Facies and Diagenesis Distribution in an Aptian Pre-Salt Carbonate Reservoir of the Santos Basin Offshore Brazil : a Comprehensive Quantitative Approach
Résumé
Aptian carbonate rocks of Santos Basin in the Brazilian southeastern continental margin show a highly heterogeneous reservoir quality conditioned by many controlling factors. This study presents a quantitative approach based on an integrated petrographic, mineralogical, and geochemical dataset of the Barra Velha Formation (BVF) to understand the spatial and temporal distribution of depositional and diagenetic aspects and the parameters controlling the nature and distribution of the diagenetic phases in Santos Basin pre-salt carbonate reservoirs. In this formation 3 units can be differentiated. The facies with higher Mg-clay content predominate in lower structural positions and regions with low relative relief. Higher proportions of spherulites occur in the transitional zones, and fascicular calcite occurs preferentially in the transitional and higher structural positions. Grainstones are observed in all depositional environments, but with variations in their composition (intraclasts, siliciclastic/volcanoclastic grain, and clay content). Stratigraphically, facies with higher Mg-clay content predominate in Units 3 (base) and 2 (intermediate), whereas Unit 1 (top) is marked by a greater proportion of grainstones and fascicular calcite. In Unit 1, fascicular calcite crusts tend to expand from the structural high, toward lower areas. The distribution of the main diagenetic products and their association with different facies highlight the role of depositional setting and primary constituents on the diagenetic processes. The tectonic and climatic context of the BVF favored the preferential precipitation and accumulations of Mg-clays in lower structural portions of the basin. The preservation of Mg-clays is also strongly linked to the structural setting, with more intense diagenetic alterations in higher structural areas. Dolomite is a major diagenetic phase in the studied samples, followed by silica. These diagenetic phases are largely associated with Mg-clay alteration and predominate in Unit 1. Mg-clays dissolution, as well as major alteration of carbonate phases, are concentrated in higher structural positions. Given the higher occurrence of faults in these areas and the presence of saddle dolomite, barite, celestine, and fluorite, which are typical products of hydrothermal alteration, part of the dissolution processes may be related to hydrothermal alteration. The depositional and diagenetic aspects indicate a close connection between the hydrochemical evolution of the lake waters with the origin and diagenesis of these deposits.