MRCSP Phase I Geologic Characterization Report - Midwest ...

132 CHARACTERIZATION OF GEOLOGIC SEQUESTRATION OPPORTUNITIES IN THE MRCSP REGION
geography of Pennsylvanian strata in Michigan, and established a
southwestward paleo-sediment transport direction of predominantly
fluvial to deltaic sediments. In a detailed study of subsurface data,
Vugrinovich (1984) established lithostratigraphy, structure, and isopach
thickness relationships, and interpreted depositional environments
for Pennsylvanian strata in the central basin area. Recently,
Westjohn and Weaver (1998) characterized the areal distribution
and lithology of the Saginaw aquifer and confining units (composite
Saginaw and Grand River Formations) on the basis of an extensive
subsurface study of well logs.
NATURE OF LOWER AND UPPER CONTACTS
General stratigraphic relationships in the Michigan basin, based
on subsurface studies, indicate a major unconformity (the base
Absaroka unconformity) underlies Pennsylvanian-age rocks in
most areas of the basin, and that another unconformity of probable
composite origin occurs at the top of the Pennsylvanian section
(Ells, 1979). However, Westjohn and Weaver (1998) and Vugrinovich
(1984) suggested that the sandstone-dominated lithofacies
at the base of the Pennsylvanian section (the Parma Sandstone of
some workers) may interfinger and constitute facies equivalents of
the Mississippian-age Bayport Limestone in the central Michigan
basin, thus obscuring this regional unconformity contact in places.
Jurassic rocks overlie portions of the Pennsylvanian section (mostly
in the western basin area); however, lithofacies in the Jurassic “red
beds” section appear similar to the highly variable lithofacies of the
underlying Pennsylvanian, making the pick between these formations
problematic in places.
LITHOLOGY AND COAL THICKNESS RELATIONSHIPS
The description of the lithologic variability in the Saginaw Formation
is based on current stratigraphic schemes used in Michigan
(see above discussion). The lithology of the entire Pennsylvanian
section is best considered here because this is the interval mapped
for this project. Wanless and Shideler (1975) described a gross lithologic
subdivision for the Pennsylvanian section in Michigan.
“Unit A” (Parma Sandstone and lower portions of the Saginaw
Formation of other workers) consists of up to 550 feet of mainly
clastics with minor amounts of coals, limestones, and evaporites.
The coal occurs in beds generally less than three feet thick, but locally,
beds as thick as eight feet thick are reported (Vugrinovich,
1984). Total net coal thickness is typically less than seven feet in
this interval.
“Unit A” is overlain by “Unit B” (Wanless and Shideler, 1975),
an interval of fine-grained clastics with predominantly dark carbonaceous
mudstone, minor coal, and limestone (Upper Saginaw
Formation and Verne Limestone Member of other workers). Coal is
present mainly in the eastern part of the basin in this interval (Wanless
and Shideler, 1975). This unit is interpreted as the updip portion
of a prograding fluvial deltaic succession with paleosediment
transport to the southwest. “Unit B” ranges from zero to185 feet in
thickness and has an antithetic thickness relationship to the underlying
“Unit A” and the overlying “Unit C.” Coal beds are generally
less than three feet in thickness with a composite thickness of less
than seven feet.
“Unit C” (Wanless and Shideler, 1975; Grand River Formation of
others) is a predominantly coarse-grained clastics unit with lesser
amounts of mudstones, limestones, gypsums, and minor coals. The
coal is irregularly distributed and is generally less than three feet in
composite thickness in this unit.
DISCUSSION OF DEPTH AND THICKNESS RANGES
Extensive truncation and deformation of the Pennsylvanian
bedrock surface (subcrop) occurred prior to and during Pleistocene
glaciation in many areas of the Michigan basin. Thus, the Saginaw
rocks lie at relatively shallow depths directly below the Pleistocene
deposits (maximum depth of 310 feet above sea level to a high of
880 feet above sea level—Figure A16-1). Although thickness relationships
of the Saginaw/Pennsylvanian section are strongly controlled
by the basin-centered subcrop surface, it is also influenced
by primary depositional controls resulting from the facies relationships
and intra-formational unconformities, with resultant complex
isopach relationships formed in probable stacked fluvial-deltaic depocenters
(Ells, 1979). The Saginaw, in general, ranges in thickness
from zero at the subcrop to more than 650 feet thick in the central
Michigan basin (Figures A16-1 and A16-2). Considering the lithologic
complexity of the rocks, a lack of consistency in stratigraphic
nomenclature, and the lack of core and outcrop data, the formation
tops and isopach thicknesses used for the Saginaw Formation in this
study (Figure A16-2) most likely represent a composite thickness of
the Pennsylvanian strata in the basin.
Maximum composite coal bed thickness for the entire Pennsylvanian
section is probably less than 12 to15 feet. This thickness may
be present only in places in the southern and eastern portions of the
study area. A maximum of five percent coal in the entire Pennsylvanian
section is a conservative estimate.
DEPOSITIONAL ENVIRONMENTS/
PALEOGEOGRAPHY/TECTONISM
The Pennsylvanian strata of Michigan were deposited during a
period of generally southwestward sediment transport in non-marine,
marginal-marine, and open-marine environments. Progradational
fluvial and deltaic depositional systems produced variable
lithofacies and thicknesses of sediments that periodically contained
marine incursions represented by laterally persistent marine limestone
and shale units. Pre-Pennsylvanian paleotopography was
rather irregular due to moderate pre-Pennsylvanian tectonic warping,
which is best illustrated by the relief on the Howell anticline, a
major structural feature in the area. This structure is represented by
a reentrant in the structure and isopach maps in the southeast corner
of the subcrop area (Figures A16-1 and A16-2). The thickness of the
Saginaw ranges from approximately 44 feet to more than 380 feet
over a distance of approximately ten miles in this area (Wanless and
Shideler, 1975). With time, these topographic undulations become
obscured by infilling sediments, thus indicating a general tectonic
quiescence that persisted during the remainder of Pennsylvanian
time. Facies and isopach thickness relationships were apparently
influenced by eustatic fluctuations that resulted in transgressive and
regressive stratigraphic relationships.
SUITABILITY AS A CO 2
INJECTION TARGET OR SEAL UNIT
The Saginaw Formation and other Pennsylvanian strata in the
Michigan basin are of interest to the MRCSP because the unminable
coal beds and, possibly, the organic-rich shales may be sequestration
targets, even though most are at shallow depths (less than 1000 feet) in
the Michigan basin. The maximum thickness of the Pennsylvanian section
mapped is between 600 and 700 feet but less than five percent of
this thickness consists of coal. Maximum coal thickness reported in any
one section is probably less than 12 to15 feet (Vugrinovich, 1984).