Heritage, which has headquarters in London, said Wednesday there were "temporary factors" constricting oil production, though gross production rates from its license area in Nigeria returned to more than 35,000 barrels of oil per day.
The company said work was under way to optimize the field's production. New wells planned for 2014 should raise the production potential to 300,000 bpd and infrastructure is in place to handle 395,000 bpd.
Heritage Chief Executive Officer Tony Buckingham said he was "delighted" with the level of production from Nigeria, which is a member of the Organization of Petroleum Exporting Countries. Work in Nigeria was progressing as planned, he said.
OPEC said in its June oil market report the level of oil produced from Nigeria was declining. Nigeria produced 1.9 million bpd, down from the 1.92 million bpd produced in April.
Nigerian Petroleum Minister Diezani Alison-Madueke in May blamed saboteurs and vandals for the loss of about 150,000 barrels of oil per day.