PENNSYLVANIAN TENSLEEP FORMATION – A POSSIBLE PROPPANT FOR HYDRAULIC FRACTURING, CARBON COUNTY, MONTANA

The Tensleep Formation is an eolian and nearshore marine/sabka quartz arenite unit with prominent outcrops along the western Pryor/Bighorn Mountain front east of Red Lodge, MT. Regionally, the formation represents one of the largest ergs in the global geologic record. High permeability makes it an important oil and gas reservoir and aquifer in south central Montana and throughout much of Wyoming. Its high percentage of quartz content and grain roundness, due to its eolian origin, make it prospective as a source for natural proppant sand.

Three continuous 4-inch cores were obtained during a cooperative project between Montana Tech and industry partners. Using stratigraphic sections, the cores, thin sections, and x-ray fluorescence (XRF) core analysis, we are exploring the usefulness and economic feasibility of the Tensleep as a minable hydraulic fracture proppant. Usefulness depends on cementation, grain shape, grain size, and depth from surface of the prospective zone. Grain shape and size is determined by thin sections, sieving, and stereomicroscope analysis. Analysis of 20 disaggregated sand samples has shown that as much as 30 percent of the grain sizes are between 30-50 mesh (medium- to fine-grained sand size) and ~45 percent of the grain sizes fall between 70–140 mesh (very fine-grained sand to coarse silt), sizes appropriate for some hydraulic fracture operations. Core descriptions and XRF data help to display the distribution of lithology and cementation. Elemental (XRF) analyses help to delineate more pure quartz sands from those with grain fractions reflecting fine-grained clastic and evaporitic inputs. The core and nearby stratigraphic sections are used to quantify overburden and the amount of resource in the area. Initial results show favorable crush strength and useable grain size and shape.