Stephen T. Whitaker
IBEX Geological Consultants Inc.
Champaign, Ill.
Oil production in the Lower Mississippian Ullin (Warsaw) limestone (Fig. 1)(57664 bytes) was first established in Illinois over 40 years ago. Exploration for hydrocarbons in this zone however, was limited until recently due to a lack of a suitable explanation for reservoir development in this interval.
The theory that Waijlsortian mound development may explain many of these reservoirs was first presented in 1990.1 Research effort led by Zakaria Lasemi at the Illinois State Geological Survey has since established an excellent model for the generation of these bioherms.
Lasemi performed numerous examinations of outcrops, cores, and well cuttings to develop his theory on the development and evolution of these biohermal features. Publications by Lasemi et al.2 and Lasemi and (]rube' presented the first idealized geologic models that could be used for Warsaw hydrocarbon exploration in Illinois.
This article presents an application of Lasemi's model to an oil field in the Illinois basin. Similar applications could improve exploration and development success in Warsaw reservoirs.
RESERVOIR DEVELOPMENT
In general, Waulsortian-type mounds developed in the lower portions of the Warsaw in relatively deep water (below storm-wave base). As the mud mound developed, it eventually reached a height sufficient to reach storm-wave base. Subsequently, the effects of waves generated from storms modified the types of sediments that were deposited. Storm-generated carbonate sandwaves, lenticular to mound-like skeletal piles, and bryozoan patch reefs became widespread during latter stages of Warsaw deposition.
It is in this upper part of the Warsaw that hydrocarbon traps are developed. Storm-generated sandwaves made of fossil debris (mostly crinoid or bryozoan fragments) formed on some of these mounds and provide the best potential for hydrocarbon traps. These sandwaves form isolated wedge-shaped lenses that cascade down the gentle slopes of the mud mound.
The porosity in these lenses varies depending on whether the fragments are crinoidal, which typically form calcite cements quickly and occlude porosity, or are bryozoan, which typically retain a good amount of porosity.
A sample bryozoan-rich grainstone facies from one Warsaw outcrop has porosity of over 30% with permeability of 1,450 md. These values are better than those commonly observed in the subsurface. The same sample lacks cements that would occlude porosity and permeability.
In the subsurface, these grainstone facies tend to occur near the crest of mud mounds which had reached storm-wave base and also along their flanks. Porous lenses of bryozoan and crinoidal debris interfinger with tight carbonate muds which help to form traps for hydrocarbons.
BROUGHTON FIELD HISTORY
Broughton field (Fig. 1)(57664 bytes) was discovered in 1951 when Middle Mississippian McClosky oolite shoals were encountered at a depth of approximately 3,275 ft. The field was abandoned in 1954 and revived in 1977, when Duke Resources drilled the 1 Van Winkle about a mile north of the previous production. This well established production from porous Warsaw zones at a depth of approximately 4,190 ft. In 1978, pays were also found in oolite shoals in the overlying Salem formation at depths of about 4,020 ft.
Total production from the eight wells completed in Warsaw at Broughton is about 300,000 bbl of oil. The best production in the field has come from the 1 Bonan and 1 Van Winkle wells (Fig. 1)(57664 bytes); 79,009 bbl and 137,236 bbl, respectively. Apparently coning of water in the Bonan was a major factor limiting its production.
WARSAW AT BROUGHTON
Structure on the Warsaw at Broughton (Fig. 2)(46332 bytes) shows a mound centering in Sec. 27. There is apparently a slight elongation to the mound in a northeast-southwest direction based on subsurface control including the 1 Hunt well, SW SW SE 22-6s-7e.
It is likely that Warsaw mud mounds tended to develop on features, such as lineaments, that were slightly high during early Mississippian time. Faults have been detected under some Warsaw fields in the basin (e.g. Johnsonville field) and are responsible for Warsaw reef development at those localities. A lineament may underlie Broughton and thus provide the reason for the elongation there as well.
Examinations of well cuttings and compensated density-neutron logs enabled the identification of facies within the field. Correlations of facies indicate that there are eight separate grainstone (sandwave) intervals above the central mud mound, although due to geometries of these grainstone lenses all eight zones never appear in any one well (Fig. 3)(79051 bytes). These eight lenses were labeled in descending order A through H.
Cross section A-B (Fig. 4)(99261 bytes) utilizes the top of the Lower Salem limestone as datum and clearly illustrates the mound nature of the Warsaw at Broughton. The porous grainstone lenses (sandwaves) cascade down the flanks of the feature and provide potential hydrocarbon traps.
Maps showing the distribution of these individual reservoirs are illustrated in Figs. 5 (148515 bytes)and 6(140807 bytes). Porosity, values are based solely on density logs since they are the most commonly available porosity indicators in the area. Although the best production was from wells that had good porosity and favorable structural position, production was also established in some wells that were structurally low. This fact supports the concept that some of these porous zones are limited to the flanks of the Warsaw feature and pinch out updip.
In general, the grainstone banks tended to form on the western side of the Warsaw mound at Broughton, indicating that storm winds were blowing predominantly from what is now the east. Similar maps in other fields could be utilized to improve development success.
CONCLUSION
Subsurface data at Broughton field support the Waulsortian mound model developed by Lasemi. Recent drilling proves that production from Warsaw mounds can be much more prolific than observed at Broughton and that these reservoirs are important targets for further exploration. Utilization of this model should provide an excellent means for improving exploration for, and development of, additional Waulsortian mounds in the Warsaw.
ACKNOWLEDGMENTS
Richard Howard and Lindell Bridges with IBEX Geological Consultants and Zakaria Lasemi with the Illinois State Geological Survey provided much-appreciated assistance in the generation of this report.
REFERENCES
1. Whitaker, S.T., and Treworgy, J.D., Nine o'clock cross section in the Illinois basin, Illinois State Geological Survey, Open File Series 1990-4.
2. Lasemi, Z., Treworgy, J.D. Norby R.D., Grube, J.P., and fluff, B.G., Waulsortian mounds and reservoir potential of the Ullin lime-stone ("Warsaw") in southern Illinois and adjacent areas in Kentucky, Illinois State Geologic Survey Guidebook 25, 1994, 65
3. Lasemi, Z., and Grube, J.P., Mississippian 'Warsaw' play makes waves in Illinois basin, OGJ, Jan. 9, 1995, pp. 47-51.
BIBLIOGRAPHY
Howard, R.H, Hydrocarbon reserve distribution in the Illinois basin, in Leighton, M.W., Kolata, D.R Oltz, D.F., and Eidel, J.J., eds Interior cratonic basins: AAPG Memoir 51,1991, p. 308.
