In the southern Piceance Basin, gas generation resulting from high heat flow caused high overpressures that produced a pervasive fracture system. Gas migrated vertically up this fracture system and all the sands within the continuous gas interval were highly gas charged. Additional charge from the deeper Mancos occurred along fault zones, resulting in elevated top of gas. This system was less effective in the northern Piceance, where the heat flow was lower and the coals are thinner. Sands intersecting faults receive high gas charge, but the other sands receive less gas charge resulting in higher water production.