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

~vhere a nearby diamond-drill hole penetrated
8 feet of this coal. In other control
drill holes the coal varied in thickness from
3 to 6 feet. Attempts to estimate the thickness
where only electric logs are available
proved unprofitable. 'The No. 6 coal bed
appears to be thickest in the southwest
quarter of the county.
The thickness of No. 5 coal bed, as determined
by diamond drilling, varies between
3 to 7 feet with an average of about 4 feet.
The interval between No. 6 and No. 5 coal
beds varies between 50 to 120 feet.
The
distributions of variations in interval are
shown by the accompanying isopach map
(fig. 23).
On the basis of approximately 1 million
tons per square mile-foot the amount of
coal in No. 6 bed, assuming an average
thickness of 5 feet over 435 square miles,
is 2175 million tons, and in No. 5 bed,
assuming an average thickness of 3 feet,
1305 million tons, or a total of 3480 million
tons. Previously, a recent estimate for
these two beds was 3s billion tons.23
Other coal beds are present below No. 5
coal, but none appears to be more than
3 feet thick. The facts about these lower
coal beds will not be known until the
Carbondale and lower Pennsylvanian
groups are explored with a diamond drill
or more accurate methods of logging other
types of drilling operations are devised.
PROTECTION OF COAL BEDS
Both No. 6 and No. 5 coal beds require
protection when drill holes are abandoned
and plugged. The structure map (~1. 9)
gives the approximate altitude above sea
level of the No. 6 bed in all parts of the
county. The altitude of the surface or the
drilling floor is available to the person responsible
for the plugging of the well. If
the altitude of the coal bed is added to the
surface altitude, which gives in this county
a negative or below-sea-level value, the depth
to the coal bed is obtained. Plugging
should extend from 50 feet above to 50
feet below the positioa of the coal bed.
'3Cady, G. H., Coal resources based on information obtained
from rotary drilling February 1, 1942 to May 3 1 ,
1943: Tllinois Geol. Survey Rept. Inv. 93, p. 38, 1944.
However, because the No. 5 coal bed is
usually about 100 feet below No. 6, the
plug should actually extend for about 200
feet, which would carry it from 50 feet
above No. 6 to 50 feet below No. 5 coal
bed.
If plugging is not required for coal beds
more than 1000 feet in depth, regardless
of their thickness, the No. 6 coal bed and
lower beds will not require plugging in
most of the northern part of the county.
This is likely to be true where the No. 6
coal bed lies 600 feet or more below sea level
or where the surface altitude exceeds 500
feet. Under this rule plugging will be
applied very unevenly because of local irregularities
of the surface, and its value, when
applied, will be correspondingly diminished.
OIL AND GAS RESOURCES
The extent to which the structure of No.
6 coal bed may reflect the presence of pre-
Pennsylvanian structures favorable to oil
and gas accumulation is of interest. An
isopach map (fig. 24) showing the distribution
of variations in interval between No.
6 coal bed and the top of "Little Menard"
limestone indicates that these variations
are relatively regular. Fairly close agreernent
between the structure of the coal bed
and that of the limestone is to be expected.
Almost every oil pool in the county is located
on a structural irregularity in the
No. 6 coal bed.
The name and location of the oil pools
in Hamilton County along with the producing
formations for each pool are given
on page 110.
The delineation of the detailed structural
features in areas occupied by the various
oil pools was made possible only by the
drilling of the pools. Elsewhere drilling
has been widely spaced and delineation of
the structure is correspondingly generalized.
The presence of what might be regarded as
structures favorable for oil accumulation in
such sparsley explored areas must be discounted
to some extent because the map is
generalized. More closely spaced drilling
would considerably modify the present
interpretation.