Oil production has been setting records in Colorado, but according to a new analysis from Bloomberg New Energy Finance (BNEF), the state’s drillers may soon be facing “a rough patch.”

The Bloomberg study analyzed break-even prices in 10 western shale oil basins and found Colorado’s Denver-Julesburg Basin, centered over Weld County, one of the most expensive. It found that the lowest average break-even point, for developing and operating a well, was in the Midland Basin, in Texas-$37 a barrel.

The average break-even point for the DJ Basin was $63 a barrel. Only Oklahoma’s Anadarko Basin was more expensive at $66 a barrel.

“The analysis shows Texas and North Dakota have some of the lowest break-evens in the market,” BNEF said. “In Texas, the Permian and Eagle Ford are the most economic, followed by the Bakken basin in North Dakota.”

The Anadarko basin in Oklahoma and the DJ Basin had break-evens closest to current prices, the analysis noted. “A retreat in prices would probably cause a slowdown in drilling there,” BNEF said.

The spot price for Western Texas Intermediate (WTI) on June 4 was $64.76.

To be sure, estimates on break-even vary widely. In a survey this spring by the Federal Reserve Bank of Dallas, oil companies operating in the Permian Basin, which includes the Midland, said they need $47 to $52 a barrel to break even, though smaller operators said they needed more than $70.

An analysis by Rystad Energy, an industry energy consultant, put the break-even price for the Niobrara, the shale formation that is key to DJ Basin operations, as low as $34 a barrel.

“Drillers have also gotten more efficient, extracting more oil from wells for less money. As a result, more wells are economic, or ‘in the money,’ even though crude futures haven’t reached $75 a barrel since 2014,” Bloomberg said.

In April, drillers were pumping a record 586,777 barrels a day from Colorado’s Niobrara, according to state data.

There are, however, factors in addition to price that may present a challenge for continued oil production, such as increased output from OPEC and increasing the output of wells in the early years, Bloomberg said.

“BNEF foresees challenges for U.S. producers,” the analysis said. “In the short term, OPEC may put more oil on the market, which would lower prices and cut into U.S. profits. In addition, the next drilling will be farther from “sweet spots,” or prime acreage, leading to lower IP rates and total output.”

Drillers have also been using more proppants recently to boost initial production, BNEF said. Proppants are sand or ceramics pumped into source rock to improve hydrocarbon flow.

While this technique raises initial flow rates and helps cut costs, it also depletes the source rock faster, according to S&P Global.

“A combination of steep declines from wells using proppants, more drilling in less-prime areas, and increased global supply from OPEC may lead to a rough patch for U.S. producers, especially in the Anadarko and Denver Julesburg basins,” BNEF said.