Pore space properties and permeability of solution surfaces in Mesozoic shallow-water carbonates, southern Apennines, Italy

Focusing on the Mesozoic and Eocene carbonate samples from the Viggiano Mt. and Raparo Mt., Southern Apennines of Italy, we examine the role of solution surfaces on the present-day porosity and permeability. Carbonate lithofacies include mudstones, packstones, grainstones, and rudstones and gas-porosimetry measurements show effective porosity up to 5 %. The value is mainly due to the secondary porosity localized along microfractures and within the rough bed-parallel and rough bed-oblique solution surfaces, contrary to what was found in the smooth bed-parallel ones, showing that roughness can control pore localization. NMR (Nuclear Magnetic Resonance) results indicate that the former group of solution surfaces include vugs characterized by subspherical to tubular shapes (pore throat r < 3 μm), and low aspect ratios (r < 2), forming pores with low sensitivity to compression. The microfractures form capillary porosity (r ≈ 1 μm) and are characterized by high aspect ratios (r > 2), typical of pores with high sensitivity to compression. Permeability measurements at room pressure reveal that samples with visible microfractures are characterized by values up 2 order magnitude greater than of those with only visible solution surfaces, showing that the pore connectivity is controlled by opening-mode, sub-mm scale microfractures. At confining pressures greater than 25 MPa, both fracture- and stylolite dominated samples show similar values of permeability, indicating that at depths larger than ca. 1 km, the rough stylolite localize effective porosity that may enhance the along-solution surface fluid flow in carbonates.