Oil production from shale basins in North Dakota and Texas are the primary reason for a decline in U.S. oil imports, data published Thursday show.

Bentek Energy, the analytical division of Platts, said July oil production from the Bakken area in North Dakota and the Eagle Ford shale play in Texas increased 3.4 percent year-on-year, or more than 86,000 barrels per day.

"Oil production gains from the Bakken and Eagle Ford shale formations are a major reason why U.S. imports of crude oil have dropped to levels not seen since the mid-1990s," Benteck Director of Energy Analysis Jack Weixel said in a statement Thursday.

The Energy Information Administration, the statistical arm of the Energy Department, said in its monthly short-term market report total U.S. crude oil production reached 8.5 million barrels per day in July, the highest monthly level since April 1987.

The rise in domestic oil production in turn means lower imports of foreign crude oil. EIA said imports nearly halved from 2005 to average 33 percent of the market share in 2013.

Texas and North Dakota are the No. 1 and No. 2 oil producers in the nation, respectively. Combined, they produced 120 million barrels of oil in April, the last full month for which data are available from EIA.