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36 | Frac Sand Insider May 2015 www.rockproducts.com Geology The principal stratigraphic units that are mined for premium frac sand ("Northern White" and "Ottawa") are in the Midwest and include the Middle and Upper Ordovician St. Peter Sandstone, the Upper Cambrian and Lower Ordovician Jordan Formation, and the Upper Cambrian Wonewoc and Mount Simon Formations. Sand- stone units that meet some, but not all the specifcations ("Brady" or "Brown") are from the Upper Cambrian Hickory Sandstone Member of the Riley Formation in central Texas. Less economic deposits of the Middle Ordovician Oil Creek Formation of the Simpson Group in Oklahoma (a partial St. Peter Sandstone equivalent) are also used in the frac sand industry. The effort to meet the growing demand for hydrocarbon in- dustry proppants that meet the requirements of increasingly deeper wells has fostered the development of a new industry in alternative proppants. These alternative proppants include resin-coated sand and synthetic proppants engineered from high-strength ceramic materials such as sintered bauxite (Dolley, 2012). The highly so- phisticated proppants of today result in increasingly optimal results in hydrocarbon recovery as proppants can be customized for each particular reservoir, well, or treatment design. This frac sand report begins with a description of the unique physical properties of frac sand, then focuses on the geology and the spatial relationships of frac sand sources in the United States. It tracks recent published efforts to examine the potential for less conventional frac sand sources, reviews the current and future competing sources in Canada, and discusses the emergence of alternative proppants and their role in the market. Furthermore, it summarizes the processes of origin for frac sand deposits, and provides geologic guidelines to assist in the search to identify potential new sources of frac sand. This report contains infor- mation and map fgures excerpted from a U.S. Geological Sur- vey Open-File Report that is currently in press. (Benson, M.E., and Wilson, A.B., Frac Sand in the United States—A Geological and Industry Overview), which relies upon digital data from the U.S. Geological Survey State Geology Map Compilation Ge- odatabase of the Conterminous United States (SGMC) that is in preparation (J.D. Horton, C.A. San Juan, and D.B. Stoeser, unpub. data). Whenever units of measurement are provided in this report, the frst value given is in units as published in the ref- erenced citation; and where converted to other units, these val- ues are shown subsequently in parentheses. The goals of this compilation are to provide basic information about the geologic units that are currently yielding high- and medium-quality frac sand and substrate (seed) sand best suited for resin coating, and to recognize for future examination some of the additional sources that have limited suitability. The intent of this report and the maps included is to convey information about frac sand as a resource; and to graphically illustrate the geographic distribution of geologic units that contain major current and potential future frac sand sources in the United States. The specifcations for frac sand are based upon the standards for proppants determined by the American Petroleum Institute (API) and the International Organization for Standards (ISO) (American Pe- troleum Institute and others, 2008). These frac sand standards were modeled after the properties of the "Ottawa" or "Northern White" sand (Figure 2) and the "Brady" or "Brown" sand (Zdunczyk, 2014). The "Ottawa" sand, a synonym for the St. Peter Sandstone mined from Ottawa, Illinois (Maslowski, 2012), is used as a standard by the American Society of Testing Materials (ASTM) (Zdunczyk, 2014). The frac sand specifcations include criteria for mineralogy (high percent silica content), grain size range from medium to coarse, sphericity/roundness of 0.6 or greater, high crush resistance, low solubility, low turbidity, and good friability (American Petroleum Insti- tute and others, 2008). 2. A sample of "Northern White" sand well-rounded HIGH SILICA CONTENT The premium frac sand is >99 percent quartz or silica (SiO 2 ) (Zdunczyk, 2007, 2014), although a great deal of sand used as frac sand falls within the range of 95-99 percent silica content. Miner- alogical purity as to silica content is a characteristic of mature and super-mature sand or sandstone, which has been highly reworked and well sorted, so that mechanically and chemically less-resistant minerals and fne particles have been dissolved or winnowed away (Pettijohn and others, 1972). GRAIN SIZE Generally, a range of grain sizes from 0.1 to >2 millimeters (mm) diameter is desirable. Size is based on "U.S. Standard Sieve Series sizes" or "U.S. Mesh," in which the smaller the grain-size number, the coarser the grain (Beckwith, 2011). Grain size rang- es for sand samples are designated by notations such as 20/40, 30/50, 40/70, and so forth, that indicate the end-member mesh sizes for >90 percent of the sample. In the case of 20/40 sand, >90 percent of the sand passes through the 20 mesh (0.850 mm) sieve and is retained by the 40 mesh (0.425 mm) sieve (Zdunczyk, 2014). Larger sand grains provide better permeability, but smaller sand grains are typically stronger (Rupke, 2014). Grain size require- ments for frac sand are determined by downhole conditions and completion design, but 20/40 has been the most in demand (Mont- gomery and Smith, 2010; Beckwith, 2011). Typically, 20/40 and OPTIMAL PHYSICAL PROPERTIES FOR FRAC SAND