Insights into shallow and deep fluid circulation of the Southern Apennines seismic belt (Italy) using borehole pore pressures

Pore pressures at depth are usually described in relation with hydrostatic pressures, implying an interconnection between pores and fractures from the earth's surface up to a certain depth. In some cases, pore pressures exceed hydrostatic values, and these overpressures can be interpreted as an equilibrium between geological pressurization mechanisms (e.g., under compaction, tectonics, hydrocarbon generation, dehydration reactions, various sources of fluids, etc.) and pressure dissipation processes, which mainly depend on rock properties (e.g., hydraulic diffusivity).

In actively deforming regions, other subsurface mechanisms may favor the generation of overpressure (e.g., parallel shortening of strata) and in addition, surface topography may drive meteoric groundwater to flow from positive reliefs to nearby lowlands, interacting with deeper fluids.

Within the framework of the PRIN FLUIDS project, the research presented here aims to study the pore pressures collected in 30 exploration wells of the Sannio and Irpinia regions (Southern Apennines thrust-and-fold belt, Italy), with the objective of clarifying if and how deep fluids (e.g., free gas phases such as CO₂ and HCs, as well as saline paleo/formation waters with Na-Cl chemistry and high pCO₂) interact with shallow waters and to investigate the relation between shallow and deep crustal fluid dynamics and seismogenesis. In the proposed study, pressures, normalized to a hydrostatic profile, have been first retrieved from borehole pressure data, and then projected on five geological transects, to recognize the spatial distribution of the pressure trends (i.e., hydrostatic, over-pressured and hydrostatic over-pressured zones) underneath the Apennines range (from the internal to the external thrust belt) and the Plio-Pleistocene Bradano foredeep. In addition to the structural features, we also used other information available from well profiles (i.e., litho-stratigraphy, geochemical data, thermal data and petrophysical parameters) and open sources (i.e., geothermal gradient and sedimentary facies distribution maps). This material was integrated with the distribution, at the surface, of deep-derived fluids (gas manifestations, thermal springs, CO₂-rich groundwater) to calibrate the system. Moreover, the overall data enabled deepening the comprehension of the role of the pressurized layers in acting as possible vertical and lateral barriers to/for fluid migration, and estimating the possible origin and depths reached by the thermal circuits. Finally, with respect to the distribution of pore pressure zones, two other aspects related to the active deformation and fluid leakage were addressed: vertical stress magnitudes at depth and distribution pattern of low-magnitude background seismicity of the area. The analysis on these topics and the preliminary results will be shown at the end of the proposed workflow.