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Seismicity at Newdigate, Surrey, during 2018-2019: A candidate mechanism indicating causation by nearby oil production
  • Rob Westaway
Rob Westaway
University of Glasgow

Corresponding Author:robert.westaway@glasgow.ac.uk

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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, ~4 km and ~8 km from these wells. The pattern, with earthquakes concentrated during production from this 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 the Portland 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 Coulomb failure criterion and causing seismicity. In principle this model is testable, but required data, notably the history of pressure variations in the wells, are not currently in the public domain. The recognition that this instance of seismicity is arguably caused by human activity may well help inform understanding of anthropogenic seismicity in other settings. The initial response, including claims that any connection between this seismicity and oil production was implausible, before any geomechanical analysis was done, was inappropriate.