Abstract

The purpose of this study is to determine the reservoir quality and heterogeneity of the Jurassic Navajo Sandstone in the Devils Canyon area of the San Rafael Swell as an analogue to the Navajo in the central Utah thrust belt. Objectives include (1) an outcrop-based reservoir characterization study in the Justensen Flat/Devils Canyon area, and (2) characterization of facies heterogeneity within the Navajo erg system. The interbedding of erg interdune and dune facies has produced sedimentologic heterogeneities in the reservoir (i.e., baffles and barriers) that can potentially partition the reservoir, depending on the extent of the structural closure within a given oil field. Interdune facies were deposited by a variety of processes and display a number of different sedimentary structures. We propose eight facies (three dune and five interdune) for the Navajo Sandstone based on the sedimentary characteristics observed in the Justensen Flat/Devils Canyon area. Interdune facies generally have lower porosity and permeability than the dune facies. However, the lateral extent of interdune facies is variable, ranging from meters to kilometers. Low-permeability interdune facies are concentrated in the lower 50 m (150 ft) of the section. The interdune facies in the lower Navajo are baffles to fluid flow. The upper Navajo is more homogeneous with few interdune baffles. Separating the lower and upper Navajo Sandstone is a low-permeability interdune unit (Wavy Algal Mat [WAM] facies) that forms a barrier to flow, potentially compartmentalizing the reservoir into two units. Because of the different character of the lower and upper Navajo, the reservoir production and completion strategy will be different.