Field-scale permeability and temperature of volcanic crust from borehole data: Campi Flegrei, southern Italy

We report combined measurements of petrophysical and geophysical parameters for a 501-mdeep borehole located
on the eastern side of the active Campi Flegrei caldera (Southern Italy), namely (i) in situ permeability by
pumping tests, (ii) laboratory-determined permeability of the drill core, and (iii) thermal gradients by distributed
fiber optic and thermocouple sensors. The borehole was drilled during the Campi Flegrei Deep Drilling Project (in
the framework of the International Continental Scientific Drilling Program) and gives information on the least explored
caldera sector down to pre-caldera deposits. The results allow comparative assessment of permeability
obtained from both borehole (at depth between 422 a 501 m) and laboratory tests (on a core sampled at the
same depth) for permeability values of ~10−13 m2 (borehole test) and ~10−15 m2 (laboratory test) confirm the
scale-dependency of permeability at this site. Additional geochemical and petrophysical determinations (porosity,
density, chemistry, mineralogy and texture), together with gas flow measurements, corroborate the hypothesis
that discrepancies in the permeability values are likely related to in-situ fracturing. The continuous
distributed temperature profile points to a thermal gradient of about 200 °C km−1. Our findings (i) indicate
that scale-dependency of permeability has to be carefully considered in modelling of the hydrothermal system
at Campi Flegrei, and (ii) improve the understanding of caldera dynamics for monitoring and mitigation of this
very high volcanic risk area.