Reservoir Heterogeneity as a Function of Accumulation and Preservation Dynamics, Upper Tensleep Sandstone, Bighorn Basin, Wyoming

Mary Crabaugh, Thomas L. Dunn

The processes controlling accumulation versus preservation of the Tensleep Sandstone in the Bighorn Basin are distinct, and as such have different effects on reservoir heterogeneity. Accumulation is the short-term net deposition of sediments within an eolian system, whereas preservation marks the entrance of the sediments into the stratigraphic record. Accumulation of the Upper Tensleep Sandstone in the Bighorn Basin occurred within a dry eolian system, that is one in which accumulation occurs as a result of aerodynamic controls, and is unaffected by the local water table. Conversely preservation of the Upper Tensleep, was caused by the relative rise of sea level that placed the area below the regional baseline of erosion. The orientation and character of the bounding sur aces and internal laminations of eolian facies in the Tensleep indicate that the ergs were dominated by large compound bedforms that were affected by annual fluctuations in airflow.

Reservoir heterogeneity can be considered on a number of scales within the Upper Tensleep Sandstone. Small-scale heterogeneities, such as internal bounding surfaces and lateral variations in microscopic pore structure, are the result of variations in the character of the bedforms during accumulation. These variations in internal fabric, along with drive type and fluid mobility ratio, govern sweep efficiency and ultimate recovery. Large-scale heterogeneities, represented by the areally extensive limestones within the Upper Tensleep, are controlled by the longer term variations in preservation space as influenced by relative sea level fluctuations. These features govern overall reservoir continuity and along with tectonic fabric, reservoir size. Delineation of these sedimentary processe and the resulting rock fabrics will enhance future incremental production from the Upper Tensleep Sandstone, Wyoming.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995