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<br />• Authigenic minerals are common in varying amounts and consist of quartz overgrowths where
<br />matrix development is anomalously low; illite and mixed illite and smectite, which occur as
<br />alteration products of lithic fragments and as pore linings; and rare iron-rich chlorite (Pitman, et.
<br />al., 1989, p. G25). The lazge proportion of matrix has prohibited the development of silica cement
<br />inmost areas. Calcite, dolomite, or both are frequently present in amounts up to nearly 25%, and
<br />where common they replace framework grains and fill pores (Pitman, et. al., 1989, p. G25).
<br />Virtually all Rollins Sandstone samples the writer has tested from the surface in Coal Canyon and
<br />along the escarpment east of Palisade, from underground in the old Cameo, Roadside North, and
<br />Roadside South mines (including clastic dikes intruding the Cameo coal beds which are assumed
<br />to have originated from the Rollins), and from cores on leases C-020740 and C-024998 (Roadside
<br />South) ,have been at least slightly calcareous. In most cases, however, cementation has been weak,
<br />and test fragments immersed in water frequently disintegrate, leaving considerable clay in
<br />suspension. Pitman, et. o!. (1989, p. G25), refemng to the Mesaverde shoreline-marine sandstones
<br />generally, state that "The relative abundances of both authigenic and detrital clay have significantly
<br />reduced porosity and permeability." In discussing the potential of these sandstones as natural gas
<br />reservoirs, they further state:
<br />Porosity in the shoreline-marine zone is 1.6-8.9 percent aad probably resulted mostly from
<br />dissolution of rock fragments and matrix material ... Although some open pores exist
<br />between detrital framework grains ... ,most inter- and intragranular pores show extensive
<br />authigenic clay-mineral growth. No textural evidence indicates that dissolved framework
<br />• grains were replaced by carbonate prior to dissolution even in sandstones containing abundant
<br />carbonate. A tortuous, disconnected pore network resulting from clay bridges and pore
<br />throats filled with clay ...and small capillaries has significantly reduced fluid flow,
<br />and permeability determined by conventional laboratory techniques is low (<0.01 mD~j.
<br />[Omissions are figure references; emphasis added.]
<br />Electric logs from core holes drilled in Rapid and Cottonwood canyons indicate an increase in
<br />resistivity at the top of the Rollins Sandstone. Such an increase can be caused by a number of
<br />factors, including high coalified plant debris content, presence of natural gas, empty pore space, or
<br />fresh water. SP, gamma density, and natural gamma logs do not indicate any of these, which would
<br />produce a corresponding decrease in density and natural gamma and varying SP responses. It is
<br />most likely that these logs reflect the abundance of clay matrix material in the uppermost part of the
<br />Rollins.
<br />Gas production from the Cozzette Sandstone is attributed by Pitman, et. a/., to naturally-occurring
<br />fractures (1989, p. G25). In the Plateau Creek gas field, the western edge of which is only about five
<br />miles northeast of Cameo, an average permeability of 0.06 mD has been reported for the Rollins
<br />Sandstone (Johnson, 1989, p. E29). Johnson (]989, p. E30) states that Mesaverde sandstones
<br />(specifically the Corcoran, Couette, and Rollins) have low permiabilities as the result of intense
<br />regional diagenesis, which filled pore spaces with quartz, dolomite, calcite, illite, mixed-layer
<br />illite/smectite, kaolinite, authigenic feldspar, and iron-rich chlorite. He goes on to state that "A
<br />• growing amount of evidence indicates that natural fractures aze the primary conduits for fluid
<br />v Millidarcy.
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