Depositional Characteristics and Formation Mechanisms of Deep-Water Canyon Systems along the Northern South China Sea Margin

Submarine canyon systems are sites for coarser clastic sediment accumulations in the deep-water domains, having the most potential for hydrocarbon reservoirs. Based on the interpretation of high resolution 2 D/3 D seismic and drilling data, depositional characteristics of three large deep-water canyon systems on the South China Sea northern margin have been analyzed. The Central Canyon System has a deep incision geomorphology extending from east to west, featured by distinct canyon segmentations, multi-provenance sediment supplies and multi-stage canyon fillings. The Pearl River Canyon System’s formation is closely related to the development of Pearl River Delta. Its vertical stacking and migrating canyon patterns have changed over time. The depositional architectures and evolution of the recent Penghu-Gaoping Canyon System respond to tectonic movements along the Taiwan-Luzon convergent continental margin. The main controlling factors of the formation and evolution of these three canyon systems include the tectonic setting, sediment supply, sea level change and paleo-geomorphology, among which the former two are dominant. The Penghu-Gaoping Canyon System formed along the subduction structural zone, directly indicating a typical tectonic origin. Numerous seismic data show that the Central Canyon and Pearl River Canyon systems are obviously affected by tectonics, associated local topography and sediment supply.

Characteristics and origin of continental marginal fault depressions under the background of preexisting subduction continental margin,northern South China Sea,China

Based on the new seismic and drilling data and the recent related research results,this paper systematically analyzes the diversity and complexity of evolution process of crustal lithosphere structure and basin structure in the Pearl River Mouth Basin on the northern margin of the South China Sea.Three types of detachment faults of different structural levels exist:crust-mantle detachment,inter-crust detachment and upper crust detachment.It is considered that different types of extensional detachment control different subbasin structures.Many fault depressions controlled by upper crust detachment faults have been found in the Zhu I Depression located in the proximal zone.These detachment faults are usually reformed by magma emplacement or controlled by preexisting faults.Baiyun-Liwan Sag located in the hyperextension area shows different characteristics of internal structure.The Baiyun main sag with relative weak magmatism transformation is a wide-deep fault depression,which is controlled by crust-mantle detachment system.Extensive magmatism occurred in the eastern and southwest fault steps of the Baiyun Sag after Middle Eocene,and the crust ductile extensional deformation resulted in wide-shallow fault depression controlled by the upper crust detachment fault.Based on the classical lithosphere extensional breaking and basin tectonic evolution in the Atlantic margin,it is believed that the magmatic activities and pre-existing structures in the Mesozoic subduction continental margin background are important controlling factors for the diversified continental margin faulted structures in the northern South China Sea.

Middle and Late Tertiary Palaeogeography and Palaeoenvironment on the Northern Continental Margin of the South China Sea

Based essentially on research results of calcareous nannofossils, combined with some other microfossil da-ta and several secondary depositional breaks, this paper discusses the criteria of division and comparison of themiddle and late Tertiary marine sediments, palaeogeographical and palaeoenvironmental evolution andpalaeoclimates on the northern continental margin of the South China Sea, comprising the Tainan basin, PearlRiver Mouth basin. Southeast Hainan basin and Beibu Gulf basin. Study shows that the upper Oligocene toPliocene strata in the whole area consist essentially of marine sediments except in the Beibu Gulf basin. Theyinclude littoral. neritic and deltaic sediments as well as carbonate rock-bioherm limestone. The sea advancedfrom southeast to northwest. During the transgression there appeared three culminations coinciding to thestages of deposition of nannofossil zones NN4-5, NN11 and 13-15.

Petroleum geology controlled by extensive detachment thinning of continental margin crust: A case study of Baiyun sag in the deep-water area of northern South China Sea

The relationships between crustal stretching and thinning,basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic,gravity,magnetic,ocean bottom seismic(OBS),deep-water exploration wells and integrated ocean drilling program(IODP).During the early syn-rifting period,deep-water area was a half-graben controlled by high angle faults influenced by the brittle extension of upper crust.In the mid syn-rifting period,this area was a broad-deep fault depression controlled by detachment faults undergone brittle-ductile deformation and differentiated extension in the crust.In the late syn-rifting period,this area experienced fault-sag transition due to saucer-shaped rheology change dominated by crustal ductile deformation.A broad-deep fault depression controlled by the large detachment faults penetrating through the crust is an important feature of deep-water basin.The study suggests that the broad-deep Baiyun sag provides great accommodation space for the development of massive deltaic-lacustrine deposition system and hydrocarbon source rocks.The differentiated lithospheric thinning also resulted in the different thermal subsidence during post-rifting period,and then controlled the development of continental shelf break and deep-water reservoir sedimentary environment.The high heat flow background caused by the strong thinning of lithosphere and the rise of mantle source resulted in particularities in the reservoir diagenesis,hydrocarbon generation process and accumulation of deep-water area in northern South China Sea.

南海北部被动陆缘洋陆转换带张裂–破裂研究新进展

文章基于深地震探测数据与大洋钻探资料,研究了南海北部被动陆缘洋陆转换带张裂–破裂机制科学问题。2018-2023年在国家基金委共享航次的协助下,实施了三维深地震探测实验,共投放海底地震仪52台次,海底电磁仪2台次,放炮数量达8750炮,累积获得了294Gb的第一手数据,并取得了一系列的创新性成果:1)首次获得南海北部陆缘洋陆转换带深达上地幔的三维速度结构,精准确定了研究区洋陆转换带三维空间展布范围宽度为10~20km;2)揭示南海北部陆缘异常活跃的岩浆作用,提出了不同时间段的构造与岩浆相互作用方式,岩浆活动随着伸展作用不断向洋迁移并逐渐占据主导地位;3)建立了边缘海独特的张裂–破裂地质演化模型,最后一期强烈的岩浆活动导致岩石圈破裂,形成由玄武岩为主的“鳄鱼嘴”构造,同时张裂–破裂过程沿其陆缘走向又存在显著的构造–岩浆差异性。

The deep thermal characteristic of continental margin of the northern South China Sea

Heat flow plays an important role in the study ot thermal structure and thermal evolutionof continental margin of the northern South China Sea. The analysis of heat flow value shows that margin heat flow in the northern South China Sea is relatively high setting, but the percentage of crustal heat flow is lower than 35% in terrestrial heat flow. The terrestrial heat flow exhibited a current of rise from the Northern Continental Margin to the Southern Central Basin. However, the proportion of crustal heat flow in terrestrial heat flow slowly dropped down in the same direction. It is suggested that the main factor causing high heat flow setting is the moving up of hot material from asthenosphere.

Geodynamic characteristics of tectonic extension in the northern margin of South China Sea

Based on the geothermal and gravitation methods, this paperinvestigated the rheological and thermal structure of the lithosphere under the northern margin of South China Sea. The result shows that the temperature of the upper crust is 150-300℃ lower than that of the lower crust, and the viscous coefficient of the upper crust is 2-3 orders of magnitude larger than that of the lower crust. It reveals that the upper crust is characterized by brittle deformation while the lower crust by ductile deformation. A channel of lower-viscosity should be formed between the upper and lower crust when the lithosphere is scattered and spreads out toward ocean from northwest to southeast along the northern margin of South China Sea. And, a brittle deformation takes place in the upper part of the lithosphere while a ductile deformation takes place in the lower part of the lithosphere due to different viscous coefficients and temperature. The layered deformation leads the faulted blocks to rotate along the faulting and the marginal grabens to appear in the northern margin of South China Sea in Cenozoic tectonic expansion.

Stretching characteristics and its dynamic significance of the northern continental margin of South China Sea

To investigate the thinning of the whole crust, and the contribution of the upper versus lower crust to the stretching since Cenozoic, we calculated the stretching factor of the northern margin of South China Sea with data such as whole crustal thickness, depth of Cenozoic sedimentary basement and the horizontal displacement of faults. An isometric line drawing on whole crustal stretching factor is then obtained. Along the seismic Line 1530 in Baiyun sag, we also calculated the stretching factors of the upper and lower crust. The results suggest that the whole crustal thickness decreases seaward while the whole crustal stretching factor increases from shelf to slope. The Moho upwells highest beneath where the crust is thinned most. The value of the whole crustal stretching factor ranges from 1.5 to 6. Two areas were thinned intensely: the center of Yinggehai Basin, and the Baiyun sag in the Pearl River Mouth Basin. The calculation of the upper and lower crustal stretching factors from DSP1530 in Baiyun sag shows that the original crust of Baiyun sag should be thinned before deformation. Its pre-Cenozoic evolution as well as tectonic position during Cenozoic might be responsible for that.

Deep Structure and Dynamics of Passive Continental Margin from Shelf to Ocean of the Northern South China Sea

To study the deep dynamic mechanism leading to the difference in rifting pattern and basin structure from shelf to oceanic basin in passive continental margin, we constructed long geological sections across the shelf, slope and oceanic basin using new seismic data. Integrated gravity-magnetic inversion and interpretation of these sections were made with the advanced dissection method. Results show that the basement composition changes from intermediate-acid intrusive rocks in the shelf to intermediate-basic rocks in the slope. The Moho surface shoals gradually from 31 km in the shelf to 22.5 km in the uplift and then 19 km in the slope and finally to 13 km in the oceanic basin. The crust thickness also decreases gradually from 30 km in the northern fault belt to 9 km in the oceanic basin. The crustal stretching factor increases from the shelf toward the oceanic basin, with the strongest extension under the sags and the oceanic basin. The intensity of mantle upwelling controlled the style of basin structures from shelf to oceanic basin. In the Zhu 1 depression on the shelf, the crust is nearly normal, the brittle and cold upper crust mainly controlled the fault development; so the combinative grabens with single symmetric graben are characteristic. In the slope, the crust thinned with a large stretching factor, affected by the mantle upwelling. The ductile deformation controlled the faults, so there developed an asymmetric complex graben in the Baiyun （白云） sag.

南海北部陆缘盆地深水区油气勘探新认识及攻关方向

南海北部陆缘盆地深水区已成为我国海洋油气勘探增储上产的重要领域,但由于构造演化史复杂,该区不同凹陷的沉积—构造特征、烃源岩生烃—储层成岩演化和成藏模式均存在明显差异。为准确认识该区油气勘探潜力和方向,基于前人研究成果和勘探实践,在综合分析烃源岩、储层、差异成藏等地质条件的基础上,剖析了南海北部陆缘盆地深水区成藏特征、油气勘探作业面临的挑战,并指出了下一步油气攻关研究方向。研究结果表明:①受地壳拆离作用及强烈薄化影响,该区地层沉积特征特殊,岩石地球物理性质多样;②软流圈差异抬升形成深水区独特的高热流背景,促使区域性湖相、煤型及叠合型等3类烃源灶快速、高强度生烃,生烃门限变浅;③深水盆地群的演化和成盆规模差异大,构造演化控制了盆地几何形态和储集体时空展布,深水区独特的板块构造位置、复杂的构造演化史、温压变化剧烈的地质特征等控制了油气成藏模式。结论认为:①该区油气勘探面临地震资料解析度低、规模有效储层预测困难、勘探成本高等技术与经济的双重挑战;②需要创新形成高地温背景下优质储层差异化分布规律的研究思路;③亟需提升勘探设备能力建设,攻关高效地震采集、“重磁电震”地球物理一体化采集处理、双井架钻探作业、多井段“一趟钻”等勘探关键技术,为我国海洋深水油气勘探开发的高质量发展做好理论和技术支撑。

Dongsha erosive channel on northern South China Sea Shelf and its induced Kuroshio South China Sea Branch

Based on a data base of multi-channel seismic profiles covered over Dongsha plateau of the northern South China Sea margin, we found that the sea bed morphology of northern South China Sea margin had been changed dramatically after Dongsha uplifting, that sedimentary layer since Miocene age had been eroded with maximum eroded thickness more than 1000 m, and that an erosive channel had been formed of 20 km in width and 200 km in length and several hundreds meters in depth on the outer shelf of northern South China Sea. The erosive channel is parallel to the 600 m isobath line, stretching from northeast to the southwest north of Dongsha uplift. The Kuroshio intrudes the South China Sea through Luzon Strait both in winter time and summer time, and in the northern South China Sea margin area, the intruded Kuroshio Branch takes the form of Pacific-Indian Ocean Through Flow (PITH) in winter time, while the Luzon Strait Subsurface Inflow (LSSIF) in summer time, the routes of both PITH and LSSIF coincide well with the distribution of the erosive channel. After climbing from the northern slope up to the northern shelf, and after joined by the southward flow from the middle northern shelf of South China Sea, the Kuroshio Branch is strengthened and thus is able to erode the sea floor, and the shape of the erosive channel is a result of the long-term interaction between the Kuroshio South China Sea Branch and the Dongsha outer shelf sea floor.

南海南缘礼乐盆地在南海扩张前的位置恢复

本文依据洋中脊两侧磁条带的对称性和同期磁条带的等时性,利用南海已有的磁异常条带资料,假定华南大陆连同南海北缘位置相对固定,南海南缘从32 Ma开始向南漂移,恢复了南海的扩张历史和礼乐盆地在不同时期的相对位置。结果表明,礼乐盆地在南海扩张前的位置紧邻珠江口盆地东南缘。这一结果从礼乐盆地与南海北缘盆地在基底性质、中生代地层的发育情况和沉积特征以及新生代构造活动和沉降沉积特征的对比得到了进一步佐证,为礼乐盆地的构造演化、沉积充填特征和沉积物源分析、石油地质条件研究提供了重要参考。

南海北部陆缘超深水区珠四坳陷地质结构及其对烃源岩发育的控制

珠江口盆地珠四坳陷整体水深超过1500 m,属极低勘探程度区域,钻井发现的油气显示证实了该区存在烃源岩。地震资料揭示古近系是主力烃源岩发育层系,沉积厚度及面积均较大。珠四坳陷作为南海北部向超深水迈进的油气勘探新区,其各凹陷的石油地质条件亟待剖析。在珠江口盆地6种断陷形成机制认识的基础上,通过对新采集地震资料的系统分析,将珠四坳陷划分大西洋贫岩浆型大陆边缘的壳间韧性流变型、大西洋富岩浆型大陆边缘的壳上岩浆底侵型、大西洋贫岩浆型大陆边缘的壳上先存拼合型3类断陷结构。运用2Dmove软件及去压实技术恢复各个洼陷沉降速率和伸展速率,结合大量地震剖面深入研究了珠四坳陷地质结构及其对烃源岩发育的影响,明确了不同断陷结构下烃源岩发育背景的差异,以此认为荔湾、鹤山凹陷烃源岩发育条件优越,可作为南海北部超深水地区的重要勘探突破点。

华南北东向断裂在南海北部陆架的延伸

南海北部陆缘发育大量含油气盆地,且南海北部陆坡存在丰富的天然气水合物资源,厘清其总体构造格局,对理解南海北部陆缘的构造演化及动力学机制、地震灾害评估与资源开发等具有十分重要的意义。本文通过重磁异常、二维地震资料解译,结合地震活动及构造地貌特征分析,发现华南陆块和南海北部陆缘的NE向主干断裂延伸方向一致且特征可进行对比,即华南发育的滨海、长乐-南澳、政和-大埔及邵武-河源-阳江四条NE向主干断裂可自然延伸至南海北部陆缘。延伸至南海北部的四条主干断裂控制了相应盆地的发育,其中,滨海断裂为珠二凹陷的东界,长乐-南澳断裂横穿珠江口盆地的珠一和珠二凹陷,政和-大埔断裂位于珠一、珠二坳陷的西界以及北礁凹陷的东界,河源-阳江断裂为琼东南盆地的西界。南海北部陆缘盆地群与华南陆缘盆地群在成因上具有相似性,皆为拉分盆地。南海北部陆缘分布的NE向右行断裂对盆地的发育以及展布起决定性作用,为控盆断裂。NEE向断裂控制盆地内部坳陷的充填样式,为控坳断裂。

南海北部边缘盆地CO_2成因及充注驱油的石油地质意义

在获取大量地质、地球化学资料的基础上 ,对南海北部边缘盆地CO2 的成因及充注驱油特征进行了分析 ,结果表明 :南海北部边缘盆地CO2 可划分为壳源型岩石化学成因、壳源型有机成因、壳幔混合型及火山幔源型成因 4种成因类型 ;其中 ,莺歌海盆地壳源型及壳幔混合型CO2 运聚成藏主要受控于泥底辟热流体晚期分层分块多期局部上侵活动与上新统—中新统海相含钙砂泥岩的物理化学综合作用 ;琼东南盆地东部及珠江口盆地火山幔源型CO2 成藏主要受控于幔源型火山活动及沟通深部气源的基底深大断裂的展布 ;CO2 运聚成藏中 ,其充注驱油过程主要受运聚输导条件及气源供给等诸多地质关键因素的制约和控制。由于CO2 充注驱替往往导致油气藏中油气再分配或重新组合 ,并引起原来的油气产出及产状特征发生变化 ,故容易形成新的油藏或气藏。因此 ,可以将CO2 充注驱油特征作为判识油气成藏动态过程的示踪标志 ,用于预测油气运聚状态 。

南海北部新生代盆地群构造特征及其成因

南海北部陆缘自西向东分布有北部湾、琼东南、珠江口和台西南等新生代盆地。前人认为这些盆地是华南大陆东南缘裂解直至南海北部被动陆缘形成过程中逐渐形成的,但大量地震剖面揭示,南海北缘主控盆断裂倾向陆地,与典型的被动陆缘的主断裂倾向海盆的特征明显不符。因而,南海北部陆架盆地成因显然不是被动大陆边缘的Mckenzie伸展机制。为此,基于大量陆地调查和海域地震剖面资料的对比,揭示了南海北部陆缘至少在34Ma之前不是被动大陆边缘,早期陆缘断裂十分发育,主控断层为NE—NNE走向,和陆地同期走滑断层具有连续性。这些NNE—NE向断裂右行右阶走滑控制了拉分盆地内的EW或NEE方向的次级断裂,并控制了盆地内部近EW向的次级构造单元展布。因此,新生代南海北部陆缘的一系列盆地是动力学成因上具有密切联系的右行右阶拉分盆地群。这个拉分成因模式与南海北部陆缘新生代盆地内部沉积沉降中心迁移、构造跃迁、岩浆展布等特征非常一致。而南海北部真正成为典型被动大陆边缘的时间是在15Ma之后,但此时南海却停止了扩张,而且大约在10～5Ma由于菲律宾海板块沿吕宋岛弧-台湾造山带逐步楔入欧亚板块导致最后的弥散性NWW向断裂切割南海北部所有构造。从盆地动力学考虑,南海北部陆架盆地的成因主要与太平洋板块的动力学联系较为紧密。

南海北部陆缘地壳结构探测结果分析

深部地震和重力资料反演揭示了南海北部陆缘地壳结构在总体上由北部的华南沿海（厚约30km）向南部的洋盆（5──8km）逐渐减薄。南海的近SN向拉张不仅造成南北方向地壳结构的巨大变化，也造成东西向的明显变化。在南海北部陆缘的西部，局部拉张产生了一系列裂谷构造。西沙海槽作为一条狭窄的陆内裂谷向西延伸，海槽南北两侧地壳厚度超过25km，海槽中部地壳减薄至不足10km。西端的莺歌海盆地地壳厚仅5km，缺少明显的壳内反射－折射。在珠江口盆地中部，地壳厚度在下陆坡明显减薄，地壳下部存在较薄的（3──4km）高速层（地震波速7．2──7．5km·s－1）；在珠江口盆地东部，地壳底部存在约 10km厚、300km宽的高速层。在台湾地区，由于弧陆碰撞，曾经减薄的陆壳在碰撞带增厚，莫霍面深度超过30km。南海北部陆缘在裂谷拉张和海底扩张期间岩浆活动平静，表明南海北部陆缘为非火山型陆缘。

南海北部陆缘西沙海槽—台湾恒春半岛地学断面

利用南海北部陆缘及邻区的地质、地球物理资料,综合编制了印支半岛东缘—西沙海槽—南海北部陆坡—台湾恒春半岛—菲律宾海地学断面,从地形地貌、地球物理场、地质构造特征和时空演化等多个方面综合揭示了南海北部陆缘洋陆转换带岩石圈结构及其相互作用,对深入认识南海北部陆缘构造格局和深水区油气地质具有重要意义。

琼东南盆地地壳伸展深度依赖性及其动力学意义

地壳或岩石圈尺度内伸展因子随深度变化特征对于理解岩石圈演化有重要的指示意义.我们利用南海北部大陆边缘琼东南盆地区深反射地震剖面的地壳分层模型,计算了沿剖面上地壳与全地壳的伸展因子.结果表明:琼东南盆地区具有明显的地壳尺度内伸展的深度相关性(上地壳尺度伸展因子变化范围为1.0～2.0,全地壳尺度的伸展因子变化范围为1.2～2.5);琼东南盆地各构造单元内的上地壳与全地壳伸展具有明显的非均一性(长昌凹陷上地壳尺度伸展最大,乐东陵水凹陷其次,松南—宝岛凹陷最小;长昌凹陷和松南宝岛凹陷的地壳尺度伸展因子较乐东陵水凹陷大)与各向异性(南东北西剖面较之北东南西向剖面地壳伸展因子大).这些结果预示着琼东南盆地区地壳伸展优势方向为北西向,盆地区东四部的伸展过程或伸展机制可能差异较大拟或存在太平洋岩石圈俯冲角空间差异或地幔岩浆产出时空差异.结合研究区相关研究成果,推断地壳伸展因子的深度相关性可能是共轭大陆边缘低角度拆离控制的简单剪切系统内伴随地幔挤出的动力学现象.

张裂型盆地地热参数的垂向变化与琼东南盆地热流分布特征

海洋热流数据是开展海洋地球动力学研究和油气资源评价的基础数据.为深入认识琼东南盆地的地热特征,本文首先利用耦合沉积作用与岩石圈张裂过程的数值模型分析了张裂型盆地主要地热参数的垂向变化特征;并通过钻孔资料的详细分析,获得了琼东南盆地44口钻孔的热流数据;结合海底地热探针获取的热流数据,对琼东南盆地地热特征及其主要影响因素进行了简要分析.结果表明：沉积作用的热披覆效应对表层热流有较明显的抑制作用,由于沉积物生热效应与披覆效应的共同作用,同一钻孔处海底表层热流与钻孔深度3000~4000m处热流或与海底间的平均热流差异很小,可以一起用于分析琼东南盆地的热流分布特征;莺歌海组、乐东组热导率随深度变化小于黄流组及其下地层热导率的变化,钻孔沉积层平均热导率约为1.7 W·（m·K）-1,钻孔地层生热率一般低于2.5μW·m-3,平均生热率为1.34μW·m-3,平均地温梯度主要介于30~45℃/km,热流介于50~99mW·m-2,陆架区热流主要集中于60~70mW·m-2,深水区钻孔具有较高的地温梯度和热流值;从北部陆架与上陆坡区往中央坳陷带,热流值从50~70mW·m-2,增高为65~85mW·m-2,并且往东有升高趋势,在盆地东部宝岛凹陷、长昌凹陷与西沙海槽北部斜坡带构成一条热流值高于85mW·m-2的高热流带.进一步分析认为,琼东南盆地现今热流分布特征是深部热异常、强烈减薄岩石圈的裂后冷却作用、晚期岩浆热事件、地壳与沉积层的生热贡献以及沉积作用的热披覆效应等多种主要因素综合作用的结果.