IP 119 S IA-. GS ■ IP 111 C.3 O^Sma*^ Paleochannel across Louden Anticline, Fayette County, Illinois: its relation to stratigraphic entrapment of petroleum in the Cypress Sandstone Robert M. Cluff and Zakaria Lasemi Illinois Institute of Natural Resources STATE GEOLOGICAL SURVEY DIVISION, URBANA Illinois Petroleum 119 Jack A. Simon, Chief 1980 Cluff, Robert M. Paleochannel across Louden Anticline, Fayette County, Illinois: its relation to stratigraphic entrapment of petroleum in the Cypress Sandstone / by Robert M. Cluff and Zakaria Lasemi. - Urbana, 111. : State Geological Survey Division, 1980. 21 p. ; 28 cm. - (Illinois petroleum ; 119) 1. Louden Anticline, 111. 2. Petroleum-Geology-Illinois. 3. Geology, Strati- graphic-Mississippian I. Lasemi, Zakaria. II. Title. III. Series. ILLINOIS STATE GEOLOGICAL SURVEY Printed by authority of the State of Illinois/1980/2500 3 3051 00004 9407 Paleochannel across Louden Anticline, Fayette County, Illinois: its relation to stratigraphic entrapment of petroleum in the Cypress Sandstone Robert M. Cluff and Zakaria Lasemi CONTENTS Abstract Introduction Characteristics of the "false Barlow" Relationship of the "false Barlow" to massive sandstones in the Cypress Origin of the "false Barlow" and related features Prospects for further Cypress development References Acknowledgments Appendix 1 2 5 5 11 18 20 20 21 Figures 1 . Structure of the top of the Karnak Limestone Member 2 2. Generalized geologic column for the Lower Chesterian and subjacent strata of the Louden Anticline 3 3. Elevation of the Beech Creek Limestone 4 4. Thickness of the interval between the base of the Beech Creek Limestone and the top of the Karnak Member of the Ste. Genevieve Limestone 6 5. Thickness of the interval between the top of the Beech Creek Limestone and the top of the Karnak Limestone Member of the Ste. Genevieve Limestone 7 6. Thickness of the Beech Creek Limestone 8 7. Net thickness of sandstone in the Cypress 9 8. Area of oil production from Cypress sandstones 10 9. SW-NE electric log cross section across northwestern portion of paleochannel crossing Louden Anticline 12 10. SW-NE electric log cross section across middle portion of paleochannel crossing Louden Anticline 1 1 . SW-NE electric log cross section across southeastern portion of paleochannel crossing Louden Anticline 12. NW-SE cross section across Louden Anticline based on electric log interpretations 13. Paleogeography during main phase of Cypress Sandstone deposition 14. Paleogeography during early Beech Creek ("false Barlow") deposition 15. Diagrammatic cross section of Louden Anticline during Cypress sand deposition Table 1 . Discovery wells of extensions to the Louden Oil Field, 1949-1950 12 14 16 17 17 18 19 Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/paleochannelacro119cluf ABSTRACT Structure mapping on the base of the Beech Creek (Barlow) Limestone of Chesterian age (upper Missis- sippian) across Louden Oil Field, Fayette County, Illinois, has revealed a northwest-trending saddle more than 1 mile ( 1 .6 km) wide and 4 miles (6 km) long that crosses the Louden Anticline. This appar- ent depression results from the abrupt appearance of a thick, fine-grained, argillaceous limestone (so- called "false Barlow") subjacent to the thin bed of coarse-grained, bioclastic upper Beech Creek Lime- stone. Sandstone beds in the underlying Cypress Sandstone were found to be thin or absent beneath this area of "false Barlow." This saddle is believed to be a major tidal chan- nel that breached deposits of shallow marine or eolian sands that had accumulated along the crest of the anticline. The trend of the channel (per- pendicular to the anticlinal axis) and the chan- nel's restriction to the crestal area (with no ap- parent extension off-structure) strongly suggest that the Louden Anticline was topographically high during Cypress deposition. The channel was filled during late Cypress and early Beech Creek deposition by marine shales and fine-grained lime- stone ("false Barlow"). During the main phase of sand deposition, the channel profoundly influ- enced local sandstone depositional patterns, and two offshore sandbars or barrier islands accumu- lated near its southeastern terminus along the flank of the anticline. These flanking sand bodies pinch out up-dip against lagoonal shales and are true stratigraphic traps, which have since produced sev- eral million barrels of petroleum. The recognition of large marine bar sand bodies in the Cypress opens new prospects for oil explo- ration in the Illinois Basin. Henceforth, Cypress sandstones should not be regarded as massive blanket sands or overlapping fluvial channel sands, but rather as complex sequences of shallow marine sandstones. Favorable areas to explore for large stratigraphic traps in the Cypress include the flanks of major anticlines, areas of thick "false Barlow," and areas near linear gaps in the distribution of Cypress production. PALEOCHANNEL ACROSS LOUDEN ANTICLINE INTRODUCTION The Cypress Sandstone (Chesterian) is a litholog- ically heterogenous unit of sandstone, sandy shale, and shale that is present throughout most of the Illinois Basin. Massive sandstone bodies within the Cypress are among the Basin's most important hydrocarbon-producing (pay) zones, and have produced about 500 million barrels of oil. Sand- stones in the Cypress (locally called "Weiler sands") are the principal pay zones in the Louden Oil Field (fig. 1 ) -one of the few giant oil fields in the Illinois Basin. These productive sandstones do not blanket the entire Louden Anticline, however, and in one elongate area trending perpendicular to the major axis of the structure they are entirely absent. This area where the Cypress is all shale also coincides with an area where the superjacent Beech Creek Limestone is abnormally thick. The base of the Beech Creek Limestone (com- monly called the "Barlow lime") is a widely used structure mapping horizon in the Illinois Basin. In certain areas, however, its usefulness as a mapping horizon is impaired by the anomalous occurrence of as much as 50 feet ( 1 5 m) of fine-grained, argil- laceous limestone that is subjacent to a regionally normal thickness (10 to 25 feet; 3 to 8 m) of coarse-grained limestone (fig. 2). This thick lower part of the Beech Creek, often called "false Barlow" by petroleum industry geologists, is manifested on electric logs by a "dangling base" of lower than normal resistivity (Bristol, 1968). The term "false Barlow" is a misnomer, however, because this low resistivity limestone is truly part of the Beech Creek Limestone. One such area of the "false Barlow" occurs at Louden Field, where Bristol (1968) mapped a major saddle trending at right angles to the major axis of the Louden Anticline (fig. 3). Several years later, while mapping the structure of the top of the Karnak Member of the Ste. Genevieve Limestone (Valmeyeran) across the same area. R2E R3E R4E T9N T8N T7N T6N T5N Figure 1 . Structure of the top of the Karnak Limestone Member (Ste. Genevieve) in the vicinity of Louden Anticline (after Bristol and Howard, 1976), Effingham and Fayette Counties, Illinois. Louden Oil Field (shaded) and the study area are shown. ILLINOIS STATE GEOLOGICAL SURVEY/ILLINOIS PETROLEUM 119 Brief petrographic description Shale, medium-gray (N5), fissile, poorly indurated, weakly calcareous; minor red, very calcareous shale / Limestone, pale yellowish-brown (10YR6/2) to light- gray (N7), medium- to coarse-grained, fossiliferous oolitic, pelletoidal, moderately to well-sorted bio- calcarenite; tightly cemented by sparry calcite; sandy and glauconitic in lower part Limestone, dark yellowish-brown (10YR3/2) to medi- um dark-gray (N3), very fine-grained, micritic, oolitic, possibly pelletoidal, argillaceous and shaly;thin sandy limestone bed at base Shale, medium light-gray (N6), soft, fissile, carbona ceous, slightly silty, mixed with red and green varie gated shale chips Sandstone, light-gray (N7), very fine-grained, well- rounded and sorted, weakly calcareous cemented, argillaceous in places, sparsely fossiliferous in some samples (bryozoans, brachiopods, crinoids) Limestone, light-gray (N7), medium- to coarse-grained biocalcarenite, with crinoid fragments and echinoid spines, coarse spar cement; interbedded with sand- stone, light -gray (N7), fine-grained, quartzose, rounded, well-sorted, calcareous cemented, fair intergranular porosity; and shale, medium-gray (N5), fissile, soft, mixed with red and pink massive shale chips Limestone, very light-gray (N8), medium- to coarse- grained biocalcarenite, pelletoidal?, coarse spar cement Shale, medium-gray (N5), soft, fissile, abundant car- bonaceous plant fragments, considerable red shale in lower part Sandstone, light brownish-gray (5YR6/1 ), fine- to very fine-grained, subrounded, argillaceous, weakiy calcite cemented, good intergranular porosity Limestone, very light-gray (N7) to pale yellowish- brown (10YR7/2), dense, micritic, sandy, may be interbedded with very calcareous sandstone Sandstone, light brownish-gray (5YR6/1), fine-grained, subangular, well-sorted, very weakly calcareous, fria- ble, interbedded with shaly streaks in lower part Limestone, yellowish-brown (10YR5/2), medium- grained biocalcarenite, well-sorted and rounded, dense, micritic (packstone), coarse spar cement Figure 2. Generalized geologic column for the Lower Chesterian and subjacent strata on the Louden Anticline, Fayette County, Illinois. Elec- trical resistivity and spontaneous potential curves are composites based on several well logs in the area. Petrographic descriptions are based on visual sample studies and thin-section studies of several wells. PALEOCHANNEL ACROSS LOUDEN ANTICLINE R3E .82° Structure contour (interval = 20 feet) Figure 3. Elevation of the base of the Beech Creek (Barlow) Limestone. Modified from Bristol's work map (1968), this map utilizes all avail- able drill holes penetrating the Barlow. The pronounced saddle across the Louden Anticline in Sections 21 and 27 is an artifact of the "false" base of the Barlow in this area. ILLLINOIS STATE GEOLOGICAL SURVEY/ILLINOIS PETROLEUM 119 Bristol and Howard (1976) noted a pronounced thinning of the stratigraphic interval between the base of the Beech Creek and the Karnak (fig. 4). Both Bristol and Howard noted the presence of what was thought to be a pre-Beech Creek paleo- channel crossing the anticline, but the feature was not studied further until Howard's unpub- lished Beech Creek to Karnak thickness map came to our attention during the updating and revision of Bristol's work maps ( 1 968). The question then arose as to whether the "false Barlow" represented simple thickening of the limestone, or a facies relationship between the Beech Creek Limestone and the underlying Cypress Sandstone. We selected the area of "false Barlow" at Louden Field as an area favorable for detailed investigation because of the ample data offered by dense drilling and numerous electric logs. CHARACTERISTICS OF THE "FALSE BARLOW" A structure map of the base of the Beech Creek Limestone (fig. 3) shows a saddle trending approx- imately perpendicular to the major axis of the Louden Anticline and traversing Sections 21, 22, 27, and 28, T. 8 N., R. 3. E., Fayette County. Examination of the stratigraphic relationships in this area, however, shows the apparent structural saddle to have the attributes of a channel rather than a tectonic feature. For example, the thick- ness of the interval between the base of the Beech Creek and the top of the Karnak Limestone Mem- ber of the Ste. Genevieve (fig. 4) ranges from 230 to 280 feet (70 to 85 m) across most of the Louden Field, varying gradually and somewhat irregularly. Within the area of the saddle (fig. 3), however, this interval rapidly thins (fig. 4) to less than 180 feet (55 m). The thin area trends slightly east through Sections 26 and 25. Thus mapped, the channel is at least 4 miles (6 km) in length and locally exceeds 1 mile (1.6 km) in width. No evidence has been found to suggest that the feature extends beyond the anticline to the east or west. The interval from the top of the Beech Creek to the top of the Karnak thickens regionally to- ward the south and thins only very slightly (about 20 feet, or 6 m) in the area of the channel (fig. 5). The structure of the top of the Beech Creek does not show a prominent sag (fig. 3), and the slight decrease in thickness in this area is probably caused by differential compaction. The thickness of the Beech Creek increases dramatically to over 70 feet (21 m) in Section 27 (fig. 6), almost solely because of the addition of the "false Barlow" at its base. Sample studies and thin sections of this interval show that the "false Barlow" is a fine-grained, sparsely fossiliferous, peloidal, partly oolitic, and micritic limestone (fig. 2). The characteristically low resistivity and positive spontaneous potential suggest that the "false Barlow" is very shaly; usually, the Beech Creek Limestone is also shaly where it crops out around the southern margin of the Illinois Basin. The high resistivity bed at the top of the Beech Creek is present throughout the Louden area and varies only slightly in thickness (fig. 6). This upper limestone is a coarse-grained, fossiliferous (mostly crinoidal), peloidal, and partly oolitic biocalcarenite (fig. 2). RELATIONSHIP OF THE "FALSE BARLOW" TO MASSIVE SANDSTONES IN THE CYPRESS For the interval from the top of the Beech Creek to the top of the Karnak to remain as constant as that shown in figure 5, the "false Barlow" must displace an almost equal thickness of underlying strata in this area. The net thickness of sandstone beds in the Cypress, based on electrical self-poten- tial logs, varies from to more than 60 feet (18 m) across Louden Field and averages 10 to 30 feet (3 to 9 m) (fig. 7). The sandstone thickens and thins irregularly in many areas, with several north west -trending thick areas; then the thickness abruptly decreases to in the same area as the area of thick "false Barlow." Because the reservoir sand- stones are absent, the Cypress is also not pro- ductive (fig. 8). Electric log cross sections perpendicular to the axis of the channel illustrate the reciprocal rela- tionship between the thickness of the sandstones in the Cypress and the thickness of the Beech Creek Limestone (figs. 9, 10, and 1 1). These cross sections show how rapidly the sandstones thin toward the margins of the channel; in some areas the sand thins from 30 to feet (9 to m) between wells only a few hundred feet apart (fig. 7). The "false Barlow" first appears where the sandstones pinch out, although in some areas a PALEOCHANNEL ACROSS LOUDEN ANTICLINE R3E • Datum point . 250 -^ Thickness contour (interval = 20 feet) Figure 4. Thickness of the interval between the base of the Beech Creek (Barlow) Limestone and the top of the Karnak Member of the Ste. Genevieve Limestone on the Louden Anticline. ILLINOIS STATE GEOLOGICAL SURVEY/ILLINOIS PETROLEUM 119 R3E ISGS 1980 ^260 Datum point -Thickness contour (interval = 20 feet) Figure 5. Thickness of the interval between the top of the Beech Creek (Barlow) Limestone and the top of the Karnak Limestone Member of the Ste. Genevieve Limestone. The slight thinning centered in section 21 is due to compactional thinning of shale within the paleo- channel. PALEOCHANNEL ACROSS LOUDEN ANTICLINE R3E SGS 1980 « Datum point •Thickness contour (interval = 20 feet) 1 -20 feet 40-60 feet :•:•:•:! 20-40 feet mm >60feet Figure 6. Thickness of Beech Creek (Barlow) Limestone (including "false Barlow"), interpreted from electrical resistivity logs. 8 ILLINOIS STATE GEOLOGICAL SURVEY/ILLINOIS PETROLEUM 119 R3E • Datum point Thickness contour (interval - 20 feet) 1 40-60 feet 60-80 feet > 80 feet | 1 feet l: : :'-: : :l 1 -20 feet [ I 2040 feet Figure 7. Net thickness of sandstones in the Cypress, interpreted from electrical self-potential logs. The procedure used to prepare this map was as follows. A clean shale base line was drawn on the basis of the SP response of overlying and underlying Chesterian shales; sand- stone was then defined as those intervals exhibiting a -40 mV or greater deflection left of the shale base line. Forty mV was selected on the basis of examining several logs with both clean and shaly sands present; our evaluation indicated that a 20 mV cutoff would include several zones of shaly sands that are usually nonproductive in this area, and a 10 mV cutoff (as used by Potter, 1962) would include considerable shale. Many of the minor irregularities and isolated areas of zero sand shown on this Figure may be artifacts due to the use of this procedure, variability in hole conditions, and the choice of an arbitrary cutoff; caution is therefore advised in interpreting this map. Only wells that penetrated the entire Cypress Sandstone were used in the preparation of this map. PALEOCHANNEL ACROSS LOUDEN ANTICLINE R3E 8 -^- Cypress Sandstone dry hole • Well used in cross section accompanying this report Area of oil production from Cypress Sandstone Figure 8. Area of oil production from Cypress sandstones (stippled). Lines of cross sections (figs. 9, 10, 11, and 12) are shown. 10 ILLINOIS STATE GEOLOGICAL SURVEY/ILLINOIS PETROLEUM 119 thin interval of "false Barlow" overlaps the sand- stone (fig. 9, well #5). In many areas the base of the "false Barlow" roughly coincides with the projected top of the sandstones on either side of the channel (fig. 9). The electrical resistivity curves suggest that the"false Barlow" grades into the underlying shale (figs. 10 and 1 1), and grades lat- erally into slightly calcareous shale in the upper part of the Cypress on the sides of the channel (fig. 9, wells 5 and 6). No evidence of interfingering of the channel-fill sediments with the sandstones on either side of the channel was found; therefore, the channel was apparently devoid of sediment while the sands were deposited. A second area of thin or missing sandstone in the Cypress occurs at the southern edge of the study area in Sections 5, 6, 7, and 8, T. 7 N., R. 3 E. (figs. 7 and 8). Five to 10 feet (2 to 3 m) of "false Barlow" stretch across this area (fig. 6) and persist southward to outside the study area. Although this feature is not nearly as well defined as the channel to the north, it probably represents a second, older, and partially sand-filled channel parallel to the larger channel. The net sand thickness map (fig. 7) and the map of Cypress oil production (fig. 8) reveal two thick oil-productive sandstone bodies in the Cypress along the southeast flank of the Louden Anticline. In Section 2, T. 7 N., R. 3 E., the southern-flanking sandstone body is more than 80 feet (24 m) thick (fig. 7). Because most of the drill holes in this area penetrate only the upper portion of Cypress pay, the total sandstone thick- ness could not be accurately mapped. The extent and thickness of sandstone to the southeast could not be determined precisely because of the paucity of off -structure tests below the oil-water contact; however, the sandstone appears to continue east- ward without interruption into Effingham County and remains about 100 feet (30 m) thick (figs. 7 and 12). A major area that holds no sandstone and that trends northeast through Sections 3 and 9, T. 7 N., R. 3 W., and Section 34, T. 8 N., R. 3 E., separates this thick sandstone bar from the main sandstone body atop the anticline. This area, however, does not coincide with an area of "false Barlow," unlike the sand-free, channel crossing the anticline (figs. 6 and 7). The more northerly of the two flanking sand- stone bodies is centered on Sections 24 and 25, T. 8 N., R. 3 E., where it is more than 20 feet (6 m) thick (fig. 7). This sandstone body is not as well developed as the sandstone bar to the southwest; however, it does yield oil (fig. 8). Spontaneous potential electric logs show that this sandstone is not entirely isolated from the main sandstone body atop the anticline by an area with no sand (fig. 7); instead, it is separated by a narrow area of thin and very shaly sandstones. Apparently, the shale beds are persistent enough to prevent communication between the two reservoirs. This area of shaly sandstones also lacks thick "false Barlow" (fig. 6). ORIGIN OF THE "FALSE BARLOW" AND RELATED FEATURES The abrupt termination of sandstone beds in the Cypress and the parallel development of the "false Barlow" indicates that the origin of the thick "false Barlow" at the Louden Field was re- lated to Cypress depositional patterns. The trend of the channel perpendicular to the anticlinal axis and the restriction of the channel to the crestal area, with no extension off-structure, strongly suggest that the Louden Anticline was topographically high during Cypress deposition. Several other Chesterian units, including the massive sandstones of the Yankeetown Sand- stone and some thin limestone beds in the Paint Creek Group (fig. 2), are thin or missing in one or more areas along the anticlinal crest; their deposi- tion was also apparently influenced by early topo- graphic relief (fig. 1 2). Although outside the scope of this study, detailed mapping of discrete Chest- erian sandstone, shale, and limestone bodies would probably provide additional evidence and insight into the timing and magnitude of syndepositional uplift of major structures such as the Louden Anticline. We interpret the paleochannel to have been a large tidal channel which cut across a shallow marine sand bar or eolian dune field that was accu- mulating along the crest of an island of low relief (fig. 13). The irregular sand thickness pattern along the anticlinal crest (fig. 7), with several northwest- trending thick and thin areas, may have been the result of tidal currents across the sand shoal during high tides. Two of the most prominent thick areas, lying on either side of the chanel in Sections 22 and 28, might represent natural levees or channel margin bars (Hubbard, Oertel, and Nummendal, 1979). The channel itself was probably swept PALEOCHANNEL ACROSS LOUDEN ANTICLINE 11 Fraileys Shale ^. 'tiyy^rrrrv , paleochannel Beech Creek Limestone \ Cypress Sandstone Paint Creek - Group ^^2 B^^ sandban Ridenhower Formation- Bethel Sandstone Yankeetown Sandstone Figure 9. SW-NE electric log cross section across northwestern portion of paleochannel crossing Louden Anticline. Lithologic interpretations are based on sample studies. Well names and locations are listed in the appendix. Paint Creek Group Fraileys Shale Beech Creek Limestone ^^^ - - ^S i.i.i ,i LLLJJ-— 7 i"=>_ Downeys Bluff Limestone Yankeetown Figure 10. SW-NE electric log cross section across middle portion of paleochannel crossing Louden Anticline. Lithologic interpre- tations are based on sample studies. 12 ILLINOIS STATE GEOLOGICAL SURVEY/ILLINOIS PETROLEUM 119 A' Fraileys Shale Beech Creek Limestone Cypress Sandstone Ridenhower Formation- Bethel Sandstone Downeys Bluff Limestone Yankeetown Sandstone ft m OtO 50--15 100-^30 Calcareous sandstone Sandy limestone ^ Coarsely crystalline fossiliferous limestone Fine-grained argillaceous limestone [»_] Shale Sandstone Sandy shale Partly oolitic limestone Scale and legend apply to both figures 9 and 10. Downeys Bluff Ls Yankeetown Ss PALEOCHANNEL ACROSS LOUDEN ANTICLINE 13 ft m OtO ioo-^o Fraileys Shale 'I --. I ~| 50 '-15 Beech Creek Limestone (P L ■ ' Ridenhower Formation Paint Creek Group Cypress Sandstone Downeys Bluff Limestone ^ /.[ i ^? ! 1 .. i 1 .. 1 ! ?."^ Yankeetown Sandstone H x^n -r i > 1 -J — - - 1 .. - Calcareous sandstone Sandy limestone Coarsely crystalline fossiliferous limestone Fine-grained argillaceous limestone Figure 11. SW-NE electric log cross section across southeastern portion of paleochannel crossing Louden Anticline. Lithologic interpretations are based on sample studies. clean of sediment by strong tidal currents and in some areas may have been floored by lithified marine limestones or calcareous sandstones of the upper part of the Paint Creek Group (figs. 9, 10, and 1 1). Deposition within the channel apparently did not begin until significant marine inundation occurred and shale was deposited across the entire area (fig. 14). The shale in the lower part of the channel is therefore believed to be contempor- aneous with the lower portion of the shale in the Cypress overlying the sand bars on either side. The thick sequence of fine-grained argillaceous lime- stone forming the "false Barlow" deposited in the slightly deeper water of the channel is apparently a lateral facies of the uppermost Cypress shale on the sides of the channel. This argillaceous limestone grades upward into the more open marine, coarse- grained, bioclastic, upper part of the Beech Creek Limestone, which blanketed the anticline and surrounding area. The occurrence of a few feet of "false Barlow" throughout the area surrounding the Louden Anticline suggests that the "false Barlow" is a deep water facies and that the anti- cline was expressed above the surrounding area during Cypress deposition. The stratigraphic position of two thick sand- stone bodies down-dip and physically separated from the main sandstone body atop the anti- cline, along with the direction of their trends (parallel to the axis of the structure), suggest that these sands accumulated as offshore bars or barrier islands. Sand transported from the north or northeast (Potter, 1962) was apparently diverted by the tidal channel and deposited by longshore currents as bars north and south of the south- eastern terminus of the channel (fig. 13). The narrow area of shale deposition separating these flank sands from the crestal sands is suggestive of a back-barrier lagoon. The up-dip pinchout of the flank sands against shale qualifies them as true stratigraphic traps (figs. 12 and 15). 14 ILLINOIS STATE GEOLOGICAL SURVEY/ILLINOIS PETROLEUM 119 paleochannel 17 18 19 C Fraileys Shale Beech Creek Limestone Cypress Sandstone Ridenhower Formation Yankeetown Sandstone dstone [_ jj Sandy shale Partly oolitic limestone ISGS 1980 Most studies of the Chesterian sandstones in the Illinois Basin have interpreted the sands to be fluvial-deltaic sequences (Potter, 1962; Swann, 1 964). These interpretations are based on mappings of sandstone thickness,which often reveal a dom- inant northeast trend to the sandstone bodies with an elongated distributary-like pattern, as well as studies of cross bedding orientation and physical sedimentary structures. Potter and Swann did not, however, map the thick lower Chesterian and Val- meyeran sandstones (Cypress, Bethel, Yankeetown, and Aux Vases). Both Potter and Swann noted that these sandstones did not display the simple geo- metry of the overlying upper Chesterian sandstones, and suggested that they were complex, multistory sandstones deposited during several cycles of fluvial deposition (Potter, 1962, 1963). The Cypress Sandstone bodies mapped in this study suggest that, in fact, these sands may have been deposited by more open marine processes during periods when the Michigan River delta had been extensively innundated and shifted far to the northeast. Generally, the Cypress Sandstone bodies at Louden are unlike fluvial sandstones, in which most sand deposition occurs within shifting channels. At the Louden Field, the channels are filled with shale and limestone and are bounded on all sides by sandstone— just the reverse of a typical fluvial sand body. The limited length and depth of the channel, the absence of a basal conglomerate, and the fact that no significant unconformity with associated erosional valleys has ever been found at the top of the Cypress, are all arguments against a subaerial-erosional origin for Cypress paleochannels at Louden. Unfortunately, we must base our paleoenvi- ronmental interpretation almost exclusively on the mapped patterns of sand and limestone thickness, and on examination of well cuttings. Cores through the Cypress in the Louden Field have not been preserved in a manner suitable for detailed paleo- environmental analysis, and cores through the PALEOCHANNEL ACROSS LOUDEN ANTICLINE ILLINOIS STATE 8F0L0G1CAL SURVEY LIBRARY 15 Paint Creek Group dnOJQ JJ83J0 HHBd E< ^ £ o ■g c ■5 o u ^ o. E « o s 43 s u .5 E o a a •n T> dl D. h 3 tl 1 u »~ o< z 5 Wl o c S C e| a u O _o O i u c •7 tl o u ■^3 a. c a. u n n c o >> o -c c •o o y o u JO M ed d -o c C c E c C/i < s 3 s € S3 ■a 3 3 o t^ -J u y V) J3 aj O C o o o . -c UJ U * J on j>; M 16 ILLINOIS STATE GEOLOGICAL SURVEY/ILLINOIS PETROLEUM 119 tidal channel sheltered lagoon sea level beach or dunes on low-relief island barrier island dal currents 4 ► Longshore currents Figure 1 3. Paleogeography during main phase of Cypress Sandstone deposition. Sands accumulated along the crest of the rising anticline in beach or eolian environments, while barrier islands (as show in this illustration) or submerged offshore bars (not illustrated) accu- mulated to the east and merged downdip into a subtidal sand sheet. A major tidal channel breached the crestal sands and acted as a pathway for sand transport across the structure. partially-filled tidal channel pond mud flats or shallow subtidal muds oolite bar Figure 14. Paleogeography during early Beech Creek ("false Barlow") deposition. Shale and calcareous shale accumulated in the shallow areas along the anticlinal crest, possibly in mud flat environments as shown here. Argillaceous limestone ("false Barlow") was deposited in the deeper water surrounding the structure and within the partially shale-filled channel crossing the structure. Small oolite bars probably formed along the margin of the anticline and near the mouth of the paleochannel, as shown by sparse oolites in many samples of "false Barlow." PALEOCHANNEL ACROSS LOUDEN ANTICLINE 17 NW SE eolian dunes lagoon barrier island Figure 15. Diagrammatic cross section of Louden Anticline during Cypress sand deposition. Beech Creek Limestone are not available from this area. Although the Cypress is one of the major productive sandstones in the Illinois Basin, there are disappointingly few published studies that interpret its depositional environment. Smoot (1960) summarized the then current state of knowledge concerning Cypress deposition by stating that the lower part of the Cypress (pro- bably including part of the Ridenhower Formation) represented offshore marine environments, whereas the upper part represented near-shore and possibly "terrestrial" deposition. Marine fossils are appar- ently rare in the Cypress, although our thin-section studies of Cypress sands in the Louden area revealed fragmented bryozoans, brachiopods, and crinoids. Thin coal beds are also present in the Cypress in southern Illinois and have been cited as evidence for "terrestrial" or continental deposition, although we suspect they may in fact represent deposition in lagoons or small coastal swamps. Sedimentary structures in Cypress sands include cross bedding, cut-and-fill structures, and in one core (from another area) examined by the authors, poorly developed bidirectional (herringbone) cross bedding and flame structures. Bioturbation is apparently rare and few detailed analyses or de- scriptions of physical sedimentary structures have been made. Most of the published data are fully consistent with a shallow marine, tidally influenced origin for the Cypress Sandstone. PROSPECTS FOR FURTHER CYPRESS OIL DEVELOPMENT Although drilling data are very sparse, the patterns of sandstone and shale deposition in the Cypress along the northwest flank of the Louden Anticline resemble the depositional patterns along its south- east flank. Several wells at the northwest edge of production encountered no sandstone, thin sand- stone, or shaly sandstone (fig. 7). This situation is similar to one at the southeast edge of the major producing sandstones which extends from Section 9, T. 7 N., R. 3 E., northeast to Section 24, T. 8 N., R. 3 E. An area of thick shale with little or no sand may therefore trend along most of the northwest flank of the structure, although the continuity of this shale interval is questionable. Several wildcat wells have been drilled west of Louden Field in T. 9 N., R. 1 E.;T. 9 N., R. 2 E.; T. 8 N., R. 2 E.; Fayette County. Most of these wells encountered between 15 and 30 feet (5 to 9 m) of sandstone in the Cypress with a total thickness range of to over 50 feet (15 m). These sandstones probably blanket most of the area west of the Louden Field and may represent a subtidal marine sand sheet. Although these sand- stones are not as thick as the productive bar sand- stones on the southeastern flank of the Louden Anticline, stratigraphic traps might occur at their up-dip extremities on the flank of the structure. 18 ILLINOIS STATE GEOLOGICAL SURVEY/ILLINOIS PETROLEUM 119 Drilling along the steep northwest flank of the Louden Anticline has been sparse (fig. 8). Most of this drilling was conducted in the late 1930s-just after oil was discovered on the anticline in 1937. Few wells were drilled more than a few hundred feet from producing wells, and geophysical logs were not run on most of these distant wells. None of the samples were of suitable quality to determine the presence or absence of potential reservoir sands on the northwest flank. The recognition that certain aspects of Cypress Sandstone deposition are best explained by shallow marine depositional models, and that the tectonic uplift of large anticlines in the Illinois Basin may have been syndepositional, opens new prospects for stratigraphic trap exploration. The two off- structure bar sandstones at Louden were not discovered until late 1949, when the W. L. Belden # 1 Grames wildcat encountered the northern sand- stone bar. In the following months, several addi- tional tests found the southern sandstone bar and outlined the general extent of the bars (table 1). Since their discovery in 1949, the southern sand bar has produced more than 6 million barrels of oil, and the northern sand bar more than 2 million barrels of oil. Many of the large anticlines in the Illinois Basin are sparsely tested along their flanks, much as the Louden Anticline was in 1948. Explo- ration for flanking sandstone bodies along known structures might result in significant new discoveries. Even in areas with no structural closure, strat- igraphic traps could occur within the Cypress. The Cypress sandstone bodies at Louden Oil Field resemble ancient marine barrier bar sands found in other petroliferous basins. For example, several major oil and gas fields have been found in marine bar sandstones within the Cretaceous Muddy Sand- stone of eastern Wyoming and Montana, and most of these fields occur in areas with no structure other than regional dip— the entrapment of hydro- carbons is entirely stratigraphic. The productive bar sandstones are often cut by major shale-filled paleochannels trending normal to the axis of the sand body (e.g., Curry and Curry, 1972, South Glenrock Oil Field; Woncik, 1972, Recluse Oil Field; McGregor and Biggs, 1972, Bell Creek Oil Field), and in at least one field (South Glenrock), smaller sand bars accumulated at both ends of one of these paleochannels. The relationship between areas of "false Barlow" and Cypress sand cutouts discussed in this report may be useful as an exploration tool in other areas of the Illinois Basin. Channel-like trends of "false Barlow" may indicate major pathways of sand transport, and thick sand accumulations may occur near the ends of the channels. Linear cutouts in the areal distribution of Cypress production should be investigated for possible associated strat- igraphic traps. Despite the fact that Chesterian sandstones have been one of the primary objectives of oil and gas exploration in the Illinois Basin for over 50 years, there is still considerable potential for discovering significant new reserves of strat- igraphically entrapped petroleum. Table 1. Discovery wells of extensions to the Louden Oil Field, 1949-1950 Company and farm Location County Depth to top of pay (ft) Initial production BO/BW PD Completion date W. L. Belden #1 W. Grames SW NE NW 25-8N-3E Fayette 1528 130/0 9/27/49 Reynolds & Henson #1 Kuppels SE SE SE 35-8N-3E Fayette 1612 88/0 10/11/49 M. H. Richardson et al. #1 J. A. Siebert SESESW 3-7N-3E Fayette 1556 180/0 10/25/49 Jones & Simpson #1 Phillips SENENW19-8N-4E Effingham 1509 50/24 1/24/50 M. H. Richardson #1-A R. Lily NENENE18-8N-4E Effingham 1579 10/30 3/07/50 Claypool Drlg. #1 J. W. Doty NWNWNE 7-8N-4E Effingham 1560 6/25 11/07/50 PALEOCHANNEL ACROSS LOUDEN ANTICLINE 19 REFERENCES Bristol, H. M., 1968, Structure of the base of the Mississippian Beech Creek (Barlow) Limestone in Illinois: Illinois State Geological Survey Illinois Petroleum 88, 12 p. Bristol, H. M., and R. H. Howard, 1976, Structure of the top of the Karnak Limestone Member (Ste. Genevieve) in Illinois: Illinois State Geological Survey Illinois Petroleum 109, 6 p. Curry, W. H., and W. H. Curry III, 1972, South Glenrock Oil Field Wyoming: Prediscovery thinking and postdiscovery description, in R. E. King [ed.] , Stratigraphic oil and gas fields-Classifi- cation, exploration methods, and case histories: American Association of Petroleum Geologists Memoir 16, p. 415-427. Hubbard, D. K., G. Oertel, and D. Nummedal, 1979, The role of waves and tidal currents in the development of tidal-inlet sedimentary structures and sand body geometry: Examples from North Carolina, South Carolina, and Georgia: Journal of Sedimentary Petrology, v. 49, p. 1073-1092. McGregor, A. A., and C. A. Biggs, 1972, Bell Creek oil field, Montana, in R. E. King [ed.] , Stratigraphic oil and gas fields- Classification, exploration methods, and case histories: American Association of Petroleum Geologists Memoir 16, p. 367-375. Potter, P. E., 1962, Late Mississippian sandstones of Illinois: Illinois State Geological Survey Circular 340, 36 p. Potter, P. E., 1963, Late Paleozoic sandstones of the Illinois Basin: Illinois State Geological Survey Report of Investigations 217, 92 p. Smoot, T. W., 1960, Clay mineralogy of Pre-Pennsylvanian sand- stones and shales of the Illinois Basin. Part III. - Clay minerals of various facies of some Chester formations: Illinois State Geological Survey Circular 293, 19 p. Swann, D. H., 1964, Late Mississippian rhythmic sediments of Mis- sissippi Valley: American Association of Petroleum Geologists Bulletin, v. 48, p. 637-658. Woncik, J., 1972, Recluse Field, Campbell County, Wyoming, in R. E. King [ed.] , Stratigraphic oil and gas fields-Classifi- cation, exploration methods, and case histories: American Association of Petroleum Geologists Memoir 16, p. 375-382. ACKNOWLEDGMENTS Richard H. Howard, Howard R. Schwalb, and Mark L. Reinbold participated in numerous discussions of the ideas and conclusions presented in this report ; their advice and criticisms are gratefully acknowl- edged. P. E. Potter kindly reviewed the manuscript. 20 ILLINOIS STATE GEOLOGICAL SURVEY/ILLINOIS PETROLEUM 119 APPENDIX Numbered wells used in cross sections (figs. 9, 10, 11, and 12). All wells are located in Fayette County, Illinois. Well no. Well name Approximate location within section Section, township, range 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Carter Oil #7 G. Raymond Humble Oil #4 Weaber-Horn Carter Oil #3D H. Beck Carter Oil #5D M. E. Hogan Carter Oil #5D Presbyterian Church Carter Oil #10D J. B. Dreese Carter Oil #5D E. Marshall Carter Oil #3D M. Tucker Carter Oil #3D C. M. Dial Carter Oil #5D J. Hogan Carter Oil #3D J. B. Tucker Carter Oil #3D Hopper-Cummins Carter Oil #3D M. Myers Carter Oil #14DM. Mills Carter Oil #3D H. Zetsche Carter Oil #4D A. Fortner Carter Oil #6D J. Hogan Humble Oil #27-4WT M. Rhoades Carter Oil #3 G. Durbin Carter Oil #7W F. W. Botterbusch Carter Oil #SWD-2 K. Owens Carter Oil #5D K. Birdie Carter Oil #3D R. Weber Carter Oil #1 2D M. Mills Carter Oil #10DS. Dial Joe Dull #A-1 L. Watson Carter Oil #1 S. Durbin M. L. Niemeyer #1 Mills Smith Oil #1 E. K. Smith E/2 SE NE N/2NENW CSESW N/2 NW SE E/2 SE NE CNENW CSENW CSWNE CSENE CNWNW S/2 NE NW CSESW CNWSE CSWSE CNESE CNWSW CSWNW SENWNE CSWSW SW NE SE CNESW E/2 NW SE W/2 SE SE CSESW CNENW NE SW SW SW NE SE E/2 SW SW NE NE SE 29-8N-3E 28-8N-3E 21-8N-3E 21-8N-3E 21-8N-3E 22-8N-3E 28-8N-3E 28-8N-3E 28-8N-3E 27-8N-3E 27-8N-3E 22-8N-3E 22-8N-3E 28-8N-3E 28-8N-3E 27-8N-3E 27-8N-3E 27-8N-3E 23-8N-3E 30-8N-3E 29-8N-3E 29-8 N-3E 29-8N-3E 28-8N-3E 33-8N-3E 34-8N-3E 34-8N-3E 35-8N-3E 35-8N-3E PALEOCHANNEL ACROSS LOUDEN ANTICLINE 21 ILLINOIS GEOLOGICAL SURVEY 1\ YEARS C«olojic ,«se„ c h ,„ d „„ ice fo , , he citons of ^^ ^