Origin and Distribution of Fractures in Lower Tertiary and Upper Cretaceous Rocks, Piceance Basin, Colorado, and Their Relation to the Occurrence of Hydrocarbons
Published:January 01, 1986
Janet K. Pitman, Eve S. Sprunt, 1986. "Origin and Distribution of Fractures in Lower Tertiary and Upper Cretaceous Rocks, Piceance Basin, Colorado, and Their Relation to the Occurrence of Hydrocarbons", Geology of Tight Gas Reservoirs, Charles W. Spencer, Richard E. Mast
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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 δI3C 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. δI8O 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.
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Geology of Tight Gas Reservoirs
Tight gas reservoirs occur in low-permeability, gas-bearing formations that are present to some extent in all gas-producing basins worldwide. This is the first volume to bring together data on tight reservoirs for a variety of basins and different geologic settings. The papers in this volume discuss characteristics of some of the most significant tight gas areas in the United States; however, these data are equally applicable to many other recognized and unrecognized tight gas provinces in other nations. In general, tight reservoirs in the United States are grouped into tight gas sandstones and eastern Devonian shales. The Devonian shale sequences are dominantly marine shale but include some siltstone and sandstone. Tight gas sandstone formations of other than Devonian age are present throughout the United States and consist primarily of fluvial and marine sandstones and siltstones. In addition, gas also occurs in low-permeability marine carbonate reservoirs. The 14 papers in this volume cover such topics as: coal-bed methane and tight gas sands interrelationships; gas-bearing shales in the Appalachian basin; exploration and development of hydrocarbons from low-permeability chalks; and geologic characterization of low-permeability gas reservoirs.