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南海西北陆坡区新生代碳酸盐台地周缘深水沉积体系研究
田洁
学位类型博士
导师吴时国 ; 吕福亮
2015-05-18
学位授予单位中国科学院大学
学位授予地点北京
学位专业海洋地质学
关键词深水沉积体系 重力流 底流 西沙海域 碳酸盐台地 南海北部
摘要被动陆缘深水区沉积物因能反映深水沉积动力学、古海洋学等地质信息,深水沉积体系的研究逐渐受到地质学家的高度关注;又因其是油气及天然气水合物资源的重要储存场所而广受石油行业的关注。现今关于南海北部陆坡区西沙海域的研究主要集中在三个方面,一是碳酸盐台地的发育演化特征及其控制因素,二是西沙海域陆坡区水道发育演化及其控制因素,三是西沙海域深水区块体搬运体系(MTDs)地震特征及其控制因素,缺乏对与碳酸盐台地相关的深水沉积体系的发育演化及其所反映的构造、沉积、古海洋学等信息的系统分析。本文通过高分辨率2D、3D(部分)地震数据,结合西沙海域钻井资料,在层序地层分析的基础上,辅以地球物理手段,分析西沙海域现今水深为300-1500 m区域的碳酸盐台地周缘深水沉积体系特征。对西沙海域碳酸盐台地周缘不同类型深水沉积体系地震特征、地球物理特征及钻井资料进行分析总结,并对其发育演化机制进行探讨,研究其形成过程中所受的控制因素,建立相应的沉积模式。
1、西沙海域琼东南盆地深水区北礁凹陷地区NW-SE方向和NE-SW方向的地震剖面上均发育丘形反射特征,其主要发育于T41-T40层位内,对应中中新统梅山组地层。NW-SE方向地震剖面上丘形反射在中部较为明显而向NW和SE方向丘形顶界面变平坦。NE-SW方向地震剖面上具有类似的丘形反射特征,并且具有不对称性及东翼较陡的特征。丘形地质体在平面上表现出底部为圆形-椭圆形的特征。古海洋学分析认为丘形地质体所在的北礁凹陷为半深海-深海相。其波阻抗值远远小于LH11-1生物礁油气田灰岩的波阻抗值,钻井A也证实其岩性为钙质泥岩,排除了丘形地质体为生物礁的可能性,并暗示来自西沙碳酸盐台地的碎屑沉积物被后期改造。综合分析认为西沙碳酸盐台地北部斜坡边缘丘形地质体是重力流与底流共同作用的结果。中新世以来不同构造单元沉降速率的差异造成斜坡坡度变陡,经重力流搬运作用输送的沉积物在斜坡处发生沉积变形而产生似波状沉积物。中中新世时南海底流流速有可能极大,从而造成对深海沉积物的侵蚀、搬运及再沉积作用。
2、西沙海域碳酸盐台地前缘发育大型深水水道沉积体系。西沙碳酸盐台地与广乐碳酸盐台地之间的南北向低洼地带-中建海槽内发育大型南北向中建深水水道。由于中建海槽内古地貌高点的存在,中建深水水道被分为南北两个分支。中建深水水道北分支可分为5期,分别为三亚组(I期)、梅山组(II期)、黄流组(III期)、莺歌海组(IV期)和乐东组(V期)。每期水道内充填特征不同,且水道具有明显的迁移特征。中建深水水道南分支可分为4期,依次为梅山组(I期)、黄流组(II期)、莺歌海组(III期)和乐东组(IV期)。单期水道内可识别出强振幅、连续性好的浊流沉积体和弱振幅、连续性差-杂乱的反射结构的MTDs。水道表现出侵蚀-充填-废弃的旋回性。
3、西沙海域碳酸盐台地周缘水道类型多样。礁缘水道发育于碳酸盐台地内生物礁周缘。广乐碳酸盐台地斜坡以发育多条相互平行的水道(冲沟)为主。西沙碳酸盐台地南部大型水道自三亚组开始发育。水道发育后期被二次侵蚀,该水道自北向南流入西沙东断坳。东部发育至今仍未沉积的自西向东流的峡谷水道。西沙碳酸盐台地东北部大型环礁周围发育梅山组及乐东组小型水道。
4、古构造分析认为西沙碳酸盐台地周缘水道的物源主要来自西沙隆起及广乐隆起。西沙隆起受南海扩张的影响,起源于华南陆块,中新世以后由于西沙海槽和琼东南盆地中央坳陷带的负地形及南部隆起带的隔挡作用,研究区受陆源碎屑影响较小。受越南东部边界断裂带影响,早中新世时广乐隆起与越南陆架经南北向的狭长坳陷分离,而当时越南隆起边缘的陆架-陆坡体系还未形成,陆源碎屑物质无法越过坳陷进入广乐隆起。因此,西沙碳酸盐台地周缘水道的物源主要来自西沙隆起及广乐隆起。西沙海域沉积相分析认为,早中新世-中中新世物源以西沙隆起和广乐隆起的碳酸盐、生物礁碎屑及火成岩碎屑为主;晚中新世至现今由于相对海平面上升导致碳酸盐台地向构造高部位迁移而面积减小,无法提供大量浅海碎屑物质,物源以深海沉积物为主。
5、西沙海域隆凹相间的古地貌对水道的发育起到控制作用。发育于西沙隆起及广乐隆起之间的南北向低洼地带限制了水道的流向,古凸起对中建深水水道具有分流作用,将其分为南北两个分支,南分支由北向南流入中建坳陷,北分支由南向北流入华光凹陷。南部限制性空间抑制了水道的迁移,而北分支在北部较为开阔的空间迁移现象较明显。
6、相对海平面变化控制水道的发育演化,底流对水道后期埋藏废弃起到重要作用并对其沉积物有再改造作用。西沙海域三级海平面变化与全球海平面变化一致,水道的侵蚀期均发育于海平面下降的海退阶段。三级海平面周期性的旋回变化控制了水道的侵蚀-充填-废弃周期性演化,三级海平面下降时,重力流侵蚀地层,水道发育;三级海平面上升时,重力流作用减弱,底流产生的漂积体埋藏水道。西沙海域二级海平面变化与全球海平面变化相反,裂后期构造沉降加速是其主要原因。二级海平面的持续上升,碳酸盐台地由广泛发育进入淹没阶段,从而使得水道内充填岩性发生变化。二级海平面上升减弱了重力流的侵蚀能力,因此上新世至今中建深水水道南分支下切深度减小。限制性地形的存在使得底流流速增大,从而使底流对水道沉积物进行再改造作用。
其他摘要Geoscientists have focued on the deep-water sedimets recently as deep-water sediments can record some geological information such as the sedimentary dynamics, ancient geological oceanography and so on. Meanwhile, it also received intense attention by the oil industry because it is important reservoir for the oil and gas as well as hydrate resources. However, studies about periplatform deep-water sedimentary systems of the Xisha area in the South China Sea is rare. In that thesis, we used high resolution of 2D and part of 3D seismic data, well data and combined with geophysical interpretation technologies to analyze the characteristics of different deep-water sedimentary system around carbonate platforms in the Xisha area, South China Sea.
1. Mound-shaped reflections were identified both in NE-SW and NW-SE oriented seismic profiles, and the mounds are most prominent in the central part of NW–SE oriented seismic profiles with undulating tops that flatten out towards the NW and SE. The mound-shaped reflections are mostly asymmetric with steeper eastern flanks in the NE–SW oriented seismic profiles. They are approximately round in planform. Paleogeographic analysis shows that the Xisha uplift was dominated by tropical shallow carbonate platforms, while the Beijiao depression became a bathyal environment in the middle Miocene. The absolute value of the wave impedance of the sediments is much smaller than that of the LH11-1 Reef Oilfield, indicating that the sediments were not derived from reefs. Integrated analysis suggests that the periplatform sedimentary mounds were generated by the combined work of gravity flow and bottom currents. The different subsidence rates between the QDNB and the Xisha uplift oversteepened the paleo-slope gradient and sediments transported downslope generated mounds on the slope that were deformed consistently to the north. The velocity of bottom current in the middle Miocene may have been high enough to flow into the SCS anderode, transport and redeposit deep-sea sediments.Sediments transported from the Xisha carbonate platforms crept downslope and were laterally reworked by bottom currents.
2. Zhongjian deep-water channel developed in the graben lying between Xisha uplift and Guangle uplift, and the channel was separated apart into two branches because of the existence of a paleo-highland. The north branch can be divided into five stages with significant migration, including Sanya Formation(I), Meisha Formation (II), Huangliu Formation (III), Yinggehai Formation(IV) and Ledong Formation (V).The south branch can be divided into four stages(Meisha Formation (I), Huangliu Formation (II), Yinggehai Formation(III) and Ledong Formation (IV)) and with inner high amplitude, continuous reflections. Each stage of it show an erosion-infill-abandon sedimentary cycle. Paleogeography analysis shows that the channel-filled deposits compose of carbonate and reefs debris and volcaniclastic rocks.
3. High resolution seismic data shows that there widely exist periplatform channel system in the Xisha area, northern south China sea. Channels around reefs display strong amplitude reflections, and were filled with weak-strong and continuous reflections, chaotic seismic reflection can be found at the bottom of the channels. Small-scale channels on the slope show a serious of V-shaped reflections with incised shallow on the underlying strata. A channel initiated from early Miocene located in the southern slope of the Xisha carbonate platforms. On the east of the Xisha carbonate platform, an unfilled canyon exist.
4. Palaeo-geology analysis suggest that the sediment supply mainly come from the Xish and Guangle uplift. The Xisha uplift originated from the south china continental in respond to the expending the South China Sea and the exist of the Xisha Trough, Central Depression and the southern uplift belt of the Qiongdongnan Basin stop the sediment from the south china continental from Miocene. The Guangle uplift is separated with Vietnam uplift by a narrow depression developed along the trace of the East Vietnam Boundary Fault Zone. The inexistence of shelf - slope system of the Vietnam uplift made it impossible for the sediments from Vietnam uplift to be transported into the Guangle uplift. Sedimentary facie show that early-middle Miocene, carbonate rocks, reef clastic and igneous clastic were the main sediment supply for the channels. Late Miocene to present the carbonate platforms decrease and not so much tropical sediments supplied, so the channels mainly composed of deep-sea sediments.
5. Palaeogeomorphology of the Xisha area greatly controlled the development the deep-water channel. The low-lying belt between the Xisha and Guangle uplift restrict the flow direction of the channel. The ancient rise separated the Zhongjian deep-water channel into two branches: the southern one to the Zhongjian depression and the northern one to the Huaguang depression. The confined space limited the migration of the channel, however, the northern branch channel migrated unidirectionally because of the open space in the north.
6. The third order sea-level changes controlled the erosion-backfilled-abandoned cycle evolution of the Zhongjian deep-water channel, while the second order sea-level changes controlled the component and the scale of it. Bottom current play an important role in the burying of the channel and reworked the sediments of the channel.The third order sea-level changes the same with global eustasis periodically, and the Zhongjian channel developed in regression period when the sea-level fell. The falling and rising of the third order sea-level play an import role in the cycle development of the channel. The second order sea-level changes were against with the global eustasis, and as with the consistent rising of the second order sea-level, carbonate platform became drowned eventually from its flourish stage. As a consequence, the sediment filled in the channel changed from carbonate rocks to deep-sea mudstones. The erosion of gravity flow decrease because of the rising of the second order sea-level, which lead to decreases of the cut depth in the southern branch of Zhongjian deep-water channel.
学科领域深水沉积与油气地质
语种中文
文献类型学位论文
条目标识符http://ir.qdio.ac.cn/handle/337002/23272
专题海洋地质与环境重点实验室
作者单位中国科学院海洋研究所
第一作者单位中国科学院海洋研究所
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田洁. 南海西北陆坡区新生代碳酸盐台地周缘深水沉积体系研究[D]. 北京. 中国科学院大学,2015.
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