Flipping through field notebooks…
ABSTRACT The Eagle basin occupies a part of the Maroon trough that began to develop in Early Pennsylvanian time. The trough extended across northwestern Colorado and was bounded by the positive elements of the ancestral Front Range uplift on the northeast and the Uncompahgre uplift on the southwest. The early sediments are clastics, carbonates, and evaporites deposited during cyclic marine transgressions and regressions in the narrow trough. The evaporites are of the basin-center type according to Sloss (1958), but are 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 clastic sediments to the trough during the regressions; these sediments are interbedded with the evaporites. The coarse clastics from the ancestral Front Range and Uncompahgre uplift continued to influence strongly the sedimentary types of the Maroon basin throughout Triassic and Jurassic deposition, the predominant type being redbeds. In Cretaceous time, the area was inundated by a sea in which the sediments being deposited were black shale, marine sandstones, bars, and beach deposits. During Late Cretaceous time, the entire basin became a shallow sea with paludal and lagoonal sediments, including coal, shale, and underclay that were interbedded with shoreline sandstones in response to fluctuations of the sea. Some of the older structural features, such as the Douglas Creek arch, as well as the Uncompahgre uplift, began to influence sedimentation during Late Cretaceous time, as evidenced by thinning in the Mancos Shale over the arch. After prolonged sedimentation, the Eagle basin was uplifted at the end of Mesozoic time, and folded and faulted during the Laramide orogeny. The Sawatch Range, White River uplift, and the Uinta Mountains came into being in successive stages of the orogeny, and separated the Maroon basin into the present tectonic basins, namely, the Piceance basin, Carbondale basin, Axial basin, Coyote basin, and 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. The types of sediments have been very important in determining the reservoirs for the accumulation of oil and gas after the Laramide orogeny. Virtually all of the oil and gas in the Paleozoic in the Eagle basin has been produced from the Weber Sandstone. It is a facies having some permeability and effective porosity that was, at the same time, sufficiently brittle to fracture. The intergranular porosity and permeability alone in this formation are insufficient in many of the fields to support commercial rates of production. Thus, in addition to the types of sediments, folding and fracturing have been all-important in establishing substantial reserves in the Paleozoic reservoirs. The same control of accumulations by fracturing and sedimentary type applies to Cretaceous and Tertiary reservoirs in the Piceance basin, though folding was not a prime prerequisite to accumulation. All important Cretaceous and Tertiary oil and gas fields are related to the local improvement of porosity and permeability greater than that normal in the sediments encasing the reservoirs. In contrast to the normal relationships of gas, oil, and water found in the Paleozoic reservoirs, many of these relationships are reversed in the Cretaceous and Tertiary reservoirs. The relationships are similar to those in the San Juan basin of New Mexico and may have a similar explanation. More realistically, the explanation may be the indigenous nature of the oil and gas, where the only migration has been to local reservoirs in juxtaposition with the source beds.
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