Petrology and Petrophysics of the Middle Lance Formation (Upper Cretaceous), American Hunter Old Road Unit No. 1, Sublette County, Wyoming

Webb, John C.¹, Suzanne G. Cluff¹, Catherine M. Murphy¹, Alan P. Byrnes^2
1 The Discovery Group, Inc, Denver, CO
² Kansas Geological Survey, University of Kansas, Lawrence, KS

The American Hunter Old Road Unit no. 1 was drilled in 1981. It is located 10 miles south of Jonah field. Drill stem test data in the Middle Lance indicated a low permeability formation with probable formation damage.

Core in the Middle and Lower Lance recovered thin to medium bedded, fine to medium sandstone and chert pebble conglomerate, interbedded with silty mudstone and shale. Conglomeratic beds are cross-bedded, massive and graded. Sandstones are massive, trough cross-bedded and ripple laminated, with local rooted zones. Small meandering rivers deposited the sandstones and conglomerates. Mudstones are root mottled, homogeneous, and locally carbonaceous, consistent with deposition in overbank, swamp and lacustrine settings.

Sandstones are sublithic to lithic arenite, with cementation by moderate amounts of mixed-layer illite-smectite and quartz, rare siderite, pyrite, ferroan calcite and kaolinite. Porosity consists of moderate to rare mesopores, moderate to common clay-lined intergranular microporosity) and kaolinite-filled natural fractures.

Mean core porosity is 10.4%, mean in situ gas permeability is 0.81 md. “Irreducible” water saturations (Swi) in the sandstones range from 21% to 50%, and closely agree with log-derived saturations, indicating that most of the potential reservoir sandstones are at or within 10-15% of Swi. In situ relative permeability to gas measured at Swi ranges from 46% to 99% of the absolute permeability. The common pore-bridging and pore-filling clay cement is responsible for the low absolute and relative permeabilities, and relatively high values of Swi for the lower permeability rocks and moderate to severe susceptibility to formation damage.