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MAY 2015

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www.rockproducts.com Frac Sand Insider May 2015 | 43 Geology South-central Frac Sand SourceS Hickory Sandstone Member of the Riley Formation The Hickory Sandstone is the basal member of the Upper Cambrian Riley Formation of the Moore Hollow Group (Figure 4) of central Texas that was deposited on the unconformable surface of the Proterozoic and crops out along the western fank of the Llano uplift (Kyle and McBride, 2014) (Figure 10) (Plate 1). - the Llano uplift of central Texas, and the Middle Ordovician Oil Creek Formation (pink) of the Simpson Group (a partial St. Peter equivalent) in southern Okla- subsurface unit in west Texas, crops out in the Beach and Baylor Mountains. Towns appear as red dots. Frac sand that is referred to as "Brown" or "Brady" sand is mined from local quartz arenites that are in the marine subarkosic to arkosic Upper Cambrian Hickory Sandstone Member (Kyle and McBride, 2014). In the Voca area, sand production is generally from a 50 to 65 ft. (15 to 20 m) near-surface lower interval of the Hick- ory Sandstone Member (Kyle and McBride, 2014). The suitability of this sand as a proppant is due to a combination of its depo- sitional, burial, and diagenetic histories. The marine transgressive sands of the Hickory Member were derived from the Precambrian basement rocks and deposited on an extensive unconformity (Kyle and McBride, 2014). Some of the Hickory sands were reworked from older fuvial deposits and aeolian dunes and were redeposited as quartz arenites that have well-sorted and well-rounded grains (Kyle and McBride, 2012). Although they originally had a high feld- spar content, some of the arkosic sandstones underwent diage- netic alteration that removed the feldspars, resulting in an elevated quartz content (Kyle and McBride, 2014). Furthermore, the sand- stones remained friable near the Llano uplift, where they were nev- er buried deeper than 1,500 ft. (about 1 km) (Kyle and McBride, 2014). Compared to the more suitable "Ottawa" or "Northern White" sands, these "Brown" or "Brady" sands have a higher percentage of coarser grades (8/16, 12/20, and 16/30) (Texas Frac Sands, 2014). Also, they are less crush resistant than the "Ottawa" sands because, rather than consisting of monocrystalline quartz, they are polycrystalline quartz grains (Levson and others, 2012). Oil Creek Formation The Middle Ordovician Oil Creek Formation is one of fve for- mations within the Simpson Group (Wright, 1965; Suhm and Eth- ington, 1975) (Figure 5) that were deposited in the Permian Basin of west Texas and southeast New Mexico, and in Oklahoma's Anadar- ko Basin (Jones, 2009) (Figure 10) (Plate 1). The southernmost de- pocenter for the Simpson Group was a broad, shallow, gently dip- ping depression referred to as the Tobosa Basin, a tectonic basin that was the precursor to the Permian Basin (Galley, 1958; Adams, 1965; Wright, 1965; Jones, 2009). At the time of Simpson Group deposition, the Tobosa Basin was separated from the Anadarko Basin by a peninsula that was an extension of the Transcontinen- tal arch, also referred to as the Texas arch (Wright, 1965; Jones, 2009). The Simpson Group has been generally correlated with the St. Peter Sandstone of the Mississippi Valley (Dake, 1921; Suhm and Ethington, 1975). The Simpson Group contains three sand units (the Connell, Waddell, and McKee) (Figure 5) that are oil-producing reservoirs in both the Permian and Anadarko Basins, with the greater production occurring in southern Oklahoma (Jones, 2009). These clean quartz sands are the basal members of the Oil Creek, McLish, and Tulip Creek Formations, respectively (Figure 5) (Suhm and Ethington, 1975; Jones, 2009). These three basal sands are described as containing 95 percent or more quartz, are well sorted and rounded, and have frosted grains (Howe, 1959; Suhm and Ethington, 1975). They tend to range in thickness from 20 to 50 ft. (6 to 15 m) (Jones, 2009). In west Texas, surface exposures of the Connell Sandstone Member of the Oil Creek Formation occur in the Beach Mountains and the Baylor Mountains in Culberson County (Figure 10) (Plate 1), with thicknesses that range from 10 to 30 ft. (3 to 9 m), re- spectively (Suhm and Ethington, 1975). Although the Connell Sand- stone Member has properties similar to equivalent-age midconti- nent sandstones of the St. Peter Sandstone (Howe, 1959; Suhm and Ethington, 1975), the Connell in Texas does not appear to be currently mined as frac sand. The Connell Sandstone Member is present mostly in the subsurface of the Tobosa Basin, where it was named for a sand that produced oil in Ector County (Schweers, 1949; Wright, 1965; Suhm and Ethington, 1975). In south-central Oklahoma, the Oil Creek Formation (Figure 10) (Plate 1) has been recognized in outcrops along the fanks of the Arbuckle Mountains in Murray and Johnston Counties (Buttram, 1913). The basal unnamed sand unit of the Oil Creek Formation is mined by U.S. Silica at Mill Creek in Johnston County, Oklahoma, for multipurpose sand used in glass, foundry molds, well stimulation (fracking), and building products sand (U.S. Silica, 2014b).

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