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Abstract
2004. The American Association of Petroleum Geologists. All rights reserved.
Petrology and Petrophysics of the Lance Formation (Upper Cretaceous), American Hunter, Old Road Unit 1, Sublette County, Wyoming
J. C. Webb,1 S. G. Cluff,1 C. M. Murphy,1 A. P. Byrnes2
1The Discovery Group, Inc., Denver, Colorado, U.S.A.
2Kansas Geological Survey, The University of Kansas, Kansas, U.S.A.
ACKNOWLEDGMENTS
This chapter would not have been possible without the donation of core to the U.S. Geological Survey Core Research Center by American Hunter, Ltd., Calgary, Canada. The staff at the Core Research Center is responsible for archival preservation and maintenance of the core samples and analytical data. Triple O. Slabbing, Denver, Colorado, slabbed cores from the middle and lower Lance. Basin Exploration and Stone Petroleum funded core slabbing. Snyder Oil Company supported special core analysis. Core analysis and DST data were provided by the operator and are on file with the Wyoming Oil and Gas Conservation Commission, Casper, Wyoming, and at the U.S. Geological Survey Core Research Center, Denver, Colorado. Drafting of the core description was completed by Ken Graff.
ABSTRACT
The American Hunter Old Road unit 1 was drilled to test multiple, stacked sandstones in lower Tertiary and Upper Cretaceous sandstones but was completed in 1981 as a water well producing from a shallow aquifer. Poor fluid recoveries from a drillstem test in the middle Lance Formation indicated a low-permeability reservoir with probable formation damage. Two cores, totaling 106 ft (32 m), were cut in the middle and lower Lance Formation. The cored intervals consist of thin- to medium-bedded, fine- to medium-grained sandstone and chert pebble conglomerate interbedded with silty mudstone and shale. Conglomeratic beds are cross-bedded and massive to normally and inverse graded. Sandstones are trough cross-bedded, massive and ripple laminated, and locally root mottled. Mudstones are rooted and burrowed. Conglomerate and sandstone were deposited in fining-upward genetic units by small meandering rivers. Mudstones were deposited in flood plain, swamp, lacustrine, and brackish to normal marine environments.
Sandstones consist of sublithic to lithic arenite and are cemented by moderate amounts of mixed-layer illite-smectite and quartz, sparse siderite, pyrite, ferroan calcite, and kaolinite. Feldspars are very sparse. Rock fragments are dominated by chert, with sparse limestone, dolomite, shaly and silty mudstone, reworked glauconite, and phosphate grains. Porosity consists of moderate to sparse, modified primary intergranular, clay-lined and clay-filled intergranular, secondary intergranular and grain-moldic pores, and natural fractures. Most natural fractures are filled by kaolinite but retain some permeability. Core-measured in-situ porosities range from 4 to 13%. In-situ Klinkenberg permeabilities range from 0.001 to 2.66 md. Irreducible
water saturations, representing water saturations at gas column heights of approximately 750 ft (230 m), range from 21 to 72%, in close agreement with log-derived data, indicating that most of the potential reservoir sandstones are at or near irreducible water saturation. Relative gas permeability at irreducible water saturation ranges from 46 to 99% of the absolute permeability. Common clay-filled microporosity is responsible for low measured permeability, relatively high values of irreducible water saturation, and moderate susceptibility to formation damage.
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