The Greeley Tribune reports that more than 89 percent of the state's production this year is coming from Weld. That's up from 85 percent last year.
Analysts say operators are getting more oil from every well by drilling the best parts of the basin. They're also using improved techniques for well fracturing, or fracking.
The Colorado Oil & Gas Conservation Commission says that for the first half of 2015, Weld oil production averaged 8.7 million barrels per month.
Statewide oil production for 2015 so far is at 79.46 million barrels.
Rio Blanco is the second-largest oil county in Colorado with 2015 production of 2.6 million barrels.