Mechanical stratigraphy controls fracture pattern and karst epigenic dissolution in folded Cretaceous carbonates in semiarid Brazil

Abstract

Carbonate reservoirs are often affected by fractures and karst that increase rock's permeability. An accurate characterization of reservoir architecture, fracture attributes and distribution of karst features are critical to modeling carbonate reservoirs, predicting flow behavior and estimating geofluids recoveries. In this contribution, we describe a case study of epigenic karst dissolution concentrated in a fold hinge zone and controlled by stratigraphic and structural processes in Cretaceous layered carbonates in the Potiguar Basin, Brazil. The stratigraphic setting and petrographic/petrophysical properties of host rocks were defined before fracture analysis. Two main joint sets were recognized: master joints (persistent fold hinge parallel set) and cross-joints (orthogonal to fold hinge and confined within master joints). The quantitative mesoscale fracture analysis was performed on a hinge-parallel joint set in 10 field sites distributed across the fold using scanlines to determine two parameters: Fracture Spacing Ratio in individual beds and the Fracture Spacing Index of bed packages. Fracture density and intensity of cross-joints were analyzed in one site using digital scanlines and scanareas on drone images. Results indicate that (i) systematic fractures consist of strata-bound joints, mostly oriented parallel to fold hinge, (ii) fracture intensity does not depend on intrinsic petrophysical properties of individual beds, and (iii) fracture intensity increases within a ∼2.5 km-wide fold hinge zone, consistent with previous studies performed via remote sensing analysis. By integrating our new results with published data, we provide a conceptual model that considers the role of regional-scale tectonic process and local-scale mechanical stratigraphy in controlling the size, shape and distribution of epigenic karst dissolution. Our findings indicate that facture permeability anisotropy, inferred from the karst pattern, is imparted by the combination of bed's attributes (mechanical stratigraphy) and fold-related fracturing. These results are an effective tool to predict the meso- and macro-scale porosity systems in fractured and karstified hydrocarbon reservoirs.