Abstract Gas production in the lower Tertiary Wasatch Formation and Upper Cretaceous Mesaverde Group in the Piceance basin, Colorado, is controlled principally by a network of open and partly mineralized natural fractures. The Piceance Creek field, situated on the Piceance Creek anticline, and the Rulison and Divide Creek fields all have extensive fractures. These fractures formed in response to high pore-fluid pressures that developed during hydrocarbon generation and to widespread tectonic stress associated with periods of uplift and erosion that occurred during the late Tertiary. Sandstone beds commonly contain vertical extension fractures that are cemented with fine- to coarse-crystalline calcite and locally with quartz, barite, and dickite. These fracture-fill minerals cut detrital grains, and authigenic mineral cements indicating that fracture development and mineralization occurred during the later stages of diagenesis. The δ I3 C compositions for calcite vary over a wide range (from — 5.0 to — 11.6‰ for the Wasatch and from — 0.7 to — 10.4‰ for the Mesaverde) and may reflect the original isotopic composition of matrix carbonate that was present in nearby sandstone beds. δ I8 O values for fracture-fill calcite generally are light, ranging from — 9.5 to — 14.9‰ for the Wasatch and from — 13.3 to — 17.7‰ for the Mesaverde. Most gas encountered in Tertiary and Cretaceous rocks was generated in situ from interbedded carbonaceous and coaly shales and tongues of organic-rich lacustrine rock. In areas that are extensively fractured, gas may comprise a mixture from different sources due to migration along open faults and fractures.