Geologists say Horse Hill oil site likely to blame for earthquakes

Press release from the Weald Action Group

As an earthquake rocked the Charlwood area this morning [1], scientists from Edinburgh University have made a submission to Surrey County Council [2] which points the finger at the Horse Hill oil site for 13 quakes in Surrey in 2018.

In December the Oil and Gas Authority ruled out a human cause for the shallow quakes, which were without precedent in the region. It said nothing was going on at Horse Hill, so it excluded it from its analysis. But the Edinburgh team said one possible cause could be release of pressure at Horse Hill well site: “We infer that management of this pressure (probably by bleeding the well annulus prior to testing) likely altered the Horse Hill fault stress balance, which then impacted on the Newdigate fault, causing the earthquakes”.

The Edinburgh University team’s research shows that there was work on the site before the quakes started in April and “the earthquakes in August and September coincide exactly with the deployment of a perforation gun downhole to improve flow”. They have produced a timeline of activity.

They conclude: “our assessment supports the concern that Horse Hill oil exploration triggered the earthquakes. We infer that future oil exploration and production close to critically stressed faults in the Weald is likely to result in similar earthquake events”.

The Weald Action Group [3] says the company behind the drilling should provide its well and site engineering logs for the relevant period. So far Horse Hill Developments Ltd has not provided any information.

James Knapp of the Weald Action Group said: “The Oil and Gas Authority appears to have been less than rigorous in researching what caused these quakes. Horse Hill Developments Ltd hasn’t provided the information that’s needed for a proper assessment and what information there is available clearly points the finger at their well site.

With plans afoot for a massive increase in oil extraction at this site, this matter needs to be thoroughly investigated before any decisions are made. We support calls for a moratorium on drilling and on decisions on planning applications which might cause quakes. They have caused a lot of concern locally and the process of analysing the cause of the quakes has been far from transparent”.


[1] Magnitude 2.4

Felt in the Charlwood area – residents report a large bang and it being “scary”

[2] Further Potential for Earthquakes from Oil Exploration in the Weald, by Dr Andrew Cavanagh, Dr Stuart Gilfillan and Professor Stuart Haszeldine, School of GeoSciences, The University of Edinburgh, 5 Feb 2019

Submitted to Surrey County Council with reference to the current planning application.

[3] The Weald Action Group is an umbrella organisation for community groups across the South East opposing extreme oil and gas extraction.

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