Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism Indicating Causation by Nearby Oil Production

Abstract

During 2018–2019, oil was intermittently produced from the Late Jurassic Upper Portland Sandstone in the Weald Basin, southeast England, via the Horse Hill-1 and Brockham-X2Y wells. Concurrently, a sequence of earthquakes of magnitude ≤3.25 occurred near Newdigate, ∼3 km and ∼8 km from these wells. The pattern, with earthquakes concentrated during production from this Portland reservoir, suggests a cause-and-effect connection. It is proposed that this seismicity occurred on a patch of fault transecting permeable Dinantian limestone, beneath the Jurassic succession of the Weald Basin, hydraulically connected to this reservoir via this permeable fault and the permeable calcite ‘beef’ fabric within the Portland sandstone; oil production depressurizes this reservoir and draws groundwater from the limestone, compacting it and ‘unclamping’ the fault, reaching the Mohr-Coulomb failure criterion and causing seismicity. In principle this model is fully testable, but required data, notably the history of pressure variations in the wells, are not currently in the public domain. Quantitative estimates are, nonetheless, made of the magnitudes of the variations, arising from production from each well, in the state of stress on the seismogenic Newdigate fault. The general principles of this model, including the incorporation of poroelastic effects and effects of fault asperities into Mohr-Coulomb failure calculations, may inform understanding of anthropogenic seismicity in other settings.

Earthquake hazard and risk analysis for natural and induced seismicity: towards objective assessments in the face of uncertainty

“…co. uk/ news-publi catio ns/ news/ 2018/ oga-newdi gate-seism icity-works hop-3-octob er-2018/), the workshop report stating that, with one exception "The workshop participants concluded that, based on the evidence presented, there was no causal link between the seismic events and oil and gas activity" (the exception being the lead author of the letter). A study published subsequently in a mainstream seismological journal concluded that it was indeed unlikely that the earthquakes had been induced (Hicks et al 2019), although dissenting views have been expressed in a chapter of a slightly obscure book (Westaway 2020) and on a blog (www. geosi erra.…”