3D geological reconstruction of serpentinite bodies in Tuscany: insights for in-situ CO2 sequestration

Anthropogenic greenhouse gas emissions may be offset by sequestering carbon dioxide (CO2) through the carbonation of magnesium silicate minerals (i.e., Mg2SiO4 olivine, Mg3Si2O5(OH)4 serpentine, CaSiO3 wollastonite) to form magnesium carbonate minerals (i.e., MgCO3 magnesite, MgCa(CO3)2 dolomite, CaCO calcite, FeCO siderite, NaAl(CO)(OH) dawsonite). This technology attempts to mimic natural low-temperature alteration (carbonation) of widespread silicate rocks (i.e., peridotite, serpentinite, basalt) that trap safely CO2 over geological times. Southern Tuscany presents more than 3 Gt of magnesite - equivalent to 1,6 Gt of trapped CO2 - that have been formed in the past thought the carbonation of serpentinite (BOSCHI et alii, 2009). In addition, extensive outcrops of unaltered serpentinites (Ligurian Units) as well as deeper occurrences are found at the base of, or embedded in, dominantly argillitic units, largely consisting of low-permeability shales and marls. Considering the outcropping rocks, exposed in Tuscany over an area of about 230 km2, a mineralogical sequestration up to 100 Gt of CO2 is theoretically possible - equivalent of about 200 years of Italian GHG emissions. Even assuming a lower performance of mineralogical GHG sequestration in Tuscan ophiolite, it is clear that the potentiality of this technology is very interesting. Here, we present a 3D geological reconstruction of a continuous serpentinite layer, buried under 300 m of argillitic formations, at Gabbro (Southern Tuscany). This reconstruction is the first important step to count realistically the potentiality of an in situ CO2 sequestration in Tuscany. We selected the study area from all the lithologic and stratigraphic wells data extracted from The Italian National Geothermal Database, managed at our Institute (http://geothopica.igg.cnr.it/). The database contains data for 3193 wells and 586 thermal springs for the whole Italian territory, and, in particular, 770 wells from Tuscany. In the study area, the Ligurian Units are represented by a complex stacking sequence, made up by several tectonic units that include ophiolites, sedimentary covers (cherts, limestones and shales), and turbiditic sediments. The main exposed ophiolitic bodies, dominated by serpentinites with minor gabbros and basalts, form an ENE trending, discontinuous outcrop alignment (from Bolgheri to Casole d'Elsa). The Ligurian units lie tectonically above a stack formed by units of continental affinity (Tuscan units). The latter (Tuscan Nappe and the Palaeozoic-Triassic metamorphic basement) crop out extensively to the south of the study area, while in the Gabbro area they are buried at a depth of about 1300-1500 m below the sea level, as indicated by the geothermal exploratory wells. Owing to this geological setting, ophiolities in the Gabbro area are buried at several hundred meters depth and are potentially exploitable for in-situ CO2 sequestration. Wells containing ophiolites were selected from the database, and lithological, stratigraphical, as well as temperature data were retrieved. The result of these queries provided a map showing the most suitable place that follow the main requirements to have efficient and safe carbonation: temperature around 150 °C, moderate depth and argillitic envelope. Some wells provide these requirements displaying serpentinite/gabbros/basalt embedded in argillitic units at a ranging depth from 150 m to 400 m, and a temperature of about 80°-170 °C. The peculiarity of the study area (Fig. 1) is the proximity to the Larderello Geothermal field with its anomalous heat flow that allows reaching significant temperature also at relatively shallow depth, enhancing spontaneously the reaction of carbonation in presence of CO2. The data from the selected wells have been used as input data for the 3D geological reconstruction, that has been performed using a 3D geological modeling software, "3DGeomodeler". This software was developed by BRGM and Australian Intrepid Geophysics. Input data could be geological maps, geological cross-sections, borehole, as well as geophysical data (i.e. gravimetric and magnetic data). 3DGeomodeler uses the implicit surfaces to model the geological surfaces; this method is based on the potential field theory, where a set of smoothly curving, sub-parallel geological surfaces in 3D spaces con be seen to be analogous to a set of iso-potential surfaces of a scalar (potential) field (MCINERNEY et alii, 2005). Once the geological model is ready the result block diagram can be visualized, as horizontal slices or vertical cross-sections. Our resulting 3D geological model highlights the presence of well-confined serpentinite bodies with lateral continuity that are potentially exploitable for in-situ CO2 sequestration. Our on-going research will continue with a detailed petrographic and structural studies of ophiolites exposed at surfaces and cored by exploratory wells in order to evaluate the permeability of these rocks.