North Dakota is now producing 1 million barrels of oil per day, with a large part of the credit due to the rich Bakken shale formation in the western part of the state.

The state's Department of Mineral Resources on Tuesday released the April production numbers that show the oil fields' record level. In March, the state produced 977,000 barrels per day.

The director of the department, Lynn Helms, says only Texas, Alaska and California have also reached a million-barrel-a-day production.

The Bakken and the Three Forks formation below it account for the vast majority of North Dakota's oil production. Experts knew the Bakken held millions of barrels of crude, but it wasn't until oil prices reached record levels that the technology was developed to be able to exploit it.