Detachment faulting and petroleum prospectivity in the Rio Muni Basin, Equatorial Guinea, West Africa

Abstract

Abstract The change from rifting to drifting is generally signified by a characteristic end-rift, or break-up, unconformity. In the Rio Muni Basin, two such unconformities bound an Aptian-Cenomanian transitional sequence of allochthonous carbonates and clastics considered unique among the South Atlantic passive margins. The transitional sequence comprises allochthonous thin-skin ‘rafts’, up to 2500 m thick, of shallow water carbonate and clastic sediment emplaced in a relatively deepwater setting along shallow detachments. A new model for the rift to drift development of the Rio Muni Basin interprets the raft tectonics as a response to a combination of end-rift-early post-rift uplift of the basin margins and deep off-shore bathymetry c. 1800 m. Seismic interpretations reveal spectacular hanging wall geometry in the rafts as a consequence of their movement across a geometrically complex, ramp-flat basal detachment. In places, the hanging walls of the rafts display an ‘eggbox’ topography of closely spaced anticlines, often with four-way dip closure. Information from wells and analogy with adjacent basins indicates potential source rocks in the Aptian and Turonian. Several migration paths are identified, including the juxtaposition of hanging wall reservoirs with the source-bearing stratigraphy across transverse fault zones. Raft tectonics has considerable isostatic and thermal consequences, with attendant implications for predicting the timing of potential source rock maturity. Modelling maturity at variously located wells and pseudowells indicates the principal effect of the raft tectonics to have been a very rapid episode of burial in the basin centre and isostatic rebound of the unloaded margins.