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Tectonic dynamics of Cenozoic sedimentary basins and hydrocarbon resources in the South China Sea

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Abstract

This paper discusses the distributing characteristics of petroleum in the South China Sea by comparing on the tectonic environments, the assemble of reservoirs, source and cap rocks between the southern and northern South China Sea in order to demonstrate the exploration guide of the petroleum. Petroleum potential in the southern margin of the South China Sea is better than that in the northern margin according to present petroleum exploration. Possible reasons causing such a difference may be grouped into the following aspects. Firstly, the southern South China margin belongs to an extruding environment in Cenozoic time, where the volume of source rocks is bigger than that in the northern margin belonging to a passive environment. Secondly, heat flow and geothermal gradient in the southern margin are higher than those in the northern margin, for example, heat flow in the Pearl River Mouth basin on the shelf of northern margin is 53-87 mW/m2 with an average value of 67 mW/m2; but in the Zengmu basin in the southern margin, an average heat flow is 97 mW/m2 with a maximum value of 130 mW/m2. This fact indicates that the maturation of source rocks in the southern margin is higher than that in the northern margin. Thirdly, many large scale oil-bearing structures in the southern margin are formed due to the extruding environments in Cenozoic time, but trap structures in the northern margin are relatively little in quantity and small in scale in extension environments. Finally, the coordination between the formation time of trap structures and hydrocarbon-generating time in the southern margin is also better than that in the northern margin. Present hydrocarbon exploration also shows that a number of big oil and gas fields exist in the southern margin, only medium-little oil and gas fields occur in the northern margin. Therefore, petroleum potential in the southern margin is richer than that in the northern margin of the South China Sea.

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... With its uniquely complicated geohistory and great hydrocarbon potential, the SCS has become a hot spot attracting geoscientists from both academia and the petroleum industry. Over the years, many authors have published their studies on the classification schemes and hydrocarbon systems of the SCS Basin (e.g., Hayes, 1980, 1983;Tapponnier et al., 1982Tapponnier et al., , 1986Zhang, 1985;Liu, 1988;Briais et al., 1993;Liu, 1993;Li, 1997, 2004;Xia and Huang, 2000;Gong et al., 2001;Lee et al., 2001;Packham, 2003;Hao et al., 2004;Hutchison, 2004;Xia et al., 2004;Yao et al., 2004;Wang et al., 2005;Wei et al., 2005;Zhou et al., 2005b, c;Metcalfe, 2006;Li and Li, 2006;Droust and Sumner, 2007;Wang and Li, 2009;Cullen et al., 2010). Many investigators of the SCS Basin have primarily focused on the Cenozoic sedimentary basins in the Cenozoic plate tectonic setting instead of on the polyhistory and dynamic nature of the basins generated and overprinted at different geologic times (Kingston et al., 1983a, b). ...
... As part of the China-Indochina continent during the Late Cretaceous -middle Eocene, a series of basins was developed and characterized by a half-graben structural style. Many geologists have studied them in the SCS and would call them ''rift basins'' (e.g., Gong et al., 1989;Liu, 1993;Li, 1997, 2004;Packham, 2003;Hutchison, 2004;Yao et al., 2004;Wang et al., 2005;Li and Li, 2006;Droust and Sumner, 2007). Based on their plate tectonic configuration, we define them as the kind of graben basins formed in a convergent tectonic setting ( Figure 9B) (Table 1). ...
... After the birth of the oceanic crust in the SCS, a series of new basins was developed from the earlier basins. At the northern margin, the so-called rift basins actually experienced two stages (e.g., Gong et al., 1989;Liu, 1993;Li, 1997, 2004;Zhang and Bai, 1998;Lee et al., 2001;Packham, 2003;Hutchison, 2004;Yao et al., 2004;He et al., 2005;Qiu et al., 2005;Wang et al., 2005;Wei et al., 2005;Li and Li, 2006;Droust and Sumner, 2007), which are an earlier rift stage associated with the structural style of half graben in the late Eocene -Oligocene and a later regional thermal subsidence stage since the Miocene. Here, we classify them as rift basins in the late Eocene -Oligocene and as divergent marginal subsidence since the Miocene in a divergent tectonic setting. ...
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The greater South China Sea (SCS) Basin is composed of basins of different generations and styles. These polyhistory basins formed in complicated geologic settings and evolved through different tectonic regimes. Based on a classical basin classification scheme and data from previous studies, we summarize the evolution of tectonic environments of the SCS in the Mesozoic – Cenozoic into a Late Triassic – middle Eocene divergent-convergent cycle and a late Eocene – present divergent-convergent cycle. The two cycles are in turn composed of four evolutionary phases, which are (1) Late Triassic – Middle Jurassic divergent continental margin setting, (2) Late Jurassic – middle Eocene convergent intracontinental setting, (3) late Eocene–Miocene divergent continental margin setting, and (4) Pliocene–present con-vergent continental margin setting. We identify temporal sequence and spatial distribution of major polyhistory basins in the SCS associated with the four basin evolutionary phases in the two tectonic cycles. Each basin corresponds to a specific pressure, space, and temperature, and overprinting of the basin caused changes in pressure, space, and temperature with time. Unraveling this complex and dynamic nature of the polyhistory basins can be instrumental in assessing the hydrocarbon potential and exploration risk in the SCS.
... In the Oligocene, under the control of the regional stress field, the basin rifting scope was further expanded, and the basin turned into a depression fault structure. At the end of the Middle Eocene (approximately 42.5 Ma), the Indian plate collided completely and gradually wedged into the Eurasian plate [41]. The Indosinian block rotated and extruded to form a large-scale escape structure. ...
... Based on the proportion diagram of the extension in different periods corresponding to each section (Figure 8 In the Oligocene, under the control of the regional stress field, the basin rifting scope was further expanded, and the basin turned into a depression fault structure. At the end of the Middle Eocene (approximately 42.5 Ma), the Indian plate collided completely and gradually wedged into the Eurasian plate [41]. The Indosinian block rotated and extruded to form a large-scale escape structure. ...
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Cenozoic extension rates were calculated based on 20 seismic profiles across the Qiongdongnan Basin, South China Sea. The results confirmed that the Cenozoic rifting in the Qiongdongnan Basin exhibited multistage extension and spatiotemporal variation. In terms of the N–S striking seismic profiles, the structural forms of the northern and southern sags of the basin were characterized by narrow half grabens, while the structure at the center sag of the basin was characterized by wide and gentle grabens. The fault strikes in the west of the basin were mainly northeast–southwest trending, whereas those in the east of the basin changed from northeast–southwest trending to nearly east–west trending. The extension rate in the east sag was higher than that in the west area. The extension rate in the middle part was lower relative to the east and west sags. This was because the rifting was controlled by the distribution of the main boundary fault along the basin. Temporally, the Cenozoic extension could be divided into three periods: Eocene, Oligocene, and Miocene. The amount of stretching in the three extension stages was unevenly distributed in the entire basin. The maximum was mainly in the Oligocene Lingshui and Yacheng Formations. The Oligocene extension occurred in the entire basin, and the Eocene extension was limited to the Ledong and Changchang sags. Significant fault activity could be observed during the deposition period of the Yacheng and Lingshui Formations and could be attributed to strong extensional activity. The rifting tectonics controlled the distribution of source rocks and oil-generating window as well as hydrocarbon generation, reservoir formation, and accumulation.
... Wan'an Basin is 85.3 Â 10 6 t in ultimate recoverable oil reserves, 230 Â 10 9 m 3 in ultimate recoverable natural gas reserves, 83.9 Â 10 6 t in remaining recoverable oil reserves and 208 Â 10 9 m 3 in remaining recoverable natural gas reserves, with 26 oil gas fields discovered, oil and gas fields distributed around the depression in the middle, including tectonic units such as upheaval in the north, fault terrace zone in northwest, upheaval in the middle, having achieved commercial oil and gas flow with high yield. Palawan Basin in northwest is 37.8 Â 10 6 t in ultimate recoverable oil reserves, 102 Â 10 9 m 3 in ultimate recoverable natural gas reserves, 25.8 Â 10 6 t in remaining recoverable oil reserves and 811 Â 10 9 m 3 [21] in remaining recoverable natural gas reserves. 7 wells have been drilled in Lile Basin at present, with 1 oil bearing structure (Sampaguita) discovered and 2 wells available for gas production [20]. ...
... It is observed that deepwater oil-rich basin group along Atlantic Ocean is in distribution from south to north and that deepwater gas-rich basin group along Neo-Tethyan tectonic domain is in distribution in nearly east-west direction [21]. ...
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It is observed from distribution of oil and gas-rich basins as well as summary and analysis of main control factor for hydrocarbon accumulation in global deepwater that oil and gas-rich basins in deepwater in the world shows “one horizontal and two vertical” in distribution pattern and that “one vertical” of deepwater basin group chiefly distributed in Atlantic Ocean from north to south of deepwater oil rich-basin group in the world is under the influence of fault basin group and that deepwater gas-rich basin group along Neo-tethyan tectonic domain and epicontinental basin group in East Africa shows “one horizontal and one vertical” in distribution, being under the influence of “fault basin group in transitional facies”; Comparative analysis and research of main factors for hydrocarbon accumulation in main deepwater oil and gas-rich basins in the world in a systemic way show that main control factors for hydrocarbon accumulation in deepwater oil and gas-rich basin in the world can be reduced to following five types: (1) Reservoir under common control of salt structure, passage system and large turbidite fan; (2) Reservoir under control of source control area and large reservoir body; (3) Reservoir under control of source rock and cap rock control area and large delta; (4) Reservoir under control of source and cover control zone and reef flat; (5) Reservoir under control of source heat control zone, passage system and trap. Keywords: Deepwater basin, Atlantic, Neotethys, East Africa, Main factors controlling hydrocarbon accumulation
... The study area is located at the continental margin slope area in the deep-water of the northern South China Sea ( Figure 1). The area is affected by the interaction of the Pacific plate, the India-Australia plate, the Eurasian plate, and the expansion of the South China Sea [28][29][30][31][32][33][34]. Previous studies have shown that three tectonic events controlled the formation and evolution of the area since the Cenozoic ( Figure 2): the continental margin cracking at the end of the Mesozoic, the South China Sea expansion in the late Oligocene, and the plate wedging event at the end of mid-Miocene [35][36][37]. ...
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The fault system is one of the structural carrier systems of gas hydrate accumulation, which plays a vital role in controlling the distribution of natural gas hydrate (NGH) accumulation. The previous studies mainly focus on summarizing the vertical migration mode of high flux fluid along the fault with obvious geophysical response characteristics on the seismic profile, such as “fault with gas chimney,” “fault with mud diapir,” and “fault with submarine collapse”, but lack of evaluation methods for the fault carrier system. We use the X sag in the deep-water continental margin slope area of the northern South China Sea as an example to study the fault systems closely related to NGH. This paper puts to use attribute technologies, such as coherence, curvature, and fusion, to analyze the characteristics and combination of the fault systems. We discussed migration patterns and evaluation methods of dominant fault carrier systems. This research proves that the strike-slip fault system in the platform area can directly connect the gas source bed with high-quality hydrocarbon generation to the gas hydrate stability zone (GHSZ). The activity of this fault system is more conducive to the accumulation of hydrocarbon in the GHSZ. This area has a good site for pore-filling gas hydrate prospecting and a preferential favorable fault carrier system. The composite fault system, consisting of a normal dip-slip fault system and a polygonal fault system, in the slope area can jointly communicate the biogenic gas-rich reservoir. Its activity and well-migration performance are the main reasons for the submarine gas leakage and collapse. It is a secondary favorable fault carrier system in the study area. There may be massive and vein natural gas hydrate formation in fractures in the leakage passage, and pore-filled gas hydrate may exist in the submarine nonleakage area. In this work, a three-factor evaluation method of the fault carrier system is proposed for the first time. This method is of great significance for the evaluation and exploration of NGH reservoirs in the continental margin slope area of the northern South China Sea.
... A total of 540 samples were obtained from the Sabah Fig. 2. The Pre-Cenozoic strata histogram in the northern Borneo and basin of the southern South China Sea (after Kudrass et al., 1986;Liu and Zhan, 1994;Wu and Yang, 1994;Gong et al., 2001;Liu, 2000;Yao et al., 2004;Zhou et al., 2005Zhou et al., , 2011Liu et al., 2007;Zhang C et al., 2007;Yan et al., 2008). and Sarawak areas in north Borneo for field geological surveys and laboratory research. ...
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Based on the volume magnetic susceptibility and specific gravity measurements and mineral and lithologic identification results for 540 samples, the rock type, density, and magnetic susceptibility of rocks from northern Borneo were analyzed, and the applicability of gravity and magnetic data to the lithologic identification of the Mesozoic strata in the southern South China Sea was assessed accordingly. The results show that there are 3 types and 25 subtypes of rocks in northern Borneo, mainly intermediate‐mafic igneous rocks and exogenous clastic sedimentary rocks, with small amounts of endogenous sedimentary rocks, felsic igneous rocks, and metamorphic rocks. The rocks that are very strongly–strongly magnetic and have high–medium densities are mostly igneous rocks, tuffaceous sandstones, and their metamorphic equivalents. The rocks that are weakly magnetic‐non‐magnetic and have medium–very low densities are mostly conglomerates, sandstones, siltstones, mudstones, and coal. The rocks that are weakly magnetic‐diamagnetic and have high‐medium densities are mostly limestones and siliceous rocks. The Cenozoic rocks are characterized by low densities and medium susceptibilities; the Mesozoic rocks are characterized by medium densities and medium–high susceptibilities; and the pre‐Mesozoic rocks are characterized by high densities and low magnetism. Based on these results and the distribution characteristics of the various rock types, it was found that the pre‐Mesozoic rocks produce weak regional gravity anomalies; the Mesozoic sedimentary rocks produce negative regional gravity anomalies; whereas the Mesozoic igneous rocks produce positive regional gravity anomalies; and the Cenozoic igneous rocks produce positive regional gravity anomalies. The regional high magnetic anomalies in the southern part of the South China Sea originate from the Mesozoic mafic igneous rocks and their metamorphic equivalents; and the regional medium magnetic anomalies may be produced by the felsic igneous rocks and their metamorphic equivalents. Accordingly, the identification of the Mesozoic lithology in the southern South China Sea shows that the Mesozoic sedimentary rocks are distributed over a large area of the southern South China Sea. Thus, it is concluded that the Mesozoic strata in this area have the potential for oil and gas exploration.
... ,即陆 壳在北缘离散阶梯状下降,在南缘拼贴增生。洋壳在中央海盆新生,在东侧马尼拉海沟 消减。陆缘地堑在扩张中形成,岛弧-海沟在挤压中发育 [14] 。 南海大陆架北部及西北部属于水深小于 200 m 的陆架海域,在地质构造和沉积物源 特征上与中国华南大陆关系密切,其中北部湾为半封闭的浅海盆地 (北部湾盆地) ,水深 在 20~50 m;南海南部为宽广的巽他大陆架,其宽度超过 300 km,坡度平缓,水深小于 150 m [15] ;南海西部为中南半岛大陆架,大陆架沿越南东海岸呈带状分布 [16] ,宽度约 405 0 km,大陆架外缘水深小于 200 m [17][18] ;南海东部为带状分布的狭窄岛架,分布于吕宋 岛、巴拉望岛边缘,以急剧变化的地形坡度向吕宋海槽和巴拉望海槽过渡。 南海大陆坡位于大陆架的周边外缘,面积达 120×10 4 km 2 ,系南海区域分布最广阔的 大地单元,深度介于 150~3600 m 之间 [19] 。深海盆地南北两侧大陆坡是块状断裂下沉形成 的阶梯状大陆坡,相对高出深海盆地数千米,表现为海底高原;深海盆地东西两侧陆坡 狭窄而坡度陡峭,西、北吕宋海槽、马尼拉海沟均分布于东侧陆坡,其具有阶梯状下降 的特点。 南海海盆位于南海中部偏东,面积达 40×10 4 km 2 [20] ,中央水深超过 4000 m,盆地中 分布着由孤立的海底山组成的高出海盆底部 3400~3900 m 的海底山群,以及 27 座高度超 过 1000 m 的海山和多座 400~1000 m 的海丘 [21] 。海盆北部边缘为华南陆架,其内部有一系 列阶梯状正断层以及地堑和地垒交错分布,盆地中填充了巨厚的沉积,是拉张型边缘; 南海海盆南部为南沙海槽,南沙海槽是特提斯的残留海,自燕山时期开始向南消减,在 加里曼丹岛北部形成褶皱带及冲断层带,为挤压型边缘;南海海盆西部为狭窄的中南半 岛大陆架,与海岸线大致平行,陆架上有一系列平直的阶梯状断层,系剪切拉张所形 成;南海海盆东部有近似 S-N 向的马尼拉海沟与台湾-菲律宾岛弧相分隔,海沟位于向 陆一侧,有别于西太平洋边缘其他海沟-岛弧体系,具有挤压型边缘特征。因此,南海 海盆自西北向东南裂离蠕散,后缘阶梯状拉张,前缘挤压,两侧剪切,这是南海海盆边 缘的基本构造特征 [15,22] Abstract: The latitude/longitude coordinates of eleven dotted-line segments in The Location Map of the South China Sea Islands (Nanhai zhudao weizhi tu, in Chinese) of the scale 1:4,000, 000, produced by the then-Chinese government in 1947, were determined by affine transformation in the geographic information system. Based on this map, a three-dimensional terrain model of the South China Sea was built and then the seafloor topographic characteristics were analyzed with the spatial overlay algorithm. ...
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The latitude/longitude coordinates of eleven dotted- line segments in The Location Map of the South China Sea Islands (Nanhai zhudao weizhi tu, in Chinese) of the scale 1:4,000,000, produced by the then- Chinese government in 1947, were determined by affine transformation in the geographic information system. Based on this map, a three-dimensional terrain model of the South China Sea was built and then the seafloor topographic characteristics were analyzed with the spatial overlay algorithm. Results show that the lengths and intervals of the dotted- line segments vary with their geographic locations, with larger lengths in the east and smaller lengths in the west. The range and shape of the dotted- line segments, mostly located on the continental slope and shelf, are parallel to the underlying topography. Three principles for demarcation of the dotted line are further summarized: (1) the principle of "equidistance midline" is adopted for the shallow sea basins and canyons; (2) in areas with dramatic reliefs, it follows the axis of troughs; (3) on the southwestern continental slope and shelf of the South China Sea, the shoreline and underlying topography control its distribution. These results could provide Chinese government a scientific basis for the solution of disputes in the South China Sea.
... Oil/gas resources in the SCS are very rich, and the prospecting of Cenozoic oil/gas has achieved fruitful result (Yao et al., 2004), at the same time, Mesozoic sedimentary strata are widespread in the SCS. A number of wells in the SW Taiwan Basin, Peikang Uplift and Liyue Bank have revealed Mesozoic strata and oil/gas structures. ...
Article
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A series of drilling, dredge, and seismic investigations indicate that Mesozoic sediments exist in the South China Sea (SCS) which shows a bright prospect for oil and gas exploration. In order to study the distribution of Mesozoic strata and their residual thicknesses in the SCS, we carried out an integrated geophysical study based mainly on gravity data, gravity basement depth and distribution of residual Mesozoic thickness in the SCS were obtained using gravity inversion constrained with high-precision drilling and seismic data. In addition, the fine deep crustal structures and distribution characteristics of Mesozoic thicknesses of three typical profiles were obtained by gravity fitting inversion. Mesozoic strata in the SCS are mainly distributed in the south and north continental margins, and have been reformed by the later tectonic activities. They extend in NE-trending stripes are macro-controlled by the deep and large NE-trending faults, and cut by the NW-trending faults which were active in later times. The offset in NW direction of Mesozoic strata in Nansha area of the southern margin are more obvious as compared to the north margin. In the Pearl River Mouth Basin and Southwest Taiwan Basin of the north continental margin the Mesozoic sediments are continuously distributed with a relatively large thickness. In the Nansha area of the south margin the Mesozoic strata are discontinuous and their thicknesses vary considerably. According to the characteristics of Mesozoic thickness distribution and hydrocarbon potential analyses from drilling and other data, Dongsha Uplift-Chaoshan Depression, Southwest Taiwan Basin-Peikang Uplift and Liyue Bank have large thickness of the Mesozoic residual strata, have good hydrocarbon genesis capability and complete source–reservoir–cap combinations, show a bright prospect of Mesozoic oil/gas resources.
... In the southwest border, there is a northsouth Zhongnan seamount which is 100 km long in north-south and 40 km wide in east-west, and it is 4000 m high above the seabed. This seamount separates the Central basin and the Southwest basin [13] . ...
Article
Seismic oceanography has been widely used to detect many kinds of oceanographic features as a new oceanographic prospecting method. Inversion for temperature and salinity distributions from seismic data is a very important research field in seismic oceanography. We analyzed low frequency seismic data from one multichannel seismic (MCS) line acquired in the scope of the European GO Project (line GOLR-12) combined with the simultaneously acquired XBT (eXpendable BathyThermograph) and CTD (Conductivity-Temperature-Depth) data, which are located to the southwest of the Iberian Peninsula. We used a post-stack constrained impedance inversion method to derive temperature and salinity distributions of sea water and the result demonstrated that this method can indeed provide reliable temperature and salinity distribution profiles every 6.25m along the seismic line, with the temperature resolutions of 0.16°C. This method applied to good quality MCS data can provide high lateral-resolution temperature and salinity profiles for oceanographic research, using only a few oceanographic measurements to constrain the inversion procedure.
... Organic reefs, which have good sourcereservoir-cap combination of the configuration and favorable conditions for hydrocarbon migration developed well in Xisha area, and organic reefs in this area also have good porosity, permeability, source rock, passage for hydrocarbon migration, and high quality cover. Thus, Xisha area is an important target area for petroleum exploration (Sun et al. 2008;Yao et al. 2004;Yang et al. 2011). However, there are numerous volcanoes and igneous rocks in this area because of multistage structure evolution (Ma et al. 2009). ...
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Organic reefs, the targets of deep-water petroleum exploration, developed widely in Xisha area. However, there are concealed igneous rocks undersea, to which organic rocks have nearly equal wave impedance. So the igneous rocks have become interference for future exploration by having similar seismic reflection characteristics. Yet, the density and magnetism of organic reefs are very different from igneous rocks. It has obvious advantages to identify organic reefs and igneous rocks by gravity and magnetic data. At first, frequency decomposition was applied to the free-air gravity anomaly in Xisha area to obtain the 2D subdivision of the gravity anomaly and magnetic anomaly in the vertical direction. Thus, the distribution of igneous rocks in the horizontal direction can be acquired according to high-frequency field, low-frequency field, and its physical properties. Then, 3D forward modeling of gravitational field was carried out to establish the density model of this area by reference to physical properties of rocks based on former researches. Furthermore, 3D inversion of gravity anomaly by genetic algorithm method of the graphic processing unit (GPU) parallel processing in Xisha target area was applied, and 3D density structure of this area was obtained. By this way, we can confine the igneous rocks to the certain depth according to the density of the igneous rocks. The frequency decomposition and 3D inversion of gravity anomaly by genetic algorithm method of the GPU parallel processing proved to be a useful method for recognizing igneous rocks to its 3D geological position. So organic reefs and igneous rocks can be identified, which provide a prescient information for further exploration.
... The Dongsha event, which occurred in Late Miocene, formed a regional unconformity in the Dongsha region, although its area of influence was fairly limited, and was accompanied with uplift, faults, and magmatic activity (Cai et al., 2010;Li, 1993). Most previous studies focused on the evolution of the SCS from Mesozoic to Paleocene (Cai et al., 2010;Huang et al., 2001;Li, 1993;Lin et al., 2006;Lüdmann and Wong, 1999;Wu et al., 2004;Yao et al., 2004), whereas little has appeared that deals mainly with the Dongsha event. In addition, the Dongsha area and adjacent depressions of the Pearl River Mouth Basin (PRMB) are sites of focused fluid flow, which is an important process for the formation of gas hydrates in sedimentary basins Sattler et al., 2004;Sun et al., 2012aSun et al., ,b, 2013Wu et al., 2009). ...
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An important post-rifting tectonic event with extensive uplift and fluid flow took place at the northern margin of the South China Sea (SCS) during the late Miocene. Based on an analysis of two- and three-dimensional seismic data plus drilling well data, the tectonic event mainly affected the Dongsha Rise and the adjacent depressions. Tectonic deformation likely occurred in the Dongsha Rise, the eastern part of the Panyu Swell and several depressions of the Zhu I Sag. The post-rifting deformation in the study area was characterized by faulting, erosion, igneous activity and hydrothermal fluid flow. The tectonic movement caused abundant secondary NW and WNW tensional and transtensional faults, with lengths of 1 to 10 km. It also created a clear angular unconformity which marks the time of the tectonic movement. Late Neogene igneous rocks are found in the strata in the Dongsha Rise only close to the continent─ocean transition zone. Geothermal activity occurred in the Liuhua carbonate platform and adjacent depressions. Deformation is more intense in the eastern area than in the western area. According to sequence stratigraphy, we infer that the tectonic event mainly occurred in the late period of Late Miocene and ended at the Mio/Pliocene boundary (5.5 Ma). The dynamic mechanism for tectonic movement could be associated with subduction of the South China Sea slab beneath the Philippine Sea Plate at the Manila trench. When subduction of lower density crust within the continental─ocean transition zone was initiated, large resistance stresses may have led to lithosphere bending and crustal uplift in the Dongsha area.
... The Xisha Uplift was subaerially exposed prior to the Miocene, but subsided during the late Oligocene to early Miocene period of seafloor spreading (Fig. 1). Crustal thickness in the Xisha Uplift varies from 27 km to 6.8 km (Taylor and Hayes, 1983;Xia et al., 1998;Yao et al., 2004). The Xisha Uplift had experienced rifting since the late Cretaceous and its crest in the early Miocene was broken into small, fault-controlled uplifted blocks and intervening grabens (Sun et al., 1996). ...
Article
Newly acquired seismic data allow improved understanding of the architecture and evolution of isolated carbonate platforms on the continental slope of the northern South China Sea. The Xisha carbonate platforms initiated on a basement high, the Xisha Uplift, in the early Miocene and have remained active to the present. Their distribution is limited to pre-existing localized, fault-bounded blocks within the Xisha Uplift so individual platforms were small in size at the beginning of the Miocene. However, during the middle Miocene, the platform carbonate factories flourished across an extensive area with 55,900 km2. The platforms began to backstep in response to a relative sea-level rise in the late Miocene. Platform-edge reefs, patch reefs, pinnacle reefs, atoll reefs and horseshoe reefs, all developed on various platforms. The distribution of platform carbonates shrank significantly during Pliocene-Quaternary time to isolated carbonate platforms, represented today by Xuande Atoll and Yongle Atoll. Tectonics and eustasy were the two main controls on platform development. Tectonics controlled both the initial topography for reef growth and the distribution of platforms, including those that survived the drowning event associated with the late Miocene rapid relative sea-level rise. Eustasy controlled high-frequency carbonate sequence development.
... Besides, the basement of Zhongsha and Xisha Islands and Nansha Area can also be compared with the metamorphosed basement in middle Viet Nam, which proved that they may all belong to Indochina Block. Thickness of Xisha basement changed from over 20 km to about 8 km (Taylor and Hayes, 1983;Qiu and Wen, 2004;Qiu et al., 2005;Xia et al., 2004;Yao et al., 2004). Basement of Xisha Islands was composed by deep grey granite gneiss and its absolutely age is 689 Ma which belong to Precambrian metamorphics (Wang et al., 1979). ...
Article
The Xisha carbonate platforms, which include the modern-day Xisha Atoll, occur upon the northern continental margin of the South China Sea (SCS). In this study, we identify the seismic characteristics about various sedimentary facies of the carbonate platform and different types of reef in this area, based on the 2-D seismic data, reefs show high-amplitue moundy continuous reflection at the top and weak chaotic reflections inside. Carbonate platforms show high-amplitude continuous reflection at the top and high reflection alternative with weak reflection. The above analysis provided information about the influence of fluctuations in relative sea level, tectonic movements, paleotopography and the development mechanism of the carbonate platform. Based on the seismic data and data from drillholes (Xichen-1, Xiyong-1), we propose a schematic sedimentary model of the Xisha carbonate platforms at the northern margin of the SCS and outline five stages of development for the carbonate platform. Its sedimentary evolution consists of Initial establishment stage, development stage, exposure stage, drowning stage and large atoll reefs development stage. We also propose that several phases tectonisms supply proper environment and structural position for carbonate platform development, they can also destroyed the exist platforms. Besides, eustasy change was also the main influential factor on the development of the platforms.
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Coal-type source rocks include both coal and terrigenous marine source rocks. By studying the distribution of secondary depressions, uplifts, as well as the characteristics of peat formation and accumulation in the northern marginal sea basin of the South China Sea, and combining them with coal formation characteristics observed in other basins, five genetic theories on the relationship between peat accumulation and dispersed organic matter accumulation are proposed. The northern marginal sea basin of the South China Sea is characterized by “disadvantageous coals formation and favorable terrigenous marine source rocks formation.” This paper provides a distribution map of coal seams and terrigenous marine source rocks in the Qiongdongnan Basin and determines their distribution patterns. Research shows that the migration of sedimentary facies in the basins and inner depressions led to the formation and migration of the peat accumulation centers. In addition, the vertical migration of the peat accumulation centers led to planar migration, which is actually a type of coupling relationship.Previous research results have revealed that the formation of coal-type source rock is multi-phased. The marginal sea basin is composed of several fault-depression basins, with each basin developing a second order of depression and uplift. There is no unified basin center or depositional center to be found. As a result, the concentration centers of coal-forming materials also vary greatly. Based on the distribution characteristics of coal-type source rocks in different basins within the marginal sea basins of the South China Sea, the research results have practical significance and provide guidance for exploring coal-type oil and gas reservoirs in this area.
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The Yinggehai Basin is a unique NNW-trending petroliferous basin in the northwestern South China Sea. This paper mainly utilized stratigraphic, tectonic and seismic data by characterizing the geological structures and conducting the geo-mechanical analysis to study the formation, evolution and dynamics of the Yinggehai Basin. The study indicates that the Ailaoshan-Truong Son extruded terrane is composed of multiple secondary extruded bodies. The Red River fault zone, located within the Qiangtang-Simao-Yinggehai mantle flow channel and basin zone, experienced transform-type sinistral strike-slip motion before the basin forming stage and formed a NW-trending extruded mantle uplift, which activated the Yinggehai basin. After experiencing the rift depression, fault depression, and fault subsidence, the basin eventually formed large-scale, thick sedimentation features with ideal hydrocarbon-forming conditions at the end of the Miocene. Later, the basin dynamically transformed and entered a period of tectonic superposition, reworking, and thermal subsidence. Superposition of the NNW thrust sinistral strike-slip fault zone on the northern Hanoi sub-basin complicated the basin structure. Since the Pliocene, the southern Yinggehai main basin has been transformed into an extensional dextral strike-slip environment that hosted numerous mud diapirs. The thin crust and high geothermal gradient provide favorable conditions for the large-scale accumulation of natural gas.
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The natural gas hydrate exploration and evaluation of south China sea are mainly concentrated in continental margin slope deep water area of the northern south China sea for these 20 years. Major breakthroughs in the exploration and production of natural gas hydrate have been made in Shenhu area, Dongsha area and Lingshui-songnan area of Qiongdongnan Basin, where we have found three billions of cubic meters of reserves of natural gas hydrate reservoir. A preliminary evaluation prediction about the south China sea natural gas hydrate resources are scale of 80 billion tons of oil equivalent, which has made major achievements in the natural gas hydrate exploration stage of south China sea. However, where is the gas hydrate resources beneficial areas to further deepen and expand exploration in the South China Sea? In particular, where are the strategic replacement areas for sustainable rolling exploration? Is there any geological condition for gas hydrate formation in the continental slop-Ocean Continent Transition zone (OCT) or even ocean basin area which is closely adjacent to the exploration of oil, gas and hydrate in deep water of continental slope at present? This paper intends to make a preliminary analysis and discussion on the key issues affecting and determining the future deployment and trend of gas hydrate exploration. We hope that it can be of some benefit to the exploration and evaluation of gas hydrate resources in the South China Sea and the selection of strategic replacement areas.
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A prominent seismic reflector is associated with a widespread Late Oligocene-Early Miocene carbonate platform and patchy reefs (Nido limestone) that developed on the southeastern continental margin of the South China Sea. It coincides with the history of seafloor spreading in the South China Sea. To understand the tectonic controlling factors, based on a large number of wells, dragnet, and seismic data, we characterize the sedimentary facies, distribution, and analyze the controlling effect of tectonic deformation on it. The results show that the Nido carbonate platform is distributed continuously in the northeast to southwest of the study area. Bounded by the Ulugan fault, the Nido carbonate platform in the northeast tends to be northwest, controlled by the top of the tilted fault blocks during rifting and the weak tectonic activity in the spreading stage. The Nido carbonate platform in the southwest of the Ulugan fault trends to the southeast, and it is mainly located on the top of uplift, the Late Oligocene formed titled blocks, and the migrated flexural forebulge caused by the flexural forebulge during Oligocene-Early Miocene. The age and distribution of the carbonate platform are zonal distribution that coincides with the tectonic history in the southeast of the South China Sea. This feature is related to the pre-existing tectonic background and evolution process at different tectonic locations, reflecting the diachronous breakup of the South China Sea and the discordant subduction-collision process.
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This study investigates the evolution of the Miocene Guangle carbonate platform (or Triton Horst) of the northwestern South China Sea margin. The platform is located at a junction area surrounded by Yinggehai basin, Qiongdongnan basin and Zhongjiannan basin. Well and regional geophysical data allow the identification of the morphologic and stratigraphic patterns. The Guangle carbonate platform was initiated on a tectonic uplift during the early Miocene. The early platform was limited at Mesozoic granitic basement, pre‐Paleogene sediments localized tectonic uplift and was small extension at the beginning stage. While during the middle Miocene, the carbonate buildup flourished, and grow a thrived and thick carbonate succession overlining the whole Guangle uplift. The isolated platforms then united afterward and covered an extensive area of several tens of thousands of square kilometers. However, it terminated in the late Miocene. What are the control factors on the initiation, growth and demise of the Guangle carbonate platform? The onset of widespread carbonate deposits largely reflected the early Miocene transgression linked with early post‐rift subsidence and the opening of the South China Sea. Stressed carbonate growth conditions on the Guangle carbonate platform probably resulted from increased inorganic nutrient input derived from the adjacent uplifted mainland, possibly enhanced by deteriorated climatic conditions promoting platform drowning. Therefore, tectonics and terrigenous input could be two main controlling factors on the development of the Guangle carbonate platforms and main evolution stages.
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The study of source rocks is a key component in the exploration and discovery of hydrocarbon plays in the offshore petroliferous basins of China. Geochemical analyses, drilling, seismic, and microfossil data were integrated to document the formation mechanism of lacustrine source rocks contained in the Wenchang Formation in the Zhu I Depression of the Pearl River Mouth Basin, China. Several factors that control the development of lacustrine source rocks were evaluated in this study, including tectonics, sedimentary conditions, palaeoclimate, the supply of organic matter, and redox conditions. The deposition and occurrence of source rocks is a combined function of these factors. Lacustrine source rocks are characterized by high organic matter contents (average TOC value = 1.33 wt.%) and are dominated by type I and type II1–2 kerogen, which primarily originate from planktonic algae. The rifting of basement rocks, syn‐sedimentary faults, and palaeoclimate played significant roles in controlling the geochemical properties of the source rocks (e.g., controlling the abundance and types of organic matter). The Eocene lacustrine source rocks were deposited when the palaeoclimate was characterized by humid to semi‐humid, subtropical to tropical weather that did not have a stable temperature or humidity. Controlled by large‐scale faults, “Half‐graben patterns” and “Graben patterns” are the two main palaeotopographic patterns where the thermal stratification of deep lake water resulted in flourishing planktonic algae in its surface zones. Dysoxic to anoxic conditions prevailed during the deposition of the lacustrine source rocks. An underfilled basin situation caused the occurrence of abundant organic matter during the period of maximum surface flooding.
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In order to analyze crustal structures and features in the conjugate margins of South China Sea (SCS), we collected large numbers of studied results from the ocean bottom seismometers experiments, onshore-offshore seismic experiments, gravity and magnetism anomalies. This paper firstly constructs three land-sea super cross-sections from east to west in the northern margin of SCS. The results show the onshore-offshore transitional zone is a border separating the unstretched and the stretched continental crust. The low velocity layer (LVL) in the middle crust was widely imaged in the unstretched continental crust. However, the high velocity layer (HVL) in the lower crust was detected in the stretched continental crust. By analyzing the mechanisms of the LVL in the middle crust and HVL in the base of crust, we believe the crustal structures had distinctly different attributes in the continental South China and in the northern SCS, which indicates that the littoral fault zone (LFZ) could be the boundary fault between them. Then, we reveal the crustal features in the Liyue Bank based on water depth, gravity and magnetism anomalies. The Moho depth thins from about 24 km in the Liyne bank to 11 km in the oceanic basin. The ocean-continent transitional zone generally widens from east to west in the southern margin. Finally, we devide the crustal structure in the conjugated margin of SCS into four types, including the unstretched continental crust, the stretched continental crust, the ocean-continent transitional zone, and the ancient ocean crust.
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The Liyue block is composed of two parts, namely, Reed Bank and Liyue basin, which is separated by Zhongnan fault in west, Palawan trough in southeast and fault scarp in the north of Nansha area. Based on gravity and seismic data set, a systematic investigation on major tectonic and crust structure units in the Liyue block is made. Multichannel seismic data can better our understanding of the stratum and fault structure located in the sediment basement. In the Reed Bank, composed of reefs, the faults are undeveloped and the stratum is flat. In the basin area, the early faults are developed with two kinds of structures: tilted fault block and low relief anticline. The thickness of the Mesozoic strata in basin area decreases from SW to NE. But the thickness of rifting strata increases from SW to NE, which indicates the fault activity strength in the rift period increases from southwest to northeast. Gravity inversion is performed to understand the geometry of the MOHO surface and the crustal thicknesses beneath. The region is characterized by a large positive Bouguer gravity anomaly (60 to 140 mGal), and the MOHO depth generally varies from 16 to 27 km. In general, crustal structures can be distinguished from the thinned continental crust. The Liyue block is characterized by stretching factors ranging from 1.3 to 2.0, which indicates that the local region is lowly stretched.
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Huizhou depression is one of the major petroleum contribution areas in Pearl River Mouth basin, and it is significant to the research of abundant hydrocarbon depression with the further confirmation of source rock characteristics and petroleum resources. Comprehensive analysis of the abundance, type, maturity of organic matter, effective thickness, organic carbon recovery factor and production rate, shows that Paleogene Wenchang formation and Enping formation are good source rocks. The quantitative prediction technology of source rock TOC seismic data makes up for the lack of less core and measured sample discontinuous distribution. TOC seismic data predict quantitatively TOC contents and variations in the different depths and different time planes, which has laid a good foundation for the more accurate assessment of source rocks and petroleum resources. Based on the quantitative prediction technology of source rock TOC seismic data and the basic stratigraphy unit of third-order sequence, that, petroleum resources quantities of Paleogene source rocks through the genetic method in Huizhou Depression, are 1.698 times last one. The Paleogene transfer zones which are close to source rocks have the advantage of near-source for hydrocarbons with great exploration potential.
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Spectral analysis and continuous wavelet analysis were conducted on the natural gamma ray logging data from the Middle Miocene-Pliocene Hanjiang, Yuehai and Wanshan formations of well HZ0811 and HZ1811, which locate at northern Huizhou depression and Huilu low uplift at Zhuyi depression, respectively. Significant Milankovitch sedimentary cycles were identified, and "floating" astronomical time scales of well HZ0811 and HZ1811 were established through Gaussian band filtering of short eccentricity signals. The Wanshan, Yuehai and Hanjiang formations record 35, 56 and 64 short eccentricity cycles, which indicate the duration of Wanshan, Yuehai and Hanjiang formations can be estimated as 3.5, 5.6 and 6.4 Ma, respectively. The sedimentary rate of Hanjiang-Wanshan formations of well HZ0811 and HZ1811 were calculated according to floating astronomical time scale. The variations of sedimentary rate indicate significant 1.0 Ma and 0.4 Ma cyclicities, which may be related to the sea level change controlled by long eccentricity cycles.
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The evolution of rift-drift-subsidence can usually be illuminated by sedimental style, subsidence history and structural style, so it's possible to understand basin evolution through analysis of depositional process and coupling tectonic-sedimentary. In this paper, infill stratigraphy, subsidence history and structural characteristics are analyzed. The results show that Liyue basin is characterized by typical "tow-layer" structural pattern, which is separated by a regional breakup unconformity (T60). The horsts and grabens are in low basin overlain by upper depressions of the Neogene. And the half-grabens characterized by asymmetric wedge-shape are bounded by both normal faults and large-scale listric faults, which controlled the sedimentary cycles. The subsidence method is mainly tectonic subsidence before 23.8 Ma but is thermal subsidence after Late Oligocene. There were three regional subsidence episodes, and each subsidence rate was from slow to fast. Liyue basin is primarily composed of three regressive cycles of rift infilling influenced by regional tectonic, which correspond to three stages of basin development. The first stage, characterized by extensional stress, is related to early rift development from approximately 45 Ma until Early Oligocene. Featured by both extension and strike-slip stress, the second is associated with rift-drift development from initiation of sea floor spreading in South China Sea at 30 Ma until the end of sea floor spreading at Middle Miocene. And the third is late post-rift subsidence from approximately 16 Ma until 5.5 Ma, which is controlled by transpressive stress field.
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The ancient Pearl River delta in the early Eocene shelf of Pearl River Mouth Basin is defined as "setting source", the canyon channels in the slope as "transporting paths", and the deep-water fan in the slope as "sedimentary sink". They form a sedimentary system from source to sink. The sedimentary system can be divided into shelf subsystem and slope subsystem. The relative variation to sea level, regional structural movement and source supply strength controlled the development and evolution of the sedimentary system. In third-order sequence framework, the delta in the shelf subsystem developed into a shelf-edge delta in the LST, and the delta front crossed over shelf break to connect with the canyon channels in the slope subsystem. The transporting mechanism of clastic material is changed: from the traction current controlled by river, wave and tide to the gravity current controlled by gravity and physiognomy. Baiyun deep-water fan "sedimentary sink" of the slope subsystem is formed in the upper slope, and the sedimentary development mode of "two bodies-one connection path" is formed. In TST and HST phases the delta devoloped to be shelf delta, the delta front can not cross over the break and deposits again. In the slope subsystem, marine mud deposits because of no outer source supply, so the sedimentary development mode of "one body-one connection path" is formed. During the evolution process of whole sedimentary system, clastic material transports between the subsystems under different time-space matching relationships, which makes the shelf subsystem connect with the slope subsystem. So the whole sedimentary system is an episodic sedimentary mode.
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Since Late Mesozoic, the united South China Sea region has experienced at least two tectonic cycles of marginal seas, i.e. the cleavage to convergence of the paleao-South China Sea, and the cleavage to convergence of the neo-South China Sea, which formed a couple of structural units, such as the northern continental margin of the South China Sea, the oceanic basin of the neo-South China Sea, the Nansha block, the residual oceanic basin of the palaeo-South China Sea, the northern continental margin of Borneo, and the western and eastern continental margins of the South China Sea. The tectoic framework developed in the cleavage of the palaeo-South China Sea was afterwards truncated and reworked to form a new tectoic framework because of the difference in locations of the palaeo-South China Sea and the neo-South China Sea. The superimposition of these two cycles controls properties of these different units, types of overlying basins and geological conditions of hydrocarbons. The northern continental margin of the South China Sea was of a passive continental margin in both the cycles though its scale (including the Nansha block parting away during the second cycle) was larger in the first cycle than the present residual part of the first cycle, which, only as an intracontinental rift part, was superimposed by the transitional marine rift developed in the late cycle to form tensional basins. Source rocks of the early cycle dominated by semi-deep lake strata, are distributed offshore and mainly generate oil that accumulate in nonmarine or marine facies, while those of the late cycle, composed of mainly transitional deltaic deposits and marine mudstones, are distributed infralittorally and generate oil that are reservoired in marine facies. The Nansha block used to be a rift basin that developed marine source rocks and was located at the northern continental margin of the South China Sea in the early cycle, afterwards it drifted south-ward for thousand kiometers and lay between the palaeo-and neo-oceanic basins, remaining in an under compensation status. During the late cycle it was compressed and reworked, its deposits are dominated by marine fades of the early cycle, with occurrences of marine source rocks and reservoirs. The southern continental margin of the palaeo-South China Sea used to be a passive continental margin during the early cycle but was reworked into an active continental margin during the late cycle. Stretched basins formed in the early cycle are superimposed by compressional basins developed in the late cycle, and strata of the early cycle are deeply buried and of metamorphism, while strata deposited in the late cycle consist mainly of large-scale deltas and marine mudstones, with occurrences of both oil and gas in marine facies. The western continental margin of the South China Sea was formed in an intracontinental environment during the early cycle, and then reworked into a transformational continental margin during the late cycle, where tenso-shear basins developed with restricted terrestrial strata deposited in the early cycle and transitional deltaic strata deposited mainly in the late cycle. This area has a high value of heat flow and is apt to generate gas. Oil-gas reservoirs such as diapirs, buried hills and reefs are well developed in this area. Presently the eastern continental margin of the South China Sea is subducting with the development of accretionary wedge basins. The superimposition of the two different cycles leads to a great difference in exploration potential of different areas. The southern continental margin of the palaeo-South China Sea experienced extension and compression in the early and late cycles, respectively, resulting in the formation of large-scale basins, the richness of mature source rocks and enormous resources of hydrocarbons. Two suits of source rocks are well developed in rift basins of the northern and western South China Sea and they have generated an abundance of hydrocarbons. The Nansha block was located at the northern passive margin of the palaeo-South China Sea in the early cycle and restrained between the remnant oceanic basin of the palaeo-South China Sea and the oceanic basin of the neo-South China Sea. It contains well-developed structural straps due to regional compression and shows a great potential in petroleum resources. Presently, the degree of petroleum exploration is high in shallow water areas of the South China Sea, especially for structural traps and organic reefs, with the exception of non-structural traps. However, the petroleum exploration in deepwater and ultra-deepwater areas of the South China Sea remains yet at an infant stage though some significant discoveries have been made. Hence, the exploration potential of the South China Sea is still considerably great in the future.
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Huaguang Sag is located in the deep seawater area of Qiongdongnan Basin, and its tectonic position belongs to the intersection of NE-trending, SN-trending and NW-trending tectonic systems in the continental margin of the Northwest South China Sea. To investigate the initial rifting process and further more the dynamics mechanism of Huaguang Sag, this paper sets up the structure model of basement which mainly makes up with several depression-controlling faults, and simulates the initial rifting process of Huaguang Sag by the FLAC software. The simulation results show that only affected by the S-N trending extensional stress, the rifting center appears in northern boundary basement faults (two NEE-trending and NWW-trending faults) of Huaguang Sag while does not take place at the NNE-trending and NE-trending basement fault zone in the middle sag, and doesn’t match the current pattern that the basement fault plays a main role in controlling the sediment. In the other case, affected by the S-N trending and E-W trending extensional stress at the same time, the areas of the northern boundary faults zone and internal NNE-trending basement faults zone come to be rifting center quickly, the sedimentary is controlled by the main basement faults to different degrees, and is consistent with the tectonic-sedimentary framework of Huaguang Sag which obtained by the data of geophysical interpretation. In combination with the analysis of regional tectonic background, the paper proposes that two remote tectonic effects occurred by the collision of India-Eurasian Plate: One remote effect was the rotational extrusion of IndoChina Block, which led to form a series of NE-trending and NNE-trending basement faults, as well as the E-W trending tensile stress field in Huaguang Sag. The other remote effect was that the deep mantle material of South China Block flowed southward, which resulted in the S-N trending extensional rifting of the lithosphere in northern South China Sea, and finally formed a series of EW-trending and NEE-trending basement faults and the S-N trending tensile stress field in Huaguang Sag. Affected by the above tensile stress fields and the basement faults, the initial rifting occurred in E-W and nearly S-N directions along the pre-existed basement faults (the weak structural zones) in Huaguang Sag.
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The northern continental margin of the South China Sea (SCS) is located within the tectonic system of Southeast Asia, an area with a great deal of tectonic migration due to the regional tectonic movements. The available geological and geophysical data of the area are comprehensively analyzed in order to demonstrate the typical migration patterns of the Cenozoic tectonics in the northern SCS caused by the episodes of the Cenozoic tectonic movement. Furthermore, the lateral variation characteristics of the strata and the different evolution patterns of the main basins’ features are assessed. It primarily focus on: (1) the Cenozoic episodic rifting from north to south in the continental margin of the northern SCS; (2) the rifting and depression time of the main basins progressively become younger as one goes from north to south, signifying that the migration of both the tectonics and the sediments within the northern SCS travelled from north to south during the Cenozoic; and (3) the lateral tectonic migration on the direction of EW is not regular in total, but in some local areas the trending of the tectonic migration is from west to east. The analysis of the tectonic migration features of the northern SCS, in combination with the regional tectonic evolution background, indicates that the observed remote lagging effect, resulted from the India-Eurasia plate collision, is the main dynamic mechanism involved in the tectonic migration within the northern SCS. The tectonic migration has significant influence on both the organization of petroleum deposits and on the hydrocarbon accumulation within the basins in the northern SCS; comprehensive understanding of this dynamic system is of great reference value in predicting the hydrocarbon accumulation and has the potential to have an enormous impact in discovering new deep reservoirs for the future oil-gas exploration.
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Thermal anomalies can be caused by fast and intensive tectonism in sedimentary basins. So such thermal anomalies should be eliminated when the thermal history is used to reveal the geodynamic process at depth. In this work, based on the dynamic-instantaneous heat conduction theory, geothermal correction of 3 typical wells in the Bozhong Depression was carried out. The results indicate that the Paleogene rapid subsidence in the Bozhong depression leads to the present geothermal disequilibrium, where average heat flow at present is about 60.9 mW·m−2, but a higher heat flow 67.4 mW·m−2 is yielded after correction of the low thermal anomalies. The average thermal lithosphere thickness in the Bozhong depression is about 70 km, calculated by using the corrected heat flow, which is 13∼28 km thinner than before correction (82∼100 km), consistent with that from geophysical and xenoliths analyses.
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The formation and evolution of South China Sea has been one of the hot spot of attention at home and abroad. The expansion of the South China Sea happened in early Oligocene-early Miocene, and the expansion of South China Sea includes ridge transition once at least. This paper uses a model of rising divergent mantle flow and convection coupling to simulate the expansion of eastern sea basin of the South China Sea, the ridge jump, and the process of lithosphere thermal structure evolution and melting of magma after expansion. Numerical simulation results indicate that a ridge jump is an important process of the South China Sea expansion. Ridge jump forms a magma chamber between the two ridges, and the existence of the magma chamber makes the submarine volcanoes relatively more and the terrain relatively high between the ridges. At the same time, it also leads to the terrain of both sides of south side ridge and the distribution of submarine volcano asymmetric, which in turn can demonstrate the reasonability of ridge jump model.
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Three distinct continental marginal basin types developed in the South China Sea, i.e. divergence, transform-extensional and flexural-extensional complex types, due to different boundary conditions. Differences of tectonic evolution in these basins resulted in great variations in sedimentary fill and associated hydrocarbon accumulating conditions. Source rocks of lacustrine and marine mudstones mainly developed in northern diverging continental marginal basins in the northern South China Sea. Although there developed good lacustrine sources, limited hydrocarbon accumulation occurred in the adjacent areas around these separated depressions with a relative small scope distribution. In the southern flexural-extensional complex continental marginal basins of the South China Sea, the Zengmu basin underwent the peripheral foreland tectonic stage earlier than other basins such as the Liyue, Beikang and Nanweixi basins. An available hydrocarbon accumulating condition occurred in those basins, where there are not only large areas and thicknesses of depressions with the sources of coal-bearing and marine mudstones, but also formation of a broad carbonate and reef reservoirs resulted from slow subsidence during the terminating period of the South China Sea spreading (about 15.5 Ma). The transform-extensional basins in western continental margins of the South China Sea are characterized by natural gas accumulation due to very thick sediments and high heat flow values. Considering all kinds of geological conditions mentioned above, we suggest that the hydrocarbon-bearing prospects in southern continental marginal basins are superior to the northern continental marginal basins.
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The northern South China Sea is a rifted continental margin. It has experienced complicated geological evolution and has been proved to have the prospect of oil and natural gas. In this paper, the velocity structures of strata in the northern South China Sea are obtained and analyzed by the velocity structure calculation of far offset refractive wave method using the long cable multi-channel seismic data collected in a northern region of South China Sea and in addition, the conception of non first-arrival refractive wave is proposed. In this method, refractive horizons are recognized; travel times of the same horizon in different common offset sections and different horizons in the same common offset section are collected. The velocity structure of the same horizon is calculated by the difference of travel-time in different common offset sections. The application of this method in a northern region of South China Sea reveals the whole velocity structure and the velocity change prosperities of the Cenozoic strata of different ages. The application provides a fresh sight using far offset refractive velocity structure to study the Cenozoic geological evolution of South China Sea. The results show that far offset refractive wave method supplies more geological information compared with the conventional first-arrival refractive wave method; the preliminary application provides a new insight for the research of far offset refractive wave method.
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