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Abstract

AAPG Bulletin, V. 86, No. 12 (December 2002), P. 2007-2021.

Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

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

Shirley P. Dutton,1 Christopher D. White,2 Brian J. Willis,3 Djuro Novakovic4

1Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, 78713; email: [email protected]
2The Craft and Hawkins Department of Petroleum Engineering, CEBA Building Room 3516, Louisiana State University, Baton Rouge, Louisiana, 70803; email: [email protected]
3Texas A&M University, Department of Geology and Geophysics, College Station, Texas, 77843; email: [email protected]
4The Craft and Hawkins Department of Petroleum Engineering, CEBA Building Room 3516, Louisiana State University, Baton Rouge, Louisiana, 70803; email: [email protected]

AUTHORS

Shirley P. Dutton received a B.A. degree from the University of Rochester and an M.A. degree and Ph.D. from the University of Texas at Austin, all in geology. She is a senior research scientist at the Bureau of Economic Geology. Her research focuses mainly on sedimentology, sedimentary petrology, and clastic diagenesis, particularly the effect of diagenesis on reservoir quality.

Christopher White is a petroleum engineer with research interests in reservoir engineering and statistics. He earned his Ph.D. from Stanford University in 1987. White is an assistant professor of petroleum engineering at Louisiana State University and formerly worked at the Bureau of Economic Geology and at Shell Development Company.

Brian Willis is an assistant professor in geology at Texas A&M University. His research interests include understanding sequence stratigraphic controls on clastic deposition and quantifying sedimentologic variations to predict reservoir heterogeneity. He received a B.S. degree from the University of Minnesota and an M.S. degree and Ph.D. from Binghamton University (New York). He has been a research fellow at the Smithsonian's Nature History Museum, a research scientist at the Bureau of Economic Geology, a visiting professor at SUNY-Oswego, and a geologist with BP Canada Energy Company.

Djuro Novakovic is a petroleum engineer with professional interest in reservoir engineering. He holds an M.S. degree in petroleum engineering and is a doctoral candidate at Louisiana State University.

ACKNOWLEDGMENTS

This research is a product of the Clastic Reservoirs Group at the Bureau of Economic Geology, University of Texas at Austin, sponsored by Amoco Production Company; BP Exploration Operating Company Limited; Chevron Oil Field Research Company; Conoco, Inc.; Elf Exploration Production; Exxon Production Research Company; Intevep S.A.; Japan National Oil Company; Maxus Energy Corporation; Occidental International Exploration and Production, Inc., and OXY USA, Inc.; Oryx Energy Company; Saga Petroleum; Statoil; and Union Oil Company of California. Parts of this research were supported by the Craft and Hawkins Department of Petroleum Engineering at Louisiana State University. Partial support of publication costs was provided by the Owen-Coates Fund of the Geology Foundation, University of Texas at Austin. Janok P. Bhattacharya interpreted the regional stratigraphic setting of the Frewens and other Frontier sandstones in the outcrop and adjacent subsurface area. James Jennings provided guidance and insight into geostatistical procedures. Sharon Gabel, Charl Broquet, and Christopher Swezey did fieldwork, mapped, and helped interpret the geology. Yugong Gao prepared the digitized outcrop diagrams. Isotopic analyses were done at the Stable Isotope Laboratory, directed by Peter Swart, at the University of Miami's Rosenstiel School of Marine and Atmospheric Science. Reservoir simulation software was provided by Computer Modeling Group, Ltd., and Schlumberger Technology Company. Illustrations were prepared by the graphics staff of the Bureau of Economic Geology under the direction of Joel Lardon, graphics manager. Thomas L. Dunn, Earle F. McBride, an anonymous reviewer, and Bulletin editor John C. Lorenz provided constructive reviews that improved this article. Published by permission of the director, Bureau of Economic Geology, University of Texas at Austin.

ABSTRACT

Precipitation of extensive calcite cement during burial diagenesis can strongly modify the depositional permeability of a sandstone reservoir and affect fluid flow during production. To predict subsurface flow through cemented reservoirs, permeability distributions used in fluid-flow models must reflect this diagenetic overprint. Calcite cements in sandstones commonly occur as irregularly distributed concretions, which makes it difficult to predict diagenetic permeability modifications in the subsurface from typically spaced wells. Outcrops can provide a continuous image of heterogeneity produced by concretionary calcite cements.

The size and distribution of calcite concretions were mapped in outcrops of the Frewens sandstone, Frontier Formation, in central Wyoming. Large, tabular calcite concretions in this deltaic sandstone generally follow basinward-inclined bedding. Median thickness of the concretions is 0.6 m, length is 4.2 m, and width is 5.3 m. The highest cement fraction is in the high-permeability facies at the top of the sandstone body. Concretion centers are approximately Poisson distributed within the sandstone. The upward-increasing cement fraction is caused by upward-increasing concretion size. Lateral variation in the fraction of the sandstone cemented by calcite has a normal distribution, with a mean of 12% (s = 5%).

Spatial distribution of calcite cement in the Frewens sandstone was modeled using indicator geostatistics. Variograms were inferred from outcrop maps of cement. 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. Stochastic images of cement were created using indicator simulation with vertically varying cement proportion.

Flow models indicate that concretions make flow paths more tortuous and retard flow in the coarser facies near the top of the sandstone. The fastest path through the sandstone is in the lightly cemented, high net-to-gross center of the sandstone body. Because the cement mainly occurs within the highest permeability facies in the sandstone body, a model based on depositional facies alone would overestimate upscaled permeability of the Frewens sandstone.

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