GEOLOGIC CONTROLS ON MOVEMENT OF SURFACE AND SUBSURFACE BRINE AT THE OSPER “A” SITE, SKIATOOK LAKE, OSAGE COUNTY, OKLAHOMA

USGS research on the soil, ground water, surface water, and ecosystem impacts of past oil production at its Osage-Skiatook Petroleum Environmental Research site “A” began in 2001. Commercial oil production at this site began in 1913 and continued through at least 1973. Produced fluids were separated into wooden tanks in the upper part of the site, then the brine and crude oil were moved by trench to two downslope pits, one of which temporarily held the brine and the other of which held the crude until it could be pumped into a tank truck. Under accepted oilfield practices at the time, brines in the one pit were allowed to evaporate, seep through the pit bottom, or overflow the pit through a notch on the north berm of the pit. These brine releases moved downslope following topography into a small stream and downdip in the subsurface following 1) channels at the contact between surficial sediments and underlying bedrock, and 2) permeable sandstone layers and fractures in the gently northwest-dipping bedrock. Like the sandstones, interbedded shales in the upper part of the section became salt saturated with time. Downward movement of brine through bedrock was locally limited by layers of heavily dolomite-cemented sandstone. The salt moved deeper stratigraphically, probably along fractures, as it migrated laterally through bedrock downdip. A large salt scar formed downslope to the north of the pits. The upper part of this salt scar starts where a less permeable shale layer forces brine, moving through permeable weathered sandstone immediately underlying the pits, to the near surface. This salt scar can be observed about 80 percent developed in the earliest aerial photos available for the site (1936). The impacts of production thus include a highly saline creek (as seen in older aerial photos and reported anecdotally, the creek was later flooded by the creation of Skiatook Lake), salt scars, and a subsurface volume of saline bedrock that extends at least 120 m north, northwest, and west of the pit source and to depths of at least 12 m. The limits of the volume of saline bedrock derived from this source are not delineated to the north and northwest of existing drillholes and saline bedrock likely extends out under Skiatook Lake.