Geophysical Studies of Brine Contamination in and near the East Poplar Oil Field, Northeastern Montana
Areas of brine contamination within the shallow aquifers in and near the East Poplar oil field, northeastern Montana, were delineated by the U.S. Geological Survey (USGS) in cooperation with the Fort Peck Assiniboine and Sioux Tribes. The success of this study is attributed to the use of a combination of geophysical methods integrated with hydrologic and geologic studies. Ground electromagnetic methods were first used during the early 1990s to delineate about 12 square miles of saline-water contamination in a portion of the East Poplar oil field area. A helicopter electromagnetic survey of nearly 106 square miles was conducted in 2004. Borehole electromagnetic induction and natural gamma logs collected at wells throughout the study area in 1993, 2004, and 2005 provided information on vertical electromagnetic conductivity and lithologic controls on brine movement. Direct-current resistivity surveys completed in 2006 defined the location of gravel channels within the shallow aquifers. Ground electromagnetic methods were used again in 2008 to delineate movement of a discrete brine contamination plume near the City of Poplar.
Hydrogeologic data corroborated findings from the geophysical data. About 700 wells and geologic controls were used to map the bottom of the shallow aquifers. Water-quality samples were collected from wells throughout the study area during 2003-08 to compare geophysical measurements with the chemical composition of water from shallow aquifers. Helicopter, ground, and borehole conductivity data were used to delineate subsurface areas of high electromagnetic conductivity and compared to hydrogeologic data to identify areas of contamination.
The USGS concluded that handling and disposal of brine in and near the East Poplar oil field resulted in contamination of not only the shallow aquifers, but also the Poplar River. In the 10 years since the first delineation, the quality of water from wells completed in the shallow aquifers in and near the East Poplar oil field has changed markedly and continues to change. Ground electromagnetic surveys and borehole geophysical logging are planned in 2009 to document the progress of plume remediation that began in 2008. Use of these methods to monitor remediation progress is unprecedented, and these techniques might be used to supplement future industry standards for monitoring remediation.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009