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Effects of Thermal Sulfate Reduction on Permeability Distributions of the Norphlet Formation

DUNN, THOMAS L., and RONALD C. SURDAM, University of Wyoming, Laramie, WY

Framework grain coatings are common in the Norphlet. Clay coatings are present throughout the depth range (16,000 to 22,000 ft) over which significant variations of permeability occur. Pyrobitumen coatings occur within the deep, low-permeability interval (approximately 18,000-20,000 ft) and the deeper (greater than 20,000 ft), more permeable interval. Both types of coatings may be important in preserving porosity during portions of the burial history of the Norphlet sandstones; however, their occurrence does not correlate with observed variations in permeability.

Diagenetic reactions associated with thermal sulfate reduction provide a mechanism for the dissolution of carbonate cements in deep

zones characterized by enhanced permeabilities. Protons generated from dissociation of H(2)S produced during sulfate reduction results in the dissolution of carbonate cements. To be effective, this process must remove cements that precipitated after grain coatings. Uncoated quartz grains produce quartz overgrowths.

Vertical permeability distributions within the Norphlet suggest that early and intermediate diagenetic carbonate and sulfate cements, sourced from the intercalated, interdunal pond strata, were redistributed throughout the dune sands. Portions of carbonate cements were either dissolved or the extent of their precipitation was reduced as thermal decarboxylation was closely followed by the initiation of sulfate reduction. Hence, variations in Norphlet permeability distributions are in part the result of diagenetic reactions associated with thermal sulfate reduction and, therefore, can be predicted using kinetic modeling of sulfate reduction.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)