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.