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Muddy sandstones form lenticular reservoirs that contain water at both the up-dip and down-dip margins. As in many other stratigraphic traps, oil is produced in the central part of the reservoir where permeability is higher, whereas only water occurs at the margins where permeability is reduced. Consequently, wells that produced oil at the rate of 1800 bbls per day were offset by tests that recovered water at the rate of 1800 bbls per day from the same sandstone and at the same elevation.
The principal Muddy reservoirs are of two types. One is a narrow, sinuous, fluvial sandstone body of limited extent. The other is a fine-grained sandstone that was deposited close to the shoreline during a marine transgression, and this sandstone forms wider, ovate bodies parallel to strike. In the latter section, grain size and quartz content decrease upward while clay matrix and bioturbation increase upward. Similar changes occur laterally so that permeability is reduced from an average 400 md to 17 md or less at the margins of the field. Lateral changes within the reservoir are confirmed by interpretation of logs and cores, and by analysis of shut-in pressures from drill-stem tests.
The distribution of oil and water within the reservoir can be accounted for by capillary-pressure and potentio-metric gradients. The observed oil column is somewhat greater than 130 ft. Calculations show that about 20 ft. was trapped by capillary-pressure changes where permeability is reduced, and 110 ft was trapped by down-dip hydrodynamic flow. These calculations explain the recovery of water adjacent to oil production and suggest that fluid pressure relationships should not be neglected in either exploration for or development of stratigraphic traps.
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