A recent study indicates that over 100 small earthquakes in Surrey, occurring from 2018 to 2019, may have been caused by oil extraction from a nearby well.
A recent study suggests that oil extraction from a nearby well might have caused over 100 small earthquakes in Surrey during 2018 and 2019, according to researchers from UCL.
The tremors, which took place in Newdigate and nearby areas from April 2018 to early 2019, were recorded between magnitudes of 1.34 and 3.18. They resulted in damage such as cracked walls and ceilings, and many residents reported their homes and beds shaking.
Geologists have been uncertain about whether these earthquakes could be linked to activities at the Horse Hill oil well, located approximately 5 to 10 km away in Horley.
Some researchers previously believed that the earthquake patterns did not correspond with oil extraction activities, noting that increases in extraction did not always coincide with spikes in seismic events.
The new study, published in Geological Magazine, involved running over a million simulations to estimate the likelihood of earthquakes based on the timing and volume of oil being extracted. The findings indicated that the predicted earthquake frequency aligned closely with actual events, hinting at a relationship between oil extraction and the earthquakes.
Dr. Matthew Fox, the lead author from UCL Earth Sciences, commented, “Our findings suggest there may be a connection between the earthquakes and the oil extraction at Horse Hill, but we cannot entirely dismiss the possibility that this correlation is by chance. Further research is needed to clarify whether this is a case of cause and effect. Nevertheless, our results make it reasonable to consider that oil extraction could have instigated the earthquakes.”
The oil extraction operations at Horse Hill ceased in October following the annulment of planning permission due to a significant legal ruling. Judges from the Supreme Court determined that the complete climate effects of the project should have been evaluated before Surrey County Council granted approval.
In contrast, another project has been authorized by the council to conduct exploratory oil drilling in Balcombe, West Sussex. However, this is facing legal opposition from a local residents’ group, with hearings expected in the Court of Appeal this month [January 2025].*
The extraction of oil alters the pressure within the rocks from which the oil is taken. This pressure shift can move through hundreds of kilometers underground and may interact with faults (fractures where two blocks of rock meet). The rate of pressure change is influenced by the rock’s permeability (the speed at which fluids can flow through it). Since faults typically experience significant stress, even minor changes can trigger the swift movement of rock that results in an earthquake.
Using a mathematical model, the researchers assessed potential earthquake occurrences based on the volume of oil extracted, while considering two different types of rocks involved.
Due to the lack of certain vital information—such as the delay between oil extraction and the onset of earthquakes—the researchers utilized a Bayesian machine learning technique to randomly alter unknown factors like this time lag.
Building on the research of the late geologist Dr. Rob Westerway from the University of Glasgow, the team accounted for oil extraction transitioning between two rock types. Notably, one type, Portland rock, is far more permeable than Kimmeridge rock, suggesting that the time lag between extraction and subsequent seismic activity could be just days, rather than weeks.
Although the earthquakes began shortly before oil extraction commenced, the researchers posited that preparatory actions—such as pressure testing in the well—might have triggered similar pressure changes that affected the fault line.
The study’s complexity is underscored by the fact that seismometers (devices used to measure ground vibrations) were only installed after the initial larger earthquakes had taken place. Consequently, any smaller earthquakes that may have occurred before the seismometers were set up went undetected. This scenario highlights the importance of comprehensive seismic monitoring prior to commencing oil extraction at potential sites.
Co-author Professor Philip Meredith from UCL Earth Sciences noted, “Our research underscores the necessity of monitoring seismic activity in regions where oil extraction may happen before any work is initiated. It is essential to proceed with caution. It’s unwise to assume there are no problems when there potentially could be.”
“Surrey has not witnessed significant seismic activity for decades, making these earthquakes unusual. However, rare events can occur in nature, meaning we cannot completely eliminate the chance that the timing with oil extraction was coincidental.”
*Lead author Dr. Matthew Fox has testified as an expert witness in the ongoing legal case.
