The Mesozoic strata in the northern South China Sea have good potential for oil and gas exploration. The Dongsha Waters, where the study area is located, have complex seismic and geological characteristics; in particular, turbulence is very prominent in this area, so it is difficult to implement 3D seismic data collection. The “single-source and single-cable quasi-3D seismic survey” method integrates some key technologies in acquisition and processing, thus improving the quality of seismic imaging. Based on the interpretation of the existing research results and new data, structure B-1 has good source–reservoir–cap combination conditions. The oil–gas accumulation mode is predicted, and the drilling well B-1-1 is given. In addition, the large-scale distribution of bottom-simulating reflectors (BSRs) and the discovery of gas seepage areas in the study area suggest the presence of gas hydrate. We suggest that deep thermogenic gas from the Mesozoic strata has migrated into the overlying strata along the fault system and mixed with microbial gas to form hydrate.

Despite extensive pore-scale studies on oil–water displacement, quantitative understanding of the dynamic evolution of residual oil morphology and waterflooding efficiency in geologically heterogeneous sandstones remains limited, particularly under large water-injection multiples. To better understand pore-scale oil–water distribution and its influence on enhanced oil recovery, this study utilized Micro-CT combined with SEM-EDS to examine the 3D pore structure and oil–water phase evolution in a heterogeneous sandstone sample from the Xiayang Formation, Wushi Sag, Zhanjiang. Mineralogical analyses reveal that dolomite cementation and vermicular kaolinite infilling introduce strong pore-scale heterogeneity by selectively reducing pore connectivity and permeability, posing challenges for uniform fluid displacement. A 30% KI solution was used to enhance X-ray attenuation of the aqueous phase, enabling clear discrimination between oil and water. Micro-CT reconstructions reveal a relatively uniform pore network dominated by medium-to-large intergranular pores. As the water-injection multiple increases, water progressively invades larger pores, while residual oil is immobilized by capillary forces within micro-throats, forming isolated clusters. The oil-droplet size distribution broadens from a narrow range (50–100 µm) to a wider one (200–300 µm), indicating interfacial destabilization and droplet coalescence. Quantitative analysis indicates that oil saturation decreases from approximately 90% to 36%, while waterflooding efficiency increases rapidly to ~45% at 1 PV and gradually approaches a plateau of ~60% beyond 500–1000 PV. This waterflooding plateau is attributed to capillary trapping and pore-scale connectivity limitations imposed by mineral-induced heterogeneity, which prevent further mobilization of residual oil despite continued water injection. This study advances pore-scale waterflooding research by combining mineralogical heterogeneity with long-term micro-CT imaging, revealing the pore-scale mechanisms controlling residual oil evolution and ultimate waterflooding limits in realistic sandstone.

The Chaoshan Depression, situated in the northern South China Sea, is a Mesozoic residual depression beneath the Cenozoic Pearl River Mouth Basin. Borehole LF35-1-1 has confirmed the existence of marine Jurassic layers rich in organic carbon within this depression. However, the understanding of petroleum geology in this area is limited due to the complex interplay of Mesozoic and Cenozoic tectonic activities and the poor quality of seismic imaging from previous surveys, which have obstructed insights into the characteristics of Mesozoic reservoirs and the processes of oil and gas accumulation. Recent quasi-3D seismic data have allowed for the identification of Mesozoic bioherms and carbonate platforms in the Middle Low Uplift of the Chaoshan Depression. This research employs integrated geophysical data (MCS, gravity, magnetic) and well data to explore the factors that influenced Middle Jurassic reef development and their implications for reservoir formation. The seismic reflection patterns of reefs and carbonate platforms are primarily characterized by high-amplitude discontinuous to chaotic reflections, with occasional blank reflections or weak, sub-parallel reflections, as well as significant high-velocity, high Bouguer gravity and low reduced-to-pole (RTP) magnetic anomalies. Atolls, stratiform reefs, and patch reefs are located on the local topographic highs of the platform. Three vertical evolutionary stages have been identified based on the size of atolls and fluctuations in relative sea level: initiation, growth, and submergence. The location of bioherms and carbonate platforms was influenced by paleotectonic topography, while their horizontal distribution was affected by variations in relative sea level. Furthermore, the reef limestone reservoirs from the upper member of the Middle Jurassic, combined with the mudstone source rocks from the Lower Jurassic and the lower section of the Middle Jurassic, as well as the bathyal mudstone caprocks from the lower part of the Late Jurassic, create highly favorable conditions for hydrocarbon accumulation.

The Chaoshan Depression situated in the Dongsha Sea region, is an important Mesozoic residual depression beneath the Cenozoic Pearl River Mouth Basin in the northern South China Sea, showing significant potential for hydrocarbon exploration. However, current drilling efforts have not resulted in hydrocarbon finds due to the absence of high-quality reservoirs. Recent quasi-3D seismic data has allowed for the first identification of Mesozoic bioherms and carbonate platforms on the Middle Low Uplift of the Chaoshan Depression. A thorough analysis was performed on their horizontal sedimentary distribution and vertical evolution. The findings indicate that the seismic reflection patterns of reefs and carbonate platforms in this area are primarily characterized by high-amplitude discontinuous to chaotic reflections, sometimes showing blank reflections or weak, sub-parallel reflections, along with notable high-velocity anomalies. Atolls, stratiform reefs, and patch reefs are found on local topographic highs of the platform, with atolls mainly in the central part of the quasi-3D area, stratiform reefs in the northeast, and patch reefs in the central-eastern region. Four vertical evolutionary stages have been identified: initial development, flourishing, decline, and submergence. The spatial distribution of bioherms and carbonate platforms was influenced by paleotectonic topography, while their developmental scale was affected by changes in relative sea level. Furthermore, reefs limestone reservoirs within the upper part of the Middle Jurassic exhibit excellent conditions for hydrocarbon accumulation, following a "lower source, upper reservoir, and upper seal" model. These reservoirs have favorable geological conditions for the formation of medium- to large-scale hydrocarbon accumulations, suggesting significant exploration opportunities.

The Beikang Basin is a large petroliferous sedimentary basin in the southern South China Sea, and is regarded as an important frontier area for deepwater hydrocarbon exploration in the Nansha region. Previous exploration by foreign oil companies has demonstrated its significant hydrocarbon potential. However, no wells have yet been drilled by China in the Beikang Basin. In this underexplored, well-free setting, sandstone reservoir prediction is hindered by several key challenges, including low velocity-picking accuracy, difficulty in constructing reliable low-frequency models, and limited inversion accuracy. These limitations collectively reduce the reliability of reservoir identification and thus increase exploration risk. To address the challenge of accurately predicting sandstone reservoirs in underexplored areas without well control, this study proposes a reservoir prediction workflow that integrates horizon-controlled gridded tomographic velocity modeling with seismic multiple integration. The workflow includes (1) pre-stack gather optimization, (2) tomographic velocity inversion to construct a precise velocity field, (3) multiple integration to supplement missing medium- to low-frequency information, and (4) pre-stack amplitude-versus-offset (AVO) simultaneous inversion to derive P-wave and S-wave impedances. Combined with petrophysical analysis from adjacent areas to establish a lithology interpretation framework, this integrated approach enables quantitative prediction of sandstone reservoirs in the Beikang Basin. Application to the study area demonstrates that the predicted sandstone thickness distribution is consistent with the regional geological conditions and is supported by drilling results from surrounding areas. The proposed method, therefore, provides a reliable technical approach for reservoir evaluation in the Beikang Basin, and has practical value for sandstone reservoir prediction in other underexplored frontier basins with limited or no well control.