TEXTURAL EVIDENCE OF MULTIPLE GENERATIONS OF SULFIDES IN THE MIDDLE DEVONIAN MARCELLUS FORMATION (WEST VIRGINIA)

Quantifying iron sulfide morphologies in the organic-rich black shales of the Marcellus Formation (Middle Devonian) is important for exploring the role of thermochemical and biogenic processes in the development, growth, and habit of pyrite in mudrock matrices. Scanning Electron Microscopy (SEM) was used to gather backscattered electron image suites from three thick sections. Samples were cut from sidewall cores carefully selected to adequately represent, the geochemical properties, and structure of mudrock lithofacies of the Marcellus Shale. Image analysis software Image J and ilastik machine learning classifier workflows were employed to inventory pyrite morphology and composition by means of thresholding pixel intensity values and segmenting labeled morphologies. X-ray diffraction (XRD) performed on powdered sidewall core was used to determine the bulk mineral composition at each depth. When combined with petrophysical and geochemical composition data, sub-micron scale textural details provide useful insights into how textures can be characterized with depth. In interrogating the samples, strong correlations are observed between pyrite morphologies and depth. Correlating to selected intervals, pyrite texture sizes, habits, and abundances varied noticeably. With increasing depth, framboidal pyrite morphologies display loss of intergranular pore space between monocrystals, increased pyritization of organic matter, and increased size of euhedral pyrite grains. These observations suggest the Marcellus Formation has undergone complex diagenesis after lithification. This research serves to better resolve the identification of pyrite morphologies during SEM analysis and observe qualities of diagenesis occurring in sulfides hosted in organic-rich mudrock lithofacies as a function of depth.