Thermomechanical Evolution of Potiguar Basin - RN, Brazil

Ulisses T. Mello

Abstract

The Potiguar Basin lies on the northeastern coast of Brazil. This basin is divided operationally by Petrobrás into two areas, onshore and offshore. This study deals with quantitative aspects of the Potiguar basin evolution.

Extensional factors of the uniform extensional model (McKenzie's Model) and of the depth dependent stretching model (Royden and Keen's Model) were obtained. These models explain successfully the tectonic subsidence of the basin. The formation of basin onshore area may be interpreted as a geometrical accommodation of a hanging-wall block rollover along an intracrustal listric fault. Such fault might be related to the basement structural framework inherited from the Pan-African events in the area. Extensional tectonics related to the East Brazilian rifting event reactivated the listric faults in the Early Neocomian. The offshore area is a passive marginal basin generated by the Atlantic Equatorial rifting in the Late Neocomian. McKenzie's Model accounts well for the backstripped subsidence of the offshore area of the basin.

Thermal maturity and burial-history calculations were performed to provide information about the basin hydrocarbon generation window. Lopatin's model indicates that the Pendênçia Formation had thermal potential to generate oil since the Barremian and that the Alagamar Formation became mature in the Campanian.

Cross-spectral techniques were used to analyze the relationship between free air gravity anomalies and bathymetry along six profiles of the offshore basin area. The comparison between the resulting admittance function and the theoretical admittance based on the Airy and Flexure Models of isostasy revealed that the basin gravity anomalies are successfully explained by the Flexural Model. This study suggests that the average effective elastic thickness of the analyzed area through time is around 5 km.

Tentative two-dimensional modeling of basin stratigraphy was performed with the purpose of detecting the most important factors that control the evolution of stratal patterns in Potiguar basin. In the passive continental margin the complex interaction of erosion, sea- level variation and sediment supply emerged as most significant, whereas in the onshore area lateral heat flow, regional isostatic compensation, and also sea-level variation and erosion were interpreted as the main controlling factors.