Reactivation of compressional pre-existing structures in an extensional regime by wastewater re-injection induced seismicity: a Southern Appennine (Italy) oilfield case study

Abstract:

The Val d’Agri Quaternary basin is located within the Southern Apennine extensional belt, hosting the largest oilfield in Europe. Since 2006 wastewaters coming from oil production were re-injected by a single disposal well into the reservoir, represented by the carbonates of the Apulian platform. Consequently, induced micro-seismicity (219 events, ML < 2.3) was recorded close to the beginning of injection and within 5 km of the well. Afterwards, a detailed analysis of the oilfield underground dataset was performed to define a 3D geological model of the injection site. A pre-existing thrust system inherited by a Plio-Pleistocene compressional tectonic phase mainly constituted by NE-verging high-angle thrusts and SW-verging back-thrusts interesting the Apulian carbonates was interpreted. Below the injection well a mainly E-dipping back-thrust completely developed between two major thrusts can be modeled. The depth conversions of seismic interpretation based on sonic logs, VSP and seismic check-shots highlighted that the back-thrust geometry is compatible with accurate 3D absolute location and high precision relative location of induced events. Such analysis also show that wastewater injection firstly caused the reactivation of the deep portion of the back-thrust, and then seismicity mainly migrated up-dip toward shallower depths and interesting some portions of other faults, but remaining almost confined within the reservoir volume, and close to the injection well. These combined results also suggest that induced events presumably occurred on a small and high-permeability patch of the fault zone, favorably oriented with respect to the present-day SW-NE trending extensional stress field. Results can be read in the light of understanding the impact that induced seismicity due to the reactivation of pre-existing/inactive faults might have on seismic hazard and seismogenic potential evaluations, especially in tectonically active areas like Italy