Birch Creek field is located on the LaBarge arch in the western Green River basin, and is a part of the Big Piney-LaBarge producing complex. Oil and gas are produced in this area from sediments ranging in age from Jurassic through Paleocene. Of concern here are the trap geometry and source of hydrocarbons of the shallower productive intervals within the Mesaverde and "Almy" Formations.

The Upper Cretaceous Mesaverde and upper Hilliard Formations together form a typical regressive sequence of, from northwest to southeast, lagoonal coal-bearing siltstone and thin sandstone, a littoral sandstone complex, and marine siltstone and shale. Uplift along the LaBarge arch during the Late Cretaceous or very early Paleocene resulted in the erosion of a portion of this sequence and an unknown thickness of Late Cretaceous rocks. A combination of the truncated edge of the upturned littoral sandstone complex in conjunction with lateral permeability changes and the gentle structure of the LaBarge arch forms the traps within the Mesaverde.

The Paleocene sequence, assigned to the "Almy" Formation by operators in the area, is composed of clastic sediments derived from two separate and distinct source areas. An upper sequence, consisting of conglomerate, varicolored shale, and siltstone, was derived from a sedimentary terrane probably in the rising thrust belt on the west. This sequence is characterized by pebbles and cobbles of limestone and quartzite and would be better assigned to the Chappo Member of the Wasatch Formation. The major (or lower) part of the Paleocene rocks at Birch Creek was derived from a granitic source on the east, probably in the vicinity of the Wind River Mountains, and is characterized by abundant mica and feldspar. The Paleocene forms a regressive lacustrine sequence of the following facies: (1) l custrine shale; (2) marginal lacustrine sandstone; (3) paludal shale, siltstone, and thin sandstone; and (4) variegated mudstone and thick sandstone, probably deposited in a fluvial environment. A more logical formation assignment of this latter sequence would be to the Fort Union Formation.

Significant oil production and the major part of the gas production from the Paleocene at Birch Creek and surrounding fields are from reservoirs within the marginal lacustrine sandstone facies. Traps result from the updip and lateral pinchout of individual sandstones into lacustrine shale along the eastern flank and crest of the LaBarge arch. Structural modification of this basic trapping mechanism is present at some nearby fields such as LaBarge.

The close association of significant oil product and the greater portion of the gas production with the Paleocene lacustrine shale body strongly suggests that this shale is the source of the hydrocarbons produced from these reservoirs. The close association of production from the Mesaverde with the unconformity and the overlying shale body, the lack of a distinct difference between oils from Cretaceous and Paleocene reservoirs, and the lack of significant production from areas where shale is absent or poorly developed suggest that the source of hydrocarbons produced from these

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Cretaceous reservoirs also may be the Paleocene lacustrine shale rather than the underlying Cretaceous marine shales and siltstones.

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