Palynology of Upper Cretaceous (uppermost Campanian–Maastrichtian) deposits in the South Yellow Sea Basin, offshore Korea

Prospective prediction and exploration situation of marine Mesozoic-Paleozoic oil and gas in the South Yellow Sea

The crustal velocity structure in the South Yellow Sea (SYS) Basin is crucial for understanding the basin’s geological structure and evolution. OBS (ocean-bottom station) data from the OBS2013 line have been used to determine the crustal velocity structure in the SYS. The velocity model of the upper crust in the northern SYS was determined using first-arrival traveltime tomography. The model showed a higher resolution shallow crustal velocity structure but a lower resolution middle-lower crustal velocity structure. The crustal velocity structure, together with the Moho discontinuity in the SYS Basin, was also constructed using a human–computer interactive traveltime simulation, and the result was highly dependent on the prior knowledge of the operator. In this study, we reconstructed a crustal velocity model in the SYS Basin using a joint tomographic inversion of the traveltime and its gradient data of the reflected and refracted waves picked from the OBS data. The resolution of the inverted velocity structure from shallow-to-deep crust was improved. The results revealed that the massive high-velocity body below the Haiyang Sag of the Jiaolai Basin extends to the Qianliyan Uplift in the SYS; the low-velocity Cretaceous strata directly cover the pre-Sinitic metamorphic rock basement of the Sulu orogenic belt; and the thick Meso-Paleozoic marine strata are retained beneath the Meso–Cenozoic continental strata in the northern depression. The Moho depth in the SYS Basin ranges from 28 to 32 km.

Tectonic evolution of the eastern margin of the Northern South Yellow Sea Basin in the Yellow Sea since the Late Cretaceous

The South Yellow Sea Basin is the main body of the lower Yangtze area in which marine Mesozoic–Paleozoic strata are widely distributed. The latest geophysical data were used to overcome the limitation of previous poor-quality deep data. Meanwhile, the geological characteristics of hydrocarbon reservoirs in the marine Mesozoic–Paleozoic strata in the South Yellow Sea Basin were analyzed by comparing the source rocks and the reservoir and utilizing drilling and outcrop data. It is believed that the South Yellow Sea Basin roughly underwent six evolutionary stages: plate spreading, plate convergence, stable platform development, foreland basin development, faulted basin development, and depression basin development. The South Yellow Sea Basin has characteristics of a composite platform-fault depression geological structure, with a half-graben geological structure and with a ‘sandwich structure’ in the vertical direction. Four sets of hydrocarbon source rocks developed–the upper Permian Longtan–Dalong formation, the lower Permian Qixia formation, the lower Silurian Gaojiabian formation, and the lower Cambrian Hetang formation/Mufushan formation, giving the South Yellow Sea Basin relatively good hydrocarbon potential. The carbonate is the main reservoir rock type in the South Yellow Sea area, and there are four carbonate reservoir types: porous dolomitic, reef-bank, weathered crust, and fractured. There are reservoir-forming horizons similar to the typical hydrocarbon reservoirs in the Yangtze land area developed in the South Yellow Sea, and there are three sets of complete source-reservoir-cap rock assemblages developed in the marine strata, with very good hydrocarbon potential.

Assessment of Mesozoic and Upper Paleozoic source rocks in the South Yellow Sea Basin based on the continuous borehole CSDP-2

Effective hydrocarbon-bearing geological conditions of the Permian strata in the South Yellow Sea Basin, China: Evidence from borehole CSDP-2

To advance oil and gas exploration in the South Yellow Sea Basin, characteristics of Triassic, including its correlation with regional strata, distribution, lithology characteristics, and karst development characteristics, are studied by geological mapping. Results show that there exist Zhouchongcun Formation and Qinglong Formation in the Triassic. Zhouchongcun Formation corresponds to Dongma Formation in the lower Yangtze Region, Badong Formation in the middle Yangtze Region, and Lekoupo Formation in the upper Yangtze region. The upper Qinglong Formation corresponds to the Nanlinghu Formation and Longshan Formation in Xiyanzi district, and Jialingjiang Formation in the middle and upper Yangtze region. Its lithology is limestone, intercalated with marl, mudstone, and argillaceous siltstone. The lower Qinglong Formation corresponds to the Yinkeng Formation at the lower Yangtze group. The lower Qinglong group has the complex lithologic characteristics, which can be divided into six categories from top to bottom, including limestone weathering crust layer, limestone, marl with mudstone, mudstone with limestone, marl with limestone, and mudstone interbedding limestone and mudstone. Karst phenomenon is well developed in the lower Qinglong Formation, and the thickness of weathering crust, revealed by cz35-2-1 well, reaches 60.5 m. Triassic in the South Yellow Sea Basin is mainly distributed in the central uplift and southern depression, showing the regional differences in distribution. Karstification is probably the main reason for this significant difference. Identification of karst phenomenon and stratigraphic division method towards Triassic drilling coring are two important directions of Triassic study in the South Yellow Sea Basin.

The development of the sedimentary basin in the South Yellow Sea that lies between the Chinese mainland and the Korean Peninsula well documents the tectonic evolution of eastern China. However, the Moho morphology and its relationship with the basin in this area remain poorly understood. Here we used high‐resolution 2D seismic lines and well data to map the geometry, structure, and distribution of the sedimentary basin in the South Yellow Sea. Our results suggest that the South Yellow Sea Basin can be divided into two distinct depressions and three uplifts. The depressions consist of eight depositional sags that are largely controlled by ENE‐, EW‐, and WNW‐striking normal faults. The basin depth reaches 9,500 m in the depressions, but is only <2000 m in the uplift zones. Calculated using a gravity stripping method, the Moho depth varies between 27.4 and 31.5 km. The basin is isostatically compensated and the Moho morphology approximately mirrors that of the basin basement. The Moho lows correspond to the uplift zones and Moho highs correspond to the depression zones, with exception of a sag in the Northern Depression. This exception is caused by the presence of a geological body of high velocity and high density beneath the sag. This body is likely to represent high‐pressure metamorphic rock that initially formed during the collision between the Sino‐Korean and Yangtze cratons and are currently overlain by the sedimentary basin. Based on the stretching factors and strain rates of the South Yellow Sea Basin, we propose that development of the basin was primarily driven by the subduction and retreat of the Pacific Plate since the Late Cretaceous, combined by far‐field effects from the convergence between the Indian and Eurasian plates during the Cenozoic.

The first oil and gas well in the South Yellow Sea Basin was completed in 1961. In 1984, 2.45 tons of light oil were obtained from the Cenozoic strata. However, it remains the only large oil and gas basin in China’s offshore area without industrial oil and gas discoveries. Although the consensus is that the South Yellow Sea Basin is a foreland basin, and the oil and gas exploration prospects are promising, the research on the regional structure and the tectonic evolution of the foreland basin system is weak, which seriously hinders the process of industrial oil and gas discoveries. This paper reports the results of over 30 years of onshore and offshore investigations and well-seismic joint interpretation in the study area: for the first time, the mountains and basins formed by the collision of the North China and Yangtze plates were discovered in the geological survey of the northern islands of the South Yellow Sea Basin; the C-type eclogite chronology of Qianliyan Island, the characteristics of the foreland basins and intracontinental foreland basins around the South Yellow Sea, and the tectonic evolution characteristics and models of the basins were clarified. Through the zircon/phosphate fission track analysis of the deep black Jurassic strata in the Qianyuan S-2 well, it was revealed that the collision and subduction of the Pacific Plate against the Eurasian Plate since the Late Cretaceous–Paleogene led to large-scale uplift movements, and more than 3000 m of strata were eroded in the basin area. This is consistent with the multiple unconformities of E/N, K/N, and T2/N identified by well-seismic joint interpretation, and is also the main reason why oil and gas have been difficult to preserve in the South Yellow Sea Basin since the Middle Triassic–Jurassic. Deep prototype oil and gas exploration in the basin may be the preferred option for current oil and gas exploration deployment, which is conducive to achieving industrial oil and gas discoveries.

Lacustrine environmental evolution and implications on source rock deposition in the Upper Cretaceous-Paleocene of the South Yellow Sea Basin, offshore eastern China

By selecting typical seismic sections to carry out detailed structural interpretation, the structural style features of the Wunansha Uplift in the South Yellow Sea basin were systematically combined, and the compressional structures (imbricate, opposite/back thrust, and Y-shaped structures), strike-slip faults (positive flower-shaped faults), and extensional normal faults (listric-shaped normal faults) were identified. On this basis, combined with the characteristics of the regional stress field and the background of deep geodynamics, the genetic mechanism and structural evolution of the structural style in the Wunansha Uplift were defined. The stress mechanism of the compressional structures originated from the initial high-speed and low-angle NW subduction of the paleo-Pacific plate during the early Yanshanian movement in the Early Jurassic. The regional strike-slip fault was mainly a positive flower structure with compression and torsion characteristics, and its stress mechanism originated from sinistral shear caused by the Early Cretaceous low-angle NNW subduction of the paleo-Pacific plate. The Tan-Lu fault in eastern China also had sinistral shear characteristics in this period. The extensional normal fault was characterized by a listric shape, which developed along the northern boundary of the Wunansha Uplift, that is, the connection between the Wunansha Uplift and the Southern Depression of the South Yellow Sea basin. The stress mechanism was derived from the transition of the paleo-Pacific plate from low-angle to high-angle subduction during the late Yanshanian movement in the Late Cretaceous. Simultaneously, the tectonic stress system in eastern China also changed from compressional to tensional.

Structural characteristics and evolution of the South Yellow Sea Basin since Indosinian

Hydrocarbon preservation conditions in Mesozoic–Paleozoic marine strata in the South Yellow Sea Basin

The South Yellow Sea basin is a large superposed basin composed of a Paleozoic‐Mesozoic marine sedimentary basin and Mesozoic‐Cenozoic terrigenous sedimentary basin. This work studied the structure and stratigraphic distribution of the lower crystal basement and the upper folded basement of the two kinds of basins based on integrated geological and geophysical data. New seismic data interpretation combined with drilling data and geological correlation between land and sea are used to identify the marine Paleozoic‐Mesozoic stratigraphic sequence. The depth of the top surface of the marine Paleozoic‐Mesozoic strata is determined by seismic interpretation; undulation of the bottom surface is derived by magnetic inversion. The residual thickness of the Triassic Qinglong limestone and the upper Permian strata is also analyzed to understand the distribution and structure features of the marine Paleozoic‐Mesozoic strata. The thickness and distribution of the marine sequence are mainly controlled by the undulation of the basement, and also influenced greatly by the Indosinian movement. The thickness of the marine Paleozoic‐Mesozoic strata is comparatively stable in the central uplift. The lower Triassic Qinglong formation and the upper Permian Dalong and Longtan formations are widely distributed in the southern depression and the Wunansha uplift of the South Yellow Sea basin, while the distribution of this succession in the northern depression is very limited; in the central uplift, little is are left due to uplift and denudation. At present, the residual thickness of the lower Paleozoic remains unknown due to limited data available.

The South Yellow Sea Basin (SYSB) has multiple sets of proven source rocks and good hydrocarbon prospects, but no industrial oil and gas has been explored at present. To solve this puzzle for petroleum geologists, we systematically investigated the marine hydrocarbon geological conditions based on cores and testing data from borehole CSDP-2, the first exploration well with continuous coring in SYSB. The qualities of source rocks are evaluated in detail according to organic matter abundance, type, and maturity. The reservoir characterization mainly includes porosity, permeability, and reservoir space. Displacement pressure test and stratum thickness are the main foundations for defining the caprocks. Then, the oil-source rock correlation in the Permian and stratum model are analyzed to determine the favorable source-reservoir-caprock assemblages. The results show that three sets of effective source rocks (the Lower Triassic, Upper Permian, and Lower Permian), two sets of tight sandstone reservoirs (the Upper Permian and Lower Silurian-Upper Devonian), and two sets of caprocks (the Lower Triassic and Carboniferous) combine to constitute the hydrocarbon reservoir-forming assemblages of “lower-generation and upper-accumulation” and “self-generation and self-accumulation”, thus laying a solid foundation for promising petroleum prospects. The three sets of marine source rocks are characterized by successive generation and expulsion stages, which guarantees multistage hydrocarbon accumulation. Another three sets of continental source rocks distributed across the Middle Jurassic, Upper Cretaceous, and Paleogene depression areas, especially in the Northern Depression, may supplement some hydrocarbons for the Central Uplift through faults and the Indosinian unconformity. The favorable Permian exploration strata have been identified in the Central Uplift of SYSB. First, the Lower Permian and Upper Permian source rocks with high organic matter abundance and high thermal maturity supply sufficient hydrocarbons. Secondly, the interbedding relationship between the source rocks and sandstones in the Upper Permian strata ensures that hydrocarbons have been migrated into the nearby Upper Permian sandstones, reflecting near-source hydrocarbon accumulation. Finally, the good sealing property of the Lower Triassic Qinglong Formation caprocks plays an indispensable role in hydrocarbon preservation of the Permian reservoirs. This conclusion is supported by direct oil shows, gas logging anomalous layers, and hydrocarbon-bearing fluid inclusions.