Abstract

Over the past three decades, a significant number of small-magnitude and shallow earthquakes have occurred in the Western Canada Sedimentary Basin and are located along its western flank near areas of oil and gas production. One of the better documented examples is the swarm of earthquakes associated with the Strachan field, in the Alberta foothills. A model based on Segall's poroelastic theory is developed to account for the occurrence of earthquakes below the Strachan reservoir. Using this methodology, we show that the earthquake of 19 October 1996, underneath the Strachan field, was most probably triggered by gas extraction. The numerical model also implies that gas extraction would cause subsidence and localized changes in in situ stress magnitudes. There is a strong correlation between rates of production and the number of seismic events, but the onset of major seismic activity postdates the commencement of production by approximately 5 years. Poroelastic modelling can account neatly for this observed delay. The modelled stress changes due to gas extraction point to a regime which favours reverse or thrust faulting that is compatible with stress magnitude measurements in the area. The proposed mechanism involves volume changes which decrease the vertical stress Sv and increase the larger horizontal stress SHmax. The mean stress increase beneath the reservoir appears to be small, but increasing the deviatoric stress permits Mohr-Coulomb failure. As a result, the initially high rate and long history of gas extraction appear likely to be the main trigger for the seismicity beneath the Strachan field.

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