Interpretation of gravity and magnetic data from western edge of Potiguar basin

Abstract

This paper presents qualitative and quantitative interpretations of geophysical data in the onshore western part of Potiguar sedimentary basin, NE Ceara. These geophysical data were performed in order to understanding tectonostratigraphy relationships involving these part of Potiguar Basin based on determination of main geophysical lineaments, geometry and depth of sources and the important determinations of geophysical domains. The study area shown structural lineaments partitioning characterized by lineaments in the NE-SW direction, E-W and NW-SE inflexions. The spatial arrangement of geophysical fields is related to the Brasiliano lineaments that occur in the region. Gravity modeling over the profiles shows the geometry of the study area which has a graben feature that may be associated with the accumulation of hydrocarbons such as the Fazenda Belem oil field. Introduction The Potiguar basin internal geometry shows a preferencial ENE-WSW direction, showing asymmetric grabens separated by internal basement highs. The structural framework of Potiguar basin is a result of changes that occurred during evolution in rift and drift stages and recent magmatism. (Bertani et al., 1990). The study area is inserted in the western portion of the Potiguar basin in its portion onshore and within the boundaries of the Ceara state. The region where is inserted the study area has a positive gravimetric anomaly and a significant magnetic anomaly (Pedrosa et al., 2010). This peculiar structure is not well defined and is near Fazenda Belem oil field. Regional Geological Context The Potiguar basin is inserted in the Precambrian basement complex of northeastern Brazil named Borborema Province (Almeida et al., 1981). This basin is part of a series of small to medium rift basins which form the Northeast Brazilian Rift System and its origin is linked to the South Atlantic opening in the Cretaceous (Matos 1992). The main stratigraphic units in the study area are, from base to top: 1) the Paleoproterozoic basement represented by supracrustal sequence as mica schist and quartzites of the Santarem Formation into Oros zone; 2) In the southeast of study area outcrop sandstones of the Acu Formation and limestones of the Jandaira Formation, which are into the Apodi Group; 3) most of the area is covered by sandstones and conglomerates of the Barreiras Formation; e 4) recents sediments associated with alluvial sediments and coastal Aeolian sediments along the entire coast (Cavalcante et al., 2003) (Figure 1). The Jaguaribe lineament or shear zone (JL) was inferred from structural data from Cavalcante et al., (2003). The Ponta Grossa-Fazenda Belem lineament (PGFBL) was determinate by Sousa (2002), who identified strongly deformed sector of Barreiras Formation, which alignement has coincided with Brasiliano shear zones that outcrop in the south of study area. Figure 1: Simplified geological map of the study area. JL: Jaguaribe lineament; PGFBL: Ponta Grossa-Fazenda Belem lineament.(Modified from Cavalcante et al., 2003). Geophysical datasets Airborne magnetic data The aerogeophysical survey provided by the Brazilian Petroleum National Agency (ANP) is named the Potiguar Basin Project. This dataset was undertaken between 1986 and 1987 and covered an area of 44,600 km2 with N20°W oriented flight lines and 2.0 and 4.0 km spacing between these lines. The nominal flight height was 500 m and the sampling rate 100 m. The aeromagnetic data were corrected for diurnal and main component of the geomagnetic field variations IGRF (International Geomagnetic Reference Field). These data were then interpolated into 500 m regular grid by the bidirectional method in order to generate the anomalous field. After this procedure, several filtering techniques were applied to improve the signal/noise relationship and to highlight specific features of the magnetic sources.