The Japanese company revealed the Minami-Kuwayama-3 well had hit a 24-metre oil column at a depth of roughly 3900 metres.
Wireline logs and samples confirmed the well would be capable of producing oil.
The Minami-Kuwayama field lies about 20 kilometres south-east of the city of Niigata and has produced roughly 1 million barrels of oil since 2004.
Inpex is carrying out drilling at the field in the hope of tripling production which currently averages between 300 and 380 barrels per day.
