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Analysis and modeling of intermediate-scale reservoir heterogeneity based on a fluvial point-bar outcrop analog, Williams Fork Formation, Piceance Basin, Colorado

Matthew J. Pranter,1 Amanda I. Ellison,2 Rex D. Cole,3 Penny E. Patterson4

1Department of Geological Sciences and Energy and Minerals Applied Research Center, University of Colorado, UCB 399, Boulder, Colorado 80309; [email protected]
2Department of Geological Sciences, University of Colorado, UCB 399, Boulder, Colorado 80309; present address: ExxonMobil Upstream Research Company, 3120 Buffalo Speedway, URC-N328, Houston, Texas 77098; [email protected]
3Department of Physical and Environmental Sciences, Mesa State College, 1100 North Avenue, Grand Junction, Colorado 81501; [email protected]
4ExxonMobil Exploration Company, 233 Benmar, Houston, Texas 77060; [email protected]


This study presents results of outcrop characterization and modeling of lithologic heterogeneity within a well-exposed point bar of the Williams Fork Formation in Coal Canyon, Piceance Basin, Colorado. This deposit represents an intermediate-scale depositional element that developed from a single meandering channel within a low net-to-gross ratio fluvial system. Williams Fork outcrops are analogs to petroleum reservoirs in the Piceance Basin and elsewhere. Analysis and modeling of the point bar involved outcrop measurements and ground-based high-resolution light detection and ranging data; thus, the stratigraphic frameworks accurately represent the channel-fill architecture.

Two- and three-dimensional (2-D and 3-D) outcrop models and streamline simulations compare scenarios based on different lithologies, shale drapes, observed grain-size trends, petrophysical properties, and modeling methods. For 2-D models, continuous and discontinuous shale drapes on lateral-accretion surfaces result in a 79% increase and 24% decrease in breakthrough time (BTT), respectively, compared to models without shale drapes. The discontinuous shale drapes in the 2-D and 3-D models cause a 30% and 107% decrease, respectively, in sweep efficiency because they focus fluid flow downward to the base of the point bar. For similar reasons, 2-D models based on grain size exhibit 67–267% shorter BTT and 44–57% lower sweep efficiency compared to other model scenarios. Unlike the 2-D models, the continuous shale drapes in the 3-D models cause the fluid front to spread out and contact more of the reservoir, resulting in 42–53% longer BTT and 41–52% higher sweep efficiency compared to the other models. These results provide additional insight into the significance of intermediate-scale heterogeneity of fluvial reservoirs.

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