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Recently, drilling wells have encountered rich gas hydrates in fine-grained sediments in the northern South China Sea. Gas hydrate in fine-grained sediments is very heterogeneous, and its physical properties are different from those of oil and gas reservoirs. The reliability of the classical logging saturation evaluation models established for diagenetic reservoirs is questionable. This study used four wells in GMGS3 and GMGS4 to evaluate the effects of the application of three typical methods for evaluating saturation with different principles in the unconsolidated fine-grained sediments: nuclear magnetic logging, sigma logging, and the Archie formula. It was found that the value of the lithologic capture cross-section in sigma logging and the rock's electrical parameters in the Archie formula affect the accuracy of the model. Therefore, to obtain a reliable saturation value for fine-grained sediments, an innovative method for the calculation of resistivity and acoustic time is proposed to estimate gas hydrate saturation based on logging data, which is most consistent with the results of core analysis. The overall relative error of the verification well was 5.87%, whereas that of the density NMR logging method was 56%, showing that the accuracy of the newly proposed resistivity DT logging method's saturation formula was significantly improved. Finally, a new model-based cross chart was developed, which can rapidly differentiate gas saturation during drilling. [ABSTRACT FROM AUTHOR]

In 2018, Guangzhou Marine Geological Survey conducted the fifth gas hydrate drilling expedition (GMGS5) in the northwestern continental slope of South China Sea. We use pore water geochemistry, pressure core CT scanning, degassing quantification, seismic data and in situ temperature test at four drill sites (W01, W07, W08 and W09) to investigate the gas hydrate characteristics and discuss the dynamics of the gas hydrate system. Based on the CT scanning and pressure core degassing, we demonstrate the coexistence of pore-filling and vein-like gas hydrates. Gas hydrate saturation estimated based on the chloride concentration (0–92%) is higher than that based on the degassing quantification (0–52.9%). The saturation discrepancy is mainly resulted from the sampling discrimination. Based on the average gas hydrate saturation estimated using chloride concentration and degassing, the methane gas volume contained in the gas hydrate of one cubic meter sediment at drill sites W07, W08 and W09 are calculated to be 13.7 m3, 26.82 m3 and 22.24 m3 and 3.03 m3, 14.30 m3 and 16.25 m3. The geothermal gradients estimated based on the in situ temperature measurements at sites W07, W08 and W09 (102–111 °C/km) are more than 50% higher than that at the background site W01 (65 °C/km). It is inferred that the fractures and faults in the gas chimney at sites W07, W08 and W09 are the major contributors for the efficient fluid pathways and high geothermal gradients. The pore water sulfate profiles and inferred sulfate-reduction methane-oxidation interface (SMI) depths indicate that methane flux is lowest at site W01 and highest at site W08. The SMI depths and top of gas hydrate occurrence is linearly correlated which provides an experienced function for understanding the local gas hydrate distribution. Kick-type sulfate profiles at sites W07 and W09 indicate an ongoing increase of methane flux and corresponding shoaling SMI depths. In addition, chloride enrichment in the shallow sediment at site W08 indicates a recent fast gas hydrate formation process. The kick-type sulfate profiles, positive chloride anomalies and authigenic carbonates at multiple depths suggest that the system experienced dynamic changes in fluid flux and multi-stage gas hydrate evolution on time scale from months to thousands of years. Understanding the controlling mechanism and periodicity of the composite pressure system is important for evaluating the gas hydrate resource and carbon cycling. • Gas hydrates were drilled for the first time in the Qiongdongnan Basin. • Pore-filling and vein-like gas hydrates co-exist in the cores. • Kick type sulfate profiles indicate the transit state of methane flux. • Positive pore water chloride anomalies indicate the recent fast gas hydrate formation. • Dynamic fluid flux and multi-stage gas hydrate evolution happened on time scale from months to thousands of years. [ABSTRACT FROM AUTHOR]

The Shenhu area is the key target area for marine gas hydrate industrialization in China. Significant progress has been made with respect to gas hydrate exploration and production in this area in recent years. However, a comprehensive understanding of the reservoir characteristics and hydrate accumulation mechanism are still lacking; in particular, the hydrate enrichment factors are not, as yet, completely clear. In this study, we systemically characterize the geological controls on gas hydrate accumulation in the Shenhu area, including the sedimentary environment, gas sources, fluid and gas pathways, and reservoir-caprock system. We subsequently provide a quantitative evaluation of some critical factors influencing hydrate enrichment. The depositional environment in the study area was characterized by low energy (sedimentary hydrodynamic conditions), and anoxic environment, and the gas hydrate accumulations were closely associated with turbidites. The methane captured in the gas hydrates in the GMGS1 area and at the SHSC-4 and SHSC2-6 production test sites was mainly derived from the microbial reduction of CO 2 , while thermogenic gas was a more important source of hydrocarbon gases in the GMGS3 and GMGS4 areas. In addition, we used a combination of 8-slice spiral X-ray computed tomography (CT) and micro-CT techniques for the first time for the in-situ investigations of the microfractures and foraminifera in the hydrate cores. The results indicate that microfractures and foraminiferal shells are crucial to the occurrence and accumulation of gas hydrates in fine-grained sediments of the Shenhu area. Interestingly, microfractures were found to be more developed in the hydrate layers where most sediment particles were relatively small in size. Hydrate saturation increased with i) a decrease in the specific surface area of sediment grain, ii) an increase in the median pore size, and iii) increased foraminifera abundance in the reservoir. Our findings could help locate high-saturation gas hydrate layers ('sweet spot') in fine-grained marine sediments of the Shenhu area, South China Sea. • Microscopic factors influencing the hydrate enrichment are comprehensively discussed. • A conceptional model for hydrate accumulation was proposed in the Shenhu area. • Microfractures prefer to develop in the hydrate layers where the sediment particles have relatively small grain size. • Thermogenic gas makes an important contribution to hydrate accumulation in turbidites connecting deep faults. [ABSTRACT FROM AUTHOR]

Gas hydrates and deep-sea cold seeps are of interest to industry and academia for several reasons, including their implications for offshore geohazards and unconventional resources, and for global climate change and carbon cycling. Guangzhou Marine Geological Survey (GMGS) implemented its fifth gas hydrate drilling expedition in the QiongDongNan Sea Area (QDN-SA) of the northern South China Sea (SCS) from June to September 2018. Multidisciplinary investigations, including Logging While Drilling (LWD), pressure and non-pressure coring, gas composition analyses, in situ temperature and permeability measurements, pore water geochemical analyses, and a regional seismic survey, reveal a complex distribution of gas hydrate within a vertical chimney structure. LWD profiles record high resistivity anomalies and high P-wave velocity variations at 9–174 mbsf, suggesting the coexistence of free gas and gas hydrate in the sediment column throughout the investigated gas chimney. Hydrate-bound and void gas is an admixture of methane (70.1%–98.3%) and heavier hydrocarbons (C 2 H 6 : 0.16%–17.59%, C 3 H 8 : 0.05%–9.88%, i-C 4 H 10 : <0.001%–2.06%, n-C 4 H 10 : <0.001%–0.85%, i-C 5 H 12 : <0.001%–0.12%, n-C 5 H 12 : <0.001%–0.02%), indicating a thermogenic origin for the gas. Based on the high content of C 2 + hydrocarbons in the gas composition, it is suggested that sII/sH gas hydrate exists in the study area. The gas chimney is interpreted to provide efficient pathways for fluid migration from an intermediate gas reservoir below the BSR to the gas hydrate stability zone (GHSZ). Gas hydrate crystallization decreases the permeability of the sediment, impeding fluid flowing into the upper sediment layers. Bivalve fragments embedded within carbonate rocks were recovered at the seafloor and multiple levels at two main sediment levels of 3mbsf and ∼54mbsf, indicating recurrent seepage activity. • First results from the 5th gas hydrate drilling expedition in the QiongDongNan Area of the South China Sea in 2018. • Gas hydrates co-exist with free gas between 9 mbsf and 174 mbsf at the drilling site. • High C 2 + (>10%) in the hydrate-bound gas indicate the presence of sII/sH gas hydrates and a thermogenic gas source. • Authigenic carbonates with cold seep bivalve fragments at multiple depths indicate recurrent seepage in the past 0.54 Ma. [ABSTRACT FROM AUTHOR]

Abstract Structure type, gas composition and cage occupancy are important gas hydrate characteristics which play an important role in calculating gas hydrate stability and estimating the amount of global hydrate-bound methane. China conducted the Fourth gas hydrate drilling campaign (GMGS4) in the Shenhu Sea Area (SSA) of the South China Sea in 2016, during which comprehensive Logging-While-Drilling (LWD) data and natural gas hydrate samples were obtained. The LWD data shows high electrical resistivity and high sound velocity at various depth, indicating the occurrence of gas hydrate in this area. Coexistence of structure I (sI) and structure II (sII) gas hydrates in the South China Sea was confirmed for the first time by using Raman spectra analysis. The methane cage occupancies of large cages and small cages in sI hydrate are 0.95 and 0.96, respectively. The hydration number of sI hydrate is 6.03. Gas composition analysis shows that methane (>98.7%) is the dominate gas, with small amount of heavier hydrocarbon gases (ethane: 0.006%–0.976%, propane: 0.033%–0.216%, isobutane: 0.004%–0.047%) and other non-hydrocarbon gases (carbon dioxide: <0.008%). Gas composition (C 1 /(C 2 +C 3): 84–2333) and isotope composition (δ13C: −50.9‰∼-46.46‰, δD: −177.69‰∼-148.90‰) indicate hydrate-bound gas is dominated by thermogenic source, mixed with certain amount of biogenic source. Existence of sII hydrate potentially deepen the base of sI gas hydrate stability zone in the marine sediment. Coexistence of sI and sII gas hydrates increases the complexity of the gas-water-hydrate system which need to be considered when studying the gas hydrate distribution, estimating the amount of global natural hydrate-bound gases and modeling the gas production. Highlight • sII gas hydrate is discovered for the first time in the South China Sea. • Gas composition and isotope composition indicate hydrate-bound gas is mainly thermogenic origin. • LWD data shows that high saturation gas hydrate of SC-W01 is mainly distributed between 151 and 173 mbsf. [ABSTRACT FROM AUTHOR]

The second gas hydrate expedition of the Guangzhou Marine Geological Survey drilled 13 sites and recovered a large amount of gas hydrate in the east of the Pearl River Mouth basin of the South China Sea. In this study, we examine three of these sites (sites 05, 08, and 16) using multichannel seismic data, LWD logging data, and multi-beam echo sounder data. The geophysical data suggest and core samples confirm that gas hydrates occur near the BSR at all three sites, whereas near-surface gas hydrates exist at sites 08 and 16 with high fluid flux. We present a conceptual model for the formation and accumulation of gas hydrates in our study area, controlled by the activity of dissolved and free gas-rich fluid. Interpreted abundant gas sources below the BSR contribute to the generation of overpressure, resulting in the movement of gas-rich fluid. Dissolved gas-rich fluid forms pore filling hydrates near the BSR, and the relatively deep hydrate layer thickens over time by making its way through the growing hydrate layer and adding to the top, if the fluid is supplied sufficiently. Free gas-rich fluid is more favorable to generate near-surface nodules or massive hydrates, which could be stable in natural conditions. As sediments deposit, the relatively shallow hydrates are buried deeper and keep their morphologies. Furthermore, gas hydrate formation and stability imply that gas hydrates may be increasingly considered as a potential energy resource. [ABSTRACT FROM AUTHOR]