Gravity and magnetic anomalies field characteristics in the South China Sea and its application for interpretation of igneous rocks

Igneous rocks in the South China Sea have broad prospects for oil and gas exploration.Integrated geophysical methods are important approaches to study the distribution of igneous rocks and to determine and identify igneous rock bodies.Aimed at the characteristics of gravity and magnetic fields in the South China Sea,several potential field processing methods are preferentially selected.Reduction to the pole by variable inclinations in the area of low magnetic latitudes is used to perform reduction processing on magnetic anomalies.The preferential continuation method is used to separate gravity and magnetic anomalies and extract the gravity and magnetic anomaly information of igneous rocks in the shallow part of the South China Sea.The 3D spatial equivalent distribution of igneous rocks in South China Sea is illustrated by the 3 D correlation imaging of magnetic anomalies.Since the local anomaly boundaries are highlighted gravity and magnetic gradients,the distribution characters of different igneous rocks are roughly outlined by gravity and magnetic correlation analysis weighted by gradient.The results show the distribution of igneous rocks is controlled and influenced by deep crustal structure and faulting.

Integrated gravity and magnetic study on patterns of petroleum basin occurrence in the China seas and adjacent areas

The China seas and adjacent areas contain numerous petroleum basins.One of the main challenges for future oil and gas exploration is to identify the inherent patterns of petroleum basin distribution.The formation and evolution of petroleum basins along with the migration and accumulation of oil and gas are often closely related to the tectonic environment.The gravity and magnetic fields with high lateral resolution and wide coverage provide important data for regional tectonic research.Based on the gravity data in the Global Satellite Gravity Anomaly Database(V31.1)and magnetic data from the Earth Magnetic Anomaly Grid(2-arc-minute resolution)(V2),this study uses integrated gravity and magnetic field technique to obtain integrated gravity and magnetic field result for the China seas and adjacent areas,and then adopts the normalized vertical derivative of the total horizontal derivative technique to conduct partition.Finally,it identifies the relationship between the partition characteristics and tectonics as well as the patterns of petroleum basin occurrence.The research shows that the partition of gravity and magnetic field integrated result has a good correlation with the Neo-Cathaysian tectonic system and tectonic units.The petroleum basins are characterized according to three blocks arranged from north to south and four zones arranged from east to west.The north-south block structure causes the uneven distribution of oil and gas resources in the mainland area and the differences in the hydrocarbon-bearing strata.Petroleum basins are more abundant in the north than in the south.The ages of the main oil-and gas-bearing strata are“Paleozoic–Mesozoic,Paleozoic–Mesozoic–Cenozoic,and Paleozoic–Mesozoic”,in order from north to south.The difference in the overall type of oil and gas resources in all basins is controlled by the east–west zonation.From east to west,the oil and gas resource type exhibits a wave-like pattern of“oil and gas,gas,oil and gas,gas”.The vertical distribution is characterized by an upper oil(Mesozoic–Cenozoic)and lower gas(Mesozoic–Paleozoic)structure.Within the study area,the Paleozoic marine strata should be the main strata of future natural gas exploration.

A new algorithm based on the background field for red tide monitoring in the East China Sea

Remote sensing has been proven to be an effective means of monitoring red tides. The spectral information is an important basis for establishing a model to monitor red tides. The spectral curves of red tide events are analyzed and compared with multiyear monthly averaged spectral curves based on MODIS data from July 2002 to June 2012, as well as spectral differences at the same location during red tide presence and absence. A red tide monitoring algorithm is developed based on the background field to extract the red tide information of the East China Sea （ECS）. With the application of the algorithm in the ECS, the results reveal that the developed model can effectively determine the location of red tides, with good correspondence to the results from an official bulletin. This demonstrates that the algorithm can effectively extract the red tide information.

On the Tectonic Problems of the Southern East China Sea and Adjacent Regions:Evidence from Gravity and Magnetic Data

In this paper, two sets of gravity and magnetic data were used to study the tectonics of the southern East China Sea and Ryukyu trench-arc system： one data set was from the ＇Geological-geophysical map series of China Seas and adjacent areas＇ database and the other was newly collected by R/VKexue Ⅲ in 2011. Magnetic and gravity data were reorganized and processed using the software MMDP, MGDP and RGIS. In addition to the description of the anomaly patterns in different areas, deep and shallow structure studies were performed by using several kinds of calculation, including a spectrum analysis, upward-continuation of the Bouguer anomaly and horizontal derivatives of the total-field magnetic anomaly. The depth of the Moho and magnetic basement were calculated. Based on the above work, several controversial tectonic problems were discussed. Compared to the shelf area and Ryukyu Arc, the Okinawa Trough has an obviously thinned crust, with the thinnest area having thickness less than 14km in the southern part. The Taiwan-Sinzi belt, which terminates to the south by the NW-SE trending Miyako fault belt, contains the relic volcanic arc formed by the splitting of the paleo Ryukyu volcanic arc as a result of the opening of the Okinawa Trough. As an important tectonic boundary, the strike-slip type Miyako fault belt extends northwestward into the shelf area and consists of several discontinuous segments. A forearc terrace composed of an exotic terrane collided with the Ryukyu Arc following the subduction of the Philippine Sea Plate. Mesozoic strata of varying thicknesses exist beneath the Cenozoic strata in the shelf basin and significantly influence the magnetic pattern of this area. The gravity and magnetic data support the existence of a Great East China Sea, which suggests that the entire southern East China Sea shelf area was a basin in the Mesozoic without alternatively arranged uplifts and depressions, and might have extended southwestward and connected with the northern South China Sea shelf basin.

Typhoon-induced wind waves in the northern East China Sea during two typhoon events:the impact of wind field and wave-current interaction

We examined the influences of the wind fi eld and wave-current interaction(WCI)on the numerical simulation results of typhoon-induced wind waves in the northern East China Sea(NECS)using the coupled Simulating Waves Nearshore+Advanced Circulation(SWAN+ADCIRC)model.The simulations were performed during two typhoon events(Lekima and Muifa),and two widely used reanalysis wind fields,the Climate Forecast System Version 2(CFSv2)from the National Centers for Environmental Prediction(NCEP)and the fifth-generation European Centre for Medium-Range Weather Forecasts(ECMWF)Reanalysis(ERA5),were compared.The results indicate that the ERA5 and CFSv2 wind fields both reliably reproduced the wind variations measured by in-situ buoys,and the accuracy of the winds from ERA5 were generally better than those from CFSv2 because CFSv2 tended to overestimate the wind speed and the simulated significant wave height(SWH),particularly the peak SWH.The WCI effects between the two wind field simulations were similar;these effects enhanced the SWH throughout the nearshore NECS during both typhoons but suppressed the SWH on the right side of the Typhoon Muifa track in the deep and off shore sea areas.In summary,variations in the water depth and current propagation direction dominate the modulation of wave height.

A three-dimentional model of the northeast East China Sea, with application to the M2 current field

A simple three-dimensional tidal model is used to examine the M2 tidal current distribution in a northeastern part of the East China Sea, especially the vertical variation of the current in the region. Computed M2 current is compared with observations available and found to be in good agreement.Main features of the calculating method in this study are: (1) Vertical variation of the tidal current is taken as a funetion of the depth-mean velocity: (2) the method is applicable to a variety of the vertical eddy viscosities; (3) it has a fine vertical resolution, especially near the sea bootom. So, this method not only enables us to get a steady state solution easily but also depicts effects of the friction on the vertical variation of the current much better.

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.

Derivation of sea surface current field from sequential satellite images of the East China Sea

A series of NOAA AVHRR data over the East China Sea were collected from the ground station of the Second Institute of Oceanography, Hangzhou, China. Three methods, including a functional analytic method (FAM), a maximum cross correlation (MCC)'method and a correlation relaxation (C - R) method, are applied to derive the sea surface current field from sequential satellite images in the area of the East China Sea. Several preprocessing steps, such as geometric correction, SST determination, image projection, image navigation and grey value normalization as well as land and cloud mask are performed. The results from the three methods reflect the general current system in this area reasonably.

STUDY ON THE FRACTAL CHARACTER OF MAGNETIC ANOMALY FIELDS AND THE DELINEATION OF TECTONIC ELEMENTS IN THE SOUTH CHINA SEA

This is a quantitative method for studying the fractal character of magnetic anornaly fields and delineating tectonic elements using fractal theory . For an area with different geological and geophysical features , the definition of Hausdorff dimension is used to calculate the fractal dimension of the area’s magnetic field , then delineate geological tectonic elements by comparing this value . Use of 1978- 1986 geophysical survey data in this method applied to the South China Sea yielded three first grade tectonic elements and nine second grade ones , in six of which subsecond grade elernents were found.

Tidal analysis in the venturi-shaped area between Zhenhai and Shenjiamen in the East China Sea

In order to better understand the general tidal features in the venturi-shaped area between Zhenhai and Shenjiamen in the northern coastal region of Zhejiang Province in the East China Sea, the tidal data were obtained from both the three permanent tide stations of Zhenhai, Dinghai and Shenjiamen, and four temporary tide stations of Mamu, Chuanshan, Guoju and Liuheng, along with the current speed being observed at Luotou Waterway. Results from harmonic analysis show that: (1) The area was dominated by shallow water tides with irregular semi-diurnal features, and the smallest tidal range occurred in the area near a crossing line between Zhenhai and Dinghai stations, indicating that a tidal node existed in the southern Hangzhou Bay; (2) Formulae, HS2/HM2 >0.4 and gM2-(gK1+gO1)=270° (where H and g are harmonic constants), could be used as judging criteria for high and low tidal level diurnal inequalities; (3) The duration difference between ebb and flood tides could be roughly assessed by the ratio of HM4 vs. HM2; and the larger the ratio is, the bigger the duration difference is. At the same time, the duration period could be assessed by 2gM2-gM4, the epoch difference between M2 and M4 tidal constituents. If 2gM2-gM4 <180°, then the ebb duration is longer than the flood duration; if 180°< 2gM2-gM4 <360°, the result is reversed; (4) Taking Dinghai station as a center point, the highest tidal levels and the average high tidal levels, as well as the average tidal ranges at all stations became higher and larger both southeastwards and northwestwards, while the lowest tidal levels and the average low tidal levels appeared to be lower both southeastwards and northwestwards; and (5) The tidal patterns were not all in line with the tidal current patterns. As a conclusion, the smallest tidal range occurred in the narrow part of the venturi-shaped area. Along the both sides of the area, the highest tidal level and tidal range became higher and larger, while the lowest tidal level became lower with the increase of the distance from the narrow throat area. This is somehow different from the theory that the tidal level increases gradually when it moves towards the top narrow area of a V-shaped bay or estuary.

Adaptability Study of Intelligent Well Systems in East China Sea Oil Field

Intelligent well system is the well that has a set of equipment fixed in the down hole including sensing devices, data transmission system and operating devices for information acquiring, data gathering and decision analysis. By this remote control process, the smart well system can ultimately optimize well deliverability; it is used more and more often in oil fields with its stability and control technique. At present, the main intelligent well systems in the worm include SCRAMS, Direct Hydraulic, Digital Hydraulic that belongs to WellDynamics Company, InForce and InCharge that belongs to Baker Oil Tools Company, RMC that belongs to Schlumberger Company. This paper compares different types of systems and their characteristics, recommending the InCharge system as the intelligent well system for East China Sea Oil Field according to its geological and reservoir conditions.

Characteristics and Origin of Yacheng Gas Field in Qiongdongnan Basin,South China Sea

The Yacheng gas field lies in the foot wall of the No. 1 fault, the boundary fault between the Yinggehai and Qiongdongnan basins. An overpressured system developed in the Meishan Formation near the No. 1 fault in the gas field and in the adjacent Yinggehai basin. Away from this fault into the Qiongdongnan basin, the overpressure diminishes. Below 3 600 m in the gas field, an obvious thermal anomaly occurs. The gases show obvious compositional heterogeneities which reflect reservoir filling process and origin of the gas field. The gas field was charged from both the Qiongdongnan and the Yinggehai basins but mainly from the former. Hydrocarbons sourced from the Qiongdongnan basin have relatively low maturities while hydrocarbons from the Yinggehai basin have relatively high maturities.

FEATURES OF GRAVITY AND MAGNETIC ANOMALIES IN CENTRAL AND NORTHERN PARTS OF THE SOUTH CHINA SEA AND THEIR GEOLOGICAL INTERPRETATION

The gravity and magnetic survey lines of about 13,500 km were carried out in the centraland northern parts of the South China Sea from 1977 to 1978. The results obtained showthat the Bouguer gravity and magnetic anomalies have a tendency to increase gradually theirvalues from the northern continental shelf, through the slope, to the central abyssal basin of theSouth China Sea. The change in free-air gravity anomaly values coincides to a certain degreewith the undulation of the sea-bottom topography. The primary factor determining regionalvariation of the Bouguer gravity anomayl values is the Moho depth. The main factor deter-mining the magnetic anomly values is the nature of the basement rock. The high magnetieand Bouguer gravity anomaly values observed in some fault basin areas are inferred to becaused by draping the basic and ultrabasic magma extruding along the faults on the basementof the metamorphic rock,or by intrusion of the same magma into the basement.

东海陆架盆地中部隆起带北西向大断层识别及成因机制

东海陆架盆地不同地区均揭示有局部北西走向断层的存在,但由于对断层缺乏盆地整体研究,导致断层运动学和动力学认识存在争议。以中部隆起带为重点研究区,通过对区域重磁和地震资料的综合分析开展了盆地尺度北西向大断层识别和分布规律研究,探讨了区域北西向大断层构造演化过程及成因机制。结果表明:①中部隆起带发育有10条北西向大断层,该类断层具有分布范围广,断层规模“南大北小”和断面相对低角度的特点,并控制了区域内中生界及古新—中下始新统厚度的南北差异;②北西向大断层形成时间早于北东、北北东向断层,主要经历中生代燕山期正断层发育、中生代末期至早中始新世伸展-转换断层发育、中晚始新世到渐新世断层整体消亡和中—上新世至今断层局部活化四个演化阶段;③北西向大断层的形成与中国东部印支—燕山期先挤压逆冲、后伸展回滑作用有关,反映印支期华南与华北板块南北碰撞向东南方向深入影响至东海陆架盆地区域,形成大量北西走向低角度逆冲断层,燕山期在北西—南东向区域挤压及地壳增厚背景下,存在广泛的北北东—南南西向伸展逃逸,最终控制了北西向低角度正断层的形成。上述研究为区域内深层及基底潜山领域的油气勘探研究提供了支撑。

基于井眼清洁效果的大位移井钻井参数与钻井液性能优化方法研究与实践

针对东海油气田大位移井在钻井过程中易形成岩屑床,井眼清洁困难,导致摩阻扭矩大甚至频繁出现憋泵憋扭矩等复杂的问题,提出了一种基于井眼清洁效果的钻井参数设计和钻井液性能优化方法。首先,分析大位移井井眼清洁的主要影响因素,使用VIRTUALMUD软件对影响大位移井井眼清洁性能的钻井液排量、机械钻速等工程参数和钻井液6转3转读数等钻井液参数进行优化设计。然后,通过优选合适的基础油、乳化剂和重晶石等方法对现场使用的油基钻井液进行性能改进。在大位移井A井应用优化后的钻井参数及钻井液体系,井眼清洁效果好,钻井液携岩效率高,在作业过程中未发生憋钻卡钻等复杂情况,起下钻顺利。现场应用情况表明,该方法具有较好的可行性和有效性,对于提高钻井效率和降低钻井成本具有重要意义。

Deep crustal structures of eastern China and adjacent seas revealed by magnetic data

Through reduction to the North Pole and upward continuation of the total field magnetic anomalies, we analyze magnetic patterns and spatial distributions of different tectonic blocks and crustal faults in eastern China and adjacent seas. Depths to the Curie isotherms are further estimated from radially averaged amplitude spectra of magnetic data reduced to the pole. Data reductions effectively enhance boundaries of regional tectonic belts, such as the Dabie ultra-high metamorphic belt, the Tanlu Fault, and the Diaoyudao Uplift. Curie depths are estimated at between 19.6 and 48.9 km, with a mean of 31.7 km. The Subei Basin and the south Yellow Sea Basin in the lower Yangtze block show relatively deep Curie isotherms, up to about 35 km in depth, whereas in the surrounding areas Curie depths are averaged at about 25 km. This implies that the lower Yangtze Block has experienced a unique tectonic evolution and/or has unique basement lithology and structures. From a regional perspective, sedimentary basins, such as the Subei Basin, the south Yellow Sea Basin, and the East China Sea Basin, normally show deeper Curie isotherms than surrounding uplifts such as the Diaoyudao Uplift and the Zhemin Uplifts. Curie isotherms also upwell significantly in volcanically active areas such as the Ryukyu Arc and the Cheju Island, confirming strong magmatic and geothermal activities at depth.

Physical Mechanism of Formation of the Bimodal Structure in the Meiyu Front System

The bimodal structure of the Meiyu front system is readdressed after Zhou et al.（2005）. The physical mechanism of the formation of the bimodal distribution is discussed. The bimodal structure of the Melyu front system considerably results from atmospheric moisture gradients, though atmospheric temperature gradients are also not negligible. According to the definition of equivalent potential temperature, and by scale analysis, we find that atmospheric equivalent potential temperature gradients, which could be regarded as an indicator of the Meiyu front system, could be mainly attributed to the variations of atmospheric potential temperature gradients with a scaling factor of 1 and moisture gradients multiplied by a scaling factor of an order of about 2.5 × 10^3, which means that small variations of atmospheric moisture gradients could lead to large variations of equivalent potential temperature gradients, and thus large variations of the Meiyu front system. Quantitative diagnostics with a mesoscale simulation data in the vicinity of the Meiyu front system show that moisture gradients contribute to equivalent potential temperature gradients more than potential temperature gradients.

晚更新世以来南海中央海盆沉积物的磁学特征:对物源和东亚季风演化的指示

南海沉积物记录了丰富的高原隆升剥蚀、古海洋、东亚季风和区域构造演化信息,是研究东亚季风和古环境演化的理想材料.但是,由于南海沉积环境复杂,物质来源多样,使得沉积物定年和环境气候探讨的难度加大.为此,本文选取南海中央海盆的SCS-01钻孔作为研究对象,建立其准确的年代框架,综合利用磁学和地球化学分析方法,探讨沉积物物源及其对东亚季风演化的指示.首先,通过对沉积物样品的岩石磁学特征分析,表明沉积物的主要载磁矿物为低矫顽力的假单畴磁铁矿,载磁矿物颗粒大小均一、含量变化小,基本符合建立地磁场相对古强度(Relative Paleointensity,RPI)曲线的"均一性"标准.在此基础上利用磁化率和非磁滞剩磁对天然剩磁进行归一化处理得到RPI,将钻孔的RPI曲线与全球或区域的标准RPI曲线(Sint-200,NAPIS-75和SCS-PIS)进行比较,得到了6个年龄控制点,并结合AMS-^(14)C测年结果,建立了南海中央海盆75 ka以来的时间框架.SCS-01钻孔的RPI记录和其他全球性观测结果的一致性表明,南海的沉积物记录了全球尺度的地磁场古强度行为模式.综合稀土元素和沉积物记录的磁学信息对沉积物的来源进行探讨,结果表明SCS-01钻孔的沉积物主要来源于台湾地区,少量来自吕宋岛区域,这为进一步全面了解南海各区域的沉积物物源分布提供了数据支撑.另外,SCS-01钻孔的环境磁学记录可以较好地反映75 ka以来东亚季风在冰期-间冰期的变化,为该钻孔地区千年尺度上的气候变化提供可靠指标.

随钻核磁共振技术在深层水平井地质导向中的应用

南海东部油田陆丰某油气藏属于深层岩性圈闭油气藏,也属于边水油气藏,经过多年的开发,剩余油分布存在不确定性,在调整井水平井钻进过程中存在钻进风险。随钻核磁共振技术能提前预测储层岩性、能准确识别储层流体性质,在南海东部LFX-X-H1井深层水平井地质导向中应用时,成功指导现场随钻过程中的井身轨迹调整,提高了钻井效率,实现了油田增产。

东海海域带鱼矢耳石形态特征与生长研究

矢耳石作为重要的硬组织,被广泛用于鱼类的生长、种群结构和生活史等研究。根据2020年10月—2021年10月东海海域采集的331尾带鱼样本,测量了15个矢耳石形态特征参数,通过主成分分析、Pearson相关分析、独立样本t检验、方差分析和最小显著差异法对矢耳石形态特征与生长进行了研究。结果表明,东海海域带鱼矢耳石呈长梭形,基叶、翼叶区分明显;其矢耳石的主要形态特征参数为基叶到翼叶的长度(DRA)、基叶到矩口的长度(DRI)、基叶到后端的长度(DRP)、翼叶到后端的长度(DAP)和矩口到后端的长度(DIP);其矢耳石的生长受性别、体型大小和性腺发育等因素影响。东海海域带鱼矢耳石特征参数及其生长影响因素的确定,为后续利用矢耳石研究东海海域带鱼渔业生态学及资源评估提供了科学依据。