Fluid geochemistry and geothermometry in the unexploited geothermal field of the Vicano–Cimino Volcanic District (Central Italy)

Highlights

• We studied the geochemical features of fluids emerging around the Lake Vico (Italy).

• Waters emerge from both shallow (cold) and deep (thermal and CO₂-rich) reservoirs.

• Gas phase is dominated by CO₂ which is mainly originated by decarbonation reactions.

• The structural framework plays a key role in the upraise of deep-originated fluid.

• Secondary processes significantly affect the chemistry of the uprising deep fluids.

Abstract

The Vicano–Cimino Volcanic District (VCVD) is related to the post-orogenic magmatic activity of the peri-Tyrrhenian sector of Central Italy. The chemical and isotopic compositions of 333 water discharges and 25 gas emissions indicate the occurrence of two main sources: 1) cold Ca-HCO₃ to Ca(Na, K)-HCO₃ type waters from relatively shallow aquifers hosted in volcanic and sedimentary formations; and 2) thermal Ca-SO₄(HCO₃) type waters located in a deep CO₂-pressurized reservoir, hosted in carbonate–evaporite rocks and separated from the shallow aquifers by thick sequences of low-permeability formations. Carbon dioxide is mainly produced by thermal metamorphic decarbonation within the deepest and hottest parts of the carbonate–evaporite reservoir (δ¹³C–CO₂ from − 3.1 to + 2.2‰ vs. VPDB), likely affected by a mantle-rooted CO₂. Release of CO₂-rich gases from the deep aquifer into the overlying shallow aquifers produces high-CO₂ springs and bubbling pools. The spatial distribution of thermal waters and CO₂-rich cold discharges is strongly controlled by fractures and faults located in correspondence with buried structural highs. Stable isotopes (δD and δ¹⁸O) suggest that meteoric water feeds both the shallow and deep reservoirs. The relatively low R/R_a values (0.27–1.19) indicate that He is mainly deriving from a crustal source, with minor component from the mantle affected by crustal contamination related to the subduction of the Adriatic plate. Consistently, relatively high N₂/Ar and N₂/³He ratios and positive δ¹⁵N–N₂ values (from 0.91 to 5.7‰ vs. air) characterize the VCVD gas discharges, suggesting the occurrence of a significant “excess” nitrogen. Isotopic compositions of CH₄ (δ¹³C–CH₄ and δD–CH₄ values from − 28.9 to − 22.1‰ vs. VPDB and from − 176 to − 138‰ vs. VSMOW, respectively), and composition of light alkanes are indicative of prevalent thermogenic CH₄, although the occurrence of abiogenic CH₄ production cannot be excluded. The δ³⁴S–H₂S values (from + 9.3 to + 11.4‰ vs. VCDT) are consistent with the hypothesis of H₂S production from thermogenic reduction of Triassic anhydrites. Gas geothermometry in the H₂O–H₂–Ar–H₂S system suggests that the VCVD gases equilibrated in a liquid phase at redox conditions controlled by interactions of fluids with the local mineral assemblage at temperatures lower (< 200 °C) than that and measured in deep (> 2000 m) geothermal wells. This confirms that secondary processes, i.e. steam condensation, gas dissolution in shallow aquifers, re-equilibration at lower temperature, and microbial activity, significantly affect the chemistry of the uprising fluids. Thermal water chemistry supports the occurrence in this area of an anomalous heat flow that, coupled with the recent demographic growth, makes this site suitable for direct and indirect exploitation of the geothermal resource, in agreement with the preliminary surveys carried out in the 1970's–1990's for geothermal exploration purposes.

Introduction

The Vicano–Cimino Volcanic District (VCVD) is part of a thermally anomalous area that extends from southern Tuscany down to the Campanian volcanic areas of the Phlegrean Fields and Vesuvius, where CO₂-pressurized reservoirs developed in a complex geodynamic setting (Barberi et al., 1994, and references therein). Thermal emissions and mineral springs, as well as areas characterized by an anomalously high CO₂ diffuse degassing from the soil, are spatially controlled by fractures and faults related to an extensional tectonic regime (Minissale, 2004, and references therein). Most VCVD thermal manifestations are located W of the town of Viterbo along a N10°E tectonic alignment (Fig. 1). Here, the thermal activity is known since Etruscan and Roman times and acknowledged by Dante in the Divina Commedia (Dante Alighieri, Inferno XIV, 76–84). Thermal discharges are currently used as spa thermal resorts for health therapies and public pools (Chiocchini et al., 2001). A thermal spa near Nepi (Terme dei Gracchi), constructed in Roman times (Fig. 1), exploits sulfur-rich and sparkling waters that are currently bottled for commercial use. The VCVD, as well as the whole region between southern Tuscany and northern Latium, was intensively investigated for evaluating geothermal potential (Conforto, 1954, Cataldi and Rendina, 1973, Calamai et al., 1976, Cataldi et al., 1978, Borghetti et al., 1983, Bertrami et al., 1984, Cavarretta and Tecce, 1987, Cataldi et al., 1995, Barelli et al., 2000). Starting from the early 1950's, 9 deep wells and 76 test-holes (Fig. 1) were drilled by Terni Company (Conforto, 1954) and then, by the Italian energy provider (ENEL) and the national oil company (ENI-AGIP). A maximum temperature of 218 °C was measured in the Cimino 1 well, at the depth of 2153 m. Despite the significant potential highlighted by these preliminary explorations, the geothermal resources at VCVD have not yet been exploited.

Geochemical data of some fluid discharges from VCVD were reported in studies carried out at regional scale (Baldi et al., 1973, Arnone, 1979, Duchi et al., 1985, Chiodini et al., 1995, Chiodini et al., 1999, Minissale et al., 1997a, Minissale, 2004), whereas recent investigations have dealt with the reconstruction of a hydrogeological conceptual model of the Viterbo thermal area (Piscopo et al., 2006, Baiocchi et al., 2012) and VCVD (Baiocchi et al., 2006, Angelone et al., 2009, Chiocchini et al., 2010).

This study presents an original and detailed dataset of chemical and isotopic analyses of 333 fluid discharges (cold and thermal waters) and 25 gas emissions collected from VCVD. The main goals are to: 1) evaluate the fluid contributions from different source regions and their relationship with the tectonic assessment, 2) constrain the chemical–physical conditions controlling the chemistry of the fluid reservoirs, and 3) investigate the effects of secondary processes occurring during the uprising of deep-originated fluids toward the surface. Geochemical data were integrated with the available information concerning the stratigraphical, geophysical, hydrogeological and structural setting aimed to define a detailed conceptual model for fluid circulation and provide insights into the most promising areas for geothermal exploitation.