Study of well data reveals fluid pressure distribution and origin in the upper crust of the Irpinia region (Southern Apennines, Italy)

Abstract

This study investigates subsurface pore pressures in the Irpinia region of southern Apennines, Italy, one of the central Mediterranean areas with the highest seismic activity. The Apennine thrust belt consists of stacked thrust sheets formed from both deep- and shallow-water environments during the pre-orogenic phase and later involved in the Neogene compressional phase. In the ongoing post-orogenic phase, the region is experiencing an extensional tectonic regime, as evidenced by the 1980 Ms. 6.9 normal fault Irpinia earthquake. Eleven exploration wells drilled to depth of 1.7–5.9 km intersect the main tectonostratigraphic units of the chain, providing valuable data on drilling mud weights and fluid chemical features, allowing for the reconstruction of vertical pore pressure trends and fluid circulation through the upper crust up to nearly 6 km of depth. The data analysis reveals that the carbonate platform and basin Meso-Cenozoic deposits generally exhibit hydrostatic or nearly hydrostatic conditions in the upper 2–4 km depth. However, moderate to high overpressure gradients are observed in Meso-Cenozoic basin sequences, Messinian evaporites, tectonic mélange, and foredeep Pliocene shales. These overpressures are typically associated with reverse faults and are not correlated with occurrences of gas. Conversely, slight overpressure gradients at shallower depths are related to shaly lithologies containing gas traces. Notably, the pressure profile of San Gregorio Magno-1 well, intersecting the causative fault of the 1980 earthquake, suggests a uniform distribution of fluids throughout intensively fractured nappes, including carbonate platform units and deep-water basin formations. Moreover, wells that penetrated the buried platform carbonates, known as Apulian carbonates, display hydrostatic or low overpressure gradients, even when overpressured shales seal the carbonate reservoirs, challenging previous seismological interpretations of overpressured Apulian carbonates. Finally, the analysis of diffusion mechanisms has provided insights into the timing of the geological disturbance that caused the locally observed overpressures and their maintenance.