A new study has found no clear links between the recent swarm of earth tremors in Surrey and local oil drilling activity.
According to the paper, there have been 129 seismic events centred between the villages of Newdigate and Charlwood since 1 April 2018.
But based on the available evidence, the authors concluded it was unlikely that nearby industrial activities had induced the swarm.
The study, submitted to the journal, Seismological Research Letters, uses recorded seismic data and operational information provided by operators at oil exploration sites at Horse Hill, near Gatwick Airport, and Brockham, near Dorking.
Most of these tremors were recorded on a network of temporary seismometer stations installed in the area in July and August 2018 after the first nine seismic events.
The largest tremor in the swarm had a magnitude of 3.1ML. 31 events exceeded 0.5ML but only 11 were felt, based on reports recorded by the British Geological Survey (BGS). The most recent tremor reported by the BGS was on 17 May 2019.
UK Oil & Gas, the main investor in the Horse Hill site, and Angus Energy, the operator at Brockham, have repeatedly said their activities did not cause the tremors.
The paper, which has not yet been peer-reviewed, has been posted online and lead author, seismologist, Dr Stephen Hicks, of Imperial College, welcomes scientific feedback. The final published version may change.
The authors said the average depth of the tremors was 2.3km, below the target formations at Horse Hill. The well, Horse Hill-1 (HH-1) is exploring the Portland sandstone at a depth of 600m and the Kimmeridge shale at 800m, they said.
If the tremors had been induced it was a novel mechanism that has not been documented in scientific literature, Dr Hicks told BBC Radio.
He said the tremors, based on data available at present, were likely to be part of the background seismicity of the UK. Ancient faults could be stressed by tectonic forces in the Alps and Mediterranean, he said. This could cause seismic slip to be released along faults running east-west in the Weald.
The study’s conclusions were based on six key points:
The authors said the sequence of seismic events started before subsurface activity and flow testing/production began at HH-1 in 2018
The earth tremors were 3km from the nearest oilfield operations at HH-1 and deeper than the target formations. This would be an unusual location for induced seismicity caused by the current scale of operations, the authors said. They found no clusters of seismicity immediately close to the drill site.
- Oil activities
Based on operational logs provided by the operators, the authors said they found no clear link between the seismicity rate and cumulative oil production or activities at either HH-1 or Brockham. Some events happened when the HH-1 well was shut-in, they said. But if this were a factor, any correlation was not yet robust and such a stress transfer mechanism was unclear, Dr Hicks said.
The authors did not find any abnormal faulting mechanisms that had been previously observed for production-induced seismicity, such as at Groningen in the Netherlands. The observations were consistent with reactivation of a pre-existing fault, they said.
- Volume and stress
The cumulative volumes of net production reported by the operators were much smaller than past reported cases of extraction-induced seismicity, the authors said.
- Fluid pathways
There were no obvious connections between Horse Hill and Newdigate Faults, which could plausibly offer a permeability pathway from HH-1 to the earthquakes, the paper concluded. The borehole does not directly intersect the Horse Hill fault, it said.
The study recommended seismic monitoring should be carried out close to hydrocarbon development and production sites. Operational activities, such as well shut-in periods, and production volumes and rates should be reported to reduce uncertainties for similar cases in future, it added.