Petrology and Petrophysics of the Lance Formation (Upper Cretaceous), American Hunter, Old Road Unit 1, Sublette County, Wyoming
J. C. Webb, S. G. Cluff, C. M. Murphy, A. P. Byrnes, 2004. "Petrology and Petrophysics of the Lance Formation (Upper Cretaceous), American Hunter, Old Road Unit 1, Sublette County, Wyoming", Jonah Field: Case Study of a Tight-Gas Fluvial Reservoir, John W. Robinson, Keith W. Shanley
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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 glau-conite, 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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The discovery of a giant natural gas field within a mature petroleum province is a significant event. Understanding the factors that control such an accumulation is important if the oil and gas industry is to continue to develop natural gas resources. Jonah field, in the Greater Green River basin of southwest Wyoming, is the largest natural gas discovery in the onshore United States in the last 10-15 years with recoverable reserves ranging from 8 to 15 tcf natural gas. Since beginning widespread field development in August 1992, Jonah has produced approximately 1 tcf gas, 10.3 million barrels of oil, and 3.7 million barrels of water. Field production is still increasing with daily production presently at 666 MMCFGPD, 5800 BOPD, and 4000 BWPD from approximately 600 wells. Active drilling continues within the field as operators consider widespread downspacing. By virtue of being a tight-gas field, Jonah is, in many respects, nontraditional. Recent assessments of natural gas potential, for both the U.S. and the world, strongly suggest that most future gas resources will come from low-permeability sandstones in the deeper portions of sedimentary basins, and from fields that will undoubtedly share characteristics with Jonah. The subtle structure, the low-permeability nature of the reservoir, the challenging petrophysics, and the environmental sensitivity surrounding Jonah may foreshadow what explorationists have to look forward to as the demand for natural gas increases, not only in the United States, but throughout the world. This volume brings together previously unpublished material on Jonah field and attempts to integrate all aspects including geology, geophysics, reservoir engineering, drilling and completion, and regulatory affairs. As such, this is a definitive collection that provides a truly integrated perspective of this giant field.