Comment on “Estimation of potential distribution of gas hydrate in the northern South China Sea” by Chunjuan Wang et al.

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.     

Study of the Surface Morphology of Gas Hydrate

Study of the surface morphology of gas hydrate is of great importance in understanding its physical properties and occurrence.In order to investigate the surface morphology of different types(sI and sII)and occurrences(pore-filling and fracture-filling)of gas hydrate,both lab-synthesized and drilled-gas hydrate samples were measured using cryo-scanning electron microscopy(cryo-SEM).Results showed that the surface of s I hydrate was relatively smooth,and spongy nano-pores(200–400 nm)gradually occurred at the surface during continuous observation.The surface of sII hydrate was more compact,showing a tier-like structure.Hydrate occurred in quartz sand and usually filled the pores of the sediments and both hydrate and sediments were cemented with each other.SEM observation of the gas hydrates collected from the South China Sea showed that the surface morphology and contact relation with sediments varied with hydrate occurrence.For instance,hydrates dispersed in sediments mainly filled the pores of the sediments.The existence of microorganism shells,such as foraminifera,was beneficial to the formation of gas hydrate.When hydrate occurred as a massive or vein structure,it was easily distinguished from the surrounding sediments.The surface of hydrate with massive or vein structure showed two distinct characters:one was dense and smooth,the other is porous(several to tens of micrometers in diameter).The occurrence of different hydrate morphologies was probably caused by the supplement rates of methane gas.     

Placement Optimization Method of FPSO Gas Detectors Based on Leakage Risk

As the first step of the fire/gas-detection systems of floating production storage and offloading(FPSO)units is to iden-tify leakage accidents,gas detectors play an important role in controlling the leakage risk.To improve the leakage scenario detection rate and reduce the cumulative risk value,this paper presents an optimization method of the gas detector placement.The probability density distribution and cumulative probability density distribution of the leakage source variables and environmental variables were calculated based on the Offshore Reliability Data and the statistical data of the relevant leakage variables.A potential leakage sce-nario set was constructed using Latin hypercube sampling.The typical FPSO leakage scenarios were analyzed through computational fluid dynamics(CFD),and the impacts of different parameters on the leakage were addressed.A series of detectors was deployed according to the simulation results.The minimization of the product of effective detection time and gas leakage volume was the risk optimization objective,and the location and number of detectors were taken as decision variables.A greedy extraction heuristic algo-rithm was used to solve the optimization problem.The results show that the optimized placement had a better monitoring effect on the leakage scenario.     

Estimation of potential distribution of gas hydrate in the northern South China Sea

Gas hydrate research has significant importance for securing world energy resources, and has the potential to produce considerable economic benefits. Previous studies have shown that the South China Sea is an area that harbors gas hydrates. However, there is a lack of systematic investigations and understanding on the distribution of gas hydrate throughout the region. In this paper, we applied mineral resource quantitative assessment techniques to forecast and estimate the potential distribution of gas hydrate resources in the northern South China Sea. However, current hydrate samples from the South China Sea are too few to produce models of occurrences. Thus, according to similarity and contrast principles of mineral outputs, we can use a similar hydrate-mining environment with sufficient gas hydrate data as a testing ground for modeling northern South China Sea gas hydrate conditions. We selected the Gulf of Mexico, which has extensively studied gas hydrates, to develop predictive models of gas hydrate distributions, and to test errors in the model. Then, we compared the existing northern South China Sea hydrate-mining data with the Gulf of Mexico characteristics, and collated the relevant data into the model. Subsequently, we applied the model to the northern South China Sea to obtain the potential gas hydrate distribution of the area, and to identify significant exploration targets. Finally, we evaluated the reliability of the predicted results. The south seabed area of Taiwan Bank is recommended as a priority exploration target. The Zhujiang Mouth, Southeast Hainan, and Southwest Taiwan Basins, including the South Bijia Basin, also are recommended as exploration target areas. In addition, the method in this paper can provide a useful predictive approach for gas hydrate resource assessment, which gives a scientific basis for construction and implementation of long-term planning for gas hydrate exploration and general exploitation of the seabed of China.     

Numerical analysis of wellbore instability in gas hydrate formation during deep-water drilling

Gas hydrate formation may be encountered during deep-water drilling because of the large amount and wide distribution of gas hydrates under the shallow seabed of the South China Sea. Hydrates are extremely sensitive to temperature and pressure changes, and drilling through gas hydrate formation may cause dissociation of hydrates, accompanied by changes in wellbore temperatures, pore pressures, and stress states, thereby leading to wellbore plastic yield and wellbore instability. Considering the coupling effect of seepage of drilling fluid into gas hydrate formation, heat conduction between drilling fluid and formation, hydrate dissociation, and transformation of the formation framework, this study established a multi-field coupling mathematical model of the wellbore in the hydrate formation. Furthermore, the influences of drilling fluid temperatures, densities, and soaking time on the instability of hydrate formation were calculated and analyzed. Results show that the greater the temperature difference between the drilling fluid and hydrate formation is, the faster the hydrate dissociates, the wider the plastic dissociation range is, and the greater the failure width becomes. When the temperature difference is greater than 7℃, the maximum rate of plastic deformation around the wellbore is more than 10%, which is along the direction of the minimum horizontal in-situ stress and associated with instability and damage on the surrounding rock. The hydrate dissociation is insensitive to the variation of drilling fluid density, thereby implying that the change of the density of drilling fluids has a minimal effect on the hydrate dissociation. Drilling fluids that are absorbed into the hydrate formation result in fast dissociation at the initial stage. As time elapses, the hydrate dissociation slows down, but the risk of wellbore instability is aggravated due to the prolonged submersion in drilling fluids. For the sake of the stability of the wellbore in deep-water drilling through hydrate formation, the drilling fluid with low temperatures should be given priority. The drilling process should be kept under balanced pressures, and the drilling time should be shortened.     

Revision of P-wave Velocity and Thickness of Hydrate Layer in Shenhu Area,South China Sea

To revise P-wave velocity and thickness of the hydrate layer in the Shenhu area of the South China Sea, acoustic and resistivity logging curves are reanalyzed. The waterlogging phenomenon is found in the shallow sediments of five drilling wells, which causes P-wave velocity to approximate the propagation velocity of sea water（about 1500 m s-1）. This also affects the identification of the hydrate layer and results in the underestimate of its thickness. In addition, because there could be about a 5 m thick velocity ramp above or below the hydrate layer as interpreted by acoustic and resistivity logging curves, the recalibrated thickness of this layer is less than the original estimated thickness. The recalibrated P-wave velocity of the hydrate layer is also higher than the original estimated velocity. For the drilling well with a relatively thin hydrate layer, the velocity ramp plays a more important role in identifying and determining the thickness of the layer.     

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.     

Estimation of extreme marine hydrodynamic variables in western Laizhou Bay

Laizhou Bay and its adjacent waters are of great importance to China's marine oil and gas development. It is therefore crucial to estimate return-period values of marine environmental variables in this region to ensure the safety and success of maritime engineering and maritime exploration. In this study, we used numerical simulations to estimate extreme wave height, sea current velocity and sea-level height in western Laizhou Bay. The results show that the sea-level rise starts at the mouth of the bay, increases toward west/southwest, and reaches its maximum in the deepest basin of the bay. The 100-year return-period values of sea level rise can reach 3.4–4.0 m in the western bay. The elevation of the western part of the Qingdong Oil Field would remain above the sea surface during extreme low sea level, while the rest of the oil field would be 1.6–2.4 m below the sea surface. The return-period value of wave height is strongly affected by water depth; in fact, its spatial distribution is similar to the isobath's. The 100-year return-period values of effective wave height can be 6 m or higher in the central bay and be more than 1 m in the shallow water near shore. The 100-year return-period values of current velocity is about 1.2–1.8 m s-1 in the Qingdong Oil Field. These results provide scientific basis for ensuring construction safety and reducing construction cost.     

A New Inversion Method of Sedimentary Strata For Deriving Double Parameters and Its Applications

The inverse problem of wave equation is the importance of study not only in seismic prospecting but also in ap-plied mathematics. With the development of the research, the inverse methods of 1 - D wave equations have been trending to-wards the multiple parameters inversion . We have obtained an inverse method with double - parameter, in which medium density and wave velocity can be derived simultaneously. In this paper, to increase the inverse accuracy, the method is im-proved as follows. Firstly, the formula in which the Green Function is omitted are derived and used. Secondly, the regulari-zing method is reasonable used by choosing the stable function. With the new method, we may derive elastic parameter and medium density or medium density and wave velocity. Thus, lithology parameters for seismic prospecting may be obtained. After comparing the derived values from the new method with that from previous method, we obtain the new method through which substantially improve the derived accuracy . The new     

Comprehensive Investigation of Submarine Slide Zones and Mass Movements at the Northern Continental Slope of South China Sea

Multi-beam bathymetry and seismic sequence surveys in the northern slope of the South China Sea reveal detailed geomorphology and seismic stratigraphy characteristics of canyons, gullies, and mass movements. Modern canyons and gullies are roughly elongated NNW–SSW with U-shaped cross sections at water depths of 400–1000 m. Mass movements include slide complexes, slide scars, and debris/turbidity flows. Slide complexes and slide scars are oriented in the NE–SW direction and cover an area of about 1790 and 926 km~2, respectively. The debris/turbidity flows developed along the lower slope. A detailed facies analysis suggests that four seismic facies exist, and the late Cenozoic stratigraphy above the acoustic basement can be roughly subdivided into three sequences separated by regional unconformities in the study area. The occurrence of gas hydrates is marked by seismic velocity anomalies, bottom-simulating reflectors, gas chimneys, and pockmarks in the study area. Seismic observations suggest that modern canyons and mass movements formed around the transition between the last glacial period and the current interglacial period. The possible existence and dissociation of gas hydrates and the regional tectonic setting may trigger instability and mass movements on the seafloor. Canyons may be the final result of gas hydrate dissociation. Our study aims to contribute new information that is applicable to engineering construction required for deep-water petroleum exploration and gas hydrate surveys along any marginal sea.     

Rock physics and seismic modeling of shale reservoirs with horizontal fractures

The presence of horizontal fractures enhances seismic anisotropy of shales.Calculation based on the effective medium theory indicates that horizontal fractures have little effects on velocities along the direction pa-rallel to fractures,but can significantly reduce velocities along the direction normal to fractures.Seismic respon-ses of shales with horizontal fractures are calculated based on the reflector model and the anisotropic propagator matrix method,in which the reflections are a combination of the contrast in impedance due to the variations in fracture density,anisotropic propagation of waves within the shales,and the tuning and interferences associated with layer thickness.Calculated results indicate that seismic reflections are sensitive to reservoir layer thickness and fracture density.Anisotropic propagation alters amplitudes and phases of reflections.It corresponds to high-er reflection amplitudes for the case of surrounding sandstone with higher velocity because the increase in frac-ture density increases the contrast in impedance between the shale and sandstone.In contrast,the surrounding sandstone with lower velocity corresponds to lower reflection amplitudes for the increase in fracture density.     

Spatio-temporal Variations of Sea Surface Wind in Coral Reef Regions over the South China Sea from 1988 to 2017

The seasonal and interannual variabilities of sea surface wind(SSW) in the South China Sea(SCS), especially in coral reef regions such as Nansha Islands, Xisha Islands, Zhongsha Islands and Dongsha Islands were investigated in detail using the Blended Sea Winds dataset(1988–2017). Annual and monthly variations of SSW and sea surface temperature(SST) in the four zones were investigated. Empirical Orthogonal Function(EOF) analysis of wind field was performed to aid in better understanding the different spatial patterns. The results indicate that, as observed in the spatial distribution of the first mode of monthly mean wind speed anomaly, the magnitudes in the four island zones are all negative and are similar to each other, showing that the variations of SSW in the four island zones are consistent. In the second mode, the magnitudes in Nansha Islands are opposite to those in the other three zones. The spatial distribution of the third mode reflects regional differences. The maximum annual SSW appears in Dongsha Islands, and the minimum appears in Nansha Islands. The interannual variations of SSW in all island zones are basically concurrent. The island zones with high SSW mostly have low SST, and vice versa. There may be an inverse relationship between SSW and SST in coral reef regions in the SCS. The multiyear monthly variations of SSW in the island zones present a ‘W'-shaped structural variation. Each island undergoes two months of minimum SSW every year, one during March–May(MAM) and the other during September–November(SON). Both months are in monsoon transition periods. During the months with low SSW, high SST appears. The SST peaks almost correspond to the SSW troughs.This further indicates that SSW and SST may have opposite changes in coral reef regions. Coral bleaching events often correspond to years of high SST and low SSW.     

Research Progress of Seafloor Pockmarks in Spatio-Temporal Distribution and Classification

Seafloor pockmarks are important indicators of submarine methane seepages and slope instabilities.In order to promote the understanding of submarine pockmarks and their relationship with sediment instabilities and climate changes,here we summarize the research results of pockmarks in the spatio-temporal distributions and shaping factors.Most of pockmarks occur along active or passive continental margins during the last 25 kyr B.P..Circular and ellipse are the most common forms of pockmarks,whereas pockmarks in a special crescent or elongated shape are indicators of slope instabilities,and ring-shape pockmarks are endemic to the gas hydrate zones.Further researches should be focused on the trigger mechanism of climate changes based on the pockmarks in the high latitudes formed during the deglaciation periods,and the role of gas hydrates in the seafloor evolution should be elucidated.In addition,the feature of pockmarks at their early stage(e.g.,developing gas chimneys and gas driving sedimentary doming)and the relations between pockmarks and mass movements,mud diapirs could be further studied to clarify the influences of rapid methane release from submarine sediments on the atmospheric carbon contents.     

Spatiotemporal evolution and driving forces of changes in rural settlements in the poverty belt around Beijing and Tianjin: a case study of Zhangjiakou city,Hebei Province

The rural settlement is one of important aspects to understand regional human-environment relationships. Zhangjiakou city, located in the poverty belt around Beijing and Tianjin, was used as an example to analyze the evolution of rural settlements during 1985–2010 through the use of statistical and spatial analysis methods. The results demonstrate the following:(1) the area of rural settlements expanded from 665.76 to 928.14 km2 during 1985–2010. Rural settlements in prefecture city of Zhangjiakou were significantly agglomerated in terms of their spatial distribution, which presented an approximately normal distribution over the entire period. Rural settlements were usually distributed within regions where the terrain niche had an elevation of between 900 and 1500 m, and a slope from 5? to 20?;(2) there were significant regional differences in the density and size distribution of rural settlements in Zhangjiakou. The density of rural settlements in 2010 was higher than that in 1985, while there was a clustered distribution of rural settlements in 2010. There was a significant high value cluster in the size distribution, and a local negative correlation between the size and density distribution of rural settlements in Zhangjiakou;(3) rural economic development, investment in agricultural technology, and infrastructure improvement played important roles in the evolution of rural settlements. There were spatial differences in the relationship between the distribution of rural settlements and socio-economic factors in Zhangjiakou. This study provides practical guidance for the achievement of urban and rural integration development and the promotion of a new form of countryside construction.     

Joint inversion of magnetotelluric and fullwaveform seismic data based on alternating cross-gradient structural constraints

Magnetotelluric(MT)inversion and seismic inversion are important methods for the interpretation of subsurface exploration data,but separate inversion of MT and seismic produces ambiguous and non-unique results due to various factors.In order to achieve accurate results,the authors propose a joint inversion method of two-dimensional MT and seismic data in the frequency domain.The finite element method is used for numerical simulation of electromagnetic data in the forward modelling,and the Gauss-Newton method is used for the inversion.The 9-point-finite-difference method is used to solve the seismic wave field in the acoustic wave equation,and the inverse problem of seismic data is solved by full waveform inversion with a conjugate gradient,a simple and fast method.Cross gradient functions are used to provide constraint structure between resistivity and velocity parameters to carry out the joint inversion.The joint inversion algorithm is tested by double-rectangular model synthesis data,and the accuracy of the algorithm is verified.The results show that the joint inversion results are better than those from separate inversion.The algorithm is applied to a geophysical model of a metalliferous deposit in Jinchuan and is compared with the separate inversion results.It shows that the results obtained with joint inversion are much closer to the real model.     

FloatingEscherichia coli by Expressing Cyanobacterial Gas Vesicle Genes

Gas vesicles are hollow, air-filled polyprotein structures that provide the buoyancy to cells. They are found in a variety of prokaryotes. In this study, we isolated a partial gas vesicle protein gene cluster containing gvp A and gvp C20Ψ from Planktothrix rubescens, and inserted it into an expression vector and expressed it in E. coli. The gas vesicle was developed in bacterial cells, which made bacterial cells to float on medium surface. We also amplified gvp A and gvp C20Ψ separately and synthesized an artificial operon by fusing these two genes with the standardized gene expression controlling elements of E. coli. The artificial operon was expressed in E. coli, forming gas vesicles and floating bacteria cells. Our findings verified that the whole set of genes and the overall structure of gas vesicle gene cluster are not necessary for developing gas vesicles in bacteria cells. Two genes, gvp A and gvp C20Ψ, of the gas vesicle gene cluster are sufficient for synthesizing an artificial operon that can develop gas vesicles in bacteria cells. Our findings provided a wide range of applications including easing the harvest of cultured microalgae and bacteria, as well as enriching and remediating aquatic pollutants by constructing gas vesicles in their cells.     

Distribution and Chain Pattern of Liquefied Natural Gas Industry in China

The production and consumption of natural gas in China has been developing rapidly in recent years.It is expected that the annual growth rate of the demand for natural gas will reach 12% in the next 15 years,and the gas consumption in the primary energy will increase from 0.3% to 10% or more by 2020.However,since the supply of natural gas cannot satisfy the requirements,China has begun to build liquefied natural gas(LNG)terminals in the coastal regions such as Guangdong and Fujian,and solve this problem by importing LNG from foreign countries.LNG needs to be transported by LNG ships from abroad.With the rapid growth of global gas production,the volume of LNG trade also increases,and the interregional production increased from 0.3% in 1970 to 26.2% in 2004.So,we need LNG ships more than before.This article puts forward the distribution of LNG ships and the speculation of the future of LNG transportation based on the studies on foreign LNG production,the LNG trade,the building of LNG ships,the LNG transportation,the chain model of LNG distribution,etc.     

Effects of rock fragments on yak dung greenhouse gas emissions on the Qinghai-Tibetan Plateau

Dung deposition is an important pathway of nutrient return and redistribution in alpine grasslands of the Qinghai-Tibetan Plateau.To date,information on the greenhouse gas emissions of yak dung on alpine grasslands,especially where there are large amounts of rock fragments,is limited.Our aim,therefore,was to evaluate variations in N_2O,CH_4,and CO_2 emissions from yak dung(CCD),and compare it to dung placed on rock fragments(RCD),alpine steppe soil(CSD),and a soil and rock fragment mixture(RSD) over a 30-day incubation period.The results showed that the total N_2O emissions from treatments without soil were significantly(P 0.05) lower than those from treatments with soil.The highest total CH_4 emissions were detected in the CSD treatment,while CH_4 losses from treatments without rock fragments were significantly(P 0.05) greater than those with rock fragments.The total CO_2 emissions from the RSD treatment was 6.30%–12.0% lower than those in the other three treatments.The soil beneath yak dung pats elevated the globalwarming potential(GWP),while the addition of rock fragments to the soil significantly(P 0.05) decreased the GWP by reducing emissions of the three greenhouse gases.We therefore suggest that interactions between rock fragments and alpine steppe soil are effective in decreasing yak dung greenhouse gas emissions.This finding indicates that rock fragments are an effective medium for reducing greenhouse gas emissions from dung pats,and more attention should therefore be paid to evaluate its ecological impact in future studies.These results should help guide scientific assessments of regional GHG budgets in agricultural ecosystems where the addition of livestock manure to soils with large amounts of rock fragments is common.     

Methane seepages in the northern South China Sea: evidence from in situ measurements

Continental margins in world oceans contain large amounts of marine gas hydrates. Changes in the temperature and pressure of sediment may destabilize the methane hydrate, leading to its release and seepage into the sea. This process would increase the dissolved methane in the ambient seawater. In this study, a methane sensor was used to detect methane anomalies in the water column in southwestern Dongsha and Shenhu, northern slope of the South China Sea. Methane plumes were detected at stations SCS001 and T001 in the southwestern Dongsha area, and station SCS002 in the Shenhu area, respectively. The maximum methane concentrations were 8.8 nmol/L in southwestern Dongsha and 10.1 nmol/L in Shenhu, which are about 4–5 times higher than the background methane concentration. This indicates that there are active methane seepages present in both the southwestern Dongsha and Shenhu areas, which are likely related to a methane hydrate reservoir beneath the seafloor. A methane sensor with a low detection limit is a practical instrument with which to detect methane plumes in oceanic environments and to monitor methane leakage from the seafloor.     

The biogeochemistry of carbon cycle in summer of the Prydz Bay, Antarctic I ： Characteristics of DOC distribution

The distributions and changes of dissolved organic carbon in the Prydz Bay and out open sea were investigated during CHINARE-15 ( the 15th Chinese National Antarctic Research Expedition). The results showed that the content of DOC was higher in the Prydz Bay and outer open sea compared to those typical of surface oceanic levels (70-80μM), average content of DOC in the surface water was 102.32 μM,the range was 68.23-125.92 μM. The vertical distribution of DOC in the water column was similar to many ocean sites, that is to say, the content of upper water is higher than deep water, a subsurface maximum persisted between 25-50 m. The DOC pool in the Prydz Bay were consisted with labile, semi-labile and refractory pools, in which refractory pools was mainly part. The concentration of refractory DOC was 92.34 and 76.89 μM in Prydz Bay and outer open sea, and account 77% and 82% for total DOC, respectively.