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The northern Pinedale field is a complex set of stacked gas-bearing sandstones in a section approximately 6000 ft (1800 m) thick. The main reservoirs occur in the Upper Cretaceous Lance Formation and the Upper Mesaverde interval above the Ericson Sandstone.

Gas-charged sands are first penetrated at the top of overpressure in the Wagon Wheel Formation of Paleocene age. The pressure gradient increases with depth through the Lance and Upper Mesaverde sections. The maximum pressure gradient in the Mesa area (T31N, R109W) is 0.8 psi/ft in the Upper Mesaverde interval. In the Stewart Point area at the north end of the anticline (T32N, R109W), the maximum pressure gradient approaches 0.85 psi/ft over the same interval.

The reservoir consists of fluvial sandstones classified as litharenites. Mineralogy is dominated by quartz, chert, and clay with minor components of feldspar, calcite, and dolomite. Clays form from 6 to 19% of the rock volume and are illite, kaolinite, and chlorite.

Ten wells were cored in the study area, including three for special core analysis. Two wells were cored with tritium-traced mud to quantify the filtrate invasion. Special core analyses were performed on preserved samples. These include partition of fluids, formation factor, resistivity index, capillary pressure, and relative permeability of gas to water.

Porosity ranges from 4 to 13% and permeability ranges from 0.0001 to 0.1 mD in reservoir sandstones under in situ conditions. The average net-to-gross for pay sandstones in the field is 21%. The Mesa area generally has more sandstone than the Stewart Point area. Average porosity and water saturation for pay sandstones are 8.7% and 34%, respectively.

At in situ reservoir conditions of 1000 to 2000 psi net mean stress, the porosity reduction is less than 6% compared to laboratory conditions of 800 psi. Permeability reduction is 5 to 60% from laboratory conditions and up to 80% during late stage depletion. The permeability reduction as a result of increasing net mean stress is governed by original permeability and clay content. Relative permeability of gas with respect to water saturation is important in tight sandstones. Water saturations exceeding 50% in clean sandstones significantly reduce gas permeability by one to two orders of magnitude.

Capillary pressure curves indicate that columns heights range from 200 to 1000 ft (60–300 m) in individual reservoir sandstones. These limited column heights combined with the increasing pressure gradients with depth indicate a series of stacked gas columns rather than a single continuous column.

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