Gas production in the lower Tertiary Wasatch Formation and Upper Cretaceous Mesaverde Group, Piceance basin, Colorado, is controlled by a network of open and partly mineralized natural 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 coarsely crystalline calcite and locally with quartz, barite, and dickite. These minerals cut detrital grains, authigenic cements, and secondary pores, indicating that fracture mineralization occurred during later stages of diagenesis. Isotopic compositions for fr cture-fill calcite in the Wasatch vary from -5.0 ^pmil to -11.6 ^pmil for ^dgr¹³C and from -9.5 ^pmil to -14.9 ^pmil for ^dgr¹⁸0. In the Mesaverde, calcite ranges from -0.7 ^pmil to -10.4 ^pmil for ^dgr¹³C and from -13.3 ^pmil to -17.7 ^pmil for ^dgr¹⁸0. These isotopic data indicate that fractures were mineralized during burial by fluids of meteoric origin, with temperatures that remained fairly constant, or by fluids that circulated at a rate that

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prohibited significant cooling. The wide range in ^dgr¹³C compositions reflects mixtures of organically derived carbon and dissolved marine carbonate. In reservoir rocks that are extensively fractured, gas generated in situ from carbonaceous and coaly shales and tongues of lacustrine rock may have migrated locally along open faults and fractures.

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