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Hydrocarbon preservation conditions in Mesozoic–Paleozoic marine strata in the South Yellow Sea Basin
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ScienceDirect Natural Gas Industry B xx (2017) 1e10 www.elsevier.com/locate/ngib
Research Article
Hydrocarbon preservation conditions in MesozoicePaleozoic marine strata in the South Yellow Sea Basin Liang Jie a,b,c, Zhang Penghui b,c,*, Chen Jianwen b,c, Gong Jianming b,c, Yuan Yong b,c b
a College of Marine Geosciences, Ocean University of China, Qingdao, Shandong 266100, China Qingdao Institute of Marine Geology, China Geological Survey//MLR Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Qingdao, Shandong 266071, China c Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao, Shandong 266071, China
Received 20 January 2017; accepted 25 May 2017
Abstract In the South Yellow Sea Basin, MesozoicePaleozoic marine strata are generally well developed with large thickness, and they are characterized by multi-source and multi-stage hydrocarbon accumulation, providing a material basis for the formation of large-scale oil and gas fields. However, no substantial breakthrough has been made in this area. Based on previous research results, the complex tectonic pattern of this superimposed basin was formed by multi-stage tectonic movements and the favorable static conditions for hydrocarbon preservation were reworked or destroyed by later superimposition. Therefore, hydrocarbon preservation conditions are the key factors for restricting the breakthrough of marine oil and gas exploration in this area. In this paper, hydrocarbon preservation conditions of marine strata in the South Yellow Sea Basin were comprehensively analyzed from many aspects, such as tectonic movement, source conditions, caprock characteristics, magmatic activities, and hydrogeological and hydrogeochemical characteristics. It is indicated that the complex tectonic pattern of the South Yellow Sea Basin is resulted from tectonic events in multiple stages, and the development and evolution of regional source rocks are mainly controlled by two stages (i.e., the stable evolution stage of MesozoicePaleozoic marine basin and the MesozoiceCenozoic tectonic pattern transformation and basin formation stage), so the characteristics of differential oil and gas preservation are presented. Besides, better marine hydrocarbon preservation preconditions in this area are weaker tectonic reworking, development of high-quality thick source rocks, good vertical sealing capacity of caprocks, weaker magmatic activity and confined hydrogeological conditions. It is concluded that the Laoshan Uplift in the central part of the South Yellow Sea Basin is structurally stable with weaker faulting and magmatic activities, so it is better in oil and gas preservation conditions. Besides, several large-scale structural traps with good petroleum geological conditions and complete sourceereservoirecaprock assemblages are developed in this area. Therefore, this area is the most promising region for Paleozoic marine oil and gas exploration in this basin. © 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: South Yellow Sea Basin; MesozoicePaleozoic; Hydrocarbon preservation conditions; Marine strata; Tectonic movement; Caprock characteristic; Magmatic activity; Hydrogeological and hydrogeochemical characteristics; Laoshan uplift
The regional geology as well as gravity and magmatic data analysis show that the South Yellow Sea Basin (hereinafter referred to as SYS Basin) is not only the extension of Yangtze
* Corresponding author. Qingdao Institute of Marine Geology, China Geological Survey//MLR Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Qingdao, Shandong 266071, China. E-mail address: [email protected] (Zhang PH). Peer review under responsibility of Sichuan Petroleum Administration.
block in the offshore area but also the main part of Lower Yangtze block [1e4]. Although 29 wells have been drilled (23 wells by China and 6 by South Korea) in the SYS Basin in the past 5 decades, there is no industrial oil and gas flow up to now. In the recent years, along with the deepening of resource evaluation in the area, more and more research results reveal that the SYS Basin contains complete and very thick MesozoicePaleozoic marine strata and corresponds to a depositional history similar to the Sichuan Basin, so it holds the
https://doi.org/10.1016/j.ngib.2017.05.013 2352-8540/© 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Liang J, et al., Hydrocarbon preservation conditions in MesozoicePaleozoic marine strata in the South Yellow Sea Basin, Natural Gas Industry B (2017), https://doi.org/10.1016/j.ngib.2017.05.013
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material basis to form large accumulations and thus presents a great exploration potential [3,5]. The SYS Basin is a typical superimposed basin. As a result of the superimposed reworking by multistage multi-episode tectonic movements, the geological conditions were complicated, and the good static hydrocarbon preservation conditions in the prototype basin were reformed or damaged. The hydrocarbon preservation conditions are the primary control factor to determine the hydrocarbon enrichment in the superimposed basin [6], and also the key to bring about petroleum exploration breakthroughs in the SYS Basin. The hydrocarbon preservation conditions comprise a whole process of hydrocarbon generation, migration, accumulation and dispersion. However, up to now, there is still no systematic recognition upon the hydrocarbon preservation conditions in the marine strata in the SYS Basin. In this paper, its hydrocarbon preservation conditions were discussed from many aspects, such as tectonic movement, caprock characteristics, magmatic activity, and hydrogeological and hydrogeochemical characteristics, in order to provide reference for traps evaluation and exploration deployment in the basin. 1. Geological setting As the main part of Lower Yangtze block, the SYS Basin is located in the South Yellow Sea in the eastern Yangtze platform, with a structural pattern of “one uplift in two depressions” [3,7], and covering an area of 18 104 km2. It can be divided into three structural units from north to south: Yantai depression, Laoshan uplift and Qingdao depression (Fig. 1-a). Through the analysis of drilling and seismic data as well as marineecontinental correlation, it can be found that the SYS Basin has similar MesozoicePaleozoic depositional evolution and lithological assemblage features to the onshore Lower Yangtze area. The SYS Basin contains complete and well-preserved marine strata, including Neoproterozoic Sinian, Paleozoic Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian, and Mesozoic Triassic, but no MiddleeLower Devonian (Figs. 1-b and 2). 2. Tectonic movement, source rock conditions and hydrocarbon preservation Zhu Xia [8] indicated that “the variation of kinetic mechanism is an important condition for petroliferous basin formation”. He also proposed that there are two kinds of kinetic mechanism in the SYS Basineonshore Lower Yangtze area, which are especially different in the behaviors before and after Indosinian movement [9,10], showing the stable evolution stage of MesozoicePaleozoic marine basin and the MesozoiceCenozoic tectonic pattern transformation and basin formation stage. The common action of Caledonian, Indosinian, Yanshan and Himalayan movements finally shaped the complicated structural pattern in the SYS Basineonshore Lower Yangtze area. The tectonic evolution can help to organically combine the petroleum geologic conditions and control the differences of reservoir damage and preservation in
different zones, which further result in the diversity and complexity of trap types. 2.1. Stable evolution stage of MesozoicePaleozoic marine basin 2.1.1. Caledonian (ZeS) In the Early SinianeMiddle Ordovician (ZeO2), the Lower Yangtze block generally took on a configuration of “one uplift in two depressions” [9], receiving platform sediments in the central area and slope-shelf sediments in the south and north. The lower Cambrian Mufushan Fm mainly contains black carbonaceous shale, taupe siliceous mud shale and sallow sandy shale [11], mostly 50e200 m thick and widely distributed, and it is a set of regional source rocks with hydrocarbon generation potential in the marine strata in the SYS Basin (Table 1). This set of source rocks made great contribution to the formation of Anyue and Weiyuan large gas fields in the Sichuan Basin. In the Late OrdovicianeSilurian (O3eS), along with the closing of South China Ocean and subduction of Qingling Ocean, foreland basin and intra-platform depression began to develop in the Yangtze platform [9,12]. The widespread dark mud shale in the lower Silurian Gaojiabian Fm in the SYS Basin shows high organic matter abundance and moderate maturity. It is regarded as a set of good source rocks throughout the study area (Table 1), and acts as the gas source rocks for Wubaiti, Wolonghe, Shapingchang, Jiaoshiba and other large gas fields and shale gas fields in the Sichuan Basin. The Lower Cambrian and lower Silurian source rocks represent two sets of the best regional source rocks in the lower structural marine layer in the SYS Basin. The Caledonian movement (Guangxi event) in the late Silurian made the Lower Yangtze block (including the SYS Basin) uplift as a whole and suffer erosion, with the erosion intensity strong in the south and weak in the north [9,13]. As a result, the deposits of upper Silurian and middleelower Devonian were absent intensively, and slight folding occurred. 2.1.2. Hercynianeindosinian (DeT2) In the DevonianeEarly Carboniferous (DeC1), influenced by the closing of Qingling ocean and the opening of Tethys ocean, the SYS Basin and onshore Lower Yangtze area presented a generally stable epicontinental sea depositional environment in the Late Paleozoic [9,12,13]. In the Late CarboniferouseEarly Permian (C2eP1), along with the extension of Tethys ocean and the opening of South Qingling trough, the Lower Yangtze block suffered rifting or faulting [9,12]. It received transgression in the Early Permian Qixia stage, giving rise to a set of carbonate sedimentary formations with rich asphalt and siliceous materials in the middleesouthern SYS Basin. In the Late PermianeMiddle Triassic (P2eT2), influenced by the extension of South Qingling trough, the rift further developed, and a set of regional source rocks featured by black and grayish black siliceous shale deposited in a deep-water environment was formed in the Upper Permian Longtan
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Fig. 1. Tectonic zonation (a) and seismic sequence division (b) of MesozoicePaleozoic marine strata in the SYS Basin.
FmeDalong Fm (Table 1). This set of source rocks is the primary source rocks in the upper structural marine layer in the SYS Basin and also the major hydrocarbon supply strata for Puguang, Yuanba, Longgang, Moxi, Tieshanpo, Dukouhe and Luojiazhai gas fields in the Sichuan Basin.
The Indosinian movement in the end of Middle Triassiceinitial stage of Late Triassic thoroughly changed the regional structural and depositional patterns of Lower Yangtze block and terminated the stable evolution history of a marine depositional basin [9,12]. Thus the marine strata suffered diverse reformation, and the MesozoicePaleozoic faulting mainly happened in the Indosinian stage with thrust and overthrust faults developed (Figs. 1-b and 3), bringing the SYS Basin into an era of later reformation.
PermianeLower Triassic formations were eroded to different extent or even locally absent. During the J3eK1 tectonic event, the Yangtze block showed distinctive differentiation in the east and west. To the east of Qiyueshan fault, the Middle and Lower Yangtze areas suffered compressional deformation and folding thrust reformation as well as uplifting erosion. To the west of Qiyueshan fault, the Sichuan Basin exhibited continuous settlement without obvious folding deformation [17,18]. In this period, the MesozoicePaleozoic (including Middle and Lower Jurassic) in the SYS Basin experienced compressional deformation, deflection and order change, recording as the most intense reformation in the MesozoicePaleozoic strata. Especially, the reformation was the strongest in the piedmont of orogenic belt and near the Tanlu fault zone, while the Laoshan uplift was relatively weakened by the reformation, laying a foundation for the fundamental MesozoicePaleozoic structural shape of the SYS Basin.
2.2.1. Late Triassiceearly cretaceous (T3eK1) During the T3eJ2 tectonic event, influenced by the compression of Yangtze block in the south and north, the SYS Basin was in the foreland basin developing stage [9,12]. It presented uplifting as a whole, with the tectonic intensity stronger than the Upper Yangtze block. As a result, the Upper
2.2.2. Late CretaceousePaleogene (K2eE) The SYS Basineonshore Lower Yangtze area experienced multistage tensional rifting, giving rise to several dustpan-like or graben basins of single fault or half graben types [9]. The erosion thickness was small in the faulted sag, thus the marine hydrocarbon preservation system could be rebuilt in the
2.2. MesozoiceCenozoic structural pattern transformation and basin forming stage
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Fig. 2. Marine strata and major sourceereservoirecaprock assemblages in the SYS Basineonshore Yangtze area. Note: * represents the formation encountered in the SYS Basin.
superimposed area. The primary hydrocarbon reservoir was prone to be damaged and destroyed on the massif between the uplift and the faulted sag [12,18]. The Jurong and Huangqiao reservoirs represent the hydrocarbon accumulation in this stage, and the marine formations in the SYS Basin show differential hydrocarbon preservation in the same stage.
deposition was stable, which affected a little on oil/gas preservation in the marine strata.
2.2.3. Neogeneequaternary (NeQ) In this stage, the SYS Basin generally settled and formed a united depression when the faulting tended to be weak and the
Caprocks, as a core element in hydrocarbon preservation research, comprise direct caprocks (inner caprocks) and regional caprocks. The direct caprock is the rock layer which
3. Hydrocarbon preservation conditions 3.1. Caprocks
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Table 1 Regional source rock characteristics in the representative wells in the SYS Basineonshore Lower Yangtze area.
Note: I represents sapropel-type, II represents mixed type, II1 represents sapropel-prone mixed type; III represents humic type, modified from Refs. [3,9,12,14e16].
can directly prevent the hydrocarbon from escape. The regional caprock refers to the overlying strata constituted by different types of rocks that generally protect the hydrocarbon system [19,20]. The real sealing capacity of caprock depends on the relative sealing of the reservoirecaprock assemblage [21], and the sealing performance directly affects the hydrocarbon reservoir formation, scale and preservation. A good regional caprock can effectively prevent the massive escape of oil and gas, so that oil and gas can be reallocated and preserved under the regional caprock, even if the direct caprock is poor [22]. The MesozoicePaleozoic caprocks in the SYS Basin involve (gypsum) salt rocks, tight (mud) limestones and mud shale. (Gypsum) Salt rock presents high breakthrough pressures as well as large plasticity coefficients and compressibility coefficients, thus no fracture may occur even if there is large deformation under high temperature and high pressure. It is the most effective caprock. Tight (mud) limestones present small breakthrough pressure, thus fragile cracks may generate because the sealing capacity is relatively poor. Mud shale shows breakthrough pressure and mechanical property between (gypsum) salt rocks and tight (mud) limestones, thus its sealing capacity falls in between them [19e23]. According to seismic data interpretation, marinee continental correlation and drilling data, there are three sets of caprock sequence in the marine MesozoicePaleozoic in the SYS Basin (Fig. 4). They are mud shale in the Lower Silurian Gaojiabian Fm, mud shale in the Upper Permian Longtan FmeDalong Fm, and marl and gypsum salt rocks in the Lower Triassic Qinglong Fm. These caprocks can provide sealing conditions for marine strata.
According to the available Lower Yangtze onshore outcrop and drilling data, the mud shale in the Lower Silurian Gaojiabian Fm presents varying horizontal thickness but continuous distribution. It was usually formed earlier than the hydrocarbon generation and migration. Because of old strata, large burial depth and strong diagenesis, the mud shale represented by Gaojiabian Fm is also called highly evolving mud shale whose sealing capacity is often considered to be poor. The poor sealing is attributable to the fact that the mud shale is buried beyond a certain depth (1500e3500 m) where the compaction approaches limit and the increasing fragility can easily produce micro fractures, thereby reducing capillary sealing. Based on the researches into the relation between the burial depth and the caprock sealing parameter, the relation between the burial depth and the regional caprock sealing performance in the triaxial resistance to shearing and compressing experiment, and the discovered Paleozoic reservoirs in the Sichuan Basin, the highly evolving mud shale caprock shows high plasticity under deep burial depth, and the sealing capacity is closely related to the compaction degree. Its sealing capacity could be good, provided that it was not reworked by strong reformation activities (e.g. faulting) in later stages [24,25]. During the MesozoiceCenozoic structural pattern reformation and basin forming stage, although the mud shale in the Gaojiabian Fm was affected by the tectonic movement such as overthrust faults, the structural traps below the sliding surface could keep pattern integrity due to the sliding surface inside the overthrust fault [26], causing limited continuous destruction of caprocks. For example, Well N1 in Jurong area reveals oil and gas shows in the CarboniferousePermian below the sliding surface. In addition, core observation shows
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Fig. 3. Distribution of marine MesozoicePaleozoic faults in the SYS Basin. Boundary range shown in Fig. 1, modified from Ref. [10]
that micro-fractures in the mud shale of Gaojiabian Fm affect a little on the caprock sealing due to limited micro-fracture extension. According to the caprock parameter analysis in the onshore Lower Yangtze drilled Gaojiabian Fm, the breakthrough pressure is more than 12 MPa in Well Xingcan 1, more than 16 MPa in Well C, and 12e16 MPa in Well Jucan 2, showing a good sealing capacity. The mud shale in the Upper Permian Longtan FmeDalong Fm drilled by Well B in the SYS Basin demonstrates a cumulative thickness of more than 100 m (Fig. 4), and a
displacement pressure of 18e25 MPa estimated according to logging interpretation, reflecting a good sealing capacity. On the seismic section, the Longtan FmeDalong Fm shows continuous parallel reflection [26], with good tracking and correlation laterally and a certain continuity on the plane. This set of caprocks is relatively developed in the central and southern parts of the basin. The marl and gypsum salt rocks of the Lower Triassic Qinglong Fm exist locally in the SYS Basin, and still can serve as a local caprock due to good sealing conditions. For
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Fig. 4. Caprocks in the SYS Basineonshore Lower Yangtze area. From left to right: Wells A, B, C
example, Well A encounters Qinglong Fm, revealing that limestones are interbedded with marls, calcareous mudstones are also developed, and some limestones present gypsum pseudomorph, thus holding a certain sealing capacity. 3.2. Magmatic activity Magmatite and magmatic activities affect hydrocarbon preservation in four aspects [19,22,27,28]. 1) Magmatic activities happened prior to the massive hydrocarbon generation, and the maturing action of magmatic thermal event on the source rocks can facilitate hydrocarbon generation. 2) The impact of magmatic activities after the massive hydrocarbon generation on hydrocarbon preservation is closely related to its occurrence distribution. If the magmatic intrusion goes upward to induce a series of
tensional faults or fractures in the overlying caprocks, the sealing capacity will be reduced. If the magmatite directly goes through the accumulation, oil and gas quality will be damaged. If the magmatite distributes in parallel with the formations, there is little influence on hydrocarbon preservation. 3) Magmatic intrusion can trigger some special types of traps for oil and gas accumulation. 4) Magmatite itself and its erosion zone may contain pores, vugs and fractures for oil and gas accumulation. The magmatic intrusive activity in the SYS Basineonshore Lower Yangtze area as well as in the adjacent areas initiated in the Middle Jurassic, but it was relatively weak. During the J3eK1 tectonic event, the magmatic activity enhanced, and magmatite was predominantly intermediate and weakly-acid intrusive rocks, distributed in the eastern SYS Basin and the areas close to South Korea (Fig. 3). These rocks are very
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sensitive to magmatic activities and may damage the reservoirs formed earlier. In other areas, hydrocarbon preservation is affected a little by the magmatic activity. 3.3. Hydrogeological and hydrogeochemical characteristics The underground hydrodynamic field of petroliferous basin is controlled by climate, depositional environment and structural evolution, which can indicate the process of hydrocarbon migration and accumulation [19,29,30]. The hydrogeological cycle of sedimentary basins is usually divided into two major periods (Fig. 5): ① the depositional burial compacting centrifugal flow period, which refers to the whole period from the regional subsidence and water invasion to the deposition and sedimentary water burial, when the relatively enclosed environment is favorable for hydrocarbon preservation, with higher salinity, Cle content and other indexes [31,32]; and ② the uplifting erosion meteoric water infiltration centripetal flow period, which refers to the whole period from the regional uplift and regression to the erosion and meteoric water infiltration [29], when the conditions are affected greatly by meteoric and surface water and the salinity is small, being unfavorable for hydrocarbon preservation. There are only a few wells with hydrogeological data encountering marine strata in the SYS Basineonshore Lower Yangtze area, which cannot support the overall evaluation on the hydrogeological and hydrogeochemical conditions for hydrocarbon preservation in the area. Based on the data of limited wells, the marine MesozoicePaleozoic water salinity in the Subei Gaoyou sag and Su'nan Jurong area is generally small and does not vary apparently with the depth. It is inferred that sealing of meteoric water was dominant [29,33]. This indicates that the MesozoiceCenozoic tectonic pattern deformation and basin forming stage greatly affects the underground water chemical properties. First, the ancient leaching meteoric water extensively infiltrates along the weathering surface during the uplifting erosion. Second, the
Fig. 5. Hydrogeological cycle pattern of the sedimentary basin evolution. modified from Ref. [29]
faulting provides the passage for the surface water infiltration [29], making the reservoir damaged, thus being unfavorable for oil and gas preservation. In Well Rong 3 in the Jurong area, the Lower Triassic formation water has salinity of 21.44 g/L and Cle content up to 10.54 g/L, and the well produced several cubic meters of crude oil during well testing [33], suggesting a certain hydrodynamic sealing capacity. 4. Petroleum exploration direction The SYS Basin contains rigid ancient continental nucleus, with rigid crystalline basement [3,34,35]. Although the SYS Basin is similar to the onshore Lower Yangtze area in terms of textures and structures, it shows a weaker tectonic intensity and activity in the MesozoiceCenozoic tectonic pattern deformation and basin forming stage since the Indosinian movement. This implies that the SYS Basin has a better stability. Especially, gentle folds are developed in the central and southern parts of the basin (Fig. 1-b). The faults in the Laoshan uplift demonstrate the least range, less distribution in plane, the weakest intensity, and relatively simple structures (Fig. 3), which are beneficial for hydrocarbon accumulation and preservation in the marine formations. Based on the structural evolution, source rocks and caprocks in the SYS Basin, there are mainly three sets of sourceereservoirecaprock assemblages in the marine formation (Fig. 2), i.e. the assemblage of Lower Cambrian Mufushan Fm mud shale (source rock) þ Middle Cambrian Paotaishan FmeOrdovician dolomite and limestone (reservoir) þ Lower Silurian Gaojiabian Fm mud shale (caprcok), which is completely preserved in the Laoshan uplift where the structures are relatively stable; the assemblage of Lower Silurian Gaojiabian Fm mud shale (source rock) þ MiddleeUpper Silurian Fentou Fm and Maoshan Fm sandstone as well as CarboniferouseLower Permian Qixia Fm limestone (reservoir) þ Upper Permian Longtan FmeDalong Fm shale (caprock), which is well preserved in the southern Laoshan uplift; and the assemblage of Upper Permian Longtan FmeDalong Fm shale (source rock) þ Upper Permian Longtan Fm sandstone and Lower Triassic Qinglong Fm dolomite (reservoir) þ Lower Triassic Qinglong Fm marl and gypsum salt rock (caprock), which may be well preserved in the superimposed basin area. Through processing and interpretation of 2D multi-track seismic data acquired in the recent years, six large structural traps with excellent geological conditions have been discovered in the marine MesozoicePaleozoic formations in the Laoshan uplift, with an area of 33e220 km2, where the former two sourceereservoirecaprock assemblages are relatively complete. In addition, three sets of source rocks hold a good hydrocarbon generation potential and the shale gas geologic conditions are favorable, making the northwest of Laoshan uplift a Paleozoic shale gas prospect [11,36]. Compared to the Laoshan uplift, the Qingdao depression and Yantai depression contain poorer hydrocarbon preservation conditions. The Qingdao depression may have the conditions to form a reservoir similar to the Huangqiao primary residual reservoir, while the Yantai depression may contain the
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conditions to form a reservoir similar to the Jurong reservoir rebuilt in the late stage [12]. To sum up, the marine formation in the Laoshan uplift is characterized by clear seismic reflection, small burial depth, stable structures and large traps, as well as good sourceereservoirecaprock assemblages. Thus the Laoshan uplift is the top prospect for marine exploration in Paleozoic in the SYS Basin. 5. Conclusions 1) The multistage tectonic events shaped the complicated structural pattern of the SYS superimposed basin, mainly involving two stages, i.e. the stable evolution stage of MesozoicePaleozoic marine basin and the MesozoiceCenozoic tectonic pattern deformation and basin forming stage, both of which control the regional source rock development and evolution with the feature of differential hydrocarbon preservation. 2) The hydrocarbon preservation conditions in the marine formations in the SYS Basin include weak structural reformations, thick source rocks with good quality, good caprock sealing capacity in vertical direction, weak magmatic activities, and relatively closed hydrogeological conditions. 3) The marine MesozoicePaleozoic formation in the SYS Basin is more stable than that in the onshore Lower Yangtze area, with good configuration of source rocks and caprocks, corresponding to three sets of sourceereservoirecaprock assemblages. The hydrocarbon preservation evaluation shows that the Laoshan uplift in the central basin contains a stable structure, a good sourceereservoirecaprock assemblage, weak magmatic activities and superior hydrocarbon preservation conditions, thus it is a top prospect for marine hydrocarbon exploration in the Paleozoic in the SYS Basin.
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Research Article
Hydrocarbon preservation conditions in MesozoicePaleozoic marine strata in the South Yellow Sea Basin Liang Jie a,b,c, Zhang Penghui b,c,*, Chen Jianwen b,c, Gong Jianming b,c, Yuan Yong b,c b
a College of Marine Geosciences, Ocean University of China, Qingdao, Shandong 266100, China Qingdao Institute of Marine Geology, China Geological Survey//MLR Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Qingdao, Shandong 266071, China c Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao, Shandong 266071, China
Received 20 January 2017; accepted 25 May 2017
Abstract In the South Yellow Sea Basin, MesozoicePaleozoic marine strata are generally well developed with large thickness, and they are characterized by multi-source and multi-stage hydrocarbon accumulation, providing a material basis for the formation of large-scale oil and gas fields. However, no substantial breakthrough has been made in this area. Based on previous research results, the complex tectonic pattern of this superimposed basin was formed by multi-stage tectonic movements and the favorable static conditions for hydrocarbon preservation were reworked or destroyed by later superimposition. Therefore, hydrocarbon preservation conditions are the key factors for restricting the breakthrough of marine oil and gas exploration in this area. In this paper, hydrocarbon preservation conditions of marine strata in the South Yellow Sea Basin were comprehensively analyzed from many aspects, such as tectonic movement, source conditions, caprock characteristics, magmatic activities, and hydrogeological and hydrogeochemical characteristics. It is indicated that the complex tectonic pattern of the South Yellow Sea Basin is resulted from tectonic events in multiple stages, and the development and evolution of regional source rocks are mainly controlled by two stages (i.e., the stable evolution stage of MesozoicePaleozoic marine basin and the MesozoiceCenozoic tectonic pattern transformation and basin formation stage), so the characteristics of differential oil and gas preservation are presented. Besides, better marine hydrocarbon preservation preconditions in this area are weaker tectonic reworking, development of high-quality thick source rocks, good vertical sealing capacity of caprocks, weaker magmatic activity and confined hydrogeological conditions. It is concluded that the Laoshan Uplift in the central part of the South Yellow Sea Basin is structurally stable with weaker faulting and magmatic activities, so it is better in oil and gas preservation conditions. Besides, several large-scale structural traps with good petroleum geological conditions and complete sourceereservoirecaprock assemblages are developed in this area. Therefore, this area is the most promising region for Paleozoic marine oil and gas exploration in this basin. © 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: South Yellow Sea Basin; MesozoicePaleozoic; Hydrocarbon preservation conditions; Marine strata; Tectonic movement; Caprock characteristic; Magmatic activity; Hydrogeological and hydrogeochemical characteristics; Laoshan uplift
The regional geology as well as gravity and magmatic data analysis show that the South Yellow Sea Basin (hereinafter referred to as SYS Basin) is not only the extension of Yangtze
* Corresponding author. Qingdao Institute of Marine Geology, China Geological Survey//MLR Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Qingdao, Shandong 266071, China. E-mail address: [email protected] (Zhang PH). Peer review under responsibility of Sichuan Petroleum Administration.
block in the offshore area but also the main part of Lower Yangtze block [1e4]. Although 29 wells have been drilled (23 wells by China and 6 by South Korea) in the SYS Basin in the past 5 decades, there is no industrial oil and gas flow up to now. In the recent years, along with the deepening of resource evaluation in the area, more and more research results reveal that the SYS Basin contains complete and very thick MesozoicePaleozoic marine strata and corresponds to a depositional history similar to the Sichuan Basin, so it holds the
https://doi.org/10.1016/j.ngib.2017.05.013 2352-8540/© 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Liang J, et al., Hydrocarbon preservation conditions in MesozoicePaleozoic marine strata in the South Yellow Sea Basin, Natural Gas Industry B (2017), https://doi.org/10.1016/j.ngib.2017.05.013
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material basis to form large accumulations and thus presents a great exploration potential [3,5]. The SYS Basin is a typical superimposed basin. As a result of the superimposed reworking by multistage multi-episode tectonic movements, the geological conditions were complicated, and the good static hydrocarbon preservation conditions in the prototype basin were reformed or damaged. The hydrocarbon preservation conditions are the primary control factor to determine the hydrocarbon enrichment in the superimposed basin [6], and also the key to bring about petroleum exploration breakthroughs in the SYS Basin. The hydrocarbon preservation conditions comprise a whole process of hydrocarbon generation, migration, accumulation and dispersion. However, up to now, there is still no systematic recognition upon the hydrocarbon preservation conditions in the marine strata in the SYS Basin. In this paper, its hydrocarbon preservation conditions were discussed from many aspects, such as tectonic movement, caprock characteristics, magmatic activity, and hydrogeological and hydrogeochemical characteristics, in order to provide reference for traps evaluation and exploration deployment in the basin. 1. Geological setting As the main part of Lower Yangtze block, the SYS Basin is located in the South Yellow Sea in the eastern Yangtze platform, with a structural pattern of “one uplift in two depressions” [3,7], and covering an area of 18 104 km2. It can be divided into three structural units from north to south: Yantai depression, Laoshan uplift and Qingdao depression (Fig. 1-a). Through the analysis of drilling and seismic data as well as marineecontinental correlation, it can be found that the SYS Basin has similar MesozoicePaleozoic depositional evolution and lithological assemblage features to the onshore Lower Yangtze area. The SYS Basin contains complete and well-preserved marine strata, including Neoproterozoic Sinian, Paleozoic Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian, and Mesozoic Triassic, but no MiddleeLower Devonian (Figs. 1-b and 2). 2. Tectonic movement, source rock conditions and hydrocarbon preservation Zhu Xia [8] indicated that “the variation of kinetic mechanism is an important condition for petroliferous basin formation”. He also proposed that there are two kinds of kinetic mechanism in the SYS Basineonshore Lower Yangtze area, which are especially different in the behaviors before and after Indosinian movement [9,10], showing the stable evolution stage of MesozoicePaleozoic marine basin and the MesozoiceCenozoic tectonic pattern transformation and basin formation stage. The common action of Caledonian, Indosinian, Yanshan and Himalayan movements finally shaped the complicated structural pattern in the SYS Basineonshore Lower Yangtze area. The tectonic evolution can help to organically combine the petroleum geologic conditions and control the differences of reservoir damage and preservation in
different zones, which further result in the diversity and complexity of trap types. 2.1. Stable evolution stage of MesozoicePaleozoic marine basin 2.1.1. Caledonian (ZeS) In the Early SinianeMiddle Ordovician (ZeO2), the Lower Yangtze block generally took on a configuration of “one uplift in two depressions” [9], receiving platform sediments in the central area and slope-shelf sediments in the south and north. The lower Cambrian Mufushan Fm mainly contains black carbonaceous shale, taupe siliceous mud shale and sallow sandy shale [11], mostly 50e200 m thick and widely distributed, and it is a set of regional source rocks with hydrocarbon generation potential in the marine strata in the SYS Basin (Table 1). This set of source rocks made great contribution to the formation of Anyue and Weiyuan large gas fields in the Sichuan Basin. In the Late OrdovicianeSilurian (O3eS), along with the closing of South China Ocean and subduction of Qingling Ocean, foreland basin and intra-platform depression began to develop in the Yangtze platform [9,12]. The widespread dark mud shale in the lower Silurian Gaojiabian Fm in the SYS Basin shows high organic matter abundance and moderate maturity. It is regarded as a set of good source rocks throughout the study area (Table 1), and acts as the gas source rocks for Wubaiti, Wolonghe, Shapingchang, Jiaoshiba and other large gas fields and shale gas fields in the Sichuan Basin. The Lower Cambrian and lower Silurian source rocks represent two sets of the best regional source rocks in the lower structural marine layer in the SYS Basin. The Caledonian movement (Guangxi event) in the late Silurian made the Lower Yangtze block (including the SYS Basin) uplift as a whole and suffer erosion, with the erosion intensity strong in the south and weak in the north [9,13]. As a result, the deposits of upper Silurian and middleelower Devonian were absent intensively, and slight folding occurred. 2.1.2. Hercynianeindosinian (DeT2) In the DevonianeEarly Carboniferous (DeC1), influenced by the closing of Qingling ocean and the opening of Tethys ocean, the SYS Basin and onshore Lower Yangtze area presented a generally stable epicontinental sea depositional environment in the Late Paleozoic [9,12,13]. In the Late CarboniferouseEarly Permian (C2eP1), along with the extension of Tethys ocean and the opening of South Qingling trough, the Lower Yangtze block suffered rifting or faulting [9,12]. It received transgression in the Early Permian Qixia stage, giving rise to a set of carbonate sedimentary formations with rich asphalt and siliceous materials in the middleesouthern SYS Basin. In the Late PermianeMiddle Triassic (P2eT2), influenced by the extension of South Qingling trough, the rift further developed, and a set of regional source rocks featured by black and grayish black siliceous shale deposited in a deep-water environment was formed in the Upper Permian Longtan
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Fig. 1. Tectonic zonation (a) and seismic sequence division (b) of MesozoicePaleozoic marine strata in the SYS Basin.
FmeDalong Fm (Table 1). This set of source rocks is the primary source rocks in the upper structural marine layer in the SYS Basin and also the major hydrocarbon supply strata for Puguang, Yuanba, Longgang, Moxi, Tieshanpo, Dukouhe and Luojiazhai gas fields in the Sichuan Basin.
The Indosinian movement in the end of Middle Triassiceinitial stage of Late Triassic thoroughly changed the regional structural and depositional patterns of Lower Yangtze block and terminated the stable evolution history of a marine depositional basin [9,12]. Thus the marine strata suffered diverse reformation, and the MesozoicePaleozoic faulting mainly happened in the Indosinian stage with thrust and overthrust faults developed (Figs. 1-b and 3), bringing the SYS Basin into an era of later reformation.
PermianeLower Triassic formations were eroded to different extent or even locally absent. During the J3eK1 tectonic event, the Yangtze block showed distinctive differentiation in the east and west. To the east of Qiyueshan fault, the Middle and Lower Yangtze areas suffered compressional deformation and folding thrust reformation as well as uplifting erosion. To the west of Qiyueshan fault, the Sichuan Basin exhibited continuous settlement without obvious folding deformation [17,18]. In this period, the MesozoicePaleozoic (including Middle and Lower Jurassic) in the SYS Basin experienced compressional deformation, deflection and order change, recording as the most intense reformation in the MesozoicePaleozoic strata. Especially, the reformation was the strongest in the piedmont of orogenic belt and near the Tanlu fault zone, while the Laoshan uplift was relatively weakened by the reformation, laying a foundation for the fundamental MesozoicePaleozoic structural shape of the SYS Basin.
2.2.1. Late Triassiceearly cretaceous (T3eK1) During the T3eJ2 tectonic event, influenced by the compression of Yangtze block in the south and north, the SYS Basin was in the foreland basin developing stage [9,12]. It presented uplifting as a whole, with the tectonic intensity stronger than the Upper Yangtze block. As a result, the Upper
2.2.2. Late CretaceousePaleogene (K2eE) The SYS Basineonshore Lower Yangtze area experienced multistage tensional rifting, giving rise to several dustpan-like or graben basins of single fault or half graben types [9]. The erosion thickness was small in the faulted sag, thus the marine hydrocarbon preservation system could be rebuilt in the
2.2. MesozoiceCenozoic structural pattern transformation and basin forming stage
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Fig. 2. Marine strata and major sourceereservoirecaprock assemblages in the SYS Basineonshore Yangtze area. Note: * represents the formation encountered in the SYS Basin.
superimposed area. The primary hydrocarbon reservoir was prone to be damaged and destroyed on the massif between the uplift and the faulted sag [12,18]. The Jurong and Huangqiao reservoirs represent the hydrocarbon accumulation in this stage, and the marine formations in the SYS Basin show differential hydrocarbon preservation in the same stage.
deposition was stable, which affected a little on oil/gas preservation in the marine strata.
2.2.3. Neogeneequaternary (NeQ) In this stage, the SYS Basin generally settled and formed a united depression when the faulting tended to be weak and the
Caprocks, as a core element in hydrocarbon preservation research, comprise direct caprocks (inner caprocks) and regional caprocks. The direct caprock is the rock layer which
3. Hydrocarbon preservation conditions 3.1. Caprocks
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Table 1 Regional source rock characteristics in the representative wells in the SYS Basineonshore Lower Yangtze area.
Note: I represents sapropel-type, II represents mixed type, II1 represents sapropel-prone mixed type; III represents humic type, modified from Refs. [3,9,12,14e16].
can directly prevent the hydrocarbon from escape. The regional caprock refers to the overlying strata constituted by different types of rocks that generally protect the hydrocarbon system [19,20]. The real sealing capacity of caprock depends on the relative sealing of the reservoirecaprock assemblage [21], and the sealing performance directly affects the hydrocarbon reservoir formation, scale and preservation. A good regional caprock can effectively prevent the massive escape of oil and gas, so that oil and gas can be reallocated and preserved under the regional caprock, even if the direct caprock is poor [22]. The MesozoicePaleozoic caprocks in the SYS Basin involve (gypsum) salt rocks, tight (mud) limestones and mud shale. (Gypsum) Salt rock presents high breakthrough pressures as well as large plasticity coefficients and compressibility coefficients, thus no fracture may occur even if there is large deformation under high temperature and high pressure. It is the most effective caprock. Tight (mud) limestones present small breakthrough pressure, thus fragile cracks may generate because the sealing capacity is relatively poor. Mud shale shows breakthrough pressure and mechanical property between (gypsum) salt rocks and tight (mud) limestones, thus its sealing capacity falls in between them [19e23]. According to seismic data interpretation, marinee continental correlation and drilling data, there are three sets of caprock sequence in the marine MesozoicePaleozoic in the SYS Basin (Fig. 4). They are mud shale in the Lower Silurian Gaojiabian Fm, mud shale in the Upper Permian Longtan FmeDalong Fm, and marl and gypsum salt rocks in the Lower Triassic Qinglong Fm. These caprocks can provide sealing conditions for marine strata.
According to the available Lower Yangtze onshore outcrop and drilling data, the mud shale in the Lower Silurian Gaojiabian Fm presents varying horizontal thickness but continuous distribution. It was usually formed earlier than the hydrocarbon generation and migration. Because of old strata, large burial depth and strong diagenesis, the mud shale represented by Gaojiabian Fm is also called highly evolving mud shale whose sealing capacity is often considered to be poor. The poor sealing is attributable to the fact that the mud shale is buried beyond a certain depth (1500e3500 m) where the compaction approaches limit and the increasing fragility can easily produce micro fractures, thereby reducing capillary sealing. Based on the researches into the relation between the burial depth and the caprock sealing parameter, the relation between the burial depth and the regional caprock sealing performance in the triaxial resistance to shearing and compressing experiment, and the discovered Paleozoic reservoirs in the Sichuan Basin, the highly evolving mud shale caprock shows high plasticity under deep burial depth, and the sealing capacity is closely related to the compaction degree. Its sealing capacity could be good, provided that it was not reworked by strong reformation activities (e.g. faulting) in later stages [24,25]. During the MesozoiceCenozoic structural pattern reformation and basin forming stage, although the mud shale in the Gaojiabian Fm was affected by the tectonic movement such as overthrust faults, the structural traps below the sliding surface could keep pattern integrity due to the sliding surface inside the overthrust fault [26], causing limited continuous destruction of caprocks. For example, Well N1 in Jurong area reveals oil and gas shows in the CarboniferousePermian below the sliding surface. In addition, core observation shows
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Fig. 3. Distribution of marine MesozoicePaleozoic faults in the SYS Basin. Boundary range shown in Fig. 1, modified from Ref. [10]
that micro-fractures in the mud shale of Gaojiabian Fm affect a little on the caprock sealing due to limited micro-fracture extension. According to the caprock parameter analysis in the onshore Lower Yangtze drilled Gaojiabian Fm, the breakthrough pressure is more than 12 MPa in Well Xingcan 1, more than 16 MPa in Well C, and 12e16 MPa in Well Jucan 2, showing a good sealing capacity. The mud shale in the Upper Permian Longtan FmeDalong Fm drilled by Well B in the SYS Basin demonstrates a cumulative thickness of more than 100 m (Fig. 4), and a
displacement pressure of 18e25 MPa estimated according to logging interpretation, reflecting a good sealing capacity. On the seismic section, the Longtan FmeDalong Fm shows continuous parallel reflection [26], with good tracking and correlation laterally and a certain continuity on the plane. This set of caprocks is relatively developed in the central and southern parts of the basin. The marl and gypsum salt rocks of the Lower Triassic Qinglong Fm exist locally in the SYS Basin, and still can serve as a local caprock due to good sealing conditions. For
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Fig. 4. Caprocks in the SYS Basineonshore Lower Yangtze area. From left to right: Wells A, B, C
example, Well A encounters Qinglong Fm, revealing that limestones are interbedded with marls, calcareous mudstones are also developed, and some limestones present gypsum pseudomorph, thus holding a certain sealing capacity. 3.2. Magmatic activity Magmatite and magmatic activities affect hydrocarbon preservation in four aspects [19,22,27,28]. 1) Magmatic activities happened prior to the massive hydrocarbon generation, and the maturing action of magmatic thermal event on the source rocks can facilitate hydrocarbon generation. 2) The impact of magmatic activities after the massive hydrocarbon generation on hydrocarbon preservation is closely related to its occurrence distribution. If the magmatic intrusion goes upward to induce a series of
tensional faults or fractures in the overlying caprocks, the sealing capacity will be reduced. If the magmatite directly goes through the accumulation, oil and gas quality will be damaged. If the magmatite distributes in parallel with the formations, there is little influence on hydrocarbon preservation. 3) Magmatic intrusion can trigger some special types of traps for oil and gas accumulation. 4) Magmatite itself and its erosion zone may contain pores, vugs and fractures for oil and gas accumulation. The magmatic intrusive activity in the SYS Basineonshore Lower Yangtze area as well as in the adjacent areas initiated in the Middle Jurassic, but it was relatively weak. During the J3eK1 tectonic event, the magmatic activity enhanced, and magmatite was predominantly intermediate and weakly-acid intrusive rocks, distributed in the eastern SYS Basin and the areas close to South Korea (Fig. 3). These rocks are very
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sensitive to magmatic activities and may damage the reservoirs formed earlier. In other areas, hydrocarbon preservation is affected a little by the magmatic activity. 3.3. Hydrogeological and hydrogeochemical characteristics The underground hydrodynamic field of petroliferous basin is controlled by climate, depositional environment and structural evolution, which can indicate the process of hydrocarbon migration and accumulation [19,29,30]. The hydrogeological cycle of sedimentary basins is usually divided into two major periods (Fig. 5): ① the depositional burial compacting centrifugal flow period, which refers to the whole period from the regional subsidence and water invasion to the deposition and sedimentary water burial, when the relatively enclosed environment is favorable for hydrocarbon preservation, with higher salinity, Cle content and other indexes [31,32]; and ② the uplifting erosion meteoric water infiltration centripetal flow period, which refers to the whole period from the regional uplift and regression to the erosion and meteoric water infiltration [29], when the conditions are affected greatly by meteoric and surface water and the salinity is small, being unfavorable for hydrocarbon preservation. There are only a few wells with hydrogeological data encountering marine strata in the SYS Basineonshore Lower Yangtze area, which cannot support the overall evaluation on the hydrogeological and hydrogeochemical conditions for hydrocarbon preservation in the area. Based on the data of limited wells, the marine MesozoicePaleozoic water salinity in the Subei Gaoyou sag and Su'nan Jurong area is generally small and does not vary apparently with the depth. It is inferred that sealing of meteoric water was dominant [29,33]. This indicates that the MesozoiceCenozoic tectonic pattern deformation and basin forming stage greatly affects the underground water chemical properties. First, the ancient leaching meteoric water extensively infiltrates along the weathering surface during the uplifting erosion. Second, the
Fig. 5. Hydrogeological cycle pattern of the sedimentary basin evolution. modified from Ref. [29]
faulting provides the passage for the surface water infiltration [29], making the reservoir damaged, thus being unfavorable for oil and gas preservation. In Well Rong 3 in the Jurong area, the Lower Triassic formation water has salinity of 21.44 g/L and Cle content up to 10.54 g/L, and the well produced several cubic meters of crude oil during well testing [33], suggesting a certain hydrodynamic sealing capacity. 4. Petroleum exploration direction The SYS Basin contains rigid ancient continental nucleus, with rigid crystalline basement [3,34,35]. Although the SYS Basin is similar to the onshore Lower Yangtze area in terms of textures and structures, it shows a weaker tectonic intensity and activity in the MesozoiceCenozoic tectonic pattern deformation and basin forming stage since the Indosinian movement. This implies that the SYS Basin has a better stability. Especially, gentle folds are developed in the central and southern parts of the basin (Fig. 1-b). The faults in the Laoshan uplift demonstrate the least range, less distribution in plane, the weakest intensity, and relatively simple structures (Fig. 3), which are beneficial for hydrocarbon accumulation and preservation in the marine formations. Based on the structural evolution, source rocks and caprocks in the SYS Basin, there are mainly three sets of sourceereservoirecaprock assemblages in the marine formation (Fig. 2), i.e. the assemblage of Lower Cambrian Mufushan Fm mud shale (source rock) þ Middle Cambrian Paotaishan FmeOrdovician dolomite and limestone (reservoir) þ Lower Silurian Gaojiabian Fm mud shale (caprcok), which is completely preserved in the Laoshan uplift where the structures are relatively stable; the assemblage of Lower Silurian Gaojiabian Fm mud shale (source rock) þ MiddleeUpper Silurian Fentou Fm and Maoshan Fm sandstone as well as CarboniferouseLower Permian Qixia Fm limestone (reservoir) þ Upper Permian Longtan FmeDalong Fm shale (caprock), which is well preserved in the southern Laoshan uplift; and the assemblage of Upper Permian Longtan FmeDalong Fm shale (source rock) þ Upper Permian Longtan Fm sandstone and Lower Triassic Qinglong Fm dolomite (reservoir) þ Lower Triassic Qinglong Fm marl and gypsum salt rock (caprock), which may be well preserved in the superimposed basin area. Through processing and interpretation of 2D multi-track seismic data acquired in the recent years, six large structural traps with excellent geological conditions have been discovered in the marine MesozoicePaleozoic formations in the Laoshan uplift, with an area of 33e220 km2, where the former two sourceereservoirecaprock assemblages are relatively complete. In addition, three sets of source rocks hold a good hydrocarbon generation potential and the shale gas geologic conditions are favorable, making the northwest of Laoshan uplift a Paleozoic shale gas prospect [11,36]. Compared to the Laoshan uplift, the Qingdao depression and Yantai depression contain poorer hydrocarbon preservation conditions. The Qingdao depression may have the conditions to form a reservoir similar to the Huangqiao primary residual reservoir, while the Yantai depression may contain the
Please cite this article in press as: Liang J, et al., Hydrocarbon preservation conditions in MesozoicePaleozoic marine strata in the South Yellow Sea Basin, Natural Gas Industry B (2017), https://doi.org/10.1016/j.ngib.2017.05.013
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conditions to form a reservoir similar to the Jurong reservoir rebuilt in the late stage [12]. To sum up, the marine formation in the Laoshan uplift is characterized by clear seismic reflection, small burial depth, stable structures and large traps, as well as good sourceereservoirecaprock assemblages. Thus the Laoshan uplift is the top prospect for marine exploration in Paleozoic in the SYS Basin. 5. Conclusions 1) The multistage tectonic events shaped the complicated structural pattern of the SYS superimposed basin, mainly involving two stages, i.e. the stable evolution stage of MesozoicePaleozoic marine basin and the MesozoiceCenozoic tectonic pattern deformation and basin forming stage, both of which control the regional source rock development and evolution with the feature of differential hydrocarbon preservation. 2) The hydrocarbon preservation conditions in the marine formations in the SYS Basin include weak structural reformations, thick source rocks with good quality, good caprock sealing capacity in vertical direction, weak magmatic activities, and relatively closed hydrogeological conditions. 3) The marine MesozoicePaleozoic formation in the SYS Basin is more stable than that in the onshore Lower Yangtze area, with good configuration of source rocks and caprocks, corresponding to three sets of sourceereservoirecaprock assemblages. The hydrocarbon preservation evaluation shows that the Laoshan uplift in the central basin contains a stable structure, a good sourceereservoirecaprock assemblage, weak magmatic activities and superior hydrocarbon preservation conditions, thus it is a top prospect for marine hydrocarbon exploration in the Paleozoic in the SYS Basin.
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Please cite this article in press as: Liang J, et al., Hydrocarbon preservation conditions in MesozoicePaleozoic marine strata in the South Yellow Sea Basin, Natural Gas Industry B (2017), https://doi.org/10.1016/j.ngib.2017.05.013