AVO character research of natural gas hydrates in the East China Sea

Natural gas hydrates are considered as strategic resources with commercial potential in the 21st century. Obvious BSR characteristics will be shown on seismic profiles, if there exist natural gas hydrates. The AVO method is one of the methods which can be used to identify and forecast lithologic characteristics and fluid properties by using the relationship between Amplitude and Offset. AVO anomaly is one of the significant signs to check out whether or not there is free gas below the BSR, so it can be used to detect natural gas hydrates from the seismic profile. Considering the geological and geophysical characteristics of the Okinawa Trough and making use of the techniques mentioned above, we can conclude that the conditions there are favorable for the formation and concentration of natural gas hydrates. By analyzing the data collected from the study area, one can discover many different anomalous phenomena on the seismic profile which are related to the existence of natural gas hydrates. Preliminary estimation of the natural gas hydrates in the Okinawa Trough shows that the trough is rich in natural gas hydrates and may become a potential important resources exploration area.     

歧口凹陷深层地震岩石物理特征分析及应用

为明确歧口凹陷深层的岩石物理及AVO响应特征,分析歧口地区全波测井资料,优选该区敏感岩石物理参数;根据分岩性拟合求取纵横波转换经验公式,并采用流体置换原理进行叠前地震道集正演后提取AVO属性.结果表明:泊松比、纵横波速度比为该区有利储层预测的敏感参数,含气砂岩与含水砂岩的数值范围差异明显;歧口深层AVO响应特征可划分为3类:第一类是与气层相对应的反射振幅随着偏移距的增大而负向急剧增大;第二类是与油气同层相对应的反射振幅随着偏移距的增大而负向缓慢增大;第三类是与油水层及干层相对应的反射振幅随着偏移距的增大而负向减小.该结果可为歧口凹陷深层开展流体预测可行性提供依据.     

Distribution and Characteristics of Hazardous Geological Features in the Marine Coastal and Offshore Areas of Zhejiang Province,East China Sea

Newly acquired high-resolution shallow seismic profiles(7069 km in length) in the coastal and offshore areas of Zhejiang Province, East China Sea, China, have revealed eight marine hazardous geological features: shallow gas, sand ridges, erosion ditches, scarps, irregular bedrock features, underwater shoals, buried paleo-channels, and submarine deltas. Based on the seismic profiles, we have constructed a marine geological map of these hazardous features. Shallow gas accumulations are common and occur mainly in two separate nearshore regions that cover 4613 and 3382 km~2 respectively. There are also scattered shallow gas accumulations in the offshore area, typically accompanied by paleo-channels that occur mainly in the middle of the study area. Sand ridges, erosion ditches, scarps, and irregular bedrock features are found mainly in the northeast of the study area in association with each other. In the southeastern part of the study area, the sand ridges have a linear form and trend NW–SE, representing the western part of the linear sand ridges in the East China Sea. The maximum slope gradient is 1?, which suggests that this area is prone to landslides. These hazardous marine geological features are important to marine and engineering activities in this region.     

Estimation of potential distribution of gas hydrate in the northern South China Sea

Gas hydrate research has significant importance for securing world energy resources, and has the potential to produce considerable economic benefits. Previous studies have shown that the South China Sea is an area that harbors gas hydrates. However, there is a lack of systematic investigations and understanding on the distribution of gas hydrate throughout the region. In this paper, we applied mineral resource quantitative assessment techniques to forecast and estimate the potential distribution of gas hydrate resources in the northern South China Sea. However, current hydrate samples from the South China Sea are too few to produce models of occurrences. Thus, according to similarity and contrast principles of mineral outputs, we can use a similar hydrate-mining environment with sufficient gas hydrate data as a testing ground for modeling northern South China Sea gas hydrate conditions. We selected the Gulf of Mexico, which has extensively studied gas hydrates, to develop predictive models of gas hydrate distributions, and to test errors in the model. Then, we compared the existing northern South China Sea hydrate-mining data with the Gulf of Mexico characteristics, and collated the relevant data into the model. Subsequently, we applied the model to the northern South China Sea to obtain the potential gas hydrate distribution of the area, and to identify significant exploration targets. Finally, we evaluated the reliability of the predicted results. The south seabed area of Taiwan Bank is recommended as a priority exploration target. The Zhujiang Mouth, Southeast Hainan, and Southwest Taiwan Basins, including the South Bijia Basin, also are recommended as exploration target areas. In addition, the method in this paper can provide a useful predictive approach for gas hydrate resource assessment, which gives a scientific basis for construction and implementation of long-term planning for gas hydrate exploration and general exploitation of the seabed of China.     

Processing and interpretation of data from a sonobouy refraction seismic section in the East China Sea shelf marginal ridge

Refraction seismic data obtained from a sonobuoy (SB7) in the East China Sea shelf marginal ridge was processed and interpreted. Intercept times and layer velocities were used to set up a one-dimension depth-velocity model based on which a two-dimenssion depth-velocity crustal model was established through two-dimension ray-tracing. The computation results showed that the upper crustal depth-velocity model beneath lineSB7 is composed of four layers with primary wave velocities of 1.80–2.20, 2.85–2.90, 3.24–3.34 and 4.55–6.00 m/s respectively. Neighbouring reflection seismic data and data on the East China Sea (ECS) regional geology were used to study the relationship between the velocities and their formation ages and also the contact relationship between the velocity boundaries. Contribution No. 3259, from the Institute of Oceanology, Chinese Academy of Sciences. Project 49376266 supported by NSFC.     

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PROCESSING AND INTERPRETATION OF DATA FROM A SONOBUOY REFRACTION SEISMIC SECTION IN THE EAST CHINA SEA SHELF MARGINALRIDGE

INTRODUCTIONThemannerefraCtionseisAncmehodisirnP0Ftantf0rstudyofmarinecrustandregionalgeo1ogicalteCtonics.Mufauchi(1968),Leyden(l973),Ludwig(l973),JinXianglongetal.(l986)reportedsonobu0yreffoCtionseisAncrmurernentSintheEesandtheOkinawaTroughregion,studiedthetaltatarysequenceandthickness0fthebasernent,andcomPataltheveoCityugeoftheuppercrustallayers.beetal.(l98O)werethefustto0btainaMohodepthofl5kminthecentralaxisoftheOkinawaTroughbyusingtwo-shipreffoCtionseisAncrneh0ds.Usingl8ocanb…     

Late Cenozoic Tectonic Deformation in the Dongsha Islands and Adjacent Sea 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.     

Focused fluid flow in the Baiyun Sag, northern South China Sea: implications for the source of gas in hydrate reservoirs

The origin and migration of natural gas and the accumulation of gas hydrates within the Pearl River Mouth Basin of the northern South China Sea are poorly understood. Based on high-resolution 2D/3D seismic data, three environments of focused fluid flow: gas chimneys, mud diapirs and active faults have been identified. Widespread gas chimneys that act as important conduits for fluid flow are located below bottom simulating reflections and above basal uplifts. The occurrence and evolution of gas chimneys can be divided into a violent eruptive stage and a quiet seepage stage. For most gas chimneys, the strong eruptions are deduced to have happened during the Dongsha Movement in the latest Miocene, which are observed below Pliocene strata and few active faults develop above the top of the Miocene. The formation pressures of the Baiyun Sag currently are considered to be normal, based on these terms: 1) Borehole pressure tests with pressure coefficients of 1.043–1.047; 2) The distribution of gas chimneys is limited to strata older than the Pliocene; 3) Disseminated methane hydrates, rather than fractured hydrates, are found in the hydrate samples; 4) The gas hydrate is mainly charged with biogenic gas rather than thermogenic gas based on the chemical tests from gas hydrates cores. However, periods of quiet focused fluid flow also enable the establishment of good conduits for the migration of abundant biogenic gas and lesser volumes of thermogenic gas. A geological model governing fluid flow has been proposed to interpret the release of overpressure, the migration of fluids and the formation of gas hydrates, in an integrated manner. This model suggests that gas chimneys positioned above basal uplifts were caused by the Dongsha Movement at about 5.5 Ma. Biogenic gas occupies the strata above the base of the middle Miocene and migrates slowly into the gas chimney columns. Some of the biogenic gas and small volumes of thermogenic gas eventually contribute to the formation of the gas hydrates.     

Distinct BSRs and their implications for natural gas hydrate formation and distribution at the submarine Makran accretionary zone

To investigate the nature of gas hydrates in the Makran area,new high-resolution geophysical data were acquired between 2018-2019.The data collected comprise multibeam and two-dimensional multi-channel seismic reflection data.The multibeam bathymetry data show East-North-East(ENE) ridges,piggy-back basins,canyon and channel systems,and the morphology of the abyssal plain.Continuous and discontinuous bottom simulating reflectors(BSRs) occur in the piggy-back basins on most of the seismic profiles available.The BSRs cut the dipping layers with strong amplitude and reversed polarity.Discontinuous BSRs indicate a transition along a dipping high-permeable sand layers from gas-rich segment to the gas hydrate-bearing segment and sugge st alternating sediments of fine and relatively coarse grain size.Double BSRs are highly dynamic and attributed to slumps occurring in the study area.The BSRs induced by slumps are located both at deep and shallow depths,responding to the temperature or pressure variation.For the first time,BSRs are observed in the abyssal plain of the Makran area,being associated with anticline structures,which do not show large spatial continuity and are strongly conditioned by structural conditions such as anticlines and fluid migration pathways,including deep fault,gas chimney,and high-permeable sedimentary layer.Our results may help to assess the gas hydrate potential within the piggy-back basins and to determine the most promising target areas.Moreover,results about the abyssal plain BSR may help to locate hydrocarbon reservoirs in the deep ocean.     

Offshore Fault Geometrics in the Pearl River Estuary,Southeastern China: Evidence from Seismic Reflection Data

The Pearl River Estuary (PRE) is located at the onshore-offshore transition zone between South China and South China Sea Basin, and it is of great significant value in discussing tectonic relationships between South China block and South China Sea block and seismic activities along the offshore active faults in PRE. However, the researches on geometric characteristics of offshore faults in this area are extremely lacking. To investigate the offshore fault distribution and their geometric features in the PRE in greater detail, we acquired thirteen seismic reflection profiles in 2015. Combining the analysis of the seismic reflection and free-air gravity anomaly data, this paper revealed the location, continuity, and geometry of the littoral fault zone and other offshore faults in PRE. The littoral fault zone is composed of the major Dangan Islands fault and several parallel, high-angle, normal faults, which mainly trend northeast to northeast-to-east and dip to the southeast with large displacements. The fault zone is divided into three different segments by the northwest-trending faults. Moreover, the basement depth around Dangan Islands is very shallow, while it suddenly increases along the islands westward and southward. These has resulted in the islands and neighboring areas becoming the places where the stress accumulates easily. The seismogenic pattern of this area is closely related to the comprehensive effect of intersecting faults together with the low velocity layer.     

东海盆地西湖凹陷沉积记录的天文旋回响应

基于东海陆架盆地古新世-始新世前人解释的三级层序成果,认为东海陆架盆地现有的三级层序划分不足以满足当下研究的需要,由此开展了旋回地层与层序地层研究,重新将三级层序界面的与基于对岩性及地震相特征变化的解释相结合,加入了天文旋回驱动的机制,阐明海平面变化驱动三级层序的特征。以东海陆架盆地西湖凹陷的BSH-1井及其相邻的NB-25-2-1井为研究对象,结合地层学研究基础,选取GR测井曲线为古气候替代指标,应用旋回地层学的理论及其时间序列分析、频谱分析技术的方法,对两口典型钻井进行了天文旋回分析,借助邻井时间锚点建立了有效的天文年代标尺,探讨了斜率周期信号变化的振幅调制的约1.2 Ma长周期与海平面变化以及三级层序发育的关系,认为东海陆架盆地三级层序受控于稳定的约1.2 Ma的斜率振幅调制周期。最终形成一套绝对天文年代标尺与一套三级层序划分方案。      

高速岩体反射地震探测中的数据采集与处理

为了调查内蒙古阿拉善某高放废物地质处置库预选场址中目标花岗岩体的空间展布、内部结构和边界接触关系,应用人工反射地震探测技术,采集数据并进行处理,获得了针对大型高速岩体的近地表速度剖面和地震反射波法成像剖面。针对崎岖地表工区近地表速度建模与层析静校正、强散射干扰波压制等问题,引进基于MSFM射线追踪算法的初至波旅行时层析反演与层析静校正技术、改进的矢量分解压噪方法等,并在常规高分辨反射地震数据处理流程的基础上形成了适合于高速岩体地震探测的精细数据处理流程。     

Characters of the Pco2 and CO2 Flux in the East China Sea in Autumn

Pco2 of air and seawater samples from the East China Sea(ECS) were measured in situ in autumn, 1994,Ocean currents,terrestrial fluviation,biological activities,etc.,Pco2 char-acters in air and seawater were investigated,CO2 flux and its character in the East China Sea are discussed on the basis of the Pco2 profiles of air and seawater,It was clear that the nearshore was the source of CO2;and tht the oulter sea area was the sink of CO2; and that the shelf area of the EXS is a net sink for atmospheric CO2 in autumn.     

Chemical hydrography of coastal upwelling in the East China Sea

Based on the field data obtained during cruises on the shelf of the East China Sea from 1997 to 1999, seasonal variations of coastal upwelling on the inner shelf are discussed by using cross-shelf transect profiles and horizontal distributions of chemical and hydrographic variables. Results show that the coastal upwelling was year-round, but the areas and intensities of the upwelling were quite different in season. The coastal upwelling occurred in all of the coastal areas of the region in spring and summer, but in autumn only in the area off Zhejiang Province, and in winter in the area off Fujian Prov- ince. It was the strongest in summer and the weakest in winter. Geographically, it was the strongest in the area off Zhejiang Province and the weakest in the southmost or northmost parts of the East China Sea. The estimated nutrient fluxes upward into euphotic zone through coastal upwelling were quite large, es- pecially for phosphate, which contributed significantly to primary production and improved the nutrient structure of the coastal ecosystem in the East China Sea.     

Numerical simulation for accuracy of velocity analysis in small-scale high-resolution marine multichannel seismic technology

When used with large energy sparkers, marine multichannel small-scale high-resolution seismic detection technology has a high resolution, high-detection precision, a wide applicable range, and is very flexible. Positive results have been achieved in submarine geological research, particularly in the investigation of marine gas hydrates. However, the amount of traveltime difference information is reduced for the velocity analysis under conditions of a shorter spread length, thus leading to poorer focusing of the velocity spectrum energy group and a lower accuracy of the velocity analysis. It is thus currently debatable whether the velocity analysis accuracy of short-arrangement multichannel seismic detection technology is able to meet the requirements of practical application in natural gas hydrate exploration. Therefore, in this study the bottom boundary of gas hydrates (Bottom Simulating Reflector, BSR) is used to conduct numerical simulation to discuss the accuracy of the velocity analysis related to such technology. Results show that a higher dominant frequency and smaller sampling interval are not only able to improve the seismic resolution, but they also compensate for the defects of the short-arrangement, thereby improving the accuracy of the velocity analysis. In conclusion, the accuracy of the velocity analysis in this small-scale, high-resolution, multi-channel seismic detection technology meets the requirements of natural gas hydrate exploration.     

A probe heat flow value of the East China Sea shelf

Bottom temperature variation (BTV) is a serious problem in determining the thermal gradient and heart flow of the sediments in shallow seas. The water depth of the East China Sea shelf is mostly below 150m, and the heat flow measurement is strongly affected by BTV. Following a statistical algorithm, we rechecked the temperature and thermal conductivity data of the cruises KX90-1 and KX91-1, carried out by a cooperation program of China and Japan, and calculate the heat flow in a site without long-term temperature record. The calculated heat flow in the site was 58.6±3.6 mW/m², being just within the range of the drill heat flow value of East China Sea shelf. The inversed amplitude spectrum of BTV has a peak in frequency of 1/10 per year, and the annual component is also an important part. Comparison with two lakes of Lake Greifensee and Lac Leman (i.e. Lake Geneva), which are in different water depth, revealed that with increasing water depth, the peak of amplitude spectrum moved towards low frequency components. The heat flow values calculated in this paper and from petroleum bore hole in East China Sea shelf are much more close to that in southeast China than in Okinawa Trough. Supported by the National High Technology R&D Program of China (2004 AA616060)     

Prediction for potential landslide zones using seismic amplitude in Liwan gas field,northern South China Sea

The Liwan (Lw) gas field located in the northern slope of the South China Sea (SCS) is extremely complex for its sea-floor topograghy, which is a huge challenge for the safety of subsea facilities. It is economically impractical to obtain parameters for risk assessment of slope stability through a large amount of sampling over the whole field. The linkage between soil shear strength and seabed peak amplitude derived from 2D/3D seismic data is helpful for understanding the regional slope-instability risk. In this paper, the relationships among seabed peak, acoustic impedance and shear strength of shallow soil in the study area were discussed based on statistical analysis results. We obtained a similar relationship to that obtained in other deep-water areas. There is a positive correlation between seabed peak amplitude and acoustic impedance and an exponential relationship between acoustic impedance and shear strength of sediment. The acoustic impedance is the key factor linking the seismic amplitude and shear strength. Infinite slope stability analysis results indicate the areas have a high potential of shallow landslide on slopes exceeding 15° when the thickness of loose sediments exceeds 8 m in the Lw gas field. Our prediction shows that they are mainly located in the heads and walls of submarine canyons.     

Accumulation and exploration of gas hydrate in deep-sea sediments of northern South China Sea

The large deep-sea area from the southwestern Qiongdongnan Basin to the eastern Dongsha Islands,within the continental margin of northern South China Sea,is a frontier of natural gas hydrate exploration in China.Multiform of deep-sea sedimentations have been occurred since late Miocene,and sediment waves as a potential quality reservoir of natural gas hydrate is an most important style of them.Based on abundant available data of seismic,gravity sampling and drilling core,we analyzed the characteristics of seismic reflection and sedimentation of sediment waves and the occurrence of natural gas hydrate hosted in it,and discussed the control factors on natural gas hydrate accumulation.The former findings revealed the deep sea of the northern South China Sea have superior geological conditions on natural gas hydrate accumulation.Therefore,it will be of great significance in deep-sea natural gas hydrate exploration with the study on the relationship between deep-sea sedimentation and natural gas hydrate accumulation.     

浅地震反射剖面揭示地震小区划覆盖层厚度及隐伏断裂——以九江芳兰规划区为例

以九江芳兰规划区为例，利用浅层地震反射方法获取小区划工区的覆盖层厚度，并查明周边隐伏断裂位置及特征。结合钻孔资料对比解译，对地震小区划工作提供基础地震地质信息，同时为该区的活断层探测和地震危险性评价提供参考资料。