Guangzhou Marine Geological Survey Research Articles

Representative Dynamic Accumulation of Hydrate-Bearing Sediments in Gas Chimney System since 30 Kyr BP in the QiongDongNan Area, Northern South China Sea

A stratigraphic complex composed of mass transport deposits (MTDs), where the gas occurrence allows for the formation of a gas chimney and pipe structure, is identified based on seismic interpretation in the QiongDongNan area of the northern South China Sea. During the Fifth Gas Hydrate Drilling Expedition of the Guangzhou Marine Geological Survey, this type of complex morphology that has close interaction with local gas hydrate (GH) distribution was eventually confirmed. A flow-reaction model is built to explore the spatial–temporal matching evolution process of massive GH reservoirs since 30 kyr before the present (BP). Five time snapshots, including 30, 20, 10, and 5 kyr BP, as well as the present, have been selected to exhibit key strata-evolving information. The results of in situ tensile estimation imply fracturing emergence occurs mostly at 5 kyr BP. Six other environmental scenarios and three cases of paleo-hydrate existence have been compared. The results almost coincide with field GH distribution below the bottom MTD from drilling reports, and state layer fracturing behaviors always feed and probably propagate in shallow sediments. It can be concluded that this complex system with 10% pre-existing hydrates results in the exact distribution and occurrence in local fine-grained silty clay layers adjacent to upper MTDs.

A Flat‐Lying Transitional Free Gas to Gas Hydrate System in a Sand Layer in the Qiongdongnan Basin of the South China Sea

AbstractMost marine gas hydrate systems follow a vertical pattern with hydrate overlying free gas. Here we document the discovery of a gas to hydrate system in a horizontal sand layer in the Qiongdongnan Basin of the South China Sea. Eight wells were drilled by the Guangzhou Marine Geological Survey in 2021–2022 to investigate the occurrence and mechanisms responsible for the formation of the system. We describe a free gas‐bearing sand reservoir at the center of the system sustained by advecting hot fluids and gas; away from the advecting zone, the cooler, surrounding sand reservoir is filled with hydrate. Observations at this site show that advective heat has a large control on hydrate formation in sands and may be a key mechanism which allows gas migration within the hydrate stability zone and the formation of high‐saturation hydrate in sand layers.

Effects of Depositional Processes in Submarine Canyons and Distribution of Gas Chimneys on Gas Hydrate Accumulation in the Shenhu Sea Area, Northern South China Sea

Previous gas hydrate production tests conducted by the Guangzhou Marine Geological Survey (GSGM) in 2017 and 2020 indicated the great potential of gas hydrates in the Shenhu Sea area in the Pearl River Mouth Basin (PRMB), China. In this study, the effects of deposition processes in submarine canyons and the distribution of gas chimneys on gas hydrate accumulation were investigated using high-resolution two- dimensional (2D) and three-dimensional (3D) seismic data. Four intact submarine canyons were identified in the study area. Five deepwater depositional elements are closely related to submarine canyons: lateral accretion packages (LAPs), basal lags, slides, mass transport deposits (MTDs), and turbidity lobes. MTDs and lobes with multiple stages outside the distal canyon mouth reveal that the sedimentary evolution of the canyon was accompanied by frequent sediment gravity flows. Gas chimneys originating from Eocene strata are generally up to 3 km wide and distributed in a lumpy or banded pattern. The analysis of seismic attributes confirmed fluid activity in these gas chimneys. Gas hydrates are mainly distributed in ridges among different canyons. Based on the gas sources of gas hydrates and depositional evolution of submarine canyons, depositional processes of sediment gravity flows in submarine canyons and the distribution of gas chimneys significantly affect the accumulation of gas hydrates. Based on these findings, this study establishes a conceptional model for the accumulation of gas hydrate, which can provide guidance in the prediction for favorable gas hydrates zones in the area and nearby.

Geomechanical properties of gas hydrate-bearing sediments in Shenhu Area of the South China Sea

It is essential to understand the in-situ geomechanical properties of natural gas hydrate-bearing reservoirs for hydrate drilling, the design of reservoir stimulation and production schemes, geologic disaster prevention, and environmental impact assessment. In this way, it serves as the basis for the development and utilization of gas hydrate resources. In 2016, the Guangzhou Marine Geological Survey conducted the fourth natural gas hydrate drilling (Expedition GMGS4) in the north of the South China Sea, during which the effective stress of gas hydrate-bearing reservoirs in the study area was ascertained and gas hydrate-bearing pressure cores were collected. The effective stress was determined to be 700–800 kPa by in-situ piezocone penetration tests. The saturation, permeability, and shear strength of gas hydrates in the pressure core samples were measured under the in-situ stress state using the shipboard multi-sensor core logger (MSCL), pressure core analysis and transfer system (PCATS), and PCATS Triaxial system. Based on this, the distribution characteristics and the relationship of the permeability and gas hydrate saturation of the gas hydrate-bearing reservoirs were explored. Furthermore, the effects of effective stress and gas hydrate saturation on shear strength of gas hydrate-bearing clayey-silt sediments were mainly analyzed, as well as the stress–strain characteristics of gas hydrate-bearing sediments. The results are as follows: (i) the gas hydrate saturation and reservoir permeability in the study area are 0.7%–55% and 10−3–10−1mD, respectively, indicating low-permeability reservoirs; (ii) the gas hydrate saturation and reservoir permeability have obvious anisotropy in horizontal and vertical directions; (iii) the undrained shear strength of gas hydrate-bearing sediments is 0.24–0.52 MPa, and the shear strength of fine-grained sediments is more sensitive to the changes of in-situ stress. In the case that deviatoric stress continues increasing, micro-fractures may occur inside gas hydrate-bearing sediments, and this can lead to drastic changes in the strength of gas hydrate-bearing reservoirs. Therefore, it is necessary to take into account the effects of micro-fractures in gas hydrate-bearing reservoirs on engineering safety during long-term hydrate production.

Mixed gas sources induced co-existence of sI and sII gas hydrates in the Qiongdongnan Basin, South China Sea

Crystal structure and gas source are two important aspects in studying natural gas hydrate systems. In 2018, Guangzhou Marine Geological Survey carried out its fifth natural gas hydrate drilling expedition (GMGS5) in the Qiongdongnan basin of the South China Sea. In this paper, analyses of gas composition, stable carbon isotopes and Raman spectroscopy were conducted on twelve gas hydrate samples recovered at drill site W08, at depths of 17–170 m below seafloor. The results show that the main component of the hydrate-bound gas is methane (72.5–97.5%), mixed with other hydrocarbon gases (ethane: 1.9–12.1%, propane: 0.1–11.3%, i-butane: 0.0–3.5%, n-butane: 0.0–0.2% and C5+: 0.0–0.2%). The δ13C of methane, ethane and propane in the hydrate-bound gases are −63.6 ~ −51.9‰, −29.2 ~ −26.6‰ and −25.6 ~ −23.7‰, respectively. The hydrate-bound gas is inferred to be a mixture of microbial and thermogenic gases. Raman spectroscopic analysis indicates that structure I and structure II gas hydrates co-exist in the samples. Structure II gas hydrate is increasingly being discovered in the South China Sea, and it is suggested that natural gas hydrate resources might be underestimated and need to be re-evaluated.

Numerical Analyses on Controlling Factors for Hydrate Differential Accumulation in the Shenhu Drilling Area, South China Sea

In 2007, the first marine gas hydrate drilling expedition of the Guangzhou Marine Geological Survey (GMGS-1) was carried out in the Shenhu area on the northern continental slope of the South China Sea, collecting geophysical and drilling data from eight different sites. No hydrate sample was recovered at site SH5, even though it locates close to site SH2 where the hydrate was drilled. The reason for the different distribution of hydrate is not thoroughly understood. On the basis of the actual exploration data, the key factors controlling hydrate formation in the Shenhu area were analyzed by employing the numerical modeling. To evaluate the key controlling factors among gas flux, water depth, temperature gradient, and cap rock location, sensitive studies were conducted. The results show that the gas flux is the prerequisite factor determining the hydrate quantity. The seawater depth does not affect the hydrate formation significantly. The temperature gradient plays an important role in hydrate quantity and saturation. The cap rock location controls the gas migration to the gas hydrate stability zone (GHSZ). According to the sensitivity analyses and drilling data, we proposed a mechanism of hydrate differential accumulation in the Shenhu area. The relatively high geothermal gradient and the cap rock located below the GHSZ cause the absence of hydrate at site SH5, leading to the differential accumulation.

Characteristics of gravity and magnetic fields and deep structural responses in the southern part of the Kyushu-Palau Ridge

The southern part of the Kyushu-Palau Ridge (KPR) is located at the conjunction of the West Philippine Basin, the Parece Vela Basin, the Palau Basin, and the Caroline Basin. This area has extremely complex structures and is critical for the research on the tectonic evolution of marginal seas in the Western Pacific Ocean. However, only few studies have been completed on the southern part, and the geophysical fields and deep structures in this part are not well understood. Given this, this study finely depicts the characteristics of the gravity and magnetic anomalies and extracts information on deep structures in the southern part of the KPR based on the gravity and magnetic data obtained from the 11th expedition of the deep-sea geological survey of the Western Pacific Ocean conducted by the Guangzhou Marine Geological Survey, China Geological Survey using the R/V Haiyangdizhi 6. Furthermore, with the data collected on the water depth, sediment thickness, and multichannel seismic transects as constraints, a 3D density model and Moho depths of the study area were obtained using 3D density inversion. The results are as follows. (1) The gravity and magnetic anomalies in the study area show distinct zoning and segmentation. In detail, the gravity and magnetic anomalies to the south of 11°N of the KPR transition from high-amplitude continuous linear positive anomalies into low-amplitude intermittent linear positive anomalies. In contrast, the gravity and magnetic anomalies to the north of 11°N of the KPR are discontinuous and show alternating positive and negative anomalies. These anomalies can be divided into four sections, of which the separation points correspond well to the locations of deep faults, thus, revealing different field-source attributes and tectonic genesis of the KPR. (2) The Moho depth in the basins in the study area is 6–12 km. The Moho depth in the southern part of KPR show segmentation. Specifically, the depth is 10–12 km to the north of 11°N, 12–14 km from 9.5°N to 11°N, 14–16 km from 8.5°N to 9.5°N, and 16–25 km in the Palau Islands. (3) The KPR is a remnant intra-oceanic arc with the oceanic-crust basement.which shows noticeably discontinuous from north to south in geological structure and is intersected by NEE-trending lithospheric-scale deep faults. With large and deep faults F3 and F1 (the Mindanao fault) as boundaries overall, the southern part of the KPR can be divided into three zones. In detail, the portion to the south of 8.5°N (F3) is a tectonically active zone, the KPR portion between 8.5°N and 11°N is a tectonically active transition zone, and the portion to the north of 11°N is a tectonically inactive zone. (4) The oceanic crust in the KPR is slightly thicker than that in the basins on both sides of the ridge, and it is inferred that the KPR formed from the thickening of the oceanic crust induced by the upwelling of deep magma in the process of rifting of remnant arcs during the Middle Oligocene. In addition, it is inferred that the thick oceanic crust under the Palau Islands is related to the constant upwelling of deep magma induced by the continuous northwestward subduction of the Caroline Plate toward the Palau Trench since the Late Oligocene. This study provides a scientific basis for systematically understanding the crustal attributes, deep structures, and evolution of the KPR.©2021 China Geology Editorial Office.

Grain-size characteristics of fine-grained sediments and association with gas hydrate saturation in Shenhu Area, northern South China Sea

Fine-grained gas hydrate (GH) reservoirs are extensively studied worldwide, among which the Shenhu Area (located on the northern slope of the South China Sea) is a world-class GH exploration area. However, the lithology, physical properties, and depositional origins of the fine-grained GH reservoirs are not well known. Understanding how sediment grain-size parameters affect the fine-grained GH reservoir quality could provide and important breakthrough for reservoir evaluation. Eight cores, recovered from various expeditions of the Guangzhou Marine Geological Survey, can be combined with 2D/3D seismic data to provide a rare opportunity to systematically investigate the grain-size characteristics of the GH reservoir, as well as the surrounding sediments.A combination of lithology, grain size characteristics (mean size, sorting, skewness, kurtosis), high-resolution seismic features, and associated bivariate and cluster analysis results support the identification of two distinct intervals of fine-grained sediments that were deposited by different sedimentary processes. There is a relatively higher content of coarser silt in the lower interval than in the upper interval, and their boundary depths are highly consistent with those of the GH-bearing layer and the overlying non-GH-bearing layer. With respect to the unconsolidated GH-bearing sediments from Well G, both the porosity (52%–64%) and sorting coefficient (1.68–2.2) have limited variation, while high GH saturation (>30%) occurs at the top layer. The positive correlation between saturation and the coarsest one-percentile grain size (R = 0.55) reveals that an increase in the coarse fraction/particle size favours the development of a larger pore throat diameter and improves the initial permeability and reservoir properties.The seismic features and cross-plots of the coarsest one-percentile and median values indicate that the lower thin-bedded fine-grained sediments with hydrate may be fine-grained turbidite complexes, including channels/levees/lobes and mass transport deposits. If this is the case, then it may be inferred that turbidite sediments provide good reservoir physical properties, favourable for GH formation and accumulation. These insights into the relationship between the Quaternary fine-grained turbidites and GH saturation may promote a clearer understanding of the characteristics and development of fine-grained GH reservoirs globally, including in the Shenhu Area of the South China Sea.

Monitoring and Early Warning Technology of Hydrate-induced Submarine Disasters

With the depletion of fossil energy sources, such as coal and petroleum, natural gas hydrate has been proposed as an alternative energy source. However, the synthesis of natural gas hydrate may cause marine engineering disasters, such as submarine cable breaks and the collapse of sub-sea oil rigs, and even submarine earthquakes, triggering tsunamis and seriously endangering human safety. Hydrate test production monitoring in various countries focuses on the temperature, pressure, and methane concentration near test production wells. Effective observation equipment and methods for the monitoring and early warning of submarine geological hazards are lacking. This paper reviewed the possible geological hazards caused by deep seawater hydrate mining and the monitoring and early warning technologies for deep seawater hydrate mining in various countries and introduced the in-situ Surveying Equipment of Engineering Geology in Complex Deep Sea (SEEGeo). The equipment uses 3D resistivity measurement and data inversion technology, seabed sediment acoustic measurement and data inversion technology, seabed sediment pore water pressure measurement and data inversion technology, and renewable energy seawater-dissolved oxygen battery power supply technology. SEEGeo can realize the in situ, real-time, long-term, effective monitoring and early warning of geological disasters induced by hydrate development. At present, the equipment has conducted two sea trial experiments on the continental slope of the South China Sea, equipped with the “Marine Geology No. 6” scientific research vessel of the Guangzhou Marine Geological Survey and the “Dongfanghong No.3” scientific research vessel of the Ocean University of China, and obtained in situ engineering geological data of the South China Sea.

Characteristics of gas chimneys and their implications on gas hydrate accumulation in the Shenhu area, northern south China sea

Gas hydrates have captivated extensive concentration from scholars all over the world as potential alternative energy sources. In 2017, the Guangzhou Marine Geological Survey (GMGS) carried out a successful gas hydrate production test in the Shenhu area, which was of considerable significance to the study of gas hydrates in China. Several studies describing gas chimneys as the primary vertical gas migration pathways in the Shenhu area were mainly based on two-dimensional (2D) seismic data. In this research, on the basis of high-resolution three-dimensional (3D) seismic data newly acquired in 2018, combined with well logging data acquired during the 2007 and 2015 expeditions, the contribution of gas chimneys to gas hydrate accumulation was determined. As a result, four types of gas chimneys with various top shapes were identified, and their internal seismic reflection characteristics were predominantly chaotic, acoustic blanking, and high-amplitude reflection from bottom to top. Moreover, thick hydrate layers were discovered above some of the gas chimneys. Further analyses on the hydrocarbon generation and tectono-sedimentary evolution in the Baiyun Sag indicated the evolution of gas chimneys could be divided into five stages. Furthermore, as the primary vertical gas migration pathways in the Shenhu area, gas chimneys could be sourced from the Wenchang formation and Enping formation and could pierce into the Quaternary gas hydrate reservoirs. This resulted in deep thermogenic gas directly migrating upward into the gas hydrate stability zone (GHSZ) and accumulating into thick hydrate layers on or above the top of gas chimneys.

Sill-related seafloor domes in the Zhongjiannan Basin, western South China Sea

Magmatism can exert significant impact on sedimentary basins such as the Zhongjiannan Basin (ZJNB), western South China Sea. We have evaluated multibeam bathymetric and multichannel seismic reflection data acquired by the Guangzhou Marine Geological Survey in recent years, in order to investigate the distribution, the characteristics and the subsurface structures related to seafloor domes found in the northeastern ZJNB. Our data revealed forty-two domes at water depths between 2312 m and 2870 m, which are clustered around volcanic mounds, large seamounts and along the edge of the central depression in the study area. These domes are generally circular to elongate or irregular in plan view with large basal areas, and they also have gentler flanks (dips of 1.46°–7.73°) with vertical reliefs ranging from tens to hundreds of meters. In seismic sections, majority of the domes are underlain by variably shaped and complex magmatic sills, which provide a cause-effect relationship between domes formation and igneous intrusions. These intrusions heat surrounding organic-rich sediments, release hydrocarbons, fluidize sediment pore waters and form gas-rich fluids, which fill in sediment and uplift overlying strata immediately above the sills to form forced folds, which are manifested as seafloor domes. These sill-folds-dome structures have important implications for understanding geomorphologic features caused by sills emplaced at depth.

Geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the first offshore gas-hydrate production test region in the Shenhu area, Northern South China Sea

Abstract 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.

Origin of natural gases and associated gas hydrates in the Shenhu area, northern South China Sea: Results from the China gas hydrate drilling expeditions

Geochemical data for hydrate gases acquired from the GMGS3 and GMGS4 gas hydrate drilling expeditions conducted by the Guangzhou Marine Geological Survey (GMGS) are used to explore the origin of hydrate gases and their relationship to deep hydrocarbon reservoirs, and to evaluate the contribution of different genetic types of gases to the formation and accumulation of gas hydrates in the Shenhu area of the northern part of the South China Sea (SCS). Compositionally, methane is the dominant gas (>90%) in the void gas and pressure core gas. In addition, as much as ~3% of the gas is composed of C2+ hydrocarbons, including ethane, propane, iso-butane, butane, iso-pentane, and n-pentane. The δ13C-CH4 and δD-CH4 values indicate a mixed biogenic-thermogenic origin for the hydrate-forming gas. The methane isotope correlation indicates that the source of the hydrate gas is closely related to the deep conventional gas reservoirs discovered in the Baiyun Sag-Panyu Low Uplift area. Both the hydrate gases and the deep reservoir gases are sourced from the hydrocarbon kitchens in the Baiyun Sag, revealing a paragenetic relationship within the same petroleum system. The composition of the hydrocarbons and the isotopic variation of methane with depth suggest that the thermogenic gas was likely affected by compositional and isotopic fractionation during the long-distance migration from the deep source rocks to the shallow gas hydrate stability zone (GHSZ). The impact of biodegradation on a solely thermogenic gas could also affect the final composition of the hydrate-forming gas. Analysis of the GHSZ based on gas hydrate compositions suggests that the occurrence of thermogenic gas could also indicate the coexistence of structure I (SI) and structure II (SII) gas hydrates in the Shenhu area, with the SII hydrates accumulating in or below the lower part of the SI GHSZ. The confirmed presence of SII hydrates in the Shenhu area relocated the base of the GHSZ deeper than was indicated by the bottom simulating reflector, which warrants further study in future explorations for gas hydrates in the Shenhu area.

Characteristics and dynamics of gas hydrate systems in the northwestern South China Sea - Results of the fifth gas hydrate drilling expedition

Abstract 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.

Chemical and structural characteristics of gas hydrates from the Haima cold seeps in the Qiongdongnan Basin of the South China Sea

Crystalline structure, cage occupancy, and gas composition are important gas hydrate characteristics, which can be used to calculate the gas hydrate’s stability and to estimate the energy potential. In May and October of 2015, a Guangzhou Marine Geological Survey team investigated two sites in the Haima cold seeps in the Qiongdongnan Basin, northern South China Sea. They recovered massive gas hydrate samples by conventional gravity coring. After the Shenhu Sea area and Dongsha Sea area, the Qiongdongnan Basin became a new area on the northern slope of the South China Sea where gas hydrates have been found. In order to reveal the structural and geochemical characteristics of the natural gas hydrates, samples were analyzed by micro-Raman spectroscopy and X-ray diffraction under ambient pressure and low-temperature conditions. The results indicate that the gas hydrate samples from the Haima cold seeps are typical structure I hydrates with a hydration number of 6.12–6.19. In addition, trace amounts of H2S trapped in the hydrate were identified based on its characteristic vibrational signature. The gas composition and δ13C-CH4 of the hydrate-bound gas samples were analyzed for gas-source correlation. All of the gas samples are dominated by methane with small amounts of ethane and propane and had relatively light δ13C-CH4, indicating that all of the hydrate-bound gases are mixtures of biogenic and thermogenic gas. The thermogenic gas is inferred to be mainly derived from the coal layers of the Late Miocene-Pliocene Yacheng Formation in the northern Lingshui sag.

Complex gas hydrate system in a gas chimney, South China Sea

Abstract 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 (C2H6: 0.16%–17.59%, C3H8: 0.05%–9.88%, i-C4H10:

Characteristics and formation mechanism of seafloor domes on the north‐eastern continental slope of the South China Sea

Multibeam echo‐sounding and seismic data were obtained during three geophysical surveys by Guangzhou Marine Geological Survey (GMGS) on the north‐eastern slope of the South China Sea (SCS). Nineteen seafloor domes and numerous elongated pockmarks were distinguished in the study area. The seafloor domes vary on number of summits, shape, and spatial interrelation, based on which they were classified into four types which are single‐summit, double‐summits, elongated, and connected. Bottom simulating reflectors and blanking zone underneath were observed in the seismic profiles, indicating the potential existence of gas hydrate and free gas. Multiple faults and diapirs exist beneath the seafloor domes and pockmarks, which were inferred to provide efficient pathways for fluid migrating from deep subsurface. The seafloor domes exhibits high backscatter feature, which is potentially associated with the seep‐related gas hydrate and authigenic carbonate in the shallow sediment. It is speculated that fault and diapir activity, fault sliding caused by region extension and the interaction between neighbour domes control and affect the activity and formation of the domes. However, further investigations, including geological sampling and seafloor observation, are still needed to confirm whether they are mud volcanoes.

Distribution of gas hydrate reservoir in the first production test region of the Shenhu area, South China Sea

ABSTRACT In May and July of 2017, China Geological Survey (CGS), and Guangzhou Marine Geological Survey (GMGS) carried out a production test of gas hydrate in the Shenhu area of the South China Sea and acquired a breakthrough of two months continuous gas production and nearly 3.1 × 105 m3 of production. The gas hydrate reservoir in the Shenhu area of China, is mainly composed of fine-grained clay silt with low permeability, and very difficult for exploitation, which is very different from those discovered in the USA, and Canada (both are conglomerate), Japan (generally coarse sand) and India (fracture-filled gas hydrate). Based on 3D seismic data preserved-amplitude processing and fine imaging, combined with logging-while-drilling (LWD) and core analysis data, this paper discusses the identification and reservoir characterization of gas hydrate orebodies in the Shenhu production test area. We also describe the distribution characteristics of the gas hydrate deposits and provided reliable data support for the optimization of the production well location. Through BSR feature recognition, seismic attribute analysis, model based seismic inversion and gas hydrate reservoir characterization, this paper describes two relatively independent gas hydrate orebodies in the Shenhu area, which are distributed in the north-south strip and tend to be thicker in the middle and thinner at the edge. The effective thickness of one orebody is bigger but the distribution area is relatively small. The model calculation results show that the distribution area of the gas hydrate orebody controlled by W18/W19 is about 11.24 km2, with an average thickness of 19 m and a maximum thickness of 39 m, and the distribution area of the gas hydrate orebody controlled by W11/W17 is about 6.42 km2, with an average thickness of 26 m and a maximum thickness of 90 m.

Characteristics and generation mechanism of gullies and mega-pockmarks in the Zhongjiannan Basin, western South China Sea

The paleoceanography and deep-water circulation process of the South China Sea (SCS) is still poorly understood. We have evaluated multibeam bathymetry and multichannel seismic reflection data acquired by Guangzhou Marine Geological Survey in recent years, and we investigate the characteristics, distribution, and generation mechanism of submarine seabed gullies and mega-pockmarks in the northern Zhongjiannan Basin of the SCS. Our data reveal that there are northwest–southeast-trending gullies and randomly or linearly arraying mega-pockmarks in the northern depression of the Zhongjiannan Basin. Gullies are 1 km or more in width and tens of kilometers in length. The long-axis diameter for the largest mega-pockmarks can reach up to 4293 m. The mega-pockmarks found in the west block of the northern depression are the manifestation of focused fluid flow migrating along shallow inclined faults induced by slumps that reached the seabed. Gullies are trending southeast and perpendicular to the contours of the continental slope, suggesting that gravity flows have played an important role in the evolution of gullies. Our data demonstrate that the gullies have a close relationship with mega-pockmarks, and that they were formed through the interaction of intense focused fluid flow and gravity flows. The pockmarks formed first, subsequently growing in size and number under the effect of subsurface fluid flows. The southeastward moving gravity flow caused discrete pockmarks to coalesce and form immature gullies. After being fully converged, gullies finally become smooth-bottomed and matured. Because gullies and mega-pockmarks are the manifestation of the geologic process of fluid escape at the seabed, they should be taken into account when assessing the potential for gas hydrates and seafloor instability.

Gas hydrate accumulation and saturations estimated from effective medium theory in the eastern Pearl River Mouth Basin, South China Sea

Pore- and fracture-filling gas hydrates were identified from the core samples at several sites during the second Guangzhou Marine Geological Survey (GMGS2) expedition. Well logs indicated that gas hydrate occurred in three distinct layers at site GMGS2-08. The gas hydrate saturations calculated from well-log data and the seismic responses for the three gas hydrate-bearing layers, especially within the middle carbonate layer, were poorly known. We estimated gas hydrate saturations using isotropic and anisotropic models based on the mineral composition of the sediments and the effective medium theory. In the upper and lower gas hydrate-bearing layers, saturations estimated from anisotropic models are close to those estimated from pressures cores and chlorinity data. The average saturation using an anisotropic model in the upper (fracture-dominated) hydrate layer is approximately 10% with a maximum value of 25%. In the lower (fracture-dominated) layer, the horizontal and vertical gas hydrate-filled fractures and visible gas hydrate were formed with a maximum saturation of approximately 85%. For the middle layer, well logs show high P-wave velocity, density, high resistivity as well as low gamma ray, porosity, and drilling rate, together indicating a carbonate layer containing gas hydrate. The hydrate saturations calculated from isotropic models assuming hydrate formed at grain contacts are less than 20%, which fit well with two values calculated from chlorinity data for this layer. The upper gas hydrate layer shows no clear seismic response and probably consisted of small fractures filled with gas hydrate. The middle carbonate and lower fracture-filled gas hydrate-bearing layers show pull-up reflections, with the carbonate layer exhibiting relatively higher amplitudes. Pore-filling gas hydrate was also identified just above the depth of the bottom-simulating-reflector (BSR) from the GMGS2-05 drill site. Below the BSR, the push-down reflections, polarity reversal, and enhanced reflections indicate the occurrence of free gas in the study area.