THE MAGNETIC FABRIC OF NORTH AMERICAN SHALES: INSIGHTS INTO DIAGENETIC AND BURIAL PROCESSES

Anisotropy of magnetic susceptibility (AMS) studies were performed on cores from the Haynesville, Marcellus, Wolfcamp and Woodford shales. In general, compaction shapes the magnetic fabric of these shales with the long axis of grains oriented parallel to the bedding plane and oblate shapes. Interestingly, these units also contain vertical and sub-vertical fabric horizons with predominantly prolate shapes. By evaluating the processes shaping fabric development in shales, we hope to better understand the origin of these vertical and sub-vertical magnetic fabrics. Magnetic susceptibility values and high-field experiments suggest the AMS signal are produced by paramagnetic minerals (i.e. clays, pyrite and ferroan carbonates). Microstructural observations suggest the origin of vertical/sub-vertical fabrics in the Marcellus are caused by particulate flow and brecciation. Vertical fabrics in the Haynesville correspond to dolostone facies. These AMS signatures are likely a product of ferroan carbonate minerals which are known to produce inverse magnetic fabrics. The Woodford shale shows vertical fabrics associated with pervasive mineralized fracture horizons. X-ray computed tomography (XRCT) confirm the origin of these fabrics are largely controlled by the orientation of minerals in these fractures. Preliminary data from a Wolfcamp core shows distinct magnetic fabric signatures associated with specific facies. Mudrock facies show planar oriented, oblate magnetic fabrics and carbonates show vertical/sub-vertical oriented, prolate fabrics. Inch scale sampling across the core Wolfcamp will also evaluate lateral variability of AMS fabrics. Significant vertical variability in the degree of magnetic anisotropy (P') (a proxy for aspect ratio of grains) occurs across all units. These trends are likely related to over consolidation of sediment (high P'), under consolidation (low P') or mineralogical variability. AMS coupled with advanced imaging techniques, can provide valuable information on seal integrity, map pathways or barriers for fluid flow and as a result, shed light on whether shales behave as open or closed geochemical systems.