Seismic and well log characterization of fractures for geothermal exploration in hard rocks

We study the well log and seismic responses of intensively fractured portions of deep intrusive/metamorphic rocks in southern Tuscany (Italy), which constitute the main drilling targets of the geothermal exploration in the Larderello–Travale area. In particular, the target we consider is located near the contact between a deep Pliocene granitic intrusion and the overlying Palaeozoic metamorphic basement. Sonic, density and borehole image logs are analysed together with post-stack reflection attributes and reflection amplitude versus source to receiver azimuth (AVAZ) responses. It turns out that the intense fracturing in the contact zone causes significant decreases in the density and P-wave velocity, and that fracture planes exhibit very high dips and a common preferential direction. The fractured zone found by the well coincides with peculiar alignments of high-amplitude signals in the 3-D seismic stack volume, which are particularly visible on the reflection strength and instantaneous phase time slices. The normal incidence synthetic seismogram based on the log data matches the observed stack trace nearest to the well and confirms that the high-amplitude reflection occurs at the fractured zone. We then consider the pre-stack domain to study the same reflections on bin gathers that are close to the well and coincident with the anomalies in the 3-D volume. In particular, we perform AVAZ analysis to detect possible anisotropic features in the reflected amplitudes due to the preferential orientation of the fractures, and we study the effect of crack density on the seismic responses and on velocity and density values. To this end, we build simplified models where a level with vertical fractures is encased in tight isotropic rocks. Notwithstanding the suboptimal quality of the seismic data, we estimate the overall matching between the borehole information and the seismic response as fair. In particular, the azimuthal amplitude variation of the reflections from the studied fractured zone has a sinusoidal trend that is quite consistent with the fracture planes’ orientation as indicated by the image logs. Moreover, the comparison between the actual AVAZ response and the AVAZ responses of synthetic seismograms generated on models with different crack densities suggests that it may be feasible to estimate crack density values from the azimuthal amplitude variation of the observed reflections, within the resolution of the seismic data.