MRCSP Phase I Geologic Characterization Report - Midwest ...

APPENDIX A: DEVONIAN ORGANIC-RICH SHALES
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14. DEVONIAN ORGANIC-RICH SHALES
A very complex sequence of mudstones of Middle and Late Devonian
age occurs in both the Appalachian and Michigan basins.
In the western two-thirds of the MRCSP study area, these shales
are primarily known as the Ohio, Chagrin, or Antrim Shales. In
the eastern third of the area—the Appalachian basin of Maryland,
Pennsylvania, and West Virginia—the shales have an array of names
such as Millboro, Marcellus, Harrell, Brallier, Genesee, Sonyea, and
Rhinestreet, to name the most important (Figure 5).
The Ohio Shale members with the highest organic content and
the most widespread occurrence in the Appalachian basin are the
Cleveland and Huron Members shales (Figure A14-1). In general
the lower-most portion of the overall Ohio Shale is black organicrich
shale, which grades upward and eastward into dominantly gray,
silty shale in areas of thicker accumulation in the Appalachian basin.
In other areas, broad regional structural arches controlled stratigraphy
resulting in thinner, but overall dominant, black organic-rich
shale. Correlative shales include the New Albany Shale of the Illinois
basin and the Chattanooga Shale of the southern part of the
Appalachian basin.
In the Michigan basin, there are three formal members and one
informal member of the Antrim Shale, the Lachine, Paxton, and
Norwood Members, and an unnamed upper member (Gutschick and
Sandberg, 1991). The Paxton Member is a gray calcareous shale. All
the other units are black shale with varying organic content.
ORIGIN OF NAMES, TYPE SECTION, SIGNIFICANT
EARLIER STUDIES ON THIS INTERVAL
Nomenclature of the shale interval varies widely within the
MRCSP study area from north to south and west to east, based
primarily on the recognition of black, carbonaceous units with
generally high gamma-ray response. Stratigraphic studies of Devonian
shales are often based on the occurrence of volcanic ash beds,
siltstones, and Foerstia and Tasmanites (algal megaspores) zones.
Figure A14-1 is a correlation chart of the major Devonian shale
units in the Appalachian basin. Table A14-1 lists the major units,
the original reference, and type section. The reader is also referred
to de Witt, 1993 as a good overview of Devonian shale stratigraphy
for the Appalachian basin. Extensively studies on Devonian shales
have occurred by many workers since in the 1800s; thus, no attempt
was made to trace the various stratigraphic revisions or changes in
unit correlations for this interval in the MRCSP region.
Much of the research on the Devonian-age shales of the eastern
United States was performed by the state geological surveys and
directed by the U.S. Department of Energy as part of the Eastern
Gas Shales Project from 1976 to 1981. Also, studies by the U.S.
Geological Survey and investigations sponsored by the Gas Research
Institute (now Gas Technology Institute) of Chicago, Illinois,
have greatly contributed to the understanding of these Devonian
organic-rich, gas-bearing rocks. A few of the more recent studies of
significances include work by Decker and others (1992), Roen and
Kepferle (1993), Boswell (1996), Milici (1996), and Ryder (1996).
NATURE OF LOWER AND UPPER CONTACTS
Within the Appalachian basin portion of the MRCSP study area,
Devonian shales unconformably and conformably overlie carbonates
or shales of Upper Silurian or Lower or Middle Devonian age.
Figure A14-1.—General stratigraphic nomenclature for the Middle and Upper Devonian black shales in the Appalachian basin.
Organic shales are shaded black. F = Foerstia Zone. Black triangles indicate major ash beds. Redrawn from de Witt and others
(1993). See also Roen and Kepferle, 1993, Plate 2, cross section A-A'.