Pore space properties of solution surfaces in shallow-water carbonates

Diagenetic and tectonic processes taking place in platform carbonates produce significant textural and mineralogical modifications through time, controlling the pore space in terms of dimension, geometry, shape, and connectivity of single pores, and influencing both total and effective porosity. Focusing on the different types of solution surfaces, this study is conducted on Lower Jurassic, Cretaceous, and Eocene, carbonates exposed at the Viggiano Mt. and Raparo Mt., southern Apennines, Italy. Microscale analyses show that the primary porosity of these rocks was occluded by pervasive blocky cements, which precipitated during burial diagenesis of the carbonates. We aim to assess the role exerted by the roughness of bed-parallel and low-angle to bedding solution surfaces, on the pore properties and permeability values of a variety of carbonate lithofacies such as mudstones, packstones, grainstones and rudstones. Specifically, we show the results of Nuclear Magnetic Resonance (NMR), gas-porosimetry and water-permeability tests conducted on plugs cored either orthogonal or parallel to bedding interfaces. All the study plugs show an amount of effective porosity lower than 5%, with mean values of ca. 3%. Excluding larger microfractures, and sporadic intrafossil and intercrystal molds, among the various types of solution surfaces we document that the rough, seismogram-type stylolites localize secondary porosity, while smooth, wave-type stylolites do not. The seimogram type stylolites, due to the non-selective carbonate’s dissolution, form a poorly connected vuggy porosity, and the NMR results the pores are subspherical to tubular (r<3 µm), with low aspect ratios (stiff pores), differently from the pores associated to open fractures (soft pores). Connectivity in the seismogram-type stylolites-related pores is due mainly to small microfractures forming capillary porosity (pore throat ca. r=1 µm). The results of permeability measurements at room pressure indicate that the amount of bed-perpendicular permeability is generally low (10-1 and 10-3 mD). The results of permeability measured at increasing confining pressure show that the in stylolite-dominated the bed-parallel samples have slightly higher values with respect to the bed-orthogonal ones and, at increasing confining pressure conditions, the permeability decreases of less than one order of magnitude. Differently, the fracture-dominated plugs show a permeability decrease of two orders of magnitude. These results are therefore consistent with a pore connectivity affected by open fractures only at shallow depths (<25 MPa) and influenced by both stylolites and primary pores at depths. Results of ongoing X-ray tomography analyses will better clarify the pore distribution along stylolites and at the fracture-stylolite intersections.