The evolution of the Albian volcanic rifted margin in the Northeast of Brazil and its potential for oil generation

Abstract

In this article, an interpretation is given to the easternmost portion of Northeastern Brazilian basins, where intense crustal extension occurred, associated with profuse intra plate volcanic activity in the early Albian period. The occurrence of new depocenters, arising from this tectonic stage and having the potential to generate exploration in the basins along the Eastern Brazilian coast resulted in acquisition of extensive seismic reflection data with records that enable crust imaging as deep as 30km. The interpretation of this dataset, associated with data from the exploratory wells, has enabled the identification of new crustal structures, among which are the early Albanian rifts in the distal por-tion east of the Northeastern Brazilian basins. This set of data also permits the visualization of the ge-ometry of the deep continental crust under these basins, their internal structures and the continental/oceanic crustal transition zone. In this context, the prominent feature is the occurrence of the hyper-extended continental crust, under a wide-ranging subsidence area, with depocenters that extend to around 200km beyond the presently recognized crustal limit. The seismic sections also interpreted two notable interfaces – the upper/lower crust and the crust/mantle border. The distinct rheological contrast between the upper and lower continental crusts reflects brittle and ductile deformation pat-terns and determines, principally, the zone of the occurrence and the nature of the faults – planar or listric, wide or narrow angle, as well as faulted or rotated blocks. The ductile horizon, however, characterizes the top of the lower crust, where the surfaces of the large fault displacements con-verge. They disappear lower down. The continental–oceanic border crust delimits the area of the new depocenters, presumably new oil generating areas, This has been confirmed by the drilling of exploratory wells in the deep waters of the Ser-gipe Basin, which also permitted the recognition of igneous events associated with the sag phase of the Albian rift (c.a. 104 Ma -40Ar/39Ar), in addition to the discovery of oil and gas in the Upper Cretaceous reservoirs. Accordingly, this association is seen to be highly encouraging: a new exploratory play connected with the tectonic depressions can already be tracked in the north-eastern Brazilian coast and in the coastal basins connected to the african side, with prospects for the discovery of accumulations of oil and gas.