AUTHORS Matthew J. Pranter  Department of Geological Sciences and Energy and Minerals Applied Research Center, University of Colo- rado, UCB 399, Boulder, Colorado 80309; matthew.pranter@colorado.edu Matt Pranter is an assistant professor at the University of Colorado at Boulder and head of the Reservoir Characterization and Modeling Laboratory. He received his B.S. degrees in geology and geological engineering from Okla- homa State University and the Colorado School of Mines, respectively, his M.S. degree in geol- ogy from Baylor University, and his Ph.D. in geology from the Colorado School of Mines. He was previously with ExxonMobil Upstream Research Company and Conoco. His research interests are in reservoir geology and geophysics, sedimentary geology, and reservoir modeling. Amanda I. Ellison  Department of Geo- logical Sciences, University of Colorado, UCB 399, Boulder, Colorado 80309; present address: ExxonMobil Upstream Research Company, 3120 Buffalo Speedway, URC-N328, Houston, Texas 77098; amanda.ellison@exxonmobil.com Amanda Ellison is a geologist and geologic modeler at ExxonMobil Upstream Research Company. She received her B.S. degree in geology from Colorado State University and her M.S. degree in geology from the University of Colorado at Boulder. Her interests are in reservoir characterization and modeling. Rex D. Cole  Department of Physical and Environmental Sciences, Mesa State College, 1100 North Avenue, Grand Junction, Colorado 81501; rcole@mesastate.edu Rex Cole is a professor of geology at Mesa State College in Grand Junction, Colorado. He obtained his A.S. degree in geology from Mesa Junior College, his B.S. degree in geology from Colorado State University, and his Ph.D. in geology from the University of Utah. Previous employers include Unocal Corporation, Multi- Mineral Corporation, Bendix Field Engineer- ing Corporation, Southern Illinois University– Carbondale, and Asarco Corporation. 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, Amanda I. Ellison, Rex D. Cole, and Penny E. Patterson ABSTRACT This study presents results of outcrop characterization and mod- eling of lithologic heterogeneity within a well-exposed point bar of the Williams Fork Formation in Coal Canyon, Piceance Basin, Col- orado. 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 an- alogs to petroleum reservoirs in the Piceance Basin and elsewhere. Analysis and modeling of the point bar involved outcrop measure- ments 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 lithol- ogies, shale drapes, observed grain-size trends, petrophysical prop- erties, and modeling methods. For 2-D models, continuous and dis- continuous shale drapes on lateral-accretion surfaces result in a 79% increase and 24% decrease in breakthrough time (BTT), respective- ly, 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 AAPG Bulletin, v. 91, no. 7 (July 2007), pp. 1025–1051 1025 Copyright #2007. The American Association of Petroleum Geologists. All rights reserved. Manuscript received August 31, 2006; provisional acceptance November 7, 2006; revised manuscript received January 19, 2007; final acceptance February 1, 2007. DOI:10.1306/02010706102