Salt control on the kinematic evolution of the Southern Basque-Cantabrian Basin and its underground storage systems (Northern Spain)

Underground gas storage is one of the solutions required for reducing CO2 emissions and ensure the transition to a renewable power supply. The Hontomín pilot-plant in N-Spain is a well-studied CO2 facility located in the Burgalesa Platform, a fold belt in the Southern Basque-Cantabrian Basin. Previous works agree that crustal extension and evaporite migration controlled the subsidence in the area. The discrepancies lie in the salt expulsion models proposed during syn-rift and post-rift stages and the basin deformation during Cenozoic contraction. The models range from thin-skinned tectonics, basement-involved thrusting, strike-slip faulting, and interference between thick- and thin-skinned thrusting with lateral extrusion. There is multiple working hypothesis often with contradictory predictions of the underground structure. Understanding these structural styles is critical for identify the main salt structures suitable for gas storage in the Burgalesa Platform. We integrate multidisciplinary data from 2D seismic to wellbore, gravity and field data, allowing us to construct three restored cross-sections that illustrate the process of progradational loading and salt expulsion towards the SW during the Early Cretaceous, which led to the development of the Hontomín and other trap structures within the salt overburden. Differential loading occurred diachronously along strike during the Cenozoic, younging to the SE by evacuation of the remaining salt, which caused diapirism and salt welding. We found that most of the structures in the cover were formed by salt migration, whereas Cenozoic shortening caused the rejuvenation of salt-structures and local diapir squeezing. Shortening in the basement is consumed in the frontal thrust that overrides the Duero foreland and in the partial reactivation of the former Mesozoic transfer faults as transpressional faults. The results of this work have implications for a further understanding of the tectonic evolution of the external area of rift structures, and as tectonic analogues in other salt-bearing basins