Prediction of Reservoir Properties Using Diagenetic Analysis of a Template Unit: Example from Upper Cretaceous Sandstones in Powder River Basin, Wyoming

A. Umran Dogan, Robert L. Brenner

Depositional and postdepositional histories of the Parkman formation in the Powder River basin, Wyoming, were studied in detail and compared with other Upper Cretaceous lenticular sandstone units of the Teapot, Sussex, and Shannon sandstones. Petrographic analysis was done using light, cathodoluminescent, scanning, scanning transmission, and backscattered microscopic techniques. X-ray microanalysis was done using energy and wavelength-dispersive spectroscopy systems.

The primary diagenetic events observed in these Upper Cretaceous sandstones include ductile-grain deformation and original porosity reduction; formation of authigenic chlorite, kaolinite, illite, and smectite; quartz overgrowths; formation of authigenic feldspar; alteration of feldspar; carbonate cementation; and pyrite and iron oxide precipitation. The major effects upon reservoir properties include: porosity and permeability reduction due to formation of authigenic clays, quartz, and carbonate cement; and early formation of chlorite coatings preventing complete destruction of porosity by quartz overgrowths. Diagenetic alterations appear to be strongly influenced by depositional facies and chemistries of original interstitial waters. However, sources for authigenic silica and clays w re predominantly exogenic, although some authigenic minerals had endogenic sources such as feldspar alteration to clay minerals. Authigenic minerals that have exogenic sources appear to have precipitated from fluids generated during diagenesis of the surrounding mud rocks. For this reason, major diagenetic trends in these lenticular sandstones are similar.

A diagenetic model developed from the results of analysis of the Parkman formation was successfully used to predict reservoir properties in the Teapot, Sussex, and Shannon sandstones. Therefore, the Parkman formation can be used as a template to predict reservoir properties of other sandstone units that were deposited in similar depositional and tectonic settings.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.