Abstract
We tested synthetic seismic reflection modelling along a seismic line (CROP-18A) in the geothermal field at Larderello (Italy). This seismic line is characterized by a discontinuous but locally very bright seismic marker, named K-horizon, which has been associated with various geological processes, including the presence of fluids at supercritical conditions. Geological and geophysical data were integrated in order to develop a 3D subsurface model of a portion of the Larderello field, where extremely high heat flow values have been recorded. In the study area, the K-horizon is particularly shallow and supercritical deep conditions were accessed at depth. The 2D model of the main geological units up to the K-horizon was extracted from the 3D model along the CROP-18A and used to generate the synthetic TWT stacked seismic sections which were then compared with the observed stacked CROP-18A seismic section. To build the synthetic sections, generated through the exploding reflector approach, a 2D velocity model was created assigning to each pixel of the model a constant P-wave velocity corresponding to the related geological unit. The geophysical parameters and the geological model reconstructions used in the modelling process derive from a multidisciplinary integration process including geological outcrop analogues, core samples, and geophysical and laboratory information. Two geophysical models were used to test the seismic response of the K-horizon, which is associated with (1) a lithological discontinuity or (2) a physically perturbed layer, represented by a randomized velocity distribution in a thin layer. For the latter geophysical model (i.e., the physically perturbed layer), we have tested three different scenarios changing the shape and the thickness of the modelled layer. Despite the reliable calibration implied by the use of homogeneous units, the seismic modelling clearly shows that the physically perturbed layer provides a better explanation of the reflectivity features associated with the K-horizon.
Highlights
Reducing uncertainties in geothermal exploration is essential in order to support the growth of this promising and sustainable energy source
It has been possible to define the major geological units of the study area characterized by specific seismic velocities and potentially discernible along seismic sections
The Tuscan Nappe plus tectonic wedge complex (TWC) is always present in the study area, and the top of this unit is well constrained by the wells available in the area
Summary
Reducing uncertainties in geothermal exploration is essential in order to support the growth of this promising and sustainable energy source This is true when dealing with the exploration of supercritical geothermal systems in which the reservoir fluids are expected to be in a supercritical state (for pure water, T > 374°C and P > 22 MPa) (e.g., [1,2,3]). Following the results of the San Pompeo 2 well (temperature > 400° and pressure > 24 MPa extrapolated to the depth of 2930 m, [22]), which reached the vicinity of the K-horizon, the presence of supercritical fluids confined in a relatively thin layer has been proposed to explain its high reflectivity (e.g., [10]) This hypothesis was used to explain teleseismic converted wave behaviour [23] and was recently explored by a deep drilling project [24]. Our study had four main aims: (1) the calibration of a 2D seismic lines CROP-18A, acquired in the Italian deep crust seismic project [25], through the use of a 3D geological-geophysical model, (2) the definition of a conceptual model of the area based on a rock physics model; (3) the modelling of the seismic response of the 3D geological-geophysical model with associated implications for processing the seismic reflection data, and (4) the definition of the seismic signature of reservoir rocks possibly hosting supercritical fluids
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