Energean said it had discovered 8 billion cubic metre (bcm) natural gas in its Athena exploration well which lies between its Karish and Tanin fields.
The figure however falls short of the company's estimates of a 21 bcm resource, which it included in an investor presentation last month.
Energean shares traded in London were up 4% by 0821 GMT.
Athena could be developed by linking it to the floating production and storage vessel (FPSO) Energean plans to use to send gas into the Israeli market from Karish starting in the third quarter, the company said.
