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

EDWARDS COUNTY 57
electric logs indicate the position of the
thicker or thickest limestone bench when two
or three benches are present. The thicker
of two benches may be at the top or at the
bottom.
Where there has not been stratigraphic
shortening as a result of faulting, the top
of the West Franklin limestone is between
231 and 299 feet above the Herrin (No.
6) coal bed-that is, the base of the Mc-
Leansboro group (table 6). In the northern
two tiers of townships the township
average interval declines to between 245
and 263 feet, as compared with 268 and
279 feet in the part of the county south of
the base line.
"No. 7" coal bed.-The thin coal designated
as "No. 7" lies 200 to 225 feet
below the West Franklin limestone. The
intervening beds are a more or less monotonous
succession of dark gray, carbonaceous
and micaceous siltstones and shales in the
control drill holes (figs. 8, 9, 10). Across
this part of the succession electric logs usually
show low resistivity, indicative of uniformly
shaly beds. At the position of the
"No. 7" bed there is usually a slight peak
in the normal curve and a recession of the
third curve. The opposing patterns of the
normal and third curves combine characteristically
at this postion and are in evidence
in several of the logs of the control drill
holes (Nos. 1, 34, fig. 8; Nos. 3, 9, fig. 9;
Nos. 8, 10, fig. 10). On the basis of observations
at control drill holes and from the
study of the pattern of electric logs, the
"No. 7" coal bed is in general believed to be
not more than 2 to 3 feet thick. Black
shale or "slate" 1 to 3 feet thick is reported
above the coal bed in 6 of the control drill
holes.
The interval between "No. 7" and No.
6 coal beds in the twelve townships of the
county from which information is available
is between 26 and 83 feet, with township
averages varying from 36 to 69 feet,
being slightly greater south of the base
line (table 6). "No. 7" coal bed was not
reported in control drill hole No. 106 (fig.
lo), perhaps because the observer failed to
discover it. Occasionally, however, it appears
to be missing in drill holes for which
electric logs are the best records available.
Hel-)-in limestone and Herrin (No. 6)
coal bed.-The recorded depth to the Herrin
(No. 6) coal bed in 320 drill holes in
Edwards County is generally between 766
and 1021 feet, depending upon the surface
altitude and the altitude of the coal bed.
The thickness of this bed, as determined
from drilling-time observations and cuttings
studies of the 14 control drill holes, is believed
to be between 3 and 5 feet. The bed
appears generally to be overlain by 1 to 3
feet of black shale "slate," above which lies
usually 3 to 5 feet of impure, dark gray to
black, earthy limestone, the Herrin limestone.
It is ordinarily impossible, on the
basis of the rate of drilling, to differentiate
the black "slate" and the coal bed. In
general, thicknesses must be estimated
roughly by the amount of coal obtained in
the cuttings, but at best the estimate is not
satisfactory. In an electric log, it is generally
assumed that a swing to the right of the
third curve indicates a thickness of the bed
greater than the electrode spacing of 53
inches. In most electric logs of drill holes
in Edwards County the third curve shows a
peak pointing to the right. There is an area
of about two square miles in sec. 36, T. 2
S., R. 10 E., and in secs. 1, 2, 11, and 12,
T. 3 S., R. 10 E., where No. 6 coal bed
appears to be absent (pl. 4).
The electric-log pattern is variable.*
Two peaks on the resistivity curve of some
logs (Nos. 3, 5, 91, fig. 9; Nos. 7, 8, fig.
10) made it possible to differentiate the
limestone above and the black "slate" and
coal bed below. On the other hand in some
logs (Nos. 1, 34, fig. 8 ; No. 8, fig. 10) a
single peak appears to 'mark the position of
the combined limestone, black "date," and
coal bed. In such logs the base of the limestone
is placed at a depth representing a
reasonable compromise between the position
of maximum resistance (the peak of
the normal curve) and the positio,n of the
maximum millivolts of negative self-potential
(the peak of the opposed self-potential
curve) if the two points are not at the same
depth. If the two depths agree, the position
of the base of the limestone is known.
4Tar-lor, Earle F., Pullen, M. William, Sims, Paul K..
and Payne. J. Norman. Methods of subsurface study of
the Pennqylvanian strata encountered in rotary-drill holes:
Illinois Geol. Survey Rept. Inv. 93, pp. 16-19, 1941.