Rifting Along the South Atlantic Margins and Some General Principles for Exploration in Rifts on Continental Margins

Abstract

The main rifting in the South Atlantic occurred in a 5–10 M yr period with subsidence creating deep lakes where high quality source rocks developed. Axial drainage with fine clastic deposition predominated in these early rifts, but once the uplifted footwall blocks began to erode, large alluvial fans poured into the grabens. These alluvial fans can contain large basement boulders (2–3 m) and reach up to 2 km in thickness. They scatter seismic energy and the top of the fan is often mis-picked as the top of the intact crystalline basement. Even when the fans are drilled they can be mistaken for intact basement, as the basement clasts are large and there is little matrix. These ‘hidden reservoirs’ host significant oil reserves on both sides of the Atlantic (e.g. Rio do Bu Field Recôncavo, M'Boundi Congo). Exploration for these reservoirs is inherently very high risk, but can bring large rewards in onshore mature basins, where costs are low. If the removal of large amounts of footwall crests does not redeposit sediment in the adjacent hangingwall half-graben, but instead flows down the dip slope away from the basin towards an adjacent half-graben, then the whole of the basin can be uplifted by 1 km or more. This is due to the unloading of the crust which acts as a flexed beam. This may in turn give rise to an unconformity, even in the middle of the half-graben. The unloading of footwalls and the rebound effect produces an unconformity, which is at the top of the syn-rift sequence. Many authors refer to this as the ‘break-up’ unconformity since the introduction of this concept by Falvey (1972). However there is no known physical explanation why the onset of ocean spreading should lead to a basinwide uplift. We suggest that the top rift unconformity simply marks the erosion of uplifted footwall block down to approximately the same level. Where basement weaknesses (shear zones) intersect the graben at high angles and are sub-parallel to the rift opening direction then important hard-linked basement involved transfer faults can develop. The transfer faults can separate rift segments of opposing polarity with complex transtensional and transpressional flower structures. Large oil fields can preferentially develop along these zones where the high angle transfer faults provide oil migration and sediment pathways across the rift (e.g. Mata-Catu Fault Recôncavo Basin). Similar transfer faults occur in the Kwanza Basin, but these zones are still to be explored.

AAPG Datapages/Search and Discovery Article #90194 © 2014 International Conference & Exhibition, Istanbul, Turkey, September 14-17, 2014