Abstract: Lacustrine Turbidite Deposits in the Lower Portion of the Green River Formation, Monument Butte Field, Uinta Basin, Utah

Susan J. Lutz, Dennis L. Nielson, John D. Lomax

Cores and Formation MicroImager^TM logs from the lower portion of the Eocene Green River Formation at Monument Butte field in the Uinta basin indicate the presence of thick (15 ft or 5 m) packages of oil-stained, planar-laminated, fine-grained sandstone exhibiting various degrees of dewatering and soft-sediment deformation. The sandstones commonly contain complete Bouma sequences and are interpreted as moderate- to low-density turbidite channel deposits.

Fluidized turbidite channels, debris flows, and slumps in the lowest portion of the interval suggest deposition along a sublacustrine slope. The rippled top of the sandstone and the overlying fining-upward sequence of sandstones and bioturbated mudstones suggest wave-reworking and more shelf-like conditions upward. Using carbonate marker beds on well logs to package the interval, these sandstone units can be correlated to shales that grade upward into regressive fluvial-deltaic deposits.

The sandstone packages occur as discrete bodies within the shale interval. In the Monument Butte field, the sandstone unit has a funnel-shaped geometry with a localized, channelized base and a broad, flat top. Isopach thicks correspond with the location of syndepositional faults that offset older strata by as much as 50 ft (17 m). Offset along these faults created a slope in the generally shallow sublacustrine environment, caused the shelf margin to become unstable, and created accommodation space for the deposition of the turbidites. Inspection of logs from other fields reveals that turbidite deposits are important reservoirs in this stratigraphic interval throughout the southern part of the Uinta basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994