Shares in Union Jack Oil fell more than 20% on the publication of an assessment of the Wressle oil field in North Lincolnshire this morning.

The study, commissioned by the company from Gaffney Cline, saw potential for an extra 2.4m barrels of potentially recoverable resources, based on its analysis of a part of the reservoir currently producing oil and gas.

It also indicated further hydrocarbon potential in an undeveloped reservoir at Wressle.

But it pointed to uncertainties about reservoir size and flow rates.

The report estimated a constrained plateau production rate of 800 barrels of oil per day for approximately five years.

Earlier this year, the site operator, Egdon Resources, said Wressle production could top 1,500 barrels per day.

The industry regulator has not yet published any formal production data for Wressle.

If the Gaffney-Cline production estimate were realised, Wressle would rank as the fifth highest UK onshore oil producer, after Wytch Farm, Welton, Singleton and Stockbridge.

Oil was discovered at Wressle in 2014 from three reservoirs: the Penistone Flags, Wingfield Flags and Ashover Grit.

The Gaffney-Cline report’s best estimated of oil reserve in the Ashover Grit and Wingfield Flag was 670,000 barrels.

It said there was “potentially significant increased estimate of stock tank oil initially in place of 10.12 million barrels of oil and a recoverable resource of 2.43 million barrels” from the Ashover Grift speculative deeper oil-water contact.

But it said not all the estimated volume may be recoverable from the Wressle-1 location. The trapping mechanism and spill-point were uncertain, it said.

There was also uncertainty about extent and size of the Ashover Grit trap. There was also “significant uncertainty” on the initial flowrate from the Penistone Flags reservoir.

The report said there was hydrocarbon potential in the Santon Sandstone. This formation was not tested in the Wressle-1 well. Further data was required from future drilling before any development could be planned, the report said.

The current field development plan for Wressle envisages continued production from the Ashover Grit and Wingfield Flags, with development of the Penistone Flags from two or three additional wells.

Union Jack’s executive chairman, David Bramhill, described the content of the report as “highly encouraging for the future of the Wressle development”:

“GaffneyCline’s findings, observations and technical work in respect of the potential upside in respect of the Ashover Grit reservoir have gone a long way in answering our questions regarding future potential upside and prolonged productivity.”

At the time of writing, the Union Jack share price was down nearly 16% at 42.88p.