Occurrence of Potash-bearing strata (Sylvinite) in the Salina A-1 Evaporite in the Central Michigan Basin
Potash occurs in the deep, center of the Michigan basin and contained exclusively in the Salina A-1 Evaporite formation as Sylvinite in the middle or upper portions of the formation. The Potash occurs as Sylvite (in relatively pure beds as much as 30 feet thick or as Sylvite (KCl) intimately intermixed with Halite (NaCl) as Sylvinite. These deposits were first described by Dow Chemical Company scientists in the early 1970's, based on samples from a core in Midland County and nearby well Gramma Ray logs. The Potash zones are located between 7000 and 9000 feet deep and occur in all or parts of 17 counties in the northern half of Michigan's Lower Peninsula. Commercial Potash mining in Michigan began at a solution mining facility near Hersey, Michigan in 1997. The plant was designed for production of 160,000 tons of Potash per year primarily from a 20-30 foot thick interval, near the top of the A-1 Evaporite, of relatively pure Potash locally known as the “Borgen Bed”. The grade of this Potash deposit is one of the highest in the world at nearly 70% KCl by volume. It is also very pure containing negligible Carnallite and about 0.5% insolubles. Potash production at the Hersey mine ceased in 2013 although the facility still does produce Salt (NaCl). In 2008, a significant collection of core from the A-1 Evaporite in the commercial mining area as well as throughout the northern half of Michigan was donated to Western Michigan University. The collection contained over 11,300 linear feet of conventional, 4-inch core sealed in plastic sleeves, from 77 different wells in 9 counties. In addition to the cores, several hundred modern wireline logs are available from wells that pass through the Potash zone. Gamma Ray, Compensated Neutron and Lithodensity curves are all useful in defining the stratigraphic occurrence and approximate thickness of the Potash-bearing strata. The “Borgen Bed”, near the top of the formation is widespread in the western portion of the area, whereas there is a sequence of thinner Sylvite and Sylvite/Halite interbeds in the middle of the formation to the eastern portion of the area. By supplementing the core data with wireline logs, it can be estimated that Michigan's Potash deposits are contained in 17 counties in the central and northern part of the Michigan basin. Based on thickness of Sylvite intervals and grade values from core analysis, it is likely that commercial production can come from 9 counties representing 2.9 million acres.
AAPG Datapages/Search and Discovery Article #90195 © 2014 Eastern Section Meeting, London, Ontario, Canada, September 27-30, 2014