Formation of the Zengmu and Beikang Basins,and West Baram Line in the southwestern South China Sea margin

The Zengmu and Beikang basins,separated by the West Baram Line(WBL)in the southwestern South China Sea margin,display distinct geological and geophysical features.However,the nature of the basins and the WBL are debated.Here we explore this issue by conducting the stratigraphic and structural interpretation,faults and subsidence analysis,and lithospheric finite extension modelling using seismic data.Results show that the WBL is a trans-extensional fault zone comprising normal faults and flower structures mainly active in the Late Eocene to Early Miocene.The Zengmu Basin,to the southwest of the WBL,shows an overall synformal geometry,thick folded strata in the Late Eocene to Late Miocene(40.4-5.2 Ma),and pretty small normal faults at the basin edge,which imply that the Zengmu Basin is a foreland basin under the Luconia and Borneo collision in the Sarawak since the Eocene.Furthermore,the basin exhibits two stages of subsidence(fast in 40.4-30 Ma and slow in 30-0 Ma);but the amount of observed subsidence and heat flow are both greater than that predicted by crustal thinning.The Beikang Basin,to the NE of the WBL,consists of the syn-rift faulted sub-basins(45-16.4 Ma)and the post-rift less deformed sequences(16.4-0 Ma).The heat flow(~60 mW/m2)is also consistent with that predicted based on crustal thinning,inferring that it is a rifted basin.However,the basin shows three stages of subsidence(fast in 45-30 Ma,uplift in 30-16.4 Ma,and fast in 16.4-0 Ma).In the uplift stage,the strata were partly folded in the Late Oligocene and partly eroded in the Early Miocene,which is probably caused by the flexural bulging in response to the paleo-South China Sea subduction and the subsequent Dangerous Grounds and Borneo collision in the Sabah to the east of the WBL.

Discovery of pockmarks in the Zengmu Basin,southern South China Sea and the implication

The Zengmu Basin located in the shallow water area of the southern South China Sea,is rich in oil and gas resources,within which faults and mud-diapir are developed,but it is unknown whether oil and gas migrate to the seafloor surface.The newly collected multibeam data across the Zengmu Basin reveal a large number of depressions,with depths of 2-4 m,widths of several tens of meters,large distribution range of 1.8-8 km along survey line,up to~50 km,and their backscatter intensity(-26 dB)is much greater than that of the surrounding area(-38 dB).Combined with the developed mud-diapir and fracture structures,and abundant oil and gas resources within this basin,these depressions are presumed to be pockmarks.Furthermore,more than 110 mono-sized small circular pockmarks,with a depth of less than 1 m and a width of 5 m,are observed in an area of less than 0.03 km2,which are not obliterated by sediment infilling with high sedimentation rate,implying an existence of unit-pockmarks that are or recently were active.In addition,seismic profiles across the Zengmu Basin show characterization of upward migration of hydrocarbons,expressed as mud-diapir structures,bright spots in the shallow formation with characteristics of“low frequency increase and high frequency attenuation”.The subbottom profiles show the mud-diapir structures,as well as the gas-bearing blank zones beneath the seafloor.These features suggest large gas leaking and occurrence of large amounts of carbonate nodules on the seafloor.This indicates the complex and variable substrate type in the Zengmu Basin,while the area was once thought to be mainly silty sand and find sand.This is the first report on the discovery of pockmarks in the Zengmu Basin;it will provide basic information for submarine stability and marine engineering in China’s maritime boundaries.

Seafloor topography and landform characteristics of Zengmu Shoal waters and their evolvement

The topography data of the Zengmu Shoal and its adjacent area are acquired by the SeaBat-8111 multibeam system. The full survey covering over the Zengmu Shoal area gives us new knowledge about the area. It is a flat underwater platform consisting of coral reefs, its fathom line is like a trap in near SN direction and its shape is like an isosceles triangle with the acute angle pointing to the north. The east and west sides of the shoal are steep while its south side is more gentle. As viewed from geological evolution, the Zengmu Shoal is a typical biologic coral landform caused by tectonic uplift.

南海西南海域曾母盆地新生界沉积充填演化研究

曾母盆地位于南海西南海域,是一个总面积约17×104km2的新生代含油气盆地。在综合研究该盆地新生界沉积特征和沉积体系发育状况以及模拟沉积层的沉降速率和沉降史的基础上,分析和研究了新生界沉积层的充填演化特征和充填序列。晚始新世—渐新世时该盆地基本上为东陆西海,位于西部的坳陷为海相沉积,其它地区多为陆相或海岸平原相沉积。中渐新世末至中新世末由于纳土纳等隆起没于水下,该区主物源方向发生改变,致使该盆地早期的沉积环境为南陆北海,而后因古巽他河的作用自南向北逐渐形成三角洲前积复合体。该三角洲沉积复合体随时间由南往北不断推进,形成大范围的三角洲沉积充填序列;与此同时,盆地东部和西部发育碳酸盐岩台地和生物礁,围区深坳部位为巨厚的滨-浅海和半深海充填沉积。从晚始新世至中新世末,该区的沉降速率和沉降中心不断发生变化。上新世和第四纪主要为浅海陆架及半深海充填沉积。

南海南部海域曾母盆地油气地质特征

曾母盆地是位于南海南部的一个大型新生代沉积盆地,具有面积大、沉积速率快、沉积厚度大的特点。国内外油气勘探和研究表明,曾母盆地具有十分良好的油气地质条件,主要发育渐新统海陆过渡相炭质页岩、煤层、海相泥岩和下—中中新统海相泥岩两大套烃源岩,以及渐新统—中新统砂岩和中—上中新统灰岩或礁灰岩两大套储层。康西坳陷和东巴林坚坳陷为盆地两个主要生烃区。纵向上,油气主要赋存于下中新统砂岩和中上中新统碳酸盐岩,前者以含油为主,后者以含气为主;横向上,曾母盆地具有南油北气的特征,气田主要分布在东部南康台地上和西部斜坡上,油田主要赋存在巴林坚地区。油气分布具有明显的分区性,主要与盆地不同构造单元构造和沉积作用的差异性相关。

南海西南部曾母盆地早中新世以来沉降史分析

采用回剥法和局部均衡模式研究曾母盆地早中新世以来的沉降史,并探讨了该盆地构造演化特征。曾母盆地自早中新世以来经历了17.5～11.6Ma、11.6～5.5Ma、5.5～3.0Ma和3.0～0Ma的4次快速沉降作用,其构造演化受控于曾母地块与南沙地块及婆罗洲地块的碰撞和盆地两侧的万安—卢帕尔断裂与廷贾走滑断裂的综合作用,可划分为南北双向挤压(晚始新世—早中新世)、走滑改造(中中新世—晚中新世)和区域沉降(上新世—第四纪)3个演化阶段。

南海南部曾母盆地新生界煤系烃源岩生油条件

在广泛调研前人研究的基础上,对曾母盆地新生界典型煤系烃源岩的有机地球化学、有机岩石学以及沉积成因特征进行了系统分析,进而探讨了该盆地煤系烃源岩的生油潜力。微观显微组分组成及成因分析,结合宏观沉积相研究,揭示出曾母盆地具有良好生烃潜力的下中新统煤系烃源岩发育于下三角洲平原带的红树林沉积环境中。对显微组分定量数据、基本生烃指标以及烃源岩热演化阶段的分析表明,该套煤系烃源岩具有在低熟阶段生油以及排烃的良好条件。另外,由于烃源岩中丰富的渗出沥青体生成而产生的内部增压,导致了大量相互连通的微裂隙发育,这有助于煤系烃源岩中析出油滴的汇聚以及排出。

南海曾母盆地南部陆架边缘三角洲沉积特征

曾母盆地作为发育于古巽他陆架上的盆地,南部的古巽他河所携带的巨量沉积物和曾母盆地的陆架特征使得陆架边缘三角洲在曾母盆地南部发育成为必然。通过对曾母盆地中中新世以来陆架边缘三角洲的地震、钻井及进积特征分析,认为曾母盆地内陆架边缘三角洲的沉积形态呈分别向陆和向海减薄的楔形体,古巽他河携带多期次的沉积物向陆坡沉积过程中形成多套沉积复合体,这些沉积体由于具有良好的生储条件,因而具有重要的油气勘探价值。

曾母盆地西部岩石圈特性与有效弹性厚度:来自构造模拟的约束

曾母盆地是南海南部最大的新生代沉积盆地,保留了南海新生代共轭边缘演化历史以及性质的重要信息。岩石圈有效弹性厚度Te是了解大陆地壳张裂时岩石圈强度的一个关键指标。本文选取曾母盆地西部的三条典型地震剖面,在构造解释的基础上,按照挠曲均衡原理,通过对三条剖面的反演模拟,对Te进行了敏感性测试。在此基础上利用弹性梁模型对三条剖面进行了正演模拟,来模拟盆地的形态以及主要地层单元的展布。模拟结果与实测剖面的对比表明,Te值取3～5km较为合理,盆地西部的脆性/韧性地壳转换深度为15km。正演和反演模拟中的拉张因子β具有不同的构造含义,正演模拟拉张因子β代表了脆性上地壳的拉伸程度,反演模拟的拉张因子则表示整个岩石圈的拉张作用,拉张因子同时呈向北增大的趋势。

曾母盆地东巴林坚坳陷烃源岩评价及油源探讨

在广泛调研前人研究的基础上,系统分析了曾母盆地东巴林坚坳陷烃源岩的分布及地球化学特征,从有机质丰度、类型和成熟度等方面对东巴林坚坳陷烃源岩进行了综合评价,结合东巴林坚坳陷部分原油与烃源岩的生标特征对比分析,探讨其对油源的贡献。结果表明,曾母盆地东巴林坚坳陷主力烃源岩为渐新统、下-中中新统煤系泥岩、海相泥页岩,其厚度大、分布广,有机质丰度较高,Ⅱ-Ⅲ型有机质,目前主要处于成熟阶段,且烃源岩质量明显受到沉积相控制。Patricia原油与煤系烃源岩生标特征具有良好的相似性,两者可能具有亲缘关系。揭示了曾母盆地巴林坚坳陷生烃潜力良好,勘探前景广阔。

南海曾母盆地新生代晚期构造沉降分析

曾母盆地是一个ＮＷ向走滑型沉积盆地，新生代晚期经历了强烈沉降（Ｅ３－Ｎ１１）、平缓沉降（Ｎ１２－Ｎ１３）、加速沉降（Ｎ２）和差异升降（Ｑ）四个时期，其构造演化主要取决于加里曼丹与南沙两地块间的相互作用所引起的ＮＷ向断裂的活动方式．

南海曾母盆地油气地质条件及其分布特征

通过分析曾母盆地近几十年来的油气田（井）勘探资料和近年来的地球物理调查成果，在研究区域地质、沉积特征与烃源岩演化关系的基础上，论述了曾母盆地的油气地质条件及其分布特征，并总结其分布规律性。

婆罗洲地质构造特征及其对南海南部盆地的影响

南海南部曾母、文莱—沙巴盆地油气资源丰富,已发现一大批油气田及含油气构造。曾母盆地南部与文莱—沙巴盆地油气的生成、运移和聚集与婆罗洲地块息息相关。婆罗洲地块地质特征复杂,自古新世开始,古南海逐渐消失,与婆罗洲地块发生碰撞,中新世开始隆升造山。在这种地质背景下,婆罗洲拉让河和巴兰河为曾母盆地南部与文莱—沙巴盆地的主要物源,发育三角洲相、滨浅海相、浊积扇相、浅海相砂岩储层;同时,压扭型构造发育,不仅形成断块与背斜型油气藏,同时具有良好的油气运移条件。

南沙海域南部第三纪三角洲演化与油气聚集

利用钻井、测井及地震资料,重建了南沙海域南部第三纪三角洲沉积的时空演化,并讨论了主控因素。研究表明,自晚渐新世—上新世,研究区三角洲在平行岸线方向(NE—SW)自西向东从西巴兰断层西侧向东侧迁移;在垂直岸线方向(NW—SE)则整体表现出从陆向海进积推进。这一演化过程主要受控于古南海自西向东"剪刀式"俯冲引起的造山带的迁移和海平面的升降。并且发现研究区烃源岩、储层和圈闭具有分期、分带特征。通过综合分析,在研究区划分出3个与三角洲有关的油气聚集区,即曾母盆地巴林坚三角洲油气聚集区、文莱—沙巴盆地巴兰三角洲油气聚集区、梅丽干—冠军三角洲油气聚集区。

南沙海域曾母盆地构造样式特征及其与油气聚集和圈闭的关系

南沙海域曾母盆地发育各种类型的构造样式,本文研究与油气聚集和圈闭关系密切的典型构造样式。以盆地构造运动的力学性质为依据,将该盆地的构造样式分为三大类,即伸展构造、挤压构造和走滑构造。伸展构造样式有断块构造和箕状构造;挤压构造样式有褶皱构造、冲断构造和底辟构造;走滑构造样式有花状构造。各种构造样式特征分析结果表明,大部分构造样式可形成能够俘获、圈闭油气的局部构造,部分构造样式除了能形成圈闭以外,其中充填的沉积物还有利于油气的生成。由于这些构造样式的发育时间早于烃源岩的生烃期和大规模的排烃期,因此极易俘获油气而成为有利的油气聚集带和有效的油气圈闭。

南海近5年油气勘探进展与启示

采用统计学方法对南海最新收集数据进行系统分析,并对典型油气田进行解剖,同时结合南海油气地质背景,对南海周边国家近5年油气勘探进展与趋势进行了系统分析,研究发现:(1)南海新发现油气藏主要分布于南海北部珠江口盆地、琼东南盆地、莺歌海盆地和南海中南部湄公、万安、曾母和文莱沙巴等盆地;(2)以三角洲、扇三角洲砂岩为储层的构造油气藏发现数量最多,但以深水沉积体为储层的岩性油气藏累计储量最大;(3)油气主要富集于中中新统,其次为上新统与上中新统。研究结果表明:(1)深水区已逐渐成为南海下一步勘探主战场和新增储量接替区;以深水沉积体为储层的岩性油气藏和以生物礁为储层的岩性油气藏已成为南海新增储量的主要贡献者。(2)直接位于源上或源侧的中新统生物礁是南海中南部盆地最为现实的勘探目标,前古近系潜山油气藏是南海西北部重要的潜在勘探目标;差异压实岩性体与上倾尖灭体是陆坡深水区深水沉积体相关油气藏的主要圈闭类型。

南沙海域曾母盆地西部地质构造特征

利用在曾母盆地西部所采集的大量地震、重磁及有关的钻井、地质资料 ,对该区的地震反射特征、沉积特征、区域地质和构造发展史进行了论述 ,对地质构造特征进行了较为详细的综合分析。文章认为 ,曾母盆地是发育在块体会聚带上的周缘前陆盆地 ;其构造演化经历了前陆拗陷、走滑改造和区域沉降三个阶段 ;在地震剖面上存在 T2 ,T3,T31 ,T4 ,T5(Tg) 5个区域不整合界面 ;盆地内断裂和局部构造均十分发育 ,局部构造主要有泥底辟、断块、花状。

曾母盆地西部构造样式初步分析

曾母盆地属周缘前陆盆地，发育于两个不同块体会聚带上，后期又经过走滑体系的作用，其特殊的构造部位和特定的沉积环境，造就了独特、丰富的构造样式，主要有断块构造、花状构造、底辟构造、反转构造、潜山构造等。这些各具特色的局部构造发育于不同时期和部位，空间上相互叠置，为油气聚集的理想场所。本文在对曾母盆地西部近万公里地震资料综合分析解释的基础上，结合区域地质背景资料，对该区的主要构造样式和成因机制以及它们与油气的关系进行了初步分析和探讨。

Seismic Characteristics and Hydrocarbon Accumulation Associated with Mud Diapir Structures in a Superimposed Basin in the Southern South China Sea Margin

Mud diapirs are significant structures often associated with hydrocarbon migration and accumulation as well as gas hydrate formation in sedimentary basins.A total of 30 mud diapirs were observed in the northwestern Zengmu Basin in the southern South China Sea based on 2D multichannel seismic data.The structures are distributed along an NW-SE trend near the shelf break of the Kangxi depression in the Zengmu Basin.The mud diapirs were divided into the following according to their vertical shapes,intrusion heights,and effusive activities:tortoise,piercing,and nozzle types,which represent different evolutionary stages of diapirism.The mud diapirs in the study area suggest that the driving forces behind the mud diapirs considerably exceed the rock rupture limit of the overlying strata.Diapir formation can be divided into two steps:the accommodation of a large amount of mud in the Zengmu Basin and the movement of plastic mud induced by gravity-driven flow and regional tectonic compression.Combining seismic interpretations with analyses of the regional geology and tectonic subsidence,the current study proposes that rapid subsidence in the Kangxi depression sufficiently accommodated the vast amount of sediments deposited since the Middle Miocene,which provided favorable conditions for the growth of mud diapirs.Furthermore,the N-S directional stress field in the Zengmu Basin and the gravity slide northward along the southern slope of the depression further facilitated the development of mud diapirs since the late Miocene.The mud diapirs in the southern South China Sea margins were accompanied by the accumulation and release of overpressured fluid.

曾母拉分盆地的油气成藏规律:廷贾-西巴兰线断裂的控制

南海南部廷贾-西巴兰线断裂是一条规模巨大的北西向走滑断裂,渐新世开始活动,早期具有强烈的右旋走滑特征,晚中新世以来走滑方向变为左旋,更新世至今,走滑方向变为右旋,是曾母盆地与北康盆地、南沙海槽的分界。为了理清廷贾-西巴兰线断裂特征与曾母盆地油气成藏的关系,通过对断裂两侧地震剖面特征、重磁特征、地热流分布特征以及地壳性质和岩石圈厚度的分析,探讨该断裂在曾母盆地形成演化及油气成藏中的控制作用。研究表明:断裂活动为曾母盆地后期的沉积提供了丰富的物源,断裂走滑作用导致曾母盆地具有良好的圈闭,后期的反向走滑使得盆地的沉积中心由南向北迁移;断裂沟通深部热源,有利于油气生成,形成的伴生断层是盆地油气运移的主要通道。