ABSTRACT
Primary patterns of reservoir-rock porosity and permeability characteristics in the Pennsylvanian Tensleep Sandstone are the effects of grain-packing fabrics and early-diagenetic cements that are controlled by the environments in which the formation was deposited. These environments are a regressive sequence of shoreface to eolian and sabkha interbedded sandstones and dolomites. The formation has laterally correlatable tabular layers with strongly directional permeability in the eolian section.
Tensleep reservoir characteristics can be interpreted from resistivity density and sonic log responses once calibrated to porosity, permeability, saturation, and grain-density analyses from a local full-section core. Directional permeability can be interpreted from dip logs.
Because the controls on deposition are systematic, primary reservoir characteristics are predictable. These characteristics may be summarized as follows: lower Tensleep is moderately permeable shoreface sands interbedded with nonreservoir, relatively brittle dolostones; upper Tensleep is significantly more permeable cross-stratified eolian sands with approximately 10:1 ratio of along-bedding-plane vs. through-bedding-plane permeability, appearing cyclically in composite sets of strata, which have interdune sabkha, nonreservoir, horizontal to wavy-bedded, dolomitic sands at their bases.
