Tectonic-stratigraphic evolution of the Cretaceous section in the Barreirinhas Basin, Brazilian Equatorial Margin
Rifts; Romanche Fracture Zone; Barreirinhas Basin; Transtension; Lower Cretaceous, Break-up.
In recent years, the Brazilian Equatorial Margin (MEB) basins have been the subject of studies motivated by the discoveries of hydrocarbon accumulations in the counterpart African basins, representing a new exploratory frontier. In this context, the present study is focused on the Cretaceous interval of the Barreirinhas basin, on the mapping of the sedimentary section related to the rift implementation and on the transition from rift to post-rift stages. The recognition and mapping of the main unconformities and the analysis according to sequence stratigraphy concepts, aiming to understand the contemporaneity and the relationship between tectonics and depositional systems, contributes not only to the recognition of elements related to petroleum systems, but also to the prediction of the occurrence of these features. In addition, the geometric analysis of structures, the location of depocenters, as well as the characterization of the associated deformation are also essential approaches to the interpretation of the evolution of sedimentary basins and their exploratory evaluation. So, seismic sections (2D) were interpreted with a stratigraphic and structural bias. A previous stage of data treatment, application of filters and calculation of attributes was necessary to increase the signal/noise ratio and highlight features with geological significance. Wells were tied to the seismic sections through checkshots or synthetic seismograms (depending on the availability of sonic profiles and density) and the main unconformities with seismic expression were mapped. Despite the scarcity of seismic data in the onshore portion of the Barreirinhas basin, the density of wells in this sector of the basin allowed the creation of isopach maps from chronostratigraphic markers and structural contour maps for the main discordances of the stratigraphic chart. Structurally complex, the Barreirinhas Basin is an example of a rhombic transtensional basin, whose structural framework is directly influenced by the Romanche Fracture Zone (ZFR), resulting in the same basin, structures related to transtensional regimes, mostly and transpressional in a subordinate manner. In the framework mapped in the seismic sections, to the south of the ZFR there are features such as semi-grabens, horsts, tilted blocks, listric faults and negative flower structures, suggesting the predominance of a transtensional regime. The evolution of the depocenters installed in the onshore portion of the basin can also be observed in the isopach maps, where the greatest sedimentary thicknesses are accumulated. As it approaches the ZFR, close to the shelf break, an increase in the intensity of deformation is evident due to the increase in the density of the faults.