Title: Geology of Jonah Field – New Drilling, New Perspectives
Speaker: Dean P. DuBois, EnCana Oil & Gas (USA) Inc.
Date: July 19, 2002
Publication: The Outcrop, July 2002, p. 4
Jonah Field is located in Sublette County, Wyoming and lies on the southwestern flank of the Hoback Basin, a northwestern extension of the Greater Green River Basin. The field is confined by the intersection of two sub-vertical shear fault systems 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 some 2,000 to 3,000 ft. above regional occurrence. Adjacent to the field, overpressure conditions are found near the top of the Upper Cretaceous Mesaverde Group.
The sandstones in the Lance Formation comprise 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 occasionally productive. The gross thickness of the Lance Formation increases toward the downdip limit of the field. Near the updip termination, the Lance is 2,000 ft. thick, while at the northeastern side of the field it attains a thickness in excess of about 3,000 ft.
Overpressure increases storage capacity and gas saturation in the reservoir and allows for subtle preservation of better porosity relative to Lance sandstones outside the field boundary. Sub-compartments formed by additional faults inside the field exhibit better per-well recovery near the updip edges grading to poorer performance in downdip regions. Pore pressure within the overpressure compartment increases by about 1 psi/ft of depth or more than twice the normal hydrostatic gradient. Liquid condensate yield from the gas production also increases with depth. Despite high pressures, production performance from the deepest Lance sands tends to be poor due to low permeability and the impact of hydrocarbon liquids on relative permeability.
The lenticular nature of the fluvial Lance Formation sandstones presents a significant challenge to the gas recovery process. There is poor connectivity as indicated by difficulty in correlation of individual sand bodies between wellbores positioned for 40-acre spacing. Two models for sand body geometry have been analyzed and the one that characterizes a mix of braided and meandering fluvial deposition is preferred.