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The Piceance and Eagle basins represent the present expression of the Maroon trough or basin that started to develop in Early Pennsylvanian time. It extended across northwest Colorado and was bounded by the positive elements of the Front Range on the northeast and the Uncompahgre on the southwest. The early sediments are clastics, carbonates, and evaporites resulting from cyclic marine transgressions and regressions in the narrow trough. The evaporites are of the basin-center type, according to Sloss, but strongly affected by contemporaneous clastic deposition. The growth of the Front Range in Morrowan time and the uplift of the Uncompahgre in Desmoinesian time contributed large volumes of sediments to the trough during the regressions, which became interbedded with the e aporites.
The coarse clastics from the Front Range and Uncompahgre continued to strongly influence the sedimentary types of the Maroon basin throughout the Triassic and Jurassic deposition, with the predominant type being arkoses and redbeds. In Cretaceous, the area was inundated by the Cretaceous sea where the sedimentation was black shales, marine sands, bars and beach deposits. In Upper Cretaceous the entire basin became a shallow sea with paludal and lagoonal sediments, such as coals, shales, and underclays being interbedded with shoreline sands in response to fluctuations of the sea. Some of the older structural features, such as the Douglas arch, began to influence sedimentation during Upper Cretaceous, as evidenced by thinning in the Mancos Shale over the arch.
After prolonged sedimentation, the Maroon basin was uplifted at the end of Mesozoic time, and folded and faulted by the Laramide orogeny. The Sawatch Range, White River uplift, and the Uinta Mountains came into being in successive stages of the orogeny to separate the Maroon basin into the present tectonic basins; namely, the Piceance basin, the Eagle basin, the Axial basin, the Coyote basin, and the Sand Wash basin. Many of the present-day structural features were formed during the long period of orogeny from Late Cretaceous to early Tertiary time.
Sedimentary types have been most important in determining the reservoirs for the accumulation of oil and gas following the Laramide period of orogeny. Virtually all the oil and gas in the Paleozoic in the Maroon basin has been produced from the Weber Sandstone. It is a lithologic facies having some permeability and effective porosity, that was at the same time sufficiently brittle to fracture. The porosity and permeability alone are insufficient in many of the fields to support commercial rates of production. In addition to the sedimentary type, folding has been all important in establishing substantial reserves with respect to the Paleozoic accumulations.
The same control of the fracturing and sedimentary type applies to Cretaceous and Tertiary reservoirs in the Piceance basin, even though the role of folding is not a prime prerequisite to accumulation. All the important Cretaceous and Tertiary oil and gas fields are related to the local improvement of porosity and permeability above and beyond that normal to the marine shales encasing the reservoirs.
In contrast to the normal relationships of gas, oil, and water found in the Paleozoic reservoirs, many of the same relationships are reversed in the Cretaceous and Tertiary reservoirs. The relationships are similar to those in the San Juan basin and may have a similar explanation. More realistically, the explanation may be the indigenous nature of the oil and gas, where the migration has been limited to reservoirs in juxtaposition with the source beds.
Successful exploratory programs must be cognizant of the fact that the location of the areas of improved porosity and permeability became the primary reason for success. That success can be frustrated by inappropriate drilling and completion methods.
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