Petrophysical Signatures and Water-Reservoir Potential of Southern Apennines Turbiditic Sandstones Suites

Abstract

After the seismic event of Irpinia (Southern Apennines), occurred on November 23rd 1980, the Pavoncelli tunnel, that used to supply with water the Puglia Region, was seriously damaged. Because of the strategic importance of this civil engineering work, a new water tunnel named Pavoncelli-bis that should substitute the previous one, was built and finally tested during the end of the 2019. The geological features of the area, highlighted the presence of three main siliciclastic formations (Corleto, pre-Numidian/Numidian and Castelvetere formations), crossed by the tunnel trace, that were petrophysically analyzed through the mercury injection porosimetry for a better definition of their petrophysical signatures and related water-reservoir potential at regional scale as hydrocarbon-reservoir potential. Data highlighted the presence of two clusters: one represented by the Corleto and Pre-Numidian Fms. showing good permeability (respectively 181.70 mD and 122.25 mD in median) and low porosity (respectively 4.56 % and 3.94 % in median); the other, represented by the Castelvetere Fm. highlighting good porosity (9.58 % in median) and a very low permeability (4.84 mD in median). Permeability, more than porosity, affects the cluster differentiation, whereas the other variables (average pore diameter and nanopores volume) do not show any particular trend. All these petrophysical parameters were compared with the sandstone composition (Qm, F, Lt), highlighting that porosity results negatively correlated to monocrystalline quartz (Qm) and positively correlated with feldspars (F). The lithic fragments values (Lt) are sparse and do not show any specific trend. The variability of porosity is directly linked with the diagenetic processes that interested the clusters of the Corleto and pre-Numidian Fms. (rich of Qm) and the Castelvetere Fm. (rich of F). Particularly, where quartz is more abundant, quartz cementation and overgrowth are more efficient and widespread, resulting in a reduction of the porosity. On the contrary, where feldspars are abundant, during diagenesis their dissolution can create new pore spaces and also clay-minerals with a better porosity than the previous feldspars. Moreover, clay-minerals can coat the quartz grains present in the sandstones inhibit the porosity loss of the quartz cementation and overgrowth. However, despite an increase in porosity, permeability decreases because the authigenic clays is mainly formed by isolated and blind pores.