1 Introduction

The Yangtze region has developed very thick marine Mesozoic–Paleozoic strata on the platform margins and in the depressions and also developed terrestrial sediments in the Mesozoic–Cenozoic fault depression, giving it great potential for forming large- and medium-scale oil and gas fields (Pang et al., 2003; Cai et al., 2005; Li et al., 2014a; Cai et al., 2016; Li et al., 2016a; Pang et al., 2016, 2019). Results from the analysis of regional geological, gravity, and magnetic data show that the South Yellow Sea Basin is not only the extension of the Yangtze Plate in the sea area but also the main part of the lower Yangtze block (Zhang and Liang, 2014; Liang et al., 2017; Yuan et al., 2018b). Hydrocarbon investigation and exploration in the South Yellow Sea Basin started in the 1960s, with 30 wells in total (24 in China and 6 in Korea) having been drilled. However, no industrial hydrocarbon flow has been discovered up till now. In recent years, with the deepening of the investigation of hydrocarbon resources, more and more research results have indicated that the thick Mesozoic–Paleozoic marine strata developed completely in the South Yellow Sea Basin (Zhang and Liang, 2014; Li et al., 2016a; Liang et al., 2017; Pang et al., 2017a). They provide the material base for formation of large hydrocarbon reservoirs, thus have good exploration potential for hydrocarbon resources (Zhang and Liang, 2014; Li et al., 2016b; Zhao et al., 2017). The South Yellow Sea Basin is a typical superimposed basin, and it has undergone superi-mposed reformation by multiphase, multi-episode tectonic movements, so the geological conditions are complex and static hydrocarbons well preserved in the original basins would have been reformed or destroyed to different extents (Wan and Hao, 2010; Xu et al., 2014; Liu et al., 2016). However, the systematic understanding on the hydrocarbon geological conditions of the marine strata in the South Yellow Sea Basin remains lacking. Therefore, in this paper we comprehensively analyzed the hydrocarbon geological conditions of the marine strata in the South Yellow Sea Basin through sea-land comparison from aspects such as basin evolution, source rock characteristics, reservoir characteristics, capping bed characteristics, magmatic activity, and structural traps to provide reference for exploration deployment in this basin.

2 Geological Setting

The South Yellow Sea Basin, with the area of 18×10⁴ km², is located on the east of the Yangtze platform; it has a tectonic framework of 'one uplift sandwiched in between two depressions' (Li et al., 2014b; Chen et al., 2016a; Yuan et al., 2016; Liang et al., 2017). It is divided into three tectonic units from north to south: the Yantai Depression, the Laoshan Uplift, and the Qingdao Depression (Fig. 1). It was found through analysis of drilling and seismic data and sea-land comparison that the South Yellow Sea Basin and the lower Yangtze land area had similar sedimentary evolution and lithologic association characteristics in the Mesozoic–Paleozoic strata (Chen et al., 2016; Liang et al., 2017; Yuan et al., 2018a). Neoproterozoic Sinian, Paleozoic Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian, and Mesozoic Triassic strata were deposited from the bottom up, with the missing of middle and lower Devonian strata.

3 Samples and Methods 3.1 Sea-Land Comparison

The South Yellow Sea Basin and the upper Yangtze Si-chuan Basin have similar crystalline basements (Yang et al., 2003; Qi et al., 2013; Wang et al., 2013; Yuan et al., 2017). Both of them were reformed by identical tectonic movements. During the Paleozoic, both regions had basically consistent tectonic evolution histories (Fig. 2). Their sedimentary characteristics, stratigraphic characteristics, and geological structures were very similar to each other during the marine sedimentary period (Fig. 2). The marine sediments in the South Yellow Sea Basin and the Sichuan Basin in different periods are mainly neritic platform facies. Only parts of the upper Paleozoic and the lower Triassic strata were obtained through drilling in the South Yellow Sea and no lower Paleozoic strata were disclosed, but the Mesozoic–Paleozoic sedimentary formations in the lower Yangtze land area are basically the same as those in the Sichuan Basin, and the Mesozoic–Paleozoic sedimentary formations are comparable group by group between these two regions (Yuan et al., 2017).

3.2 Field Source Rock Samples and Logging Analysis

During the field geological investigation, 85 source rock samples in total were collected in the Yangtze region (Table 1), and they were mainly collected from the lower Cambrian, the lower Silurian, the lower Permian, and the upper Permian. The source rock samples were analyzed as follows: 85 were used for pyrolytic analysis, 47 were analyzed via gas chromatography-mass spectrometry (GC-MS) for saturated hydrocarbons, 47 were analyzed via GC-MS for aromatic hydrocarbons, and 47 were used for Ro determination (Table 1). The research object is marine highmaturity source rock, so it is poor for evaluating maturity using the routine method. In recent years, it has been found that high-maturity source rock can be well evaluated by using the Ro value converted from the ratio of pristane diastereomers in isoprenoid hydrocarbon compounds (PIR). Furthermore, if parameters such as S1+S2, total hydrocarbon contents, and chloroform bitumen 'A' are used for evaluation of an outcrop sample, the result obtained may not be a source rock. Therefore, the only reliable index for judging the abundance of organic matter is the amount of total organic carbon (TOC).

Core observation, logging interpretation, and well-to-seismic calibration were conducted for seven wells encountering Mesozoic–Paleozoic strata in the South Yellow Sea Basin. The reservoir types were determined based on the well data and outcrops in the Yangtze land area. In addition, mudstone samples in the core were analyzed to obtain the capping bed breakthrough pressure.

3.3 Seismic Interpretation

The marine Mesozoic–Paleozoic strata in the South Yellow Sea is characterized by large thickness, deep burial depths, complex tectonics, strong vertical heterogeneity of the lithology, energy shielding on a shallow strong-reflection interface, and weak energy inside the carbonate rock layer (Yuan et al., 2018b; Qiu et al., 2019). In recent years (Chen et al., 2016b), a 'high-rich-strong' (high number of coverage times, rich low frequency, and strong seismic source) (Chen et al., 2016b) seismic exploration technology suitable for the seismic and geological conditions in the study area has been gradually applied because of its continuous key technological innovation, and marine Mesozoic–Paleozoic reflection data with relatively high quality have been obtained. In this paper, regional tectonic interpretation of new collected seismic data was made based on an early geological survey (Lei et al., 2016, 2018; Chen et al., 2018a, 2018b; Zhang et al., 2018) (Fig. 3). The pattern and the formation and evolution processes of the basin and the development of seismic tectonic layers were analyzed.

4 Results 4.1 Basin Evolution 4.1.1 Stable evolution stage of the Mesozoic–Paleozoic marine basin

1) Caledonian age (Z–S)

The development and evolution of the basin started from the breakup of the global Rodinia supercontinent in the early Neoproterozoic (Xu et al., 2014). The formation of the paleo-China continent in the end of the Silurian demarcates the end of this stage (Xu et al., 2014). In the Sinian–middle Ordovician (Z–O₂), the lower Yangtze block exhibited a tectonic framework of 'one uplift and two depressions' as a whole, with sedimentary strata of platform facies in the middle and slope-continental shelf facies in the south and north (Liang et al., 2017; Fig. 4a). The period with the most severe transgression was the early Cambrian, when a set of black source rock layers containing carbonaceous mudstone and shale was formed. In the late Ordovician–Silurian (O₃–S), a foreland basin and intra-platform depression developed in the Yangtze platform, with the closure of the South China Ocean and the subduction of the Qinling Ocean Plate (Liang et al., 2017; Tan et al., 2018). From the late Ordovician to the early Silurian, intense subsidence occurred again in the platform, the sea was deepened evidently, and noncompensated shale deposits of continental shelf facies developed, which constituted the second set of source rock layers in this area (Yao et al., 2005; Zhao, 2007; Liang et al., 2008). The Caledonian movement in the late Silurian caused the lower Yangtze block, including the South Yellow Sea Basin, to uplift as a whole and to be denuded. The denudation intensity was high in the south and low in the north, leading to the loss and slight folding of a large area of the upper Silurian and middle–lower Devonian strata (Liang et al., 2017; Fig. 4a).

2) Hercynian–Indosinian age (D–T₂)

At this stage the basin changed with the formation, development, and disappearance of the Paleotethys Ocean and the formation of the paleo-Asia continent, and it ended the marine sedimentation in the south of China through the Indosinian movement between the middle and late Triassic (Xu et al., 2014; Zhang et al., 2015). In the Devonian–early Carboniferous (D–C₁), affected by the closure of the Qinling Ocean and the opening of the Tethys Ocean, the South Yellow Sea Basin and the lower Yangtze land area exhibited relatively stable epicontinental sedimentation in the late Paleozoic (Yao et al., 2005, 2010; Zou et al., 2016). In the late Carboniferous–early Permian (C₂–P₁), with the expansion of the Tethys Ocean and the opening of the south Qinling trough, there was a rift or fault depression generated in the lower Yangtze block (Liang et al., 2017; Tan et al., 2018). In the early Permian Qixia period, the lower Yangtze block underwent transgression and one set of sedimentary formations enriched in bituminous and silicious carbonate rocks developed in the middle and south of the South Yellow Sea Basin. In the late Permian–middle Triassic (P₂–T₂), affected by the expansion of the south Qinling trough, the rift developed further, filling one set of regional source rocks characterized by black and gray black silicious shale with deep-water sediments developed in the upper Permian Longtan formation–Dalong formation (Zhang et al., 2015; Liang et al., 2017).

4.1.2 The Mesozoic-Cenozoic tectonic reformation and basin formation stage

1) Late Triassic–early Cretaceous (T₃–K₁)

Late Triassic–middle Jurassic (T₃–J₂): Affected by the squeezing effects on both southern and northern sides of the Yangtze block, the South Yellow Sea Basin was in a foreland basin development period, with an uplift state as a whole, and exhibited upper Permian–lower Triassic strata that were denuded to different extents and were even missing locally (Liang et al., 2017; Fig. 4b).

Late Jurassic–early Cretaceous (J₃–K₁): In this period, the Mesozoic–Paleozoic strata, including those of the middle and lower Jurassic, in the South Yellow Sea Basin underwent squeezing deformation to different extents. In this period, the deformation of the Mesozoic–Paleozoic strata was the strongest (Fig. 4b; He et al., 2011; Jin et al., 2012; Pang et al., 2017b, 2017c).

2) Late Cretaceous–Paleogene (K₂–E)

The lower Yangtze region underwent multiphase tensional fault depression in this period, forming several single-fault or half-graben basins or graben-type basins (Xu et al., 2014; Liang et al., 2017; Fig. 4b). The marine strata in the South Yellow Sea Basin exhibited some hydrocarbon preservation features in this period (Liang et al., 2017).

4.2 Source-Reservoir-Cap Assemblage 4.2.1 Marine hydrocarbon source rock

1) Lower Cambrian source rock

The lower Cambrian Qiongzhusi formation source rock in the upper Yangtze Sichuan Basin is mainly black carbonaceous shale (Xu et al., 2011; Wu et al., 2019a; Fig. 5a). This set of source rock layers has relatively large thickness, averaging about 100 m, and has relatively strong hydrocarbon generation potential (Xu et al., 2011). The organic matter has reached a mature stage, and the hydrocarbons generated are mainly gaseous. The field outcrop samples have a mean Ro of 1.57% and a mean TOC of 1.4% (Table 2). This set of source rock made a great contribution to the formation of large gas fields in the Sichuan Basin such as the Anyue and Weiyuan (Liang et al., 2017). The lower Cambrian Mufushan formation–Hetan formation in the lower Yangtze land area is dominated by black carbonaceous mudstones (Fig. 5b), which are distributed widely with the thickness of 50–200 m, and the field outcrop samples have a mean Ro of 1.77% and a mean TOC of 3.58% (Yuan et al., 2018b). The source rock in the northern Jiangsu Basin has Ro values of 2.95%–4.01% and TOC values of 2%–9%. This is a set of good source rocks in the lower Yangtze and is a set of regional source rocks with hydrocarbon generation potential in the marine strata of the South Yellow Sea Basin (Liu et al., 2018; Tan et al., 2018; Yuan et al., 2018b).

2) Lower Silurian source rock

The Lower Silurian Longmaxi formation source rock in the Sichuan Basin is black shale (Xu et al., 2011; Han et al., 2016; Wu et al., 2019b; Fig. 5c). The source rock has relatively high degree of thermal evolution and is primarily at high-maturity or over-maturity stage, and the field outcrop samples have a mean Ro of 2.03% and a mean TOC of 1.7% (Table 2). This set of source rocks has become the major gas source in large gas fields and shale gas fields in the Sichuan Basin such as the Wubaiti, Wolonghe, Shapingchang, and Jiaoshiba (Chen et al., 2016a; Liang et al., 2017). The lower Silurian Gaojiabian formation in the lower Yangtze land area is dark mudstone (Fig. 5d), and the field outcrop samples have a mean Ro of 1.56% and a mean TOC of 1.23% (Yuan et al., 2018b). Moreover, the source rocks of the Huangqiao gas field in northern Jiangsu have TOC values of 1%–2% and a mean Ro of 1.9%, and they are at high-maturity stage, locally reaching the over-maturity stage, similar to the source rocks in the upper Yangtze (Tan et al., 2018). This set of source rocks has relatively high hydrocarbon generation potential in the lower Yangtze region. According to seismic data interpretation, the lower Silurian Gaojiabian formation developed and was widely distributed in the South Yellow Sea Basin, which can be regarded as good source rocks distributed throughout the whole region. The upper strata of the Gaojiabian formation were encountered during drilling of well CSDP-2, with the thickness of > 400 m. They were dominated by dark gray mudstone, and had Ro values of 2%–2.5% and TOC values of 0.21%–0.47% (Cai et al., 2017). The TOC content was relatively low because the main source rocks in the lower member of the Gaojiabian formation were not encountered during drilling.

3) Lower Permian source rock

The lower Permian Qixia formation carbonate source rock in the upper Yangtze Sichuan Basin had a mean thickness up to 240 m (Xu et al., 2011), and the field outcrop samples had a mean Ro of 2.00% and a mean TOC of 1.36% (Table 2), so the source rock entered an evolution stage from high maturity to over-maturity, and the source rock in this stage will mainly evolve into cracked gas. The source rock in the lower Yangtze land area has a medium degree of thermal evolution and has a mean Ro of 1.8% (Fig. 5e). Wells CZ35-2-1 and CSDP-2 in the South Yellow Sea Basin disclose that the Qixia formation is composed of black limestone and the thicknesses are 205 and 72 m, respectively. In well CZ35-2-1, the mean Ro is 2.45% and the mean TOC is 1.09% (Chen et al., 2016a; Yuan et al., 2017). In Well CSDP-2, the mean Ro is 0.92% and the mean TOC is 1.44% (Cai et al., 2017). This set of limestones is a set of good source rocks (Tan et al., 2018; Yuan et al., 2018b).

4) Upper Permian source rock

The Upper Permian Longtan formation source rock in the upper Yangtze Sichuan Basin is gray shale and mudstone, and their contents are 3%–7% (Xu et al., 2011). The mean Ro value is > 2.0%, the maturity is universally high, and the natural gas type is mainly cracked gas (Xu et al., 2011; Yuan et al., 2017). This set of source rocks is the main hydrocarbon supplying beds for large gas fields in the Sichuan Basin such as the Puguang, Yuanba, Long-gang, Moxi, Tieshanpo, Dukouhe, and Luojiazhai (Liang et al., 2017). The source rock in the lower Yangtze land area has the thickness of 50–200 m, and the field outcrop samples have a mean Ro of 1.72% and a mean TOC of 2.90% (Yuan et al., 2018b) (Figs. 5f and 5g). In well CSDP-2 in the South Yellow Sea Basin, the Longtan and Dalong formations were encountered during drilling. The Longtan formation has a thickness of 450 m and a mean TOC up to 2.13% (Cai et al., 2017). The Dalong formation has a mean Ro of > 0.7% and a mean TOC of 1.655% (Cai et al., 2017). In wells WX5-ST1 and CZ35-2-1, the Long-tan and Dalong formations were also encountered during drilling; they are characterized by thicknesses of > 260 m, high abundance of organic matters, and Ro values ranging from 1.5% to 3.0% (Yuan et al., 2017). The upper Permian source rock in the South Yellow Sea Basin is a set of good source rock, and it is the main source rock in the upper marine structural layers.

4.2.2 Reservoir characteristics

The main marine Mesozoic–Paleozoic strata with carbonate rock reservoirs in the South Yellow Sea included the upper Sinian Dengying formation, the middle–upper Cambrian, the Ordovician, the Carboniferous, the Permi-an, and the lower Triassic (Yuan et al., 2017; Peng et al., 2018; Yuan et al., 2018b; Figs. 6 and 7). The clastic rock reservoirs mainly developed in the middle-upper Silurian, the upper Devonian, and the Permian Longtan formation (Fig. 6).

1) Porous dolomite reservoir and fractured reservoir: In well WX5-ST1, there are dolomites with thicknesses of 25 m in the lower Triassic Qinglong formation, including dolomites with high porosity and high permeability in the interval of 2302–2327 m (Liang et al., 2011; Wu et al., 2016; Yuan et al., 2017). Their porosity are 6%–8% (Yuan et al., 2017), and oosparites and pelsparites are found universally in the formation. Gray limy dolomites, with abundant dissolved pores, were discovered at near 2672 m in well CSDP-2 (Fig. 8a). And limestones with abundant high-angle fractures were discovered in the interval of 2355– 2364 m (Fig. 7a).

2) Reef-bank reservoir: In well CZ12-1-1 (Wu et al., 2016; Yuan et al., 2017), reef-bank reservoirs were found in the Carboniferous strata (Fig. 7b). Light gray fine to silty biological algal-lump limestone and gray fine to silty bioclastic or micritic limestone developed in the top part of the Chuanshan formation and brown-gray fine to silty algal-lump limestone and dark organism-bearing silty limestone developed in the middle and lower parts of Chuanshan formation. The thickness is 133 m (Wu et al., 2016; Yuan et al., 2017), and the spontaneous and resistivity logging curves vary intensely in this interval. There is bioclastic limestone and hydrocarbons in the upper Carboniferous Chuanshan formation in well CSDP-2 (Figs. 6d and 6e).

3) Weathered crust reservoir: There are two sets of weathered crusts, i.e., the Indosinian weathered crust and the Hercynian weathered crust, to be found in the South Yellow Sea (Wu et al., 2016; Yuan et al., 2017; Yuan et al., 2018b). There are large quantities of reservoir spaces in the weathered crusts. In well CZ35-2-1, the Qixia formation weathered crust (Hercynian face) is relatively thin, with only surface karst zones and phreatic karst zones developed (Liang et al., 2011; Wu et al., 2016; Yuan et al., 2017). The Qinglong formation weathered crust (Indosinian face) in wells CZ35-2-1 and CSDP-2 is composed of dense earth yellow, light gray, and gray limestone, which is weathered seriously and fragile, with microcrystalline and fine crystalline structures (Fig. 6a).

4) Clastic rock reservoir: The Permian Longtan formation in well CZ35-2-1 has the porosity ranging from 4% to 8% and the clay content of 4%–14% (Liang et al., 2011). The physical properties of the Longtan formation sandstone reservoir in the sea are similar to those of the sandstone reservoir on the land. In addition, the Longtan formation in well CSDP-2 is gray fine sandstone, associated with hydrocarbon (Fig. 6b).

The wells in the northern Jiangsu region disclosed that there are hydrocarbons to be seen in the Silurian to Devonian sandstone that has some storage capacity. The upper Devonian Wutong formation in well CSDP-2 in the South Yellow Sea Basin is dominated by gray mudstone, followed by gray fine sandstone and silty sandstone, and there are gray fine sandstones in the middle and upper Silurian. The Silurian to Devonian sandstones are also very important clastic rock reservoirs in the South Yellow Sea Basin.

4.2.3 Preservation conditions

1) Magmatic activity

Magma intrusion in the lower Yangtze region mainly started in the middle Jurassic, but the magmatic activity then was relatively weak. In the late Jurassic–early Cretaceous, magmatic activity became strong, and the magmatic rocks were dominated by moderately acid, weakly acid intrusive rocks (Liang et al., 2017). Magmatic activity in this period was mainly distributed in the east of the South Yellow Sea Basin and near Korea. For example, in well Kachi-1 in the Yantai Depression, rhyolitic volcanic rock and light gray granite were encountered during the drilling of the Cretaceous strata (Pang et al., 2016). The Cenozoic magmatic rock, dominated by basalt, was mainly distributed in the east of the basin. For example, in well WX13-3-1 in the Qingdao Depression, black basalt was encountered during drilling of the Oligocene strata in the Paleogene (Pang et al., 2016).

2) Capping bed characteristics

There are three sets of capping bed sequences in the marine Mesozoic–Paleozoic strata in the South Yellow Sea Basin: lower Silurian Gaojiabian formation muddy shale (Fig. 8c), upper Permian Longtan formation–Dalong formation muddy shale (Fig. 6c), and lower Triassic Qinglong formation muddy limestone and gypsum-salt bed (Liang et al., 2017; Yuan et al., 2017).

The lower Silurian Gaojiabian formation capping bed: The breakthrough pressure of the Gaojiabian formation in the wells in the lower Yangtze land area is > 16 MPa, and the breakthrough pressure in well CSDP-2 in the South Yellow Sea Basin is 16.5–18.5 MPa (Liang et al., 2017). The Gaojiabian formation muddy shale was influenced by overthrust faulting in the Mesozoic–Cenozoic tectonic reformation and basin formation stage, but the overthrust faults mostly appeared as bedding slips, which were favorable for keeping the morphological integrity of the structural trap under the glide planes, so the destruction effect on the continuity of the capping bed was limited (Liang et al., 2016). For example, the lower Silurian in well CSDP-2 was characterized by low-angle detachment fault (Fig. 8b).

The upper Permian Longtan formation–Dalong formation capping bed: In well CZ35-2-1, the Longtan formation was encountered at 270 m during drilling, with mudstone content reaching 70%, and the Dalong formation was encountered at 115 m during drilling, with mudstone content reaching 93.92% (Yuan et al., 2017). The displacement pressure was estimated to be 18–25 MPa based on the logging interpretation. In addition, the core analysis of mudstone from well CSDP-2 shows that the breakthrough pressure of the Upper Permian Longtan formation mudstone is 15.0–15.5 MPa (Liang et al., 2017). This reflects the relatively good sealing capability of this set of mudstones. The Longtan formation and Dalong formation in the basin on seismic profiles mostly exhibit parallel and continuous reflection and the reflection interface has some continuity. This set of capping beds developed widely in the middle and south of the basin.

The Triassic capping bed: The lower Triassic Qinglong formation muddy limestone and gypsum-salt bed only developed locally in the South Yellow Sea Basin, but it could still be regarded as the local capping bed owing to its relatively good sealing conditions. In well WX5-ST1, the Qinglong formation was encountered during drilling, and interbeds of limestone and muddy limestone, with carbonaceous mudstone beds developed. Some limestones had the false appearance of gypsum, so the capping bed has some sealing capability (Liang et al., 2017).

Analysis of the source rocks, reservoirs, and capping bed conditions in the South Yellow Sea basin shows that there were four sets of main source reservoir cap assemblages in the marine strata (Fig. 9).

4.3 Hydrocarbon Generation and Accumulation 4.3.1 Material basis

Among four sets of source-reservoir-cap assemblages (Fig. 9), the first assemblage is of upper source lower reservoir type, with the Sinian Dengying formation dolomite as the main reservoir and the lower Cambrian Mufushan formation mud shale as the cap. The second assemblage is of lower source upper reservoir type, with the middle Cambrian–lower Ordovician dolomite, bioclastic limestone, and fracture dissolution-type limestone as the reservoir and the lower Silurian mud shale as the cap. The third assemblage contains the lower Silurian Fentou formation, the middle Silurian Maoshan formation, and the upper Devonian Wutong formation sandstone and the Carboniferous Hezhou formation–Chuanshan formation dolomite, bioclastic limestone, and fracture dissolution-type limestone as the reservoir and the upper Permian Longtan– Dalong formation thick-layer mudstone as the cap. The fourth assemblage contains the upper Permian Longtan formation sandstone and the lower Triassic Qinglong formation dolomite as the reservoir and the Lower Triassic Qinglong formation marlstone as the cap (Chen et al., 2016a; Yuan et al., 2017; Zhang et al., 2018).

Based on the simulation of hydrocarbon generation history in a single well, the oil and gas generation peak periods of lower Cambrian, lower Silurian, and Permian source rocks in the South Yellow Sea Basin have been determined (Chen et al., 2018b). The lower Cambrian Mufushan formation source rocks reached its oil generation peak in the late Ordovician and its gas generation peak in the late Devonian. The lower Silurian Gaojiabian formation source rocks reached its oil generation peak in the late stage of the early Permian and its gas generation peak in the middle Triassic. The Permian source rocks reached its oil generation peak in the Middle Cretaceous (Chen et al., 2018c).

4.3.2 Structural traps

Based on the tectonic interpretation of seismic data in the whole region of the South Yellow Sea Basin, the characteristics of faults and the formation of the fault system have been made clear, and numerous local structural traps have been defined (Fig. 10). The structural trap types include anticline, faulted anticline, faulted nose, and faulted block (Fig. 11). The Mesozoic–Paleozoic structural traps were mostly developed in the shallow and middle layers, and only a few developed in the deep part.

Mudstone and shale developed in the Paleozoic Silurian Gaojiabian formation, and these plastic strata became the regional detachment surface. Affected by this detachment surface, the fault throw of tension-shearing normal faults decreased downward to zero in the deep part of the Paleozoic strata in their late stage. Therefore, only a few normal faults with relatively large fault throw cut through the Cambrian, Ordovician, and Sinian (Fig. 11b). In the location with none or few faults, the morphology of the structural traps was even more complete. The structural trap types were dominated by anticline and faulted anticline, followed by faulted nose and faulted block. The number of traps was relatively small, but the areas of single traps were somewhat large, with most traps having areas of > 20 km².

The structural characteristics of the upper Paleozoic basically carry on the macroscopic laws of the lower Paleozoic. Because the local traps were affected by normal faults in the late stage, multiple complex structural traps developed in anticline, faulted anticline, and faulted blocks in the Devonian, Carboniferous, and Permian. The trap types were dominated by faulted anticline and faulted nose, followed by local faulted block. The number of traps was larger than that in the lower Paleozoic, but the mean trap area was slightly smaller than that in the deep layer.

The structural characteristics had evident differences between the Mesozoic and the Paleozoic, which were mainly reflected as follows: different uplift and subsidence activities occurred in local areas owing to the tension stress in the late Yanshanian period. The uplifted areas underwent different degree of denudation, and the tectonic movement of overall squeezing and uplift in the Himalayan period resulted in regional denudation. Therefore, there were only small-scale structure traps of faulted anticline, faulted nose, and faulted block types in the Triassic. The Mesozoic and Cenozoic structural traps had areas that were mostly < 20 km².

4.3.3 Possible hydrocarbon reservoir types

With knowledge of the main reservoir types of marine hydrocarbon in the Yangtze region, it was predicted that there were three possible hydrocarbon reservoir types in the marine Mesozoic–Paleozoic strata in this basin.

1) Primary residual-type hydrocarbon reservoir: The source rocks associated with the primary reservoir in the deep part is those in the lower Cambrian Mufushan formation, and the reservoir is mainly the upper Sinian Dengying formation and the lower Cambrian Kunlun formation. The generated hydrocarbons were preferably migrated and accumulated in the paleo-uplift formed in the Hercynian along the faults and reservoir layers. Tectonic movements in various phases adjusted and reformed the early accumulated hydrocarbons, but the original hydrocarbon system was not broken (Chen et al., 2018a). The reservoirs of this type are characterized by early hydrocarbon generation, early accumulation, and late adjustment by tectonic movements.

2) Shallow reformed-type hydrocarbon reservoir: Some structural traps were formed in the Indosinian and were uplifted in the Yanshanian and the Himalayan. The lower Triassic and the lower Permian layers were denuded, so the shallow lower Triassic and upper Permian hydrocarbon reservoirs were largely destroyed. However, the strata from the Carboniferous to the lower Permian Qixia formation were retained completely, providing the limestone reservoir for the underlying Permian source rocks. In addition, the gas reservoir was formed in the early lower Silurian Gaojiabian formation, which easily became a concurrent oil and gas reservoir under the driving of the oil in the shallow traps.

3) Composite-type hydrocarbon reservoir: The gas reservoirs in the early upper Sinian Dengying formation and lower Ordovician Lunshan formation underwent superimposed reformation during the Indosinian, Yanshanian, and Himalayan periods, so the primary hydrocarbon reservoirs were adjusted, with some hydrocarbons being retained in the traps and some migrating upward to the upper reservoirs along the faults. Furthermore, the hydrocarbons generated from the lower Silurian, lower Permian, and upper Permian source rocks entered the traps nearby, forming composite-type hydrocarbon reservoirs.

5 Discussion

There was a rigid craton developed in the South Yellow Sea Basin and it belonged to a rigid crystalline basement (Lee et al., 2006; Liang et al., 2017). Although the South Yellow Sea Basin is similar to the lower Yangtze land area with respect to structural and tectonic evolution, the tectonic intensity and tectonic activity in the South Yellow Sea Basin were weaker in the Mesozoic–Cenozoic tectonic reformation and basin formation stage since the Indosinian movements (Liang et al., 2017; Chen et al., 2018b). This shows that the South Yellow Sea Basin has greater stability than the lower Yangtze land area (Chen et al., 2016a). In particular, only gentle folds developed in the middle and south of the basin. The fault activity in the Laoshan Uplift are smallest in scale and weakest in intensity, with little plane distribution and simple structures, thus being more favorable for the preservation of hydrocarbon reservoir formation in the marine strata (Liang et al., 2017).

Based on scientific processing and interpretation of the two-dimensional multichannel seismic data acquired in recent years, six large structural traps with excellent hydrocarbon geological conditions were found in the marine Mesozoic–Paleozoic strata in the Laoshan Uplift, with the areas ranging from 33 to 220 km² (Chen, 2016; Liang et al., 2017; Fig. 10). The hydrocarbon preservation conditions in the Qingdao Depression and the Yantai Depression were poorer than in the Laoshan Uplift (Liang et al., 2017). The Qingdao Depression likely had suitable conditions for the formation of a primary residual-type hydrocarbon reservoir, similar to the Huangqiao gas field (Xu et al., 2014; Liang et al., 2017). The Yantai Depression likely had suitable conditions for the formation of a shallow reformed-type hydrocarbon reservoir, similar to the Jurong oil field (Xu et al., 2014; Liang et al., 2017). The Paleozoic strata in the Laoshan Uplift, with relatively clear seismic reflection, is relatively shallowly buried, and has relatively stable structures and large traps, showing good configuration of source reservoir cap assemblages (Chen et al., 2016a; Chen et al., 2018b). So the Laoshan Uplift is the preferred prospect zone for hydrocarbon exploration in the marine Paleozoic in the South Yellow Sea Basin (Liang et al., 2017; Chen et al., 2018b). In addition, according to 'source control theory', regional tectonic evolution and sedimentary evolution could be used to guide the selection of favorable exploration areas, and this is also an important direction for future works.

6 Conclusions

1) There are mainly two deposition systems in the South Yellow Sea Basin. One is deposited at the stable evolution stage of the Mesozoic–Paleozoic marine basin before the Indosinian movementand the other is formed at the Mesozoic–Cenozoic tectonic reformation and basin formation stage after the Indosinian movement. These two stages are the main periods for the development and evolution of the regional source rocks. Being comparable to the source rocks in the Yangtze land area, four sets of main source rocks developed in the South Yellow Sea Basin.

2) The reservoirs were relatively widely distributed in the marine Mesozoic–Paleozoic strata in the South Yellow Sea Basin, and the reservoir rocks were dominated by carbonate rocks, followed by clastic rocks. Three sets of capping beds in the Mesozoic–Paleozoic developed vertically had relatively good sealing capability. In the Mesozoic–Paleozoic, not only the magmatic activity is relatively weak, but also multiple types of structural traps developed. All of the above provided a good basis for formation of hydrocarbon reservoirs.

3) In the marine strata in the South Yellow Sea Basin, there are four sets of main source reservoir cap assemblages, which include three reservoir formation types: primary residual-type hydrocarbon reservoir, shallow reformed-type hydrocarbon reservoir, and composite-type hydrocarbon reservoir. The Laoshan Uplift in the middle of the basin has a stable structure, complete source reservoir cap assemblages, relatively weak magmatic activity, and large structural traps, so it is the preferred prospect zone for hydrocarbon exploration in the marine Paleozoic in this region.

Acknowledgements

The study is supported by the Project of China Geological Survey (Nos. DD20160152, DD20160147, GZH 200800503, DD20190818), the National Natural Science Foundation of China (Nos. 41506080, 41702162), the Project of China Ministry of Land and Resources (Nos. XQ-2005-01, 2009GYXQ10), and the Postdoctoral Innovation Fund Project of Shandong Province (No. 201602004).