PREDICTING CONTEMPORANEOUS SAND BODIES FOR MINE DEWATERING IN THE FORT UNION FORMATION OF THE POWDER RIVER BASIN, WYOMING

The Powder River Basin (PBR) is known for its vast coal reserves of Sub-Bituminous coal. A mine north of Gillette Wyoming, mines the Anderson and Canyon Seams of the Wyodak Coal Zone located within the Tongue River Member of the Paleocene Fort Union Formation. During the Paleocene the PBR had multiple vast peat mires which were intersected with fluvial and lacustrine features. Subsequent burial of these sediments and the coalification of the peat created the economically viable resource that is sought by multiple mining operations in the PBR.

While most of the mining and geologic assessment of these resources are directed to the coal itself, little is studied of the complex facies that make up the overburden material. Evident lacustrine and large sand bodies or “no coal channels” can be discovered through exploration drilling, however secondary and tertiary channels can be missed by drilling. These channels are important to identify due to the potential of being perched to unconfined aquifers that can hold such a significant yield of groundwater to be detrimental or hinder mining activities. To properly dewater ahead of pit advancement these structures must be identified. Detailed analysis of Gamma-Logs procured during drilling can show potential facies changes. After the identification of these facies, a stochastic modeling approach can be used to quantify azimuthal orientation as well as thickness for a particular sand body. In addition a qualitative modeling approach using existing mine modeling software, can aid in planning potential dewatering test holes.