Abstract

The processes controlling accumulation and preservation of the Tensleep Sandstone eolian units in the Bighorn Basin are distinct, and have different effects on reservoir heterogeneity. Accumulation is the 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 base-level of erosion. The orientation and character of the bounding surfaces 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 different scales within the Upper Tensleep Sandstone. Small-scale heterogeneities, such as stratification type, 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 fabric, along with drive type and fluid mobility ratio, govern sweep efficiency and ultimate recovery. Large-scale heterogeneities, represented by the areally extensive marine dolomites within the Upper Tensleep, are controlled by 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 processes and the resulting rock fabrics will enhance future incremental production from the Upper Tensleep Sandstone, Wyoming.