Microgravity Characterization of the Hontomín CO2 Storage Site (Spain). Integration with 3D Seismic Results

Hontomín hosts the first Spanish CO₂storage Technology Development Plant. The area, located in the southern part of Mesozoic Basque–Cantabrian Basin, presents E-W and ESE-WNW faults formed during a Permian-Triassic extensional event. The latter were reactivated during the opening of the Bay of Biscay, while a new set of NNE-SSW faults developed. Fractures were reactivated again during the Alpine compression. The resulting configuration is a dome-like structure that includes the Mesozoic succession (Upper Triassic to Lower Cretaceous) and is crowned by Upper Cretaceous and Eocene rocks lying unconformably. The target injection pointis located at 1500 m depth, within a Jurassic carbonate saline formation.

Several multidisciplinary studies have been carried out in Hontomín aiming to obtain a thorough geological characterization. Among these, a microgravity survey, acquired under the umbrella of the CIUDEN foundation, has provided us with a complete 3D image of the site. A 4x4 km²area, coincident with that surveyed by 3D seismic reflection, has been sampled using a dense grid with a station spacing of 100 m.

The result is a high resolution Bouguer anomaly gravity map capable of offering insights into the subsurface geology down to the depth of the injection point. The application of mathematical procedures to the data has further enhanced its potential for interpretation. The calculated regional anomaly indicates that the dome structure strikes E-W to ENE-WSW, sub-parallel to a major fault: the South Fault, part of the Ubierna fault system. The resulting residual anomaly enhances a number of NW-SE features that have also been interpreted as faults and that can be observed after performing vertical and horizontal derivatives to the data. Calculation of the Euler solutions confirms the previous results and brings out a new NNW-SSE feature, namely the East Fault.

Integration with 3D seismic data suggests that faults affect different levels of the sedimentary sequence. Whereas the South and the East faults, identified in both datasets, appear to affect the basement and the entire succession, the NW-SE fault system, not identified in the seismic dataset, only affects shallow sediments. Contrarily, N-S and E-W faults identified in the seismic data set do not have a gravity response, thus suggesting they have a limited offset.