Renee L Love, Reed Lewis, Spencer H Wood, Dennis Feeney, Mark Schmitz

The sedimentary deposits north of the western Snake River Plain host Idaho’s first and only producing oil and gas field. They consist of the mid-Miocene Payette Formation, the mid-late Miocene Chalk Hills Formation, and the Pliocene to early Pleistocene Glenns Ferry Formation. Using new geochronology, palynomorph biostratigraphy, and geologic mapping, we connect up-dip surface features to subsurface petroleum play elements. The Payette Formation is potentially the source of the hydrocarbons and acts as one of the reservoirs in the basin. Here we redefine the Payette Formation as 900 m of mudstone with lesser amounts of sandstone overlying and interbedded with the Columbia River Basalt Group and Weiser volcanics. Index palynomorphs, including Liquidambar and Pterocarya, present in Idaho during and immediately after the mid-Miocene climatic optimum, and new U/Pb dates of 16.39 and 15.88 Ma, help establish the thickness and extent of the formation. For the first time, these biostratigraphic markers have been defined for the oil and gas wells. The Chalk Hills Formation is a tuffaceous siltstone, claystone, and sandstone that is ~300 to 520 m thick. U/Pb dates are 9.00, 9.04, and 7.78 Ma. The Chalk Hills Formation acts both as a reservoir and the sealing mudstone facies. The overlying siltstone to fine conglomerate of the Glenns Ferry Formation acts as the overburden and sealing facies to the petroleum system in the subsurface but was important to the formations burial and hydrocarbon maturation. Both the Chalk Hills and Glenns Ferry Formation were deposited within ancient Lake Idaho during an overall increase in aridity and cooling after the mid-Miocene climatic optimum.