Geohydrology of bedrock aquifers in the Northern Great Plains in parts of Montana, North Dakota, South Dakota, and Wyoming

Abstract

The Regional Aquifer-System Analysis (RASA) program was started in 1978 after a congressional mandate to develop quantitative appraisals of the major ground-water systems of the United States. The RASA program represents a systematic effort to study a number of the Nation's most important aquifer systems that, in aggregate, underlie much of the country and that represent an important component of the Nation's total water supply. In general, the boundaries of these studies are identified by the hydrologic extent of each system and accordingly transcend the political subdivisions to which investigations have often been arbitrarily limited in the past. The broad objective for each study is to assemble geologic, hydrologic, and geochemical information, to analyze and develop an understanding of the system, and to develop predictive capabilities that will contribute to the effective management of the system. The use of computer simulation is an important element of the RASA studies, both to develop an understanding of the natural, undisturbed hydrologic system, and the changes brought about in it by human activities, and to provide a means of predicting the regional effects of future pumping or other stresses.The final interpretive results of the RASA program are presented in a series of U.S. Geological Survey Professional Papers that describe the geology, hydrology, and geochemistry of each regional aquifer system. Each study within the RASA program is assigned a single Professional Paper number, and, where the volume of interpretive material warrants, separate topical chapters that consider the principal elements of the investigation may be published. The series of RASA interpretive reports begins with Professional Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available.

Geologic framework for the national assessment of carbon dioxide storage resources: Williston Basin, Central Montana Basins, and Montana Thrust Belt study areas

“…The SAU does not extend into the southeastern Williston Basin because the regional seal is absent and does not extend into Canada because this is a U.S. national assessment. These interpretations are supported by structure contour and isopach maps from Sandberg (1961), Thompson and others (1961), Carlson and Anderson (1965), and Downey (1986), and are supplemented by the Geologic Atlas of the Rocky Mountain Region maps and cross sections (Baars, 1972).…”

“…The Bakken Formation was deposited in a restricted-marine environment, and is divided into two organic-rich, black shale units that are separated by a layer that is predominantly calcareous siltstone, but that may also contain sandstone or dolostone (Sandberg, 1961;Ballard, 1963;Macke, 1993;Schmoker, 1996). Although the shales of the Bakken Formation form a thick (more than 100 ft; Downey, 1986), regional seal within the area of the Upper Devonian storage reservoir of the Williston Basin, it also serves as a major oil and gas producer, and therefore it has been fractured and has abundant well-penetrations, which could affect the integrity of the seal in some locations.…”

“…Previous investigations of Devonian rocks in the Williston Basin report TDS values within formation waters of these rocks that on average range from 19,000 to 120,000 mg/L (ppm), but may be more than 400,000 mg/L (ppm) in the area of the SAU (Barnes, 1952;Downey, 1986). Additionally, wells sampled within the confines of the SAU indicate formation waters have TDS values that are saline and over 400,000 mg/L (ppm) in some locations (Breit, 2002), all of which exceed the 10,000 mg/L (ppm) lower regulatory limit (U.S. Environmental Protection Agency, 2009).…”

“…However, one well out of hundreds of wells that were sampled within the SAU showed a TDS value slightly below 10,000 mg/L (ppm) (Breit, 2002). Based upon the background literature (Barnes, 1952;Downey, 1986), and considering this suspected freshwater-well is surrounded by wells showing TDS values well above 10,000 mg/L (ppm), its validity is questionable; therefore, it is most likely that 100 percent of the Three Forks Formation and Jefferson Group SAU is potentially suitable for storage of CO 2 , with a minimum likelihood of 98 percent and a maximum likelihood of 100 percent.…”

“…The boundary of the Three Forks Formation and Jefferson Group SAU is defined by the 3,000-ft reservoir top-depths taken from 4,263 well penetrations (IHS Energy Group, 2010;Nehring Associates, Inc., 2010); the United States and Canada international border; and the lateral extent of the Bakken Formation seal where it is at least 75 ft thick, as determined from differencing formation tops from 2,882 wells (IHS Energy Group, 2010) and from a published isopach map (Downey, 1986). The rocks within the SAU deepen and thicken toward the center of the basin, and on average range from 700 to 825 ft thick with a most likely thickness of 750 ft, as determined from differencing formation tops from 2,152 well penetrations located throughout the SAU (IHS Energy Group, 2010).…”