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Natural gas hydrate is a potential clean energy source and is related to submarine geohazard, climate change, and global carbon cycle. Multidisciplinary investigations have revealed the occurrence of hydrate in the Qiongdongnan Basin, northern South China Sea. However, the spatial distribution, controlling factors, and favorable areas are not well defined. Here we use the available high-resolution seismic lines, well logging, and heat flow data to explore the issues by calculating the thickness of gas hydrate stability zone (GHSZ) and estimating the inventory. Results show that the GHSZ thickness ranges between mostly ∼200 and 400 m at water depths >500 m. The gas hydrate inventory is ∼6.5×109-t carbon over an area of ∼6×104 km2. Three areas including the lower uplift to the south of the Lingshui sub-basin, the Songnan and Baodao sub-basins, and the Changchang sub-basin have a thick GHSZ of ∼250–310 m, 250–330 m, and 350–400 m, respectively, where water depths are ∼1 000–1 600 m, 1 000–2 000 m, and 2 400–3 000 m, respectively. In these deep waters, bottom water temperatures vary slightly from ∼4 to 2 °C. However, heat flow increases significantly with water depth and reaches the highest value of ∼80–100 mW/m2 in the deepest water area of Changchang sub-basin. High heat flow tends to reduce GHSZ thickness, but the thickest GHSZ still occurs in the Changchang sub-basin, highlighting the role of water depth in controlling GHSZ. The lower uplift to the south of the Lingshui sub-basin has high deposition rate (∼270–830 m/Ma in 1.8–0 Ma); the thick Cenozoic sediment, rich biogenic and thermogenic gas supplies, and excellent transport systems (faults, diapirs, and gas chimneys) enables it a promising area of hydrate accumulation, from which hydrate-related bottom simulating reflectors, gas chimneys, and active cold seeps were widely revealed. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Using MCSEM data, t he seabed resistivity is imaged at the Qiongdongnan Basin. • The thickness of GHSZ is analyzed using the resistivity and thermodynamic conditions. • The anisotropic characteristics of the seabed is revealed in the study area of QB. • The investigation provides evidence for the existence of microbial gas. Abstract The Qiongdongnan Basin (QB) is an important prospecting area for gas hydrates in the South China Sea (SCS). However, it is difficult to infer gas hydrates using a bottom simulating reflector (BSR) under the influence of flat seabed, multiples and bubble effects. In the QB, the upper limit of the gas hydrate stability zone (GHSZ) and the distribution of gas hydrates in the shallow seabed are still not well understood. A marine controlled-source electromagnetic (MCSEM) survey was completed in order to collect electric resistivity information along a 4.5-km profile. Electromagnetic data were acquired and processed at nine sites. The seafloor receiver orientations were evaluated using the orthogonal procrust rotation analysis (OPRA) method. As a result of isotropic, vertical transverse-isotropic (VTI), and triaxial anisotropic inversion of electric field Pmax data, a 2-D resistivity image of the seabed was presented. Preliminary results show that the shallow seabed can be divided into four electric layers. According to thermodynamic conditions in the QB, the estimated thickness of GHSZ is approximately 320 m for biogenic methane and 272 m for thermogenic gas in the study area. The BSR and hydrates were interpreted by integrating the resistivity model along with thermodynamic calculation and seismic reflection data. The MCSEM result indicates there are abundant natural gas originating or partly originating from a microbial source beneath the GHSZ. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Petroleum systems modeling is conducted in the gas hydrate exploitation region. • There is a gap between thermogenic gas migration phase and formation period of GHSZ. • Generation of the biogenic gas correlates with the formation of GHSZ. • A function relationship between water depth and GHSZ thickness is built. Abstract Two-dimensional gas hydrate petroleum system modeling was conducted to understand gas hydrate accumulation mechanisms and spatial distribution related to geologic and geochemical processes in the first experimental gas hydrate exploitation region in the Shenhu area of the South China Sea. Based on the gas production in core samples, a new biogenic methane generation model was created and incorporated into a kinetic model. With the results of a rock pyrolysis experiment, a thermogenic gas kinetic model for deep formations in the study area was also developed. The entire dynamic evolution process from gas generation to migration and gas hydrate formation and accumulation in the perspective area was simulated with the petroleum system modeling method. The results showed that the gas hydrate stability zone (GHSZ) began to form after 3.88 Ma, and its thickness was greatest when the paleowater depth peaked at 3.66 Ma. The mass of thermogenic gas generation in deep formations is large, and that of biogenic gas in shallow formations is relatively small. Formations that overlap the gas hydrate bearing layers act as cap rocks that prevent the gas from migrating vertically to shallow layers. There was a geological time gap between thermogenic gas generation, migration and GHSZ formation. However, the timing of biogenic gas generation, which is typically of recent generation in the shallow layer, matched well with the formation of the GHSZ. Finally, coupling of the age and space of gas migration and GHSZ formation has allowed gas hydrate accumulation at structural highs in the Shenhu area. The evaluation technique of petroleum system modeling is of great significance for the improvement of gas hydrate exploration in frontier areas in the future, particularly where survey data are limited. [ABSTRACT FROM AUTHOR]

To investigate the relationship among methane seepage, subsurface gas hydrate, and heat flow and estimate hydrate-related gas amount in the Dongsha region, northeastern South China Sea, in this study we first identified Bottom Simulating Reflector (BSR) using multi-channel seismic data collected in 2004. Subsequently, the thickness of gas hydrate stability zones (GHSZ) was calculated from the observed BSR using time–depth conversion, ranging from 60 to 300 m in seawater depths of 600–1200 m. Additionally, a seismic feature of double BSRs is locally observed in this region, which is possibly caused by either the presence of mixed gas source (thermogenic and biogenic) or migration of a pre-existing base of GHSZ. Geothermal gradient, heat flow and hydrate-related gas volume are calculated based on the thickness of GHSZ. The estimated geothermal gradient and heat flow varies from 20 to 100 °C/km with an average of 48 °C/km, and from 20 to 120 mW/m 2 with an average of 54 mW/m 2 , respectively. Both estimated geothermal gradient and heat flow are generally consistent with those from measurements. High geothermal gradient (up to 60 °C/km) and heat flow anomaly (up to 80 mW/m 2 ) are present in one methane seepage site, where BSR is clear and reliable. High heat flow could result in thin GHSZ in the continental slope. In this case, gas hydrate could form in the shallow sediments, making dissolved gas from hydrate and/or free deep-seated gas easy to migrate upwards to seafloor through faults, and thus resulting in the occurrence of methane venting. The hydrate-related gas volume is estimated as ∼6 × 10 11 m 3 over an area of ∼400 km 2 with possible occurrence of BSR. This finding would shed lights on understanding potential amount of subsurface methane in the Dongsha region and place useful constraints for modeling GHSZ from measured heat flow data, especially in regions where no BSRs are present. [ABSTRACT FROM AUTHOR]

Abstract: Based on the relationship among water depth, sea water temperature, geothermal gradient and geological setting, the gas hydrate potential in the South China Sea (SCS) is estimated. The thickness of the gas hydrate stability zone (GHSZ) and the minimum sea water depth in order to form the gas hydrate structures I (100% CH4), II (95.9% CH4) and H (90.4% CH4) are calculated and defined by the relationship among sea water depth, seawater temperature, geothermal gradient and gas composition. The average thickness of the GHSZ in the SCS is estimated to be 225m, 270m and 365m for the gas hydrate structures I, II and H, respectively. The calculation also shows that the gas hydrate accumulation is at the water depth equal to or deeper than 600m, 400m and 300 for gas hydrate structures I, II and H, respectively. Maximum thickness area of the GHSZ in the SCS is also defined in the water depth ranging from 1200 to 2300m. By assuming that gas hydrate is distributed in one third of the calculated GHSZ area, the volume of gas hydrate reservoir is estimated to contain 1.38×1014 m3, 1.41×1014 m3 and 1.7 1014 m3 of methane gas at the standard temperature and pressure (STP) for the gas hydrate structures I, II and H, respectively. [Copyright &y& Elsevier]

Herein we would like to comment on the paper 'Estimation of potential distribution of gas hydrate in the northern South China Sea' by Wang et al. 2010 in Chinese Journal of Oceanology and Limnology, 28(3): 693-699. The purpose of this comment is to point out that the given probabilities of gas hydrate occurrence in the northern Zhujiang Mouth Basin and the Yinggehai Basin in the figure of Wang et al. (2010) are improper. After introducing our work of estimation of gas hydrate stability distribution in the northern South China Sea, we suggest that Wang et al. (2010) dismissed the basic P-T rule for the existence of gas hydrate. They should consider more the variables of water depth, seabed temperature and geothermal gradient in their gas hydrate distribution model in future studies. [ABSTRACT FROM AUTHOR]