Hydrothermal silicification along faults affecting carbonate-sandstone units and its impact on reservoir quality, Potiguar Basin, Brazil

We analyzed hydrothermal silicification along the Afonso Bezerra strike-slip fault system in the Potiguar Basin, equatorial margin of Brazil, to assess the role of Si-rich fluids in fault geometry, properties, and evolution. The fault cut across the whole basin, including its crystalline basement, a late Cretaceous siliciclastic unit and a lower carbonate unit, and a Paleogene-Neogene upper carbonate unit. Intense fault-controlled silicification caused by the upward migration and diffusion of hydrothermal fluids occurred repeatedly. The fault zone is characterized by multiple events of syntectonic (dynamic) silicification with hydraulic brecciation at all scales in the fault core and static silicification, in which no or little lattice orientation or deformation occurred in the damage zone. In the carbonate units, silicification resulted in the complete replacement of the carbonate mineralogy by quartz, chalcedony and opal, and the SiO2 content increased from 3 to 15% in the host carbonate units to as much as 94-97% in the silicified portions of the fault. The silicified fault zone presents an abrupt widening from 150 m in the siliciclastic unit to as much as 800 m in the lower carbonate unit. The matrix of the siliciclastic unit exhibits a porosity reduction from 27% to 4%. By contrast, the matrix porosity of the lower carbonate unit exhibits a primary porosity of approximately 5-10%, which is either reduced to similar to less than 1% or increased to more than 15% in the silicified zone. In the upper and lower carbonate units, centimeter-scale vuggy porosity along fractures occurs in the silicified zone. This anomalous vuggy porosity could serve as a reservoir pore space. The collective field and petrological evidence suggests that volcanic intrusion plays an important role in silicification. This work may shed light on the origin and likely geometry of the fault-controlled silica distribution causing heterogeneities within carbonate and siliciclastic reservoirs.