Eastern South China Sea Research Articles

Evaluation of Key Development Factors of a Buried Hill Reservoir in the Eastern South China Sea: Nonlinear Component Seepage Model Coupled with EDFM

The HZ 26-B buried hill reservoir is located in the eastern part of the South China Sea. This reservoir is characterized by the development of natural fractures, a high density, and a complex geological structure, featuring an upper condensate gas layer and a lower volatile oil layer. These characteristics present significant challenges for oilfield exploration. To address these challenges, this study employed advanced embedded discrete fracture methods to conduct comprehensive numerical simulations of the fractured buried hill reservoirs. By meticulously characterizing the flow mechanisms within these reservoirs, the study not only reveals their unique characteristics but also establishes an embedded discrete fracture numerical model at the oilfield scale. Furthermore, a combination of single-factor sensitivity analysis and the Pearson correlation coefficient method was used to identify the primary controlling factors affecting the development of complex condensate reservoirs in ancient buried hills. The results indicate that the main factors influencing the production capacity are the matrix permeability, geomechanical effects, and natural fracture length. In contrast, the impact of the threshold pressure gradient and bottomhole flow pressure is relatively weak. This study’s findings provide a scientific basis for the efficient development of the HZ 26-B oilfield and offer valuable references and insights for the exploration and development of similar fractured buried hill reservoirs.

Reproductive features of data-deficient yellowfin snapper, Lutjanus xanthopinnis (Actinopterygii: Eupercaria: Lutjanidae), from east-coast of Peninsular Malaysia: Implications for sustainable fisheries management

Understanding the reproductive biology of fishes is crucial to making accurate and scientifically sound recommendations for fisheries management. The presently reported study is the first to report the reproductive characteristics of the data-deficient and recently described yellowfin snapper, Lutjanus xanthopinnis Iwatsuki, Tanaka et Allen, 2015, collected from the eastern South China Sea, Malaysia. A total of 572 fish specimens were sampled monthly from March 2022 to April 2023 using a gillnet and a trawl net. Subsequently, these specimens were measured and weighed. Their total length (TL) ranged from 15.3 to 26.8 cm (19.25 ± 2.04 cm; mean ± SD). Their body weight (WB) ranged from 53.5 g to 279.7 g (114.5 ± 40.3 g; mean ± SD). For the reproductive biology study, the following parameters were determined: the sex ratio, gonadosomatic index (GSI), hepatosomatic index (HSI), spawning period, fecundity, condition factor (K), length at maturity, and the gonadal maturity stages. In the sex ratio, males (M) were significantly dominant over females (F) (M:F = 1:0.75) (χ2 = 11.18). Consequently, the monthly mean GSI of pooled sex revealed a consistent trend from January to August, indicating spawning seasons with peak periods in March and April. The batch fecundity of 67 mature females (16.0–25.2 cm TL; 64.5– 279.3 g W), was determined to range from 16 405 to 94 357 oocytes. The fecundity increased with gonad weight in contrast to weight and length. The length at first maturity of females and males was 14.53 and 20.56 cm, respectively. Females matured earlier than males. Macroscopic and histological examination of gonads revealed monthly variation in the ratio of gonad stages for males and females. This study offers crucial data on the reproductive biology of L. xanthopinnis, which will help with sustainable fishery management in this area and can be used as a reference for the management of similar fish populations in other regions of the world.

Discovery and inspiration of large- and medium-sized glutenite-rich oil and gas fields in the eastern South China Sea: An example from Paleogene Enping Formation in Huizhou 26 subsag, Pearl River Mouth Basin

Based on the practice of oil and gas exploration in the Huizhou Sag of the Pearl River Mouth Basin, the geochemical indexes of source rocks were measured, the reservoir development morphology was restored, the rocks and minerals were characterized microscopically, the measured trap sealing indexes were compared, the biomarker compounds of crude oil were extracted, the genesis of condensate gas was identified, and the reservoir-forming conditions were examined. On this basis, the Paleogene Enping Formation in the Huizhou 26 subsag was systematically analyzed for the potential of oil and gas resources, the development characteristics of large-scale high-quality conglomerate reservoirs, the trapping effectiveness of faults, the hydrocarbon migration and accumulation model, and the formation conditions and exploration targets of large- and medium-sized glutenite-rich oil and gas fields. The research results were obtained in four aspects. First, the Paleogene Wenchang Formation in the Huizhou 26 subsag develops extensive and thick high-quality source rocks of semi-deep to deep lacustrine subfacies, which have typical hydrocarbon expulsion characteristics of “great oil generation in the early stage and huge gas expulsion in the late stage”, providing a sufficient material basis for hydrocarbon accumulation in the Enping Formation. Second, under the joint control of the steep slope zone and transition zone of the fault within the sag, the large-scale near-source glutenite reservoirs are highly heterogeneous, with the development scale dominated hierarchically by three factors (favorable facies zone, particle component, and microfracture). The (subaqueous) distributary channels near the fault system, with equal grains, a low mud content (<5%), and a high content of feldspar composition, are conducive to the development of sweet spot reservoirs. Third, the strike-slip pressurization trap covered by stable lake flooding mudstone is a necessary condition for oil and gas preservation, and the NE and nearly EW faults obliquely to the principal stress have the best control on traps. Fourth, the spatiotemporal configuration of high-quality source rocks, fault transport/sealing, and glutenite reservoirs controls the degree of hydrocarbon enrichment. From top to bottom, three hydrocarbon accumulation units, i.e. low-fill zone, transition zone, and high-fill zone, are recognized. The main area of the channel in the nearly pressurized source-connecting fault zone is favorable for large-scale hydrocarbon enrichment. The research results suggest a new direction for the exploration of large-scale glutenite-rich reservoirs in the Enping Formation of the Pearl River Mouth Basin, and present a major breakthrough in oil and gas exploration.

China’s aggressive tactics: A comparative analysis of the South China Sea Conflict and the Sino-India Border Dispute in Eastern Ladakh

Conflicts, confrontations, and disputes have long been a part of international relations which often includes skirmishes and their consequences being the reality of states unable to find peaceful resolutions to their disputes to which the entire world pays attention. The complex nature of international relations lies more in the differences and contradictions in foreign policy. These materialize as security dilemmas, which arise due to high stakes, mutual distrust, and policy overreactions. Such a security dilemma persists between China and its neighboring states and the rest of the world due to its aggressive, coercive, and adversarial tactics and ambitions. This paper assesses the situation in the South China Sea and Eastern Ladakh following the Chinese occupation of several areas across the Line of Actual Control (LAC) in 2020 and China’s muscle stretching in South China’s Sea. The expansionist policy of China led to an incident in Galwan River Valley and the South China Sea, resulting in the deaths of both sides. This study outlines the course of events and do a comparative analysis of conflict occurred in Eastern Ladakh and South China Sea and its impact, where negotiations were partially successful and seemed deadlocked in achieving the status quo. This paper seeks to examine the larger border game, challenges for future political diplomacy, and recommendations for the future course of action.

Study on Seasonal Characteristics and Causes of Marine Heatwaves in the South China Sea over Nearly 30 Years

Marine heatwaves (MHWs) are becoming more frequent and intense in many regions around the world, as well as in China’s marginal seas. However, the seasonal characteristics and associated physical drivers of MHWs are largely unknown. In this study, we analyze, based on multiple reanalysis and numerical model data, the seasonal characteristics and causes of MHWs in the South China Sea (SCS) over a near 30-year period (1991–2022). There exist significant seasonal variabilities in the spatiotemporal features and formation mechanisms of MHWs. MHWs in the SCS show significant increasing trends in terms of frequency, duration, and intensity. MHWs during the summer half-year are stronger than the winter half-year as a whole, with them being more likely to occur over the eastern SCS in the summer half-year and the western region in the winter half-year. However, the increasing trend of MHWs in the winter half-year exceed those in the summer. Additionally, we find that MHWs are associated with the unusually strong west Pacific subtropical high (WPSH) both in the summer and winter half-years. Nevertheless, the dominant factors for MHWs are different in the varied seasons. According to upper ocean temperature equation analysis, surface heat flux anomalies (especially shortwave radiation flux) are major effect factors in the summer half-year, while ocean dynamic processes play the main role in the winter half-year. An analysis of the typical MHWs also proves this conclusion. Moreover, MHWs occurring in winter are often accompanied by temperature anomalies within the mixed-layer depth. The findings imply that the formation mechanisms and space–time distribution of MHWs exist with a seasonal contrast in the SCS, rather than simply being due to large-scale circulation and flux anomalies. This may provide a useful reference for a deeper understanding and forecasting of MHWs under different seasons and weather.

Deepwater PDC Jetting Bit-Drilling Technology Based on Well Structure Slimming

Growing global energy demand and limited reserves of traditional energy resources are causing a growing energy shortage. In order to meet future energy needs, new energy resources must be continuously explored. Deepwater drilling research has emerged as one of the key ways to address this issue, and well structure slimming is an effective way to increase drilling speed and reduce costs. The hole size of the second section of deepwater wells decreases from a conventional 660.4 mm to 444.5 mm and increases from 500–800 m to 800–1200 m, creating problems where the conventional 660.4 mm cone bit cannot be used, the rate of penetration (ROP) of the cone bit is low, and the service life is short. To solve these problems, a 444.5 mm artificial polycrystalline diamond compact (PDC) was designed for the first time for use at home or abroad, and according to the characteristics and operation requirements of the 914.4 mm conductor jetting process, a unique anti-collision gauge protector was designed, an innovative bypass nozzle was configured, and a hydraulic design to prevent bit balling in shallow soft mudstone was formulated. PDC jetting bit-drilling technology based on well structure slimming was successfully applied to eight deepwater wells in the eastern South China Sea, which successfully jetted a 914 mm conductor and greatly improved the ROP of their second-section holes. When the below-mudline depth of the second-section hole increased by 37.01%, the average ROP increased by 227.84%. These technical achievements have successfully realized deep drilling with seawater, increased speed and efficiency, achieved good application results, and accumulated valuable experiences that can be used for reducing the cost and increasing the efficiency of offshore drilling operations.

Application of fluid modulus inversion to complex lithology reservoirs in deep-water areas

It has been a challenge to distinguish between seismic anomalies caused by complex lithology and hydrocarbon reservoirs using conventional fluid identification techniques, leading to difficulties in accurately predicting hydrocarbon-bearing properties and determining oil-water contacts in reservoirs. In this study, we built a petrophysical model tailored to the deep-water area of the Baiyun Sag in the eastern South China Sea based on seismic data and explored the feasibility of the tri-parameter direct inversion method in the fluid identification of complex lithology reservoirs, offering a more precise alternative to conventional techniques. Our research found that the fluid modulus can successfully eliminate seismic amplitude anomalies caused by lithological variations. Furthermore, the seismic data-based direct inversion for fluid modulus can remove the cumulative errors caused by indirect inversion and the influence of porosity. We discovered that traditional methods using seismic amplitude anomalies were ineffective in detecting fluids, determining gas-water contacts, or delineating high-quality reservoirs. However, the fluid factor Kf, derived from solid-liquid decoupling, proved to be sensitive to the identification of hydrocarbon-bearing properties, distinguishing between high-quality and poor-quality gas zones. Our findings confirm the value of the fluid modulus in fluid identification and demonstrate that the tri-parameter direct inversion method can significantly enhance hydrocarbon exploration in deep-water areas, reducing associated risks.

Provenance of deep-sea sediments in the eastern South China Sea since Marine Isotope Stage 5 (MIS 5) and implications for climate change and volcanic activity

The South China Sea deep-sea basin receives large amounts of marine sediments and volcanic materials from the surrounding continents and islands, and it is an ideal region to study the effects of climate change and volcanic activity on sediment provenance. Core K2 was collected in a water depth of 4078 m in the eastern South China Sea and is mainly composed of diatoms, tephra, volcanic glass and slag. The results of a total of 102 analysed grains show that the depositional processes were dominantly suspension, uniform suspension and overbank-pool facies suspension, with no rolling. Six accelerator mass spectrometry (AMS) 14C ages from planktonic foraminifers and fifty-four δ18O data points of G. ruber suggest that the age of Core K2 is 125 kyr before present (BP). Forty-four clay mineralogical data points show that the clay minerals in the sediments are mainly illite+chlorite (24.3%–49.7%), with a small amount of smectite (2.6%–11%), suggesting a sediment source mainly from Taiwan with a small amount of sediment from Luzon. Forty-five samples yield 143Nd/144Nd and 87Sr/86Sr ratios ranging from 0.512250 to 0.512769 and from 0.704383 to 0.710690, respectively, indicating that sediments in K2 represent a mixture of volcanic inputs from in or around Luzon and sediments from Taiwan and Luzon. Based on calculations of clay minerals and SrNd isotopic data, the relative proportions of sediments from Taiwan and Luzon and volcanic input vary from 8% to 88%, from 1% to 23% and from 0% to 91%, respectively, and the average proportions of sediments from Taiwan and Luzon and volcanic input are 55%, 12% and 33%, respectively. Significant temporal changes in SrNd isotopes are attributed to variations in the volcanic eruption intensity or distance to the sampling location. Moreover, high εNd values and low 87Sr/86Sr ratios of the sediments in Core K2 during interglacial periods could represent volcanic eruptions.

Research on Production Performance Prediction Model of Horizontal Wells Completed with AICDs in Bottom Water Reservoirs

With the advancement of completion technology for horizontal wells in bottom water reservoirs, Autonomous Inflow Control Devices (AICDs), which have achieved good results in recent years, have been widely used in the oil fields of the eastern South China Sea. Although some mathematical methods can be used to predict the production performance of horizontal wells, there is no dynamic prediction method for the production performance of horizontal wells completed with AICDs. In this work, a mathematical model of porous flow in the reservoir, nozzle flow in the AICD, and pipe flow in the horizontal well is established, and then a new model is presented for predicting the dynamic performance of horizontal wells completed with AICDs in bottom water reservoirs. The new coupling model is compared with two horizontal wells completed with AICDs in the bottom water reservoirs of the eastern South China Sea, and the results indicate that the accuracy of the new model is sufficiently high to provide theoretical support for the further prediction of horizontal wells in the eastern South China Sea.

Atmospheric impacts of local sea surface temperatures versus remote drivers during strong South China Sea winter cold tongue events

AbstractSea surface temperatures (SSTs) in the western part of the South China Sea (SCS) are cooler than in the eastern part in boreal winter, owing to a winter climatological cold tongue (CT). In this study, using a regional atmospheric model configured for the Maritime Continent, we assess the atmospheric impacts of local (or SCS) SSTs versus those from remote drivers (e.g., western tropical Pacific SSTs) during strong CT events with anomalously cool SSTs. In the local run, more rainfall is observed over the eastern SCS, but no significant atmospheric impacts are found over the CT region when SSTs associated with strong CT events are imposed within the SCS while climatological conditions are imposed elsewhere. SCS SST anomalies during strong CT events do not significantly modify the regional wind circulation. The lack of atmospheric response to SSTs over the CT region may be explained by the wintertime mean SSTs (i.e., &lt;27–28°C) over the CT region that are inadequate to trigger deep atmospheric convection, while eastern SCS SSTs are high enough. The increase of anomalous positive moist static energy (MSE) near the sea level over the eastern SCS indicates underlying warm eastern SCS SST anomalies could be influencing positive rainfall anomalies. In the remote run, imposing climatological SCS SSTs but remote SSTs and lateral boundary conditions linked to strong CT events results in cyclonic wind and positive rainfall anomalies over the eastern SCS and Philippines, which are a Matsuno–Gill response to the diabatic heating anomalies over the warm western tropical Pacific SST anomalies. Positive rainfall and cloud cover anomalies associated with the cyclonic wind anomalies are due to the anomalous positive MSE import into the eastern SCS by horizontal advection.

A Millennial-Scale Tephra Event-Stratigraphic Record of the South China Sea since the Penultimate Interglacial

Abstract Large volcanic eruptions have significant impacts on climate and environmental changes. The deposition of tephra in marine sediments may serve as an eruption recorder, but it has not been extensively studied in the western Pacific. This study explored a millennial-scale tephra event-stratigraphy with multiple indicators in a sediment core collected from the eastern South China Sea (SCS) basin. The magnetic susceptibility (MS), Fe and Mn concentrations determined by X-ray fluorescence (XRF), and identification of individual ash particles were used to identify tephra layers and reconstruct the history of volcanic activity. Nine visible volcaniclastic units (VVU) and two cryptotephra layers have been identified based on their distinct features, as manifested by high MS, Fe, and Mn concentrations and single-peak grain size distribution. The VVUs and cryptotephra layers reveal elevated volcanic activities. Using the radiocarbon age model and oxygen isotope stratigraphy, these episodes could roughly correspond to the following periods: 1-11 ka, 16-17 ka, 27-31 ka, 41-42 ka, 45-46 ka, 49-50 ka, 77-80 ka, 90-91 ka, 97-99 ka, 116-126 ka, and 132-140 ka. The alkenone-derived SST has significant glacial cycles and good synchronicity with other SCS SST records, which could partially help build the preliminary age model. Despite possible age errors larger than 1 kyr, the discovery and timing of tephra layers provide a preliminary framework to further investigate the impact of historical volcanic eruptions on climate changes.

A New Multi-Objective Optimization Design Method for Directional Well Trajectory Based on Multi-Factor Constraints

The design of the wellbore trajectory directly affects the construction quality and efficiency of drilling. A good wellbore trajectory is conducive to guiding on-site construction, which can effectively reduce costs and increase productivity. Therefore, further optimization of the wellbore trajectory is inevitable and necessary. Based on this, aiming at the three-segment, five-segment, double-increase-profile extended reach wells, this paper considered the constraints of formation wellbore stability; formation strength; and the determination of the deviation angle, deviation point position, and target range by the work of deflecting tools. In addition, the optimization objective function of the shortest total length of the wellbore, minimum error of the second target, lowest cost, minimum friction of the lifting and lowering string, and minimum torque of rotary drilling were proposed and established. The objective function of the longest extension limit of the horizontal section of the extended reach well is established. Taking the 14-8 block of the Lufeng Oilfield in the eastern South China Sea as an example, the actual data of the field were modeled, and the independence of the objective function was verified by comparing the number of non-inferior solutions of the two objective functions. By normalizing simplified to double-, three-, and four-objective functions, using a genetic algorithm and particle swarm optimization results, it can be found that the new method of optimization design established in this paper has an obvious optimization effect compared with the original design.

Spatio-temporal distribution and mechanism of Cenozoic magmatism in the South China Sea and adjacent areas: Insight from seismic, geochemical and geochronological data

ABSTRACT Cenozoic magmatism was highly active in the South China Sea (SCS), and mostly dominated by basalts. The magmatic rocks are mainly distributed in the lower slope and subbasins of the SCS, in addition to sporadic and small-scale Cenozoic volcanism on the SCS shelf. This paper presents a complete map of the Cenozoic magmatic rocks of the SCS with magmatic rock boundaries. This paper also presents an analysis of geological survey and published data in terms of seismic profiles, ages, geochemistry and isotopic systematics of the Cenozoic magmatic complexes of the SCS and adjacent areas. Many of wells or dredges are located in magmatic rocks identified by seismic profiles. Three periods of magmatic activity are distinguished: (1) a pre-spreading period (>33 Ma); (2) a syn-spreading period (33–15 Ma) and (3) a post-spreading period (<15 Ma). The thickest volcanic rocks encountered more than 1000 m (possibly Oligocene to Miocene) in the Boayan 1/1A well in the Palawan Trough of the southern SCS during Cenozoic. The oldest magmatic rocks are andesites (62.5 ± 2.2 Ma) in the northern SCS (offshore) and andesites (65 ± 3 Ma) in Penghu (on land). The latest modern magmatic activity was detected in 1923 offshore of a submarine volcanic eruption (10°09´29″N, 109°00′50″E, 20 m below sea level) and along the shore of a volcanic eruption in the Philippines. Cenozoic magmatic rocks of the SCS include at least 1008 rock bodies: the smallest is 0.00012 km2, the largest is 1528 km2 and the total area covers 46,309 km2. The Yangjiang-Yitong fault and Zhongnan-Reedbank fault divide the magmatic rocks into two different sections in the eastern and western SCS. The ages of the magmatic rocks in the eastern SCS are mostly older than those in the western SCS. The magmatic rocks in the northern, southern, eastern, western SCS and adjacent areas, and SCS basin are related to extension, subduction+collision, subduction, extrusion, extension+cooling+subsidence, respectively.

A forearc pull-apart basin under oblique arc-continent collision: Insights from the North Luzon Trough

The North Luzon Trough, on the East Asian continental margin, is under transpression due to the oblique convergence between the Eurasian and Philippine Sea plates. Based on a seismic profile across the Manila subduction system and other geological data, we propose a strike-slip pull-apart basin model to account for the formation of the North Luzon Forearc Basin. The subduction system experienced four tectonic evolutionary stages: (1) initial subduction (~22–20 Ma): the Eurasian Plate began to subduct beneath the Philippine Sea Plate, and the incipient Manila Trench-North Luzon Volcanic Arc system started to form; (2) upper crustal deformation (~20–15 Ma): the overriding Philippine Sea Plate was strongly deformed by continuous subduction, and thrust faults, volcanic activity, and possibly strike-slip faulting weakened the forearc areas; (3) initiation of strike-slip faults (~15–6 Ma): continuous rotation of the Philippine Sea Plate initiated strike-slip faults in the weak zones of the forearc areas; (4) pull-apart basin formation (~6–0 Ma): the pull-apart basin formed in the forearc due to sinistral movement of two sets of strike-slip faults. We propose an indentation-escape process involving coupling between the sinistral strike-slip faults in the Taiwan-Luzon area and dextral strike-slip faults in the East Asia continental margin, for the evolution of the eastern South China Sea Basin margin.

First record of the Philippine snubnose halfbeak, Melapedalion breve (Actinopterygii: Beloniformes: Hemiramphidae), from Bangka Belitung Islands, Indonesia

A single specimen (117.1 mm standard length) of Melapedalion breve (Seale, 1910) collected off Lepar Island, Bangka Belitung Islands, Indonesia, represents the first record of M. breve from Indonesian waters and the southernmost record of the species. Melapedalion breve has previously been recorded from the Sulu Sea (southwestern Philippines) and eastern central South China Sea.

E&amp;P Notes (January 2022)

E&amp;P Notes (January 2022)

A New Method for Research on Unsteady Pressure Dynamics and Productivity of Ultralow-Permeability Reservoirs

In the numerous low-permeability reservoirs, knowing the real productivity of the reservoir became one of the most important steps in its exploitation. However, the value of permeability interpreted by a conventional well-test method is far lower than logging, which further leads to an inaccurate skin factor. This skin factor cannot match the real production situation and will mislead engineer to do an inappropriate development strategy of the oilfield. In order to solve this problem, key parameters affecting the skin factor need to be found. Based on the real core experiment and digital core experiment results, stress sensitivity and threshold pressure gradient are verified to be the most influential factors in the production of low-permeability reservoirs. On that basis, instead of a constant skin factor, a well-test interpretation mathematical model is established by defining and using a time-varying skin factor. The time-varying skin factor changes with the change of stress sensitivity and threshold pressure gradient. In this model, the Laplace transform is used to solve the Laplace space solution, and the Stehfest numerical inversion is used to calculate the real space solution. Then, the double logarithmic chart of dimensionless borehole wall pressure and pressure derivative changing with dimensionless time is drawn. The influences of parameters in expressions including stress sensitivity, threshold pressure, and variable skin factor on pressure and pressure derivative and productivity are analyzed, respectively. At last, the method is applied to the well-test interpretation of low-permeability oil fields in the eastern South China Sea. The interpretation results turn out to be reasonable and can truly reflect the situation of low-permeability reservoirs, which can give guidance to the rational development of low-permeability reservoirs.

The Role of the Mindoro-Sibutu Pathway on the South China Sea Multi–layer Circulation

AbstractThe role of the Mindoro Strait-Sibutu passage pathway in influencing the Luzon Strait inflow to the South China Sea (SCS) and the SCS multi-layer circulation is investigated with a high-resolution (0.1° × 0.1°) regional ocean model. Significant changes are evident in the SCS upper layer circulation (250-900 m) by closing the Mindoro-Sibutu pathway in sensitivity experiments, as Luzon Strait transport is reduced by 75%, from −4.4 Sv to −1.2 Sv. Because of the vertical coupling between the upper and middle layers, closing the Mindoro-Sibutu pathway also weakens clockwise circulation in the middle layer (900–2150 m), but there is no significant change in the deep layer (below 2150 m). The Mindoro-Sibutu pathway is an important branch of the SCS throughflow into the Indonesian Seas. It is also the gateway for oceanic waves propagating clockwise around the Philippines Archipelago from the western Pacific Ocean into the eastern SCS, projecting El Niño-Southern Oscillation sea level signals to the SCS, impacting its interannual variations and multi-layer circulation. The results provide insights into the dynamics of how upstream and downstream passage throughflows are coupled to affect the general circulation in marginal seas.

Topographic Rossby Waves in the Abyssal South China Sea

AbstractTopographic Rossby waves (TRWs) in the abyssal South China Sea (SCS) are investigated using observations and high-resolution numerical simulations. These energetic waves can account for over 40% of the kinetic energy (KE) variability in the deep western boundary current and seamount region in the central SCS. This proportion can even reach 70% over slopes in the northern and southern SCS. The TRW-induced currents exhibit columnar (i.e., in-phase) structure in which the speed increases downward. Wave properties such as the period (5–60 days), wavelength (100–500 km), and vertical trapping scale (102–103 m) vary significantly depending on environmental parameters of the SCS. The TRW energy propagates along steep topography with phase propagation offshore. TRWs with high frequencies exhibit a stronger climbing effect than low-frequency ones and hence can move further upslope. For TRWs with a certain frequency, the wavelength and trapping scale are dominated by the topographic beta, whereas the group velocity is more sensitive to the internal Rossby deformation radius. Background circulation with horizontal shear can change the wavelength and direction of TRWs if the flow velocity is comparable to the group velocity, particularly in the central, southern, and eastern SCS. A case study suggests two possible energy sources for TRWs: mesoscale perturbation in the upper layer and large-scale background circulation in the deep layer. The former provides KE by pressure work, whereas the latter transfers the available potential energy (APE) through baroclinic instability.

Cenozoic Sea‐land Transition and its Petroleum Geological Significance in the Northern South China Sea

AbstractThe process of Cenozoic sea‐land changes in the northern South China Sea (SCS) controlled the sedimentary filling pattern and played an important role in the petroleum geological characteristics of the northern marginal sedimentary basins. Under the control of the opening process of the SCS, the northern SCS Cenozoic transgression generally showed the characteristics of early in the east and late in the west, and early in the south and late in the north. The initial transgression occurred in the Eocene in the Taixinan Basin (TXNB) of the eastern SCS, while the transgression occurred until the Pliocene in the Yinggehai Basin (YGHB) of the western SCS. International Ocean Discovery Program (IODP) expeditions (Expeditions 367/368) revealed that the initial transgression of the SCS basin occurred at approximately 34 Ma, which was the initial opening time of the SCS. The period of drastic changes in the sedimentary environment caused by large‐scale transgression corresponded to the opening time of the southwestern subbasin (approximately 23 Ma), which also represented the peak of the spreading of the SCS. The sea‐land transition process controls the distribution of alternating continent‐marine facies, marine facies source rocks and reservoirs in the basins. The marine facies source rocks of the basins in the northern SCS have a trend of gradually becoming younger from east to west, which is consistent with the regional process of gradual transgression from east to west. Regional sea‐level changes were comprehensively influenced by SCS opening and global sea‐level changes. These processes led to the early development in the east and south and late development in the west and north for the carbonate platform in the SCS. Carbonate platforms form another type of “self‐generating and self‐accumulating” oil‐gas reservoir in the northern SCS. The sea‐land transition controlled the depositional filling patterns of different basins and laid the foundation of marine deposits for oil and gas resources. The source‐reservoir‐cap assemblage in the northern SCS was controlled horizontally by provenance supply and sedimentary environmental changes caused by sea‐land transition and vertically by the tectonic evolution of the SCS and regional sea‐level changes.