London — UK shale gas pioneer Cuadrilla Resources has temporarily halted hydraulic fracturing -- or fracking -- at its flagship Preston New Road shale gas site in northwest England after the work triggered an earth tremor late Wednesday.

Under the UK's "traffic light" regulatory system, fracking must be halted if the work causes earth tremors of morethan 0.5 on the Richter scale.

Cuadrilla -- which only resumed fracking at Preston New Road on August 15 -- and other shale gas operators including Ineos have complained about the restrictions on fracking, saying the regulations are too strict and are hampering investment in the sector.

"We can confirm that a micro seismic event measuring 1.55 on the Richter scale occurred after we had completed thehydraulic fracturing program for the day at our Preston New Road site," a company spokesman said.

"Well integrity has been verified and we will now pause operations and continue monitoring for the next 18 hours,"he said.

'LARGE BAG OF SHOPPING'

The spokesman added that most local people would not have felt the tremor due to its "small size," comparing the equivalent ground motion to "a large bag of shopping dropping to the floor."

By comparison, earth tremors caused by drilling at the giant onshore Groningen field in the Netherlands have prompted the Dutch government to cap production from the field.

Earthquakes have damaged buildings in the Groningen region, with the latest tremor from May this year registering at 3.4 on the Richter scale.

The Cuadrilla spokesman said that "minor" movements of the level triggered Wednesday were "to be expected and are way below anything that can cause harm or damage to anyone or their property."

"All the relevant regulators were informed and we have verified that the well integrity is intact," he said.

Cuadrilla has said the fracking work would be completed by the end of November followed by flow tests -- with the first results expected early in 2020 -- as Cuadrilla looks to prove the commerciality of the site.

NEW RESEARCH

The UK shale gas industry remains in its infancy and was dealt a blow this week when new research suggested the country's recoverable shale gas reserves could be much lower than thought.

According to the research from Nottingham University published Tuesday, innovative new testing techniques have revealed that resources within the Bowland Shale formation -- the country's most prospective shale gas area -- could be up to five times lower than previous estimates suggested.

The research, supported by the British Geological Survey (BGS), said economically recoverable reserves of Bowland shale gas could be less than 10 years of current UK gas consumption -- implying a ceiling of around 800 Bcm.

In 2013, the BGS estimated that the Bowland Shale alone could hold up to 1,329 Tcf (37.6 trillion cu m) of gas in place, which with a recovery rate of 10% would mean recoverable reserves of as much as 3.8 Tcm, enough to meet the UK's current gas demand for almost 50 years.

Cuadrilla said in response that the only way to determine the capacity of UK shale reserves was to drill, frack and test the flow rate of gas from real world wells.

-- Stuart Elliott, Stuart.Elliott@spglobal.com

-- Edited by Alisdair Bowles, alisdair.bowles@spglobal.com