The declaration came after the company observed a "system anomaly" during a routine check of its loading facility on 14 July.
"We are working to ensure loading activities at the facility return to normal. We cannot speculate on any timeline for repairs," the spokesman said. "Qua Iboe Terminal is operating and production activities continue."
Nigeria has struggled to maintain its crude oil production following a spate of militant attacks and technical problems that in May pushed production briefly to 30-year lows.
