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Paleocene and Eocene lacustrine deltas are present in several Rocky Mountain intermontane basins. Deltaic deposits of the Eocene Green River Formation in the Red Wash-Raven Ridge area, northeastern Utah, have been studied intensively. Recognition of the Red Wash delta is based on distribution of (1) fluviatile red and green shale facies (Wasatch Formation), distributary and nearshore sandstone facies, and lacustrine oolitic, ostracodal limestone facies (Green River Formation); (2) geographic orientation and spatial dimensions of thick sandstone bodies projecting into the lake beds, indicating positions of entry into Lake Uinta of south-flowing streams; and (3) sedimentary structures that are in accord with a deltaic environmental interpretation (although they are present lso in other environments).
An estimated 100 million bbl of oil reserves (ultimate production) is trapped in a series of discrete sandstone bodies within the Red Wash delta. Variations in petroleum chemistry from pool to pool indicate local source beds and short-distance migration. Several billion bbl of oil in Wasatch-Green River outcrops (not including oil shale) attest to the almost incredible petroleum-generating power of Eocene lake deposits.
In the Piceance Creek basin, northwestern Colorado, Douglas Creek sandstones (basal member of the Green River Formation) were deposited at the mouth of a southwestward-flowing river, and facies associations similar to those of the Red Wash area are present. The lobate shape of sandstone deposition is not as distinctively developed as in Red Wash. An estimated 250 billion cu ft of gas is trapped at the up-dip edge of porous intervals on Piceance Creek anticline. Oil saturation is common in outcrops and is present in the subsurface although high wax content and low reservoir temperature prevent commercial production.
A similar facies association in the Wasatch-Green River section is present in the Washakie basin, southern Wyoming. A river flowing northwestward deposited thick sandstone beds that interfinger with lake beds in the vicinity of the basin axis and along the west flank of the basin. Oil and gas shows have been reported in these beds, but no commercial production has been developed.
Paleocene lakes in several basins have shoreline deposits, in part deltaic, that contain oil and gas fields and are targets for future exploration. Examples are the Fort Union Formation in the Big Piney-La Barge area, Waltman Shale lake beds in the Wind River basin, and Fort Union lake deposits in the Big Horn basin. The lobate configuration of Fort Union coarse clastics on the west flank of the Big Horn basin is very suggestive of deltaic deposits.
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