STRATIGRAPHIC ANALYSIS OF THE ARATU STAGE (LOWER CRETACEOUS), RECONCAVO BASIN (BRAZIL), WITH HYDROCARBON RESERVOIR ROCKS

The section between the 7 and 11 electric markers in the Aratu Stage of the Reconcavo Basin (Lower Cretaceous) has been studied in order to perform a stratigraphic analysis. This section bear reservoir rocks known as Imbe, Cambuqui and Miranga sandstones. It produces petroleum in fields like Miranga, Aracas, Taquipe, Miranga Norte and Imbe. The isopach map showed a general trend of increased subsidence for south and southeast. A series of structural lows surround the platform region known as QuiricO High. Southwestward, the Taquipe Canyon is an indication of the lake deepening in that region. In terms of depositional systems, it was interpreted that the Aratu Stage presented a river-dominated deltaic system all over its depositional history. In the proximal regions, located to the north and northwest, the main facies belong to the fluvial and delta plain associations and to the delta front and prodelta in the most distal system. The isolithic map and the sand percentage map indicates a main axial sand distribution. The main sedimentary inflow comes from north and northwest. Near the southeastern faulted border there is a slight contribution from the alluvial fans attached to the Salvador fault. As a natural consequence, the stratigraphic sections showed a decrease of sand south and southeastward as well as the Spontaneous Potential (SP) logs indicate a progressive change from fluvial (normal bell shapes) to deltaic patterns (inverted bell shapes). The sedimentary cyclicity is strongly depicted in the Aratu section. Part of it can be observed in resistivity and SP logs, where auto-correlation can be obtained. Peaks in the resistivity logs indicate lag deposits due to periodic lake flooding. The resistivity value, as a function of the thickness of ostracodal and fish remains calciferous sandstones and shales, allows to separate several orders of peaks (four to six orders). Conventional cyclicity analyses indicate clearly an orbital control due to precession and axial obliquity. The calculated sedimentation ratio is 60 cm/1000 years, and the time interval of 1.44 Ma for deposition of the entire section.