The explosion on Saturday sparked a huge fire at the site, where some 2,000 barrels of oil were being stored in a tank.
"An explosion happened at an oil collection and export site in east Obangue, under the Dinonga-Irondou licence, operated by the company Addax Petroleum Oil & Gas Gabon," petrol and hydrocarbons minister Etienne Dieudonne Ngoubou said in a statement.
The fire is being put out and authorities were investigating the cause of the incident, the statement added.
Two more people suffered less serious burns, the statement added.
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Obangue has been at the heart of a years-long standoff between Gabon's government and Sinopec subsidiary Addax Petroleum, which was barred from operating in December 2012 for failing to meet tax, customs and environmental obligations.
For a while the public Gabon Oil Company was the only company licensed to operate in the oil rich region, but the Sinopec subsidiary was allowed to restart operations in 2014.