subsurface geology and coal resources of the pennsylvanian system ...

INTRODUCTION 13
less than 1000 feet below the surface."ll
Earlier editions of the Rules and Regulations
defined the lower limits of workability
of a coal bed as a bed 30 inches thick at a
depth of 1000 feet, but extended the limits
of workability of beds 36 inches or more
thick indefinitely. Plugging has presumably
been carried on in line with these definitions.
Actually coal beds 3 feet or more
in thickness have not been found in the
Illinois basin below depths of 1523 feet.
In the first report of this seriesl1"n
estimate of the coal resources in the Illinois
basin was presented based upon observations
on 140 rotary-drill holes, the so-called
"control" wells or drill holes partially
logged by the Survey (table 2). The total
reserve in the 17 counties represented was
estimated at about 30 billion tons. Only the
No. 5 and No. 6 coal beds were considered
in the estimate.
Because a considerable part of the Illinois
coal field has a surface altitude between 500
and 600 feet, the structure contour line representing
500 feet below sea level for the
No. 6 coca1 bed shown on any of the maps
becomes a critical line in determining the
areas in which it is necessary to provide
protection for coal beds of workable thickness.
In general such workable beds lie
below No. 6 coal bed but rarely more than
-COO feet lower. Occasional coal beds 30
inches or more in thickness are found at
various positions above No. 6 coal bed. In
much of the Illinois basin the No. 6 coal
bed lies more than 500 feet below sea level.
Since the first estimate was made, 78
additional drill holes13 have been logged by
the Survey (table 3). Fifty of these drill
holes were located in one or another of the
following counties of. the Illinois basin:
Clay, Coles, Cumberland, Edwards, Effingham,
Hamilton, Jasper, Jefferson, Richland,
Wayne, and White. Four holes were located
in Lawrence County. Many encountered
one or more coal beds between 3, 9,
and possibly 12 feet thick at depths below
1000 feet. Coal 4 feet thick was encountered
in 34 drill holes; 5 feet thick in 20;
l1 Rules and Regulations, Illinois Department of Mines
and Minerals, adopted and approved Oct. 18, 1915. Rule
15. p. 15. Mimeographed.
lqept. Inv. 93, p. 44, I944 (as of May 30, 1913).
l3 -4s of Sept. 30, 1945.
6 feet thick in 5; 7 feet thick in two; and
8 to 10 feet in five drill holes. In the
first 140 wells drilled (the basis of the
estimate of reserves), coal beds 4 feet or
more in thickness were encountered at a
depth below 1000 feet in 49 wells.
In general, logging of the first 140 drill
holes was not continued below No. 5 coal
bed, whereas in the last 66 wells logging
commonly was continued to the base of the
Pennsylvanian system. Hence the earlier
data do not include figures on locwer beds
which subsequent observation discovered.
Had observation continued to the base of
the Pennsylvanian system, undoubtedly
beds lying below 1000 feet would have
been reported many more than 49 times.
At any rate it is quite apparent that a very
large reserve of coal is present in the
Pennsylvanian rocks at depths between
about 1000 and 1500 feet.
The assumption that a coal bed becomes
unworkable at a depth exceeding 1000 feet,
irrespective of its thickness, needs the
thoughtful consideration of those interested
in conserving the coal resources of the
State. Undoubtedly mining men will have
a very respectful attitude toward the great
difficulties involved in large-scale mining
operations 1000 to 1500 feet in depth,
~articularly so long as considerable supplies
of average-grade coal are available at much
shallower depths. Yet if evidence should
indicate that relatively thick low-ash and
low-sulfur coal is available at the greater
depths, the relative scarcity of such coal in
the State would tend to attract serious attention
to the deeply buried reserves. Unfortunately
the information available concerning
the ash and sulfur content of the
deep-lying coal beds is not very satisfactory,
because of the difficulty of obtaining samples
of such coal beds that are truly representative.
In most cases, but not in all, such
samples appear to indicate that the coal beds
are relatively high in ash and sulfur rather
than otherwise (table 1). However, it is
possible that the cuttings collected tend to
represent the denser portions of the coal
because the lighter portions are more apt
to escape in the overflow.