GEOLOGY OF THE LATE CAMBRIAN HICKORY SANDSTONE IN THE VOCA FRAC SAND DISTRICT, WESTERN LLANO UPLIFT, TEXAS

The Hickory Sandstone, the basal member of the Cambrian Riley Formation, is as much as 500 ft (150 m) thick and consists dominantly of marine tidal-flat sandstone with minor mudrock, conglomerate, and limestone, plus a local ironstone unit as much as 80 ft (25 m) thick. The Hickory Sandstone was deposited on the “Great Unconformity” developed on the Precambrian basement of Central Texas and elsewhere. The local basement consists of diverse metamorphic rocks and syn- to post-orogenic granites, ranging in age from 1300 to 1100 million years. The Hickory Sandstone is subarkose to arkose, with minor diagenetic quartzarenite from which most or all feldspar has been destroyed. Although Hickory sediments were sourced ultimately from the Precambrian basement, some grains were derived from fluvial and eolian deposits. Typical of quartz sand grains with a long history of abrasion, coarser grains are better rounded than finer grains. Some of the best-rounded grains, however, are medium-size grains, some of which have crescentic surface fractures typical of impact in air. These crescentic fractures and sparse ventifacts attest to a contribution of detritus from eolian deposits.

Despite its great age, the Hickory remains friable where it was never buried more than 1500 ft (~1 km), notably along the flanks of the Llano Uplift. The slight induration shown by most beds is the product of incipient quartz overgrowth development plus trivial suturing of grains at pressure solution contacts. Because of its poorly cemented nature, the Hickory has been a source of industrial sand for several decades. The rapid growth of enhanced petroleum recovery via hydraulic fracturing has resulted in a corresponding increase in the demand for “proppants”. Texas has multiple unconventional petroleum plays, and the production of industrial sands for this purpose has doubled in the past decade. One of Texas’ major frac sand-producing areas is near Voca, Texas, where sands are produced from the Cambrian Hickory Sandstone on the northwestern flank of the Llano Uplift. In response to regional market demand, frac sand production from the Hickory in the Voca area has steadily increased. Current production is from a near-surface 50- to 65-ft (15 to 20-m) interval in the lower Hickory. Only about 35% of mined sand has optimal grain-size properties for fracing.