Prices would have to fall lower, and stay low for a while, to turn the U.S. oil boom into a bust. Wells that are already producing won't be shut off and enormous projects with long-time horizons will still be built. Many drillers have funded next year's drilling plans by selling oil in the futures market.

Still, a $20 drop in the price of oil means $170 million less in revenue every day for the U.S. oil industry.

Investors are less willing to take on the risk of funding new expansion without hopes for a big reward and oil companies big and small are left with less money to go and drill the next well.

BP, Chevron and Shell told investors last week they would reduce spending on new development because of lower prices.

Mike McDonald, co-owner of Triad Energy, which usually operates 1 or 2 rigs in Oklahoma, says that low prices have stung and now he's not planning to get another rig going after current projects are complete.

Drilling in fields that aren't very prolific will stop because it won't be profitable. For example, drillers in North Dakota's Burke County need $81 a barrel on average to break even, according to the Department of Mineral Resources, while the price is just $28 in McKenzie County, the state's top oil producing county.