Inferred Middle Miocene Ages for Reservoir Rocks of Western Snake River Plain Production, Idaho

Abstract

Production of gas and condensate hydrocarbons is from reservoir sands deeper than 3400 ft (1040 m) in the Willow Creek Field near New Plymouth, Idaho. These sands are generally regarded as the Payette Formation, now recognized to contain both basalt sills and flows and silicic lavas intercalated with fluvial sands and lacustrine mudstones. Geologic mapping and new precision U/Pb zircon ages on exposed silicic lavas and tephra in Indian Creek and upper Big and Little Willow Creeks, 12 miles (19 km) northeast of the field provide ages ranging 7 to 16 Ma for the Payette and overlying Chalk Hills Formation. Of interest is a 15.88 Ma age on a 5-m thick welded tuff that lies near the base of a 2500 ft (760 m) thick, 16° west dipping, bentonitic silty claystone section, and 30 m above the thick rhyolite of Indian Creek (age of 16.40 Ma). This 2500-ft (760 m) thick lacustrine claystone section of Indian Creek is overlain by mudstone and arkosic fluvial sand. Problematic is correlation of this thick exposed claystone section of Indian Creek to the 1965 El Paso Natural Gas Assmussen No. 1 dry well which drilled 4024 ft (1227 m) of a dominantly sand section three miles (5 km) to the west. We have also mapped and dated two pumice-bearing marker beds within the Chalk Hills Formation with ages of 9.01 Ma and 7.78 Ma which provide constraints on the depositional age of the younger sediments and establish correlation with sediments south of the Snake River Plain. Understanding the distribution of sands within the dominantly claystone/mudstone lacustrine deposits continues to be a challenge that we will address with further mapping, age dating, and geochemical correlations of interbedded volcanic ashes.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018