The company said an audit conducted by Gustavson Associates, a geology consultant in the United States, placed the reserve estimate of the prospect at 422 million barrels of oil.
Rosemary Johnson-Sabine, vice president of exploration for the company, said Tethys was "very pleased" with the assessment. Her company said exploratory drilling in the Klymene prospect is scheduled for later this year.
The Energy Information Administration, the U.S. Energy Department's statistical arm, said Kazakhstan is estimated to hold 30 billion barrels of proven oil reserves.
Tethys said it is focused on exploiting the oil and gas potential in the Caspian region, a "highly prolific" area that the company believes has "significant potential."