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Coal Resources in the Michigan Basin: Some Suggestions for Development
CROSS, AUREAL T.
Michigan State University, East Lansing, MI
Coals in the Michigan Basin are in thin, relatively discontinuous, laterally variable seams (mostly a meter or less in thickness) with numerous partings. The coals are of non-coking, high volatile B and C bituminous rank, with a range of 10,300-12,300 Btu, 3-9 percent ash, and 1-3 percent sulfur. The coal-bearing strata, up to 200 m thick, are of Lower to Middle Pennsylvanian age. Bedrock overlying Pennsylvanian strata unconformably in the central and western part of the basin (Jurassic age “Michigan Red Beds”) is comprised of red, gypsiferous shales, gypsum pods, and coarse to fine-grained, pink to tan sandstone with yellow/orange tinge. This sequence is both a caprock seal and in some parts a large aquifer. Elsewhere, Pennsylvanian strata are overlain by unconsolidated Pleistocene and Recent sediments (gravels, silts, clays, and peat) up to 200 m thick.
Remaining reserves are in 100-1500 (mostly less than 250) acre parcels. The principal factors constraining further subsurface development would be the small area available for each mine, the thinness and irregularities of the coal and partings, splitting at the margins of fields, the weak character of roof shales, and the disturbance of significant overlying aquifers.
A three-stage exploration program is proposed: reevaluation of existing logs and cuttings of oil wells penetrating the coal within each parcel selected; the drilling of one test core with geophysical logs within each of thirty townships with demonstrated coal; and determination of gas content of each coal and the sandstone and shale above and below, documenting potential methane recovery.
Close-up ofthe Michigan Basin
A giant incomplete bull's-eye is centered on the state of Michigan. Radiating outward to Ohio, Indiana, Illinois, Wisconsin, and Ontario, this annular pattern outlines the Michigan Basin. The Basin is a bowl-shaped structure of uncertain origin that contains over 2.5 miles (4 km) of inward-dipping
strata and a veneer of
rocks. This curious basin is located in the less tectonically-active interior of the continent, bordered by the Appalachian and Rocky Mountains. It subsided rapidly from
time and filled with shallow-water marine sediments, some of which contain deposits of petroleum, coal, and salt.
Michigan has small deposits of coal, although many of our coal deposits are in thin seams, and not economically valuable for that reason.
The swamp forests of the
stored many forms of usable wealth. The sands at the bottom of the swamp, now hardened into the Parma sandstone, are a prominent reservoir for fresh water in central Michigan counties; in a small area natural gas has been obtained from it. The trees of the forest died, were buried, and became the coal of the Saginaw valley, Grand Ledge, Shiawassee, Ingham, and Jackson counties. The coal is a bit difficult to use in the ordinary furnace, but with the proper type furnace and proper firing methods, it burns well.
What was the Pennsylvanian environment, during which these coal deposits formed, like? Slow, gentle crustal movements caused the Mississippian seas to almost retreat from Michigan, leaving a shallow pool in the central part of the basin and cut off from the outside sea. The time was the Pennsylvanian, the last 45 million years of the Carboniferous, and the time of the great coal swamps. Sedimentation was partly marine, partly fresh or brackish water in swamps. A layer of sparkling sand was first spread over the last Mississippian limestone and above this in the shallow waters a luxuriant swamp forest flourished --- but the trees were quite unlike our modern trees. They were giant fern trees, ground pines, and horsetail rushes that grew to 10 meters or more in height. No birds or butterflies or flowers were there, but dark loathsome amphibians and the earliest known reptiles crawled in the muds; giant scorpions and dragonflies flew about. The climate was warm and moist. The swamp vegetation died and fell to the swamp floor, layer upon layer of plant remains accumulated, changed to peat, were buried under a blanket of dark muds which slow streams brought from forest covered lands. Thus protected from oxygen, they have become the coal beds and shales of the central counties.
michigan's potential coal resources.JPEG (29409 bytes)
For the last time the seas rather hesitatingly invaded Michigan and a layer of thin sands and marine shales was spread over the coal swamps. Hot winds of the Permian time probably dried the last thin muds of the shallow seas and made a layer of thin red shale with streaks of gypsum in it, covering the last Pennsylvania sandstone. The Michigan basin was filled, the Paleozoic era was ended. The coal of Pennsylvanian age lies under a large part of the central part of the Lower Peninsula. It lies near the surface in Ingham and Jackson Counties, and the thin overburden of glacial drift means that it can be (was) mined by open pit methods. In the center of the coal-bearing area (in Midland County) coal is found at depths of about 800 ft.
michigan's coal basin and major past coal mining.JPEG (117675 bytes)
The seams vary in thickness, but the major coal-producing strata are approximately 1 m thick. The largest deposits found (and the most coal mined in the past) were in the Saginaw Valley. The height of production as 2 million tons in 1907, but most mining operations had stopped by 1920 except for some small-scale, open pit mining near Williamston, which ended in 1952. The major problems are:
the low quality of the coal, including a weak structure that makes the coal break down into powder when transported
the limited lateral extent of productive seams
large amounts of overburden (glacial drift), which handicaps the utilization of open pit methods.
It is estimated that the coal reserves of Michigan total 220 million tons, of which half could be mined with the present technology.
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