Thermal state of the Po Plain subsurface: a first overview

Abstract

Geothermal energy capacity in Europe has doubled in the last decade and will grow more over the next ten years, according to the 2020’s market report of the European Geothermal Energy Council. This brings great potential for development in Europe as the global energy demand is turning to greener, more reliable, and non-intermittent options besides oil and gas. As of 2022, Italy is the second country in Europe with the biggest number of geothermal plants (installed, in development, or planned) with a capacity of more than 800 MWe, but this capacity almost didn't increase in the last decade with only small additions in capacity in the 2010-2020 period, according to the EGEC report. This data shows the great potential for geothermal energy development, and a thorough evaluation of new study areas in Italy with geothermal potential is necessary.One of these potential areas is the Po Plain, in northern Italy; a complex geological area that represents the foreland basin of two opposite verging orogens: the Southern Alps and Northern Apennines. This region experienced E-W extensional events during the Mesozoic, leading to the formation of several carbonatic platforms divided by relatively deep carbonate basins, followed by a tectonic inversion since the Cenozoic that changed the regime to a N-S compressive setting which deposited a clastic sedimentary succession of up to 8 km in thickness, thus hiding the outer thrust fronts both from the Northern Apennines and the Southern Alps. Because of this complexity, several hydrocarbon provinces have been discovered associated mainly with oil and thermogenic gas. These discoveries have led to extensive amounts of thermal subsurface data, especially Bottom Hole Temperatures (BHT), Drill Stem Tests (DST), and extrapolated data from temperature maps at different depths that are mainly available from public databases like the Geothopica Project. Although obtained mainly for the oil and gas industry, these data have become very important for the exploration of geothermal reservoirs. For a reliable use of BHT data, they need to be corrected before any interpretation because they are usually obtained at the bottom of the borehole right after the perforation, leading to the change of the true formation temperature due to the cooling effect of the perforation mud.We present here a basin-scale depth to the isotherm maps for the Po Plain subsurface obtained for 40°, 60°, 80° and 100°C based on the extensive database of BHT data from Geothopica integrated with other public thermal data. All the temperature values were corrected using the equations proposed in the scientific literature for the Po Plain. The isothermal maps provide a new large-scale baseline for the most common temperatures used for low-enthalpy geothermal energy applications. The temperature distribution analysis, both in a map and along the regional cross-section through the Po Plain, allows preliminary considerations about how the heat flow is transferred and distributed in the Plio-Pleistocene deposits compared with deep carbonate structures.