Structural trapping and internal geometry of fractured carbonate reservoirs: a case study in the Irati Formation, Saltinho (SP), Brazil

Trapeamento estrutural e geometria interna de reservatórios carbonáticos fraturados: um estudo de caso na Formação Irati, Saltinho (SP), Brasil

Resumen

The evolution of structural traps in fractured carbonate reservoirs is fundamentally governed by fault-zone architecture, which controls permeability, fluid pathways, and trap preservation through time. In the Irati Formation, on the eastern margin of the Paraná Basin (Brazil), normal faults compartmentalize a dolomitic succession that hosts outcrop-scale hydrocarbon accumulations. Unlike conventional systems - where source and reservoir rocks are distinct and primary porosity governs storage - the Irati dolomites represent a unconventional fractured reservoir in which hydrocarbon generation, storage, and trapping occur within the same stratigraphic interval, controlled by secondary porosity and multi-nucleated fault zones. This study examines how such reservoir behaviour is linked to the evolution of fault-zone architecture by integrating structural characterization, paleostress analysis, and U-Pb dating of syntectonic calcite to constrain the timing and conditions of brittle deformation and fluid circulation. Our results show that an early extensional phase produced a laterally connected damage-zone fracture network, establishing conduit-dominated flow along the dolomitic layer. Subsequent post-rift reactivation promoted fault-core sealing through brecciation, clay smearing, and calcite precipitation, sharply reducing cross-fault permeability and consolidating a structural trap during the Late Cretaceous (~75 Ma). A younger Paleogene event (~35 Ma) generated minor reactivation but did not breach the sealed core, demonstrating that reactivation alone does not necessarily compromise traps in mechanically layered systems. We propose a three-stage evolutionary model linking conduit formation, fault-core sealing, and trap preservation. These findings emphasize that long-term hydrocarbon retention in the Irati system is primarily an architectural outcome, and highlight the value of integrating U-Pb carbonate geochronology with structural analysis to refine trapping models in fractured carbonate reservoirs in similar settings worldwide.