Subtle partitioning of a sandstone reservoir: permeability patterns interpreted from fluid-escape geometries in the Early Jurassic Navajo Sandstone

Abstract

In the desert southwest of the United States, the 700 m-thick Navajo Sandstone stretches nearly 1,000 km north/south and 400 km east/west. This broad extent and large volume, coupled with the high porosity and permeability of the fine-to-medium, well-rounded, well-sorted, quartzose sands characteristic of this unit, make it the primary aquifer for small communities dispersed across the region. In the central Utah thrust belt, where complex structures have trapped migrating fluids, the Navajo Sandstone also serves as a productive petroleum reservoir. The texture of the eolian dune deposits composing the Navajo Sandstone not only imparted excellent reservoir characteristics, it also made the unlithified sediment susceptible to liquefaction, when saturated with water and subjected to seismic loading. This susceptibility is evidenced in the abundant soft-sediment deformation features that are distributed unevenly throughout the unit. These features derive from liquefaction, fluid escape, and fluidization processes that acted upon the upper 50 m of deposits, at many times during the accumulation history of the unit. Fluid escape and fluidization features, in particular, define fluid flow pathways that responded to permeability variations in the depositional architecture. Extraordinary exposures of ancient pathways of fluid movement through the Navajo Sandstone appear at Navajo Canyon, Waterholes Canyon, and North Coyote Buttes, in northernmost Arizona. Outcrops at these locations reveal a predictable hydraulic response to gross lithologic variations derived from differing sedimentation processes in adjacent dune and interdune environments. They also reveal a surprising sensitivity to more subtle permeability variations derived from the episodic advance of a single dune and the vertical accumulation of multiple, successive dunes. Inasmuch as these patterns of fluid escape through loose sediment constitute a proxy for patterns of flow established later in the diagenetic history of the Navajo Sandstone, a more detailed understanding of reservoir complexities can be derived from analyzing their geometries.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016