Enrichment Mechanism and Exploration Direction of Thick Plus Shallow Shale Oil: A Case Study from the Bongor Basin, Central African Rift

Abstract

Abstract The Bongor Basin in the Central African Rift System is a typical strongly inverted intracontinental rift that hosts unique "shallow-buried thick shale oil" resources. Previous studies, based only on limited data, underestimated its shale oil potential, and the controlling factors of movable oil and its enrichment mechanism remain unclear. By integrating organic geochemical analysis, reservoir physical-property measurements, high-pressure mercury intrusion porosimetry, combined artificial-intelligence-based log prediction, this study systematically analyzes the characteristics of the source rocks, reservoirs, and movable oil in three shale intervals of the Lower Cretaceous P, M, and K formations in the Bongor Basin. A reservoir classification and evaluation system, a quantitative characterization method for movable oil, and a graded shale oil resource evaluation system are established, and a shale oil enrichment model for strongly inverted rifts is constructed. The results show that the Northern Slope of basin contains 45.17×108t of shale oil geological resources and 6.96×108t of technically recoverable resources. The shale reservoirs are classified into interbedded and laminated types, and the resources are divided into four classes based on S₁, OSI, porosity, and brittleness index to scientifically assess the shale oil resource potential of the basin's Northern Slope belt. The basin has developed a shale oil enrichment model characterized by the coupling of thick shale, tectonic uplift-induced shallow burial, and multi-origin overpressure. Compared with North American marine shale oil and Chinese continental shale oil, it has advantages such as large shale thickness, and shallow burial depth, although the crude oil mobility is relatively poor. The analysis indicates that the Northern Slope belt of the basin is the core area of Class I + II high-quality sweet spots, and differentiated exploration and development by interval and reservoir type is recommended.