SUBSURFACE GEOLOGICAL AND GEOPHYSICAL PROFILES OF VOLCANIC STRATA AT THE LORELLA DEEP WELL, KLAMATH COUNTY, OREGON

The 1005-ft Lorella Deep Well (LDW), 40 km east of Klamath Falls, was drilled to provide geologic and downhole geophysical constraints on surface magnetotelluric (MT) profiles of the aquifer and to provide for water-level monitoring of two depths. Located on the Cheyne Ranch, it is a double-completion well, nominally, a 6-inch borehole with a 1.5-inch steel pipe down the center. The borehole penetrated a sequence of mudstone and ash, basalt, pyroclastic rocks, and altered basalt of Pliocene to Miocene age. Tholeiitic basalt from the 87-ft and 905-ft levels yielded 40Ar/39Ar ages of 4.58 and 5.79 Ma, respectively. Basalt from the 680-ft level is calc-alkaline. Downhole geophysical measurements found that competent basalt was characterized by low gamma counts, high electrical resistivity/low electrical conductivity, and competent borehole walls. Clay-rich sedimentary, pyroclastic, and altered basalt was characterized by high gamma counts, low electrical resistivity/high electrical conductivity, and increases in borehole diameter. MT profiles from surface surveys run prior to drilling mirrored the subsurface stratigraphy discovered through drilling. MT readily discriminated highly conductive mudstone, highly resistive basalt, and moderately conductive pyroclastic rocks above about 500 ft. MT was increasingly less discriminate below 500 ft because of increasing clay-alteration (primarily celadonite) of basalt and inherent resolution of the technique relative to layer thicknesses. Nearby production wells penetrating the same horizons have production rates that differ by a factor of ten. MT and well-log data indicate that localized clay alteration dramatically affects transmissivity.