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

HAMILTON COUNTY 99
FIG. 20.-
Matheny
1 2 3 4 5 6 7 8 9 1 0 1 1
DRILLING TIME - MINUTES
HAMILTON COUNTY
MAGNOLIA MATHENY NO I
SEC 19. T 5S., R.5E.
-One-foot drilling time log of Magnolia-
No. 1 drill hole in Hamilton County,
19-5s-5E.
and thus differs from most Pennsylvanian
limestones. Commonly the underlying
stratum is a siltstone or silty shale or even
a sandstone.
Herrin limestone.-The caprock of No.
6 coal bed is one of the most conspicuous
members of the McLeansboro group and is
encountered in most drill holes. It is called
the Herriqg the Brereton10 (western Illinois)
or the Providence1' limestone (southeastern
Illinois, Indiana and western Kentucky).
It is 2 to 6 feet thick, and drill
cuttings consist of fragments of dark gray,
bsown or black, finely granular and sparingly
fossiliferous limestone. A characteristic
small fossil, fragments of which are oc-
"Idem.
10 Savage, T. E., Significant breaks and overlaps in the
Penncyl.vanian ro-ks of Illinois: Amer. Jour. Sci., 5th Ser..
101. 14, pp. 307-316, 1921.
Glenn. L. C.. The geology and coals of Webster County:
Kentucky Geol. Survey, Ser. 6, vol. 5, p. 98, 1922.
casionally encountered in drill cuttings, is
Fusulina girtyi.12 This limestone is separated
from the No. 6 coal bed by black
"slate" and gray shale from 1 to 10 feet
thick.
Herrin (No. 6) coal bed.-The No. 6
coal bed, 5 to 9 feet thick, is the upper member
of the Carbondale group and the most
commonly recognized key bed of the Pennsylvanian
system in southern Illinois. Because
it is easily identified and commercially
important, the bed has been selected as the
datum for mapping Pennsylvanian structure
in the southern half of the State. In
electric logs the position of the bed is usually
indicated by a characteristic pattern1"
closely associated with the pattern of the
caprock limestone.
Hawisburg (iVo. 5) cod bed.-Harrisburg
(No. 5) coal bed lies 50 to 120 feet
below the bottoni of No. 6 bed. Diamonddrill
records generally show 3 to 7 feet of
coal in the No. 5 bed, but a few holes near
the eastern county line encountered no coal
at this position. A caprock is reported only
rarely in the logs; when present it is thin.
The position of No. 5 coal bed appears in
electric logs of rotary-drill holes as a
characteristic fairly prominent peak in the
normal resistivity curve.
""No. 4" coal bed.-In the coatrol wells
black "slate" and coal were usually present
at a depth 140 to 200 feet below No. 6 coal
bed. In some wells only black "slate" was
found at this position. The thickness of the
bed is usually recorded in inches, but 3 feet
of coal were reported 185 feet below No. 6
coal bed in a drill hole in sec. 19, T . 5 S.,
R. 5 E. (County No. 479). Diamonddrill
holes in the county have not reached
this coal bed. Its thickness and position are
determined primarily from data supplied by
records of control wells and electric logs of
other rotary-drill holes. In general the
pattern of "No. 4" coal bed has a fairly
prominent peak to the right in the normal
curve, less prominent, however, than that
produced by the Harrisburg (No. 5) coal
12, Dunbar, Carl 0.. and Henbeqt, Lloyd G., Pennsylvanlan
Fy111inidea of Illinois: Illinois Geol. Survey Bull.
67. n. 111. 1912.
13 Taylor, E. F., et al., Methods of subsurface study of
Pennsylvanian strata encountered in rotary-drill holes: Illinois
Geol. Survey Rept. Inv. 93, pp. 15-19, 1944.