Petrophysics of the Lance and Upper Mesaverde Reservoirs at Pinedale Field, Sublette County, Wyoming, USA
Suzanne G. Cluff, Robert M. Cluff, Daniel G. Hallau, Ryan J. Sharma, 2014. "Petrophysics of the Lance and Upper Mesaverde Reservoirs at Pinedale Field, Sublette County, Wyoming, USA", Pinedale Field: Case Study of a Giant Tight Gas Sandstone Reservoir, Mark W. Longman, Stephen R. Kneller, Thomas S. Meyer, Mark A. Chapin
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Pinedale field is a giant gas field producing from extremely low porosity and permeability sandstones. Wireline log data from 127 wells covering the entire field were studied to characterize the porosity, permeability, and water saturation of the Lance and Upper Mesaverde reservoirs. The logs were environmentally corrected and normalized, shale volume and porosities were calculated, water saturations were determined by the Dual Water model, and net pay was calculated using field-specific pay criteria.
Within the entire Lance Formation, which ranges from 3580 to 4780 ft (1090–1460 m) in thickness, the average well has 1890 ft (580 m) of net sandstone (less than 75 api units on the gamma-ray log) with an average log-determined effective porosity of 5.7%. The average permeability of all sandstones, estimated from core data-derived equations, is only 20 microdarcies (0.02 mD). The average water saturation of all sandstones is 52%. Using 5% porosity and 60% water saturation as absolute net pay cutoffs, the average net pay thickness of the Lance reservoirs at Pinedale is 1050 ft (320 m).
A substantial section of Upper Mesaverde sandstones is also gas productive at Pinedale. Out of a total section ranging from 80 to 800 ft (25–225 m) in thickness, the average well has 420 ft (130 m) of net sandstone of which 200 ft (60 m) is net pay using the same cutoffs used in the Lance. This represents just 15% of the gross Upper Mesaverde section. The average porosity of the Upper Mesaverde sandstones is 5.0% with an average water saturation of 42%.
The major difference between Jonah and Pinedale fields is the total interval thickness saturated with gas, which is almost 2.5X greater at Pinedale than at Jonah. Although the porosity and gas saturations at Pinedale are on average lower than at Jonah, the greater net thickness more than compensates for the difference in reservoir quality accounting for the very high productivity of wells at Pinedale.
Evaluation of water saturation trends versus structural elevation, both determined from as–received cores and from log modeling, shows no systematic trend in gas saturation with height. Although the apparent gas saturated section at Pinedale is over 7000 ft (2130 m) in thickness, water saturation does not decrease consistently up section as might be expected. This, combined with pressure versus depth profiles based on mud weights, indicates Pinedale is not a simple single gas column but rather is a series of separate and overlapping reservoir compartments separated by imperfect seals. The saturation attained in any given reservoir compartment is likely set by the capillary pressure characteristics of the overlying sealing facies, so that the minimum saturations observed in the field reflect only a few hundred to not more than 2000 ft (610 m) of gas column.