South China Sea

摘要： Offline bias correction of numerical marine forecast products is an effective post-processing means to improve forecast accuracy. Two offline bias correction methods for sea surface temperature(SST) forecasts have been developed in this study: a backpropagation neural network(BPNN) algorithm, and a hybrid algorithm of empirical orthogonal function(EOF) analysis and BPNN(named EOF-BPNN). The performances of these two methods are validated using bias correction experiments implemented in the South China Sea(SCS), in which the target dataset is a six-year(2003–2008) daily mean time series of SST retrospective forecasts for one-day in advance, obtained from a regional ocean forecast and analysis system called the China Ocean Reanalysis(CORA),and the reference time series is the gridded satellite-based SST. The bias-correction results show that the two methods have similar good skills;however, the EOF-BPNN method is more than five times faster than the BPNN method. Before applying the bias correction, the basin-wide climatological error of the daily mean CORA SST retrospective forecasts in the SCS is up to-3°C;now, it is minimized substantially, falling within the error range(±0.5°C) of the satellite SST data.

摘要： A number of cold seeps have been discovered in the northern South China Sea(SCS) including the Haiyang 4 cold-seep area where Core 973-5 was collected. Intact polar lipids(IPLs) and core lipids(CLs) were analyzed separately in sediments from Core 973-5. The most abundant lipid biomarkers were isoprenoidal GDGTs(iso GDGTs), with Crenarchaeol and GDGT-0 predominating. IPL-iso GDGTs and CL-iso GDGTs were mainly derived from Thaumarchaeota. IPL-iso GDGTs were mainly produced and retained in situ thus containing most of the in situ microbiological information. Branched GDGTs were predominantly derived from generated in marine production, and mixed with some terrestrial inputs. All IPLs groups presented a high value in the sulfate-methane transition zone(SMTZ). Furthermore, IPL and CL-MI, IPL-R;showed the highest values within the SMTZ, while IPL and CL-R;had the lowest values at the SMTZ, suggesting that the contribution of Methanophila and methanogenic to GDGTs increased, while the contribution of ammonia-oxidizing Archaea to GDGTs decreased at the SMTZ.

摘要： Natural gas hydrates(NGHs)are globally recognized as an important type of strategic alternative energy due to their high combustion efficiency,cleanness,and large amounts of resources.The NGHs reservoirs in the South China Sea(SCS)mainly consist of clayey silts.NGHs reservoirs of this type boast the largest distribution range and the highest percentage of resources among NGHs reservoirs in the world.However,they are more difficult to exploit than sandy reservoirs.The China Geological Survey successfully carried out two NGHs production tests in the Shenhu Area in the northern SCS in 2017 and 2020,setting multiple world records,such as the longest gas production time,the highest total gas production,and the highest average daily gas production,as well as achieving a series of innovative theoretical results.As suggested by the in-depth research on the two production tests,key factors that restrict the gas production efficiency of hydrate dissociation include reservoir structure characterization,hydrate phase transition,multiphase seepage and permeability enhancement,and the simulation and regulation of production capacity,among which the hydrate phase transition and seepage mechanism are crucial.Study results reveal that the hydrate phase transition in the SCS is characterized by low dissociation temperature,is prone to produce secondary hydrates in the reservoirs,and is a complex process under the combined effects of the seepage,stress,temperature,and chemical fields.The multiphase seepage is controlled by multiple factors such as the physical properties of unconsolidated reservoirs,the hydrate phase transition,and exploitation methods and is characterized by strong methane adsorption,abrupt changes in absolute permeability,and the weak flow capacity of gas.To ensure the long-term,stable,and efficient NGHs exploitation in the SCS,it is necessary to further enhance the reservoir seepage capacity and increase gas production through secondary reservoir stimulation based on initial reservoir stimulation.With the constant progress in the NGHs industrialization,great efforts should be made to tackle the difficulties,such as determining the micro-change in temperature and pressure,the response mechanisms of material-energy exchange,the methods for efficient NGHs dissociation,and the boundary conditions for the formation of secondary hydrates in the large-scale,long-term gas production.

摘要： The Heyuan Fault, is one of the main NE-trending faults in the southernmost South China Block and is close to the northernmost South China Sea to the south. The fault features multi-stage deformation and controlled the formation of late Cretaceous to Cenozoic basins. Based on detailed field investigations and comprehensive geochronological research, including zircon U-Pb, Rb-Sr isochron, zircon U-Th-He dating, two episodic tectono-thermal events are recognized. The first occurred during ~79 Ma to 66 Ma, which is characterized by large-scale quartz-fluid emplacement. The other occurred at ~34 Ma, which features the eruption of a set of basalts. The two events show a changing on material source from siliceous hydrotherm to basalt magma, supporting obvious lithospheric thinning of the southernmost South China Block, which shed light on the geological evolution and the interaction mechanisms between the SCB and the northern South China Sea since the Late Cretaceous.

摘要： Traditional suction anchor technology is mainly used in the fields of subsea structure bearing foundations,single-point mooring systems and offshore wind power.It is characterized by providing sufficient lateral and vertical bearing capacities and lateral bending moment.The anchor structure of a traditional suction anchor structure is improved with wellhead suction anchor technology,where a central pipe is added as a channel for drilling and completion operations.To solve the technical problems of a low wellhead bearing capacity,shallow built-up depth,and limited application of conductor jetting in the second production test of natural gas hydrates(NGHs)in the South China Sea(SCS),the China Geological Survey(CGS)took the lead in independently designing and manufacturing a wellhead suction anchor,which fulfilled the requirements of the production test.This novel anchor was successfully implemented in the second production test for the first time,providing a stable wellhead foundation for the success of the second production test of NGHs in the SCS.

摘要： The distributed acoustic sensor(DAS)uses a single optical cable as the sensing unit,which can capture the acoustic and vibration signals along the optical cable in real-time.So it is suitable for monitoring downhole production activities in the process of oil and gas development.The authors applied the DAS system in a gas production well in the South China Sea for in situ monitoring of the whole wellbore for the first time and obtained the distributed acoustic signals along the whole wellbore.These signals can clearly distinguish the vertical section,curve section,and horizontal production section.The collected acoustic signal with the frequency of approximately 50 Hz caused by the electric submersible pump exhibit a signal-to-noise ratio higher than 27 dB.By analyzing the acoustic signals in the production section,it can be located the layers with high gas production rates.Once an accurate physical model is built in the future,the gas production profile will be obtained.In addition,the DAS system can track the trajectory of downhole tools in the wellbore to guide the operation.Through the velocity analysis of the typical signals,the type of fluids in the wellbore can be distinguished.The successful application of the system provides a promising whole wellbore acoustic monitoring tool for the production of marine gas hydrate,with a good application prospect.

摘要： Blockage in water-dominated flow pipelines due to hydrate reformation has been suggested as a potential safety issue during the hydrate production.In this work,flow velocity-dependent hydrate formation features are investigated in a fluid circulation system with a total length of 39 m.A 9-m section pipe is transparent consisted of two complete rectangular loops.By means of pressurization with gas-saturated water,the system can gradually reach the equilibrium conditions.The result shows that the hydrates are delayed to appear as floccules or thin films covering the methane bubbles.When the circulation velocity is below 750 rpm,hydrate is finally deposited as a“hydrate bed”at upmost of inner wall,narrowing the flow channel of the pipeline.Nevertheless,no plugging is observed during all the experimental runs.The five stages of hydrate deposition are proposed based on the experimental results.It is also revealed that a higher driving pressure is needed at a lower flow rate.The driving force of hydrate formation from gas and water obtained by melting hydrate is higher than that from fresh water with no previous hydrate history.The authors hope that this work will be beneficial for the flow assurance of the following oceanic field hydrate recovery trials.

摘要： Understanding the pore water conversion characteristics during hydrate formation in porous media is important to study the accumulation mechanism of marine gas hydrate.In this study,low-field NMR was used to study the pore water conversion characteristics during methane hydrate formation in unsaturated sand samples.Results show that the signal intensity of T_(2) distribution isn’t affected by sediment type and pore pressure,but is affected by temperature.The increase in the pressure of hydrogen-containing gas can cause the increase in the signal intensity of T_(2) distribution.The heterogeneity of pore structure is aggravated due to the hydrate formation in porous media.The water conversion rate fluctuates during the hydrate formation.The sand size affects the water conversion ratio and rate by affecting the specific surface of sand in unsaturated porous media.For the fine sand sample,the large specific surface causes a large gas-water contact area resulting in a higher water conversion rate,but causes a large water-sand contact area resulting in a low water conversion ratio(C_(w)=96.2%).The clay can reduce the water conversion rate and ratio,especially montmorillonite(C_(w)=95.8%).The crystal layer of montmorillonite affects the pore water conversion characteristics by hindering the conversion of interlayer water.

摘要： 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.

摘要： 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.