c?Co~<^ yz£^z& 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 T\°^Sn^T^^\^TV PENNSYL- VANIAN CHESTERIAN Sre.Genevieve MISSIS- SIPPI VALMEYERAN KINDERHOOKIAN ? UPPER sKsKs^r^ Z3=x«\ T~y / i , i // / , S ALEXANDRIAN I / \ CINCINNATIAN I / \ ORDOVICIAN T~l 1 , 1,7 Galena ( Trenton ) CHAMPLAINIAN S ;/■ , /::■ Fig. 2 - Generalized columnar section above the St. Peter Sandstone. WAPELLA EAST OIL POOL, DE WITT COUNTY >- DC < DC DC < o 1 < < >- en < a. 0- co co co i s t-z L ¥ 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> Dr y no,e O -2 !