enrichment but uranium enrichments, reflecting shallow formation depths close to the seafloor. In contrast, carbonate crusts without bivalve shells and nodules contain more calcite, and are characterized by major molybdenum enrichment and different degrees of barium enrichment, agreeing with precipitation at greater depth under strictly anoxic conditions. AMS 14 C ages suggest that a major episode of carbonate precipitation occurred between 6.1 ka and 5.1 ka BP at the Haima seeps, followed by a possibly subordinate episode from approximately 3.9 ka to 2.9 ka BP. The common occurrence of dead bivalves at both sites indicates that chemosynthesis-based communities flourished to a greater extent in the past, probably reflecting a decline of seepage activity in recent times. Overall, these results confirm that authigenic carbonates from gas hydrate-bearing areas can provide insight into long-term seepage dynamics and the genesis and fate of marine gas hydrate reservoirs. [ABSTRACT FROM AUTHOR]

Research on the origin and source of hydrate-bound gas and its relationship to deep conventional oil and gas accumulation in a basin is critical to understanding the accumulation mechanism of gas hydrates and to resource evaluation of gas hydrate accumulation. In this study, the hydrate-bound gas obtained via pressure coring and the production gas recovered during a production test on a gas hydrate reservoir in the Shenhu area offshore of Southern China were tested and discussed. The geochemical analysis results indicate that methane is the predominant gas, and heavier hydrocarbons (C2+) are also present but in low concentrations. The molecular compositions of the hydrate gas recovered from two production test sites are similar to those of the hydrate-bound gas acquired via pressure coring. In addition to the isotopic composition of the methane, the carbon and hydrogen isotopes of the C2+ hydrocarbons were obtained for the first time. The δ13C isotopes of the methane range from −66.6‰ to −46.2‰, indicating that the hydrate-bound gases have a mixed origin, containing both biogenic and thermogenic gases. The plot of δ13C 1 versus δ13C 2 suggests that the biogenic and thermogenic hydrocarbons were derived from marine organic matter and terrestrial organic matter, respectively. The isotopic characterization of the hydrate-bound gas reveals that the thermogenic hydrate gas contains both humic-type gas and sapropel-type gas, but the sapropel-type gas is predominant. The source rocks of the thermogenic hydrate gas are interpreted to be both the gas-prone coal measure strata of the Enping Formation and the oil-prone medium-deep lacustrine strata of the Wenchang Formation, the latter of which contributed more to the hydrocarbon supply of the gas hydrates. In addition, the maturity of the source rocks of the thermogenic hydrate gas may be lower than that of the deeply buried conventional hydrocarbons discovered in the Baiyun Sag-Panyu Low Uplift area. The seismic profile crossing the shallow gas hydrate accumulations and deep conventional gas reservoirs clearly shows that the proven LW3-1 thermogenic gas reservoir communicated with the gas hydrate stability zone through vertical migration pathways formed by high-angle faults and gas chimneys. This indicates that there was a cogenetic relationship between the thermogenic hydrate gas and the deep conventional hydrocarbon reservoir, which was supplied the thermogenic gas derived from both the Wenchang and Enping formations. The major implications of this finding are that it confirms, rather than theorizes, the identity of the hydrocarbon source rocks of the thermogenic hydrate gas in the Shenhu area for the first time, and it demonstrates the coupling relationship between the shallow gas hydrate accumulations and the deep conventional reservoirs proposed in previous studies. The mechanisms of hydrocarbon migration and gas hydrate accumulation in the production test gas hydrate reservoirs in the Shenhu area were also identified, providing a valuable reference for subsequent exploration and for the potential exploitation of gas hydrate reservoirs in the South China Sea. • The δ13C and δD isotopes of the heavier hydrocarbons (C2+) in the Shenhu area are obtained for the first time. • The origin of the hydrate gas obtained from the second trial production well in the Shenhu area is confirmed. • The hydrocarbon source rocks of the thermogenic hydrate gas in the Shenhu area are identified. • Coupled paragenetic relationship between shallow gas hydrate and deep petroleum system is revealed. [ABSTRACT FROM AUTHOR]

• The gas production from hydrate dissociation accounting for about 85%. • Analysis of the radius of the hydrate dissociation area in this production test. • Five-year prediction of long-term influence radius of pressure drop. • Exploration of favorable areas for the formation of secondary hydrates. • Analysis of the response of pressure and temperature of hydrate reservoirs. The first offshore natural gas hydrate production test of China in 2017 has proved the feasibility of hydrate exploitation from clayey-silt reservoirs, which possesses the highest reservoirs than other types of hydrate resources. However, owing to the absence of monitoring wells in this production test, the hydrate dissociation behavior cannot be analyzed through pressure and temperature changes of hydrate reservoirs. This paper focuses on the simulation study on the detailed response of the temperature and pore pressure of hydrate reservoirs of Well SHSC-4 during the gas production by depressurization. Meanwhile, it highlights the analysis of favorable areas for the formation of secondary hydrates and the influence of the secondary hydrates on pressure and temperature field of hydrate reservoirs. The simulation results indicate that in the first 60 days, the hydrate reservoirs feature a dissociation radius of about 5 m, and the gas production from hydrate dissociation accounts for about 85%. After 1 year, 2 years and 5 years of hydrate exploitation, the influence radius of low-pressure area (<10 MPa) is 15 m, 16 m and 17 m, respectively, suggesting that the hydrate reservoirs have higher gas production efficiency in the first year. Furthermore, the temperature and pressure of hydrate reservoirs are not favorable to the formation of secondary hydrates in the first 60 days. In long-term production, secondary hydrates are mainly formed at hydrate dissociation front. This can increase the pore pressure and further decrease the effective stress in the local areas of hydrate reservoirs, thus affecting mechanical stability of the local hydrate reservoirs. [ABSTRACT FROM AUTHOR]

Studying deep-water cold seep systems is of great significance to gas hydrate exploration due to their close relationship. Various cold seep systems and related gas hydrate accumulations have been discovered in the northern South China Sea in the past three decades. Based on high-resolution seismic data, subbottom profiles, in situ submergence observations, deep drilling and coring, and hydrate gas geochemical analyses, the geological and geophysical characteristics of these cold seep systems and their associated gas hydrate accumulations in the Qiongdongnan Basin, the Shenhu area, the Dongsha area, and the Taixinan Basin have been investigated. Cold seep systems are present in diverse stages of evolution and exhibit various seabed microgeomorphic, geological, and geochemical features. Active cold seep systems with a large amount of gas leakage, gas plumes, and microbial communities and inactive cold seep systems with authigenic carbonate pavements are related to the variable intensity of the gas-bearing fluid, which is usually derived from the deep strata through mud diapirs, mud volcanoes, gas chimneys, and faults. Gas hydrates are usually precipitated in cold seep vents and deeper vertical fluid migration pathways, indicating that deep gas-bearing fluid activities control the formation and accumulation of gas hydrates. The hydrocarbons collected from cold seep systems and their associated gas hydrate reservoirs are generally mixtures of biogenic gas and thermogenic gas, the origin of which is generally consistent with that of deep conventional gas. We also discuss the paragenetic relationship between the gas-bearing fluid and the seafloor morphology of cold seeps and the deep-shallow coupling of gas hydrates, cold seeps, and deep petroleum reservoirs. It is reasonable to conclude that the deep petroleum systems and gas-bearing fluid activity jointly control the development of cold seep systems and the accumulation of gas hydrates in the northern South China Sea. Therefore, the favorable areas for conventional oil and gas enrichment are also prospective areas for exploring active cold seeps and gas hydrates. [ABSTRACT FROM AUTHOR]

Studying deep-water cold seep systems is of great significance to gas hydrate exploration due to their close relationship. Various cold seep systems and related gas hydrate accumulations have been discovered in the northern South China Sea in the past three decades. Based on high-resolution seismic data, subbottom profiles, in situ submergence observations, deep drilling and coring, and hydrate gas geochemical analyses, the geological and geophysical characteristics of these cold seep systems and their associated gas hydrate accumulations in the Qiongdongnan Basin, the Shenhu area, the Dongsha area, and the Taixinan Basin have been investigated. Cold seep systems are present in diverse stages of evolution and exhibit various seabed microgeomorphic, geological, and geochemical features. Active cold seep systems with a large amount of gas leakage, gas plumes, and microbial communities and inactive cold seep systems with authigenic carbonate pavements are related to the variable intensity of the gas-bearing fluid, which is usually derived from the deep strata through mud diapirs, mud volcanoes, gas chimneys, and faults. Gas hydrates are usually precipitated in cold seep vents and deeper vertical fluid migration pathways, indicating that deep gas-bearing fluid activities control the formation and accumulation of gas hydrates. The hydrocarbons collected from cold seep systems and their associated gas hydrate reservoirs are generally mixtures of biogenic gas and thermogenic gas, the origin of which is generally consistent with that of deep conventional gas. We also discuss the paragenetic relationship between the gas-bearing fluid and the seafloor morphology of cold seeps and the deep-shallow coupling of gas hydrates, cold seeps, and deep petroleum reservoirs. It is reasonable to conclude that the deep petroleum systems and gas-bearing fluid activity jointly control the development of cold seep systems and the accumulation of gas hydrates in the northern South China Sea. Therefore, the favorable areas for conventional oil and gas enrichment are also prospective areas for exploring active cold seeps and gas hydrates. [ABSTRACT FROM AUTHOR]

Studying deep-water cold seep systems is of great significance to gas hydrate exploration due to their close relationship. Various cold seep systems and related gas hydrate accumulations have been discovered in the northern South China Sea in the past three decades. Based on high-resolution seismic data, subbottom profiles, in situ submergence observations, deep drilling and coring, and hydrate gas geochemical analyses, the geological and geophysical characteristics of these cold seep systems and their associated gas hydrate accumulations in the Qiongdongnan Basin, the Shenhu area, the Dongsha area, and the Taixinan Basin have been investigated. Cold seep systems are present in diverse stages of evolution and exhibit various seabed microgeomorphic, geological, and geochemical features. Active cold seep systems with a large amount of gas leakage, gas plumes, and microbial communities and inactive cold seep systems with authigenic carbonate pavements are related to the variable intensity of the gas-bearing fluid, which is usually derived from the deep strata through mud diapirs, mud volcanoes, gas chimneys, and faults. Gas hydrates are usually precipitated in cold seep vents and deeper vertical fluid migration pathways, indicating that deep gas-bearing fluid activities control the formation and accumulation of gas hydrates. The hydrocarbons collected from cold seep systems and their associated gas hydrate reservoirs are generally mixtures of biogenic gas and thermogenic gas, the origin of which is generally consistent with that of deep conventional gas. We also discuss the paragenetic relationship between the gas-bearing fluid and the seafloor morphology of cold seeps and the deep-shallow coupling of gas hydrates, cold seeps, and deep petroleum reservoirs. It is reasonable to conclude that the deep petroleum systems and gas-bearing fluid activity jointly control the development of cold seep systems and the accumulation of gas hydrates in the northern South China Sea. Therefore, the favorable areas for conventional oil and gas enrichment are also prospective areas for exploring active cold seeps and gas hydrates. [ABSTRACT FROM AUTHOR]

Integrated investigations have revealed abundant resources of gas hydrates on the northern slope of the South China Sea (SCS). Regarding the gas hydrate research of northern SCS, the gas hydrate related environment problem such as seabed landslides were also concentrated on in those areas. Based on 2D seismic data and sub-bottom profiles of the gas hydrate areas, submarine landslides in the areas of Qiongdongnan, Xisha, Shenhu, and Dongsha have been identified, characterized, and interpreted, and the geophysical characteristics of the northern SCS region investigated comprehensively. The results show 6 major landslides in the gas hydrate zone of the northern SCS and 24 landslides in the Shenhu and Dongsha slope areas of the northern SCS. The landslide zones are located mainly at water depths of 200–3000 m, and they occur on the sides of valleys on the slope, on the flanks of volcanoes, and on the uplifted steep slopes above magmatic intrusions. All landslides extend laterally towards the NE or NEE and show a close relationship to the ancient coastline and the steep terrain of the seabed. We speculate that the distribution and development of submarine landslides in this area has a close relationship with the tectonic setting and sedimentary filling characteristics of the slopes where they are located. Seismic activity is the important factor controlling the submarine landslide in Dongsha area, but the important factor controlling the submarine landslides in Shenhu area is the decomposition of natural gas hydrates. [ABSTRACT FROM AUTHOR]

The gas hydrate deposit in Shenhu, Northern South China Sea (NSCS) was one of the main sources for deep-water hydrocarbon and hydrate. Based on field observations on a gas hydrate deposit among 20 × 20 km2 at Shenhu of the northern continental slope of SCS in May (before drilling) and September (after drilling) in 2015, environmental baseline, comparisons of phytoplankton community, biomass and potential environmental drivers were studied. Results indicated the deposit area was a typical low-nutrients low-chlorophyll a (LNLC) environment, characterized by low phytoplankton abundance and diversity. The maximum of abundance and Chl a were fixed at 75 m before and after drilling, but their decrease was larger from 0.53 mg/m3 to 0.40 mg/m3 and 1218 cells/L to 843 cells/L with non-significance found, respectively. After drilling, ammonium decreased significantly (2.13 μmol/L to 0.20 μmol/L), along with nitrite, salinity and dissolved methane, while pH increased significantly (8.10–8.19). Phytoplankton community was composed of 97 taxa of 47 genera in 4 classes, including diatoms, dinoflagellates, cyanobacteria and chrysophyte. The dominant diatom species before and after drilling remained the same, which included Navicula parva , Pseudonitzchia circumpora and Fragilariopsis doliolus. Dominant dinoflagellates shifted from the cosmopolitan Scrippsiella trochoidea to the warm-water adapted Gyrodinium flavum. The diazotrophic cyanobacterium Trichodesmium thiebaultii become more apparent in higher occurrence and abundance due to alleviation from lower-pH inhibition on nitrogen fixation. Redundancy analysis and Spearman correlation analysis revealed that the dominant species and Chl a were mainly associated to pH, DO and nitrite. The X-bar control chart indicated that a constrained maximum problem of phytoplankton abundance in particular layers (50 m, 75 m and 100 m). We clearly documented in the revisited stations (SH3–W2 and SH3–W14), the decrease of abundance and Chl a and the increase of pH and DO were evident in each sampling layer accompanied with lower dissolved methane associated to in situ anoxic or aerobic microniches activities, but the correlations with dissolved methane were not strong. • The gas hydrate deposit in Shenhu, NSCS was a typical low-nutrients low-chlorophyll environment accompanied with low phytoplankton abundance and diversity • Nitrite, pH and DO were major factors causing phytoplankton fluctuation, while dissolved methane had non-significant impact directly. • Variation of subsurface methane was likely resulted from in situ anoxic or aerobic microniches activities. [ABSTRACT FROM AUTHOR]

The formation of authigenic phosphorus (P) minerals near the sulfate-methane transition (SMT) represents an important pathway of P burial in marine sediments. However, our current understanding of the coupling between the formation of authigenic phosphate and carbon and iron (Fe) cycling is insufficient. In this study, a piston core recovered from the Haima cold seeps of the Qiongdongnan basin, northern South China Sea, was used to analyze pore-water parameters and solid-phase Fe and P speciation. Results from a previous study indicate that the current SMT is located at approximately 1 m below the seafloor (mbsf) and a fossil SMT located at approximately 6.5 mbsf. Dissolved phosphate concentrations and solid-phase iron oxide (Fe Ox) contents are found to decrease with depth, while Fe-bound P (P Fe) and authigenic phosphate (P Auth) contents increase with depth in the zone between the current and fossil SMTs where gas hydrate is present, suggesting that vivianite and authigenic apatite formed in this zone. Gas hydrate below the current SMT will dissociate once methane is undersaturated, which could persistently provide dissolved methane to prevent the downward shift of the current SMT where methane is continually consumed. Such a methane-rich environment maintained by gas hydrate may favor the persistent reduction of Fe Ox and the formation of vivianite. Dissolved phosphate concentrations are high, but decrease rapidly with depth at the fossil SMT. Combined with higher Fe Ox and P Auth contents and lower P Fe contents and lower P Org /total organic carbon (TOC) ratios, these results suggest the formation of authigenic apatite at the fossil SMT. The high dissolved phosphate concentration and low P Org /TOC ratios at the fossil SMT are interpreted to reflect preferential release of P during the degradation of organic matter generated by methanotrophic microorganisms. Overall, our study shows that P speciation in methane-rich sediments provides insight into the formation processes of authigenic P minerals near the SMT and associated carbon and Fe cycling processes. • P and Fe cycling was investigated between the current and fossil sulfate-methane transition (SMT). • An apparent relationship was observed between the occurrence of gas hydrate and the formation of vivianite. • Iron-driven anaerobic oxidation of methane may be the key for a co-occurrence of gas hydrate and vivianite. • Preferential release of P during degradation of organic matter can occur at fossil SMTs. [ABSTRACT FROM AUTHOR]

Highlights • Porewater geochemistry of two piston cores in hydrate drilling areas was investigated. • A numerical model was applied to quantify involved biogeochemical processes. • Methane consumed mainly originated from external sources rather than local methanogenesis. • The external methane is most likely to originate from local gas hydrate systems. Abstract Methane of various origins is widespread in the sediments of continental margins, representing a key component of the carbon cycle in the ocean. Its occurrence in shallow marine sediments can be linked to the gas hydrate reservoir in the deep sedimentary column. However, this type of methane is difficult to track due to complex reactions occurred during its long distance migration. Porewater geochemical analyses of two ∼7 m piston cores (D17-15 and W01-16) combined with a reaction-transport model were applied to quantify methane-related biogeochemical processes and to explore the linkage of shallow methane to gas hydrate reservoir in the hydrate drilling areas of Shenhu and Dongsha in the South China Sea (SCS). The model results revealed that anaerobic oxidation of methane (AOM) is the predominant biogeochemical process in both cores, consuming approximately 85% and 74% of sulfate in the modeled domain, respectively. The crossplot of dissolve inorganic carbon (DIC) accumulation versus sulfate depletion of the system suggests that methane consumed during the AOM is mainly originated from external methane sources rather than local methanogenesis. Using a δ13C DIC mass balance calculation together with model-derived fractions of different DIC sources, we estimated that the δ13C value of the external methane in core D17-15 is −66.6‰, which is in accordance with the previous reported δ13C values of hydrate-bound methane in the area. These results suggest that methane consumed in shallow sediments in the hydrate drilling areas of the SCS is mainly originated from local gas hydrate reservoir. The proposed approach has the potential to be used to differentiate the sources of methane, which will provide constraints on dynamics of methane in gas hydrate-bearing marine sediments. [ABSTRACT FROM AUTHOR]

Methane seepage activity caused by gas hydrate dissociation plays an essential role in the global carbon cycle and climate change in Earth's history. However, the triggering mechanisms of gas hydrate dissociation remain elusive because of the lack of a relatively complete history of methane seeps on long-term timescales. Here, we present major and trace elements, stable C–O isotopes, and U–Th dating analyses for cold seep carbonates from drill cores in the South China Sea. Our results reveal a 209,00-year-old history of methane seepage activity, which involves multi-stage changes in seepage intensity and methane source (microbial or thermogenic). Chronological data suggest that hydrostatic pressure reduction and dynamic depositional environment mainly controlled the gas hydrate dynamics in the northwestern South China Sea, which is different from the gas hydrate dissociation in the northeastern South China Sea controlled by bottom water warming. This discrepancy may be caused by the differences in physicochemical conditions (e.g., water depth) in different sea areas. Therefore, the findings of this study help to further clarify the relationship between oceanic hydrate reservoirs and environmental change in other world regions. • Seep carbonates from drill cores in the Qiongdongnan Basin were investigated. • Two methane seepage events were identified in the northwestern South China Sea. • Multi-stage changes in seepage intensity and methane source. • Seepage activity controlled by hydrostatic pressure and depositional environment. [ABSTRACT FROM AUTHOR]

Marine gas hydrates mostly occur in high-water content and non-diagenetic submarine sediments without tight stratigraphic traps. Therefore, the seepage capability of overlying and underlying strata (OUS) serves as a crucial parameter to analyze hydrate production behaviors. This study focuses on the numerical simulation study on the effects of the seepage capability of OUS on hydrate production behaviors in depressurization-induced production of three-phase hydrate deposits using a horizontal well. As shown by the simulation results, in the early stage of hydrate production, the permeability of the OUS had little effects on gas production rates. In long-term hydrate production, the hydrate deposit with lower permeability OUS has higher cumulative gas production, larger gas-water ratio and hydrate dissociation scope, and less cumulative water production. Moreover, the hydrate production efficiency first decreased rapidly and then tended to be stable as the permeability of OUS increased, and hydrate production behaviors are similar while the permeability of OUS larger than 1.0 mD. Herein, a water-resisting layers were constructed in overlying and/or underlying strata to decrease the seepage capability of OUS. Compared to the original hydrate deposits, the cumulative gas production and gas-water ratio increased by 31% and 147%, respectively but the water production was only 76% on the 5th year of the simulated production in which water-resisting layers were designed. Furthermore, the water-resisting layer in underlying strata produced more significant effects on hydrate production efficiency than the layer in overlying strata. All these indicate that the low-permeability OUS (especially in underlying strata) can restrain fluid invading into hydrate reservoirs, increase the pressure gradient of hydrate reservoirs and significantly improve the hydrate production efficiency during long-term hydrate production. Therefore, water-proof reformation in OUS may be an effective method to improve the energy utilization efficiency and promote the process of hydrate industrialization. However, further research is needed. • Effects of seepage capability of cap strata on hydrate production were studied. • Critical permeability of overlying/underlying strata determined to be 1.0 mD. • Hydrate system with low permeability cap strata has higher energy utilization. • Aquiclude in underlying strata having higher hydrate production efficiency. [ABSTRACT FROM AUTHOR]