Affect of Laramide Structures on the Regional Distribution of Tight-Gas Sandstone Reservoirs in the Upper Mesaverde Group, Uinta Basin, Utah

Aschoff, Jennifer L.¹; Sitaula, Raju ^*1
(1) Geology and Geological Engineering, Colorado School of Mines, Golden, CO.

New, basin-scale sequence stratigraphic correlation, syntectonic unconformity mapping and isopach maps indicate multi-phase uplift and development of the San Rafael Swell (SRS) that resulted in partitioning of the Uinta basin during deposition of upper Mesaverde Group (UMG). Such partitioning could have implications for tight-gas sand production in the Uinta basin, Utah. Sequence-stratigraphic correlation of 100 well logs, 20 stratigraphic profiles and 10 outcrop-based gamma ray profiles define four, 3rd order (~3 My duration) depositional sequences within the dominantly fluvial Upper Cretaceous Mesaverde Group in the Uinta basin, Utah. The correlation was constructed using a combination of fluvial facies and stacking patterns, chert-pebble conglomerates that mark periods of longer residence time, and tidally influenced strata that mark flooding surfaces. These surfaces were extrapolated into subsurface by matching outcrop-based GR profiles with those in the subsurface. Locally, Sequences 1 (oldest) and 4 (youngest) are entirely truncated across the SRS, whereas sequences 2 and 3 thin towards the SRS. The cycles of truncation and onlap within the sequences represent at least 4 phases of SRS uplift. Local thickening of syntectonic depositional sequences on the northeast side of the SRS, and thinning towards the west of Natural Buttes area suggests sediment ponding on the northeast side of the SRS during times of uplift on the structure. Isopach maps show another thinning trend west of Natural Buttes area that continues further south to Book Cliffs possibly caused by an incipient Laramide-style uplift. Paleocurrents are consistent with the interpretation of periodic segmentation and deflection of sedimentation. Regional paleocurrents are generally E-NE-directed in Sequences 1-2, and N-directed in Sequences 3-4. Locally, paleocurrents are highly variable near the SRS further suggesting the UMG basin-fill was partitioned by the uplift of the SRS. In conclusion, we suggest that the Uinta basin was episodically partitioned into several local depo-zones during the deposition of UMG due to the multi-phase uplift of Laramide-style structures in the basin. Understanding the affect of SRS uplift on the development of depositional sequences and basin-scale facies distribution will aid prediction of the best-producing gas reservoirs.