Fracture stratigraphy and pore space properties of Mesozoic platform carbonates, Southern Apennines, Italy

Fractured reservoir characterization has always had a great relevance for academia and industry, due to the great interest in the resources hosted in the subsurface. It is well known that the behaviour of those reservoirs is due to sub-seismic scale structural discontinuities which cannot be resolved through seismic investigation or well logging. Hence, analogue studies play an important role in filling the resolution gap. This PhD focuses on the Viggiano Mt. platform carbonates, cropping out in the Agri Valley, in the axial portion of the Southern Apennines fold-and-thrusts belt. The build-up of the Southern Apennines chain occurred since late Oligocene – early Miocene thrusting, with the development of a E to NE-verging multi-duplex structure, formed due to combined thin- and thick-skinned tectonics. Subsequently, during Pliocene, the multi-duplex was dissected by transtensional and extensional faults associated to either the Tyrrhenian Basin opening and/or to the gravitational collapse of the orogen. The Agri Valley formation resulted from the activity of two main sets of regional scale, Quaternary, high-angle, active, transtensional faults that include the NW-SE basin-bounding faults, and the NE-SW faults. Viggiano Mt. forms a Lower Jurassic (Sinemurian-Pleinsbachian-Toarcian) - Cretaceous (Cenomanian) layered carbonate succession that, at sub seismic scale is crosscut by a dense array of pressure solution seams, fractures, and veins, which together form a structural network associated with polyphasic tectonic evolution. The characterization of rock textures and fossil associations show that the Sinemurian–Pleinsbachian carbonates were originally deposited in a low-energy open lagoon, the Toarcian carbonates in a ramp setting rimmed by sand shoals, and the Cenomanian carbonates in a medium- to high-energy, lagoonal–tidal setting not far form the platform margin. The fracture-density (P20) and intensity (P21) values display similar trends in both Sinemurian–Pleinsbachian and Toarcian carbonates, consistent with the mechanical control of bed and bed-package heterogeneities. Differently, P20 and P21 that characterize Cenomanian carbonates do not show very similar variations, due to pronounced bed amalgamation. The results of the analyses of the diagenetic processes show that the grain-supported carbonates underwent to cement precipitation during very early diagenesis firstly under marine phreatic conditions, and then freshwater vadose conditions. During continuous burial of the carbonates, pervasive cementation occurred under freshwater phreatic conditions, with embrittlement of the whole carbonate succession, and formation of laterally continuous, bed parallel wavy-type stylolites and pressure solution seams. Pressure solution continued during thrusting tectonics, due to tectonic burial, forming of bed-parallel seismogram-type stylolites. Specifically, pressure solution processes were strongly affected by the grain size of single beds. Since early diagenesis, pressure solution localized within the coarser-grained carbonate beds, forming wave-like solution surfaces. During the onset of thrusting tectonics, prior to tectonic burial, small scale thrusting took place by means of shearing of the bed-parallel heterogeneities, and formation of bed-oblique slickolites forming back thrusts characterized by flat-ramp-flat geometries. The combined results of microstructural observations with those after petrographic and Nuclear Magnetic Resonance (NMR) conducted on bed-perpendicular plugs show that the studied carbonates are characterized by an amount of effective porosity lower than 5%, with mean values of ca. 3%. Focusing on the stylolite related porosity, petrographic analysis shows that in the Lower Jurassic carbonates the secondary pores mainly localize along bed-parallel, seismogram-type stylolites. Differently, in the massive Cretaceous limestones, moldic porosity is due intrafossil and intercrystal micropores. The connectivity in the studied Mesozoic limestones is mainly controlled by the presence of microfractures. The results reported in this PhD thesis show that the fracture distribution is significantly controlled by the depositional setting and the diagenetic processes that may occur very early during the rock formation. In fact, due to early cementation occurring in grain supported carbonate rocks, the different carbonate lithofacies may develop different mechanical properties during the initial stages of rock formation. Moreover, pressure solution processes can play an important role during the different diagenetic stages and during the tectonic evolution of the multi layer, forming mechanical interfaces capable of compartmentalizing fractures within single mechanical units. Finally, within the rock volumes located far away from large faults, the pressure solution interfaces can also localize significant amounts of porosity, depending upon their morphology. Further permeability measurements may shed lights on the control exerted by those interfaces in the fluid flow/storage properties of the fractured reservoirs hosted in shallow-water carbonates.