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34 | Frac Sand Insider May 2015 www.rockproducts.com Geology The past decade has seen a surge in mineral industry activity targeting the highly pure silica sand deposits of the upper Midwest of the United States for use as frac sand in hydraulic fracturing of unconventional petroleum reservoirs. This frac sand mining boom has resulted from the technological advances in the recovery of petroleum from tight reservoirs that have promoted expansion of hydraulic fracturing and horizontal drilling, all of which are driven by the increasing global demand for energy. Frac sand is a specialized sand that is added to fracking fu- ids that are injected into unconventional oil and gas wells during hydraulic fracturing (fracking or hydrofracking), a process that en- hances petroleum extraction from tight (low permeability) reservoirs. Frac sand consists of natural sand grains, having strict mineralogical and textural properties, that act as a proppant (keeping induced fractures open), which extends the time of release and the fow-rate of hydrocarbons from the fractured rock surfaces in contact with the well-bore. Although the hydrofracking process is applied in many types of tight reservoirs, the majority of fracking in North America is conducted in shale gas plays. Due to the cost of transportation, the proximity of frac sand source rocks to productive basins is a key consideration in determining the feasibility of mine development. Highly prized frac sand mined principally in Wisconsin and Minne- sota is referred to by the mining industry as "Northern White" sand or "Ottawa" sand. Slightly lower quality frac sand, referred to as "Brady" or "Brown" sand, is mined from central Texas. The principal sources of "Northern White" or "Ottawa" sand in the upper Midwest are the Middle and Upper Ordovician St. Peter Sandstone and the Upper Cambrian and Lower Ordovician Jordan Formation, with the Upper Cambrian Wonewoc and Mount Simon Formations gaining in importance. The main source for the "Brown" or "Brady" sand is the Upper Cambrian Hickory Member of the Riley Formation in central Texas. Additional sec- ondary frac sand sources include the Middle Ordovician Oil Creek For- mation in Oklahoma. Sand deposits that are less suitable as frac sand, such as the Pliocene Bidahochi Formation in Arizona and the modern Loup River sands of Nebraska, are also being used in the proppant in- dustry. As the demand for frac sand increases, new sources are being sought that may become economic with changes in technology and physical standards, despite their currently lower quality. In addition to the abrupt rise in frac sand mining and distribution, a new industry has emerged from the production of alternative prop- pants, such as coated sand and synthetic beads. Alternative prop- pants, developed through new technologies, are often more costly than frac sand, but can be more cost-effective because they have better performance in deep wells under high pressure conditions. Frac Sand Sources in the United States By Mary Ellen Benson and Anna Burack Wilson, U.S. Geological Survey, Denver, Colorado ABSTRACT