| Description |
The Permian White Rim Sandstone of southeast Utah is a distinctive, porous and permeable, white-colored unit surrounded by fine-grained red beds. In the Elaterite Basin, the White Rim Sandstone is well exposed and acts as a reservoir for tar. White Rim deposits are dominantly eolian in origin, but a thin (0.5-6 meters thick) veneer of shallow marine deposits blankets an erosional paleotopographic high in the Elaterite Basin. Superimposed on the depositional facies are three diagenetic facies that impart white (bleached), red, orange, and brown coloration in the form of disseminated and concretionary iron cement. A yellow alteration zone extends up to 20 meters (32 ft) into the underlying Permian Organ Rock Shale and overlying Triassic Moenkopi Formation. Diagenetic coloration is used as a physical proxy for fluid flow that records discrete stages of diagenesis and highlights depositional heterogeneity on multiple scales. Field, petrographic and geochemical analyses indicate that the White Rim Sandstone underwent three diagenetic stages: (1) early oxidation of the reservoir and precipitation of iron, (2) iron mobilization (bleaching) and reprecipitation of pyrite cement by hydrocarbon migration through the reservoir, and (3) oxidation of pyrite to iron oxide minerals. Field and petrographic observations, and quantitative permeability analyses indicate four scales of heterogeneity exist in the White Rim Sandstone in order of decreasing size: (1) depositional unit, (2) bed-set, (3) internal lamination, and (4) depositional grain-texture scales. Reservoir heterogeneity is highly dependent on grain packing. Authigenic cement phases also act as a record of reservoir conditions through diagenesis, providing indirect information on pH, reservoir compaction and fluid interactions. This study has three broad applications to understanding subsurface fluid behavior. (1) The physical diagenetic records provide insight into the history of hydrocarbons in a porous and permeable sandstone, and can be used in petroleum exploration to help determine hydrocarbon migration timing and pathways. (2) The mineralogic and textural characteristics of authigenic cement phases help to assess the complex history of White Rim Sandstone diagenesis through deep time. (3) Secondary iron precipitation highlights large- and small-scale heterogeneity that must be well understood for applications of CO2 sequestration in similar eolian reservoir units. |
