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Paleomagnetism provides a powerful tool for initially determining or verifying the compass orientation and top direction of conventional borehole cores from the Mid-Continent region. This is demonstrated by a study of four fully oriented diamond cores of Pennsylvanian fine-grained sandstone and siltstone taken to determine the orientation of a system of vertical fractures thought to control production in the Horizon Cleveland field, Ochiltree County, Texas. Although fractures in one of the cores were aligned 50 to 60° counterclockwise from the fractures in the other cores, suggesting the presence of two fracture directions in the field, paleomagnetic data show clearly that the core with anomalous fracture directions was improperly oriented and only one major system o vertical fractures exists. After "cleaning" by partial alternating field demagnetization, the mean directions of remanent magnetization for cores having consistent fracture orientations agree within ±6° with the expected direction of orientation, based on the known average Pennsylvanian pole position for the North American craton, which confirms the correct orientation of these cores. The mean direction of remanent magnetization for the anomalous core, however, deviates about 60° counterclockwise from the expected direction proving that the core is misoriented.
Progressive partial alternating field and thermal demagnetization studies isolate and remove from nearly all the samples a strong magnetic component oriented vertically downward that partially overprints the original remanent magnetization direction. This overprint, resulting from a vertical magnetic field produced by the drill pipe and core barrel, was used to determine that several samples had been inadvertently placed in the core box upside down.
Paleomagnetic core orientation techniques should be generally applicable in the Mid-Continent and other regions with relatively uncomplicated geologic structure located on crustal plates for which paleomagnetic polar wandering curves are well established.
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