Origin of Anomalous Magnetic Fabrics in the Woodford and Marcellus Shales

Abstract

The anisotropy of magnetic susceptibility (AMS) in most shale units is characterized by a normal magnetic fabric where the magnetic foliation is parallel to the bedding plane with a distinctly oblate shape. New AMS data from the Woodford and Marcellus shales show anomalous AMS signatures with a magnetic foliation perpendicular to the bedding plane and a mixed prolate/oblate fabric. The acquisition of these magnetic fabrics may be explained by the following processes: (1) fluid filled fractures displacing adjacent ferromagnetic and paramagnetic grains; (2) bioturbation; (3) inverse magnetic fabrics carried by single domain magnetite or goethite; (4) granular rotation of magnetic carrier minerals in localized viscoplastic zones (5) and authigenic ferromagnetic and minerals formed in association with fluid events. Preliminary observations of the Woodford shale suggest that anomalous AMS fabrics are associated with localized vertical/sub vertical fluid filled fractures however, further investigation is required. Observations of the Marcellus shale indicate an association between anomalous AMS fabrics and localized viscoplastic zones. Paleomagnetic and rock magnetic investigations reveal that the dominant magnetic carrier mineral is magnetite for both shale units. The degree of anisotropy (P′) in both the Woodford and Marcellus shales increases with depth, suggesting fabric stretching in response to overburden. The average shape factor (T) in the Marcellus shale is oblate. The average T in the Woodford shale is prolate. Significant spatial variability of magnetic susceptibility is observed in both shales, suggesting a complex magnetic mineralogy. An understanding of anomalous AMS fabric development in these shales units can provide insight into fluid flow processes.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015