DETERMINATION OF POTASSIUM SOURCES FOR AUTHIGENIC ILLITE IN MISSISSIPPIAN SANDSTONES OF THE MICHIGAN BASIN

Abundant authigenic illite cement has been identified within the Mississippian age Michigan Stray and Marshall Sandstones of the Michigan Basin. This illite has been petrographically determined to be the most recent event in the paragenetic sequence for these formations. Illite cement precipitation in sandstone has two fundamental requirements: sufficient heat to overcome the kinetic barrier for crystallization, and a supply of potassium ions. The diagenetic history of the Michigan Basin remains uncertain, and multiple hypotheses have been proposed to account for these requirements. The investigation of chemical/thermal pore-fluid conditions controlling illite precipitation can be performed using a petrographic and mineralogical approach from rock samples. This presentation will discuss the framework and initial analytical results of this approach.

Petrographic analysis of core samples collected from four wells throughout the Michigan Basin is currently underway. Petrographic light microscopy and scanning electron microscopy with backscattered electron imaging and energy dispersive X-ray spectroscopy are being used to identify and characterize minerals, identify samples containing highly illitic clay cements, and determine mineralogical input for geochemical inverse modelling of pore-fluid composition. Modelling with the USGS aqueous geochemical software package PHREEQC is underway to evaluate thermodynamic stability of illite cement in open systems as a function of various hypothesized pore-fluid compositions.

Powder X-ray diffraction analysis of highly illitic samples will be 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. This property is primarily controlled by temperature; therefore crystallinity measurements may be used to evaluate formation temperature of diagenetic illite.

Thorough understanding of a sedimentary basin’s diagenetic history is integral to future academic work and management of resources within the basin. This study presents an approach to assess and understand the most recent diagenetic event in the Mississippian units of the Michigan Basin.