Geology

"Frac sand" is mined from sandstone formations deposited about 500 million years ago, when what is now western Wisconsin and southeastern Minnesota consisted of ocean shoreline. Over time, waves formed and smoothed the sandstone into a material with characteristics highly coveted by the oil and gas industry. Mines are concentrating in areas outside the most recent glaciation (that occurred 15,000 to 30,000 years ago), which buried sandstone formations with glacial material. Certain sandstone formations currently meet different industry specifications, including the Cambrian-period Jordan, Wonewoc, and Mt. Simon formations, and the slightly younger Ordovician-period St. Peter formation. The region of western Wisconsin and southeastern Minnesota known as the Driftless Area, characterized by rugged hills and immense, picturesque bluffs overlooking the Mississippi River, remained untouched by glaciation and has no glacial deposits (known as "drift"). The sandstone is very close to the surface or partially exposed. Little topsoil or "overburden" needs to be removed, making it easy to dig up and process.

For use in hydraulic fracturing, the ideal sand gain is nearly 100% quartz and perfectly spherical in shape, providing strong crush-resistance and consistent grain size. The industry measures grain size in terms of "mesh size," which refers to the number of openings per inch on a sieve screen, and seeks specific mesh sizes depending on the type of use. Course is generally desirable for drilling, as is high crush strength. Common sand deposits, such as the sand you would typically find on a beach, are too angular and inconsistent to serve as a proppant for oil and gas drilling. The deposit should also be relatively free of silt and clay. During mining and processing, the sand is washed to remove any unwanted materials, also known as waste or "fines," which are usually returned to the mine and buried during reclamation. In addition to a geology that makes sand mining profitable, access to transportation infrastructure is key, especially access to railroads.

Resources

Brown, Bruce A. 2012. Hydrofrac sand in Wisconsin. Wisconsin Geological and Natural History Survey.

Syverson, Kent. 2012. The Frac Sand Mining Boom in Wisconsin: Geology, Impacts, and Politics. Videotaped seminar, February 17, University of Wisconsin-Eau Claire.

• This presentation is now dated, but provides a good overview by a local geologist. Generally seen as a proponent of sand mining, Syverson is the chair of the geology department at UW-Eau Claire and also works as a consultant for the frac-sand industry. 

WGNHS. 2011. "Bedrock stratigraphic units in Wisconsin." Educational Series 51. Wisconsin Geological and Natural History Survey.

Maps

WGNHS. 2013. Industrial Sand Potential in Wisconsin: Sandstone At or Near the Surface. Wisconsin Geological Survey.

Illustrations

Minnesota counties with frac sand deposits, October 2012.
Source: Minn. Department of Natural Resources

Illustration source: U.S. Silica