Joanna N. Thamke

A GIS-based vulnerability assessment of brine contamination to aquatic resources from oil and gas development in eastern Sheridan County, Montana

Water (brine) co-produced with oil in the Williston Basin is some of the most saline in the nation. The Prairie Pothole Region (PPR), characterized by glacial sediments and numerous wetlands, covers the northern and eastern portion of the Williston Basin. Sheridan County, Montana, lies within the PPR and has a documented history of brine contamination. Surface water and shallow groundwater in the PPR are saline and sulfate dominated while the deeper brines are much more saline and chloride dominated. A Contamination Index (CI), defined as the ratio of chloride concentration to specific conductance in a water sample, was developed by the Montana Bureau of Mines and Geology to delineate the magnitude of brine contamination in Sheridan County. Values >0.035 indicate contamination. Recently, the U.S. Geological Survey completed a county level geographic information system (GIS)-based vulnerability assessment of brine contamination to aquatic resources in the PPR of the Williston Basin based on the age and density of oil wells, number of wetlands, and stream length per county. To validate and better define this assessment, a similar approach was applied in eastern Sheridan County at a greater level of detail (the 2.59 km(2) Public Land Survey System section grid) and included surficial geology. Vulnerability assessment scores were calculated for the 780 modeled sections and these scores were divided into ten equal interval bins representing similar probabilities of contamination. Two surface water and two groundwater samples were collected from the section with the greatest acreage of Federal land in each bin. Nineteen of the forty water samples, and at least one water sample from seven of the ten selected sections, had CI values indicating contamination. Additionally, CI values generally increased with increasing vulnerability assessment score, with a stronger correlation for groundwater samples (R(2)=0.78) than surface water samples (R(2)=0.53).

Chemical and isotopic changes in Williston Basin brines during long-term oil production: An example from the Poplar dome, Montana

Brine samples were collected from 30 conventional oil wells producing mostly from the Charles Formation of the Madison Group in the East and Northwest Poplar oil fields on the Fort Peck Indian Reservation, Montana. Dissolved concentrations of major ions, trace metals, Sr isotopes, and stable isotopes (oxygen and hydrogen) were analyzed to compare with a brine contaminant that affected groundwater northeast of the town of Poplar. Two groups of brine compositions, designated group I and group II, are identified on the basis of chemistry and 87Sr/86Sr ratios. The solute chemistry and Sr isotopic composition of group I brines are consistent with long-term residency in Mississippian carbonate rocks, and brines similar to these contaminated the groundwater. Group II brines probably resided in clastic rocks younger than the Mississippian limestones before moving into the Poplar dome to replenish the long-term fluid extraction from the Charles Formation. Collapse of strata at the crest of the Poplar dome resulting from dissolution of Charles salt in the early Paleogene probably developed pathways for the ingress of group II brines from overlying clastic aquifers into the Charles reservoir. Such changes in brine chemistry associated with long-term oil production may be a widespread phenomenon in the Williston Basin.

USE OF BOREHOLE GEOPHYSICAL DATA IN HYDROGEOLOGIC AND CONTAMINANT STUDIES IN THE EAST POPLAR OIL FIELD AREA, FORT PECK INDIAN RESERVATION, NORTHEASTERN MONTANA

Areas of high electrical conductivity in shallow aquifers in the East Poplar oil field area (Figure 1) were delineated by the U.S. Geological Survey (USGS), in cooperation with the Fort Peck Assiniboine and Sioux Tribes to delineate areas of saline groundwater contamination. The generalized geologic section of the study area consists of electrically conductive glacial clays, more resistive sandy clays, and gravel units which are the most resistive. The glacial section, primarily east of the Poplar River (Figure 1), overlays a Cretaceous shale aquitard. West of the Poplar River, the shale is overlain in some areas by Tertiary sands and gravels and the glacial section is thin or absent. As part of this investigation, ground, airborne, and borehole geophysical data were collected in the area from 1992 through 2005 as part of an integrated hydrologic study. We present borehole geophysical data for thirty-two wells that were collected during, 1993, 2004 and 2005 in the study area. The well logs in 2004 and 2005 were collected to provide subsurface controls for interpretation of a helicopter electromagnetic survey flown over most of the East Figure 1 Location of the East Poplar Oil Field

ELECTROMAGNETIC METHODS TO DELINEATE HIGH CONDUCTIVITY IN SHALLOW AQUIFERS, EAST POPLAR OIL FIELD AREA, NORTHEASTERN MONTANA

Airborne, ground, and borehole geophysical studies by the U.S. Geological Survey (USGS), in cooperation with the Fort Peck Assiniboine and Sioux Tribes, have been used to delineate areas of saline groundwater in shallow (<40 meters) unconfined aquifers underlying the East Poplar oil field in northeastern Montana. In the 20 years since the first delineation of saline groundwater, the quality of water from wells completed in the shallow aquifers has changed markedly. The current estimated extent of saline-water plumes based on integrated geophysical and hydrologic studies differs from that delineated in the early 1990s. Ground electromagnetic surveys began in the mid-1990s using an EM-34 (10, 20, and 40 meter vertical and horizontal loops) to measure subsurface electrical conductivity. Results from the EM surves indicated broad areas of high conductivity, which when integrated with results from groundwater quality samples, led to estimates of more than 12 square miles of saline groundwater. In 2004; an airborne electromagnetic survey funded by the Ft. Peck tribes was conducted over a 106 square-mile area that included most of the southeast East Poplar oil field. These surveys provided a foundation for developing a hydrogeologic framework and saline plume mapping over a large area. These surveys, in conjunction with water quality analyses, led to the determination that handling and disposal of brine produced with oil in the East Poplar oil field area resulted in contamination of not only the deeper aquifers, but also shallower areas some of which are near the Poplar River. The integrated interpretation of hydrogeological and geophysical studies has increased the understanding of the subsurface glacial hydrostratigraphy which controls groundwater flow and migration of saline waters. In one area (termed the Biere area near Biere well #1-22), Pioneer Natural Resource (PNR) voluntarily designed and built a plume capture and remediation system consisting of fifteen saline, groundwater removal wells, five crude oil recovery wells and a deep, 7,800 foot, USEPA Class V, injection well. The brine remediation system became fully operational in August 2008 and is operating at an average daily rate of 5,100 barrels per day (214,200 gallons/day). As of January 2015 the system has removed 8,798,390 barrels (369,532,380