Geochemical aspects of the oil from the Sousa Basin (Lower Cretaceous), Northeast Brazil: geological context

Abstract

The complex of cretaceous features known as Rio do Peixe basins includes the Souza, Brejo das Freiras or Triunfo-Uiraúna and Pombal basins. They are located in western region of Paraíba State, with a total area of 1,250 km2, separated by basement highs and controlled by the Portalegre (NE-SE) and Patos (E-W) shear zones. One oil sample obtained from the Sousa Formation (Lower Cretaceous) in a well located at Salguinho Farm, was analysed with chromatography. It has characteristics of a light oil, with 81,1% of saturated compounds and a predominance of C17 and C23 n-paraffins. The analysis of biomarkers indicates the presence of tricyclic and tetracyclic terpanes, the dominance of the 17(H), 21(H), 30-Hopane (C30), gammacerane/C30 17(H), 21(H), 30-Hopane (C30) ratio of 0.23, and C30 -Hopane/C30 -Hopane (moretane) ratio reaching 80%. These aspects are pointing out to a non-biodegraded mature oil from a lacustrine freshwater environment. The medium thickness of Sousa Formation is 800 meters and occurs in two thirds of the total area of Sousa Basin – the presence of this oil, is a good indicative of a new exploratory area for the Brazilian sedimentary basins.