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Shirley P. Dutton1, Christopher D. White1, Brian J. Willis1
(1) Bureau of Economic Geology, Austin, TX

Abstract: Calcite cement distribution and its effect on fluid flow in a deltaic sandstone, Frontier Formation, Wyoming, USA

Precipitation of extensive calcite cement during burial diagenesis modifies the depositional permeability distribution in a sandstone reservoir and affects fluid flow during production. To simulate flow in cemented reservoirs, reservoir models must use permeability distributions that incorporate the diagenetic overprint. It is impossible to determine horizontal diagenetic heterogeneity from subsurface data, but outcrops can provide a continuous image of interwell-scale cement distribution.

The size and distribution of calcite concretions were mapped in a study of outcrops of the Frewens sandstone, Frontier Formation, in central Wyoming. Calcite cement in the Frewens sandstone occurs as large ellipsoidal concretions that generally follow bedding; many are located adjacent to shale beds. Median thickness of the concretions is 0.6 m, length 4.5 m, and width 5.7 m. Median volume is 5.2 m . The concretions increase in abundance toward the flooding surface overlying the sandstone and thus are most abundant in the high-permeability facies at the top of the sandstone body. Lateral variation in the fraction of the sandstone thickness cemented by calcite has a normal distribution, with a mean of 12% (^sgr = 4.5%). Indicator semivariograms of cement have a range of 30 m horizontally and 2.5 m vertically, dimensions that correspond approximately to the size of the largest concretions. Flow models indicate that concretions make flow paths more tortuous, cause breakthrough lower in the section, and reduce effective permeability by 50%.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana