Search Results (15)

The methane flux in the Dongsha area in the northern South China Sea is relatively high. The results indicate the presence of both shallow and deep gas hydrate reservoirs at the Site DS-W08. The gas hydrate reservoir in this area is mainly composed of fine-grained sediments, and high-saturation gas hydrates are present. The shallow-GHR (8–24 mbsf) exhibits a maximum hydrate saturation of 14% (pore volume). The deep-GHR (below 65 mbsf) shows a maximum hydrate saturation of 33% The suspended sedimentation process on the banks of turbidity currents and the deep-water traction current sedimentation process play potentially important roles in the enrichment of gas hydrates. To investigate the influence of sedimentary processes on gas hydrate accumulation, this study analyzed gas hydrate saturation, sediment grain size, grain compositions, biological components, and geochemical characteristics of hydrate-bearing and adjacent layers at Site DS-W08. Sediment grain size analysis suggests that the studied layer was formed through the interaction of turbidity current-induced overbank suspended deposition and traction current deposition. By comprehensively analyzing the comparison of sediment Sr/Ba ratios and the data of foraminifera and calcareous nannofossils, it is found that the bank deposits and traction current deposits triggered by turbidity currents correspond to glacial periods and interglacial periods, respectively. Analysis of biological components shows that layers with high foraminifera content and traction current-modified sediments are more favorable for gas hydrate accumulation. Hydrate reservoirs are all composed of traction current deposits, and the cap rock rich in foraminifera fossils at the top promotes hydrate formation; while the fine-grained turbidites formed during the turbidite deposition process inhibit hydrate accumulation. This study aims to deepen the understanding of the enrichment mechanism of natural gas hydrates and support the commercial development of fine-grained sediments in the northern South China Sea. Full article

Shallow water bathymetry is essential for maritime navigation, environmental monitoring, and coastal management. While traditional methods such as sonar and airborne LiDAR provide high accuracy, their high cost and time-consuming nature limit their application in remote and sensitive areas. Satellite remote sensing offers a cost-effective and rapid alternative for large-scale bathymetric inversion, but it still relies on significant in situ data to establish a mapping relationship between spectral data and water depth. The ICESat-2 satellite, with its photon-counting LiDAR, presents a promising solution for acquiring bathymetric data in shallow coastal regions. This study proposes a rapid bathymetric inversion method based on ICESat-2 and Sentinel-2 data, integrating spectral information, the Forel-Ule Index (FUI) for water color, and spatial location data (normalized X and Y coordinates and polar coordinates). An automated script for extracting bathymetric photons in shallow water regions is provided, aiming to facilitate the use of ICESat-2 data by researchers. Multiple machine learning models were applied to invert bathymetry in the Dongsha Islands, and their performance was compared. The results show that the XG-CID and RF-CID models achieved the highest inversion accuracies, 93% and 94%, respectively, with the XG-CID model performing best in the range from −10 m to 0 m and the RF-CID model excelling in the range from −15 m to −10 m. Full article

The Dongsha area, a key region in the northern South China Sea (SCS), features both diffusive deep and seepage shallow gas hydrate reservoirs. Utilizing sediment samples from gas hydrate reservoirs and adjacent layers at sites W08 and W16 in the Dongsha area, this study aims to uncover the sediment property differences between deep and shallow gas hydrate reservoirs and their impact on gas hydrate accumulation through grain size, X-ray diffraction, and specific surface area (SSA) analyses. The findings classify the study intervals into four distinct layers: shallow non-gas hydrate layer (shallow-NGHL), shallow gas hydrate reservoir (shallow-GHR), deep non-gas hydrate layer (deep-NGHL), and deep gas hydrate reservoir (deep-GHR). In the clayey silt sediment reservoirs, grain size has a minor influence on gas hydrate reservoirs. Both shallow and deep NGHLs, characterized by high smectite content and SSA, possess a complex structure that impedes gas and fluid migration and offers limited potential reservoir space. Consequently, both shallow and deep NGHLs function as sealing beds. The deep GHR, having low smectite content and SSA, exhibits a strong capacity for gas and fluid migration and greater potential reservoir space. As a result, sediment properties significantly influence the deep GHR. Seepage primarily controls the shallow GHR. Full article

Based on the third-generation wave model WAVEWATCH-III (WW3), this paper analyzes the changing trend of wave power density (WPD) in the South China Sea, which can provide necessary references for the development and utilization of wave energy resources in the future. In this study, multi-platform cross-calibrated (CCMP) wind data was used to drive WW3 to calculate the WPD of the South China Sea. The Mann-Kendall (MK) algorithm can be used to determine the mutation of WPD, and the accuracy of the CCMP wind was verified. Next, the time distribution of WPD is analyzed for the whole sea area and dominant sea area of the South China Sea, and on this basis, the dominant sea area for the development of wave energy resources in the South China Sea was studied. The results are as follows: (1) Extreme weather has a significant impact on the change of WPD in the South China Sea, and this change is likely due to the effect of extreme weather on sea temperature. (2) Dongsha Islands has the highest annual WPD value and has the greatest impact on the overall trend change of the South China Sea. (3) Integrated wave energy exploitability and safety and technology perspectives, the waters around Taiwan Strait are more suitable as the primary site for energy conversion. Full article

The Mesozoic strata in the northern South China Sea have good potential for oil and gas exploration. The Dongsha Waters, where the study area is located, have complex seismic and geological characteristics; in particular, turbulence is very prominent in this area, so it is difficult to implement 3D seismic data collection. The “single-source and single-cable quasi-3D seismic survey” method integrates some key technologies in acquisition and processing, thus improving the quality of seismic imaging. Based on the interpretation of the existing research results and new data, structure B-1 has good source–reservoir–cap combination conditions. The oil–gas accumulation mode is predicted, and the drilling well B-1-1 is given. In addition, the large-scale distribution of bottom-simulating reflectors (BSRs) and the discovery of gas seepage areas in the study area suggest the presence of gas hydrate. We suggest that deep thermogenic gas from the Mesozoic strata has migrated into the overlying strata along the fault system and mixed with microbial gas to form hydrate. Full article

Aerosol robotic network (AERONET) data from Dongsha Island (20.699N, 116.729E) and Taiping Island (35.90N, 3.03W) over the South China Sea (SCS) from January 2018 to December 2020 were used to analyze and discuss the temporal evolution properties of aerosols in the South China Sea. Surrounding AERONET stations (Hong Kong, NSPO, Nha Trang and Singapore) were also used to analyze regional characteristics. High aerosol loads over Dongsha were strongly associated with the anthropogenic fine particle transport from the southeastern coast of China and occasional advection of desert dust from Mongolian areas. The high fine aerosol loading in Taiping originates from the region between Singapore and Indonesia. Compared with other marine islands in the world, SCS was not a pure marine aerosol environment and was affected by terrestrial aerosols. In the Taiping area, aerosol optical depth τ (500 nm) was 0.17 ± 0.13 and the average Ångström exponent α (440–870 nm) was 0.96 ± 0.36. However, that of Dongsha shows the larger values of τ (0.26 ± 0.21) and α (1.1 ± 0.38), indicating that there are large fluctuations in aerosol concentration and size. Aerosol loads in different regions of the SCS due to uneven socioeconomic and complex meteorological systems, such as those of the coastal cities of China, Singapore, and the region between Singapore and Indonesia, contribute to the high optical depth. The special meteorological regime and aerosol source mechanism in the SCS leads to the obvious seasonal cycle of aerosol optical depth and Ångström index. Moreover, the loading variations of aerosols on Dongsha Island and Taiping Island were highly consistent with those of coastal cities around them, suggesting the significant effect of the aerosol in the SCS by the surrounding coastal cities, although the aerosol optical depth in these two places was much lower than that in the surrounding cities. Full article

Petroleum extraction, transportation, and consumption in the marine environment contribute to a large portion of anthropogenic oil spills into the ocean. While previous research focuses more on large oil spill accidents from oil tankers or offshore oil platforms, there are few systematic records on occasional regional oil spills. In this study, optical imagery from Landsat-8 OLI was used to detect oil slicks on the ocean surface through spatial analysis and spectral diagnosis in the northern South China Sea (NSCS). The source of the slicks was identified through datasets from traffic density and platform locations. A total of 632 oil slicks were detected in the NSCS from 2015 to 2019, where 57 were from platforms sources, and 490 were from ships. The average area of the detected slicks was 4.8 km², and half of the slicks had areas <1.7 km². Major oil spill hot spots included coastal Guangdong (ship origins), southeast and northeast Dongsha Island (ship origins), middle of south Beibu Gulf (ship and platform origins), and southeast Pearl River Estuary (platform origins). Through this study, we demonstrate the capability of medium-resolution optical imagery in monitoring regional oil spills. Such results and methods may help in near real-time oil spill monitoring and further environmental assessments. Full article

The stratigraphic-diffusive type of gas hydrate system is formed by microbial methane produced in a shallow slope space when flowing laterally into hydrate stable zones and is worth studying for both energy supply and academic understanding. A deposition production model matching the vertical and lateral seabed morphological characteristics was constructed to show the accumulation process, layer timing sequence, and reservoir quality of the stratigraphic-diffusive hydrate system in the Dongsha slope sediments since the Pleistocene. Six representative key system factors at three selected moments (1.5 Ma, 700 ka B.P., and at present) have been exhibited during debris is continuously accumulating. The coexistence of the hydrate decomposition in the lower part and the formation in the upper part, and the uneven distribution of hydrates within the slope sediment surface are explained clearly. By comparing four geological cases with diverse environments, it is shown that the diffusive hydrate system is likely to develop into moderate geological conditions. The most powerful carbon fixation ability in this system was quantified within the time range of 100−50 ka B.P. Finally, it was verified that residual methane would converge near the seafloor interface and then eventually overflow out of the seabed into the seawater. Full article

We processed the raw multi-beam bathymetry data acquired in the central and northeastern part of the South China Sea by eliminating noise and abnormal water depth values caused by environmental factors, and a high resolution bathymetric map with a 20-m grid interval was constructed. Various scales of seafloor geomorphological features were identified from the data, including an image of Shenhu canyon, which is located in the northern continental margin of the South China Sea; submarine reticular dunes in the north of the Dongsha atoll; submarine parallel dunes in the northeast of the Dongsha atoll; and several seamounts in the southwest sub-basin and in the east sub-basin. In the processing step, various anomalies in the multi-beam bathymetry data were corrected. The optimal swath filtering and surface filtering methods were chosen for different scales of seafloor topography in order to restore the true geomorphological features. For the large-scale features with abrupt elevation changes, such as seamounts (heights of ~111–778 m) and submarine canyons (incision height of ~90–230 m), we applied swath filtering to remove noise from the full water depth range of the data, and then surface filtering to remove small noises in the local areas. For the reticular dunes and parallel dunes (heights of ~2–32 m), we applied only surface filtering to refine the data. Based on the geometries of the geomorphological features with different scales, the marine hydrodynamic conditions, and the regional structure in the local areas, we propose that the Shenhu submarine canyon was formed by turbidity current erosion during the Sag subsidence and the sediment collapse. The submarine reticular dunes in the north of the Dongsha atoll were built by the multi-direction dominant currents caused by the previously recognised internal solitary waves around the Dongsha atoll. The submarine parallel dunes in the northeast of the Dongsha atoll were built by the repeated washing of sediments with the influence of the tidal currents and internal solitary waves. The conical, linear and irregular seamounts identified from the bathymetry data were formed during the spreading of the southwest sub-basin and the east sub-basin. The identified seamounts in the multi-beam bathymetry data are correlated to deep magmatic activities, the Zhongnan transform fault and the NE-trending faults. Full article

The Dongsha Shoal is one of the largest shoals in the South Yellow Sea and has important marine ecological value. The shoal extends in a south–north direction and is controlled by the regional dominating tidal currents. Recently, due to human activities and some natural factors, the geomorphic dynamics of the Dongsha Shoal has undergone drastic changes. However, few people have proposed quantitative research on the changes of tidal flat morphology, let alone the long-term sequence analysis of sand ridge lines. Hence, we attempt to take the Dongsha Shoal in the Radial Sand Ridges as the research area, and analyze the trends of the long-term morphological evolution of the sand ridge lines over the period 1973–2016 based on a high-density time series of medium-resolution satellite images. The sand ridge line generally moves from southeast to northwest, and the position distribution of the sand ridge line from north to south has gradually changed from compact to scattered. We also found that the geomorphological dynamics at different positions of the sand ridge line are inconsistent. The north and south wings are eroded on the west side, while the central area is eroded on the east side. Most of the sand ridge line is moving eastward. In addition, the change of sand ridge line is affected by multiple factors such as sediment supply, typhoon, reclamation and laver cultivation. Full article

A remarkable exposure of tubular authigenic carbonates was found on the seafloor in the Dongsha area of the South China Sea (SCS). The tubular carbonates, around 2–3 cm in diameter and usually less than 10 cm in length, represent broken fragments of once-larger pipes that now protrude from muddy sediments. The morphology, carbon and oxygen stable isotope compositions, and trace and rare earth element contents of the carbonates were analyzed to decipher the mode of carbonate formation. The tubular carbonates exhibit a dark brown coating of iron and manganese hydrous oxides, indicating prolonged exposure to oxic bottom waters. The carbonate content of the micritic pipes falls between 12.5 and 67.3 wt.% with an average of 42.0 wt.%, suggesting formation within the sediment. This inference is supported by trace and rare earth element patterns including a moderate enrichment of middle rare earth elements. Low δ¹³C values (as low as −50.3‰, Vienna Pee Dee Belemnite (VPDB)) suggest that carbonate precipitation was induced by the anaerobic oxidation of methane. The unusually positive δ¹⁸O values of the carbonates (as high as +5.3‰, VPDB) are believed to reflect the destabilization of locally abundant gas hydrate. Taken together, it is suggested that pipe formation was initiated by sediment-dwelling organisms, such as crustaceans or bivalves. The burrows subsequently acted as conduits for upward fluid migration. The lithification of the sediment directly surrounding the conduits and the partial filling of the conduits with carbonate cement resulted in the formation of tubular carbonates. Turbidity currents, sediment slumps, or the vigorous emission of fluids probably induced the fragmentation of tubular carbonates within the sediment. The carbonate fragments had been further subjected to winnowing by bottom currents. This study provides insight into the interaction of megafauna burrowing with fluid migration and carbonate formation at hydrocarbon seeps, highlighting the role of bottom currents and mass wasting on the formation of fragmented tubular carbonates. Full article

The diagenetic environment is vital in controlling reservoir quality by influencing diagenetic processes, especially porosity evolution. The Zhuhai Formation is a critical exploration target in the Huizhou Sag. However, there are considerable differences between the reservoirs in the west (Huixi) and east (Huidong) Huizhou Sag with only a 400 m difference in burial depth. To investigate causes of these differences, scanning electron microscopy (SEM), cathodoluminescence analysis, fluorescence analysis, and fluid inclusion analysis are employed for the petrological and diagenetic characterization of the target formation. Moreover, image analysis software is used to qualitatively evaluate the impacts of diagenesis on porosity. The results show that the digenetic environments in Huixi and Huidong are different. Huixi is characterized by a high content of feldspar with a stronger dissolution phenomenon than that observed in Huidong. The main cement types in Huixi are siliceous cement and kaolinite. Moreover, Huixi had a low palaeo-temperature, low palaeo-salinity, and low palaeo-pH diagenetic environment. In contrast, Huidong is accessible via deep faults and rich in lithic fragments which show visible deformation. Moreover, cements in this area include carbonate, gypsum, quartz overgrowth, kaolinite, and chlorite. This evidence confirms that the diagenetic environment in Huidong was a high palaeo-temperature and high palaeo-salinity environment with high palaeo-pH. Additionally, the palaeo-pH evolved following the sequence of weakly alkaline, acidic, alkaline and acidic. Microscopy evidence indicates that the Dongsha Uplift was a likely source of Huixi sediments during the deposition of the Zhuhai Formation. The determination of the porosity evolution shows that compaction and lithic fragment content are the main controlling factors on the reservoir quality of the Zhuhai Formation. Full article

Fe and Mn oxides and (oxy)-hydroxides are the most abundant solid-phase electron acceptors in marine sediments, and dissimilatory Fe/Mn reduction usually links with the anaerobic oxidation of methane (AOM) and organic matter oxidation (OMO) in sediments. In this study, we report the results from subsurface marine sediments in the Dongsha hydrate-bearing area in the South China Sea. The petrological and geochemical signatures show that the Fe/Mn reduction mediated by AOM and OMO might occur in sediments above the sulfate-methane transition zone. X-ray diffraction and scanning electron microscopy analyses of sediments indicate that Fe(III)/Mn(IV)-oxides and authigenic carbonate minerals coexisted in the Fe/Mn reduction zone. The lower δ¹³C values of dissolved inorganic carbon, coupled with an evident increase in total inorganic carbon contents and a decrease in Ca²⁺ and Mg²⁺ concentrations indicate the onset of AOM in this zone, and the greater variation of PO₄³⁻ and NH₄⁺ concentrations in pore water suggests the higher OMO rates in subsurface sediments. Geochemical and mineralogical analyses suggest that the previously buried Fe(III)/Mn(IV) oxides might be activated and lead to the onset of Fe/Mn reduction induced by AOM and OMO. These findings may extend our understanding of the biogeochemical processes involved in Fe/Mn reduction in continental shelves with abundant methane, organic matter, and terrigenous metal oxides. Full article

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. Full article

Natural gas hydrate accumulations were confirmed in the Dongsha Area of the South China Sea by the Guangzhou Marine Geological Survey 2 (GMGS2) scientific drilling expedition in 2013. The drilling sites of GMGS2-01, -04, -05, -07, -08, -09, -11, -12, and -16 verified the existence of a hydrate-bearing layer. In this work gas production behavior was evaluated at GMGS2-8 by numerical simulation. The hydrate reservoir in the GMGS2-8 was characterized by dual hydrate layers and a massive hydrate layer. A single vertical well was considered as the well configuration, and depressurization was employed as the dissociation method. Analyses of gas production sensitivity to the production pressure, the thermal conductivity, and the intrinsic permeability were investigated as well. Simulation results indicated that the total gas production from the reference case is approximately 7.3 × 10⁷ ST m³ in 30 years. The average gas production rate in 30 years is 6.7 × 10³ ST m³/day, which is much higher than the previous study in the Shenhu Area of the South China Sea performed by the GMGS-1. Moreover, the maximum gas production rate (9.5 × 10³ ST m³/day) has the same order of magnitude of the first offshore methane hydrate production test in the Nankai Trough. When production pressure decreases from 4.5 to 3.4 MPa, the volume of gas production increases by 20.5%, and when production pressure decreases from 3.4 to 2.3 MPa, the volume of gas production increases by 13.6%. Production behaviors are not sensitive to the thermal conductivity. In the initial 10 years, the higher permeability leads to a larger rate of gas production, however, the final volume of gas production in the case with the lowest permeability is the highest. Full article