Multiscale fracture and fault characterization of carbonate reservoirs in the Atapu Field, Santos Basin, Brazil

Carbonate reservoirs exhibit significant heterogeneity due to the combined effects of fractures, faults, lithofacies variations, and diagenetic processes, complicating reservoir characterization and fluid-flow prediction. This study evaluates fracture–fault architecture and its relationship with reservoir heterogeneity in the Atapu Field, Santos Basin, through a multiscale approach integrating 3D seismic interpretation, borehole image logs, elemental spectroscopy, petrography, and petrophysical data. Seismic interpretation defined three reservoir domains: the Mound Domain (MD), Structural High Domain (SD), and Flanks Domain (FD). The MD exhibits high fracture intensity, with an overall positive association with silica enrichment and vug fraction, whereas dolomite shows a negative association with fracture intensity. Silicification may have locally increased rock brittleness, while petrographic and mineralogical evidence indicates late-stage fluid–rock interaction associated with dissolution, vug development, and mineral precipitation. The SD exhibits localized but intense fracturing in clean carbonate-rich, clay-poor intervals of the lower Barra Velha Formation. In contrast, the FD is characterized by very low fracture intensity and vug fraction, fine-grained facies with slightly higher clay content, predominant microporosity, limited macropore connectivity, and low permeability. Fracture orientations record inherited rift-related structures in relation to the present-day NE–SW SH max orientation, with breakout data showing no systematic vertical variation in stress orientation. Structural position, lithofacies, mineralogy, and diagenetic overprinting therefore define recurring geological associations that organize reservoir heterogeneity across the Atapu Field and may provide a useful framework for reservoir characterization in structurally complex carbonate systems.