MESOSCALE HETEROGENEITY OF FLUVIAL-LACUSTRINE RESERVOIR ANALOGUES: EXAMPLES FROM THE EOCENE GREEN RIVER AND COLTON FORMATIONS, UINTA BASIN, UTAH, USA
A. W. Taylor*+ and B. D. Ritts*
*Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah, 84322-4505, USA.
+Current address: Anadarko Petroleum Corporation, 1201 Lake Robbins Drive, The Woodlands, Texas, 77380, USA.
The Eocene Uinta Basin of Utah is a dominantly lacustrine basin that affords the opportunity to survey analogue sandstone reservoirs developed within and adjacent to the lacustrine system. From its SW to NE margin, Eocene strata in the Uinta Basin record fluvial, deltaic, profundal lacustrine, and wave-dominated shallow-lacustrine environments. Of these depositional systems, analogue sandstone reservoirs are well-exposed in outcrop in the fluvial-deltaic and wave-dominated intervals. Field and petrographic examination of selected fluvial-lacustrine reservoir analogues are used to document the mesoscale facies architecture and heterogeneity. Complex depositional geometries and heterogeneity exist at a variety of scales, both within single subenvironments and between opposite sides of the basin.
Three outcrops in the Uinta Basin were studied. Outcrops One and Two are located in the SW portion of the basin in Nine Mile Canyon, and represent the interface between fluvial rocks of the Colton Formation and dominantly lacustrine strata of the Green River Formation. Outcrop Three is located at Raven Ridge in the NE of the basin, a shallow wave-dominated lacustrine margin. Photomosaics and multiple vertical measured sections were described for each outcrop, and were utilized to document lithofacies assemblages, bounding surfaces, and sandbody architecture and heterogeneity. Sandstone samples from each outcrop were analyzed petrographically to capture the small-scale heterogeneity characteristic of the three outcrops.
Sedimentary rocks at Outcrops One and Two consist of lenticular, tabular, or undulatory channel, distributary channel, and bar deposits that are partially to entirely compartmentalized, or encased, by mudstone units. These sandstone units, analogous to reservoir intervals in lacustrine basins, show various types of lateral and vertical connectivity which are dependent on the degree of amalgamation and the sand:mud ratio. Within these sandbodies, mud chip lags, large mud clast horizons, discontinuous finer-grained beds, accretion sets and amalgamation surfaces represent internal heterogeneity which could be expected to affect transmissibility in an actual reservoir. The complex geometry and heterogeneity characteristic of these reservoir analogues confirms the need for detailed reservoir characterization studies on all scales in order to improve production efficiency in such systems.
Outcrop Three is dominated by tabular and laterally extensive offshore bar and sand-sheet deposits which are separated by thick mudstone units. Large-scale heterogeneity is minimized within these reservoir analogues, in that sandbodies exhibit excellent lateral continuity and less complex amalgamation. Therefore, documentation of heterogeneity at the bed- and petrographic- scale is necessary to better address production potential in these types of reservoirs.