Petrophysics of the Lance Sandstone Reservoirs in Jonah Field, Sublette County, Wyoming
Suzanne G. Cluff, Robert M. Cluff, 2004. "Petrophysics of the Lance Sandstone Reservoirs in Jonah Field, Sublette County, Wyoming", Jonah Field: Case Study of a Tight-Gas Fluvial Reservoir, John W. Robinson, Keith W. Shanley
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Jonah field is a giant gas field producing from extremely low-porosity and low-permeability sandstones. Wire-line–log data from 62 wells near the center of the field were studied to characterize the porosity, permeability, and water saturation of the Lance reservoirs. The logs were environmentally corrected and normalized, shale volume and porosities were calculated, water saturations were determined by the dual water model, and net pay was calculated using field-specific pay criteria. Ultimate gas recovery per well was estimated by decline curve analysis of monthly production data.
Within the upper 2500 ft (760 m) of the Lance Formation, which includes the entire productive interval in nearly all wells, the average well has 1000 ft (30 m) of net sandstone, having an average porosity of 6.4%. The average permeability of all sandstones, estimated from core data-derived equations, is an astonishingly low 6 μd. The average water saturation of all sandstones is 45%.
Net pay criteria were determined from cumulative storage-capacity and cumulative flow-capacity plots. Although the average sandstone may have only 6% porosity, the low-porosity sandstones contribute an insignificant fraction of the reservoir flow capacity. We estimate that more than 95% of the flow capacity is from sandstones with greater than 6% porosity. A small percentage of high-porosity (>10%) and high-permeability rocks dominate the flow behavior of the reservoir and are probably critical to economic production. Using 6% porosity as an absolute net pay cutoff, the average net pay thickness at Jonah is 440 ft (130 m), with 9.3% porosity and 33% water saturation. The estimated average permeability of net pay is 25μd. Estimated ultimate recovery per well is approximately 4 bcf gas on current 40-ac (0.16-km2) well spacing.
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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.