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Mayo and Assoclales, LC <br />• good local subsw~face marker, it is a poor regional marker because of lateral fades changes <br />and intertonguing with the Mancos Shale. <br />The Rollins Sandstone was deposited as marine sltoreface blanket sands which are laterally <br />continuous, but thin seaward (to [he east). Landward ([o the west), these sandstones <br />terminate abruptly into the mud- and organic-rich backshore fades. In many locations, the <br />Rollins Sandstone has immediate contact with the coal seams of the overlying Lower Coal <br />Member of the Mesaverde Formation. Elsewhere sandstone bodies of the Rollins Sandstone <br />are overlain and underlain by lower shoreface and open marine shales of the Mancos Shale. <br />Thus, the sandstones are three dimensionally encased by low-permeability marine shales and <br />fine-grained carbonaceous backshore coal-bearing fades. <br />~J <br />The Rollins Sandstone is generally not capable of transmitting significant volumes of fluids <br />through the intergramtlar spaces in the sandstone (Moms, 1997). This is due in large part to <br />the pervasiveness of both calcium carbonate and kaolinite cement. Locally, fracturing within <br />the Rollins Sandstone enhances the secondary permeability of the unit. The Rollins <br />Sandstone has been given the designation "tight gas sand" by the Colorado Oil & Gas <br />Conservation Commission. <br />Lower Coal Member <br />The Lower Coal Member conformably overlies the Rollins Sandstone. This member consists <br />primarily of interbedded sandstones, siltstones, shales, and coal. Individual rock units in the <br />• Lower Coal Member are commonly letrticular in nature and are not traceable over large <br />Characterization of Groundwater Systems in the Vicinity of the West Elk Mine, Somerset. Colorado <br />29 January 1999 <br />Pate 17 <br />