Abstract: Modeling of Deep Water Structures in the Sergipe/Alagoas and Jacuípe Basins, in the Northeastern Brazilian Margin: an Integration of Potential Field and Seismic Data

Bassetto, Marcelo; Mohriak, Webster U.; Vieira, Ines S. - Petrobras/E&P

A multidisciplinary study, based in the interpretation of potential field and reflection seismic data, resulted in the recognition of the regional framework in the deep water region of the Sergipe/Alagoas and Jacuípe basins, in the northeastern Brazilian margin. Gravimetric, magnetometric and derivative maps were generated, which were complemented by gravimetric and magnetometric models along regional dip profiles, in order to support the interpretation of the seismic dataset available, in terms of the crustal architecture in deep waters.

Controlled by a large number of exploratory wells, located in shallow water regions, the seismic interpretation allowed the tracking of several stratigraphic horizons. These include drift sequence reflectors (tops of Eocene, Paleocene, Cretaceous and Turonian), transitional sequence reflectors (top and base of Aptian evaporites), and rift sequence reflectors, including the break-up unconformity and the pre-Cambrian to Paleozoic basement.

The gravity modeling procedure is based on the input of free-air gravity data obtained during acquisition of marine seismic data, and the introduction of horizons derived from seismic interpretation, depth-converted by applying appropriate interval velocities. The magnetometric maps and models were generated using a regional aeromagnetometric dataset. The magnetic models were based on profiles extracted from the aeromagnetometric grid.

Important regional aspects of Sergipe/Alagoas and Jacuípe basins were deduced from the seismic interpretation and potential modeling. The main ones are mentioned as follows: a) The depocenters of the Sergipe/Alagoas and Jacuípe basins are controlled by the Sergipe Fracture Zone and show very distinct structural characteristics; b) Both basins are characterized by rapid crustal thinning towards the deep water region, but in the Jacuípe Basin the rift blocks are narrower; c) Some of the master faults appear to detach in the lower crust, near the Moho, and severely rotate the main rift block in deep water; d) In both basins, the basement structural framework shows three preferential directions: NE-SW, N-S and E-W; e) The continental/ oceanic boundary is located close to the shelf edge, and the maximum width of the offshore rift reaches approximately 50 km in Jacuípe and 40 to 70 km in Sergipe/Alagoas; f) In both basins, reflectors diverging and dipping eastwards are identified as seaward-dipping volcanic wedges, marking the transition to oceanic crust; g) In the Sergipe/Alagoas Basin, the rift blocks are thicker in shallow waters and gradually thin basinwards; in the deep water region, most blocks are rotated counterclockwise and largely eroded; h) Evidence for salt tectonics structures is identified locally, in the deep water region of the Sergipe/ Alagoas Basin.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil