Structural characteristics of transform passive continental margin basins and their exploration domains

Transform continental margin basins have emerged as important frontier areas for global oil and gas exploration, with several significant discoveries reported in recent years. Based on a comprehensive analysis of the distribution patterns and structural styles of typical transform continental margin basins worldwide, this study proposes a three-stage basin evolution model consisting of the “rifting phase, sag phase, and continental margin phase”. Correspondingly, a tripartite tectonostratigraphic framework comprising the rift sequence, sag sequence, continental margin sequence is established to clarify the influence of basin architecture and tectonostratigraphic layering on key elements of the petroleum system, including source rock development and reservoir distribution. As indicated by the research results, during the sag phase, influenced by the barrier effects of marginal ridges or plateau structures, restricted marine environments widely developed on the landward side, providing favorable geological conditions for the large-scale development of high-quality source rocks. During the continental margin phase, depositional systems were jointly controlled by sediment supply and climatic variations, forming three major reservoir types: base-of-slope fan, large progradational delta, and platform-margin carbonate. The depositional thickness of these sedimentary successions exerted first-order control on the thermal evolution of the underlying source rocks. On this basis, three typical hydrocarbon accumulation models are proposed: base-of-slope fan reservoirs, in which efficient coupling with sag-phase source rocks result in the formation of high-quality source-reservoir assemblages, with microfracture networks facilitating hydrocarbon migration; large progradational delta reservoirs characterized by commonly developed sandstone traps such as rollover anticlines, compressional structures, and diapiric structures, providing favorable conditions for hydrocarbon accumulation; platform-margin reef reservoirs, occurring adjacent to slope source kitchens, where hydrocarbons migrate vertically along fracture corridors and accumulate in high-porosity and high-permeability reefal reservoirs. Base-of-slope fans, deltas, and platform-margin carbonates developed in transform continental margin basins along the equatorial Atlantic margin, East Africa, and both margins of the Central Atlantic exhibit enormous hydrocarbon resource potential.