Diagenetic characterization and the influence on the porosity properties of a mixed bioclastic-siliciclastic rift lacustrine deposit of the Sergipe-Alagoas Basin, northeastern Brazil

Abstract

Bioclastic and siliciclastic deposits of the Morro do Chaves Formation, Early Cretaceous of the Sergipe-Alagoas Basin, northeastern Brazil, show depositional and diagenetic analogies with the Brazilian Pre-salt rift reservoirs. A detailed petrographic study carried out on 221 thin sections from four well-cores were used to define the stratigraphic and sedimentological controls of the diagenetic events on coquinas supported by cathodoluminescence and EDS-MEV analysis. The study of cement distribution allowed identification in high- and low-frequency cycles of the subaerial exposure zones and intervals with porosity and permeability enhancement. The eodiagenetic processes and products show a good correlation with the maximum regressive events marked in the lower carbonate-dominated sequence, by the occurrence of nodules, micritization, pseudomicrokarst, mechanical compaction, three types of calcite fringes, coarse calcite cement, and three dissolution events. This sequence shows less than 25% of terrigenous constituents, thus favoring a pervasive early calcite cement precipitation and consequently an occlusion of the porosity. The upper sequence displays a predominance of poikilotopic and blocky calcite cement related to the mesogenetic stage. This sequence shows a dominance of mixed carbonate and siliciclastic constituents, which reflects directly the reduced early cementation, and the higher porosity and permeability relatively to the lower sequence. This study on cement occurrence and distribution on mixed deposits and their relationships with sedimentological and stratigraphic controls adds new and useful data for helping in the understanding of the complex coquina successions of the analogous lacustrine bioclastic reservoirs of the Pre-salt interval. The diagenetic model can support further studies regarding to predictive models of porosity and permeability distribution in other bioclastic lacustrine reservoirs.