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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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The great majority of Minnelusa Formation (Pennsylvanian-Permian) oil production in the eastern Powder River basin is derived from various types of stratigraphic traps which resulted from paleotopographic relief developed on the upper Minnelusa. This relief is mirrored by thickness variations in the overlying Opeche Shale (Permian). Construction of isopachous maps of the Opeche is one of the methods used to explore for paleotopographic traps in the Minnelusa.
Hand-contoured Opeche isopachous maps may be subject to ambiguous interpretations in areas where the data points are scattered or nonexistent. This difficulty is partially overcome when the isopachous map is produced by mathematical methods.
The upper Minnelusa paleotopography is believed by the author to reflect eolian sand dunes encased by the red shale of the Opeche. Observations from oil tests in the area indicates
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that this paleotopography has a cyclicity, with a crest wavelength of approximately 3 mi (5 km). Double Fourier transforms are most appropriately used in modeling where such a cyclicity exists.
The resulting double Fourier transform-generated computer model of the upper Minnelusa paleotopography shows a good correlation between the observed data points and the calculated best-fit surface. Additionally, the computer generated surface suggests areas away from present production and drilling which may warrant further exploration.
The computer generated surface data must, however, be integrated with other known geologic data and examined closely in areas where the control point spacing exceeds either the x or y direction fundamental wavelengths.
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