Dear BOW Followers,

According to the British Geological Survey (BGS), there had been no earthquakes in Surrey in the last 50 years; but since April 1 2018, a series of seven tremors (including three of magnitude well above 2 ML and one of magnitude 3.1 ML) has surprised and worried residents around Newdigate and the wider area between Dorking and Crawley. This is at a time of increased oil and gas activity in the county, and many people are making a connection between the two.

We think that serious questions should be asked and investigated, especially in connection with the two oil and gas sites nearest to the epicentres: Brockham and Horse Hill.

In a statement issued following the earthquakes, the BGS say that they are unable to categorically say if these earthquakes are related to hydrocarbon operations though they do not rule out that possibility.

The statement also says that, it is well known that hydrocarbon exploration and production can result in man-made or “induced” earthquakes” and that “such events usually result from either long term hydrocarbon extraction, or the injection of fluids (e.g. hydraulic fracturing) during production.”  The announcement mentions that it seems unlikely that any flow testing at the Horse Hill site would result in induced seismicity.


We have written to the BGS to point out that what the statement fails to mention is that there is a re-injection well at Brockham, where produced water had been re-injected for years from operations at the Brockham and Lidsey sites. Both sites were closed for most of 2016 and 2017, but resumed production in March and February 2018 respectively, and water reinjection also restarted at Brockham in March according to the Oil and Gas Authority (OGA) website.

 Based on analysis and studies done in US, the strongest relationship to seismicity has been found where wells re-injected waste water underground for permanent disposal. Over time, pressure can start to build up on geologic faults causing them to slip. Earthquake risk can spread miles away from the disposal wells and persist for more than a decade after re-injection stops. This presents a challenge in analysing a possible link, exacerbated by a lack of reliable earthquake data due to the fact that BGS’s closest monitoring station is located more than 50km away from the estimated epicentres.

Horse Hill

With respect to the Horse Hill site, UK Oil & Gas (UKOG) announced on 27th June that they commenced “production flow test operations.” However, since then Mr. Sanderson, UKOG’s executive chairman said that “there’s been no sub-surface activity since March 2016” on the site. The OGA have confirmed this, but failed to produce any clear evidence to support it.

Given that according to expert analysis, the Horse Hill-1 well was drilled into a fault zone, and that UKOG’s Environment Agency permit allows for injection of acid and chemicals under pressure high enough to squeeze it into the pores of the rock, we think that the activities at Horse Hill should be investigated and closely monitored in order to be certain that UKOG have not triggered the earthquakes and do not trigger them in the future.

Unfortunately, the type of flow test UKOG are intending to perform will not be caught by the OGA’s requirement to monitor seismicity in real time through a “traffic light protocol”, which is only required for sites where hydraulic fracturing is proposed. Therefore, there is no oversight mechanism from the OGA to monitor induced seismicity at Horse Hill. [1]


The convergence of the Horse Hill flow test, resumed operations at Brockham and the earthquakes is at the very least puzzling and needs to be explained.

We think that there are too many unknowns and that operations at Brockham and Horse Hill should be immediately suspended. The reasons for the earthquakes should be thoroughly investigated, the likely cause established and any mitigation required put in place before the operations can continue.

There is an urgent need for monitoring equipment positioned locally to produce better data on earthquake epicentres and depths. Detailed injection information is needed from the regulators or industry to allow for analysis.

Well integrity should be tested as well to check if the earthquakes didn’t cause damage that could lead to environmental pollution.

This is critically important at a time when Surrey is facing a proliferation of applications for hydrocarbon exploration and production, including in some of its most precious areas of outstanding natural beauty.

P.S. What can you do about it? Please see here a useful list from A Voice for Leith Hill.



Twitter: @BrockhamWatch


[1] Oil & Gas Authority: Consolidated Onshore Guidance V.1, November 2017,


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