Hydrodynamic Flow in Lower Cretaceous Muddy Sandstone, Gas Draw Field, Powder River Basin, Wyoming

Abstract

Lower Cretaceous Muddy sandstones form a simple stratigraphic trap at Gas Draw field, northeast Powder River Basin. The Muddy at Gas Draw can be subdivided into six zones. The lowest, sixth Muddy sandstone is, fluvial in origin, and the overlying fifth sandstone is a transgressive marine deposit. The fourth zone represents a fluvial origin below to shallow marine above. The third zone is interpreted to be of fluvial-overbank origin with possibly a bay-lagoonal association. The second sandstone is suggested to be a deposit of a fluvial or deltaic environment. The first zone was deposited in a lagoonal and poorly-drained marsh environment. At Gas Draw, the second Muddy sandstone is the major producing zone and has the highest average porosity and permeability of 22.6% and 209 md. It had high initial production rates of up to 1200 BOPD (191 m3/day). From correlation of well logs, the Muddy reservoirs appear to be separated by thin shales, but analysis of drill stem tests show that these reservoirs are connected at some points within the area. Analysis of drill-stem test pressures provides the basis for interpreting fluid potential relationships. The average potentiometric gradient of 32 ft/mi across the field results in a hydrodynamic oil column of 210 feet, whereas capillary-pressure differences due to permeability changes can account for only 38 feet of oil column. The ob­ served oil column over most of the field has a height somewhat greater than 250 feet which agrees well with total calculated oil column of about 248 feet. Furthermore , local decrease in permeability to oil may be responsible for water production at any place within the field, even updip from the producing area. Therefore, the trapping capacity of this stratigraphic trap is not simple but depends on rock properties and fluid-pressure relationships within the reservoir .