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Phyllosilicate Diagenesis in Mississippian Sandstones of the Michigan Basin


Abundant authigenic clay mineral cements (illite, kaolinite, chlorite), detrital mica grains (biotite, muscovite), and well crystallized metamorphic lithic fragments (phyllite, schist) have been identified within the Mississippian age Michigan Stray and Marshall Sandstones of the Michigan Basin. These phyllosilicates exhibit a variety of unique morphologies and intergrowth textures. The diagenetic history of the Michigan Basin remains uncertain, and multiple hypotheses have been proposed to account for the pore fluid conditions necessary for the unusual relationships between phyllosilicates of differing origins in these units. The investigation of chemical/thermal pore-fluid conditions controlling thermodynamic stability of this suite of minerals can be performed using a petrographic, mineralogical, and hydrogeochemical approach from rock samples and pore fluid data.

Petrographic analysis of core samples collected from four wells throughout the Michigan Basin was completed using petrographic light microscopy and scanning electron microscopy with backscattered electron imaging and energy dispersive X-ray spectroscopy. Detrital parent grains and authigenic precipitation/alteration products were identified and characterized based on morphology, fabric, and composition. Imaging was also used to identify samples appropriate for X-ray diffraction analysis and determine mineralogical input for geochemical inverse modelling of mineral-water interactions. Modelling with the USGS aqueous geochemical software package PHREEQC was completed to evaluate thermodynamic stability of phyllosilicates as a function of various hypothesized pore-fluid compositions.

Authigenic illite has been petrographically determined to be the most recent event in the paragenetic sequence for these formations. Powder X-ray diffraction analysis of highly illitic samples was conducted to evaluate crystallinity of illite cements. Illite “crystallinity” is a loosely defined term which represents the amount of illite/smectite (I/S) mixed layers in an illitic sample. The property is primarily controlled by temperature, resulting in a useful geothermometer for the formation temperature of the most recent diagenetic event.

Thorough understanding of a basin’s diagenetic history is integral to research and management of resources within the basin. This study evaluates and attempts to unravel the range of phyllosilicate mineral phases, compositions, and alterations in uniquely mica-rich sandstones.