--> Abstract: Late-Stage Diagenesis and Development of Secondary Porosity in Frontier Formation Low-Permeability Sandstones, Green River Basin, Wyoming, USA, by S. P. Dutton; #91012 (1992).

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ABSTRACT: Late-Stage Diagenesis and Development of Secondary Porosity in Frontier Formation Low-Permeability Sandstones, Green River Basin, Wyoming, USA

DUTTON, SHIRLEY P., Bureau of Economic Geology, University of Texas at Austin, Austin, TX

The Upper Cretaceous Frontier Formation produces gas from low-permeability reservoirs along the Moxa arch in the western Green River basin. Reservoir sandstones were deposited in fluvial-channel and shoreface environments and average 12% porosity and 0.24 md permeability. Petrographic examination of 247 thin sections from 13 cores is the basis for investigation of the diagenetic history. Sample depth varies from 6800 ft at the north end of the arch to 13,000 ft at the south end 90 mi away.

Illite rims and quartz overgrowths precipitated early in the burial history. Late-stage diagenesis included feldspar albitization and kaolinite, chlorite, and calcite precipitation. Dissolution of feldspar, clay clasts, chert, and biotite formed secondary pores. The extent to which calcite precipitated and later dissolved is uncertain, but most calcite cement is not corroded. Thus, most secondary porosity apparently formed by silicate dissolution, and the entire system may have been buffered by silicate reactions. Some secondary pores contain reaction productions of feldspar dissolution, kaolinite and illite in the north (<10,000 ft) and illite and chlorite in the south (>10,000 ft). Small quartz crystals that project into secondary pores indicate that some late quartz cementati n occurred.

The average volume of secondary pores is 4.5%, and they constitute 75% of the macropores. Volume of secondary porosity decreases with increasing burial depth, apparently because of decreased detrital feldspar volume to the south. Original feldspar volume was 9% at the north end of the arch but 4% at the south. Furthermore, only 33% of original feldspar remains at the north end of the arch, compared with 50% at the south. Thus, the north end of the Moxa arch has more secondary porosity because that area had more detrital feldspar and a higher percentage of the feldspar was dissolved during burial diagenesis.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)