GNGTS 2021 - Atti del 39° Convegno Nazionale

103 GNGTS 2021 S essione 1.2 TEMPERATURE DISTRIBUTION AT THE TOP OF REGIONAL CARBONATE RESERVOIR IN CENTRAL-SOUTHERN ITALY G. Gola 1* , L. Petracchini 2 , D. Scrocca 2 , A. Santilano 1 , E. Trumpy 1 , A. Manzella 1 1 Institute of Geosciences and Earth resources, National Research Council, Pisa, Italy 2 Institute of Environmental Geology and Geoengineering, National Research Council, Rome, Italy In order to infer the temperature distribution at the upper boundary of the regional car- bonate reservoir, we exploited a multidisciplinary dataset, which includes lithostratigraphic, hydrological, petrophysical and geophysical information. We reviewed and upgraded the na- tional temperature database exploiting and processing the static temperature profiles and bot- tom hole temperatures (BHT) from a huge number of geothermal and hydrocarbon exploratory wells located in the Central-Southern Italy. Integrating stratigraphic and geophysical data, we developed a new 3D geological model considering the main lithothermal units: i) the cap-rock units, ii) the regional carbonate reservoir units and iii) the basement unit. The thermal struc- ture was evaluated numerically accounting for the thermal effects of the thermally-driven con- vection, the topographically-driven groundwater flow as well as the pressure and temperature dependence of the physical properties of the water-rock system. Different scenarios were inve- stigated by varying the vertical permeability distribution and the basal thermal boundary con- ditions. The best-fitting solutions which minimized the RMSE between the simulated and the measured temperature data were chosen. Furthermore, the cut-off depth of crustal seismicity was evaluated by analysing a sub-set of the Italian earthquake catalogue with a completeness magnitude of 1.5, hypocentre errors and rms lower than 1 km and 0.8 s, respectively. Given the high density and quite uniform distribution of earthquakes, the seismicity cut-off depth, de ned as the 90 th percentile of the frequency-depth distribution, was evaluated by a moving window of dimension 100-by-100 km 2 along the N-S and E-W directions with an overlapping factor of 0.75. In order to obtain a reliable statistical analysis, only the grid-cells containing at least 40 events were considered. Finally, the thermal and rheological images were compared. Along the Apennine chain, in correspondence of the outcropping carbonate units, the tem- perature distribution is affected by a downward heat advection. Here, the seismogenic layer displays a thickness of 12-14 km. To the East, the buried sector of the Apennine fold and trust belt is characterized by a slightly increase of temperatures, suggesting a mainly conductive thermal regime. Nevertheless, the thermal structure is effected by the recent Pliocene-Qua- ternary sedimentary cycle which deepens the isotherms. In the external sector of the belt, the seismogenic layer is thicker than 18-22 km. The favourable geological conditions leading to the development of high temperature hydrothermal systems occur in the Tyrrhenian side of the peninsula far from the outcropping carbonate units. The most important thermal anomalies rely to the Roman and Campania Magmatic Provinces where, in correspondence of the mag- matic centres, the temperatures at the top of the regional carbonate reservoir are of the order of 220-280°C and the 400°C isotherm results shallower than 5-7 km. Outside the volcanic are- as, other interesting hydrothermal anomalies occur within the carbonate reservoir characteri- zed by temperatures in the range 120-180°C. The zones where hot isotherms rise toward the Earth’s surface display a seismicity cut-off shallower than 8-10 km. Although the brittle-ductile transition temperature suffers of large uncertainties, being of the order of 475±175°C, the thin seismogenic layer, located in the thinned/transitional Tyrrhenian crustal domain, mimics the geothermal anomaly and suggests very-high temperatures at the base of the regional hydro- thermal reservoir. The obtained results suggest that different tectonic processes occur across the Apennine belt and the rheological stratification reflects the lateral variations of the thermal structure and crustal setting.