Geophysical evidence for cyclic sediment deposition on the southern slope of Qiongdongnan Basin,South China Sea

Gravity flow deposits form a significant component of the stratigraphic record in ancient and modern deep-water basins worldwide. Analyses of high-resolution 3D seismic reflection data in a predominantly slope setting, the southern slope of Qiongdongnan Basin, South China Sea, reveal the extensive presence of gravity flow depositional elements in the Late Pliocene−Quaternary strata. Three key elements were observed: (1) mass transport deposits (MTDs) including slumps and debris flows, (2) turbidity current deposits including distributary channel complexes, leveed channel complexes and avulsion channel complexes, and (3) deep-water drapes (highstand condensed sections). Each depositional element displays a unique seismic expression and internal structures in seismic profiles and attribute maps. Based on seismic characteristics, the studied succession is subdivided into six units in which three depositional cycles are identified. Each cycle exhibits MTDs (slump or debris) at the base, overlain by turbidities or a deep-water drape. The genesis of these cycles is mainly controlled by frequent sea-level fluctuations and high sedimentation rates in the Late Pliocene–Quaternary. Moreover, tectonics, differential subsidence, and paleo-seafloor morphology may have also contributed to their formation processes. The present study is aimed to a better understanding of deep-water depositional systems, and to a successful hydrocarbon exploration and engineering-risk assessment.     

南海北部陆坡琼东南盆地晚中新世以来沉积物来源及输送样式

为了更好地揭示南海北部陆坡琼东南盆地晚中新世以来的沉积物输送样式,本次研究将盆地裂后期加速沉降阶段以来的沉积物充填样式作为研究对象,基于前人对这一区域潜在物源区的分析,通过对已有勘探成果的总结和归纳,对深水沉积体的类型进行识别,建立具有成因关系或相同来源的深水沉积体组合,尝试对沉积物输送样式进行划分和归类。研究结果认为,晚中新世以来,琼东南盆地主要存在海南岛物源、莺西物源、南部隆起带物源和神狐隆起物源等4个潜在物源区,沉积物输送样式可划分为垂向沉积物输送、轴向沉积物输送和转向沉积物输送3种类型。     

琼东南深水区海陆过渡相富生烃凹陷评价及优选

琼东南盆地古近系崖城组被证实为海陆过渡相烃源岩,但是深水盆地内6个凹陷的特征及演化存在显著差异,如何确定最富生烃的凹陷直接关系到深水钻探的成效。本文在深水凹陷区域构造形成机制、沉积环境演变特征以及海陆过渡相烃源岩有机质特征分析的基础上,充分利用现有钻井和地震资料,首先依据地震相模式分析方法预测了烃源岩层段沉积相分布,并根据沉积相与有机相的对应关系,预测了有机相分布;同时采用地震速度岩性定量分析技术确定出各凹陷烃源岩厚度分布,并利用地震反演速度及区域内泥岩孔隙度和烃源岩Ro的关系,定量预测了源岩热成熟度分布;然后依据烃源岩有机相、厚度和热成熟度等参数计算了崖城组各层段生气量和生气强度;最后以这两个参数为主,结合资源量和油气发现概况,建立了深水区富生烃凹陷评价标准,以此对6个凹陷进行综合评价优选。研究认为陵水、乐东、宝岛和长昌四个凹陷是Ⅰ类(最富生烃)凹陷,而松南和北礁凹陷为Ⅱ类(较富生烃)凹陷。该评价结果对南海北部深水区下一步勘探部署和目标钻探有重要的指导意义。     

南海琼东南盆地新生代构造层序研究

基于南海琼东南盆地到目前为止还没有一个统一的构造层序划分方案的问题,在前人研究工作基础上,通过大量的二维地震构造层序闭合解释,从地震不整合面和构造发育特征识别出发,对新生代主要构造层序进行详细解剖。进一步结合对南海北部琼东南盆地新生代二维地震数据的精细综合分析,重新厘定了其新生代构造层序,并进行了构造层序的识别和划分。结果表明：按古构造运动面可将盆地充填序列划分为上、中、下三个构造层序,分别对应于盆地演化的三个阶段性。着重论述了三个构造层序的结构特征、叠加构造样式、构造层序发育特征、层序分布特征、沉积体系类型和盆地断裂演化序列之间的关系等,以期为今后的研究奠定基础。     

琼东南盆地南部早中新世碳酸盐岩台地的地震解释

The southern uplift of the Qiongdongnan Basin is a deepwater area in which no wells have beens drilled. The Miocene-Quaternary strata in the Xisha Islands, which are located 40–100 km to the south, are composed of carbonate reef formations. Paleotectonic and paleogeographic analyses of the basin suggest that the southern uplift experienced favorable geological conditions for the development of carbonate reefs during the Miocene.The high-impedance carbonates have high amplitudes and low frequencies on seismic profiles. The reefs are distributed on paleotectonic highs and are thicker than the contemporaneous formations. A forward model of the variation in carbonate thickness based on lithological and velocity information from wells in nearby regions can simulate the seismic response of carbonates with different thicknesses. We identified several important controlling points for determining the thickness of carbonates from seismic profiles, including the pinchout point,the λ/4 thickness point, and the λ/2 thickness point. We depict a carbonate thickness map in the deepwater area of the southern Qiongdongnan Basin based on this model. The carbonate thickness map, the paleotectonic and paleogeographic background, and the seismic response characteristics of reefs suggest that the carbonates that developed on the southern uplift of the Qiongdongnan Basin during the Miocene were mainly an isolated carbonate platform peninsula and ramp deposits. It consisted of gentle ramp platform, steep slope platform,platform depression, gravity flow, and reef bank facies.     

Seismic features and origin of sediment waves in the Qiongdongnan Basin, northern South China Sea

Sediment waves have been documented around the world for several decades, and their origins are still debated because of their various characteristics in different settings. Based on numerous high-resolution seismic profiles and two boreholes, sediment waves are identified in deepwater areas of the eastern Qiongdongnan Basin, and their distribution and seismic features are illustrated. Combined with the bathymetry, the potential origins of these sediment waves are discussed. Drilling in the central canyon revealed that the channel infill comprises some along-slope fine-grained turbidites, which are good reservoir for gas plays. The sediment waves are distributed on the banks of the central canyon and their seismic features indicate that most of them are caused by turbidity current overflows along the canyon. Although previous researches on these sediment waves suggested that they were of westward-flowing contourite origin, detailed topographic map derived from the seafloor reflector on seismic data shows that there is a N–S trending ridge at the east part of sediment wave zones, which could block and divert the bottom current. According to the geometry of sediment waves, the flow thicknesses across the entire wave field are calculated as 280–560 m, and the current velocity falls in the range of 30–130 cm/s, which would favor a fine-grained composition and could be a good reservoir because of the better sorting of turbidites than contourites or other gravity flow deposits.     

The dynamic mechanism of post-rift accelerated subsidence in Qiongdongnan Basin, northern South China Sea

A 1-D unloaded tectonic subsidence (air-loaded tectonic subsidence) model is proposed and applied to the Qiongdongnan Basin. Results show that three episodes of subsidence exist in Cenozoic, that is, syn-rift rapid subsidence (Eocene–Oligocene) with subsidence rate at 20–100 m/m.y., post-rift slow thermal subsidence (early-middle Miocene) around 40 m/m.y., and post-rift accelerated subsidence (since late Miocene) 40–140 m/m.y., which is substantially deviated from the exponentially decayed thermal subsidence model. For exploring the mechanism of post-rift accelerated subsidence, the faulting analyses are conducted and results show that there is a dramatically decrease in the numbers of active faults and fault growth rate since 21 Ma, which indicates that no active brittle crust extension occurred during post-rift period. Furthermore, previous studies have demonstrated that the stretching of the upper crust is far less than that affecting the whole crust. Therefore, we infer that the lower crust thinned during the post-rift period and a new model of basin development and evolution is put forward to explain the post-rift accelerated subsidence and depth-dependent crust thinning in the Qiongdongnan Basin, which is supported by gravity data.     

Fine-grained Pleistocene deepwater turbidite channel system on the slope of Qiongdongnan Basin, northern South China Sea

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.     

南海北部琼东南盆地陵水段峡谷沉积建造及勘探意义

琼东南盆地中央峡谷为一大型轴向峡谷体系,具有明显的"分段性"特征,不同区段沉积充填和内部结构均存在明显的差异,其中陵水段位于峡谷西段,具有非常好的储层物性特征,是深水区峡谷勘探的重点区域。为了进一步精细刻画陵水段峡谷内部水道复合体的沉积充填及沉积微相展布特征,本文借助新增加的三维地震资料和新钻井资料,通过峡谷三级层序界面的识别,将峡谷充填划分为3个三级层序SQ1、SQ2和SQ3,并通过古生物有孔虫化石带、钙质超微化石带及井-震结合确定了峡谷底界为中中新世晚期S40界面,陵水段峡谷自西向东依次识别出6种主要的内部充填结构,且将SQ3层序细分为5个次级层序(SSQ1~SSQ5);结合属性特征,精细刻画了陵水段峡谷内沉积微相平面展布及空间演化特征,并指出点砂坝在SSQ1层序最发育,且规模较大,横向上连片分布,将是下步勘探的有利储集体。     

Sedimentary characteristics and genetic mechanism of the giant ancient pockmarks in the Qiongdongnan Basin,northern South China Sea

In the late Miocene, giant ancient pockmarks, which are fairly rare globally, developed in the Qiongdongnan Basin. In this paper, to determine the sedimentary characteristics and genetic mechanism of these giant ancient pockmarks in the Yinggehai Formation of the Qiongdongnan Basin, based on high-resolution 3D seismic data and multiattribute fusion technologies, we analyzed the planar distribution and seismic facies of the ancient pockmarks and compared the characteristics of the ancient pockmar...     

Geochemistry,origin and accumulation of natural gases in the deepwater area of the Qiongdongnan Basin,South China Sea

The Qiongdongnan Basin, South China Sea has received huge thickness (>12 km) of Tertiary-Quaternary sediments in the deepwater area to which great attention has been paid due to the recent discoveries of the SS22-1 and the SS17-2 commercial gas fields in the Pliocene-Upper Miocene submarine canyon system with water depth over 1300 m. In this study, the geochemistry, origin and accumulation models of these gases were investigated. The results reveal that the gases are predominated by hydrocarbon gases (98%–99% by volume), with the ratio of C₁/C_1-5 ranging from 0.92 to 0.94, and they are characterized by relatively heavy δ¹³C₁ (−36.8‰ to −39.4‰) and δD_CH4 values (−144‰ to −147‰), similar to the thermogenic gases discovered in the shallow water area of the basin. The C_5-7 light hydrocarbons associated with these gases are dominated by isoparaffins (35%–65%), implying an origin from higher plants. For the associated condensates, carbon isotopic compositions and high abundance of oleanane and presence of bicadinanes show close affinity with those from the YC13-1 gas field in the shallow water area. All these geochemical characteristics correlate well with those found in the shales of the Oligocene Yacheng Formation in the Qiongdongnan Basin. The Yacheng Formation in the deepwater area has TOC values in the range of 0.4–21% and contains type II_b–III gas-prone kerogens, indicating an excellent gas source rock. The kinetic modeling results show that the δ¹³C₁ values of the gas generated from the Yacheng source rock since 3 or 4 Ma are well matched with those of the reservoir gases, indicating that the gas pool is young and likely formed after 4 Ma. The geologic and geochemical data show that the mud diapirs and faults provide the main pathways for the upward migration of gases from the deep gas kitchen into the shallow, normally pressured reservoirs, and that the deep overpressure is the key driving force for the vertical and lateral migration of gas. This gas migration pattern implies that the South Low Uplift and the No.2 Fault zone near the deepwater area are also favorable for gas accumulation because they are located in the pathway of gas migration, and therefore more attention should be paid to them in the future.     

Characteristics and genesis of the polygonal fault system in southern slope of the Qiongdongnan Basin,South China Sea

Based on 3D seismic data, the polygonal fault system (PFS) discovered in the southern slope of the Qiongdongnan (QDN) Basin was studied through fine seismic interpretation and coherent cube analysis. The results show that: ① the PFS is mostly composed of small plate-type faults, with fault length of 1–3 km, maximum fault throw of 20–40 m and dip angle of 25°–45°; ② the PFS is mainly observed in the Huangliu Formation, deposited in the early stage of fast slope depression; ③ the PFS is layer-bound and can be separated into two tiers by T31′ interface with obvious channel incision features; ④ the PF of tier2 exhibit “y” structural style, including two parallel faults, master fault and associated fault, dipping in opposite directions. Their intersection relationship can be divided into three classes, and 14 sub-classes based on the fault intersection relationship of master fault and its associated fault; ⑤ the PFS is dominated by two strikes: NE60° and NW150°, indicating that it was controlled by tensile stress produced by differential settlement between the Xisha uplift and QDN basin; and ⑥ its genesis was jointly controlled by the syneresis of clay minerals and overpressure cyclical dehydration.     

南海北部琼东南盆地中央峡谷成因新认识

通过对区域构造断裂体系和逐渐连片的高分辨率三维地震资料的精细解析,认识到琼东南盆地中央峡谷的形成机制除了与晚中新世区域构造变动、大规模海平面下降、充足物源供给以及凹槽型古地形特征等因素相关之外,还存在另外一个非常重要因素:峡谷底部早期隐伏断裂带的存在。研究表明:琼东南盆地中央坳陷带发育平行于陆架坡折的大规模深水峡谷,峡谷底部发育大型走滑断层以及走滑断层派生出一系列次级断层形成的地层破碎带,认识到峡谷的形成、规模以及展布方向均受断裂带影响;相应地峡谷的充填及演化亦是受物源、海平面变化、重力流作用等多种因素共同作用和相互叠加的过程。从而为研究经历了裂陷期和坳陷期盆地演化过程形成的大型峡谷提供了科学依据。     

南海西北部琼东南盆地深水沉积环境演化及其油气勘探意义

Over the past several years, a number of hydrocarbon reservoirs have been discovered in the deepwater area of Qiongdongnan Basin, northwestern South China Sea. These oil/gas fields demonstrate that the...     

The Central Submarine Canyon in the Qiongdongnan Basin, northwestern South China Sea: Architecture, sequence stratigraphy, and depositional processes

Submarine canyons have been the subject of intense studies in recent years because of their close link to deepwater systems. The Central Canyon is a large unusual submarine canyon in the northwestern margin of the South China Sea, has a total length of about 425 km and is oriented sub-parallel to the continental slope. Using integrated 2D/3D seismic, well log, core, and biostratigraphy data, the current study documents the stratigraphic framework, internal architecture, depositional processes, and controlling factors of the segment of the Central Canyon located in the Qiongdongnan Basin.The integrated analysis shows that the canyon fill consists of four 3^rd-order sequences, SQ4, SQ3, SQ2, and SQ1. Each of them is bounded by regionally important erosional surfaces (3^rd-order sequence boundaries). Within each 3^rd-order sequence there is maximum regressive surface separating a regressive systems tract in the lower part and a transgressive systems tract in the upper part. Nine facies are identified and are further grouped into five depositional units, DU1 through DU5.The canyon evolved through four cut-and-fill stages, with a change from predominantly axial cut-and-fill to primarily side cut-and-fill. Axial cut-and-fill dominated during the first stage, and the slope-subparallel paleo Xisha Trough was intensely eroded by large-scale axial gravity flows. During the second cut-and-fill stage, the Central Canyon experienced both axial and side cut-and-fill. The third stage was dominated by side cut-and-fill. The canyon was eroded and fed by slope channels that transported sandy sediments from the shelf to the north during regression, and was covered by side-derived muddy MTCs during transgression. The last stage was also dominated by side cut-and-fill. The canyon, however, was filled predominantly by side-derived muddy MTCs.Evolution and depositional processes in the Central Canyon were likely controlled by slope-subparallel negative-relief induced by paleo-seafloor morphology, structural inversion of the Red River Fault and the slope-subparallel basement faults. Additionally, Coriolis force, sea-level fluctuations, high sedimentation rate, and rapid progradation of the slope also controlled and influenced the depositional processes, and internal architectures of the canyon.     

琼东南盆地井震地层对比分析及区域地层格架的建立

琼东南盆地历经断陷、断坳、裂后热沉降和裂后加速沉降等一系列的构造变动,沉积环境由始新世的滨海环境发展为现今的深水环境,形成了一套包括滨岸沉积、滨浅海沉积、陆架和陆坡沉积、以及半深海沉积的地层组合,具有良好的油气资源的生储盖条件,已成为当前油气资源勘探开发的重点区域。本文首先对盆地区域内钻井和地震剖面进行了主要地层界面(T20、T30、T40、T50、T60和T70)的识别和提取(点),继而结合连井地震剖面(线)和盆地区域过井地震剖面(面)对主要地层界面做了追踪对比分析,再依据古生物年代,建立了适用于琼东南盆地的区域地层年代格架。在琼东南盆地浅水区主要沉积了新近系地层(T60-T20),断裂基本不发育,地层厚度变化不大,极少有明显的上超和削截,局部地区发育有利于油气储集的三角洲沉积体系,表明琼东南盆地新近纪时期受构造作用影响较小。在深水区,新近系地层(T60-T20)和浅水区特征相似,仅反射特征有所不同;古近系地层(T100-T60)内部层序结构主要为楔状或近平行状,具有明显的上超和削截,地层厚度较大,断裂明显并导致地层错断,表明琼东南盆地深水区在古近纪时期主要受构造作用控制,并伴随着强烈的拉张和快速沉降作用,沉积环境主要为浅海。在近东西向的中央峡谷内存在有三期砂体:第一期砂体(井深3 528~3 336m,厚约192m)形成于距今11.6~5.5Ma(T40-T30),分布范围跨越中央峡谷的陵水-松南-宝岛段,沉积物构成包括浊积水道沉积、浊积席状砂、块体流沉积、深海泥质沉积、天然堤及漫溢沉积等;第二期砂体(井深4 100~3 900m,厚约200m)形成于距今5.5~4.2Ma(T30-T29),分布范围跨越中央峡谷的乐东-陵水段,以重力流沉积为主;第三期砂体(深度3 630~3 400m,厚约230m)发育于距今4.2~3.6Ma(T29-T28),分布于峡谷的乐东-莺东段,以浊积水道沉积为主。三期砂体在琼东南盆地中央坳陷带自东向西、由老到新依次展布,构成了良好的油气储层体。     

南海北部琼东南盆地冷泉区表层沉积物微量元素的分析及其对甲烷渗漏的指示

Recent studies have shown that specific geochemical characteristics of sediments can be used to reconstruct past methane seepage events. In this work, the correlation between the Sr/Ca and Mg/Ca ratios of sediment samples is analyzed and the sulfate concentration profile in Site C14 from cold-seep sediments in the Qiongdongnan Basin in northern South China Sea is obtained. The results confirmed that, sulfate at 0–247 cm below sea floor(Unit I) is mainly consumed by organic matter sulfate reduction(OSR), while sulfate at 247–655 cm(Unit II) is consumed by both the OSR and the anaerobic oxidation of methane(AOM). In addition, the bottom sediment layer is affected by weak methane seepage. The Mo and U enrichment factors also exhibit similar trends in their respective depth profiles. The responses of trace elements, including Co/Al, Ni/Al, Cr/Al and Zn/Al ratios to methane seepage allowed the study of depositional conditions and methane seepage events. Based on the results, it is speculated that the depositional conditions of Unit II changed with depth from moderate conditions of sulfidic and oxic conditions to locally anoxic conditions, and finally to suboxic conditions due to methane fluid leakage. The stable isotope values of chromium-reducible sulfide produced by AOM and those of sulfide formed by OSR in the early diagenetic environment suffered serious depletion of 34 S. This was probably due to weak methane leakage, which caused the slower upward diffusion and the effect of early diagenesis on the samples. It is necessary to consider the effects of depositional environments and diagenesis on these geochemical parameters.     

New perspective of Miocene marine hydrocarbon source rocks in deep-water area in Qiongdongnan Basin of northern South China Sea

Drilling wells reveal that the organic matter abundance of Miocene marine source rocks in shallow water area of the Qiongdongnan Basin is relatively low with poor hydrocarbon generation poten- tial. However, in some drilling wells of deep water area close to the central depression belt, Miocene marine source rocks with better organic matter abundance and hydrocarbon generation have been found, which have achieved better source rock standard based on the analysis of geochemical charac- teristics. Although there are no exploratory wells in deep water area of the research region, through the comparative analysis of geochemical data of several typical exploratory wells respectively from shallow water area in the basin, central depression belt margin in deep-water area of the basin and Site 1148 of deep sea drilling in the South China Sea Basin, it reveals that the tendency of the quality of source rocks becomes positive gradually from delta to bathyal environment, which then becomes negative as in deep oceanic environment. Owing to the lack of terrestrial organic matter input, the important controlling factors of Miocene marine source rocks in the Qiongdongnan Basin are ocean productivity and preservation conditions of organic matter. The element geochemistry data indicate that the tendency of the paleoproductivity and the preservation conditions of organic matter become positive as water depth increase from shallow area to bathyal area close to central depression belt. So it is speculated that there must exist high quality source rocks in the central depression area where the preservation conditions of organic matter are much better. Besides, in theory, in oxygen-poor zone of oceanic environment at the water depth 400-1 000 m, the preservation conditions of organic matter are well thus forming high-quality marine source rocks. The result- ing speculation, it is reasonable to consider that there are high hydrocarbon generation potential source rocks in bathyal environment of the Qiongdongnan Basin, especially at the water depth 400- 1 000 m.