Processes and Patterns of Dolomite Precipitation in Aptian Pre‐Salt Deposits of Santos Basin, Brazil

Abstract

ABSTRACT The Aptian pre‐salt lacustrine carbonates represent the most significant oil exploration target along the South Atlantic margin. Dolomitization of the Santos Basin Barra Velha Formation (BVF) is critical for reservoir quality, yet its origin, patterns, and evolution remain poorly understood. This study characterizes the petrographic features of core samples from the Sururu field, identifying four distinct dolomite types—blocky, lamellar, spheroidal, and saddle—within the diagenetic framework of the BVF and evaluating their impact on reservoir quality. Dolomite precipitation occurred predominantly during eodiagenesis through multiple pathways, mainly involving the replacement of Mg‐rich clays or cementing secondary pores. Blocky dolomite formed mostly through consumption of Mg‐clay precursors, evolving from scattered floating rhombs to coalescent mosaic textures. Spheroidal dolomite is associated with the early replacement of peloidal Mg‐clays, whereas lamellar dolomite filled shrinkage pores within the laminated clay matrix. Saddle dolomite represents a later‐stage cement, commonly linked to hydrothermal fluids. Scanning electron microscopy (SEM)–energy‐dispersive spectrometer (EDS) data indicate Ca‐enriched, nonstoichiometric compositions, with Mg mol fractions ranging from ∼0.37 to 0.45, as well as the presence of Si–Al‐rich clay remnants within crystals, supporting a close association between dolomite formation and Mg‐clay precursors. These different dolomite fabrics exert contrasting effects on reservoir properties. Blocky crystals occurring in early, partially coalescent stages, as well as lamellar dolomite associated with shrinkage pores, tend to coexist with matrix‐dissolution porosity and display a wide range of permeability values. In contrast, the development of mosaic blocky dolomite and late saddle cement significantly reduce both porosity and permeability. This study enhances the understanding of dolomite formation in alkaline lacustrine systems and provides valuable insights into reservoir quality and heterogeneity in the pre‐salt succession.