Abstract
End_Page 556------------------------------Sandstones in the Cotton Valley Group are poor-quality gas reservoirs that require massive hydraulic fracturing for economic production. Burial diagenesis of these rocks has resulted in extensive cementation and grain replacement that, with few exceptions, has reduced porosity to less than 10% and permeability to less than 0.1 md. Major diagenetic minerals are quartz and carbonates (calcite, ankerite-dolomite) with clay minerals, albite, and anhydrite present in subordinate amounts. The abundance of cement is related in part to initial sandstone composition. Cotton Valley sandstones are very fine to fine-grained, moderately to well-sorted quartzarenites and subarkoses. In quartzose sandstones, pore-filling cements average between 15 and 20% of the rock volume, whereas in feldspathic s ndstones cements usually comprise over 20% of the rock volume. These cements eliminate most of the primary intergranular porosity leaving a series of nearly isolated voids connected by submicron-size pore throats. Although blockage of pore throats by authigenic clay minerals locally contributes to low permeability, quartz and carbonate cements usually line pores and appear to more effectively block pore throats. Porosity preserved in the Cotton Valley consists of primary intergranular and secondary dissolution voids. Most secondary porosity results from dissolution of feldspar and chert; a sparse amount of secondary voids originate from dissolution of shell fragments and possibly calcite cement. Dissolution porosity is locally the most abundant type of porosity, particularly in feldspathic sandstones, but is usually associated with inadequate permeability to improve reservoir quality. Secondary voids are not interconnected because their distribution depends on the presence of soluble grains, which are dispersed in the sandstones. Highest permeability is present where primary intergranular porosity forms over 50% of the total porosity. End_of_Article - Last_Page 557------------
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