Search

The South China Sea (SCS) serves as an excellent case for studying “source-to-sink” sedimentary dynamics among the global marginal seas. Analysis of the grain size distribution enables reconstruction of the sediment transport processes and dynamic mechanisms. In this study, based on 232 surface sediment samples obtained from the distal margin of the northern SCS, grain size trend analysis (GSTA) and end-member analysis (EMA) were first applied to the deep-sea environment to reveal the sedimentary dynamics around isolated carbonate platforms. The Zhongsha Atoll and Xisha Archipelagos were the two main sources of gravity flows in the study area. The GSTA results indicate the dominant transport trends that sediments initially originated from the Zhongsha Atoll and Xisha Archipelagos, dispersed in surrounding areas, and eventually formed a deposition center in the Zhongsha Trough. Furthermore, the sediment transport trends in the northern, eastern, and southern boundaries of the study area bounded by the oceanic basin were driven by deep circulation with seasonal variations. The grain size distributions were unmixed into five end-members, with dominant modes of 0.62, 1.89, 3.31, 7.14, and 10.17 Ф, corresponding to coarse sand, medium sand, fine sand, fine silt, and clay, respectively. Gravity flows and deep circulation have a significant impact on the distribution of end-members in the study area. The contents of EM1, EM2, and EM3 decreased while EM4 and EM5 contents increased outward from the Zhongsha Atoll to deep-sea areas. Sediments with coarse grains are primarily retained in the Zhongsha Atoll, and sediments with fine grains can be transported by gravity flows from platforms to deep-sea areas and removed by deep currents. The sediment transport trends were consistent with the results of sediment sources, seafloor topography, and current observations. The results show that GSTA is applicable in deep-sea environments and its combination with EMA helps to better understand sedimentary dynamics.

Distribution, saturation and hydrate indicator of free gas beneath gas hydrate stability zone have been the difficulty in the research of gas hydrate. Amplitude versus offset (AVO) and rock physics can be used to quantitatively interpret free gas saturation. In this paper, seismic data, acquired at K-G basin, India, have been processed to preserve true amplitude information. Based on the logging data and horizon calibration, free gas saturation of NGHP01-10A is assessed with effective medium theory and AVO modeling. It is found that free gas saturation is related to its distribution patterns. Assessments of free gas saturation in homogeneous and patchy distribution are 0.3%similar to 0.4% and 3%similar to 4%, respectively. The results show good agreement with the crossplot of Poisson's ratio which are calculated by the measurement at NGHP01-10D. Free gas is further deduced to be homogeneous distribution on the base of dry matrix Poisson's ratio with the saturation of 0.3%similar to 0.4%.

Three sediment cycles were identified by 3-D seismic data within early Pleistocene to present strata on the northwestern slope (15°3000-16°3000E) of the Qiongdongnan Basin. The sediment cycles are characterized by basal mass transport deposits (MTDs) overlain by channel-levee deposits (CLDs). MTDs are characterized by mounded, hummocky, chaotic, low amplitude, and discontinuous seismic reflection, with internal thrusts and basal grooves. CLDs exhibit parallel, continuous, and local translucent seismic reflection, with meandering channels, gull-wing and over-bank levees, crevasse splays, and slumps. Original sources were inferred as the mountains of central Vietnam by analyzing morphology, gravity fluid flow direction, and river distribution in the western region of the study area. The basic condition of sediment cycle formation is the high accumulation rate on the western margin shelf and slope of the Qiongdongnan Basin. Timing and mechanism of the sediment cycles suggest a causal link between pore pressure changes influenced by eccentricity scale(400-500 ka)sea-level change and sequential emplacement of MTDs and CLDs.

The drilling GMGS-1 reveals that gas hydrates are present above a stable zone with a thickness of 10 m at site SH3 in the Shenhu area, northern South China Sea. At this place, the gas hydrate saturation estimated from observed pore-water chloride values is up to 26% in the pore space. Higher resistivity, lower P-wave velocity and slightly increasing density characterize the gas hydrate-bearing sediments. To analyze this low P-wave velocity, the simplified three-phase Biot-type equation (STPBE) is used to calculate the water-saturated P-wave velocity. The results show that at the depth of 195 m, the P-wave velocity from wireline logging is lower than that of the water-saturated velocity of the normal sediments. The Archie's parameters used to calculate gas hydrate saturation from resistivity were defined from the cross plot between density porosity and formation factor. Gas hydrate saturation estimated from resistivity using the Archie equation with a=1.1 and m=2.3 is about 5%∼20% of the pore space, with a maximum value 26.8%, which shows that the distribution of gas hydrates is inhomogeneous in vertical direction. The dissociation of gas hydrates (the mixture of free gas and water) may be caused by drilling. Either the in-situ free gas or gas released from gas hydrate dissociation can cause the low well-log P-wave velocity. Due to surface seismic data were acquired before drilling, synthetic seismograms generated using different P-wave velocities are compared with the real seismic data, which can be used to identify the cause of the low P-wave velocity observed in the well log.

This work applies the finite difference tomographic method to invert P-wave velocity structure beneath the Tengchong area using earthquake data from a temporary network and other permanent stations, and analyzes the magmatic activity of Tengchong volcanoes and the deep structural genesis of the Longling M7.3/M7.4 earthquakes. The results show the velocity structure of the crust in this area has a pronounced heterogeneity. The lower velocities in the shallow subsurface are caused by unconsolidated basin deposits, Cenozoic volcanic deposits, fluid fractures and hot springs. The high-velocity zone at 5∼15 km depth probably represents cooled and solidified magmatic intrusions or less volatile ultramafic remnants. The low-velocity structure in the deep crust indicates molten or semi-molten magma. It is probable that the magmatic activity beneath volcanoes and magma intrusions in the deep crust of the Longling earthquake zone are from the same magmatic hearth area, which is the main area of the present magmatic activities. A strongly lateral heterogeneity is observed in the Longling earthquake zone. The east of the Nujiang fault belt and the west of the Longling fault belt are characterized by high velocity, implying large strain strength of rocks, which are the main carriers of stress accumulation. The low-velocity anomalies between the two faults extend to lower crust, probably caused by magmatic intrusions in the deep crust. The magmatic activity in this area is controlled clearly by the Longling fault and the Nujiang fault. The Longling M7.3 and M7.4 earthquakes occurred near the velocity boundary beneath the faults. The main reasons for the strong earthquakes in the Longling area inlcude the heterogeneity of stress accumulation caused by the lateral variation of the crustal medium strength, the deep magmatic aggregation and dynamic actions.

The South China Sea is the biggest marginal sea in the West Pacific.The environmental conditions are favorable there to the growth of reefs.The Dongsha uplift was a shallow water area far from provenance in the Miocene.The marine conditions including temperature,salinity and water depth were excellent for reef growth.Base on the integrated study of seismic,well logging and core data,and taking into account of relative sea level changes,we believe that there were plenty of reefs developed in different horizons in the Dongsha area,due to the complicated cooperation of relative sea level change and regional tectonic evolution,which made the reef rocks good reservoir with high porosity and permeability.

Numerical simulation study of thermal structure is a supplement to field observations and an important method to verify geodynamic models. In this study, based on geological and geophysical conditions, we construct two thermal models across the East Volcano Chain (EVC) and the West Volcano Chain (WVC) along the Manila trench subduction zone and do the thermal simulations using finite-element numerical method. During these simulations, we study the thermal structure of the subduction zones affected by the frictional and shear heating and simulate the thermal structure of the subduction zones beneath the seismic lines at EVC and WVC. We also employ the results of rock experiments to predict the regions of dehydration and partial melting in the subducting slab. The simulation results show that, at 100 km depth, at the slab interface with frictional and shear heating the temperature is 865 degrees C, about 95 degrees C warmer than temperature without frictional and shear heating. Considering the frictional and shear heating, the temperatures at the slab interface are 865 degrees C and 895 degrees C and at the base of subducting oceanic crust are 560 degrees C and 605 degrees C at 100 km beneath the seismic lines of EVC and WVC. At slab surface, the depth of dehydration onset is less than 50 km. However, partial melting and dehydration may mainly occur at about 100 km depth, which is well associated to the observed location of the volcanic activity.

Mass transport deposits (MTDs) are one kind of deepwater sedimentary systems driven by the gravity flow action, and play an important role for the deep-water sedimentation in global continental margins. MTDs are not only one of the most serious deep-water geo-hazards, but also have close relationship with the formation and accumulation of marine gas hydrates and deepwater hydrocarbon. Based on the topographic and high resolution seismic data in the hydrocarbon exploration region, we identified the characteristics and distribution of MTDs in the northern slope of the South China Sea. MTDs are characterized by apparent geomorphology and sedimentary structures. In MTDs there are well developed extensional faults in the head and slides and turnover blocks in the middle. Compressive structures and thrust faults are formed in the toe region of MTDs. Several mechanisms can account for development of MTDs. However, Baiyun MTDs were inferred to be dominated by the dissolution of gas hydrate as suggested by the model of gas hydrate reservoirs and numerical modeling using the elastic-plastic finite element method for MTDs in the Baiyun sag. Six different gas hydrate dissolution models are chosen to calculate the safety factors. We found that the stability of submarine slope decreases with the increasing dissolution of gas hydrate.

Carbonate buildups started growing up in the Late Oligocene on the edge of the Xisha Islands and structural highs.Sequence analysis and drilling well facies analysis suggest that a carbonate buildup may consist of various facies,such as the lime sand island facies,reef flat facies,bindstone facies,reef framework facies and backreef lagoon facies etc.In seismic profiles,the carbonate platform usually occurs as a positive landform with high amplitude at the top,low amplitude and chaotic reflection inside,and an ambiguous boundary at the bottom.Large scale carbonate buildups usually have high amplitude,continuous reflection and a flat top.We proposed in this study a sedimentary model for the Xisha carbonate buildup with respect to relative sea level changes.The rise of relative sea level leads to the retreat of carbonate buildup toward the structure highs,while dropping of the relative sea level causes the lateral accretion of the carbonate buildup towards the sea,leaving a lagoon behind the buildup.Besides,tectonic movement,terrigenous clastic input and temperature and salinity of sea water also influence the development of the carbonate buildup.

The Xisha carbonate platforms, which include the modern-day Xisha Atoll, occur upon the northern continental margin of the South China Sea (SCS). In this study, we identify the seismic characteristics about various sedimentary facies of the carbonate platform and different types of reef in this area, based on the 2-D seismic data, reefs show high-amplitue moundy continuous reflection at the top and weak chaotic reflections inside. Carbonate platforms show high-amplitude continuous reflection at the top and high reflection alternative with weak reflection. The above analysis provided information about the influence of fluctuations in relative sea level, tectonic movements, paleotopography and the development mechanism of the carbonate platform. Based on the seismic data and data from drillholes (Xichen-1. Xiyong-1), we propose a schematic sedimentary model of the Xisha carbonate platforms at the northern margin of the SCS and outline five stages of development for the carbonate platform. Its sedimentary evolution consists of Initial establishment stage, development stage, exposure stage, drowning stage and large atoll reefs development stage. We also propose that several phases tectonisms supply proper environment and structural position for carbonate platform development, they can also destroyed the exist platforms. Besides, eustasy change was also the main influential factor on the development of the platforms. (C) 2010 Elsevier Ltd. All rights reserved.

Recently, as oil exploitation has become focused on deepwater slope areas. more multi-channel high resolution 2D and 3D seismic data were acquired in the deepwater part of the Qiongdongnan Basin, northern South China Sea. Based on 3D seismic data and coherence time slice, RMS and 3D visualization, a series of deepwater channels were recognized on the slope that probably developed in the late Quaternary period. These channels trend SW-NE to W-E and show bifurcations, levees, meander loops and avulsions. High Amplitude Reflections (HARs), typical for channel-levee complexes, are of only minor importance and were observed in one of the channel systems. Most of the detected channels are characterized by low-amplitude reflections, and so are different from the typical coarse-grained turbidite channels that had been discovered worldwide. The absence of well data in the study area made it difficult to determine the age and lithology of these channels. Using a neighboring drill hole and published data about such depositional systems worldwide, the lithology of these channels is likely to be dominated by mudstones with interbedded thin sandstones. These channels are formed by turbidity currents originated from the little scale mountain river of mid-Vietnam in SW direction and were probably accompanied by a relative sea level drop in the last glacial age. These channels discovered on the northern South China Sea slope are likely to be fine-grained, mud-dominant and low N:G deposits in a deepwater paleogeographic setting. (C) 2009 Elsevier Ltd. All rights reserved.

Many mud diapirs have been recognized in southern Okinawa Trough by a multi-channel seismic surveying on R/V KEXUE I in 2001. Gas hydrates have been identified, by the seismic reflection characteristics, the velocity analysis and the impedance inversion. Geothermal heat flow around the central of the mud diapir has been determined theoretically by the Bottom Simulating Reflectors (BSRs). Comparing the BSR derived and the measured heat flow values, we infer that the BSR immediately at the top of the mud diapirs indicate the base of the saturated gas hydrate formation zone (BSGHFZ), but not, as we ordinarily know, the base of the gas hydrate stability zone (BGHSZ), which could be explained by the abnormal regional background heat flow and free gas flux associated with mud diapirs. As a result, it helps us to better understand the generation mechanism of the gas hydrates associated with mud diapirs and to predict the gas hydrate potential in the southern Okinawa Trough.

The northern South China Sea margin has experienced a rifting stage and a post-rifting stage during the Cenozoic. In the rifting stage, the margin received lacustrine and shallow marine facies sediments. In the post-rifting thermal subsidence, the margin accumulated shallow marine facies and hemipelagic deposits, and the deepwater basins formed. Petroleum systems of deepwater setting have been imaged from seismic data and drill wells. Two kinds of source rocks including Paleogene lacustrine black shale and Oligocene-Early Miocene mudstone were developed in the deepwater basin of the South China Sea. The deepwater reservoirs are characterized by the deep sea channel fill, mass flow complexes and drowned reef carbonate platform. Profitable capping rocks on the top are mudstones with huge thickness in the post-rifting stage. Meanwhile, the faults developed during the rifting stage provide a migration path favourable for the formation of reservoirs. The analysis of seismic and drilling data suggests that the joint structural and stratigraphic traps could form giant hydrocarbon fields and hydrocarbon reservoirs including syn-rifting graben subaqueous delta, deepwater submarine fan sandstone and reef carbonate reservoirs.

The West Philippine basin (WPB) is a currently inactive marginal basin belonging to Philippine Sea plate, which has a complex formation history and various crust structures. Based on gravity, magnetic and seismic data, the tectonics in West Philippine basin is characterized by amagma spreading stage and strike slip fractures. NNE trending Okinawa-Luzon fracture zone is a large fracture zone with apparent geomorphology and shows a right-handed movement. The results of joint gravity-magnetic-seismic inversion suggest that the Okinawa-Luzon fracture zone has intensive deformation and is a transform fault. Western existence of the NW trending fractures under Ryukyu Islands Arc is the main cause of the differences between south and north Okinawa Trough. The Urdaneta plateau is not a remained arc, but remnant of mantle plume although its lava chemistry is similar to oceanic island basalt (OIB).

Sedimentary basins in the Yellow Sea can be grouped tectonically into the North Yellow Sea Basin (NYSB), the northern basin of the South Yellow Sea (SYSNB) and the southern basin of the South Yellow Sea (SYSSB). The NYSB is connected to Anju Basin to the east. The SYSSB extends to Subei Basin to the west. The acoustic basement of basins in the North Yellow Sea and South Yellow Sea is disparate, having different stratigraphic evolution and oil accumulation features, even though they have been under the same stress regime since the Late Triassic. The acoustic basement of the NYSB features China-Korea Platform crystalline rocks, whereas those in the SYSNB and SYSSB are of the Paleozoic Yangtze Platform sedimentary layers or metamorphic rocks. Since the Late Mesozoic terrestrial strata in the eastern of the NYSB (West Korea Bay Basin) were discovered having industrial hydrocarbon accumulation, the oil potential in the Mesozoic strata in the west depression of the basin could be promising, although the petroleum exploration in the South Yellow Sea has made no break-through yet. New deep reflection data and several drilling wells have indicated the source rock of the Mesozoic in the basins of South Yellow Sea, and the Paleozoic platform marine facies in the SYSSB and Central Rise could be the other hosts of oil or natural gas. The Mesozoic hydrocarbon could be found in the Mesozoic of the foredeep basin in the SYSNB that bears potential hydrocarbon in thick Cretaceous strata, and so does the SYSSB where the same petroleum system exists to that of oil-bearing Subei Basin.

Processing of a recently acquired seismic line in the northeastern South China Sea by Project 973 has been conducted to study the character and the distribution of gas hydrate Bottom-Simulating Reflectors (BSRs) in the Hengchun ridge. Analysis of different-type seismic profiles shows that the distribution of BSRs can be revealed to some extents by single-channel profile in this area, but seismic data processing plays an important role to resolve the full distribution of BSRs in this area. BSR' s in the northeastern South China Sea have the typical characteristics of BSRs on worldwide continental margins: they cross sediment bed reflections, they are generally parallel to the seafloor and the associated reflections have strong amplitude and a negative polarity. The characteristics of BSRs in this area are obvious and the BSRs indicate the occurrence of gas hydrate-bearing sediments in the northeastern South China Sea. The depth of the base of the gas-hydrate stability zone was calculated using the phase stability boundary curve of methane hydrate and gas hydrate with mixture gas composition and compared with the observed BSR depth. If a single gradient geothermal curve is used for the calculation, the base of the stability zone for methane hydrate or gas hydrate with a gas mixture composition does not correspond to the depth of the BSRs observed along the whole seismic profile. The geothermal gradient therefore changes significantly along the profile. The geothermal gradient and heat flow were estimated from the BSR data and the calculations show that the geothermal gradient and heat flow decrease from west to east, with the increase of the distance from the trench and the decrease of the distance to the island arc. The calculated 2 heat flow changes from 28 to 64 mW/m(2), which is basically consistent with the measured heat flow in southwestern offshore Taiwan.

Based on detailed processing of the gravity, magnetic, electric and seismic profile data and by applying synthetic geophysical inversion technique and interpretation, this paper studies the deep structures and tectonic styles of the Hefei Basin and the southern Tanlu fault zone. In the Hefei basin, the thrust faults occurred in the strata below Indo-China movement unconformity, while rifting system developed in the upper strata above the unconformity. Tectonic reversion in the Hefei basin happened in the late Jurassic. Rifting basin was developed in the early Cretaceous, but a reversion occurred at the latest of the early Cretaceous, and led to thrust and nappe structures. The Tanlu fault zone that bounds the Heifei basin on the east displays a style of normal faulting in the shallow part and reverse faulting in the deep part, which records the change in tectonic regimes during the Mesozoic in East China.

Side-scan sonar data collected by Cruises 99-09 Leg 2 and 00-06 Leg 1 of R/V Yokosuka were used to reveal the sedimentary processes in Zenisu deep-sea channel. The middle and lower segments of the channel are rich in turbidite and other debrite deposits. By high-resolution imaging, three sedimentary processes were distinguished with distinct acoustic features. 1. Slumps and slides occur with contrasting backscatter, rough surface textures, blockings, and acoustic shadows at head walls. They are very extensive and often in lobate form downslope. 2. Debris flow has uniform, general medium backscatter, sometimes showing marbling/lineation in lobate form. 3. Turbidity current is characterized by low backscatter confined to the channel as acoustic signal is attenuated. Regional tectonics must be the dominating factor that controls deposition pattern in this area.

Dongsha Island and the adjacent sea area locate at the northern continental margin of the South China Sea (SCS), and is connected to the east by the Manila Trench. Analyses of seismic stratigraphy and gravity, magnetic and drilling wells data led to the discovery of three post-fault sequences (V, VI, VII). Extensive tectonic uplift, magma activity and erosion occurred in Dongsha Island and the adjacent area, where most of the faults in the northeastern SCS were still active during Pliocene and Quaternary. Two groups of faults trending NEE and NW were developed during Late Cenozoic. We conclude that three important tectonic movements, especially Dongsha movement (4.4–5.2 Ma) and Liuhua movement (1.4–1.89 Ma), controlled the structural framework in the Dongsha rise; whose deformation in the east is stronger than that in the west and whose stress field variation suggests that the tectonic uplift in the study area contributed to magmato-tectonic events correlated to the main collision phases between the East China and Taiwan 5-3 and 3-0 Ma ago.

Seismic stratigraphic analysis of 150 km of high-resolution seismic profiles (GEOPULSE and 3.5 kHz) obtained along the muddy coast off Taishan, Guangdong provided valuable information on the offshore Quaternary stratigraphy, tectonics, sedimentary evolution, and environmental and engineering geology of this area. The following late Quaternary stratigraphic sequences were mapped. (1) Pleistocene alluvial sitty sand and shallow marine clay; (2) coarse fluvial channel sand; (3) nearshore and bay clayey sediments. The incision of fluvial channels occurred in two stages: during the Late Pleistocene and at the beginning of the Holocene. The Pleistocene fluvial channels have been filled by coarse sand overlain by more than 10 m of late Pleistocene clay and 20–30 m of Holocene clay. The coastal area is dissected by two major fault systems: the first group strikes ENE-WSW and controls the evolution of the adjacent Quaternary basin, while the second one trends NNE-SSW. These fault systems must be taken into account in any coastal engineering considerations. In addition, both granitic and metamorphic basements were recognized.

Seventy-seven surface sediment samples and core samples from the outer Sunda Shelf were analyzed and a number of seismic profiles of the shelf were interpreted. The bottom sediments could be divided into six types: terrigenous sand, biogenic sand, silt-sand, clay-silt-sand, clayey silt and coral reef detritus. Our seismic data showed a thick, prograding Pleistocene deltaic sequence near the shelf-break and a thin Holocene sedimentary layer on the outer shelf. Eleven thermoluminescence (TL) ages were determined. The oldest relict sediments were derived from Late Pleistocene deposits. Based on sediment types, ages, and origins, five sedimentary areas were identified: area of modern Mekong sediments; insular shelf area receiving modern sediments from small Borneo rivers; shelf area near the Natuna-Anambas islands in the southeastern Gulf of Thailand Basin off the Malay Peninsula; area of relict sediments on the outer shelf north of the Natuna Islands, and typical coral reefs and detritus sediments.