Burial-history Modeling of the Jonah Field Area: An Unusual and Possibly Unique Gas Accumulation in the Green River Basin, Wyoming
Robert J. Coskey, 2004. "Burial-history Modeling of the Jonah Field Area: An Unusual and Possibly Unique Gas Accumulation in the Green River Basin, Wyoming", Jonah Field: Case Study of a Tight-Gas Fluvial Reservoir, John W. Robinson, Keith W. Shanley
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The hydrocarbon accumulation at Jonah field is the result of a complex series of temporal and spatial geologic events. This chapter is a preliminary investigation using burial-history modeling and petroleum-systems analysis to evaluate the generative potential of source rocks in the Jonah and Pinedale anticline areas and determine the timing of generation and expulsion. To test the ability of the Lance to self-generate significant hydrocarbons through thermal maturation, burial-history models were constructed with source intervals containing type III kerogen and average initial total organic carbon values ranging from 1.00 to 1.25%. Models built using optimistic charge parameters show that in the Jonah area, the Lance is capable of generating 1.79 tcf gas and, depending on the saturation threshold applied, can only expel from 0 to 0.64 tcf gas. Source rock pyrolysis data indicate that the Lance does not contain sufficient organic material, and burial-history models calibrated with vitrinite reflectance data suggest that potential Lance source material was not exposed to the thermal conditions necessary to generate and expel the quantities of hydrocarbons estimated to be present in the Jonah trap. Generation, expulsion, and migration (vertical and/or horizontal) from the Mesaverde Group and Rock Springs Formation or deeper hydrocarbon sources such as the Mowry Shale are necessary to account for the in-place hydrocarbons. The source potential of the coal-bearing lower Mesaverde is significantly greater that that of the Lance, and modeling suggests that in the Jonah area, it is capable of generating 5.48 tcf and expelling 4.06–5.12 tcf gas.
Modeling suggests that within the Jonah field area, the source rocks included in the Lance, upper Mesa-verde, and lower Mesaverde formations can only provide the Jonah trap with between 2.61 and 3.98 tcf gas in place. This gas volume is significantly less than current in-place estimates of 8.3 tcf and suggests that gas must be generated and migrated from either deeper sources such as the Hilliard Shale or Mowry Shale and/or have migrated to the Jonah trap from a larger fetch area.
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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.