High-ResoIution Stratigraphy of the Green River Formation, NE Uinta Basin: Implications for Red Wash Reservoir Compartmentalization

Jim M. Borer and Mary L. McPherson

An extensive subsurface data base from the Greater Red Wash area and outcrops 4-8 miles from main producing areas are used to construct a high-resolution correlation framework for the lower part of the Eocene lacustrine Green River Formation. An orthogonal grid of 29 cross-sections and 11 cores comprise the subsurface database. The outcrop database consists of 12 measured sections (7,500 ft) along an oblique depositional dip profile at Raven Ridge. Outcrop gamma ray profiles collected for 4500 ft of the measured sections allow for a good outcrop to subsurface tie and provide a calibration set for interpreting subsurface log patterns.

The correlation framework is based on the identification of a hierarchical set (4 scales) of base-level cycles (changes in accommodation to sediment supply ratio). Cycle stacking patterns (lakeward-stepping, vertically-stacked and landward-stepping) provide the chronostratigraphic framework to properly evaluate depositional facies. For mapping and delineating reservoir zones, medium-scale maximum flooding surfaces (base-level rise-to-fall turnarounds) are the most useful correlation surfaces. To better understand depositional facies and heterogeneity within cycles or reservoir zones, base-level fall-to- rise surfaces are correlated in outcrop sections. This allows for the evaluation of sediment volume partitioning and facies differentiation between base-level fall and base-level rise arts of cycles at all scales.

Facies indicate a steep-gradient (high-subsidence), high-energy, storm-dominated, lacustrine shoreface profile. Best reservoirs are stacked shoreface cycles (25-35 ft thick) and occur long-term base-level fall. Reservoir facies are restricted to upper and middle shoreface, trough and hummocky cross-stratified sandstones. Another important reservoir facies is massive dispersed-pebble sandstone interpreted as high-density sediment gravity flows. Gravity flow units are concentrated near base-level rise-to-fall turnarounds due to the expansion of the lake onto the high-gradient alluvial plain.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California