--> Abstract: Clays and Cements — Their Effect on Dakota Sandstone Pores, Sand Wash Basin, Colorado, by R. J. Vinopal and G. M. Grammer; #90937 (1998).

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Abstract: Clays and Cements — Their Effect on Dakota Sandstone Pores, Sand Wash Basin, Colorado

VINOPAL, ROBERT J., Standard Geological Services, Inc.; G. MICHAEL GRAMMER, Texaco Exploration and Production Technology

Dakota Formation sandstone reservoirs in the Sand Wash Basin, northwestern Colorado, display a wide variation in porosity and permeability that is dependent upon the intensity of silica cementation and upon the type and distribution of anthigenic clay minerals in the pore system. Porosities in well sorted fluvial and marine Dakota sandstones range from 6 to 22 percent. Permeability ranges from less than 0.1 md to 140 red. Quartz overgrowths are the most abundant cement in the Dakota sandstones. Quartz cement occludes 5 to 24 percent porosity. Quartz cementation is most intense in very fine and fine grained quartzose sandstones. Medium grained sandstones, within the same well, generally display less silica cementation. Finer grained sandstones with a higher feldspar content or grain coating corrensite can retain economic porosities. Authigenic clay cements in Dakota sandstone reservoirs include kaolinite, mixed -layer illite/smectite, and corrensite. The volume of clay cements ranges from 0 to 12 percent. Kaolinite is the most widely distributed clay cement, occurring in 70 percent of the study wells. Kaolinite occurs as a patchy pore-filling cement generally in volumes of 2 to 4 percent Kaolinite is not detrimental to matrix permeability, but does present a potential migration-of-fines problem. Mixed-layer illite/smectite and corrensite occur as pore-lining and pore-filling cements. When present in amounts greater than 4 percent, these clays bridge a significant percentage of pore throats, lowering matrix permeability. Corrensite can be present in amounts up to 12 percent, forming a pore system with abundant non-effective microporosity. Mixed-layer illite/smectite and corrensite are sensitive to fresh water and can break loose and migrate during swelling. The structure of these clays and their high cation-exchange capacity cause erroneously high calculated water saturations.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah