JONAH FIELD, SUBLETTE COUNTY, WYOMING - GAS-PRODUCTION FROM OVERPRESSURED UPPER CRETACEOUS LANCE SANDSTONES OF THE GREEN RIVER BASIN

Jonah field, located in the northwestern Green River basin, Wyoming, p roduces gas from overpressured fluvial channel sandstones of the Upper Cretaceous Lance Formation, Reservoirs exist in isolated and amalgama ted channel facies 10-100 ft (3-30 m) thick and 150-4000 ft (45-1210 m ) wide, deposited by meandering and braided streams, Compositional and paleocurrent studies indicate these streams flowed eastward and had t heir source area in highlands associated with the Wyoming-Idaho thrust belt to the west. Productive sandstones at Jonah have been divided in to five pay intervals, only one of which (Jonah interval) displays con tinuity across most of the field. Porosities in clean, productive sand stones range from 8 to 12%, with core permeabilities of .01-0.9 md (mi llidarcys) and in-situ permeabilities as low as 3-20 mu d (microdarcys ), as determined by pressure buildup analyses. Structurally, the field is bounded by faults that have partly controlled the level of overpre ssuring. This level is 2500 ft (758 m) higher at Jonah field than in s urrounding parts of the basin, extending to the top part of the Lance Formation, The field was discovered in 1975, but only in the 1990s did the area become fully commercial, due to improvements in fracture sti mulation techniques, Recent advances in this area have further increas ed recoverable reserves and serve as a potential example for future de velopment of tight gas sands else-where in the Rocky Mountain region.