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Diagenesis of Mudstones: What Can We Learn From Magnetic Studies?

Abstract

Mudstone diagenesis can be complex due to a number of variables that operate across non-discrete spatial-temporal boundaries within sedimentary basins. Paleomagnetic and magnetic fabric studies of mudstones, integrated with standard diagenetic studies, can provide information on the timing and nature of physiochemical processes that occur in mudstones. The timing of fluid activity and temperature-dependent diagenetic reactions can be determined using paleomagnetism by isolation of chemical remanent magnetizations (CRMs) associated with diagenetic events. Studies of mudstones in North America reveal CRMs related to illitization of smectite, migration of orogenic fluids, maturation of organic matter, and hydrothermal fluids, and that these processes are manifested in different basins. For example, several mudstones contain a Permian CRM that can be related to alteration by hydrothermal or external fluids. In contrast, the Wolfcamp in the Midland Basin contains a Jurassic CRM that is probably related to clay diagenesis during burial diagenesis. Petrofabric analysis can offer an additional dimension to traditional textural analysis by adding information about particle alignment, their intensity and preferred orientations. Anisotropy of magnetic susceptibility (AMS) is a tool that can provide such information on depositional as well as post-depositional processes such as compaction and tectonism. Our studies suggest that clays control the AMS signal and compaction shapes their orientation in most shales with the short axis of magnetic grains oriented perpendicular to the bedding plane, creating an oblate fabric. However, some mudstones contain intervals with considerable variation in the level of magnetic anisotropy and the shapes. For example, intervals with prolate shapes (sub-vertical long axis of the AMS) occur in horizons with elevated ferroan carbonate fractions or pervasive subvertical mineralized fracture networks. Intervals with different degrees of magnetic anisotropy suggest compactional disequilibrium which could result in differential fluid-flow behaviors that could impact reservoir properties. Our studies show that integrating magnetic fabric, paleomagnetic and diagenetic data can provide information to unravel the complex paragenesis of mudstones.