MISSISSIPPIAN 'WARSAW' PLAY MAKES WAVES IN ILLINOIS BASIN

Zakaria Lasemi, John P. Grube
Illinois State Geological Survey
Champaign

Recent completions of relatively prolific wells in the mid-Mississippian Ullin limestone (Fig. 1) (19015 bytes) have generated considerable excitement about this Illinois basin play.

Reservoirs found within this limestone, commonly referred to by industry as the "Warsaw," are scattered (Fig. 2) (18434 bytes) and are prolific oil producers in some areas of the basin. The widespread development of reservoir quality facies at depths ranging from 2,400-4,400 ft and the stratigraphic proximity of thermally mature New Albany shale, the primary Illinois basin source rock, are factors that make the "Warsaw" an excellent exploration target.

The basin 'is presently undergoing a third round of "Warsaw" development after experiencing drilling booms in the late 1950s to early 1960s and in the late 1970s to early 1980s.

In the early 1990s, the completion of prolific wells in two fields, Johnsonville Consolidated in Wayne County and Enfield South in White County (Fig. 2) (18434 bytes), has once again heightened interest and encouraged drilling activity in the "Warsaw" pay. Wells flowing 200-400 b/d with sustained production are not uncommon. Open flow tests can yield as much as 2,000 b/d. Some of these wells have produced more than 100,000 bbl of oil in less than 2 years.

EXPLORATION CONCEPT

A depositional model (Fig. 3) (22608 bytes) shows reservoir facies development in the "Warsaw" and assists exploration and development in the Illinois basin.

The model, based on Waulsortian-type carbonate mud mounds recognized on the outcrop and in the subsurface of the basin, 1 2 shows the spatial relationship of mounds to reservoir facies development and hydrocarbon entrapment.

Mound development was widespread during the deposition of the lower part of the "Warsaw." Although mounds continued to develop during deposition of the upper part of the "Warsaw," they tended to be smaller than those in the lower "Warsaw." Mounds are characterized by a core facies of lime mudstone or wacke-stone flanked by a dipping packstone-grainstone facies (Fig. 4), which is dominated by bryozoan and crinoid fragments. Bryozoan and/or crinoid bioherms developed preferentially above the mounds and shed skeletal carbonates that were deposited on the flanks of the mounds or as a skeletal sandwave facies above the mound complex (Fig. 3) (22608 bytes). These skeletal carbonates form the packstone-grainstone reservoir facies of the "Warsaw."

Differential compaction of the flanking and intermound sediments causes the overlying and flanking porous beds to drape over the dense core facies of the mound, thus creating a trap. Trapping also occurs where porous and permeable flanking facies pinch out updip into the tight lime mudstone of the mound core facies.

Waulsortian mounds and associated bioherms of the lower "Warsaw" can also facilitate hydrocarbon entrapment in the upper "Warsaw" by creating local relief over which porosity zones can be draped. Subsurface data indicate that both trapping mechanisms may be important to hydrocarbon accumulation in the upper "Warsaw."

Waulsortian type mound facies similar to those in the "Warsaw" are prolific hydrocarbon reservoirs in several regions in North America, including Kentucky and Tennessee, north-central Texas, north-central Alberta,' and North Dakota.' Hydrocarbon production from these mounds is from both the porous flanking packstone and grainstone facies and from fractured and dolomitized core facies. The core facies of the Illinois mounds appears to be dense with little porosity and permeability. Mound core facies have not been commonly tested and to date have not been productive.

RESERVOIR DEVELOPMENT

Reservoir-type rocks develop as thin, discontinuous, porous zones in the upper 150 ft of the "Warsaw" (Fig. 5). These porous zones are commonly less than 10 ft thick and can stack to form porous intervals up to 40 ft thick.

The most productive "Warsaw" wells are associated with the development of multiple porosity zones in the uppermost part of the "Warsaw" (Fig. 6) (18011 bytes). In these wells only the top one or two porous zones, usually 10-20 ft thick, produce hydrocarbons. The underlying porous zones typically produce water that has a high sulfur content. Seismic modeling of the upper "Warsaw" indicates that the development of 30-40 ft of porosity in this section of carbonates should significantly change the seismic reflection character of this interval.

Two of the better fields discovered during the earlier exploration phases for "Warsaw" reservoirs are Bessie and Ewing East (Fig. 2) (18434 bytes) in Franklin County, 111. Each of these fields has produced about 1.5 million bbl of oil from the "Warsaw."

Wells in Bessie field, discovered in 1979, have average estimated reserves of 90,000 bbl/well 7 from depths of about 3,800 ft. Several wells in this field have cumulative production exceeding 250,000 bbl of oil and are presently pumping about 20 b/d.

Although wells in Ewing East field have lower reserves than those at Bessie field, at least 12 have cumulative production greater than 50,000 bbl/well from depths of about 4,000 ft. Depth to the "Warsaw" pay throughout Illinois is relatively shallow, ranging from 2,400-4,400 ft.

RESERVOIR FACIES, UPPER 'WARSAW'

Several facies that form reservoirs in the upper part of the "Warsaw" include

a sandwave facies consisting of a transported bryozoan-crinoid grainstone,
a bioherm facies dominated by bryozoans, and
a grainstone shoal

facies that consists of coated skeletal grains or pseudooolites and develops in the uppermost "Warsaw."

Sandwave facies. This facies, characterized by a wedge-shaped geometry (Fig. 5), consists of crinoid and bryozoan fragments. The proportion of crinoids to bryozoans can be quite variable. Rounding is generally poor, suggesting a lack of extensive current agitation. In outcrop this facies shows abundant hummocky cross stratification, suggesting deposition was related to storm events. The sandwave facies interfingers laterally and vertically with an intersandwave facies of dense lime mudstone.

Bryozoan-dominated bioherm facies. This facies developed preferentially on preexisting topography, including carbonate mud mounds similar to but smaller than the mounds in the lower part of the "Warsaw." In some cases these bioherms developed on mound-like, skeletal sand piles that had been previously cemented. Bryozoans are relatively well preserved in this facies (Fig. 7), an indication that little transportation or current agitation occurred during deposition. Because of the abundance of fenestrate bryozoans, this facies can have high porosity and permeability.

Pseudo-oolitic grainstone shoal facies. "Warsaw" mounds appear to have provided topographic relief that may have contributed to the formation of oolitic/pseudo-oolitic (coated grains) grainstone shoals during the deposition of the uppermost "Warsaw" limestone. The highs also may have been instrumental in development of oolitic grainstone shoals during deposition of the overlying Salem limestone. These grainstones are generally porous and show good reservoir potential.

RESERVOIR POTENTIAL, LOWER 'WARSAW'

The lower part of the "Warsaw" in part contains relatively thick bioherms (Fig. 6) (18011 bytes) of both crinoids and bryozoans that are potential reservoirs. These bioherms developed on preexisting highs that included Waulsortian-type carbonate mud mounds" or transported skeletal sand piles. The bioherms of the lower "War-saw," interpreted by Whitaker and Treworgy 8 and Whitaker 9 to be reefs and possibly related to relief along faults, are porous and appear to be permeable. These bioherms have excellent potential for reservoir development, provided an effective seal is present. Bioherms of the lower "Warsaw" have not been widely tested, and those few wells that have penetrated this interval have encountered water. Future drilling will provide a better understanding of the reservoir potential of these bioherms.

FACTORS CONTROLLING POROSITY, PERMEABILITY

There is generally a correlation between permeability and the relative abundance of bryozoans versus crinoids.

Permeability measured from a core sample dominated by crinoids was 25 md. Permeabilities measured from a core sample of crinoids and bryozoans increased to 200-300 md, whereas permeability measured from an outcrop sample dominated by bryozoans (Fig. 7) further increased to 600 md.

This correlation is supported by thin section petrography that shows reduced permeability to be directly related to preferential cementation of crinoids by syntaxial calcite. However, pseudo-oolitic coatings, common in the uppermost "Warsaw" in some areas, can prevent extensive syntaxial cementation of crinoids, thus maintaining a higher porosity and permeability.

Bryozoans are generally less susceptible to extensive overgrowth cementation. This property and the abundant intra-skeletal pore space within bryozoans are important factors that contribute to the high permeability values. In some cases, micro-fracturing also can significantly increase effective permeability in these rocks.

REGIONAL, STRUCTURAL CONSIDERATIONS

Most production is currently confined to that part of the basin where the "Warsaw" is greater than 200 ft thick (Fig. 2) (18434 bytes). The only production found to date where the "Warsaw" is less than 200 ft thick is in an area along the Illinois-Indiana line. Further evaluation of porosity development and hydrocarbon migration may expand the boundary of the play.

A review of the "Warsaw" fields in Illinois shows that a combination of structure and reservoir facies development defines the play. Structural closure on the reservoir is not critical. Either tectonic-related structures or mound-induced relief may provide the structural element necessary to trap hydrocarbons. Potential for new discoveries of these combination traps appears to be excellent considering that there are many oil fields in the Illinois basin (Fig. 8) (23831 bytes) with few or no "Warsaw" tests.

High production rates and large yields per well make the "Warsaw" an excellent exploration target. Although reservoir-type rock appears to be widespread, relatively few wells have been drilled to test this play.

ACKNOWLEDGMENTS

This research has greatly benefitted from our discussions with colleagues at the Illinois State Geological Survey - Bryan Huff, Hannes Leetaru, Rod Norby, Don Oltz, and Janis Treworgyand various people in the oil industry - Alan Henigman, Stu Lang, Tom Partin, Kevin Reimer, Steve Whitaker, and Booth Oil and Wilbanks Exploration.

REFERENCES

Lasemi, Z., Waulsortian mound, bryozoan buildup, and storm-generated sandwave facies in the Ullin limestone ("Warsaw"), in Lasemi, Z., Treworgy, J.D., Norby, R.D., Grube, J.P., and Huff, B.G., Waulsortian mounds and reservoir potential of the Ullin limestone ("Warsaw") in southern Illinois and adjacent areas in Kentucky, Illinois State Geological Survey Guidebook 25, 1994, pp. 33-51.
Lasemi, Z., Treworgy, J.D., Norby, R.D., Grube, J.P., and Huff, B.G., Waulsortian mounds and reservoir potential of the Ullin limestone ("Warsaw") in southern Illinois and adjacent areas in Kentucky, Illinois State Geological Survey Guidebook 25, 1994, 65 p.
MacQuown, W.C., and Perkins, J.H., Stratigraphy and petrology of petroleum producing Waulsortian-type carbonate mounds in Fort Payne formation (Lower Mississippian) of north central Tennessee, AAPG Bull., Vol. 66,1982, pp. 1,055-75.
Ahr, W.M., and Ross, S.L., Chappel (Mississippian) biohermal reservoirs iii the Hardeman basin, Texas, GCAGS Transactions, Baton Rouge, La., Vol. 32, 1982, pp. 187-193.
Davies, G.R., Edwards, D.E., and Flach, P., Lower Carboniferous (Mississippian) Waulsortian reefs in the Seal area of north-central Alberta, in Geldsetzer, H.H.J., James, N.P., and Tebbutt, G.F. (eds.), Reefs, Canada and adjacent areas, Canadian Society of Petroleum Geologists Memoir 13, 1989, pp. 643-648.
Burke, R., and Diehl, P., Waulsort-jan mounds and Conoco's new Lodgepole well, North Dakota Geological Survey Newsletter, Vol. 20, No. 2,1993, pp. 6-17.
Strothmann, K., Bessie field, in Suppann, C.W., and Keitli, B.D. (eds.), Geology and petroleum production of the Illinois basin, Illinois and Indiana-Kentucky Geological Societies, Vol. 2,1988, pp. 103-104.
Whitaker, S.T., and Treworgy, J.D., 9 o'clock cross section in the Illinois basin, Wayne County, Ill., to St. Clair County, Ill., Illinois State Geological Survey, 1990, Open File Series 1990-4.
Whitaker, S.T., Treworgy, J.D., and Noger, M.C., 6 o'clock cross section in the Illinois basin, Wayne County, Ill., to Gibson County, Tenn., Illinois State Geological Survey 1992, Open File Series 1992-10.

BIBLIOGRAPHY

Buschbach, T.C., and Kolata, D.R., Regional setting of Illinois basin, in Leighton, M.W., Kolata, D.R., Oltz., D.R., and Eidel, J.J. (eds.), Interior cratonic basins, AAPG Memoir 51, 1991, pp. 29-55.

Howard, R.H., Hydrocarbon reservoir distribution in the Illinois basin, in Leighton, M.W., Kolata, D.R., Oltz, D.F., and Hidel, J.J. (eds.), Interior cratonic basins, AAPG Memoir 51,1991, pp. 299327.

THE AUTHORS

Zakaria Lasemi is a geologist with the oil and gas section at the Illinois State Geological Survey. His extensive research into the paleoenvironment and diagenesis of carbonate rocks has resulted in new insights and widely accepted reinterpretations, both applied in and basic research areas. He has an MS from the University of Illinois and a PhD from Miami University (Ohio).

John Grube, a petroleum geologist with the oil and gas section, joined the survey in fall 1989. His work includes reservoir characterization and regional play analysis. Previously he worked as an exploration geologist and consultant in the Rocky Mountain and Midcontinent basins for 13 years. He has an MS in geology from the Colorado School of Mines.