Opened up to flow on 5 February, the Rawicz-12 appraisal well has been flowing at a rate of up to 4.1 million standard cubic feet (mmscf) a day.
The company said that no significant amounts of water have been encountered and gas quality matches expectations.
During the long high-rate flow period, the company identified continuous increase in well productivity, gas production rate and surface flowing pressure.
The flow testing of the appraisal well is planned to be completed on 28 February 2015, following which the company will gather pressure build-up data to determine reservoir characteristics.
San Leon executive chairman Oisin Fanning said: "This is the well we have been waiting for – a significant gas discovery in one of the highest-priced gas markets in Europe.
"The Rawicz field has existing gas infrastructure nearby and a development feasibility project has already been completed."
The company is planning to drill another three to five wells in order to develop the Rawicz field.
Palomar Natural Resources operates the Rawicz project with a 65% stake and San Leon holds a 35% interest.
San Leon chief operating officer Joel Price said: "The data quality is high, as downhole shut in is being used (improving the ability to interpret pressure build up data in a gas well), and this gives confidence in the technical interpretation so far.
"Once the full test and the final pressure build-up data have been acquired, the interpretation will be completed and will be used as input to defining the next steps for the Rawicz field."
