Michael L. Hackney¹, Ben W. Crouch¹
(1) Petroglyph Energy Inc, Hutchinson, KS

Abstract: The Castle Peak Member of the lower Green River Formation, Antelope Creek Field, Duchesne Co., Utah: An example of the effects of a migrating shoreline on the expression of an open-lacustrine carbonate facies

The Castle Peak Member in the vicinity of Antelope Creek Field provides the opportunity to study a facies gradation from an entirely clastic interval in the south (shoreward) to a carbonate-dominated (mostly open-lacustrine) interval in the north (basinward). Although many workers consider this interval troublesome for correlation, the stratigraphic relationships are clearly revealed by modern mechanical logs and cores from recent drilling in Antelope Creek Field. The interval records the interplay of a fluvial-deltaic system with an open-lacustrine system through 17 cycles of relative lake-level rise and fall. Major lake transgression during each of these cycles is marked by limestones, several of which onlap within the study area and serve to grossly define the shoreline and areal extent of Lake Uinta at that time. Comparison of the lateral extent of limestones defines variation in lake level throughout the cycles. Progradational and retrogradational assemblages can be defined within the clastic prisms between limestones, with sandstones of a fluvial/deltaic nature typically occupying episodes of lake withdrawal. South of the main study area, log analysis reveals that this geologic interval has lost all of its open-lacustrine character over a distance of only 30 km (10 mi). Because of this dramatic facies shift, studies of this interval (and probably much of the lower Green River Formation) in outcrop are probably of limited value for understanding the oil reservoirs of the Castle Peak Member.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana