3-D geological-geophysical model and synthetic seismic reflection modelling along CROP-18A line in the Larderello area

Exploration strategies of geothermal reservoirs may significantly benefit from the development of synthetic seismic reflection profiles by confirming the possibility to detect prospective features on acquired seismic reflection data and to calibrate geological-geophysical interpretation and model reconstructions. To be elaborated a synthetic seismic reflection profile requires a conceptual geological model of the subsurface structure and physical properties, which is one of the tasks of the IMAGE FP7 European project. The deep reservoir of Larderello geothermal field, Tuscany Italy, is hosted in the metamorphic basement (Batini et al., 2003; Bertini et al., 1996), and is characterized by a strong amplitude reflective signal, the well-known K-horizon, widely observed in several seismic lines (Batini et al., 1978, Accaino et al., 2005) and probably drilled by the San Pompeo 2 well (Gianelli et al., 1997). In this study, geological and geophysical available data have been integrated to develop a new 3D geological-geophysical model of the portion of the Larderello geothermal field drilled by the San Pompeo 2 well. The geological-geophysical 3D modelling, performed using Petrel software, has been used to generate a 2D model for the synthetic seismic modelling of the main seismic units up to the K-horizon along the CROP-18A seismic reflection line acquired within the CROP project (Scrocca et al., 2003). The exploding reflector approach, developed in Matlab by the CREWES consortium and partly modified by us in this project, has been used to generate the synthetic seismic sections. The geological units defined for the velocity model are respectively the Neogene Unit, the Ligurian Flysch Complex, the Tuscan Units plus Tectonic Wedge Complex and the Metamorphic Units. We have assigned to these units Vp velocity values of 2700 m/s, 3850 m/s, 5500 m/s and 4800 m/s respectively, derived from literature (Batini et al., 1978, Accaino et al., 2005). Besides an acceptable calibration of the line CROP-18A with homogeneous units characterized by the previous seismic velocities, the seismic modelling allows to hypothesize a productive geothermal horizon at depth probably constituted by a "Physical perturbed layer - PPL". The PPL is characterized by a symmetric and/or asymmetric randomized velocity distribution (around the velocity of the host rock) simulating a rock physics model with fluid inclusions corresponding to the K-horizon. The homogeneous unit seismic response indicates that: o the deeper reflected events are significantly influenced by the articulated shallow morphology of Neogene and Ligurian units; o the wedge geometry allows the fit of the line-drawing performed on the stacked section related to the top of the productive K-horizon. The responses with the PPL model indicate that: o PPL could explain the reflectivity features and pattern of productive horizons observed in the Tuscan geothermal area; o asymmetric velocity distribution (a general decrease in velocity in PPL) generates a pseudo-layer which exhibits both reflection events at the top and bottom of PPL (sometime observed as typical pattern of reflectivity); oin the case of thin layer (about 100 m) to observe diffuse reflectivity, the PPL must be characterized by symmetric randomized velocity distribution, otherwise it exhibits a response similar to a homogeneous layer; oboth symmetric and asymmetric velocity distribution with a PPL thicknesses greater than 100 m generally shows responses characterized by diffuse reflectivity.