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Most Minnelusa Formation (Pennsylvanian-Permian) oil production in the Powder River basin is from paleotopographic traps. These traps
occur where upper Minnelusa dune sands are encased in the overlying supratidal red Opeche Shale (Permian). The morphology of these sands suggests northwest-southeast-trending barchanoid sand ridges.
Thickness variations in the Opeche mirror the relief on the Minnelusa surface. Opeche isopahcous maps are one of the main methods used to explore for Minnelusa paleotopographic traps. Hand-contoured isopachous maps can be subject to ambiguous interpretations in areas of low-density control. This difficulty is partially overcome when the map is mathematically produced.
Observations from oil tests in the area indicate that Minnelusa paleotopography is cyclic with a wavelength of approximately 3 mi (5 km). Double Fourier transforms are appropriate in modeling this kind of cyclic data.
For a test township, the calculated double Fourier surfaces showed good correlation with the actual data values. This technique was then applied to a Minnelusa prospect in Campbell County, Wyoming.
Double Fourier surfaces were calculated for several structural datums and isopach intervals. Additionally, regional dip was determined from a polynomial fit, the section was restored to horizontal, and then was modeled to reveal paleotopography.
The paleotopographic-high axes and Opeche thin axes showed remarkable coincidence. This trend is believed to represent the trace of a paleo sand dune.
A test well sited using conventional geologic methods plus input from the double Fourier maps confirmed the accuracy of the calculated surface.
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