Hydrocarbons in rift basins: the role of stratigraphy

Hydrocarbon occurrence and distribution in rift basins is largely a product of the stratigraphic succession in the syn- and post-rift phases of basin evolution. Most of the known reserves of recoverable hydrocarbons occur in rifts with post-rift sag basins and in those basins that are dominated by m arine fill. Simple rifts and passive margins are significantly less prolifi c. Key factors are the style of post-rift tectonics and whether the basin f ill is dominated by non-marine or marine strata. Tectonically derived topography is the prime control on both sedimentary pr ocesses and facies distribution, which results in a consistent geographic, and stratigraphic distribution of hydrocarbon source rocks, reservoirs and seals in syn-rift successions. Potential reservoirs are abundant throughout the syn- and post-rift successions and include a wide variety of sandstone and, less commonly, carbonates. Source rocks occur less frequently and are restricted to specific stratigraphic horizons; their presence or absence i s one of the limiting factors in hydrocarbon distribution. Good seals tend to be uncommon in continental syn-rift successions and their absence often prevents the formation of hydrocarbon accumulations. Seals are more common in post-rift successions and widespread in marine syn-rift successions. The reserve distribution is largely controlled by seal distribution, which is best in sag-basin successions and in basins filled with marine strata. A strategy for the efficient exploration of rift basins can be derived from post-rift basin geometry and the nature of the stratigraphic fill. Plays c an be developed that identify the most prospective areas within the syn- an d post-rift successions by predicting the distribution of hydrocarbon sourc e rocks, reservoirs and seals from stratigraphic architecture.