Jinqiang Liang

Gas Hydrate System of Shenhu Area, Northern South China Sea: Geochemical Results

The drilling recovered high-concentration methane hydrates (maximum 26–48%) in a disseminated form in silty clay sediments in Shenhu area of Pearl River Mouth Basin, South China Sea. Combining the geochemical data, the gas hydrate-bearing sediments are 10 m to 43 m in thickness and located just above the base of the gas hydrate stability zone. The methane content is 96.10–99.91% with small amount of ethane and propane. The baseline chlorinity of pore waters shows 10% lower than that of shallow sediments below and inside the gas hydrate zone. The methane/ethane ratios are higher than 1000 above the gas hydrate zone and less than 1000 at the interval of gas hydrate zone. The depth of sulphate methane interface varies from site to site as 17 to 27 mbsf. These results show that the methane of gas hydrate was mainly originated from microbial activity and the upward methane flux is minor. This is evidenced by the values of headspace gases from the gravity piston cores and released gases from pressure cores, which range from −74.3‰  PDB to −46.2‰  PDB, with the majority less than −55%‰  PDB. The hydrate deposit is a distributed gas hydrate system in Shenhu area.

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 hydratebearing 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.

Variations of pore water sulfate gradients in sediments as indicator for underlying gas hydrate in Shenhu Area, the South China Sea

Shenhu Area is one of the most promising areas for gas hydrate exploration in the northern South China Sea (SCS). Pore water sulfate gradient, sulfate-methane interface (SMI) depth, and sulfate flux were analyzed at 53 sites in this area. SO4−2 gradient ranges between 0.33 and 4.43 mmol L−1 m−1. SMI depths are from 7.7 to 87.9 mbsf. Sulfate flux varies between 2.0 and 26.9 mmol m−2 yr−1, with a mean of 11.7 mmol m−2 yr−1. Correlation coefficient between SMI depth and methane flux for the 53 sites is −0.80, implying that methane flux regulates the rate of anaerobic methane oxidation (AMO), SMI depth, and sulfate flux. Twelve anomalous fields with high methane flux and steep sulfate gradients were recognized. Bottom simulating reflector (BSR) is distributed mainly in areas where SMI depth is less than 50 mbsf or places with sulfate flux larger than 3.5 mmol m−2 yr−1. It is suggested that the Baiyun Sag and the Southern Uplift are potential areas for gas hydrate exploration.

Accumulation and exploration of gas hydrate in deep-sea sediments of northern South China Sea

The large deep-sea area from the southwestern Qiongdongnan Basin to the eastern Dongsha Islands, within the continental margin of northern South China Sea, is a frontier of natural gas hydrate exploration in China. Multiform of deep-sea sedimentations have been occurred since late Miocene, and sediment waves as a potential quality reservoir of natural gas hydrate is an most important style of them. Based on abundant available data of seismic, gravity sampling and drilling core, we analyzed the characteristics of seismic reflection and sedimentation of sediment waves and the occurrence of natural gas hydrate hosted in it, and discussed the control factors on natural gas hydrate accumulation. The former findings revealed the deep sea of the northern South China Sea have superior geological conditions on natural gas hydrate accumulation. Therefore, it will be of great significance in deep-sea natural gas hydrate exploration with the study on the relationship between deep-sea sedimentation and natural gas hydrate accumulation.

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.

Application of AVO analysis to gas hydrates identification in the northern slope of the South China Sea

Amplitude versus offset (AVO) analysis is a conventional seismic exploration technique in geophysical and lithological interpretation and has been widely used in onshore and offshore exploration. Its use in marine gas hydrate research, however, is still in initial stages. In this study, AVO analysis is applied to seismic profiles at drilling sites where hydrate samples have been recovered. The AVO responses of free gas, bottom simulating reflector (BSRs), and gas hydrates are discussed, and the AVO attributes in relation to gas hydrates are summarized. The results show that changes in intercept, gradient, fluid factor and Poisson’s ratio clearly reflect: (i) location of free gas and the BSR, and (ii) spatial relations between blank zone, BSR, gas hydrate, and free gas.

Types, Characteristics and Significances of Migrating Pathways of Gas-bearing Fluids in the Shenhu Area, Northern Continental Slope of the South China Sea

The first marine gas hydrate expedition in China has been conducted by Guangzhou Marine Geological Survey in the Shenhu Area, northern continental slope of the South China Sea. Previous study has analyzed the P-T conditions, geophysical anomalies and saturation calculations of these gas hydrates, but has not documented in detail the migration of gas-bearing fluids in the study area. Based on the interpretations of 2D/3D seismic data, this work identified two types of migration pathways for gas-bearing fluids in the Shenhu area, i.e., vertical and lateral pathways. The vertical pathways (large-scale faults, gas chimneys and mud diapirs) presented as steep seismic reflection anomalies, which could be traced downward to the Eocene source rocks and may penetrate into the Late Miocene strata. The deeper gases/fluids might be allowed migrating into the shallower strata through these vertical conduits. However, the distributions showed distinct differences between these pathways. Large-scale faults developed only in the north and northeast of the Shenhu area, while in the drilling area gas chimneys were the sole vertical migration pathways. Since the Pliocene, normal faults, detachment faults and favorable sediments have constituted the lateral pathways in the Shenhu gas hydrate drilling area. Although these lateral pathways were connected with gas chimneys, they exerted different effects on hydrate formation and accumulation. Gas-bearing fluids migrated upward along gas chimneys might further migrate laterally because of the normal faults, thereby enlarging the range of the chimneys. Linking gas chimneys with the seafloor, the detachment faults might act as conduits for escaping gases/fluids. Re-deposited sediments developed at the early stage of the Quaternary were located within the gas hydrate stability zone, so hydrates would be enriched in these favorable sediments. Compared with the migration pathways (large-scale faults and mud diapirs) in the LW3-1 deep-sea oil/gas field, the migration efficiency of the vertical pathways (composed of gas chimneys) in the gas hydrate drilling area might be relatively low. Description and qualitative discrimination of migration pathways in the Shenhu gas hydrate drilling area are helpful to further understand the relationship between good-quality deep source rocks and shallow, mainly biogenically-produced, hydrates. As the main source rocks of the Baiyun sag, lacustrine mudstones in the Wenchang and Enping Formations may provide thermogenic methane. Gas chimneys with relatively low migration efficiency created the vertical pathways. Caused by the Dongsha tectonic movement, the release of overpressured fluids might reduce the vertical migration rates of the thermogenic methane. The thick bathyal/abyssal fine-grained sediments since the Late Miocene provided migration media with low permeability. These preconditions may cause carbon isotopic fractionation of thermogenic methane during long-distance vertical migrations. Therefore, although geochemical analyses indicate that the methane forming gas hydrate in the Shenhu area was mainly produced biogenically, or was mixed methane primarily of microbial origin, thermogenic methane still contribute significantly.

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

AbstractPore- 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 f...

Characterization of hydrate-bearing sediments recovered from the Shenhu area of the South China Sea

Abstract The characteristics of the hydrate-bearing layer are important parameters for gas hydrate exploration and exploitation, which can be provided by laboratory analysis. We have carried out a systematic analysis of modern instruments, including the laser grain size analyzer, X-ray computed tomography, scanning electron microscopy, X-ray diffraction (XRD), Raman spectroscopy, gas chromatography, and isotope mass spectrometry, on the hydrate-bearing sediment samples recovered from the Shenhu area in the South China Sea in 2016. We focused on the structure, hydration number, and gas composition of the gas hydrate samples. We also evaluated the effects of sediments on the occurrence and distribution of gas hydrate. Our results showed that the sediments are much finer than those recovered from the Shenhu area in 2007. Some samples are found to contain calcareous microfossils and foraminifera, which are beneficial for dispersed gas hydrate formation. Whereas the only visible hydrate sample (FC4) has not fo...

ESTIMATION OF GAS HYDRATE SATURATION WITH MODIFIED BIOT GASSMANN THEORY: A CASE FROM NORTHERN SOUTH CHINA SEA: ESTIMATION OF GAS HYDRATE SATURATION WITH MODIFIED BIOT GASSMANN THEORY: A CASE FROM NORTHERN SOUTH CHINA SEA

The modified Biot-Gassmann theory(BGTL) proposed by Lee(2002) is applied in this study to estimation of the saturation of gas hydroate in the deep-water unconsolidated clayey sediments from the Well A,Shenhu area,northern South China Sea.The BGTL theory assumes that the ratio of the shear to compressional velocities of an unconsolidated sediment is related to the ratio of the shear to compressional velocities of the solid matrix in the sediment and its porosity.Parameters involved in the model are related to the occurrence of gas hydrate,mineral components,pressure differentiation,porosity,and pore structure of the sediments.The cross plot of velocity from sonic logging vs.concentration of gas hydrate from core measurements suggest that the occurrence of gas hydrate in the sediments of Well A is more or less close to the matrix model.Statistics from core smear slide data suggest that the sediment matrix in well A can be simplified as three major mineral components,i.e clay,carbonate,and terrigenous clastic minerals.The elastic modulus and density values of the matrix are calculated by the elastic modulus and density values of the individual mineral components and their volume percentage.As estimated,the gas hydrate in Well A is mainly distributed in the depth interval of 195 to 220 meters below sea floor with a highest concentration of gas hydrate up to 47%,which matches well with the results from core measurements.