关键词: 北部湾盆地, 涠西南凹陷, 始新统流沙港组, 南海西部油田, 潜山, 风化裂缝带, 成藏模式

Abstract: Buried hills in Beibuwan Basin represents a key component of the exploration strategy aimed at enhancing production by 10 million cubic meters in the western South China Sea oilfields. In recent years, significant breakthroughs in oil and gas exploration have been achieved in newly recognized buried hills in Weixinan sag. A systematic analysis of the geological conditions controlling hydrocarbon enrichment and accumulation is of great significance for guiding further exploration in Beibuwan Basin. Based on rock pyrolysis, wall-core and thin-section analyses, imaging logging, formation pressure data, petroleum geochemistry, fluid inclusions, and seismic datasets, this study focuses on the X5 Paleozoic granitic buried-hill reservoirs, the X5S Proterozoic metamorphic buried-hill reservoirs, and the Paleozoic granitic buried-hill reservoirs. A comprehensive investigation was conducted on source rock conditions, high-quality reservoir-caprock assemblages, the main controlling factors of oil and gas distribution in buried hills, and accumulation characteristics. The geological conditions and accumulation models of hydrocarbons in the newly identified buried hills were characterized, and future directions for petroleum exploration in this area were further clarified. The research results show as follows. (1) Three sets of source rocks are developed in Weixinan sag. Among them, oil shale in the lower sub-member of Member 2 of Eocene Liushagang Formation represents the most favorable source rock and serves as the main contributor to hydrocarbons in the buried hill of the Weixinan low-uplift. Hydrocarbons in this type of buried hill were primarily generated from peak oil generation to late oil window in the western slope of B subsag, with secondary contributions from source rocks in the upper-middle sub-member of Member 2 of Liushagang Formation. Hydrocarbons in buried hills along the No.2 fault zone are mainly derived from the highly mature source rocks in the central B subsag. (2) Buried hills in Weixinan sag can be broadly classified into two types. The first type is metamorphic buried hill, such as the X5S buried hill located in the hanging wall of the f1 normal fault in the Weixinan low-uplift, where fractures and dissolution pores (vugs) in metamorphic rocks constitute the primary reservoir spaces, with multistage fracture systems being extensively developed and intersecting. The second type is granitic buried hill, such as the X5 granitic buried hill in the middle section of the No. 2 fault zone. Its reservoirs exhibit strong vertical zonation, with the weathered fracture zone being the best reservoir interval, characterized by widely developed, well-connected fractures and dissolution vugs as high-quality storage spaces. These two types of buried hill reservoirs form high-quality reservoir-caprock assemblages with the overlying tight slate as well as the thin dense limestone, weathered clay layer, and thick mudstone of the lower sub-member of Member 3 of Liushagang Formation. (3) Overall, hydrocarbons in the buried hills of Weixinan sag exhibit an orderly distribution pattern, characterized by high-maturity condensate gas in the subsags, high-maturity oil in proximal subsag areas, and mature oil in distal subsag regions. The hydrocarbon distributed in metamorphic buried hills of the Weixinan low-uplift is primarily controlled by structural ridge configuration and reservoir fracture development, following a "source- and ridge-controlled migration and preferential accumulation" model. In contrast, hydrocarbon distributed in granitic buried hills of the Weixinan low-uplift is mainly controlled by fault-ridge coupling, characterized by a "narrow-window fault-ridge coupling and near-source enrichment" model. The hydrocarbon distributed in buried hills within the central segment of No.2 fault zone is mainly controlled by the spatial distribution of reservoir fractures and hydrocarbon migration windows, following a "wide-window strong hydrocarbon supply and three-dimensional network migration" model. (4) The predicted favorable exploration targets of buried hills include the X-1 buried hill in the hanging wall of fa1 normal fault in the Xieyang slope, the X-2 and X-3 buried hills in the western segment of the central No. 2 fault zone, the X-4 buried hill in the proximal subsag area along with the X-5, X-6, and X-7 buried hills in the distal subsag area of Weixinan low-uplift. Buried hills are favorable direction for the next step of oil and gas exploration in Weixinan sag.

Key words: Beibuwan Basin, Weixinan sag, Eocene Liushagang Formation, western South China Sea oilfields, buried hill, weathered fracture zone, accumulation model