Strontium Isotope Evolution of Produced Water in the East Poplar Oil Field, Montana

The East Poplar oil field on the Fort Peck Indian Reservation in eastern Montana has been in production for about 60 years, mostly from the Charles Formation of the Mississippian Madison Group. The Poplar dome is in the western part of the Williston Basin and is breached by faults and modified by local dissolution of Mississippian evaporites as described by Orchard in 1987. Because of past disposal practices, groundwater in the shallow aquifers has been contaminated in places by oil-field brines with salinities up to several times that of sea water. With funding from the Fort Peck Office of Environmental Protection, the U.S. Geological Survey (USGS) is continuing hydrochemical studies by Thamke and others with the addition of strontium (Sr) isotopes as described by Peterman and others in 2010.

Samples of uncontaminated and contaminated groundwater from domestic wells define a trend with a narrow range in 87Sr/86Sr values (mean of 0.70817 ± 0.00017) but a fifty-fold increase in Sr concentrations (0.24 to 12.5 mg/L) with increasing contamination. Twenty samples of brine collected directly from producing oil wells define two isotopic groups. One group (10 samples) has a small range in 87Sr/86Sr (0.70816 ± 0.00024 with 12.2 to 338 mg/L Sr) and could represent the groundwater contaminant. The other group has a larger range in 87Sr/86Sr (0.71061 ± 0.00190 and 34.5 to 51.7 mg/L Sr) and could not have produced the contaminant trend. Two of the brine samples in this second group are from the Late Mississippian Heath Formation overlying the Madison Group and have 87Sr/86Sr values of 0.70928 and 0.70942. The remaining samples are from the Madison Group and have 87Sr/86Sr values ranging from 0.70962 to 0.71175. These brines probably originated in older clastic units and migrated upward as a result of the long-term production from the East Poplar field. This is consistent with the suggestion of Jarvie in 2001 that oil from the Bakken Formation has mixed with oil from the overlying Charles Formation.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California