Gj ten Suruj>iu STATE OF ILLINOIS WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION VERA M.BINKS, Director ' %f< Relation of Silurian Reefs to Ordovician Structure in the Patoka Oil Area Thomas W. Smoot DIVISION OF THE ILLINOIS STATE GEOLOGICAL SURVEY JOHN C. FRYE, Chief URBANA CIRCULAR 258 1958 ■iVilMin!? UmiVl GEOL OG'CAL SURVEY 3 3051 00003 8640 RELATION OF SILURIAN REEFS TO ORDOVICIAN STRUCTURE IN THE PATOKA OIL AREA Thomas W. Smoot ABSTRACT Oil has been produced from Mississippian rocks in the Patoka area, Marion County, since 1937, but only in the last three years has it been produced from the Ordovician rocks, namely the Kimms- wick ("Trenton") Limestone. Because there appeared to be good pos- sibilities of extending Kimmswick production, a study of the sub- surface geology was undertaken. The results of this study indicate that Kimmswick production might be extended northeastward and southwestward from the Fairman pool. The Silurian reef structures at Patoka and Patoka East were found to have been developed on the northeast noses of pre-existing Ordovician anticlines which apparently were a controlling factor in the development of the reefs. The presence of the Ordovician structures are reflected in iso- pach and structure maps of the Pennsylvanian and Mississippian rocks . Some structural features are noted that may indicate the pres- ence of other reef-like structures in the area. INTRODUCTION This study of the relationships between Silurian reefs and known "Trenton" structure in south-central Illinois was prompted by recent discovery of "Trenton" oil production and was undertaken with the hope that it might disclose other pos- sible accumulations of oil. The Patoka oil area, as defined in this report, consists of 132 square miles in Clinton, Fayette, and Marion counties (fig. 1). Oil is produced from six pools. The area has six pay zones, four of which produce in the Patoka pool from rocks that range in age from Ordovician to upper Mississippian. The area has produced oil since the discovery of the Patoka pool in 1937, but the impor- tance of Kimmswick production is slight as compared to that from Chester and, to a minor degree, Devonian in the Patoka pool . Kimmswick production figures from Patoka and Fairman pools indicate that the ratio of oil recovered per well has been low or in the marginal range to Jan- uary 1958. From the Patoka pool, nine wells, in production from one to two years, have yielded approximately 122, 000 barrels of oil, and eight other wells, pro- ducing less than three months, yielded approximately 51, 000 barrels of oil. [1] ILLINOIS STATE GEOLOGICAL SURVEY R I E R I W Oil pool' "Trenton" production Fig. 1. - Index map of Patoka area showing oil pools. PATOKA OIL AREA 3 In the Fairman pool, four wells have produced approximately 12, 500 barrels in less than eight months. These figures are based on the "Pipeline Production Report, " December 1957, and personal communications from oil producers. This investigation is based essentially on an isopach and structural map study. Data were obtained from micrologs and electric and induction logs. Sup- plementary information was obtained from driller's logs, drilling reports, and, to a minor extent, sample studies, all of which are on open file at the Illinois State Geological Survey in Urbana . The datum surfaces used for this investigation were chosen because 1) they are easily recognized on geophysical logs and on driller's logs, 2) they are con- tinuous in extent throughout the area, 3) the key beds show little lateral vari- ance, 4) they are a sufficient stratigraphic distance from a major erosional sur- face to show possible structural anomalies, as reflected by differential compac- tion, and 5) they afforded maximum control for each geologic system or series. Where convenient, the strata chosen were those used by Brownfield (1954) in the Centralia area which adjoins and in part overlaps the Patoka oil area. Acknowledgments Thanks are due to Wayne F. Meents of the Illinois State Geological Sur- vey who introduced the problem and who furnished much information for the study. Other Survey staff members who contributed suggestions and advice are David H. Swann, who advised me on stratigraphic interpretations, procedures, and techniques, and Lester L. Whiting, A. H. Bell, and Elwood Atherton, who des- cribed the contact between the Warsaw and Salem Formations. John Van Fossen of the Kewanee Oil Company supplied information, porosity data, and personal opinion. Ronald A. Younker and Jerry L. Williams of the Survey gave technical assistance. STRATIGRAPHY A summary of the stratigraphy of the Patoka area is presented in the col- umnar section (fig. 2). For purposes of this report, the Pennsylvanian rocks are subdivided at the base of the Shoal Creek Limestone and at the base of No. 2 Coal. The youngest sequence, that above the Shoal Creek Limestone, was not used in this study. The Shoal Creek Limestone is 8 to 12 feet thick, light colored, and well consolidated. It is characterized on electric logs by its abnormally high self- potential values. The base of the limestone has been used as a marker bed for figure 3, which indicates that Ordovician deformational and younger differential compaction structures are reflected at shallow depths. The sequence of rocks between the Shoal Creek and No. 2 Coal is com- posed of silty shales with many thin coal beds, thin limestones, and locally thin, medium-grained sandstones. No. 2 Coal is recognized on electric logs in this area by its low resisti- vity values on the 16-inch normal curve and more easily on the 64-inch normal curve. There are three similar pairs of curves in a 5 0-foot interval, the top one representing No. 2 Coal approximately 70 feet below No. 4 Coal. The oldest sequence of Pennsylvanian rocks below No. 2 Coal are charac- terized by sandy shales and thick sandstones. There are few limestones or coals, ILLINOIS STATE GEOLOGICAL SURVEY SYSTEM SERIES < > _] >- CO UJ a. a. o_ co co co CO c_> GROUP FORMATION MEMBER Shoal Creek No. 6 Coal No 5 Coal No 4 Coal No. 2 Coal _Glen Dean Hardinsburg SYSTEM SERIES Golconda Beech Creek (Barlow) Continued next column Q- CL CO CO CO CO a> o > o . o GROUP FORMATION MEMBER Cypress Paint Creek Yankeetown Aux Vases Ste Genevieve St Louis Warsaw Borden Continued next column Fig. 2. - Stratigraphic column. PATOKA OIL AREA :m GROUP SERIES FORMATION MEMBER CO CO a> >> a> 6 o > < o > Ld Q o |QJ, '?' 1 — 1 < a: _) CO c o k_ D o> o 1 CH J& O X a> < z < > o Q or o c o o c c o c (_> 1.1 Chouteau New Albany _Linqle Grandtower St Clair Sexton Cre ek Edgewood Moquoketa Kimmswick A thick sandstone is present through- out most of the area, lying 100 to 150 feet below No. 2 Goal. This sand- stone generally is underlain by a sandy shale at least 150 feet thick. The youngest Mississippian strata are a succession of alternating limestones, sandstones, and shales of the Chester Series. The sandstone units vary laterally in thickness. The Glen Dean Limestone is the highest Chester stratum present in the area, as overlying Chester formations were removed from parts of the area by pre- Pennsylvanian erosion (fig. 2). The interval between No. 2 Coal and the base of the Glen Dean Lime- stone is important because it repre- sents a time of major tectonic activi- ty in the Illinois Basin, a time when the major development of the DuQuoin monocline took place. Both No. 2 Coal and Glen Dean Limestone can be assumed to have been deposited essentially horizontally and evenly over a great area . Both are far enough stratigraphically from the pre-Penn- sylvanian erosional surface to have been little affected by pre-Pennsyl- vanian topography. The Glen Dean Limestone is light brownish gray, dense, and slight- ly sandy. It is easily recognized on electric logs that go below the sand- stone of the Tar Springs Formation or the basal sandstone of the Pennsyl- vanian. The base of the Glen Dean affords the best control of all the da- tum surfaces used in this area because it has been penetrated at many places and is easily recognized on geophys- ical logs, driller's logs, and in sam- ple sets. Its base is used as a mar- ker surface in figures 4 and 5. The top member of the Ste. Genevieve Limestone, the Levias Limestone, is recognized on electric logs as the top limestone member of the carbonate section of the Valmeyer Fig. 2. - Stratigraphlc column, continued. 6 ILLINOIS STATE GEOLOGICAL SURVEY Series below the lower Chester sequence of sandstones and shales. In the Pa- toka area, the base of the Aux Vases Formation, the oldest Chester formation, is represented by a thin shale member of variable thickness. The Levias is ap- proximately 20 feet thick and in many places is underlain by a sandstone member. The low self-potential and high resistivity of the Levias is therefore conspicuous in comparison to the overlying and underlying high self-potentials of the sand- stones . The Valmeyer Series comprises approximately 1200 feet of sediments divi- ded about equally between carbonates and silty shales. The Ste. Genevieve Limestone, the top formation of the carbonate section, is mainly oolitic, slight- ly sandy and cherty, brownish gray to light gray, and massive. The rest of the carbonate section is partly cherty. The older half of the Valmeyer Series is represented by the Borden Group which is composed mostly of silty shale and is approximately 600 feet thick. The Kinderhook Series is represented by the Chouteau Limestone and the upper portion of the New Albany Shale. The Mississippian-Devonian boundary falls within the New Albany Shale. The interval between the top of the Ste. Genevieve Limestone (Mississip- pian) and the top of the Lingle Limestone (Devonian) is significant in a structural interpretation of the area. During the time represented by this interval, little significant structural deformation occurred in this area. Southeastward deep- ening of the basin is obvious. Figure 6 is an isopach map of the interval be- tween the top of the Ste. Genevieve and the Lingle Limestone. The Lingle Limestone just below the New Albany Shale is easily recognized on electric logs by its relatively high self-potential and high resistivity values. The Devonian rocks on top of the Silurian serve to modify Silurian struc- tures, but it is difficult to determine any specific marker closer to the top of the Silurian than the Lingle. The top of the Lingle, therefore, is the best available marker for a structure and isopach study of the Silurian rocks. The upper part of the Devonian carbonate section is, for the most part, slightly sandy dolomite and limestone and is underlain by a cherty dolomite and cherty limestone sequence. The Silurian System is represented by argillaceous limestone, silty dolomite, and local, pure carbonate, reef and reef "wash" ac- cumulations . Although information is scarce, it appears that no significant structural movements took place in this area during the time represented by the interval between the Lingle and the Kimmswick ("Trenton"). Therefore, the anomalous thin rock intervals indicated in figure 8 reflect pre-existing, deformational struc- tures; the anomalous thick rock intervals are thick due to the Silurian reefs and accumulations of reef "wash." The top of the Kimmswick ("Trenton") Limestone of the Ordovician System is easily recognized on electric logs because the high resistivity of the lime- stone contrasts sharply with the low resistivity of the overlying Maquoketa (Cincinnatian) Shale. The Maquoketa (Cincinnatian) Shale and the Kimmswick (Mokawkian) Lime- stone are the only part of the Ordovician System drilled in the Patoka area. PATOKA OIL AREA R. I W R I E Datum point -Contour, interval 20 feet, datum sea level Fig. 3. - Structure map base of Shoal Creek Limestone (Pennsylvanian). 8 ILLINOIS STATE GEOLOGICAL SURVEY STRUCTURE The structural complex extending from sec. 28, T. 4 N., R. IE. south- westward to sec. 1, T. 3N., R. 1 W. is here referred to as the Patoka struc- tural complex. It consists of two main components: 1) the deformational anti- cline southwest of the town of Patoka, which is referred to as the Patoka de- formational anticline; and 2) the Patoka reef structure. The more extensive structural complex extending approximately from sec. 26, T.4N., R. lE.to at least sec. 34, T. 3 N., R. 1 W. is referred to as the Patoka East structural complex. This complex also consists of two main components: 1) the deforma- tional anticline extending southwest from the Patoka East pool through the Pa- toka South, Fairman, and Boulder East pools, referred to as the Fairman deforma- tional anticline; and 2) the Patoka East reef structure. Structure at the Base of the Shoal Creek Limestone (Pennsylvanian) Figure 3 shows a central high that includes the Patoka East structural complexes. The two structures are separated by a narrow shallow trough that is roughly parallel to the two structures. The highest points in the central por- tion of the map are closed highs at the northeast ends of the structures. These closed highs reflect underlying reef structures. The reef structures are more distinctly indicated than the anticlinal folds. However, the large closed highs associated with the reefs reflect deeper deformational structures. In T. 4 N., R. 1 W., a structural low is apparent. The center of this low is located at the Patoka West oil pool. The closed low spot, centered in sec- tion 15, apparently is a phenomenon restricted to the Pennsylvanian strata. On a structure map of No. 2 Coal, the same low would be apparent but in modified form. In the Chester it is a nearly flat, structural terrace (fig. 5); in the Ste. Genevieve it appears as a narrow terrace, and it apparently disappears below the Valmeyer rocks (fig. 7). As this low, as such, apparently is restricted to the Pennsylvanian sediments, high compaction of those sediments is suggested as a possible explanation. The elongate low trough extending northeast of this low seems to be genetically related to it. A small, high structure appears in sees. 15 and 22, T. 3 N., R. 1 E., the position of which is indicated at greater depths as a subdued, more exten- sive high (figs. 5, 7, and 9). Its sharpness, as shown at the base of the Shoal Creek, may be due to the presence of Pennsylvanian sands with low compaction values. This might have resulted in an oval high causing a "draping" effect of the Shoal Creek Limestone as the surrounding sediments were compacted to a greater degree. The structural high at Boulder East (sees. 27, 28, 33, and 34, T. 3 N., R. 1 W.) may reflect an underlying reef structure or may reflect the intersection of cross folds. This structure is discussed in more detail below. The regional dip, although not clearly shown in figure 3, is toward the east and south (Brownfield, 1954, p. 27, fig. 10; Siever, 1950). Thickness of Lower Pennsylvanian-Upper Chester Strata Figure 4 is an isopach map showing the thickness of the interval between No. 2 Coal (Pennsylvanian) and the base of the Glen Dean Limestone (Chester). The time interval represented by this thickness is structurally important in the Illinois Basin. PATOKA OIL AREA . R.iw, 6 R.I W • Datum point ^■""isopach, interval 20 feet