Abstract
A number of oil- and gas-producing leases have been operating in Italy in the last decades, many of which are located in the surroundings of tectonically active regions. Identifying human-induced seismicity in areas with high levels of natural seismicity is a difficult task for which virtually any result can be a source of controversy. We implemented a large-scale analysis aiming at tracking significant departures of background seismicity from a stationary behavior around active oil and gas development leases in Italy. We analyzed seismicity rates before and after hydrocarbon peak production in six oil-producing and 43 gas-producing leases, and evaluate the significance of possible seismicity rate changes. In a considerable number of cases seismicity rate results stationary. None of the observed cases of seismicity rate increase after the peak production is statistically significant (at as.l.= 0.05). Conversely, considering cases of seismicity rate decrease after peak production, our results suggest that the seismicity rate reduction is statistically significant (s.l.= 0.05) around one oil-producing lease (Val d’Agri, Basilicata) and around a cluster of gas-producing leases in Sicily. Our results put in evidence correlated changes between the rates of shallow seismicity and hydrocarbon production in these areas, which are then identified as hotspots requiring more detailed research; assessing actual causal relationships between these processes will require further physically-based modelling. If a physical causative link between these processes exists, then the observed seismicity rate reduction could either be due to increased seismicity during the progressive increase in production before reaching its maximum, or to an actual seismicity rate reduction after that peak. Considering that there is evidence of seismicity occurring before the start of hydrocarbon production, which contrasts with the evident reduction of events observed after the peak production, we think it likely that the seismicity inhibition is a plausible hypothesis. Using a simple model we also calculate Coulomb stress changes in planes optimally oriented for failure, and we show that under some conditions the inhibition of seismicity is feasible in at least one of these cases. We conclude that more efforts to study the mechanisms and the possible consequences of anthropogenically-driven seismicity inhibition are required.
Highlights
The complex geological setting that characterizes the Italian peninsula is the result of different geodynamical processes closely acting in time and space; today the crust in this zone is characterized by a complex stress field with tightly spaced compressional and extensional regions (Amato and Montone, 1997)
Afterwards, we proceed with the proper data analyses, namely i) identification of independent background seismicity in the lease area, ii) selection of background events located within a distance δx from the production wells, and iii) test the significance of possible seismicity rate changes correlated with changes in hydrocarbon production
On the other hand, regarding the cases exhibiting a seismicity rate decrease after tm, our results suggest that the observed seismicity rate change is statistically significant for one oil-producing lease and two gas-producing leases (Fiumetto and Rocca Cavallo)
Summary
The complex geological setting that characterizes the Italian peninsula is the result of different geodynamical processes closely acting in time and space; today the crust in this zone is characterized by a complex stress field with tightly spaced compressional and extensional regions (Amato and Montone, 1997). The most predominant geomorphologic features in this region are the Southern Alps and the Apennines mountain chains, which are characterized by thrust-and-fold belts originated from the interaction between the European and the Adriatic-African tectonics plates (see e.g., Calamita et al, 1994; Cello and Mazzoli, 1998; D’Agostino et al, 2001; Bertello et al, 2010; Handy et al, 2010; Cazzini et al, 2015; van Hinsbergen et al, 2020) Such an active and complex tectonic setting makes of Italy a seismically active region where on average, every year, more than 2,000 seismic events with magnitude ≥2.0 are located by the Italian national seismic network (see e.g., the Bollettino Sismico Italiano, Pagliuca et al, 2020). The only clear cases of seismicity linked to underground geo-resource development in Italy are the low-magnitude seismicity occurrences recorded in connection with wastewater reinjection at the Costa Molina 2 well in the High Val d’Agri, southern Italy (Valoroso et al, 2009; Improta et al, 2015), and the seismicity recorded near geothermal power plants in Tuscany (Evans et al, 2012)
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