Anisotropy of magnetic susceptibility: A petrofabric tool to measure the fabric of shales

Anisotropy of magnetic susceptibility (AMS) is a high resolution petrofabric tool that measures the shape preferred orientation (SPO) and intensity of magnetic minerals in a rock. The AMS in most shale units is characterized by a magnetic fabric oriented parallel to the bedding plane with a distinctly oblate shape. Interestingly, new AMS data from the Woodford and Marcellus shale cores show horizons with vertical and inclined magnetic fabric orientations with a dominantly prolate shape. Bulk magnetic susceptibility (K_lf) increases with depth in the Marcellus whereas the Woodford shale, K_lf increases until 13950ft followed by a decrease in K_lf. The average degree of magnetic anisotropy (P′) for both shale units is 1.1 and increases with depth suggesting subtle stretching of the magnetic fabric in response to overburden. Both units are weakly magnetic (Marcellus average K_lf = 1.2∗10^-4 [SI]; Woodford average K_lf = 3.7∗10^-5 [SI]) which suggests that AMS signal is carried predominantly by paramagnetic phases such as phyllosilicates. Previous paleomagnetic studies indicate that magnetite and pyrrhotite are the dominant ferromagnetic minerals in the Marcellus and magnetite is the dominant ferromagnetic mineral in the Woodford shale (Manning and Elmore, 2012). High-field magnetic hysteresis measurements indicate a single domain grain size in the Woodford shale and a multi-domain grain size in the Marcellus shale. Preliminary microstructural observations of vertical magnetic fabric horizons in the Marcellus shale suggest that these fabrics are controlled by flowage and brecciation. Microstructural observations of vertical magnetic fabric horizons in the Woodford shale suggest that fluid filled fractures and veins control the AMS fabrics. Qualitative analysis of X-ray computed tomography (XRCT) scans of both shales show populations of vertical/sub vertical fabrics among high density mineral phases. Additional work to quantify the SPO and spectrum of mineral phases detected by XRCT scans are underway.