The occurrence of a sequence of damaging earthquakes in May 2012 near the Cavone oil field, in Northern Italy, raised the question of whether these earthquakes might have been triggered by oil and gas activities, such as fluid extraction and injection. Here, we analyze geologic and seismic reflection data along with previous studies to determine rates, patterns and modes of deformation recorded in the folds and faults that comprise the Cavone structure, and examine the rate of ongoing deformation observed by GPS, as well as the historical rate at which earthquakes comparable to the 2012 sequence have occurred.
To estimate the change in reservoir pore pressure and stress on the regional faults due to oil production and water injection, we investigated the properties of the reservoir by means of well tests. Interference tests carried out in 2014 at the Cavone 14 injection well allowed us to constrain the range of values of reservoir permeability, and also suggested the presence of fractures intersecting the well that result in high transmissivity within a region of many tens of meters around the well. The interference tests also indicate the presence of geologic structures that impede flow at larger distances.
Here, we study, by way of numerical modeling and simulation, the potential for induced seismicity at the Cavone oil field. Using a coupled flow and geomechanics model of the field that honors reservoir geology and historical well schedule, we simulate oil production and water injection in the field for a period of three decades leading up to the earthquake sequence. We calculate the change in Coulomb stress on the bounding Mirandola fault, which sourced the May 29, 2012 M 5.8 earthquake. This quantity varies in space and evolves in time with changing pore pressure and total stress in the reservoir. A novel and important aspect of our work is the identification of a potential instability mechanism for a bounding fault at the edge of a reservoir experiencing pressure depletion. The discontinuity in pore pressure across the fault means that there is a discontinuity in effective normal stress and that, therefore, the Coulomb failure criterion must be evaluated locally on both sides of the fault. We track the evolution of the Coulomb stress at the hypocenter of the earthquake and compare it with the tectonic stressing rate in the region to conclude in favor of tectonic origin of the earthquake. In addition, analysis of the locations of aftershocks of the May 2012 sequence shows a lack of seismicity in the area where the stressing rates from contraction of the reservoir are largest. This observed lack of seismic activity within 1–2 km from the reservoir suggests that fluid production and injection from the Cavone field was not an important driver for the observed seismicity.