Petrophysical Evaluation of Low Contrast / Low Resistivity Middle Jurassic Valley-Fill (Eolian/Estuarine) Reservoirs, Powder River Basin, Wyoming


Thomas Ahlbrandt, USGS, James Fox, South Dakota School of Mines


Paleovalleys incised into the Triassic Spearfrsh Formation (Chugwater equivalent) are filled with a vertical sequence of eolian, estuarine, and marine sediments including two sandstones of the Middle Jurassic (Bathonian age) Canyon Springs Sandstone Member of the Sundance Formation. These are the eolian lower Canyon Springs Sandstone (LCS) and the estuarine upper Canyon Springs Sandstone (UCS), separated by the "Marine Limestone" and estuar- ine "Brown Shale". Examples of fields with hydrocarbon production from the Canyon Springs in paleovalleys include Lance Creek field (56 MMBO produced), Lightning Creek (z.5 MMBO produced) and the more recently discovered Red Bird field (300 MBO produced), all in the southern Powder River Basin, Wyoming. Canyon Springs reservoirs are prospective at a shallow depth (<5,ooo’) in a broad structurally elevated region of the southern Powder River basin.