Structural Styles on the Conjugate Equatorial Atlantic Margins:Similarities and Differences and Where to Go Next

Davison, Ian
Earthmoves Ltd., Camberley, United Kingdom.

Structural Styles on the Conjugate Equatorial Atlantic Margins: Similarities and Differences

Equatorial margin began major rifting in early Aptian times when pull-apart basins formed between the major St. Paul's and Romanche Fracture Zones. Good quality Aptian marine saline source rocks were developed along the eastern segment in Ceara and Potiguar at this time. Rifting continued into the Late Albian and deep marine conditions were developed with potential source rocks deposited. Rotated fault block plays are present along both conjugate margins.

Plate movement vectors changed in the latest Albian to early Cenomanian (ca. 95 Ma) and the dextral strike-slip-pull-apart transform plate boundary underwent a transpressive phase (NNW-ESE directed compression). Large WSW-ENE trending en-echelon folds developed in Barreirinhas, Brazil and the conjugate Ghana-Togo margin. Fold crests were eroded on the shelf and better quality upper Albian reservoirs removed. Hence, wells drilled on the crests of the anticlines may miss the best reservoirs, and more subtle folds with less erosion are the preferred targets.

The fold belts produced along the St Paul's and Romanche Fracture zones produced topographic barriers, so that Late Cretaceous channels were diverted to the eastern and western ends of the fold belts. The tectonic ‘corners' at the ends of the fold belts are considered more prospective for reservoir development.

The final split of the Equatorial margin was highly asymmetric with an abrupt narrow margin developed along the Para-Maranhão and Barreirinhas segment of the Brazilian margin and a broader margin developed in the prolific Ghana segment. The steep Brazilian margin resulted in gravitational instability and a large deepwater compressive-toe fold belt developed in late Cretaceous to Early Tertiary times. Compression still occurs to the present day. There is no known equivalent to this fold belt in Africa. The fold belt has been tested by a few wells with one small gas discovery recorded in an anticline in Barreirinhas. However the spectacular fold belts have distracted exploration away from the new prolific late Cretaceous stratigraphic play below the fold belt, which still remains to be tested.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012