The newly collected seismic data and the existing drilling data provide a good opportunity to better understand the carbonate platform distribution characteristics and the hydrocarbon resource potential in the Xisha sea area of the South China Sea. Based on the seismic data and the reflection characteristics of the carbonate platform’s edge, three boundary indicators were established: abrupt lithological interfaces, fault interfaces, and tidal channels. Combined with the regional geological settings, its spatial and temporal distribution was clearly identified for the first time. The development of the Miocene carbonate platform in the Xisha sea area is divided into six phases, which are further assigned to three evolutionary stages: the bloom stage, the recession stage, and the submerged stage. The sedimentary facies belt of the carbonate platform in each stage is well developed, and the reefs are mainly distributed on the west and southwest edges of the platform. The analysis of the data indicates that the area of the reef and carbonate platform reached 80,000 km² during the mature period, followed by a retreat period where the scale decreased with the platform’s decline. The Miocene carbonate rocks in the Xisha sea area are widely distributed. They have experienced multiple periods of exposure and infiltration, which further improved the quality of their physical properties for hydrocarbon reservoirs. According to the regional hydrocarbon geological conditions in this area—including the source rock, migration system and the capping layer—the hydrocarbon accumulation potential is preliminarily discussed in this paper. A reservoir model of the reef and carbonate platform is established, which is proposed as typical characteristics of “lower generation, upper accumulation”. It is pointed out that the carbonate platform in the Xisha sea area adjacent to the Huaguang Sag in the Qiongdongnan Basin and the northern Zhongjiannan Basin is a potential area for oil and gas exploration.

In this study, high-resolution seismic profiles and well data provided a good opportunity for better understanding the reefs and carbonate platforms in the Wan’an Basin, southwest of the South China Sea, and also provided valuable information for the oil–gas exploration in the reef reservoirs. Four evolutional phases, including the initial phase, the prosperous phase, the recession phase and the submerged phase, of the reefs and carbonate platforms are proposed according to our data. In the Early Miocene, a few small, isolated carbonate platforms initiated in the center of the basin. In the Middle Miocene, they flourished and mainly formed around the Northern Uplift and Central Uplift, with two belts of carbonate platforms in the western area and eastern area that were mainly platform-edge reefs, massive reefs and a few point reefs. In the Late Miocene, the carbonate platforms began to retreat towards the high topographic position because of the rising of sea level. Meanwhile, the numbers and styles of reefs increased to include platform-edge reefs, massive reefs, atoll reefs and point reefs. Since the Pliocene, most of the carbonate platforms have been covered by detrital materials from terrestrial sources. Crustal tectonic activity provides favorable topography for reef growth and the distribution of platforms, and eustasy controlled the vertical growth and lateral migration of reefs. Since the Late Miocene, the rapidly crustal tectonic subsidence and the rising of relative sea level may lead to the drowning of the carbonate platform.

Enhancing oil and gas production performance is essential for maintaining the economic sustainability of petroleum enterprises and meeting the increasing global energy requirements. In this context, subsurface production optimization constitutes a fundamental component of strategic reservoir management, directly affecting critical decisions such as well location design and the regulation of operational parameters. Nevertheless, conventional reservoir optimization approaches are frequently constrained by high computational costs and limited optimization effectiveness. To overcome these limitations, evolutionary algorithms have gained considerable attention for addressing complex optimization tasks, owing to their gradient-free nature and strong capability for parallel exploration. This paper proposes a chaotic exploitation orthogonal learning fruit fly optimization algorithm (COFOA) tailored for global optimization and oil and gas production optimization. Specifically, we integrate a chaotic exploitation mechanism and an orthogonal learning strategy to improve the balance between exploration and exploitation. Following the population update in FOA, the chaotic exploitation mechanism is first applied to help the population escape local optima and enhance search efficiency. Subsequently, the orthogonal learning strategy is employed to strengthen the algorithm’s exploitation capability. To evaluate the performance of the improved FOA, extensive experiments were conducted on benchmark functions from IEEE CEC 2017 and IEEE CEC 2022, including ablation studies, scalability tests and comparisons with state-of-the-art algorithms. The results demonstrate that the proposed FOA significantly outperforms competing algorithms in optimizing reservoir production. COFOA demonstrates consistent performance superiority over all compared algorithms in terms of mean NPV. Specifically, it achieves improvements of approximately 2.35% to 16.23% compared with existing methods. Notably, COFOA outperforms strong competitors such as mSCA and BLPSO by 2.35% and 3.81%, respectively, while achieving more significant gains over algorithms such as SCADE (15.31%) and CCMSCSA (16.23%). Even when compared with relatively competitive methods like HGWO and CCMWOA, COFOA still maintains performance improvements of 4.79% and 6.12%, respectively. These results clearly demonstrate the superior optimization capability of COFOA in terms of maximizing NPV under complex reservoir conditions.

Analysis of overpressure origins serves as the foundation for pressure prediction and hydrocarbon accumulation studies. Overpressure is widely distributed in the Dongfang area of the Yinggehai Basin, demonstrating a close yet complex relationship with hydrocarbon accumulation. It is im-perative to conduct in-depth research on the origins of overpressure and perform quantitative evaluation. Analysis utilizing well log combinations, loading-unloading curves, and acoustic velocity-density crossplots was conducted to investigate the origins of overpressure in the Dongfang area, Yinggehai Basin. Research results indicate that two-stage overpressure is widely developed in the Dongfang area of the Yinggehai Basin. The first overpressure zone occurs in strata above the mid-lower Yinggehai Formation at 1000-1500m depth，with a pressure coefficient over 1.3; the second segment is located in the Huangliu Formation below 2400m, with a pressure co-efficient exceeding 1.5. Acoustic time difference, density, and resistivity logging curves exhibit significant responses to overpressure, manifesting as reversals to varying degrees. Integrated analysis with multiple methods indicates that the overpressure in the Dongfang area of the Yinggehai Basin is primarily caused by a combination of hydrocarbon-generation pressurization and undercompaction. The magnitude of hydrocarbon-generated overpressure primarily ranges from 5 to 20 MPa, contributing 40% to 70% of the total overpressure.

The Miocene Sanya, Meishan, and Huangliu formations in the Qiongdongnan Basin (QDNB) record a complete evolutionary history of submarine fan systems, yet a continuous Miocene-scale depositional framework has not been established for the Northern Lingshui Sag. Using 3D seismic data, this study investigates the paleogeomorphological reconstruction, channel system architecture, channel-sink correspondence, and controlling factors of Miocene submarine fans within the Northern Lingshui Sag, QDNB. Paleogeomorphological reconstruction reveals three fundamentally different slope configurations. The Sanya Formation is characterized by a gentle ramp-like slope (1–2°) with high accommodation. The Meishan Formation exhibits a steep structurally partitioned slope (eastern part ~9°, western part 8° decreasing to 2°) with a confined trough in the east. The Huangliu Formation shows a mature moderate slope (3–4°) with reduced structural confinement. Channel system architecture shows fifteen NW-SE oriented channels in the Sanya, six N-S oriented channels in the Meishan, and seven bidirectional channels in the Huangliu. Channel-sink correspondence is strong in the Sanya, localized in the Meishan, and weak in the Huangliu. Fan evolution was driven by three controlling factors: relative sea-level fluctuations, sediment supply and provenance, and paleogeomorphology. Global eustatic falls at 23.0 Ma (T60), 15.5 Ma (T50), and 10.5 Ma (T40) facilitated fan development within lowstand systems tracts. The Ningyuan River supplied sand-rich sediment to the Sanya, the Lingshui River supplied mud-rich sediment to the Meishan, and dual Ningyuan and Lingshui rivers supplied low-flux mud-dominated sediment to the Huangliu. Two major transitions are identified: the Sanya-to-Meishan transition (ca. 15.5 Ma) represents channel reorientation from NW-SE to N-S, reduction from fifteen to six channels, and shift from sand-rich to mud-rich supply; the Meishan-to-Huangliu transition (ca. 10.5 Ma) represents channel reorientation from N-S to bidirectional, increase from six to seven channels, and shift from mud-rich to low-flux mud-dominated supply.