Geology of Jonah Field, Sublette County, Wyoming
D. P. Dubois, P. J. Wynne, T. M. Smagala, J. L. Johnson, K. D. Engler, B. C. McBride, 2004. "Geology of 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 located in Sublette County, Wyoming, and lies in the southeastern portion of the Hoback basin, a northwestern extension of the Greater Green River basin. The field is confined by the intersection of two subvertical shear fault zones that form a wedge-shaped structural block. The updip termination at the southwest end of the field is the apex of the block. The downdip limit is somewhat arbitrarily defined as occurring along the synclinal axis separating the basin flank from the Pinedale anticline to the northeast. Within the wedge-shaped block, overpressure conditions exist near the top of the Upper Cretaceous (Maastrichtian) Lance Formation, 2000–3000 ft (610–915 m) above regional occurrence. Immediately to the west and south of the field, overpressure conditions are present near the top of the Upper Cretaceous (Campanian) Mesaverde Group. The trap at Jonah is described as combination structural-stratigraphic. The bounding fault zones form the lateral trap, and the top seal is comprised of the mudstones that are intercalated with the reservoir sandstones of the Lance.
Sandstones in the Lance Formation are the principal reservoir at Jonah field. The Lance Formation is comprised of braided to meandering fluvial sandstones intercalated with overbank siltstones and mudstones. Similar sandstone facies in the upper Mesaverde Group are locally productive. The gross thickness of the Lance Formation increases toward the downdip limit of the field. Near the updip termination, the Lance is 2000 ft (610 m) thick, whereas at the northeastern side of the field, it attains a thickness in excess of 3000 ft (915 m).
Overpressure increases storage capacity and gas saturation in the reservoir and allows for subtle preservation of better porosity relative to sandstones outside the field boundary. Original gas in place in the Lance Formation is estimated to be more than 8.3 tcf. Subcompartments formed by faults inside the field exhibit better per-well recovery near their updip edge; poorer performance is present in downdip regions of each compartment. Pore pressure in each compartment increases by about 1 psi/ft of depth or more than twice the normal hydrostatic gradient. Pressure data suggest that migration of hydrocarbons into the Jonah field compartment is occurring currently. Liquid condensate yield from the gas production increases with depth. Despite high pressures, production from the deepest sandstones tends to be poor because of low permeability and the impact of hydrocarbon liquids on relative permeability.
The lenticular nature of the fluvial sandstones in the Lance has created highly complex reservoir architecture and is a significant challenge to the gas-recovery process. Connectivity is poor, as indicated by the difficulty in correlation of individual sand bodies between wellbores positioned on 40-ac (16-ha) spacing.