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

COUNTY 7 5
Generally the caprock is recorded by a peak
with 10 to more than 100 ohm-meters relief
on the normal curve. The underlying gray
shales are recorded as normal shale curves on
both resistance and potential sides. The
black "slate" and coal beds are recorded as
one unit by a definite peak or peaks in one
or both of the resistivity curves. Whether
the resistivity peaks are positive or reverse
depends on the thickness of the beds. If
the thickness of the bed exceeds the electrode
spacing, 2 positive peak results; if
thinner, a valley or re-entrant results. This
$pacing is 18 inches for the normal resistivity
curve, 53 inches for the third resistivity
curve No. 1, and 71 inches for the third
resistivity curve No. 2. The underclay
generally produces prominent valleys or reentrants
in the normal and third resistivity
curves. The resistivity curves commonly are
lower in value for underclay than the shale
base line, and the potential curve has a small
positive value.
The limestone, shale, black "slate," coal,
and underclay sequence produces a distinctive
and readily recognizable pattern.1°
However, in some logs the pattern thought
to represent these beds consists of a single
positive deflection of the two resistivity
curves with a slight break near the middle
~ f the . normal resistivity curve (fig. 13).
This break probably represents the gray
and black shales lying between the limestone
and coal beds. The characteristic underclay
resistivity curve cccurs below this pattern.
Hawisburg (No. 5) cord bed.-This bed
lies 90 to 122 feet below No. 6 bed and
is 4 to 5 feet thick. The usual interval
between the two beds is about 110 feet.
Several feet of dark gray shale or black
"slate" commonly overlie the coal bed.
Usually there is no limestone above the
black shale, but if one is present, it is thin
and impure. Below the coal bed there is
usually 2 to 4 feet of structureless gray
. .
underclay containing gray calcareous nodules
in the lower part.
"'T,;yloi-. Eaile I:., P~:llen. h4. William. Sims, Paul K..
;inti Px>-ne, J. xol-mnn, Methoifc of ~ubs~ii-face rtncly of the
Pcnnsyl\-ai~ inn 81 1r3i :I erlcoi~ri~ei-ecl in rot at-y-drill holes : Illinois
Geol. Survey Rept. Inv. 91, pp. 16-19, 1941.
No. 5 coal bed generally produces a
prominent peak on the normal curve with
a relief of 10 to 50 ohm-meters (pl. 6).
The third resistivity curve produces a similar
peak if the bed thickness exceeds the
width of the electrode spacing and a sharp
reversal when the bed thickness is less than
the electrode spacing. The potential curve
commonly has some negative value. This
may amount to 50 millivolts or more opposite
No. 5 coal bed.
OTHEK BEDS OF
STRATIGRAPHIC INTEKEST
Linzestones between Shoal Creek aud
Hyest Franklin.-Two thin beds of limestone
are usually penetrated in drill holes
in the interval between the Shoal Creek
(New Haven) and West Franklin limestones.
They are not known to outcrop
and their character is known only from drill
cuttings. The pattern of some electric logs
suggests the presence of limestone at the
appropriate positions (fig. 12). These beds
underlie the wedge-shaped area in which the
Shoal Creek limestone is found and are also
present in other parts of the county wherever
the West Franklin limestone is overlain by
175 feet or more of Pennsylvanian strata,
The upper limestone is about 1 foot thick
and fossiliferous, lying 400 to 460 feet
above No. 6 coal bed and 150 to 210 feet
above the West Franklin limestone. The
second bed, 2 feet thick, lies 15 to 65 feet
belox the first. These two beds were
penetrated in the diamond drill hole, White
County (No. 4), near New Haven in sec.
31, T. 7 S., R. 9 E. where the upper bed is
434 feet above No. 6 coal bed and is separated
from the lower bed by 50 feet of shale.
The electric log of the control well located
in the NE1/3, NEX, NVi7%, sec. 22,
T. 7 S., R. 9 E. (No. 133) displays a characteristic
pattern at the positions of these two
limestones, This consists of a small peak
in the third resistivity curve, accompanied
bs- a small negative potential. The presence
cf a similar pattern with similar spacing in
electric logs of other drill licles indicates the
:2resence of these limestones,