United States Geological Survey Bulletin 1787-E

From abstract: The Piceance basin of western Colorado contains large reserves of natural gas in low-permeability reservoirs of the Late Cretaceous-age Mesaverde Formation or Mesaverde Group. The gas accumulation can be divided into three general zones: a zone of surface-water invasion that extends\ud inward a few miles from present outcrops, a gas-and-waterbearing zone that extends 10-20 miles inward from the water-bearing zone, and a central, predominantly gasbearing zone