The lower Barra Velha formation (Aptian) in the Atapu field, Santos basin: Geological model for a pre-salt succession

Abstract

The characterization of facies, microfacies and facies successions allowed to develop a conceptual geological model for the lower succession of the Barra Velha Formation in the Atapu field, Aptian of the Santos Basin. The obtained data and interpretations were based on the detailed description (scale 1:10) of ca. 90 m of two almost continuous well-cores and petrographic analysis. The facies were defined based on their primary depositional attributes, which are well recognized despite diagenetic modifications. Lutites, microbialites, tufa-rocks, calcirudites constitute the main recognized facies. The clastic lutites recorded transport and deposition of terrigenous and intraformational clasts by hydrodynamic and gravitational processes in low energy and shallow lacustrine environment. Microbialites and tufa-rocks record in situ carbonate deposition in alkaline saturated waters. Injectites, diastasis cracks, loop bedding, slump folds, many of the normal and listric faults and fractures are syn-depositional structures interpreted as primarily induced by tectonics. Facies changes allow to divide the succession in three stratigraphic intervals. Lutites predominates in intervals 1 and 3 whereas tufa-rocks constitute the interval 2. In the lower part of interval 1 labile fresh siliciclastic components and tractive and gravitational structures are abundant, indicating dry climate but with sporadic rainfall during a period of relative high water level of the lake. The upward decreasing of siliciclastic fragments, the appearance of coarse microbialite and a fish mortality level in the upper part of interval 1, mark the transition to the tufa-rocks of interval 2. These latter features denote a relative dryer climate and higher alkalinity waters, and a low water level of the lake. The interval 3 lutites cover abruptly the tufa-rocks recording the drowning of the succession. This may be related to renewed rifting or to the beginning of the basin thermal subsidence.