Nigeria is among the top oil exporters in the region. The U.S. Energy Information Agency had reported recently that crude oil deliveries from Nigeria for July were down about 500,000 barrels compared to the same month last year.
Flooding in states in the oil-rich Niger Delta, meanwhile, prompted Shell and French supermajor Total to hold back on exports from Nigeria in October.
An unnamed spokesman from the Nigerian Department of Petroleum Resources was quoted by the Platts news service as saying exploration work in the country "started dropping a long time ago."
Oil reserves estimated during the third quarter, the spokesman said, declined from 37.1 billion barrels from last year to 36.5 billion barrels.
Watchdog group Global Witness, meanwhile, said it had evidence that oil majors working in the country may have bribed former government officials tied to the oil industry. Shell and Italian energy company ENI, two of the companies named by the watchdog, denied any wrongdoing.
Nigeria relies on oil for about 75 percent of its revenues.