At the end of May this year Gloucester's reserves were 28.5Mt, an increase over 21.9Mt at the end of 2006.
The company said the reserves had an average strip ratio of less than 3.5:1 and include 8.1Mt at Duralie and 2.8Mt at Stratford.
At Stratford South resources have been estimated for the first time in the Parkers Road seam at 1.6Mt of indicated resources and 1Mt inferred resources each in the Rombo and Avon Lower/Triple seams.
At Roseville West indicated resources in the Roseville and Cloverdale seams are estimated at 11.8Mt, plus 9Mt inferred resources in the Deards, Bindaoo and Marker 7 seams.
Gloucester chief executive Rob Lord said the estimates highlighted the potential of the company's identified mining areas and the longevity of its operations.
“I am particularly pleased to see the significant increase in resources of lower sulfur coking coal from the Stratford mine area, which we will be able to blend with coal from the Duralie operations to develop a more valuable product," he said.
Lord pointed out today's announcement did not include the discovery of a new volume of coking coal in the Clareval seam at Stratford South, announced in May this year.
Gloucester will continue drilling at Stratford with an update planned for later this year.
Gloucester was trading mid-morning at $A11.48.