Reservoir characterization of a fluvial sandstone : depositional environment and heterogeneities in modeling of the Colton Formation, Utah

The aim of the present study has been to gather and handle data from a sandstone outcrop as an analogue for a hypothetical fluvial sandstone reservoir. Reservoirs generally have a limited set of data and number of data points. By using records of alluvial style, architecture and heterogeneities from outcrops, supplementary data are obtained for reservoir modeling. The study object was the Paleocene/Eocene Colton Formation in the Roan Cliffs in Utah. The Colton Formation was a clastic wedge interchanging with and pinching out into the lacustrine Green River Formation at the southern margin of the Uinta Basin, reflecting progradation and later retrogradation of a coarse-clastic alluvial system from the east or southeast. Sandstone percentage and architecture are linked to base level fluctuations, depositional environment on the alluvial plain and rate of accommodation versus rate of sedimentation (A/S). During depositional time of the Colton Formation, rate of accommodation was at first high and later slowly rising. Units of lacustrine facies or sediments deposited in a distal alluvial plain setting formed during events of rise in the lake level. Overlying medial alluvial plain sediments were initiated by base-level falls leading to erosion and subsequent slow base level rise and deposition of amalgamated multistorey and multilateral channel-belt sandstone bodies with high connectedness ratio. Increasing A/S ratio further upwards led to increasing preservation of floodplain fines and lower connectedness of channel sandstone bodies, reflecting a change from medial towards distal alluvial plain environment. Data from the Colton Formation outcrop and from a well in the area are applied in modeling of sandstone architecture performed on Petrel software tool. On a macro scale, the modeling shows a high sandstone body connectedness above the datum. However, the mesoheterogeneity of the channel sandstone bodies is complex due to mud chip lags and mudstone drapes.