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 cond...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（largescale 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 biogenicallyproduced, 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 ofthermogenic 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.展开更多
The Shenhu area is one of the promise target areas for marine gas hydrate exploration in the northern margin of the South China Sea.Pore water samples were collected from two piston cores (8.75 and 8.52 m) in site HS-...The Shenhu area is one of the promise target areas for marine gas hydrate exploration in the northern margin of the South China Sea.Pore water samples were collected from two piston cores (8.75 and 8.52 m) in site HS-A and site HS-B in the Shenhu area,and their major anion and cation contents and trace element contents have been analyzed in this study.Cl- concentrations in pore waters are similar to the seawater value and no systematic change along depth profiles has been found for the Cl- content in both sites.In site HS-A,the SO 2- 4 contents show a limited range in pore waters from 0 to 3 m depths,but a dramatic decrease is documented in depth below 3 m.Other elements such as Ca 2+,Mg 2+ and Sr 2+ show similar variation patterns as the SO 2- 4,i.e.,no variation in 0-3 m,but large decrease in depth below 3 m.In site HS-B,both the SO 4 and Ca 2+,Mg 2+ and Sr 2+ display a decrease pattern along the depth profile,but in 0-3 m the variation is less significant than those in depth below 3 m.In both sites HS-A and HS-B,I contents show a dramatic increase along depth profiles.Calculations show that the SMI depths are very shallow (10 and 11 m) in both sites,and the sulfate flux and I flux are very high.These geochemical characteristics and anomalies at Shenhu are quite similar to those found in other gas hydrate locations in the world,and a genetic link is suggested to be related to the gas hydrate occurrence at depths in the study area.In summary,we suggest that combined geochemical parameters in shallow sediment pore waters are useful indicators to indicate the gas hydrate occurrence at depths.展开更多
Shenhu Area is located in the Baiyun Sag of Pearl River Mouth Basin,which is on the northern continental slope of the South China Sea.Gas hydrates in this area have been intensively investigated,achieving a wide cover...Shenhu Area is located in the Baiyun Sag of Pearl River Mouth Basin,which is on the northern continental slope of the South China Sea.Gas hydrates in this area have been intensively investigated,achieving a wide coverage of the three-dimensional seismic survey,a large number of boreholes,and detailed data of the seismic survey,logging,and core analysis.In the beginning of 2020,China has successfully conducted the second offshore production test of gas hydrates in this area.In this paper,studies were made on the structure of the hydrate system for the production test,based on detailed logging data and core analysis of this area.As to the results of nuclear magnetic resonance(NMR)logging and sonic logging of Well GMGS6-SH02 drilled during the GMGS6 Expedition,the hydrate system on which the production well located can be divided into three layers:(1)207.8–253.4 mbsf,45.6 m thick,gas hydrate layer,with gas hydrate saturation of 0–54.5%(31%av.);(2)253.4–278 mbsf,24.6 m thick,mixing layer consisting of gas hydrates,free gas,and water,with gas hydrate saturation of 0–22%(10%av.)and free gas saturation of 0–32%(13%av.);(3)278–297 mbsf,19 m thick,with free gas saturation of less than 7%.Moreover,the pore water freshening identified in the sediment cores,taken from the depth below the theoretically calculated base of methane hydrate stability zone,indicates the occurrence of gas hydrate.All these data reveal that gas hydrates,free gas,and water coexist in the mixing layer from different aspects.展开更多
Although the Shenhu sea area has been a topic and focus of intense research for the exploration and study of marine gas hydrate in China, the mechanism of gas hydrate accumulation in this region remains controversial....Although the Shenhu sea area has been a topic and focus of intense research for the exploration and study of marine gas hydrate in China, the mechanism of gas hydrate accumulation in this region remains controversial. The formation rate and evolution time of gas hydrate are the critical basis for studying the gas hydrate formation of the Shenhu sea area. In this paper, based on the positive anomaly characteristics of chloride concentration that measured in the GMGS3-W19 drilling site is higher than the seawater value, we numerically simulated the gas hydrate formation time of GMGS3-W19 site. The simulation results show that the gas hydrate formation rate positively correlates with the chloride concentration when the hydrate reaches the measured saturation. The formation time of gas hydrate in the GMGS3-W19 site is approximately 30 ka. Moreover, the measured chloride concentration is consistent with the in-situ chloride concentration, indicating that the formation rate of gas hydrate at the GMGS3-W19 site is very fast with a relatively short evolution time.展开更多
Previous work has largely discussed the relations between sediment structures and accumulation of gas hydrates in the Shenhu area of South China Sea, but has not documented why the gas hydrates occurred at the seafloo...Previous work has largely discussed the relations between sediment structures and accumulation of gas hydrates in the Shenhu area of South China Sea, but has not documented why the gas hydrates occurred at the seafloor topographic highs. Many gas hydrate exploration examples abroad also indicate that the saturation of gas hydrates was higher at seafloor topographic highs. This work aims to understand why gas hydrates accumulated at topographic highs and why their saturation is higher.展开更多
Cold seeps spread worldwide along the continental margins,which are closely related to the exploration of gas hydrates.Cold-seep carbonates have been reported to record the nature of seepage,including fluid source,sed...Cold seeps spread worldwide along the continental margins,which are closely related to the exploration of gas hydrates.Cold-seep carbonates have been reported to record the nature of seepage,including fluid source,sedimentary environment,and variation of seepage activity.We investigated the morphology,mineralogy,element compositions,and carbon and oxygen isotopes of 15 cold-seep carbonates collected from the Shenhu area,and compared them with 2 carbonates from the Haima cold seep,the South China Sea(SCS),to promote our knowledge of cold-seep system in SCS.Most of the Shenhu carbonates exhibit crust morphology,and some are in the form of chimneys and blocks.Their absolute(20%–65%)and relative carbonate mineral contents(mainly aragonite and calcite,with minor samples containing dolomite)vary significantly,indicating the multi-stage methane leakage in our study area.Some samples show a slight negative Ce anomaly,suggesting either the mixing of seawater or variation of the redox condition during the precipitation;the cooccurrence of strongly enriched U and Mo demonstrates anoxic condition during precipitation.The mixed genetic methane source was interpreted by δ^(13)C of the Shenhu carbonates to range from-22.34‰to-59.30‰Vienna PeeDee Belemnite(VPDB),and the slight ^(18)O-enrichment imprinted on the carbonates suggests the possible influence from hydrate dissociation.The Haima carbonates,with biogenic methane as the main gas source,were presumably formed in a stronger fluid flux by compared with our Shenhu samples.展开更多
To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19,extensive geophysical logging data and cores were analyzed to assess the reservoir properties.Sediment porosities were estimated from density,ne...To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19,extensive geophysical logging data and cores were analyzed to assess the reservoir properties.Sediment porosities were estimated from density,neutron,and nuclear magnetic resonance(NMR)logs.Both the resistivity and NMR logs were used to calculate gas hydrate saturations,the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data.The density porosity was closely in agreement with the core-derived porosity,and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates.The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity.For the gas hydrate reservoirs at GMGS3-W19,the porosity,gas hydrate saturation and free gas saturation was 52.7%,42.7%and 10%,on average,respectively.The various logs provide different methods for the comprehensive evaluation of hydrate reservoir,which supports the selection of candidate site for gas hydrate production testing.展开更多
Great advancement has been made on natural gas hydrates exploration and test production in the northern South China Sea.However,there remains a lot of key questions yet to be resolved,particularly about the mechanisms...Great advancement has been made on natural gas hydrates exploration and test production in the northern South China Sea.However,there remains a lot of key questions yet to be resolved,particularly about the mechanisms and the controls of gas hydrates enrichment.Numerical simulaution would play signficant role in addressing these questions.This study focused on the gas hydrate exploration in the Shenhu Area,Northern South China Sea.Based on the newly obtained borehole and multichannel reflection seismic data,the authors conducted an integrated 3D basin modeling study on gas hydrate.The results indicate that the Shenhu Area has favorable conditions for gas hydrate accumulation,such as temperature,pressure,hydrocarbon source,and tectonic setting.Gas hydrates are most concentrated in the Late Miocene strata,particularly in the structual highs between the Baiyun Sag and the Liwan Sag,and area to the south of it.It also proved the existence of overpressure in the main sag of source rocks,which was subject to compaction disequilibrium and hydrocarbon generation.It also shown that the regional fault activity is not conducive to gas hydrate accumulation due to excess gas seepage.The authors conjecture that fault activity may slightly weaken overpressure for the positive effect of hydrocarbon expulsion and areas lacking regional fault activity have better potential.展开更多
Drilling results suggest that the thickness of natural gas hydrates(NGHs)in the Shenhu Area,South China Sea(SCS)are spatially heterogenous,making it difficult to accurately assess the NGHs resources in this area.In th...Drilling results suggest that the thickness of natural gas hydrates(NGHs)in the Shenhu Area,South China Sea(SCS)are spatially heterogenous,making it difficult to accurately assess the NGHs resources in this area.In the case that free gas exists beneath hydrate deposits,the frequency of the hydrate deposits will be noticeably attenuated,with the attenuation degree mainly affected by pore development and free gas content.Therefore,the frequency can be used as an important attribute to identify hydrate reservoirs.Based on the time-frequency characteristics of deposits,this study predicted the spatial distribution of hydrates in this area using the frequency division inversion method as follows.Firstly,the support vector machine(SVM)method was employed to study the amplitude versus frequency(AVF)response based on seismic and well logging data.Afterward,the AVF response was introduced as independent information to establish the nonlinear relationship between logging data and seismic waveform.Then,the full frequency band information of the seismic data was fully utilized to obtain the results of frequency division inversion.The inversion results can effectively broaden the frequency band,reflect the NGHs distribution,and reveal the NGHs reservoirs of two types,namely the fluid migration pathway type and the in situ self-generation self-storage diffusion type.Moreover,the inversion results well coincide with the drilling results.Therefore,it is feasible to use the frequency division inversion to predict the spatial distribution of heterogeneous NGHs reservoirs,which facilitates the optimization of favorable drilling targets and is crucial to the resource potential assessment of NGHs.展开更多
Large amounts of gas hydrate are distributed in the northern slope of the South China Sea,which is a potential threat of methane leakage.Aerobic methane oxidation by methanotrophs,significant methane biotransformation...Large amounts of gas hydrate are distributed in the northern slope of the South China Sea,which is a potential threat of methane leakage.Aerobic methane oxidation by methanotrophs,significant methane biotransformation that occurs in sediment surface and water column,can effectively reduce atmospheric emission of hydrate-decomposed methane.To identify active aerobic methanotrophs and their methane oxidation potential in sediments from the Shenhu Area in the South China Sea,multi-day enrichment incubations were conducted in this study.The results show that the methane oxidation rates in the studied sediments were 2.03‒2.36μmol/gdw/d,which were higher than those obtained by sediment incubations from other areas in marine ecosystems.Thus the authors suspect that the methane oxidation potential of methanotrophs was relatively higher in sediments from the Shenhu Area.After the incubations family Methylococcaea(type I methanotrophs)mainly consisted of genus Methylobacter and Methylococcaea_Other were predominant with an increased proportion of 70.3%,whereas Methylocaldum decreased simultaneously in the incubated sediments.Collectively,this study may help to gain a better understanding of the methane biotransformation in the Shenhu Area.展开更多
Internal solitary waves(ISWs) contain great energy and have the characteristics of emergency and concealment. To avoid their damage to offshore engineering, a new generation of monitoring and early warning system for ...Internal solitary waves(ISWs) contain great energy and have the characteristics of emergency and concealment. To avoid their damage to offshore engineering, a new generation of monitoring and early warning system for ISWs was developed using technologies of double buoys monitoring, intelligent realtime data transmission, and automatic software identification. The system was applied to the second natural gas hydrates(NGHs) production test in the Shenhu Area, South China Sea(SCS) and successfully provided the early warning of ISWs for 173 days(from October 2019 to April 2020). The abrupt changes in the thrust force of the drilling platform under the attack of ISWs were consistent with the early warning information, proving the reliability of this system. A total of 93 ISWs were detected around the drilling platform. Most of them occurred during the spring tides in October–December 2019 and April 2020, while few of them occurred in winter. As suggested by the theoretical model, the full-depth structure of ISWs was a typical current profile of mode-1, and the velocities of wave-induced currents can reach 80 cm/s and30 cm/s, respectively, in the upper ocean and near the seabed. The ISWs may be primarily generated from the interactions between the topography and semidiurnal tides in the Luzon Strait, and then propagate westward to the drilling platform. This study could serve as an important reference for the early warning of ISWs for offshore engineering construction in the future.展开更多
The undrained shear strength of shallow strata is a critical parameter for safety design in deep-water operations.In situ piezocone penetration tests(CPTU) and laboratory experiments are performed at Site W18-19 in th...The undrained shear strength of shallow strata is a critical parameter for safety design in deep-water operations.In situ piezocone penetration tests(CPTU) and laboratory experiments are performed at Site W18-19 in the Shenhu area, northern South China Sea, where China's first marine hydrate exploitation operation is due to be located. The validation of the undrained shear strength prediction model based on CPTU parameters. Different laboratory tests, including pocket penetrometer, torvane, miniature vane and unconsolidated undrained triaxial tests, are employed to solve empirical cone coefficients by statistical and mathematical methods. Finally, an optimized model is proposed to describe the longitudinal distribution of undrained shear strength in calcareous clay strata in the Shenhu area. Research results reveal that average empirical cone coefficients based on total cone resistance, effective resistance, and excess-pore pressure are 13.8, 4.2 and 14.4, respectively. The undrained shear strength prediction model shows a good fit with the laboratory results only within specific intervals based on their compaction degree and gas-bearing conditions. The optimized prediction model in piecewise function format can be used to describe the longitudinal distribution of the undrained shear strength for calcareous clay within all depth intervals from the mud-line to the upper boundary of hydrate-bearing sediments(HBS). The optimized prediction result indicates that the effective cone resistance model is suitable for very soft to firm calcareous clays,the excess-pore pressure model can depict the undrained shear strength for firm to very stiff but gas-free clays,while the total cone resistance model is advantageous for evaluating the undrained shear strength for very stiff and gassy clays. The optimized model in piecewise function format can considerably improve the adaptability of empirical models for calcareous clay in the Shenhu area. These results are significant for safety evaluations of proposed hydrate exploitation projects.展开更多
Dynamics of major picoplankton groups, Synechococcus (Syn), Prochlorococcus (Pro), picoeukaryotes (Euk) and heterotrophic bacteria (Bact) was investigated by flow cytometry for the first time in the Nansha Islands are...Dynamics of major picoplankton groups, Synechococcus (Syn), Prochlorococcus (Pro), picoeukaryotes (Euk) and heterotrophic bacteria (Bact) was investigated by flow cytometry for the first time in the Nansha Islands area in the South China Sea. Averaged over the whole investigation area, depth-weighted integrated cell abundance (DWA) of Syn, Pro, Euk and Bact was 1.6 (0.4-5.7)×103, 5.4 (0.1-7.3)×104, 0.7 (0.2-2.2)×103, and 2.3 (1.4-3.2)×105 cells/mL respectively. Picoautotrophic cell abundance was low in the northwest part of the Nansha Islands where surface water temperature was low and the upper mixed layer was shallow. Concurrently, a surface maximum vertical distribution pattern was observed in this area. While in the southeast and east zones where temperatures were relatively higher and nitraclines were deeper, picoplankton is abundant and a subsurface maximum around 50-75 m is observed. Coupling of horizontal and vertical distribution patterns of picoplankton abundance and hydrological status was found, suggesting a strong influence of currents and water column structure on picoplankton distribution in the investigation area. Contrary to that in the shelf water in the East China Sea, the relationship between Pro and Bact in the Nansha Islands area in the South China Sea was not significantly negative but weakly positive. Moreover, a similar distribution pattern of Syn and Pro was observed. Possible reasons for these differences in the two marine regimes were discussed.展开更多
BSR （Bottom Simulating Reflector） occurs widely in the strata since the late Miocene in the deep-water area of the northern continental slope of South China Sea （SCS）. It is an important seismic reference mark whi...BSR （Bottom Simulating Reflector） occurs widely in the strata since the late Miocene in the deep-water area of the northern continental slope of South China Sea （SCS）. It is an important seismic reference mark which identifies the gas hydrate and its distribution influenced by the tectonic movements. Single-point basin modeling was conducted using 473 points in the study area. To discuss the relationships between the tectonic subsidence and BSR, the volume and rate of tectonic subsidence in each geological time have been simulated. The results show that there are three tectonic accelerate subsidence processes in the study area since the late Miocene, especially since 1.8Ma the tectonic subsidence accelerates more apparently. Since the Late Miocene to Pleistocene, the rate of tectonic subsidence in deep-water underwent a transformation from weak to strong. The ratio of tectonic subsidence to the total subsidence was relatively high （65-70%）. Through the superposition of the BSR developed areas and the contours of tectonic subsidence in this area, it was discovered that more than 80% of BSR tend to be distributed at the slope break or depression-uplift structural transfer zone and the average tectonic subsidence rate ranges from 70 m/Ma to 125 m/Ma.展开更多
Bottom acoustic parameters play an important role in sound field prediction. Acoustic parameters in deep water are not well understood. Bottom acoustic parameters are sensitive to the transmission-loss （TL） data in ...Bottom acoustic parameters play an important role in sound field prediction. Acoustic parameters in deep water are not well understood. Bottom acoustic parameters are sensitive to the transmission-loss （TL） data in the shadow zone of deep water. We propose a multiple-step fill inversion method to invert sound speed, density and attenuation in deep water. Based on a uniform liquid hMf-space bottom model, sound speed of the bottom is inverted by using the long range TL at low frequency obtained in an acoustic propagation experiment conducted in the South China Sea （SCS） in summer 2014. Meanwhile, bottom density is estimated combining with the Hamilton sediment empirical relationship. Attenuation coefficients at different frequencies are then estimated from the TL data in the shadow zones by using the known sound speed and density as a constraint condition. The nonlinear relationship between attenuation coefficient and frequency is given in the end. Tile inverted bottom parameters can be used to forecast the transmission loss in the deep water area of SCS very we//.展开更多
Geochemical and detrital zircon U-Pb dating data for drilled sediments from the Baiyun deepwater area of the northern South China Sea demonstrate a change of sedimentary sources from the Oligocene to the Miocene.Zirco...Geochemical and detrital zircon U-Pb dating data for drilled sediments from the Baiyun deepwater area of the northern South China Sea demonstrate a change of sedimentary sources from the Oligocene to the Miocene.Zircon ages of the pre-rift Eocene sequences are dominated by Yanshanian ages with various peak values（110–115 Ma for U1435 and L21; 150 Ma for H1）, indicating local sediment supply from the pre-existing Mesozoic magmatic belt. For the Oligocene sediments in the northern part of the basin, the rare earth elements show different distribution characteristics, indicating sediment supply from the paleo-Zhujiang River（Pearl River）, as also confirmed by the multimodal zircon age spectra of the Lower Oligocene strata in Well X28. By contrast, a positive Eu anomaly characterizes sediments from the western and southern parts of the basin, indicating potential provenances from intermediate to basic volcanic rock materials. The Baiyun Movement at the end of the Oligocene contributed to a large-scale subsidence in the deepwater area and also a northward retreat of continental shelf break, leading to deepening depositional environment in the basin. As a result, all the detrital zircon ages of the Upper Oligocene strata from Wells X28, L13, and L21 share a similar distribution, implying the possible control of a common source like the Zhujiang River. During the Miocene, whereas sediments in the northern area were mainly sourced from the Zhujiang River Delta, and those in the southern deepwater area continued to be affected by basic volcanic activities, the Dongsha Uplift could have contributed as the main source to the eastern area.展开更多
Through the investigation on radioactive activities of water, sediment and some marine organismsin the Zhujiang Estuary, adjacent sea area and the distributary mouths of the Zhujiang River, activities of total α. rad...Through the investigation on radioactive activities of water, sediment and some marine organismsin the Zhujiang Estuary, adjacent sea area and the distributary mouths of the Zhujiang River, activities of total α. radioactivity, total β radioactivity, artificial radioactive 90Srand 157Cs, and factors inflencing the distribution and the content of U in seawater are studied.The mainly radioactive pollution substances and their sources in the sea area are studied by γ spectra obtained from sediment in the sea area. The results show that the main radioactivity substances are natural radioactivity U,Th series and 40K. which were produced by the modern industry and transported into the sea through the main current of the Zhujiang River.展开更多
The relationships between crustal stretching and thinning,basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic,gravity,magnetic,ocean bottom seismic(OBS),deep-water...The relationships between crustal stretching and thinning,basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic,gravity,magnetic,ocean bottom seismic(OBS),deep-water exploration wells and integrated ocean drilling program(IODP).During the early syn-rifting period,deep-water area was a half-graben controlled by high angle faults influenced by the brittle extension of upper crust.In the mid syn-rifting period,this area was a broad-deep fault depression controlled by detachment faults undergone brittle-ductile deformation and differentiated extension in the crust.In the late syn-rifting period,this area experienced fault-sag transition due to saucer-shaped rheology change dominated by crustal ductile deformation.A broad-deep fault depression controlled by the large detachment faults penetrating through the crust is an important feature of deep-water basin.The study suggests that the broad-deep Baiyun sag provides great accommodation space for the development of massive deltaic-lacustrine deposition system and hydrocarbon source rocks.The differentiated lithospheric thinning also resulted in the different thermal subsidence during post-rifting period,and then controlled the development of continental shelf break and deep-water reservoir sedimentary environment.The high heat flow background caused by the strong thinning of lithosphere and the rise of mantle source resulted in particularities in the reservoir diagenesis,hydrocarbon generation process and accumulation of deep-water area in northern South China Sea.展开更多
Based on oceanic and atmospheric parameters retrieved by satellite remote sensing using a neural network method, air-sea heat fluxes over the western Pacific warm pool area were calculated with the advanced the advanc...Based on oceanic and atmospheric parameters retrieved by satellite remote sensing using a neural network method, air-sea heat fluxes over the western Pacific warm pool area were calculated with the advanced the advanced Coupled Ocean-Atmosphere Response Experiment 3.0 (COARE3.0) bulk algorithm method. Then, the average annual and interannual characteristics of these fluxes were analyzed. The rela- tionship between the fluxes and the South China Sea (SCS) summer monsoon onset is highlighted. The results indicate that these fluxes have clear temporal and spatial characteristics. The sensible heat flux is at its maximum in the Kuroshio area, while the latent heat flux is at its maximum in the North Equatorial Current and Kuroshio area. The distribution of average annual air-sea heat fluxes shows that both sensible and latent heat fluxes are maximized in winter and minimized in summer. The air-sea heat fluxes have obvious interannual variations. Correlation analysis indicates a close lag-correlation between air-sea heat fluxes in the western Pacific warm pool area and at the SCS summer monsoon onset. The lagcorrelation can therefore predict the SCS summer monsoon onset, providing a reference for the study of precipitation related to the monsoon.展开更多
Evaluating velocity-porosity relationships of hydrate-bearing marine sediments is essential for characterizing natural gas hydrates below seafloor as either a potential energy resource or geohazards risks.Four sites h...Evaluating velocity-porosity relationships of hydrate-bearing marine sediments is essential for characterizing natural gas hydrates below seafloor as either a potential energy resource or geohazards risks.Four sites had cored using pressure and non-pressure methods during the gas hydrates drilling project(GMGS4)expedition at Shenhu Area,north slope of the South China Sea.Sediments were cored above,below,and through the gas-hydrate-bearing zone guided with logging-while-drilling analysis results.Gamma density and P-wave velocity were measured in each pressure core before subsampling.Methane hydrates volumes in total 62 samples were calculated from the moles of excess methane collected during depressurization experiments.The concentration of methane hydrates ranged from 0.3%to 32.3%.The concentrations of pore fluid(25.44%to 68.82%)and sediments(23.63%to 54.28%)were calculated from the gamma density.The regression models of P-wave velocity were derived and compared with a global empirical equation derived from shallow,unconsolidated sediments data.The results were close to the global trend when the fluid concentration is larger than the critical porosity.It is concluded that the dominant factor of P-wave velocity in hydrate-bearing marine sediments is the presence of the hydrate.Methane hydrates can reduce the fluid concentration by discharging the pore fluid and occupying the original pore space of sediments after its formation.展开更多
基金supported by the National Natural Science Foundation of China (grants No.41576048,41202080 and 41176052)the Open Fund of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology) (grant No.PLC201402)+1 种基金the Youth Innovation Promotion Association CAS (2016312)the Scientific Cooperative Project by CNPC and CAS (2015A-4813)
文摘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（largescale 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 biogenicallyproduced, 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 ofthermogenic 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.
基金supported by National Natural Science Foundation of China (Grant No.40773029)National Basic Research Program of China (Grant No.2009CB219506)Key Scientific Project from China Geological Survey (Grant No.GZHZ200203-04-02)
文摘The Shenhu area is one of the promise target areas for marine gas hydrate exploration in the northern margin of the South China Sea.Pore water samples were collected from two piston cores (8.75 and 8.52 m) in site HS-A and site HS-B in the Shenhu area,and their major anion and cation contents and trace element contents have been analyzed in this study.Cl- concentrations in pore waters are similar to the seawater value and no systematic change along depth profiles has been found for the Cl- content in both sites.In site HS-A,the SO 2- 4 contents show a limited range in pore waters from 0 to 3 m depths,but a dramatic decrease is documented in depth below 3 m.Other elements such as Ca 2+,Mg 2+ and Sr 2+ show similar variation patterns as the SO 2- 4,i.e.,no variation in 0-3 m,but large decrease in depth below 3 m.In site HS-B,both the SO 4 and Ca 2+,Mg 2+ and Sr 2+ display a decrease pattern along the depth profile,but in 0-3 m the variation is less significant than those in depth below 3 m.In both sites HS-A and HS-B,I contents show a dramatic increase along depth profiles.Calculations show that the SMI depths are very shallow (10 and 11 m) in both sites,and the sulfate flux and I flux are very high.These geochemical characteristics and anomalies at Shenhu are quite similar to those found in other gas hydrate locations in the world,and a genetic link is suggested to be related to the gas hydrate occurrence at depths in the study area.In summary,we suggest that combined geochemical parameters in shallow sediment pore waters are useful indicators to indicate the gas hydrate occurrence at depths.
基金Jointly funded by a major research plan of National Natural Science Foundation of China(51991365)titled“Multi-Field Spatial-Temporal Evolution Laws of Phase Transition and Seepage of Natural Gas Hydrate in Reservoirs”and a geological survey project initiated by China Geological Survey(DD20190226)titled“Implementation of Natural Gas Hydrate Production Test in Pilot Test Area in Shenhu Area”.
文摘Shenhu Area is located in the Baiyun Sag of Pearl River Mouth Basin,which is on the northern continental slope of the South China Sea.Gas hydrates in this area have been intensively investigated,achieving a wide coverage of the three-dimensional seismic survey,a large number of boreholes,and detailed data of the seismic survey,logging,and core analysis.In the beginning of 2020,China has successfully conducted the second offshore production test of gas hydrates in this area.In this paper,studies were made on the structure of the hydrate system for the production test,based on detailed logging data and core analysis of this area.As to the results of nuclear magnetic resonance(NMR)logging and sonic logging of Well GMGS6-SH02 drilled during the GMGS6 Expedition,the hydrate system on which the production well located can be divided into three layers:(1)207.8–253.4 mbsf,45.6 m thick,gas hydrate layer,with gas hydrate saturation of 0–54.5%(31%av.);(2)253.4–278 mbsf,24.6 m thick,mixing layer consisting of gas hydrates,free gas,and water,with gas hydrate saturation of 0–22%(10%av.)and free gas saturation of 0–32%(13%av.);(3)278–297 mbsf,19 m thick,with free gas saturation of less than 7%.Moreover,the pore water freshening identified in the sediment cores,taken from the depth below the theoretically calculated base of methane hydrate stability zone,indicates the occurrence of gas hydrate.All these data reveal that gas hydrates,free gas,and water coexist in the mixing layer from different aspects.
基金co-funded by National Natural Science Foundation of China (41406068,41776050,41730528)China Geological Survey Project (DD20189310)+1 种基金Guangdong Special Fund for Economic Development (Marine Economy)(GDME-2018D001)Key Laboratory of Marine Mineral Resources,Ministry of Land Resources (KLMMR-2013-A- 24).
文摘Although the Shenhu sea area has been a topic and focus of intense research for the exploration and study of marine gas hydrate in China, the mechanism of gas hydrate accumulation in this region remains controversial. The formation rate and evolution time of gas hydrate are the critical basis for studying the gas hydrate formation of the Shenhu sea area. In this paper, based on the positive anomaly characteristics of chloride concentration that measured in the GMGS3-W19 drilling site is higher than the seawater value, we numerically simulated the gas hydrate formation time of GMGS3-W19 site. The simulation results show that the gas hydrate formation rate positively correlates with the chloride concentration when the hydrate reaches the measured saturation. The formation time of gas hydrate in the GMGS3-W19 site is approximately 30 ka. Moreover, the measured chloride concentration is consistent with the in-situ chloride concentration, indicating that the formation rate of gas hydrate at the GMGS3-W19 site is very fast with a relatively short evolution time.
基金funded by the National Natural Science Foundation of China(grants No.41406080,41273066 and 41106060)
文摘Previous work has largely discussed the relations between sediment structures and accumulation of gas hydrates in the Shenhu area of South China Sea, but has not documented why the gas hydrates occurred at the seafloor topographic highs. Many gas hydrate exploration examples abroad also indicate that the saturation of gas hydrates was higher at seafloor topographic highs. This work aims to understand why gas hydrates accumulated at topographic highs and why their saturation is higher.
基金Supported by the Guangdong Province Marine Economic Development(Six Major Marine Industries)Special Fund Project(No.[2021]No.58)the National Key R&D Program of China(No.2018YFC0310000)the National Natural Science Foundation of China(Nos.41776056,42076054)。
文摘Cold seeps spread worldwide along the continental margins,which are closely related to the exploration of gas hydrates.Cold-seep carbonates have been reported to record the nature of seepage,including fluid source,sedimentary environment,and variation of seepage activity.We investigated the morphology,mineralogy,element compositions,and carbon and oxygen isotopes of 15 cold-seep carbonates collected from the Shenhu area,and compared them with 2 carbonates from the Haima cold seep,the South China Sea(SCS),to promote our knowledge of cold-seep system in SCS.Most of the Shenhu carbonates exhibit crust morphology,and some are in the form of chimneys and blocks.Their absolute(20%–65%)and relative carbonate mineral contents(mainly aragonite and calcite,with minor samples containing dolomite)vary significantly,indicating the multi-stage methane leakage in our study area.Some samples show a slight negative Ce anomaly,suggesting either the mixing of seawater or variation of the redox condition during the precipitation;the cooccurrence of strongly enriched U and Mo demonstrates anoxic condition during precipitation.The mixed genetic methane source was interpreted by δ^(13)C of the Shenhu carbonates to range from-22.34‰to-59.30‰Vienna PeeDee Belemnite(VPDB),and the slight ^(18)O-enrichment imprinted on the carbonates suggests the possible influence from hydrate dissociation.The Haima carbonates,with biogenic methane as the main gas source,were presumably formed in a stronger fluid flux by compared with our Shenhu samples.
基金jointly supported by the Key Area Research and Development Program of Guangdong Province(2020B1111030003)the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou)(GML2019ZD0102)the Project of China Geological Survey (DD20221700).
文摘To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19,extensive geophysical logging data and cores were analyzed to assess the reservoir properties.Sediment porosities were estimated from density,neutron,and nuclear magnetic resonance(NMR)logs.Both the resistivity and NMR logs were used to calculate gas hydrate saturations,the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data.The density porosity was closely in agreement with the core-derived porosity,and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates.The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity.For the gas hydrate reservoirs at GMGS3-W19,the porosity,gas hydrate saturation and free gas saturation was 52.7%,42.7%and 10%,on average,respectively.The various logs provide different methods for the comprehensive evaluation of hydrate reservoir,which supports the selection of candidate site for gas hydrate production testing.
基金funded by Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)(GML2019ZD0201,GML2019ZD0104)Finance Science and Technology Project of Hainan Province(ZDKJ202019).
文摘Great advancement has been made on natural gas hydrates exploration and test production in the northern South China Sea.However,there remains a lot of key questions yet to be resolved,particularly about the mechanisms and the controls of gas hydrates enrichment.Numerical simulaution would play signficant role in addressing these questions.This study focused on the gas hydrate exploration in the Shenhu Area,Northern South China Sea.Based on the newly obtained borehole and multichannel reflection seismic data,the authors conducted an integrated 3D basin modeling study on gas hydrate.The results indicate that the Shenhu Area has favorable conditions for gas hydrate accumulation,such as temperature,pressure,hydrocarbon source,and tectonic setting.Gas hydrates are most concentrated in the Late Miocene strata,particularly in the structual highs between the Baiyun Sag and the Liwan Sag,and area to the south of it.It also proved the existence of overpressure in the main sag of source rocks,which was subject to compaction disequilibrium and hydrocarbon generation.It also shown that the regional fault activity is not conducive to gas hydrate accumulation due to excess gas seepage.The authors conjecture that fault activity may slightly weaken overpressure for the positive effect of hydrocarbon expulsion and areas lacking regional fault activity have better potential.
基金supported by the National Research and Development Fund entitled“High Precision Characterization Technology of Natural Gas Hydrates Reservoirs”(2017YFC0307406)the projects entitled“Integrated Observation Data Integration and Application Service of Natural Resource Elements”(DD20208067)+2 种基金“Comprehensive geological survey of coastal zone and reef in the Northern Jiaodong”(ZD20220604)initiated by the China Geological Surveythe project entitled“Study on Hydrocarbon Accumulation Failure and Fluid Evolution Reduction of the Permian Reservoir in the Laoshan Uplift,South Yellow Sea”(42076220)“Temporal and Spatial Distribution Characteristics and Provenance of Two Stages Paleo-deltas on the Outer Shelf in the North of the East China Sea since Late Pleistocene”(41706069)organized by the National Natural Science Foundation of China。
文摘Drilling results suggest that the thickness of natural gas hydrates(NGHs)in the Shenhu Area,South China Sea(SCS)are spatially heterogenous,making it difficult to accurately assess the NGHs resources in this area.In the case that free gas exists beneath hydrate deposits,the frequency of the hydrate deposits will be noticeably attenuated,with the attenuation degree mainly affected by pore development and free gas content.Therefore,the frequency can be used as an important attribute to identify hydrate reservoirs.Based on the time-frequency characteristics of deposits,this study predicted the spatial distribution of hydrates in this area using the frequency division inversion method as follows.Firstly,the support vector machine(SVM)method was employed to study the amplitude versus frequency(AVF)response based on seismic and well logging data.Afterward,the AVF response was introduced as independent information to establish the nonlinear relationship between logging data and seismic waveform.Then,the full frequency band information of the seismic data was fully utilized to obtain the results of frequency division inversion.The inversion results can effectively broaden the frequency band,reflect the NGHs distribution,and reveal the NGHs reservoirs of two types,namely the fluid migration pathway type and the in situ self-generation self-storage diffusion type.Moreover,the inversion results well coincide with the drilling results.Therefore,it is feasible to use the frequency division inversion to predict the spatial distribution of heterogeneous NGHs reservoirs,which facilitates the optimization of favorable drilling targets and is crucial to the resource potential assessment of NGHs.
基金jointly supported by the National Natural Science Foundation of China (42106052)Shandong Provincial Natural Science Foundation (ZR2020QD070)the project of China Geological Survey (DD20190221)。
文摘Large amounts of gas hydrate are distributed in the northern slope of the South China Sea,which is a potential threat of methane leakage.Aerobic methane oxidation by methanotrophs,significant methane biotransformation that occurs in sediment surface and water column,can effectively reduce atmospheric emission of hydrate-decomposed methane.To identify active aerobic methanotrophs and their methane oxidation potential in sediments from the Shenhu Area in the South China Sea,multi-day enrichment incubations were conducted in this study.The results show that the methane oxidation rates in the studied sediments were 2.03‒2.36μmol/gdw/d,which were higher than those obtained by sediment incubations from other areas in marine ecosystems.Thus the authors suspect that the methane oxidation potential of methanotrophs was relatively higher in sediments from the Shenhu Area.After the incubations family Methylococcaea(type I methanotrophs)mainly consisted of genus Methylobacter and Methylococcaea_Other were predominant with an increased proportion of 70.3%,whereas Methylocaldum decreased simultaneously in the incubated sediments.Collectively,this study may help to gain a better understanding of the methane biotransformation in the Shenhu Area.
基金funded by the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0307)the Marine Geological Survey Program of China Geological Survey (DD20190218, DD20221706)+1 种基金the Key Program of Marine Economy Development Special Foundation of Department of Natural Resources of Guangdong Province (GDNRC [2020] 043)the National Natural Science Foundation of China (41806074, 41730528)。
文摘Internal solitary waves(ISWs) contain great energy and have the characteristics of emergency and concealment. To avoid their damage to offshore engineering, a new generation of monitoring and early warning system for ISWs was developed using technologies of double buoys monitoring, intelligent realtime data transmission, and automatic software identification. The system was applied to the second natural gas hydrates(NGHs) production test in the Shenhu Area, South China Sea(SCS) and successfully provided the early warning of ISWs for 173 days(from October 2019 to April 2020). The abrupt changes in the thrust force of the drilling platform under the attack of ISWs were consistent with the early warning information, proving the reliability of this system. A total of 93 ISWs were detected around the drilling platform. Most of them occurred during the spring tides in October–December 2019 and April 2020, while few of them occurred in winter. As suggested by the theoretical model, the full-depth structure of ISWs was a typical current profile of mode-1, and the velocities of wave-induced currents can reach 80 cm/s and30 cm/s, respectively, in the upper ocean and near the seabed. The ISWs may be primarily generated from the interactions between the topography and semidiurnal tides in the Luzon Strait, and then propagate westward to the drilling platform. This study could serve as an important reference for the early warning of ISWs for offshore engineering construction in the future.
基金The National Natural Science Foundation of China under contract No.41606078the Taishan Scholar Special Experts Project under contract No.ts201712079+1 种基金the National Key Research and Development Plan under contract No.2017YFC0307600the Open Fund of Qingdao National Laboratory for Marine Science and Technology of China under contract Nos QNLM2016ORP0203 and QNLM2016ORP0207
文摘The undrained shear strength of shallow strata is a critical parameter for safety design in deep-water operations.In situ piezocone penetration tests(CPTU) and laboratory experiments are performed at Site W18-19 in the Shenhu area, northern South China Sea, where China's first marine hydrate exploitation operation is due to be located. The validation of the undrained shear strength prediction model based on CPTU parameters. Different laboratory tests, including pocket penetrometer, torvane, miniature vane and unconsolidated undrained triaxial tests, are employed to solve empirical cone coefficients by statistical and mathematical methods. Finally, an optimized model is proposed to describe the longitudinal distribution of undrained shear strength in calcareous clay strata in the Shenhu area. Research results reveal that average empirical cone coefficients based on total cone resistance, effective resistance, and excess-pore pressure are 13.8, 4.2 and 14.4, respectively. The undrained shear strength prediction model shows a good fit with the laboratory results only within specific intervals based on their compaction degree and gas-bearing conditions. The optimized prediction model in piecewise function format can be used to describe the longitudinal distribution of the undrained shear strength for calcareous clay within all depth intervals from the mud-line to the upper boundary of hydrate-bearing sediments(HBS). The optimized prediction result indicates that the effective cone resistance model is suitable for very soft to firm calcareous clays,the excess-pore pressure model can depict the undrained shear strength for firm to very stiff but gas-free clays,while the total cone resistance model is advantageous for evaluating the undrained shear strength for very stiff and gassy clays. The optimized model in piecewise function format can considerably improve the adaptability of empirical models for calcareous clay in the Shenhu area. These results are significant for safety evaluations of proposed hydrate exploitation projects.
文摘Dynamics of major picoplankton groups, Synechococcus (Syn), Prochlorococcus (Pro), picoeukaryotes (Euk) and heterotrophic bacteria (Bact) was investigated by flow cytometry for the first time in the Nansha Islands area in the South China Sea. Averaged over the whole investigation area, depth-weighted integrated cell abundance (DWA) of Syn, Pro, Euk and Bact was 1.6 (0.4-5.7)×103, 5.4 (0.1-7.3)×104, 0.7 (0.2-2.2)×103, and 2.3 (1.4-3.2)×105 cells/mL respectively. Picoautotrophic cell abundance was low in the northwest part of the Nansha Islands where surface water temperature was low and the upper mixed layer was shallow. Concurrently, a surface maximum vertical distribution pattern was observed in this area. While in the southeast and east zones where temperatures were relatively higher and nitraclines were deeper, picoplankton is abundant and a subsurface maximum around 50-75 m is observed. Coupling of horizontal and vertical distribution patterns of picoplankton abundance and hydrological status was found, suggesting a strong influence of currents and water column structure on picoplankton distribution in the investigation area. Contrary to that in the shelf water in the East China Sea, the relationship between Pro and Bact in the Nansha Islands area in the South China Sea was not significantly negative but weakly positive. Moreover, a similar distribution pattern of Syn and Pro was observed. Possible reasons for these differences in the two marine regimes were discussed.
基金supported by the National 973 Basic Research Program (Grant No. 2009CB219502)National Natural Science Foundation of China (Grant No. 41072084)
文摘BSR （Bottom Simulating Reflector） occurs widely in the strata since the late Miocene in the deep-water area of the northern continental slope of South China Sea （SCS）. It is an important seismic reference mark which identifies the gas hydrate and its distribution influenced by the tectonic movements. Single-point basin modeling was conducted using 473 points in the study area. To discuss the relationships between the tectonic subsidence and BSR, the volume and rate of tectonic subsidence in each geological time have been simulated. The results show that there are three tectonic accelerate subsidence processes in the study area since the late Miocene, especially since 1.8Ma the tectonic subsidence accelerates more apparently. Since the Late Miocene to Pleistocene, the rate of tectonic subsidence in deep-water underwent a transformation from weak to strong. The ratio of tectonic subsidence to the total subsidence was relatively high （65-70%）. Through the superposition of the BSR developed areas and the contours of tectonic subsidence in this area, it was discovered that more than 80% of BSR tend to be distributed at the slope break or depression-uplift structural transfer zone and the average tectonic subsidence rate ranges from 70 m/Ma to 125 m/Ma.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11434012,41561144006,11174312 and 11404366
文摘Bottom acoustic parameters play an important role in sound field prediction. Acoustic parameters in deep water are not well understood. Bottom acoustic parameters are sensitive to the transmission-loss （TL） data in the shadow zone of deep water. We propose a multiple-step fill inversion method to invert sound speed, density and attenuation in deep water. Based on a uniform liquid hMf-space bottom model, sound speed of the bottom is inverted by using the long range TL at low frequency obtained in an acoustic propagation experiment conducted in the South China Sea （SCS） in summer 2014. Meanwhile, bottom density is estimated combining with the Hamilton sediment empirical relationship. Attenuation coefficients at different frequencies are then estimated from the TL data in the shadow zones by using the known sound speed and density as a constraint condition. The nonlinear relationship between attenuation coefficient and frequency is given in the end. Tile inverted bottom parameters can be used to forecast the transmission loss in the deep water area of SCS very we//.
基金The National Natural Science Foundation of China under contract Nos 41576059,91128207 and 91528302the National Major Science and Technology Projects under contract No.2011ZX05025-006-02
文摘Geochemical and detrital zircon U-Pb dating data for drilled sediments from the Baiyun deepwater area of the northern South China Sea demonstrate a change of sedimentary sources from the Oligocene to the Miocene.Zircon ages of the pre-rift Eocene sequences are dominated by Yanshanian ages with various peak values（110–115 Ma for U1435 and L21; 150 Ma for H1）, indicating local sediment supply from the pre-existing Mesozoic magmatic belt. For the Oligocene sediments in the northern part of the basin, the rare earth elements show different distribution characteristics, indicating sediment supply from the paleo-Zhujiang River（Pearl River）, as also confirmed by the multimodal zircon age spectra of the Lower Oligocene strata in Well X28. By contrast, a positive Eu anomaly characterizes sediments from the western and southern parts of the basin, indicating potential provenances from intermediate to basic volcanic rock materials. The Baiyun Movement at the end of the Oligocene contributed to a large-scale subsidence in the deepwater area and also a northward retreat of continental shelf break, leading to deepening depositional environment in the basin. As a result, all the detrital zircon ages of the Upper Oligocene strata from Wells X28, L13, and L21 share a similar distribution, implying the possible control of a common source like the Zhujiang River. During the Miocene, whereas sediments in the northern area were mainly sourced from the Zhujiang River Delta, and those in the southern deepwater area continued to be affected by basic volcanic activities, the Dongsha Uplift could have contributed as the main source to the eastern area.
文摘Through the investigation on radioactive activities of water, sediment and some marine organismsin the Zhujiang Estuary, adjacent sea area and the distributary mouths of the Zhujiang River, activities of total α. radioactivity, total β radioactivity, artificial radioactive 90Srand 157Cs, and factors inflencing the distribution and the content of U in seawater are studied.The mainly radioactive pollution substances and their sources in the sea area are studied by γ spectra obtained from sediment in the sea area. The results show that the main radioactivity substances are natural radioactivity U,Th series and 40K. which were produced by the modern industry and transported into the sea through the main current of the Zhujiang River.
基金Supported by the Science and Technology Project of CNOOC Ltd.(YXKY-2012-SHENHAI-01)China National Science and Technology Major Project(2011ZX05025-003+1 种基金 2016ZX05026-003)the National Natural Science Foundation of China(91128207)
文摘The relationships between crustal stretching and thinning,basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic,gravity,magnetic,ocean bottom seismic(OBS),deep-water exploration wells and integrated ocean drilling program(IODP).During the early syn-rifting period,deep-water area was a half-graben controlled by high angle faults influenced by the brittle extension of upper crust.In the mid syn-rifting period,this area was a broad-deep fault depression controlled by detachment faults undergone brittle-ductile deformation and differentiated extension in the crust.In the late syn-rifting period,this area experienced fault-sag transition due to saucer-shaped rheology change dominated by crustal ductile deformation.A broad-deep fault depression controlled by the large detachment faults penetrating through the crust is an important feature of deep-water basin.The study suggests that the broad-deep Baiyun sag provides great accommodation space for the development of massive deltaic-lacustrine deposition system and hydrocarbon source rocks.The differentiated lithospheric thinning also resulted in the different thermal subsidence during post-rifting period,and then controlled the development of continental shelf break and deep-water reservoir sedimentary environment.The high heat flow background caused by the strong thinning of lithosphere and the rise of mantle source resulted in particularities in the reservoir diagenesis,hydrocarbon generation process and accumulation of deep-water area in northern South China Sea.
基金supported by the Knowledge Innovation Program of the Chinese Academy of Science(KZCX2-YW-Q11-02)the National Basic Research Program of China (2012CB417402)
文摘Based on oceanic and atmospheric parameters retrieved by satellite remote sensing using a neural network method, air-sea heat fluxes over the western Pacific warm pool area were calculated with the advanced the advanced Coupled Ocean-Atmosphere Response Experiment 3.0 (COARE3.0) bulk algorithm method. Then, the average annual and interannual characteristics of these fluxes were analyzed. The rela- tionship between the fluxes and the South China Sea (SCS) summer monsoon onset is highlighted. The results indicate that these fluxes have clear temporal and spatial characteristics. The sensible heat flux is at its maximum in the Kuroshio area, while the latent heat flux is at its maximum in the North Equatorial Current and Kuroshio area. The distribution of average annual air-sea heat fluxes shows that both sensible and latent heat fluxes are maximized in winter and minimized in summer. The air-sea heat fluxes have obvious interannual variations. Correlation analysis indicates a close lag-correlation between air-sea heat fluxes in the western Pacific warm pool area and at the SCS summer monsoon onset. The lagcorrelation can therefore predict the SCS summer monsoon onset, providing a reference for the study of precipitation related to the monsoon.
基金jointly supported by the National Natural Science Foundation of China (42076072)the project of China Geological Survey (DD20160211)the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou, GML2019ZD0506)。
文摘Evaluating velocity-porosity relationships of hydrate-bearing marine sediments is essential for characterizing natural gas hydrates below seafloor as either a potential energy resource or geohazards risks.Four sites had cored using pressure and non-pressure methods during the gas hydrates drilling project(GMGS4)expedition at Shenhu Area,north slope of the South China Sea.Sediments were cored above,below,and through the gas-hydrate-bearing zone guided with logging-while-drilling analysis results.Gamma density and P-wave velocity were measured in each pressure core before subsampling.Methane hydrates volumes in total 62 samples were calculated from the moles of excess methane collected during depressurization experiments.The concentration of methane hydrates ranged from 0.3%to 32.3%.The concentrations of pore fluid(25.44%to 68.82%)and sediments(23.63%to 54.28%)were calculated from the gamma density.The regression models of P-wave velocity were derived and compared with a global empirical equation derived from shallow,unconsolidated sediments data.The results were close to the global trend when the fluid concentration is larger than the critical porosity.It is concluded that the dominant factor of P-wave velocity in hydrate-bearing marine sediments is the presence of the hydrate.Methane hydrates can reduce the fluid concentration by discharging the pore fluid and occupying the original pore space of sediments after its formation.
