Structural Style and Petroleum Habitat Related to Tectonic Evolution of Potiguar Basin, Brazil

Renato Tadeu Bertani, Iran Garcia Da Costa

The Potiguar basin is the easternmost basin on the Brazilian equatorial margin and covers an area of 48,000 km². The basin extends from about 200 km landward of the coastline out beneath the present continental shelf and slope to a water depth of 2,000 m. It contains a sedimentary sequence more than 8 km thick, which filled the asymmetric grabens that developed at an early stage of South American/Africa breakup in the Neocomian. Presently, 28 oil fields have been discovered; 23 onshore and 5 offshore. These fields contain proved oil reserves of 35 × 10⁶ m³ and occur in structures formed during the stretching and cooling stages of the tectonic evolution of the basin. Two main groups of petroleum habitats are distinguished according to tructure type and reservoir-source rock association.

During the stretching stage, grabens were formed and filled with lacustrine, deltaic, and deltaic fan sediments. These sediments were affected by gravity sliding, northeast-southwest normal faulting, and east-west strike-slip faulting. These movements created the closures that formed the oil accumulations. Producing reservoirs consist of coarse to fine-grained sandstones closely associated with the source rocks.

The cooling of the previously heated lithosphere led to continuous subsidence and deposition of an Upper Cretaceous fluvial-marine transgressive sequence, followed by progradation of Tertiary clastics and carbonates. The closures observed in these rocks were formed mainly by reactivation of previous faults. Stratigraphic and paleogeomorphic traps were also developed during this evolutionary stage of the basin. In addition to the formation of structural traps, these faults were important in providing conduits for the migration of oil that was generated in older sediments into the overlying fluvial and deltaic clastic reservoirs.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.