Unconventional gas explorer Cuadrilla Resources on Wednesday said shale gas exploration work probably triggered minor earthquakes at its drill site near Blackpool in northwest England earlier this year.

It is highly probable that the hydraulic fracturing of Cuadrilla's Preese Hall-1 well did trigger a number of minor seismic events, a report commissioned by the company said.

The tremors were triggered by pumping vast quantities of water at high pressure 3 kilometres underground through drill holes in a process known as hydraulic fracturing, or fracking, which is designed to prop open shale rocks and release trapped gas, the company said.

It said the region's unusual geology contributed to tremors in April and May of 2.3 and 1.5 on the Richter scale, respectively.

But adds there is little risk of tremors caused by exploration work in future.

In a statement, Cuadrilla says this combination of geological factors is extremely rare and would be unlikely to occur together at future drilling sites.

Provided that assumption proved false, it capped the magnitude of any future tremors at around 3 on the Richter scale as a worst-case scenario.

Cuadrilla's water injection operations take place very far below the earth's surface which significantly reduces the likelihood of a seismic event of less than 3 on the Richter scale having any impact at all on the surface, the company said.

Responding to findings, activist groups called for a moratorium on shale gas exploration in Britain until the environmental risks have been properly assessed.

These findings are worrying, and are likely to add to the very real concerns that people have about fracking and shale gas, Nick Molho, head of energy policy at WWF-UK said.

The Lichfield-based company has valued the amount of shale gas in place -- not the recoverable volume -- at its site near Blackpool at 200 trillion cubic feet, the top end of what is estimated can be recovered from Europe's largest reserves in Poland.

(Reporting by Oleg Vukmanovic; editing by Keiron Henderson)