Regal said reserves were lower than expected from its Mekhediviska-Golotvshinska and Svyrydivske gas and condensate fields in Ukraine.
A third party stated there was a "material reduction" in proved and proved-plus-probable reserves at the natural gas fields.
Regal, a company listed on the London stock exchange, said last month that production from fields in Ukraine for the first two months of the year averaged 7.9 million cubic feet per day of natural gas.
Kiev is struggling to improve its image as a natural gas transit area following disagreements with Russian gas monopoly Gazprom. Ukraine hasn't been able to secure financial support to revamp its gas transit system but is reviewing gas deposits, particularly shale gas reserves, in the interim.
Shell in January signed a $10 billion contract to explore Ukrainian shale. Ukrainian Prime Minister Mykola Azarov said there may be enough natural gas in shale plays to meet the country's needs without imports.