Gravity Flow on Slope and Abyssal Systems in the Qiongdongnan Basin,Northern South China Sea

The study of new seismic data permits the identification of sediment gravity flows in terms of internal architecture and the distribution on shelf and abyssal setting in the Qiongdongnan Basin （QDNB）. Six gravity flow types are recognized： （1） turbidite channels with a truncational basal and concordant overburden relationship along the shelf edge and slope, comprising laterally-shifting and vertically-aggrading channel complexes; （2） slides with a spoon-shaped morphology slip steps on the shelf-break and generated from the deformation of poorly-consolidated and high water content sediments; （3） slumps are limited on the shelf slope, triggered either by an anomalous slope gradient or by fault activity; （4） turbidite sheet complexes （TSC） were ascribed to the basin-floor fan and slope fan origin, occasionally feeding the deep marine deposits by turbidity currents; （5） sediment waves occurring in the lower slope-basin floor, and covering an area of approximately 400 km2, were generated beneath currents flowing across the sea bed; and （6） the central canyon in the deep water area represents an exceptive type of gravity flow composed of an association of debris flow, turbidite channels, and TSC. It presents planar multisegment and vertical multiphase characteristics. Turbidite associated with good petrophysical property in the canyon could be treated as a potential exploration target in the QDNB.

The Gravity Flow on a Shelf-Edge-Slope-Basin-Floor Complex in Qiongdongnan Basin,Northern South China Sea

The Qiongdongnan Basin lies in the west part of the northern South China Sea.To the west the basin is bound by the Red River Fault and the Yinggehai Basin,to the east by the Pearl River Mouth Basin,and to the south by the Xisha Rise.In this study,the sequence stratigraphic framework was set up and 11 third-order sequences were distinguished.Based

Geophysical Signature of the Shallow Water Flow in the Deepwater Basin of the Northern South China Sea

Shallow water flow(SWF), a disastrous geohazard in the continental margin, has threatened deepwater drilling operations. Under overpressure conditions, continual flow delivering unconsolidated sands upward in the shallow layer below the seafloor may cause large and long-lasting uncontrolled flows; these flows may lead to control problems and cause well damage and foundation failure. Eruptions from over-pressured sands may result in seafloor craters, mounds, and cracks. Detailed studies of 2D/3D seismic data from a slope basin of the South China Sea(SCS) indicated the potential presence of SWF. It is commonly characterized by lower elastic impedance, a higher Vp/Vs ratio, and a higher Poisson's ratio than that for the surrounding sediments. Analysis of geological data indicated the SWF zone originated from a deepwater channel system with gas bearing over-pressured fluid flow and a high sedimentation rate. We proposed a fluid flow model for SWF that clearly identifies its stress and pressure changes. The rupture of previous SWF zones caused the fluid flow that occurred in the Baiyun Sag of the northern SCS.

GRAVITY-DRIVEN SEDIMENTATION ON THE NORTHWESTERN CONTINENTAL SLOPE IN THE SOUTH CHINA SEA:RESULTS FROM HIGH-RESOLUTION SEISMIC DATA AND PISTON CORES

A distinct echo-character was assigned to sedimentation processes, which were then verified using data from surface sediment samples and piston cores. of echo types on the continental slope perfectly reflecting both sediment erosion and deposition, four edimentary types have been recognized:(1) submarine clides distributed on the shelfbreak and characterized by high silt and water conten, loose struture, poor consolidation and low shearing strength; (2) slumps occurring on the shelfbreak, middle slope channel and reef margin near Dongsha Islands, but having different origins; (3) debris flow occurring either in sea areas around Dongsha Atoll, or on the continental slope’s three channels, where the transparent debris flow deposits often overlie or abruptly truncate highly stratified hemipelagic sediments;are of limited to local extent, ranging from a few square kilometers to hundreds of square kilometers in area; but on the lowr slope, usually occur as 1000 km2, about 100 km2 individual complexes; and (4) turbidites, limited on the continental slope; are occurring as migrating waves of sediments at the the of the slope, and are rhythmically-bedded, coarse-grained. Their migration is a result of overbank flow downslope through the submarine channel at the west. The slope faces are dominated by mass wasting deposition, and a few turbidite current sediments. As wasting is an important process. Some debris flow eomplexes on the west are buried by well-stratified confomable sediments, whereas others on the east appear on the present seafloor and therefore are relatively recent.

Crustal Structure across the Northwestern Margin of South China Sea:Evidence for Magma-poor Rifting from a Wide-angle Seismic Profile

We present results from a 484 km wide-angle seismic profilie acquired in the northwest part of the South China Sea （SCS） during OBS2006 cruise. The line that runs along a previously acquired multi-channel seismic line （SO49-18） crosses the continental slope of the northern margin, the Northwest Subbasin （NWSB） of the South China Sea, the Zhongsha Massif and partly the oceanic basin of the South China Sea. Seismic sections recorded on 13 ocean-bottom seismometers were used to identify refracted phases from the crustal layer and also reflected phases from the crust-mantle boundary （Moho）. Inversion of the traveltimes using a simple start model reveals crustal images in the study area. The velocity model shows that crustal thickness below the continental slope is between 14 and 23 kin. The continental part of the line is characterized by gentle landward mantle uplift and an abrupt oeeanward one. The velocities in the lower crust do not exceed 6.9 km/s. With the new data we can exclude a high-velocity lower crustal body （velocities above 7.0 kin/s） at the location of the line. We conclude that this part of the South China Sea margin developed by a magma-poor rifting. Both, the NWSB and the Southwest Sub-basin （SWSB） reveal velocities typical for oceanic crust with crustal thickness between 5 and 7 kin. The Zhongsha Massif in between is extremely stretched with only 6-10 km continental crust left. Crustal velocity is below 6.5 kin/s; possibly indicating the absence of the lower crust. Multi-channel seismic profile shows that the Yitongansha Uplift in the slope area and the Zhongsha Massif are only mildly deformed. We considered them as rigid continent blocks which acted as rift shoulders of the main rift subsequently resulting in the formation of the Northwest Sub-basin. The extension was mainly accommodated by a ductile lower crustal flows, which might have been extremely attenuated and flow into the oceanic basin during the spreading stage. We compared the crustal structures along the northern margin and found an east-west thicken trend of the crust below the continent slope. This might be contributed by the east-west sea-floor spreading along the continental margin.

ONSET AND RETREAT DATES OF THE SOUTH CHINA SEA SUMMER MONSOON AND THEIR RELATIONSHIPS WITH THE MONSOON INTENSITY IN THE CONTEXT OF CLIMATE WARMING

Global gridded daily mean data from the NCEP/NCAR Reanalysis(1948-2012) are used to obtain the onset date,retreat date and duration time series of the South China Sea summer monsoon(SCSSM) for the past 65 years.The summer monsoon onset(retreat) date is defined as the time when the mean zonal wind at 850 hPa shifts steadily from easterly(westerly) to westerly(easterly) and the pseudo-equivalent potential temperature at the same level remains steady at greater than 335 K(less than 335 K) in the South China Sea area[110-120°E(10-20°N)].The clockwise vortex of the equatorial Indian Ocean region,together with the cross-equatorial flow and the subtropical high,plays a decisive role in the burst of the SCSSM.The onset date of the SCSSM is closely related to its intensity.With late(early) onset of the summer monsoon,its intensity is relatively strong(weak),and the zonal wind undergoes an early(late) abrupt change in the upper troposphere.Climate warming significantly affects the onset and retreat dates of the SCSSM and its intensity.With climate warming,the number of early-onset(-retreat) years of the SCSSM is clearly greater(less),and the SCSSM is clearly weakened.

Mass transport deposits and processes in the north slope of the Xisha Trough,northern South China Sea

Triple mass-transport deposits （MTDs） with areas of 625, 494 and 902 km^2, respectively, have been identified on the north slope of the Xisha Trough, northern South China Sea margin. Based on high-resolution seismic reflection data and multi-beam bathymetric data, the Quaternary MTDs are characterized by typical geometric shapes and internal structures. Results of slope analysis showed that they are developed in a steep slope ranging from 5° to 35°. The head wall scarps of the MTDs arrived to 50 km in length （from headwall to termination）. Their inner structures include well developed basal shear surface, growth faults, stepping lateral scarps, erosion grooves, and frontal thrust deformation. From seismic images, the central deepwater channel system of the Xisha Trough has been filled by interbedded channel-levee deposits and thick MTDs. Therefore, we inferred that the MTDs in the deepwater channel system could be dominated by far-travelled slope failure deposits even though there are local collapses of the trough walls. And then, we drew the two-dimensional process model and three- dimensional structure model diagram af the MTDs. Combined with the regional geological setting and previous studies, we discussed the trigger mechanisms of the triple MTDs.

ON THE ORIGIN AGE OF THE SOUTHWEST BASIN IN THE SOUTH CHINA SEA

This paper propounds that the origin and evolution of the Southwet Basin (SWB) in the South China Sea (SCSh) are closely related with those of the SCS, reviews various viewpoints on its origin age with a large gap between the oldest age and the youngest age. offers some suggestions on the SWB’s origin age and gives some proposals to ascertain satisfactorily the origin and evolution, and multi-phasal and multiaxial spreading of the SCS and SWB.

An insight into shallow gas hydrates in the Dongsha area,South China Sea

Previous studies of gas hydrate in the Dongsha area mainly focused on the deep-seated gas hydrates that have a high energy potential,but cared little about the shallow gas hydrates occurrences.Shallow gas hydrates have been confirmed by drill cores at three sites(GMGS208,GMGS209 and GMGS216)during the GMGS2 cruise,which occur as veins,blocky nodules or massive layers,at 8–30 m below the seafloor.Gas chimneys and faults observed on the seismic sections are the two main fluid migration pathways.The deep-seated gas hydrate and the shallow hydrate-bearing sediments are two main seals for the migrating gas.The occurrences of shallow gas hydrates are mainly controlled by the migration of fluid along shallow faults and the presence of deep-seated gas hydrates.Active gas leakage is taking place at a relatively high-flux state through the vent structures identified on the geophysical data at the seafloor,although without resulting in gas plumes easily detectable by acoustic methods.The presence of strong reflections on the high-resolution seismic profiles and dim or chaotic layers in the subbottom profiles are most likely good indicators of shallow gas hydrates in the Dongsha area.Active cold seeps,indicated by either gas plume or seepage vent,can also be used as indicators for neighboring shallow gas hydrates and the gas hydrate system that is highly dynamic in the Dongsha area.

Geothermal investigation of the thickness of gas hydrate stability zone in the north continental margin of the South China Sea

The exploration of unconventional and/or new energy resources has become the focus of energy research worldwide,given the shortage of fossil fuels.As a potential energy resource,gas hydrate exists only in the environment of high pressure and low temperature,mainly distributing in the sediments of the seafloor in the continental margins and the permafrost zones in land.The accurate determination of the thickness of gas hydrate stability zone is essential yet challenging in the assessment of the exploitation potential.The majority of previous studies obtain this thickness by detecting the bottom simulating reflectors（BSRs） layer on the seismic profiles.The phase equilibrium between gas hydrate stable state with its temperature and pressure provides an opportunity to derive the thickness with the geothermal method.Based on the latest geothermal dataset,we calculated the thickness of the gas hydrate stability zone（GHSZ） in the north continental margin of the South China Sea.Our results indicate that the thicknesses of gas hydrate stability zone vary greatly in different areas of the northern margin of the South China Sea.The thickness mainly concentrates on 200–300 m and distributes in the southwestern and eastern areas with belt-like shape.We further confirmed a certain relationship between the GHSZ thickness and factors such as heat flow and water depth.The thickness of gas hydrate stability zone is found to be large where the heat flow is relatively low.The GHSZ thickness increases with the increase of the water depth,but it tends to stay steady when the water depth deeper than 3 000 m.The findings would improve the assessment of gas hydrate resource potential in the South China Sea.

Three-stage tectonic subsidence and its implications for the evolution of conjugate margins of the southwest subbasin,South China Sea

To reveal the tectonic characteristics of the continental margins in the southwest subbasin(SWB)of the South China Sea,a long high-resolution seismic profile was studied using empty basin subsidence.We find that tectonic subsidence features on both margins are uniformly divided into three stages:(1)slow subsidence from Tg to 18.5 Ma(synrift stage);(2)extremely slow subsidence/uplift from18.5 to 16 Ma(spreading stage);and(3)accelerated subsidence from 16 to 0 Ma(post-spreading stage).This feature differs from the classic tectonic subsidence pattern of rifted basins,which exhibits fast subsidence during synrift stage and slow subsidence during the post-rift stage.The tectonic uplift occurred during the spreading stage and the magnitude increased from the continent to the ocean,which is likely related to mantle flow during seafloor spreading.We propose that lower crustal flow played a significant role in the tectonic evolution of the continental margins of the SWB.The lower crust of the SWB margins was warmer and therefore weaker,and more prone to flow beneath the faulting center,which compensated for the upper crustal thinning caused by brittle faulting during the synrift period and thus reduced the tectonic subsidence rate.During the spreading stage,faulting attenuated rapidly,and a necking zone appeared at the continentocean transition formed by lithospheric extension.With upwelling asthenosphere,small-scale secondary mantle convection occurred under the necking zone,which raised the continental margin isotherms and increased the buoyancy.Simultaneously,secondary mantle convection lifted the overriding crust,thus the overall subsidence rate decreased sharply or even reversed to uplift.After seafloor spreading,the effect of mantle convection faded away,and sediment loading drove the lower crust to flow landward.Thermal relaxation,lower crust flow,and vanish of secondary mantle convection together led to rapid subsidence in this stage.

CONNECTION OF THE SOUTH CHINA SEA SUMMER MONSOON TO MARITIME CONTINENT CONVECTION AND ENSO

The relationship between the intensity of the South China Sea summer monsoon (SCSSM) and the Nino3.4 index and anomalous atmospheric circulation patterns associated with a strong and weak SCSSM are investigated using the NCEP/NCAR reanalysis data, Extended Reconstructed Sea Surface Temperature (ERSST) data and Climate Prediction Center Merged Analysis of Precipitation (CMAP) data. The SCSSM is significantly positively correlated with the Nino3.4 index in the succeeding northern autumn and winter. In the strong minus weak SCSSM composite, a positive East Asia-Pacific teleconnection (EAP) pattern and a negative Europe-Asian-Pacific teleconnection (EUP) pattern appear in the 500 hPa height difference field; low-level cross-equatorial flows are strengthened over the Maritime Continent (MC) region; positive (negative) precipitation anomalies occur in the South China Sea and western north Pacific (MC). A possible mechanism through which SCSSM affects ENSO is proposed. A strong (weak) SCSSM strengthens (weakens) cross-equatorial flows over the MC. The anomalous cross-equatorial flows cool (warm) the SST around the MC through enhanced (reduced) surface latent heat fluxes. The cooling (warming) further leads to suppressed (enhanced) convection over the MC, and causes the anomalous westerly (easterly) in the equatorial western Pacific, which favors the onset of El Nio (La Nia) through modulating the positive air-sea feedback process.

A 3D basin modeling study of the factors controlling gas hydrate accumulation in the Shenhu Area of the South China Sea

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.

Seismic geomorphology and main controls of deep-water gravity flow sedimentary process on the slope of the northern South China Sea

The Quaternary continental slope of the Baiyun Sag in northern South China Sea is characterized by a complex topography and abundant gravity flow sedimentation.High-resolution 3-D seismic data in this area allow for a detailed study of the seismic geomorphology and deep-water gravity flow depositional process.The Quaternary continental slope in the northern South China Sea is an above-graded slope.An intraslope basin lies within the above-grade continental slope.Slump,erosion,and deposition processes tend to develop a gentle topography and consequently a graded slope.The upper continental slope,which is above the slope equilibrium profile,is dominated by erosion and slumping.Slides,slumps and erosional channels are developed within this continental slope.The intraslope basin is located below the slope equilibrium profile and is potential accommodation space where sediments transported by gravity flows could be deposited,forming lobe aprons.Under the influence of gravity flow supply,gravity flow duration,continental slope topography,equilibrium profile,and accommodation,a slump-erosional channel-lobe depositional system is developed in the Quaternary continental slope in the Baiyun Sag.The deep-water gravity flow depositional process and the distribution of gravity flow sediments are greatly influenced by the continental slope topography,while the continental slope topography at the same time is reshaped by deep-water gravity flow depositional process and its products.The study of the interplay between the continental slope and gravity flow is helpful in predicting the distribution of the deep-water gravity flow sediments and the variation of sediment quality.

南海西北部莺琼陆坡重力流和等深流的时空关联

南海西北部的莺琼陆坡深水区近些年获取了大量油气发现,展示了广阔的勘探前景。深水沉积往往受控于重力流、等深流等多种机制,它们在时空中如何分布、如何相互作用,又在多大程度上影响或控制着沉积格局,对水动力方面的认识的差异会导致沉积相展布方面的不同认识,进而影响对砂体类型和展布的分析,制约了石油勘探开发。论文基于地震数据所呈现的地震相和地层叠置样式,确认莺琼陆坡在中新世以来发育两类沉积体系:重力(流)和等深流沉积体系。二者在时空上存在毗邻而居、有序进退的关系,重力流沉积呈“台阶状”逐层上超于等深流沉积之上,呈进积形态,而等深流沉积体系则呈退积样式;二者之间构成了特殊成因的、跨时的“相变”面;进而相应恢复了两种不同方向的沉积作用(重力流和等深流)的时空关联。它们这种“台阶状”进退关系根源于中中新世以来莺琼陆坡所经历的多方向物源体系充沛的碎屑供应和由此导致的活跃的重力流活动;南海深水循环导致的等深流机制作用的强度也有相当的作用;总体上,等深流沉积体系的分布范围受重力流作用强弱的控制,形成了二者彼进此退的时空关系。

中国大型LNG/FLNG绕管换热器研发进展与工业化应用

大型LNG/FLNG绕管换热器是FLNG和大型LNG生产建设的核心装备,技术门槛高,长期由美国APCI和德国Linde公司垄断。本文全面回顾了中国在大型LNG/FLNG绕管换热器研发和工业化应用方面的最新进展,主要包括:①构建了适用于烷烃工质冷凝与沸腾流动的高精度数值模拟方法,提升了数值计算的准确性和稳定性,数值模拟结果与实验数据相对误差在±20%以内;揭示了晃动及结构参数对烷烃工质复杂相变流动传热的影响机制,建立了烷烃工质复杂相变流型转换准则与分流型的流动换热关联式;开发了具有自主知识产权的大型LNG/FLNG绕管换热器仿真设计软件,其仿真结果与现场运行数据平均相对误差小于5%。②研制了300 m以上超长无缝换热铝合金管及3种适用于LNG/FLNG绕管换热器的新型均布器,形成了基于铝换热管的大型LNG/FLNG绕管换热器结构优化设计技术,开发出了中国首套全铝制30万m3/d LNG/FLNG绕管换热器,通过了CCS认证,并制定了相关企业标准。③搭建了全球首个LNG/FLNG绕管换热器工业化测试平台,完成了首套全铝制30万m3/d LNG/FLNG绕管换热器静止和晃荡工业化应用测试,结果表明国产LNG/FLNG绕管换热器在换热等方面性能良好。相关研究成果为未来自主设计和建造大型LNG/FLNG绕管换热器奠定了基础。

The deep thermal characteristic of continental margin of the northern South China Sea

Heat flow plays an important role in the study ot thermal structure and thermal evolutionof continental margin of the northern South China Sea. The analysis of heat flow value shows that margin heat flow in the northern South China Sea is relatively high setting, but the percentage of crustal heat flow is lower than 35% in terrestrial heat flow. The terrestrial heat flow exhibited a current of rise from the Northern Continental Margin to the Southern Central Basin. However, the proportion of crustal heat flow in terrestrial heat flow slowly dropped down in the same direction. It is suggested that the main factor causing high heat flow setting is the moving up of hot material from asthenosphere.

南海一个罗斯贝标准模态的特征与归因

南海是一个准封闭海盆,其本征值问题是理解南海动力学的重要内容。本文利用一种新的泛函工具—多尺度子空间变换,从卫星观测资料中分离得到一个南海本征模态,即罗斯贝标准模态的近似场。发现该模态的周期为6个月左右、波长近250km,在深水海盆向西传播,这与寿命为3个月左右的南海中尺度涡群体活动特征相一致。在此基础上,本文通过Liang-Kleeman信息流这一严格建立在第一性原理上的定量因果分析工具,探究南海两个最重要外部强迫,即黑潮入侵与南海季风对该罗斯贝标准模态的影响。结果表明二者与该模态均有较强因果关系,但分别影响模态的不同阶段:黑潮入侵主要影响其1/2π和3/2π位相,季风的作用则体现在3/4π位相。二者共同作用,调制该模态在近一个周期内的变化。进一步研究发现,黑潮入侵的过渡态是影响该模态的关键,这时黑潮在南海的分支与流套强度相当,有利于吕宋海峡西部形成不同极性涡旋的排列,从而影响南海内部罗斯贝标准模态。对南海季风而言,冬季风与夏季风的成熟阶段是影响该模态的重要时期,但并非整个南海的季风都发挥作用,泰国湾是季风改变南海罗斯贝标准模态的关键区域,这表明局地的强迫对激发全局模态起作用。

麻坑、泥火山在南海北部与西部陆缘的分布特征和地质意义

本文利用新的高分辨率海底多波束地形数据与以往研究资料对南海台西南盆地及以西和中建南盆地北部及以北陆缘区域内的麻坑与泥火山分布区进行了统计、分析与总结.本文提出根据麻坑的个体平面形态、规模量级和组合形式建立麻坑的三类分类体系并进行总结阐述.麻坑的直径与坑深总体具有线性变化关系但又有区域性分布特征,可能反映了不同海区底质性质、浅地层构造活动、海底倾斜程度和海底底流等因素的不同影响;同一区域泥火山的直径与高度具有线性关系而不同区域拟合斜率差异较大,可能说明同区域泥火山形态、流体成分和来源具有高度一致性,但其分布具有区域差异性特征.研究表明,麻坑、泥火山的产生与近5Ma新构造运动期内台湾南部和南海西缘区域性走滑断裂活动相一致,主要分布在构造活动较为活跃,沉积较为薄弱的近坡折带和大陆坡区域,在具有走滑性质的台西南、莺歌海和中建南盆地分布较广、规模较大,而在其他主要张裂性质盆地中分布较少,规模较小.其分布特征一方面能够说明某些区域流体的大量渗漏与逸散,另一方面也能够说明某些区域持续的构造活动有利于该区域深部流体的重新聚集与成藏,从而成为指导油气资源勘探的重要参考指标.

南海热流特征及其构造意义

根据南海 592个热流数据 ,为克服热流站位分布不均及局部异常热流的影响 ,结合各单元的地质史及其地壳厚度等资料对研究区热流特征进行了详细分析。结果表明 ,具拉张背景的区域如北部陆缘、湄公盆地以及北巴拉望盆地具有中等偏高热流 ;海沟区热流相对较低 ,东部海沟区除台西南盆地外均为低热流区 ,而南部边缘东段古海沟区处于热恢复中 ;南部边缘西区因边界断裂的扭张及深部热源的异常补给而具高热流 ;属于剪切断裂带的西部陆缘也具高热流特征 ;中沙—西沙地区热流中等偏高 ,并由NW往SE方向增加 ,而南沙地区热流较低 ,约为 60mW·m- 2 ;海盆的热流基本满足随洋壳年龄增加而降低的规律 ,东部次海盆实测热流与理论预测基本一致 ,而西南次海盆实测热流普遍低于预测值 ;在南海北部下陆坡区识别出一条高热流带 ,该带与前人给出的海盆北缘断裂带位置基本一致。研究区不同区域地热特征直接或间接地受控于其所处的构造环境。据此 ,给出了研究区的热流趋势图。