Design of a multiparameter data acquisition and control system for in situ seabed observation base stations

Abstract With the exploration, development, and research of deep‐sea resources, there is an urgent need for long‐term and continuous observation data of the deep‐sea seabed boundary layer. The traditional method of deep‐sea seabed survey and sampling based on scientific research vessels has the discontinuity of observation data in space and time scales. There are some problems in the seabed in situ observation method based on the seabed observation network for low mobility and high operation and maintenance costs, restricting the in‐depth understanding of the dynamic change process of the deep‐sea floor. To solve the above problems, an open and modular data acquisition control system was designed based on an embedded system and signal processing technology. In terms of the physical, chemical, geological, and ecosystem characteristics of the seafloor or near the seafloor boundary layer, various functional sensors and instrumentation were matched to form an independent underwater integrated measurement or experimental device, eventually realizing in situ multiparameter and long‐time series observations of the seafloor. The system data acquisition and control test were completed through laboratory experiments, which verified the feasibility of the system design. The research showed important theoretical and technical reference significance for the exploration and development of resources in the submarine boundary layer and the promotion of deep‐sea scientific research

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 delta C-13-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 8 delta C-13-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

Accumulation features and mechanisms of high saturation natural gas hydrate in Shenhu Area, northern South China Sea

Abstract: Based on the comprehensive interpretation of cores, loggings and 2D/3D seismic data of Shenhu GMGS3 drilling area in the northern South China Sea, the distribution characteristics, differential accumulation mechanism and reservoir forming mechanism of diffusion type natural gas hydrate with high saturation discovered from clayey silt reservoirs were investigated. The following findings are reached through the research: (1) Gas hydrate with high saturation often displays high resistivity, low interval transit time, and strong bottom-simulating reflectors (BSRs), and accompanies with fluid seepage phenomena beneath BSRs, such as mud diapiric structure and gas chimney. (2) The gas hydrate reservoirs are dominated by fine grained clayey silt sediments, and the reservoirs have higher porosity and permeability in local parts. (3) The gas hydrate is largely type I, whereas type II gas hydrate may exist below the type I gas hydrate. (4) The gas sources are mixed microbial and thermogenic gases, and the thermogenic gas originated from the deep formation in the center of Baiyun Sag migrated into shallow strata through faults, mud diapirs and gas chimneys, then was mixed with microbial gas in situ and continued to migrate until they accumulated in the temperature and pressure stability zone and formed diffusion type gas hydrate with high saturation finally. (5) The fluid migration system influenced and controlled the differential distribution of gas hydrate with high saturation. Key words: natural gas hydrate, high saturation, distribution characteristics, accumulation mechanism, Shenhu area, Pearl River Mouth Basin, northern South China Se

HIGH CONCENTRATION HYDRATE IN DISSEMINATED FORMS OBTAINED IN SHENHU AREA, NORTH SLOPE OF SOUTH CHINA SEA

In April-June of 2007, a gas hydrate drilling expedition was carried out by using M/V Bavenit in Shenhu Area, the north slope of South China Sea. High concentrations of hydrate (>40%) were obtained in a disseminated forms in foram-rich clay sediments at 3 selected sites. The hydrate-bearing sediments ranged several ten meters in thickness are located in the lower part of GHSZ, just above the BGHSZ, and are typically characteristic of higher sonic velocity and resistivity, and lower gamma density in wireline logging profiles. Evidences for gas hydrate include the IR cold spots and temperature anomalies, salinity and chlorite geochemical anomaly of pore water for non-pressurized cores, and X-ray imaging, high p-wave velocity and low gamma density, and high concentration of methane from the pressurized cores. Gasses are mainly methane (max. ethane 0.2-0.3%), therefore only hydrate S1 is formed. It is inferred that the foram content and other silt size grains may provide enough free water for the hydrate to happily occupy both the large spaces in the forams and for it to distribute itself evenly (disseminated) throughout the formation. It is possible that all the forams are hydrate filled. As the forams are visible does this not count for visible white gas hydrates.Non UBCUnreviewe

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

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

Geological features, controlling factors and potential prospects of the gas hydrate occurrence in the east part of the Pearl River Mouth Basin, South China Sea

Logging-while-drilling (LWD) and wireline log (CWL) data were acquired during China's second gas hydrate drilling expedition (GMGS-2) in the east of Pearl River Mouth Basin, South China Sea. Disseminated and massive gas hydrates deposits were found at different sites. Gas hydrate-bearing lithologies identified from the sample coring included the fine-grained sediments and coarse-grained sediments. LWD logs from Site GMGS2-08 indicate significant gas hydrate in clay-bearing sediments including two layers with massive gas hydrate with a bulk density near to 1.08 g/cm3. High electrical resistivities with a range of 2.5–2000.0 Ω m and high P-wave velocities are simultaneously observed in the hydrate-bearing sediments. The average gas hydrate saturation estimated from the pore water freshing analysis ranges from 45 to 55% of the pore space. Buried carbonate layers above the massive gas hydrate deposit discovered at Sites GMGS2-08 indicate that the formations are likely to have formed initially at the surface and then were buried. Significant high amplitude seismic anomalies, discontinuous bottom simulating reflection (BSR) and blanking zone are detected in the drilling zone. The hydrate-bearing sediments predominantly consist of silty clay and limestone grains in which the gas hydrates are deposited primarily in the form of laminated, massive, veins or nodule. The gas hydrates occurrences are subjected to the sediment lithology, new tectonic activities, migration of fluid and gas and also the factors such as heat flow, salinity and time which affect the nucleation of gas hydrates. Its natural morphologies present massive, laminated, nodular, nugget and disseminated, of which the former four often formed in shallow fault, inter-layer's weak cementation zone and on the seabed. The “buried” gas hydrates with high saturation are good zones for gas hydrate exploitation