Geology of Late Cretaceous Mesaverde and Paleocene Fort Union Oil Production, Birch Creek Unit, Sublette County, Wyoming

Abstract

Birch Creek Unit is on the La Barge arch in the western Green River basin, and is part of the Big Piney-La Barge producing complex. Oil and gas in this area come from rocks ranging in age from Jurassic through Paleocene. This paper is concerned with the trap geometry and the source of hydrocarbons in the shallower productive units of the Upper Cretaceous Mesaverde Formation and Paleocene section. The Mesaverde and upper Hilliard Formations together form a regressive sequence which consists, from northwest to southeast, of lagoonal coal-bearing siltstone and thick sandstone beds, a littoral sandstone complex, and a sequence of marine siltstone and shale. Uplift along the La Barge arch during the Late Cretaceous or very early Paleocene resulted in the erosion of part of this sequence and of an unknown thickness of younger Cretaceous rocks. Truncated, upturned littoral sandstone beds at the Late Cretaceous-Paleocene unconformity, lateral permeability changes, and the gentle structure of the La Barge arch together form the traps within the Mesaverde Formation. The Paleocene sequence is composed of terrigenous clastic sedimentary rocks derived from two separate and distinctive source areas. The basal unit is a relatively thin one. It is a conglomerate and varicolored shale and siltstone sequence that was derived from a sedimentary terrane, probably in the rising thrust belt on the west. The unit, tentatively assigned to the Chappo, is characterized by pebbles and cobbles of limestone and quartzite. The major (upper) 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 upper unit forms a regressive lacustrine sequence of: (1) lacustrine 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. This sequence is assigned to the Fort Union Formation. Most of the oil and the major part of the gas produced from the Paleocene at Birch Creek and surrounding fields are from reservoirs within the marginal-lacustrine sandstone facies. Traps formed as a result of updip and lateral pinchouts of individual sandstone bodies into lacustrine shale along the east flank and crest of the La Barge arch. In some nearby fields, such as La Barge, these traps have been modified by subsequent structural movements. The close association of the oil production and the greater part of the gas production with the Paleocene Fort Union lacustrine shale body strongly suggests that this shale is the source of the hydrocarbons produced from the associated reservoirs. The close association of production in Mesaverde reservoirs with the unconformity above it and with the overlying shale body, the absence of a distinct chemical difference between oils from Cretaceous and Paleocene reservoirs, and the lack of significant production from areas where the shale is absent or poorly developed suggest that the source of hydrocarbons produced from Mesaverde reservoirs also may be the Paleocene lacustrine shale rather than the underlying Cretaceous marine shale and siltstone.