Wapella East oil pool, De Witt County, Illinois; a Silurian reef

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STATE OF ILLINOIS

DEPARTMENT OF REGISTRATION AND EDUCATION

WAPELLA EAST OIL POOL,
DEWITT COUNTY, ILLINOIS-
A SILURIAN REEF

Richard H. Howard

ILLINOIS STATE GEOLOGICAL SURVEY

John C. Frye, Chief URBANA

CIRCULAR 349

1963

3 3051 00004 4366

WAPELLA EAST OIL POOL, DE WITT COUNTY,
ILLINOIS-A SILURIAN REEF

Richard H. Howard

ABSTRACT

The discovery well of the Wapella East oil pool, the
Lloyd A. Harris No. 1 T. P. Kiley, NE NW NW sec. 28,
T. 21 M., R. 3 E., DeWitt County, was completed Decem-
ber 2, 1962, for 154 barrels of oil per day (pump capacity)
from Silurian reef dolomite topped at 1112 feet. This discov-
ery is at the northern end of the Illinois Basin, and 25 miles
north of Decatur, which previously marked the northern bound-
ary of the major oil producing area of Illinois.

A geologic study of the area shows the Wapella East
pool to be situated on a broad, southward-plunging, structural
nose. There is about 100 feet of closure in the pool area.
How much of this closure can be attributed to the presence of
Silurian reef is conjectural and will remain so until a hole
that has penetrated the reef is drilled through the entire Sil-
urian. In the pool area, Devonian limestone thins 16 feet
over the reef from a thickness of 30 feet to 14 feet.

Eleven wells are producing oil from Silurian dolomite
that consists of (1) bluish gray, finely crystalline, vuggy to
dense, fractured reef core and (2) lighter gray reef wash hav-
ing many fossil cavities and apparent bedding dips of 20°.

More exploratory drilling on structural highs in the area
may re suit in the discovery of additional accumulations of oil.

INTRODUCTION

The discovery well of the Wapella East oil pool, the Lloyd A. Harris No. 1
T. P. Kiley, NE NW NW sec. 28, T. 21 N., R. 3 E., DeWitt County, was com-
pleted December 2, 19 62, for 154 barrels of oil per day (pump capacity) from Sil-
urian reef dolomite topped at 1112 feet. This discovery is located 16 miles south-
east of Bloomington and 25 miles north of Decatur, which previously marked the
northern boundary of the major oil producing area of Illinois (fig. 1).

A study of the geologic conditions present in the Wapella East area was
initiated to aid current petroleum exploration in this portion of Illinois. The report

1

2 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 349

R I E 2 3 4 5R6E

• OIL POOL

X TEXACO " CITIES
PIPE LINE

REPORT AREA

Fig. 1 - Location of the Wapella East area, showing nearest oil production and
crude oil pipe lines.

WAPELLA EAST OIL POOL, DE WITT COUNTY 3

area includes about one-half of DeWitt County and parts of McLean and Piatt
Counties. It is 20 miles long from north to south, \l\ miles wide from east to
west.

Physiographically the report area is in the southwest part of the Blooming-
ton Ridged Plain of the Till Plains Section of the Central Lowlands Province. The
area is drained southwestward by tributaries of the Sangamon River. Topographic
elevations range from 640 feet above sea level in the Salt Creek bottom at the
southwestern corner of the area to 820 feet above sea level along the northern
boundary. Most of the area is relatively flat till plain with local relief of less
than 10 feet.

General Geologic Setting

The Wapella East pool area is at the northern end of the Illinois Basin
about 15 miles west of the steepest dips of the western flank of the LaSalle Anti-
cline. Rock strata dip regionally southeastward at an average rate of approximately
25 feet per mile. The bedrock surface is overlain by 200-400 feet of glacial till,
sand, and gravel. Records of oil tests and structure tests for natural gas storage
provide geologic data on the deeper strata .

Stratigraphy

In lieu of a discussion of the general stratigraphy in the area, the reader
is referred to figures 2 and 3. Figure 2 shows the stratigraphic position, gross
lithologies, and approximate thicknesses of strata above the St. Peter Sandstone.
The term Hunton Limestone Megagroup (Swann and Willman, 19 61) is used for the
Devonian-Silurian carbonates. Figure 3 shows the electrical characteristics of the
formations encountered in the deepest test hole in the report area. Devonian and
Silurian strata in the Wapella East pool area are discussed in more detail under
Geologic Occurrence of Oil.

Structure

The structure within the report area is related genetically to that of the La-
Salle Anticline, whose steeply dipping western flank is just to the east. Hence,
the structural grain of the report area, as contoured on top of the Hunton (fig. 4),
is north -northwest to south- southeast, essentially parallel to the LaSalle flexure.
Dips on the western flank of the southward-plunging anticlinal nose on which the
Wapella East pool is located are four times those on the eastern flank. There are
two other important southward -plunging noses in the area: (1) five miles east of
the pool; and (2) three miles west of Farmer City. The axis of the major low is
just east of Clinton.

Thickness of the New Albany Shale in nearby holes was used in estimating
the Hunton top in holes that only reached the New Albany. Figure 5 shows the
structure of the Wapella East pool in larger scale.

ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 349

SYSTEM

SERIES

GRAPHIC
COLUMN

FORMATION
OR GROUP

THICKNESS
(FEET)

QUATER-
NARY

PLEISTOCENE

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PENNSYL-
VANIAN

CHESTERIAN

Sre.Genevieve

MISSIS-
SIPPI

VALMEYERAN

KINDERHOOKIAN

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UPPER

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ALEXANDRIAN

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CINCINNATIAN

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ORDOVICIAN

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Galena
( Trenton )

CHAMPLAINIAN

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Fig. 2 - Generalized columnar section above the St. Peter Sandstone.

WAPELLA EAST OIL POOL, DE WITT COUNTY

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Fig. 3 - Electrical log of the Stensel No. 1 Schwartz, N| NE NW sec. 30, T. 21 N.
R. 4 E., DeWitt County.

6 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 349

Structure Tests

-r> Dryno,e<Est
Ot Tren.cn tes,
O S, Peter tes,

Fig. 4 - Structure map of the report area on top of the Hunton Megagroup (Devonian-
Silurian carbonates).

WAPELLA EAST OIL POOL, DE WITT COUNTY 7

OIL PRODUCTION

Eleven wells are producing oil from Silurian reef dolomite in the Wapella
East pool; Devonian production has been minor so far. The crude oil has a green-
ish cast in a glass bottle, is amber-colored on the hand, and has a gravity of 30.5°
A.P.I. The oil is pumped into a 2 7/8-inch branch pipe line that carries it two
miles west, into the Texaco-Cities Service pipe line (fig. 1). Virtually no natural
gas is produced with the oil. Several of the structually lower wells are producing
slightly salty sulphur water. Analyses were made by the Illinois State Geological
Survey's Analytical Chemistry Section on brine samples from two wells in sec. 21,
T. 21 N., R. 3 E. One sample contained 2724 Ppm chlorides and 5118 Ppm total
solids. The other contained 3782 Ppm chlorides and 6907 Ppm total solids. The
total solids values conform to the isocon map (Meents, et al., 1952) of Devonian-
Silurian brines. Table 1 shows data on the 11 present producing wells in the Wa-
pella East pool.

Discovery and Development

Few oil tests were drilled in this area before 19 60. During the latter part
of 1960 and early 1961 the Union Hill Gas Storage Company, a subsidiary of Peo-
ples Gas Light and Coke Company of Chicago, drilled 27 structure tests in search

Structure contour
( interval 50 feet )
Datum: mean sea level

February, 1963

Fig. 5 - Structure map on top of the Hunton Megagroup (Dei
ates) in the Wapella East pool area.

i-Silurian carbon-

ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 349
TABLE 1 - ELEVEN PRODUCING OIL WELLS IN THE WAPELLA EAST POOL

Depth and elevation (in feet)

Devonian Silurian
.1 Location limestone top reef top Total Depth (pump capacity)

#2 T. P. Kiley SW NW NW 1085 (-286) 1101 (-302) 1117 (-318)
28-21N-3E

#3 T. P. Kiley SW NE NW 1097 (-293) 1119 (-315) 1122 (-318)
28-21N-3E

#1 Laura Kiley NE NE NE 1090 (-292) 1111 (-313) 1120 (-322)
29-21N-3E

#2 Laura Kiley NE SE NE 1100 (-297) 1117 (-314) 1123 (-320)
29-21N-3E

#1 Cora Ryan SW SW SW 1096 (-289) 1120 (-313) 1137 (-330)
21-21N-3E

#2 Cora Ryan SE SW SW 1111 (-297) 1134 (-320) 1139 (-325)
21-21N-3E

#3 Cora Ryan S% SE SE 1106 (-304) 1131 (-329) 1137 (-335)
20-21N-3E

#1 Julia Ryan SW SE SW 1110 (-304) 1134 (-328) 1164 (-358)
Heirs 21-21N-3E

WAPELLA EAST OIL POOL, DE WITT COUNTY 9

of structures with sufficient closure and areal extent to be suitable for underground
storage of natural gas. Their Mahomet dome storage project (Bell, 19 61) lies 10
miles east of the report area. They did not find the kind of structure for which
they were looking and released the structure test electrical logs and drilling sam-
ples to the Illinois State Geological Survey, where they were promptly placed on
open file.

Mr. Lloyd A. Harris, consulting petroleum geologist from Mattoon, Illinois,
while at the Survey office engaged in a subsurface study of the Devonian in this
area, found evidence of Devonian porosity down dip on the east flank of an un-
tested structure. Harris' first well drilled on this structure discovered the Wapella
East pool. The Mo. 1 T. P. Kiley topped Devonian limestone at a depth of 1090
feet and Silurian reef dolomite at 1112 feet. It filled up with 9 50 feet of oil, but
could be swabbed down. After being treated with 500 gallons of mud acid, swab-
bing at the rate of 30 barrels of oil per hour did not lower the fluid level below
900 feet. The well was completed for 154 barrels of oil per day (pump capacity).
After being treated with 500 gallons of mud acid, the fluid level in the No. 1 Laura
Kiley could not be lowered below 600 feet while being swabbed at the rate of 54
barrels of oil per hour.

Geologic Occurrence of Oil

The present structural configuration of the report area was formed by forces
initiated at the close of Mississippian time. The Wapella East pool is situated on
a broad, southward -plunging structural nose. There is about 100 feet of closure
in the pool area. How much of this closure can be attributed to the presence of
Silurian reef is conjectural, and will remain so until holes have been drilled through
the entire Silurian. Total Silurian thicknesses in holes that penetrated reef can
then be compared with total Silurian thicknesses in holes that did not encounter
reef. Four known Silurian thicknesses in the report area, none of which is within
4 miles of the pool, range from 463 feet to 495 feet. In the pool area, Devonian
limestone thins 16 feet over the reef, from a thickness of 30 feet in the SW NW SW
of section 21 to a thickness of 14 feet in the NE SW NW of section 28. Figure 6
shows 8 feet of this thinning along A-A'.

Devonian limestone in the Wapella East pool consists of three units. The
upper unit is tan, commonly finely oolitic, lithographic limestone, which is often
chalky and dolomitic. It is usually slightly sandy, contains many thin, black
shale partings (fig. 7A), is vertically fractured, and has a strong natural gas odor.
The bottom portion of this unit is commonly dolomitic, slightly vuggy and oil
stained but usually tight. The lower unit is crinoidal, bryozoan, and frequently
coralline limestone (fig. 7B), oil stained, often very sandy, commonly dolomitic
and friable. Between these fairly persistent upper and lower units a light to dark
brown, silty, shaly limestone with prolific white brachiopod development (fig. 7C)
is locally several feet thick. Corals and stromatoporoids (fig. 8A) are locally
abundant in the middle and lower units.

The Devonian- Silurian contact (figs. 8B, 8C, 9A) is marked by a sandy
Devonian detrital zone that sometimes contains large chert pebbles (fig. 8B) . The
Silurian surface is fractured and fissured. The fissures, which in some places
extend at least 2 5 feet into the Silurian, vary in width from minute (fig. 9 A) to
over 3 inches. Figure 9B shows a 1-inch fissure filled with Devonian sand and
Silurian reef rock fragments. Figure 9C shows a fissure more than 3 -inches wide
filled with green clay and sand. The depth 1176 is written on Silurian reef rock.

10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 349

Silurian strata consist of reef core, reef wash, and interreef rock. The reef
core (fig. 10A) is bluish gray, finely crystalline, vuggy to dense, fractured, un-
bedded dolomite. Reef wash (fig. 10B) is a lighter gray, fine-grained dolomite,
vuggy with many fossil cavities, and apparent primary bedding dips of 20 degrees.
Figure IOC shows a piece of typical interreef rock that is light green, chalky,
pyritic dolomite with isolated large vugs.

Coventional core analysis of 17 feet of reef wash cored in one well indi-
cated an average porosity of 14 to 18 percent and permeabilities of less than 500
millidarcies. Five feet of reef core and 11 feet of reef wash, cored in another
well, were subjected to whole core analysis. Average porosities and permeabilities
of reef core were 12-13 percent and less than 100 millidarcies, respectively. Av-
erage porosities and permeabilities of reef wash were 20-24 percent and more than
500 (up to 11,500) millidarcies, respectively.

The oil-water contact in the pool is believed to be at about 350 feet below
sea level.

Drilling and Completion Practices

According to Lloyd A. Harris, principal operator in the Wapella East area,
the cost of drilling a well with rotary tools, including coring, drill- stem testing,
and logging, is about $5,200. It costs another $7,000 to set pipe and install a
pumping unit, and another $2, 500 for a tank battery. Some drilling mud is lost
regardless of whether heavy or light mud is used. The use of heavy mud results

Fig. 6 - Geologic cross section across the Wapella East pool.

WAPELLA EAST OIL POOL, DE WITT COUNTY

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12 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 349

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WAPELLA EAST OIL POOL, DE WITT COUNTY 1!

in more clean -out problems during completion.

The usual casing procedure is to set 100-200 feet of 8 5/8-inch surface
casing. Drilling without surface casing also has been tried successfully. The
production string of 4i-inch casing is set 3 feet above the pay with the aid of a
special casing shoe. All the producers so far have been treated with 500 gallons
of mud acid. Rods and 2 -inch tubing are run and a 1 3/4 -inch pump having a
160-170 barrel per day capacity is installed.

RECOMMENDATIONS FOR FUTURE DRILLING

Water-bearing Silurian reef rock has been penetrated in several off- structure
dry holes. Reef rock is probably present in much of the area but must be found
structurally high to produce oil. More exploratory drilling along the highs shown
in figure 4 may result in the discovery of additional accumulations of oil.

REFERENCES

Bell, A. H., 1961, Underground storage of natural gas in Illinois: Illinois Geol.
Survey Circ. 318.

Meents, W. F., Bell, A. H., Rees, O. W., and Tilbury, W. G., 1952, Illinois
oil-field brines— Their geologic occurrence and chemical composition:
Illinois Geol. Survey Illinois Petroleum 66.

Swann, D. H., and Willman, H. B., 1961, Megagroups in Illinois: Am. Assoc.
Petroleum Geologists Bull., v. 45, no. 4, p. 471-483.

Illinois State Geological Survey Circular 349
15 p., 10 figs., 1 table, 1963

Printed by Authority of State of Illinois, Ch. 127, IRS, Par. 58.25.

CIRCULAR 349

ILLINOIS STATE GEOLOGICAL SURVEY

URBANA