Nonmatrix pores form high-permeability zones in carbonate layers: The Furna Feia cave in the Potiguar basin, Brazil

Studying nonmatrix porosity in reservoirs is challenging, especially between core samples and seismic scales. To fill this gap, this work studied a karst system as an analog for a nonmatrix carbonate reservoir. We used a multidisciplinary approach, analyzing host rock data (petrography, petrophysics, and tectonic history) and porosity geometry (terrestrial laser scanner, ground penetrating radar, and field analysis). The karst system occurs in Cretaceous carbonates of the Jandaíra Formation along a regional subaerial unconformity that forms the boundary between lagoonal facies (bed package II) and tidal bar facies (BP-III). BP-I (tidal bar 1) is observed at the base of the cave, while BP-II is in the middle of the cave walls, and BP-III (tidal bar 2) occurs at the ceiling. We find that matrix pores are more pervasive in BP-I. Nonmatrix pores predominate in BP-II and BP-III but mostly in the boundary between them. Horizontal dissolution (decametric-cavernous pores) is more intense along the subaerial unconformity. Metric nonmatrix pores, such as gigapores (pockets) and cavernous pores (cupolas), occur within dolomitic lagoonal facies but predominate in tidal bar facies at the cave ceiling. Most of these metric pores are elongated, and their principal axes are preferentially oriented along NW–SE-striking fractures, indicating tectonic control of the Poço Verde Fault Zone. In this way, we conclude that nonmatrix porosity follows tectonic features below a subaerial unconformity, forming high-permeability zones in carbonate layers. Dolomitization guides the fractures intensity, leading to their consequent enlargement. The features studied here led to cave evolution. These findings can guide karst reservoir prediction.