BEULAH, N.D. (AP) -- Some people in western North Dakota say it's easier to strike it rich from oil than win the lottery.

So-called "overnight millionaires" are multiplying thanks to the Bakken shale formation, a rich deposit that the U.S. Geological Survey calls the largest continuous oil accumulation it has ever assessed.

The number of North Dakota residents reporting incomes exceeding a million dollars rose from 266 in 2005 to 388 in 2006. Last year's numbers won't be known until October. Up to a-third of the population in one county could be millionaires in the next few years because of oil royalty payments.

North Dakota is on pace to set a state oil-production record this year, as a record number of drill rigs pierce the prairie. There are nearly 4,000 active oil wells.