Configuration of Shelf Sandstone Oil Reservoirs, Upper Cretaceous (Turonian) Turner Sandy Member of Carlile Shale, Powder River Basin, Wyoming

Dudley D. Rice, C. William Keighin

Oil production in the Upper Cretaceous Turner Sandy Member of the Carlile Shale on the east flank of the Powder River basin is established in two types of stratigraphic traps characterized by distinct geometries and reservoir properties. One type is medium-grained sandstone bodies as much as 4 m thick that have filled elongate (as much as 10 km), narrow (< 1.5 km), east-west-trending erosional depressions of low relief. Trough and tabular cross-stratification indicates an eastward direction of transport. This type of sandstone is interpreted as previously deposited sediments reworked by transgression following global drop in sea level in middle Turonian time. The sands were redistributed and concentrated by offshore-directed downwelling flows on inner shelf in irregula ly spaced depressions controlled by recurrent movement of basement fault blocks. Although this type is limited in areal extent, reservoirs in it are good (porosity and permeability are as much as 20% and 100 md, respectively).

In contrast, the other type of trap is generally very fine-grained sandstone that occurs in shoaling-upward sequences as much as 12 m thick. Shale intercalations decrease upward, and planar lamination and hummocky cross-stratification are prevalent. Upper parts of sequence are bioturbated and contain trace fossils of Skolithos ichnofacies. These sequences form widespread (70 km²) bodies that were deposited below fair-weather base on the storm-wave-dominated outer shelf. Although the overall sandstone bodies are extensive and resulting production is large, reservoirs in these bodies are characterized by thin beds and vertical and lateral discontinuity on a centimeter scale. In addition, sandstones are tight (average porosity and permeability are 15% and 0.5 md, respectively) because of original fine grain size and presence of authigenic clays.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.