AAPG Middle East Region Geoscience Technology Workshop, Rift Basin Evolution and Exploration

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Reducing Exploration Risk in Rift Basins

Abstract

As our ability to remotely image rifts from seismic data improves, we gain geological insights to guide petroleum system models. Continental breakup involves extension. Considering East Africa from the Gulf of Suez, via the Red Sea and south to Mozambique, we see stages from initial rift to passive margin. Onshore, we have a more recent wave of rifting that, in the future, will result in further breakup of the African continent. In the synrift, deep anoxic East African lakes accumulate source rocks. Thermal uplift and associated volcanism go with the rifting. For seismic imaging, both intrusive and extrusive volcanic rocks are a significant challenge. With crustal cooling, we enter the sag phase, often shallow marine facies. In arid climates, clastic sediment supply is reduced, and carbonate sediments are common. When evaporation leads to increased salinity, evaporites such as halite accumulate. Layered evaporites from multiple phases of evaporation represent a significant seismic imaging challenge. Thick, mobile evaporites such as halite move when subjected to differential loading giving rise to complex structures, a further seismic imaging challenge. When marine conditions return, carbonate platforms and reefs form that often show dissolution that add challenges to imaging. With crustal cooling, the basin tilts oceanward, slopes become unstable, and sediments flow from shallow shelf to deep water. Sediments interact with the underlying salt by gravity processes. This geologic complexity adds further challenges to seismic imaging. To correct for these overburden effects, the solution is to build geologically consistent property models. We use these to: • design surveys • process the data • invert the seismic images to rock properties • understand sedimentation and tectonics • predict the behaviour of the petroleum systems. Seismic processing algorithms rely on velocity models to image below complex overburden. Such modelling is typically an iterative process, working from shallow to deeper imaging. As our ability to image rifts improves, we gain new geologic insights. For example, the East African rifts provide a natural laboratory to aid our understanding of early synrift tectonics and deposition. The Gulf of Suez is an example of the next step. The Red Sea takes us further to the point of oceanic crust formation. The East African coast illustrates all stages up to passive margin development and beyond. We interpret geophysical data to refine geologic knowledge of the petroleum systems. Petroleum system models, in turn, reduce exploration risk.