Discovery of seep carbonate nodules as new evidence for gas venting on the northern continental slope of South China Sea

The geotectonic evolution of the northern South China Sea (SCS) is controlled by the Eurasian, the Pa-cific and the Indian-Australian Plates. With regard to its tectonic evolution, the northern margin composed of faulted terraces and basins and deposited …     

Nano-sized graphitic carbon in authigenic tube pyrites from offshore southwest Taiwan,South China Sea,and its implication for tracing gas hydrate

Gas hydrates are a significant energy resource and are usually detected by bottom simulating reflection and submarine geochemical anomalies. Authigenic minerals are related to gas hydrates, with carbonates, sulfates and sulfides being important tracing minerals. Authigenic tubular pyrites were collected from offshore southwest Taiwan in the South China Sea, and were investigated by scanning electron microscopy(SEM) and high-resolution transmission electron microscopy (HRTEM). Authigenic tubular pyrite was composed of framboidal pyrite, within which nanosized graphitic carbon of low crystallinity was discovered. The graphitic carbon coexisted with pyrite and had a texture similar to carbon nanotubes and nanocones, indicating that they likely precipitated from carbon supersaturated C-H-O fluid. Pyrite may act as a catalyst for the conversion of CH 4 to C. The discovery of nanosized graphitic carbon in pyrite indicated it was deposited in sediments that were supersaturated with methane fluid. Thus, nanosized graphitic carbon may be another tracing species for submarine gas hydrates. The discovery of nanosized graphitic carbon deposited in a low temperature environment will enlighten our understanding of the laboratory synthesis and industrial production of graphitic carbon.     

Geochemical characteristics of pore water in shallow sediments from Shenhu area of South China Sea and their significance for gas hydrate occurrence

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 s...     

Nd isotopic variations and its implications in the recent sediments from the northern South China Sea

The chondrite-normalized REE distribution patterns in recent sediments from the northern South China Sea and surrounding areas are similar, with the HREE values being almost equal to or slightly higher, and the La_N/Lu_N ratio being lower, than PAAS standards. However, samples from the Xijiang River, a major branch of the Pearl River system, show opposite trends, i.e., with slightly lower HREE values and higher La_N/Lu_N ratio than PAAS. The distribution of Nd isotopes in sediments from the northern South China Sea was controlled by the Pearl River and the inshore area of South China, respectively. The volume of ε_Nd(0) from inshore areas of southern China is apparently higher than that from the Pearl River, and ε_Nd(0) values in offshore sediments and Taiwan Island are between these values. The results clearly show that ε_Nd(0) volume in the northeastern South China Sea is higher than in the southwest part of the northern South China Sea, indicating different source rock types. The main source rock of the Pearl River sediment is carbonate types and affects sediment deposition in the southwest of the northern South China Sea, while the source rock in the inshore area of southern China, mainly Mesozoic and Cenozoic granites, contributes to sediments in the northeastern area of the northern South China Sea. Due to different source rock types, the HREE values and the volume of ε_Nd(0) in recent sediments supplied by the inshore area of southern China are higher than those from the Pearl River. Over 80% of the rapidly accumulated sediments on the northern slope of the South China Sea in the southeast of Dongsha Islands area were derived from Taiwan Island and from inshore areas of South China, with only less than 20% from the Pearl River. Supported by National Natural Science Foundation of China (Grant Nos. 40276019, 40238060 and 40621063), and National Basic Research Program of China (Grant No. 2007CB819501)     

Calcium isotope fractionation and its controlling factors over authigenic carbonates in the cold seeps of the northern South China Sea

In this study, we analyzed stable calcium isotope results of authigenic carbonates from two cold seep areas of the Dongsha area and the Baiyun Sag in the northern South China Sea. The stable isotopes of carbon and oxygen as well as the mineral composition of authigenic carbonates were used to investigate control calcium isotope fractionation. The δ44/40Ca ratios of the southwestern Dongsha area samples ranged from 1.21‰ to 1.52‰ and the ratio of the Baiyun Sag sample was 1.55‰ of the SRM915a isotope standard. X-ray diffraction analysis showed that the carbonate samples consisted of dolomite, calcite and aragonite, with small amounts of high-Mg calcite and siderite. The δ13C values of the carbonates of the southwestern Dongsha area varied between δ49.21‰ and δ16.86‰ of the Vienna PeeDee Belemnite (VPDB) standard and the δ18O values ranged from 2.25‰ to 3.72‰ VPDB. The δ13C value of the Baiyun Sag sample was 2.36‰ VPDB and the δ18O value was 0.44‰ VPDB. The δ13C values of the carbonates of the southwestern Dongsha area revealed there is methane seeping into this area, with a variable contribution of methane-derived carbon. The sampled carbonates covered a range of δ13C values suggesting a dominant methane carbon source for the light samples and mixtures of δ13C values for the heavier samples, with possibly an organic or seawater carbon source. The δ18O values indicated that there is enrichment in 18O, which is related to the larger oxygen isotope fractionation in dolomite compared to calcite. The results of the Baiyun Sag sample exhibited normal seawater carbon and oxygen isotopic values, indicating that this sample is not related to methane seepage but instead to precipitation from seawater. The relatively high δ44/40Ca values indicated either precipitation at comparatively high rates in pore-water regimes with high alkalinity, or precipitation from an evolved heavy fluid with high degrees of Ca consumption (Raleigh type fractionation). The dolomite samples from the Dongsha area revealed a clear correlation between the carbon and calcium isotope composition, indicating a link between the amount and/or rate of carbonate precipitation and methane contribution to the bicarbonate source. The results of the three stable isotope systems, mineralogy and petrography, show that mineral composition, the geochemical environment of authigenic carbonates and carbon source can control the calcium isotope fractionation.     

海水提升法试采南海天然气水合物的可行性分析

海洋天然气水合物的开采方法是当今天然气水合物研究的一大热点.通过对南海海域天然气水合物成藏机理的分析,提出了使用海水提升法开采海底天然气水合物的新模式.介绍了海水提升系统的组成和工作原理,提出了水力输送设备的相关参数,并对使用该方法的产气量及能效进行了简单评估.结果显示这种方法具有产量大、能效高的特点,运用该方法在南海采集天然气水合物是可行的.海水提升法开采天然气水合物克服了其他传统天然气水合物开采方法面临的技术难题,为海洋天然气水合物的开采研究提供了一种新思路及技术支撑.     

Lithospheric thermal-rheological structures of the continental margin in the northern South China Sea

Thermal structures of three deep seismic profiles in the continental margin in the northern South China Sea are calculated, their "thermal" lithospheric thicknesses are evaluated based on the basalt dry solidus, and their rheological structures are evaluated with linear frictional failure criterion and power-law creep equation. "Thermal" lithosphere is about 90 km in thickness in shelf area, and thins toward the slope, lowers to 60-65 km in the lower slope, ocean crust and Xisha Trough. In the mid-west of the studied area, the lithospheric rheological structure in shelf area and Xisha Islands is of four layers: brittle, ductile, brittle and ductile. Because of uprising of heat mantle and thinning of crust and lithosphere in Xisha Trough, the bottom of the upper brittle layer is only buried at 16 km. In the eastern area, the bottom of the upper brittle layer in the north is buried at 20 km or so, while in lower slope and ocean crust, the rheological structure is of two layers of brittle and ductile, and crust and uppermost mantle form one whole brittle layer whose bottom is buried at 30-32 km. Analyses show that the characteristics of rheological structure accord with the seismic result observed. The character of rheological stratification implies that before the extension of the continent margin, there likely was a ductile layer in mid-lower crust. The influence of the existence of ductile layer to the evolution of the continent margin and the different extensions of ductile layer and brittle layer should not be overlooked. Its thickness, depth and extent in influencing continent margin's extension and evolution should be well evaluated in building a dynamic model for the area.     

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

Heat flow plays an important role in the study of thermal structure and thermal evolution of 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.     

Diatoms from the southwestern continental slope, South China Sea, and their paleoenvironmental significance since the last glacial times

A quantitative study was undertaken on diatoms from cores (SA08-34) obtained from the southwestern continental slope of the South China Sea (SCS). A total of 165 diatom species belonging to 45 genera were identified. We constructed a stratigraphic subdivision and correlation according to the characteristics of diatom assemblages together with ¹⁴C dating and carbonate analysis. We also discuss the sedimentary environment in the sea area since the last glacial times. The research shows that the diatom assemblages coincide with interglacial and glacial times, and changes in diatom abundance reflect the instability of the climate in the southern part of the SCS, such that short-term, temperature descending events correlate with the interglacial interval. The abundance of diatoms is relevant to interglacial and glacial times, since high abundance values were associated with an interglacial interval, and low abundance values with the last glacial maximum. We assume that strong upwelling developed in the interglacial interval, the development of which was influenced by variations of monsoons in the East Asian region.     

喷射器内含湿含硫天然气形成水合物预测

针对高含硫气田天然气含湿含硫且井口压力不断降低的特点,利用高压气井的富余压力通过喷射器增压输送低压天然气。采用Fluent软件对喷射器内单相和气液两相含湿含硫天然气的沿程温度、压力进行了数值模拟,应用天然气水合物生成预测模型ZahediⅠ对天然气喷射器内部天然气水合物生成区域进行了预测分析,预测了工作流体入口温度、含硫量、含湿量对天然气水合物生成的影响。工作流体入口温度增加,喷射器内天然气水合物生成区域范围减小,硫化氢含量越高,天然气水合物生成区域范围越大,工作流体含水滴,喷射器内天然气水合物生成区域小于单相工质下的天然气水合物生成区域,进而提出了消除含湿含硫天然气喷射器内形成水合物的措施。     

南海北部陆坡NH-1孔沉积物中碳酸盐碳同位素特征及其地球化学意义

源于深部天然气藏渗漏或天然气水合物分解释放的甲烷,可导致海洋沉积物中生成δ13C明显负偏的碳酸盐矿物.对NH-1孔沉积物样品的碳酸盐含量、全岩碳酸盐及生物碳酸盐δ13C、有机碳含量等参数进行了分析.结果表明:沉积物中碳酸盐含量较高(平均6.90%);全岩碳酸盐δ13C(-6.09‰～-0.48‰)与正常海相碳酸盐相比明显负偏;浮游有孔虫(G.rube)壳体碳酸盐δ13C(-0.834‰～0.004‰)明显低于正常值.结合海域的地质特点,认为这很可能是沉积物中较高甲烷通量背景条件下自生碳酸盐的形成所导致的.有机碳、氮数据与全岩碳酸盐δ13C的相关性分析指示了有机质也可能是全岩碳酸盐δ13C偏低的根源之一.因此,NH-1孔碳酸盐碳同位素特征是较高甲烷通量背景下甲烷缺氧氧化-硫酸盐还原及有机质缺氧氧化-硫酸盐还原等地球化学过程的综合反映.全岩碳酸盐δ13C含量可用于指示沉积物中自生碳酸盐矿物及较高甲烷通量的存在.     

Carbon isotopic record of foraminifers in surface sediments from the South China Sea and its significance

Since the 1990s, the papers and data involved withthe South China Sea (SCS) have been emerging in largenumbers in the world as people pay more attention to thepaleoceanography of the SCS. There have been more than100 cores mentioned in papers containing p…     

Rifting process and formation mechanisms of syn-rift stage prolongation in the deepwater basin,northern South China Sea

Based on the latest seismic and geological data, tectonic subsidence of three seismic lines in the deepwater area of Pearl River Mouth Basin （PRMB）, the northern South China Sea （SCS）, is calculated. The result shows that the rifting process of study area is different from the typical passive continental margin basin. Although the seafloor spreading of SCS initiated at 32 Ma, the tectonic subsidence rate does not decrease but increases instead, and then decreases at about 23 Ma, which indicates that the rifting continued after the onset of seafloor spreading until about 23 Ma. The formation thickness ex- hibits the same phenomenon, that is the syn-rift stage prolonged and the post-rift thermal subsidence delayed. The formation mechanisms are supposed to be three： （1） the lithospheric rigidity of the northern SCS is weak and its ductility is relatively strong, which delayed the strain relaxation resulting from the seafloor spreading; （2） the differential layered independent extension of the lithosphere may be one reason for the delay of post-rift stage; and （3） the southward transition of SCS spreading ridge during 24 to 21 Ma and the corresponding acceleration of seafloor spreading rate then triggered the initiation of large-scale thermal subsidence in the study area at about 23 Ma.     

Oxygen and carbon isotope records of calcareous nannofossils for the past 1 Ma in the southern South China Sea

Oxygen and carbon isotopic analyses have been carried out on calcareous nannofossils from ODP Site 1143 in the southern South China Sea. The results demonstrate that the δ18O values of calcareous nannofossils for the past 1 Ma vary systematically with those of planktonic and benthic foraminifera from the same site. But the average δ18O value of nannofossils is 1.869‰ higher than that of planktonic foraminifera, and is 3.855‰ lower than that of benthic foarminifera. There are systematic differences between the δ13C values of nannofossils, planktonic foarminifera and benthic foraminifera. The average δ13C of nannofossils is 0.756‰ higher than that of benthic foraminifera, and is 0.460‰ lower than that of planktonic foraminifera. The δ13C values of nannofossils exhibit a significant positive shift beginning near isotope stage 14 and lasting until stage 8. This study also finds that there is a close ralationship between the δ13C of nannofossils and the percentage of Florisphaera profunda. This implies that the δ13C of nannofossils can be used as an indicator of sea water surface paleoproductivity.     

Characteristics of hydrocarbon accumulation and exploration potential of the northern South China Sea deepwater basins

Although the huge potential of the northern South China Sea deepwater basins has been proven by a series of discoveries that followed the exploration breakthrough of well LW 3-1-1, recent drilling and other studies have demonstrated the uniqueness and complicated nature of hydrocarbon accumulations of the deepwater basins there. Based on a review of previous work and the latest exploration activities and studies, the purpose of this paper is to discuss the critical controls for hydrocarbon accumulations in the deepwater basins of the northern South China Sea. A terrestrial-marine transitional coal-bearing source rock is proposed to be the primary source rock for the deepwater basins. A marine source rock, which was first identified as contributing to hydro-carbon generation in this region, probably plays a significant role in the deep-and ultra-deep water basins south to the Pearl River Mouth and Qingdongnan basins. The shelf margin delta depositional systems in the Baiyun Sag, sourced from the Pearl River, are currently primary exploration targets in the deepwater part of the Pearl River Mouth Basin, whereas the western Red River delta-ic-submarine fan depositional systems, initially proven by drilling, are the possible major exploration reservoirs in the Qing-dongnan deepwater areas. Current deepwater exploration targets at the large-sized structural traps and deep and ultra-deep areas in the south of the Pearl River Mouth and Qingdongnan basins will be the future exploration focus. Deepwater exploration activities and relevant fundamental studies, supporting and promoting each other, are of great importance to the national energy supply of China, the basic regional studies of the South China Sea, advancements in technology, and development of related deepwater industries, and will safeguard national sovereignty and territorial integrity.     

Barrier layer in the northeastern South China Sea and its formation mechanism

A VERTICALLY UNIFORM LAYER OF TEMPERATURE(ISOTHERMAL LAYER),SALINITY(ISOHALINE LAYER)AND DENSITY(MIXED LAYER)IS USUALLY FORMED IN THE UPPER OCEAN DUE TO THE WIND STIRRING.UNDERNEATH IS THE LAYER WITH RELATIVELY STRONG VERTICAL GRADIENT AS THE THERMOCLINE,…     

Distribution and identification of the low-velocity layer in the northern South China Sea

The low-velocity layer (LVL), closely related with tectonic activities and dynamic settings, has always been a hot topic in the deep crustal structure studies. The deep seismic (OBS/OBH) and onshore-offshore experiments have been extensively implemented in the northern South China Sea (SCS) since the 1990s. Six seismic profiles were finished on the northern margin of SCS by domestic and international cooperations. The features of crustal structures were revealed and five velocity-inversion layers were discovered. Among them three LVLs with 3.0—3.5 km?s^-1 velocity are located in the sedimentary structure (2.0—6.0 km in depth and 2.0—4.6 km in thickness) of the Yinggehai Basin and Pearl River Mouth Basin. They were identified by the reflective and refractive phases for their shallow depth. The other two LVLs with 5.5—6.0 km?s^-1 velocity generally existed in the middle crust (7.0—18.0 km in depth) with an about 2.5—6.0 km thickness in the transitional crustal structure of the northeastern and northwestern SCS. They were detected by the refractive phase from their overlain and underlying layers. We explored the possible tectonic formation mechanisms combining with previously reported results, which provided evidence for the formation and evolution of SCS.     

分形在南海北部陆坡断裂与天然气水合物关系研究中的应用

 根据分形几何学的原理,利用数盒子法计算了南海北部陆坡断裂的平面分布分维值,并以此探讨了断裂与天然气水合物的关系,结果表明：属被动陆缘的北部陆坡断裂越发育水合物异常越少,而属主动陆缘的台西南则断裂越发育水合物异常就越多。这一结果也反映了：被动陆缘环境的张性断层因缺乏相伴生的褶皱而不利于水合物的发育,并可能作为天然气的泄漏通道而破坏水合物成藏;主动陆缘环境的断裂以挤压逆冲断层为主,并伴生强烈的褶皱,其中断裂是深部烃源向上运移的通道,褶皱则是浅部天然气聚集成藏的圈闭。     

Distribution and identification of the low-velocity layer in the northern South China Sea

The low-velocity layer (LVL), closely related with tectonic activities and dynamic settings, has always been a hot topic in the deep crustal structure studies. The deep seismic (OBS/OBH) and onshore-offshore experiments have been extensively implemented in the northern South China Sea (SCS) since the 1990s. Six seismic profiles were finished on the northern margin of SCS by domestic and international cooperations. The features of crustal structures were revealed and five velocity-inversion layers were discovered. Among them three LVLs with 3.0—3. 5 km·s-1 velocity are located in the sedimentary structure (2.0—6.0 km in depth and 2.0—4. 6 km in thickness) of the Yinggehai Basin and Pearl River Mouth Basin. They were identified by the reflective and refractive phases for their shallow depth. The other two LVLs with 5.5—6.0 km·s-1 velocity generally existed in the middle crust (7.0—18.0 km in depth) with an about 2.5—6.0 km thickness in the transitional crustal structure of the northeastern and northwestern SCS. They were detected by the refractive phase from their overlain and underlying layers. We explored the possible tectonic formation mechanisms combining with previously reported results, which provided evidence for the formation and evolution of SCS.