Interplay between structural framework and diagenetic processes in the Barra Velha Formation: A study from Tupi Field, Santos Basin, Brazil

The Cretaceous pre-salt reservoirs of the eastern Brazilian passive margin basins store major oil and gas reserves, representing some of the most significant hydrocarbon discoveries of recent decades. Despite extensive research, critical questions remain about their depositional nature, post-depositional evolution, and tectonic controls. This study integrates seismic interpretation, structural restoration, petrographic analysis, and well data to reconstruct the tectono-stratigraphic architecture of the Tupi Field, Santos Basin, and assess its implications for reservoir compartmentalization and fluid flow dynamics.

The results indicate that the Tupi High functioned as a persistent structural high throughout successive rifting episodes, controlling sediment distribution and basin compartmentalization. During Rift II, an increase in crustal stretching toward the south played a key role in generating erosional or non-depositional features in northern sectors. Structural restoration and interpreted subsidence patterns support a continued tectonic influence during Rift III, extending fault activity into the upper Barra Velha Formation (BVF). These findings also reveal that deposition occurred across a mosaic of shallow, interconnected lakes, rather than a single large lacustrine system.

The diagenetic evolution of the BVF is marked by early calcite cementation, multiple dissolution episodes, dolomitization, and later silica cementation. Diagenetic patterns are primarily governed by sedimentary facies and stratigraphic position, but are also influenced by structural architecture. Mud-rich, low-permeability intervals exhibit dominant early diagenetic features with limited mineral replacement, whereas structurally elevated zones show intense late-stage diagenesis—including saddle dolomite, late dissolution, and hydrothermal quartz—associated with fault-related fluid circulation. Continued fault activity up to the top of Rift III likely enhanced vertical fluid migration. A south-to-north gradient in subsidence may have promoted fluid expulsion toward structurally higher domains post-deposition. Local controls, such as original mineralogy, facies interbedding, and porosity variability, further contributed to the observed heterogeneity.

These findings demonstrate that BVF reservoir quality reflects the multiscale interplay between depositional and tectonic architecture, early and late diagenetic processes, and post-depositional fluid dynamics—critical factors for pre-salt exploration and production strategies.