Girassol — Angola’s First Deepwater Pre-salt Discovery?

The Girassol discovery in 1996 kicked off a prolific string of giant deepwater discoveries in Oligo-Miocene turbidite reservoirs in Angola’s Lower Congo Basin. Initial work by one of us and many others indicated a post-salt Iabe Formation marine source for the hydrocarbons. However, just as a tiny sample set of oil indicators with non-marine characteristics triggered our discerning a lacustrine, pre-salt source for the Santos Basin of Brazil (Dickson et al., 2005), we saw weak signals of a similar story in Angola. We have re-analyzed piston core samples and are re-examining a handful of Lower Congo oils. We suggest that lacustrine, pre-salt oils may have contributed to the volumes discovered at Girassol and its neighbours.

Although expected because of giant pre-salt discoveries in the conjugate Santos-Campos-Espírito Santo basins, pre-salt source in the Lower Congo Basin is known only in proximal wells near the Angolan coast. Our poster illustrates data and reasoning that developed our Santos Basin understanding, pre-dating the Tupi trend discoveries. The Santos, like most Brazilian basins, bears contributions from multiple source intervals. Although post-salt indications of a lacustrine source were scant and volumetrically tiny, this was due to excellence of a regional salt seal rather than lack of source or generation.

Similar data (oils, piston cores, SAR slicks, basement depth, sediment thickness and basin structure) across the Lower Congo Basin out to the COB highlight clues to a widespread lacustrine source. In deeper water along the Angola Escarpment, we found pre-salt sourced oil within a dominant marine-sourced volume in two key re-analyzed piston cores that initially gave micro-indications of lacustrine source. Lighter oils (as in the Oligocene reservoirs at Girassol) have a dominant Iabe sourced black-oil component but the light oil/condensate component may be lacustrine derived (and over-printed) as in Lagoa Parda, Jubarte and Cachalote fields of Espírito Santo.

Our work in progress aims to confirm presence and distribution of the hypothesized lacustrine source interval which would charge both pre- and (where salt windows exist) post-salt reservoirs. The latter scenario is important for basin floor fans beyond the Angola Escarpment where the Iabe may be immature.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California