Petrophysics of the Lance Sandstone Reservoirs in Jonah Field, Sublette County, Wyoming
Cluff, Suzanne G., Robert M. Cluff
The Discovery Group, Inc, Denver, CO
Jonah field is a giant gas field producing from extremely low porosity and permeability sandstones. Wireline log data from 62 wells near the center of the field were studied to characterize the porosity, permeability, and water saturation of the Lance reservoirs. The logs were environmentally corrected, normalized, shale volume and porosities calculated, water saturations determined by the dual water model, and net pay calculated using field-specific pay criteria. Ultimate gas recovery per well was estimated by decline curve analysis of monthly production data.
Within the upper 2500 ft (762 m) of the Lance Formation, which includes the entire productive interval in nearly all wells, the average well has 1000 ft (30 m) of net sandstone with an average porosity of 6.4%. The average permeability of all sandstones, estimated from core data derived equations, is an astonishingly low six microdarcies (0.006 mD). The average water saturation of all sandstones is 45%.
Net pay criteria were determined from cumulative storage capacity and cumulative flow capacity plots. While the average sandstone may have only 6% porosity, the low porosity sandstones contribute an insignificant fraction of the reservoir flow capacity. We estimate that over 95% of the flow capacity is from sandstones with greater than 6% porosity. A small percentage of high porosity (>10%) and high permeability rocks dominate the flow behavior of the reservoir and are probably critical to economic production. Using 6% porosity as an absolute net pay cutoff, the average net pay thickness at Jonah is 440 ft (134 m) with 9.3% porosity and 33% water saturation. The estimated average permeability of net pay is 25 microdarcies. Estimated ultimate recovery per well is approximately 4 BCFG on current 40-acre well spacing.