Information published by the U.S. Energy Information Agency shows 361,000 barrels of Nigerian crude oil was shipped to the U.S. market in July, down from the 810,000 barrels from the previous year.
Patrick Kulsen, an oil-market analyst at PJK International, a research company in The Netherlands, told The Wall Street Journal that other markets were filling in the gap.
"That slice that goes to the U.S. will be less and more will go to Europe and Asia," he said.
The U.S. Energy Department said an oil boom in the Bakken play in the north of the country was contributing to an increase in the use of domestic crude oil. More Libyan oil on the market, meanwhile, may be sidelining other producers in certain markets.
Nigeria, the top oil producer in Africa, has exported more than 2 million barrels per day for most of the last five years, the U.S. Energy Department reports. Oil exports account for about 95 percent of the country's trade revenue.