Use of Electromagnetic Geophysical Techniques and Auger Drilling to Delineate Saline-Water Plumes in the East Poplar Oil Field, Fort Peck Indian Reservation, Northeastern Montana

Abstract

The East Poplar oil field encompasses about 70 mi[sup 2] in the south-central part of the Fort Peck Indian Reservation. Since oil production from the Mississippian Madison Group began in 1952, large quantities of brine (water containing greater than 35,000 mg/L of dissolved solids) have been produced. Most of the brine has been disposed of through injection wells completed in shallower Cretaceous rocks, although some brine has been evaporated in pits. Brine leaking from injection-well casing or evaporation pits has resulted in contamination of shallow water in Quaternary alluvial and glacial deposits along the Poplar River. Electromagnetic geophysical techniques were used to delineate three saline-water plumes in alluvial deposits and one plume in glacial deposits. Data-collection sites were located on 0.1-mi (or sometimes 0.2-mi) centers in grid networks. Apparent conductivity was measured at about [sup [minus]]300 sites within an area of about 19 mi[sup 2]. Vertical and horizontal variations in apparent conductivity were contoured. The apparent-conductivity data were used to locate potential monitoring-well sites. Nineteen monitoring wells were installed during 1992 using hollow-stem augering methods. Water-quality data from these wells, and from eight existing wells, verified the presence of saline water in Quaternary deposits that showed apparent-conductivity anomalies. Specific conductancemore » of water in plume areas is as large as 127,000 [mu]S/cm at 25[degrees]C. Additional data are being collected to further determine the areal extent and rate of movement of saline-water plumes.« less