Cairn Lanka bought the first of three oil exploration blocks off the coast of Mannar.
"Further drilling will be required to establish the commerciality of this discovery," Cairn Lanka said in a statement.
Cairn said its data indicated the presence of a 24 meter hydrocarbon column between the depth of 3048 to 3038 meters.
The data indicated it to be "predominantly gas bearing with some additional liquid hydrocarbon potential," the firm said.
Sri Lanka's President Mahinda Rajapaksa announced the find earlier in the day at a speech in Kandy in central Sri Lanka.
Sri Lanka first started looking for oil in the 1970s.