SCHEEVEL, JAY R., and CHRIS T. DALE, Chevron Overseas Petroleum Inc., San Ramon, CA

ABSTRACT: Models of the Evolution of the Lower Congo Basin, Offshore Cabinda (Angola) and Zaire

The Lower Congo basin offshore Cabinda (Angola) and Zaire encompasses a region of approximately 3 million hectares. The basin includes a classic example of a petroliferous rift- and passive-margin depositional system. The Lower Congo Basin's tectonic evolution and unique paleogeographical setting have combined to produce a stacked system of hydrocarbon-rich units within both the Early Cretaceous Lacustrine sediments and the late Cretaceous/Tertiary marine sediments. The regional tectonics resulted in closed ocean circulation during the late Early Cretaceous (mid-Aptian) and the deposition of a thick Aptian salt layer. The western portion of this salt unit is found on the Brazilian margin. The salt layer in the Lower Congo Basin vertically separates the two distinct rift (primarily lac strine) and post-rift (primarily marine) oil rich sections, the Pre-salt and Post-salt respectively.

Deep seismic profiles suggest two possible models of basin tectonics: (1) Detachment between the pre- and post-salt sections. This essentially limits the crustal scale faulting spatially and temporally to the pre-salt (pre-Aptian) and similarly limits deformation in the post-salt to gravity driven detachment faulting, or (2) Crustal-scale faulting with stacked, extensional systems interacting across the salt detachment. These normal-fault systems, in some cases exhibit ramp-flat-ramp geometries similar in form to thrust-belt systems. The periodic and continued reactivation of the crustal scale faulting in this model provides an alternative kinematic explanation for enigmatic "rafted" post-salt structures and high apparent subsidence rates.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.