Correlation of Discrete Minnelusa Porosity Intervals and Identification of Common Reservoirs Aided by Computer-Drawn Geologic Cross Sections, Powder River Basin, Northeast Wyoming

James H. Borgerding

The upper Minnelusa Formation in the northeastern Wyoming portion of the Powder River basin continues to challenge geologists. Oil exploration and development success depends heavily on correct correlation of discrete porosity intervals within the sand-dolomite series of the upper Minnelusa oil-bearing interval. Drill cores generally are not available. Correlation work must be performed on the basis of electric logs, drill cutting descriptions, and the expertise provided by the experienced geologist.

Correlation of discrete porosity units and reservoir delineation can be improved by greater uses of the resistivity log, which is generally available in conjunction with the sonic porosity log. The salinity of the water (R_w) commonly shows variance within the upper Minnelusa section, and thus suggests separate confined reservoirs.

Software has been developed for Apple's Macintosh computer that allows a screen display of digitized electric-log data and calculated results. Geologic markers can be picked by user interaction with the screen display. Geologic cross sections can then be printed quickly on the dot matrix printer and no costly plotter bed is required. The cross sections can be presented on either a subsea datum or a user-defined geologic marker. Detailed foot-by-foot water saturation calculations are performed on the digitized data, and plotted on the log cross sections. By means of known or "what-if" water resistivity inputs, identification of separately confined reservoirs is possible. For example, if a discrete sand interval that otherwise appears correlative in a downdip well calculates lower water saturation, that correlation then becomes suspect and a correlation change may be necessary.

Computer generated cross sections showing examples of how correlation work can be aided by detailed calculations from digitized well log data will be presented for poster display. The hardware, consisting of Apple's Macintosh computer, ImageWriter printer, and device for digitizing well logs will be displayed and demonstrated for interested viewers.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.