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Eight years after discovery of Redwash, Tertiary oil is being produced in significant amounts from only this one field. Lack of a dewaxing plant for high pour-point oil has discouraged prospecting the shallow south flank of the basin for the smaller Green River strat-traps. Standard of California Company markets Redwash oil through the Rangely pipeline, which can accept limited amounts of waxy crude. As a result, an impasse has existed: lack of market discourages exploration, and yet a large additional supply of oil is needed before a wax plant is feasible. The current gas exploration is a development which could help to end this impasse. Two near-commercial oil discoveries have been made already by wells scheduled as deeper tests to the Wasatch and Mesa Verde.
The 18-mile Wasatch gas trend may extend west or
southwest and overlap a large area underlain by shallower Green River sand lenses. Gas, apparently commercial, has been found in the Ute Trail area, which includes the Ouray, Bitter Creek, Chapita Wells, and the Southman Canyon discoveries.
Gas occurs in lenticular sands near the middle of the red, drab and gray Wasatch shale section, at depths ranging from 5,000 to 6,500 feet. Porosity averages 12% and permeability less than 1 millidarcy. Some sand bodies are continuous for four or five miles. Two or three sands 20 feet thick are generally encountered in each well.
Paleogeographic maps and cross-sections show the gas sands to lie on the north edge of a wedge of deltaic clastics brought into the basin from the south, and possibly east, and deposited in a shallow lake. This interpretation differs from published descriptions of the Wasatch as "dominantly fluviatile." Orientation of sand lenses, sorting of sand grains, and relatively great lateral extent of the sand bodies supports this concept.
Trapping is by isolation of the sand bodies and by permeability barriers between lenses of porosity within a sand zone. The trend is limited on the south by a coalescence and thickening of sand bodies, which eliminates the barriers and permits the escape of gas. Hydro-dynamic traps may be present here as well. The north edge is limited by poorer sorting of the sand depsoited in deeper water. Shales of the deeper water are generally dolomitic, black, bituminous and somewhat fossiliferous, and dark gray-green or drab with subordinate purplish "relict" red colors.
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