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A new generation of high-sensitivity cryogenic magnetometers permits paleomagnetic applications in weakly magnetized sedimentary rocks. One of the most useful paleomagnetic applications is drill-core orientation, which is important for determining fracture orientations, for stress analysis, and for determining sediment transport directions. A 2-year study involving ~600 core plug samples from five wells in three Rocky Mountain basins yielded paleomagnetic orientations that agree with those obtained using the conventional photographic "multishot" technique. The strongest paleomagnetic signal in these rocks points toward the late Cenozoic paleomagnetic pole and probably represents a secondary magnetization imposed by thermal effects associated with the late Cenozoic uplift nd tectonism in this region.
Weaker paleomagnetic signals, reflecting earlier thermal, diagenetic, or depositional magnetizations are also commonly preserved in sedimentary rocks and can also be used to orient core. For example, lower Paleozoic rocks of the southern Great Basin contain three secondary magnetizations acquired during the Late Permian (time of deepest burial), the Late Cretaceous (Sevier orogeny), and the Late Cenozoic (recent weathering). Although many different magnetizations commonly reside in the same rock sample, these magnetizations can be routinely separated by subjecting the samples to partial demagnetization, using alternating-field, thermal, or chemical "cleaning" techniques. The components of magnetization are destroyed at vastly different rates depending on whether they reside, for examp e, in trace amounts of magnetite, hematite, or goethite.
In paleomagnetic core orienting, the most precise orientations are obtained from fine-grained rocks, and the method requires some prior knowledge of the region to establish the reference magnetization direction. However, paleomagnetic core orienting requires no special downhole equipment and can selectively orient only those intervals of core that are of interest after visual inspection. The paleomagnetic core orienting technique has been successfully tested against the multishot technique in several regions of the United States and internationally.
Other paleomagnetic applications can be derived from the same plugs used for orienting drill core. Some of these applications use the "primary" magnetization acquired penecontemporaneously with deposition. For example, establishing the geomagnetic polarity reversal pattern in a sedimentary sequence elucidates the sedimentation rate (by thickening or thinning of the polarity stratigraphy) and the duration of hiatuses in deposition (by absence of segments of the reversal history). The reversal stratigraphy also provides timelines that are independent of the biostratigraphy and lithostratigraphy and that are useful in correlating beds from well to well. Other paleomagnetic applications use one or more secondary magnetizations reflecting later diagenetic and thermal events. These secondar magnetizations can have important implications regarding both permeability and thermal maturity. Finally, changes in rock magnetic properties, such as bulk magnetic susceptibility, can be used to detect mineralogic alteration associated with hydrocarbon migration.
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