Steven A. Tedesco
CST Oil & Gas Corp.
Denver
The Forest City basin is a shallow cratonic depression located in northeastern Kansas, southeastern Nebraska, southern Iowa and northern Missouri (Fig. 1).
The basin was formed when Pennsylvanian age tectonic activity emplaced the Nemaha ridge, causing the division of the North Kansas basin into two separate depressions, Saline and Forest City. During Pennsylvanian times a series of deltas prograded southward across the Forest City basins and a clastic sequence dominated by shales, sandstones, and numerous coal beds was deposited.
Historically, the Forest City basin in northeastern Kansas has been a shallow oil and gas province with minor coal production. The Iowa and Missouri portion has had minor oil production and moderate coal mining.
In recent years there has been little coal mining in the Forest City in Iowa and Kansas and only minor production in Missouri. Before 1940, gas was produced from coal beds and shales in the Kansas portion of the Forest City basin.
The Cherokee group (Atokan and Desmoinesian age) includes section containing the largest number of actively mined coals and has the greatest available data for coalbed methane evaluation.
GEOLOGY
The Forest City basin is dominated by the Nemaha ridge to the west and the Thurman-Redfield zone to the north, which was emplaced during post-Mississippian times.
The basin is bounded by the Mississippi River arch to the east, the Ozarks to the southeast, and the Bourbon arch to the south. The dominant structural trend, except in the area of the Nemaha and Thurman-Redfield features, is to the northwest.
Along the Nemaha and Thurman-Redfield zones the structural direction is predominantly to the north and northeast but can be highly variable. Faulting, which is typically normal, is common to the west and north of the basin center.
The coals of the Forest City basin are present throughout much of the Pennsylvanian section. But most of the laterally continuous and thick coal seams are found in strata of Morrowan, Atokan, and Desmoinesian ages (Fig. 2).
PENNSYLVANIAN ZONES
Pennsylvanian stratigraphic terminology in the Forest City is derived partly from: a) geologic investigations in the Cherokee basin to the south and the Illinois basin to the east, and b) local usage restricted to areas where natural resources were or are being extracted.
In addition there are numerous terminology differences in the published literature between various state surveys. Therefore, coals with similar names may not be in the same stratigraphic position from state to state.
The Iowa Geological Survey has recently attempted to define the Desmoinesian and older strata as a result of an extensive drilling program in close proximity to the Pennsylvanian outcrop area.
However, Iowa has added to the terminology and correlation confusion by redefining the top of the Cherokee formation. It was previously at the top of the Excello shale but is now placed at the base of this shale.
Many geologists and drillers will probably continue to use the top of the Excello (the base of the overlying Ft. Scott formation) as the top of the Cherokee formation.
Within the basin section the oldest stratum is tentatively identified as Morrowan in age, a clastic sequence with few persistent coals; its proposed name is the Bower formation.2
ATOKAN STRATA
Atokan age strata overlie this section, which is the Kilbourne and lower part of the Kalo in Iowa; McLouth, Burgner, and Riverton formations in Missouri; and the Riverton formation of Kansas.
These formations are dominated by a prograding delta sequence with marine deposition only in former valley axis.
The lower part of the Atoka section is dominated by the McLouth sandstone, which was restricted to the Forest City basin and did not cross the bourbon arch. The McLouth is a highly productive oil and gas reservoir in northeastern Kansas.
The source for the McLouth came from the north, northeast, and east. Evidence suggests that the Nemaha ridge did not provide source material to the McLouth fluvial system. The upper part of the Atoka sediments did cross and bury the Bourbon arch, and the first laterally persistent coals are found in this section.
These coals form a package with locally minor limestones and sandstones that can be mapped across great distances. Toward the basin's shallower areas in Iowa and Missouri stratigraphically younger sandstones have eroded down into older section, but this is a rare occurrence deeper in the basin.
The Desmoinesian age sediments comprise the Middle and Upper Cherokee and Marmaton groups of Missouri, Iowa, and Kansas (there are too many formations to name).
CHEROKEE GROUP
Several deltaic systems are present in the upper part of the Cherokee group; the most notable and dominant is the Bartlesville (Kansas) or Warner (Missouri) sandstone.
The Bartlesville/Warner sands were carried across the Forest City basin southward to Oklahoma and Arkansas. The coals that developed during Bartlesville deposition are not as extensive as the upper Atokan coals due to non-deposition or erosion or during active delta progradation.
Overlying the Bartlesville/Warner sandstone is a series of coal packages which are fairly continuous across most of the basin. These coals of upper Cherokee age are interbedded with black shales and are more common here than in the overlying Marmaton section which is dominated by marine limestones.
The Marmaton section overlies the Cherokee group and is composed predominantly of limestones and shales with minor amounts of coal. Coals in this section are more common on the basin fringe than in the center, where they are generally absent.
Overlying the Marmaton is the Pleasanton group, which is predominantly shale with minor amounts of coal, limestone, and sandstone.
COAL SECTION
The primary deltaic section of interest lies in the Cherokee and older formations. The sandstone channels or valley fill sequences have been stacked on top of one another through time, and they follow the same broad river valleys developed on the eroded and peneplained Mississippian surface.
The coals developed in extensive elevated peat stands in upper and lower and delta plain deposits. The scarcity of sands in the basin fill indicates that the fluvial sediments were confined to narrow alluvial strips, and thus clastic aggradational deposits are poorly developed.4
When marine transgressions did occur, the swamps had probably already died, and salt water invasion occurred quickly as there are few indications of peat erosion. The coals have a high sulfur and low ash content because of the lack of marine incursions and floodwaters.
The Bartlesville/Warner river system is not often found in the depocenter of the basin, where several coal swamps developed adjacent to its deposition. The coal seams typically vary from a few inches to 4 ft in thickness but have been documented as thick as 15 ft.
Several are laterally persistent throughout the basin. Cross-section A-A' transacts the basin west to east and demonstrates the increase in thickness of the coals and the decreasing thickness of the sands toward the basin depocenter (Fig. 3).
COAL QUALITY
The Cherokee coals are of high volatile C to A bituminous rank (trending from northeast to southwest), have an ash content ranging from 8 to 18% (average 10%), average sulfur content of 4%, and heating values of 10,800-15,300 Btu/lb (average 14,100, dry basis).5 6 7 8 9
Vitrinite reflectance values indicate a range of less than .4 (in Iowa along the basin margin) to more than .75 Ro (in extreme eastern Kansas and along the Bourbon arch), which roughly approximate their rank by proximate analysis.
No data have been published for the coals in the basin center or along the Nemaha ridge.
An isopach map of the total gross coal present was constructed based on existing, usable density-neutron logs from wells drilled for petroleum (Fig. 4).
Cores of coals taken in certain counties were correlated with associated well logs, and the neutron-density response for coal was used to compile Fig. 4. The isopach map indicates gross coal thickness varies from 0-90 ft in the Cherokee group.
Coal studies of Wehking gas field, Atchison County (Figs. 1, 5) and McClain oil field, Nemaha County (Figs. 1, 6) oil fields indicate that the lateral continuity of the coals was not influenced by either Mississippian topography or McLouth and Bartlesville sand deposition.
The depth to the coals ranges from zero (outcrop) to 2,400 ft in the basin center.
COALBED METHANE
The coals and black shales of Jefferson, Miami, Leavenworth, Linn, and Wyandotte counties in Kansas are documented to have had initial flows of gas 500 Mcfd to 1.5 MMcfd without stimulation from the late 1800s to at least the 1940s.10 11 12
Records of mud logs indicate that in many wells the mud system becomes gassy when drilling through the coals in northeastern Kansas.
Coalbed methane exploitation of the Cherokee coals began in southeastern Kansas during the middle 1980s. A few isolated wells were completed that produce 10-300 Mcfd of methane and 50-200 b/d of water from a seam 1-4 ft thick.13 14 These coals are part of the same Bartlesville/Warner deltaic system that extended southward out of the Forest City basin.
Operators began looking at the Forest City basin in 1990 as an extension of this play. Cores of the Cherokee age coals indicated the viability of this resource, and several operators concentrated in the existing oil and gas areas in Atchison, Jefferson, Miami, Leavenworth, and Franklin counties.
Deabsorption and absorption tests of the cored coals in northeastern Kansas indicated a gas content range of 50-435 Scf/ton.
OTHER COMPARISONS
Fig. 7 is a comparison of gas content data versus depth from southeastern Kansas, Forest City, and Black Warrior basins (the data for the Black Warrior was taken from Hewitt, 1984).15
The Department of Energy16 evaluated the Iowa portion of the basin and recommended a 10 county area for coalbed methane exploration. No permeability data are available yet, but records indicate that some of the coals can initially yield gas without stimulation or extensive dewatering.
None of the present operators has developed more than a one well program. In other basins this limited approach has proven to be insufficient for commercial coalbed methane production.
The Forest City and Black Warrior basins have similar geologic settings and environments of coal deposition (Fig. 8). The main geologic difference is that the coals of the Forest City basin were not subject to adjacent mountain building.
These coals have not been as aggressively mined as those in other basins. Therefore, the Forest City coals have limited data on which to base their coalbed methane potential.
In some areas the aggregate thickness of the Cherokee group is greater and is packed into a shorter interval than the equivalent stratigraphic sequence in the Black Warrior basin.
KANSAS DEVELOPMENTS
The Kansas Corporation Commission held a hearing in October 1991 in Topeka, Kan., at the request of Duncan Energy Co., Denver, and Farleigh Oil Properties, Casper, Wyo.; and supported by Mitchell Energy & Development Corp., The Woodlands, Tex. and Exeter Oil & Gas Inc., London, England.
These companies asked for the suspension of several regulations governing gas proration, downhole commingling, annual testing, flaring of gas to require a minimum spacing of 40 acres, and to test the gas for the sole purpose of evaluating and encouraging the development of coalbed methane in a seven county area in northeastern Kansas.17
The KCC granted all of the applicants' requests. The four companies together have leased more than 250,000 acres in northeastern Kansas.
In addition, the Kansas Geological Survey is drilling a well to 1,200 for a coalbed methane test in Leavenworth County.18 Evaluation of the Forest City basin as a potential coalbed methane producing area has been hampered by the lack of recent coal data usually acquired through a healthy and active mining industry.
The Topeka-Kansas City-Atchison area is growing in terms of natural gas consumption.
Another plus not usually associated with coalbed methane production is the presence of other petroleum targets below the coals. The coal section itself needs to be evaluated by more extensive drilling programs than have been recently implemented.
The presence of a thicker total coal sequence in a thinner section than that found in the Black Warrior basin suggest a potential of similar to greater gas reserve per drillsite and lower per well costs.
REFERENCES
- Ravn, R.L., J.W. Swade, M.R. Howes, J.L. Gregory, R.R. Anderson, and P.E. Van Dorpe, Stratigraphy of the Cherokee Group and revision of Pennsylvanian stratigraphic nomenclature in Iowa, Iowa Geological Survey, Technical Information Series No. 12, 1984, 76 p.
- Strickland, M.O., The areal distribution and depositional setting of the McLouth sandstone in Jefferson and Leavenworth counties, Kansas, Kansas Geological Survey, Open-File Report 87-36, 1987, 85 p.
- Merriam, D.F., The Geologic History of Kansas, Kansas Geological Survey Bull. 162, 1988, 317 p.
- Murphy, T.D., Strandplain and deltaic deposits and related coals of the Middle Cherokee Subgroup of Southeastern Kansas and Western Missouri, unpublished masters of arts thesis, University of Texas at Austin, 1978, 126 p.
- U.S. Department of Commerce, Analysis of Missouri coals, Bureau of Mines technical paper 366, 1926, 41 p.
- U.S. Department of Commerce, Analysis of Kansas Coals, Bureau of Mines technical paper 455, 1929, 52 p.
- U.S. Department of Commerce, Analysis of Kansas Coals, Bureau of Mines technical paper 706, 1929, 65 p.
- Landis, E.R., and O.J. Van Eck, Coal resources of Iowa, Iowa Geological Survey technical paper No. 4, 1976, 141 p.
- Wedge, W.K., D.M.S. Bhatia and A.W. Rueff, Chemical analysis of selected Missouri coals and some statistical implications, Missouri Department of Natural Resource R.E. 60, 1976, 36 p.
- Haworth, E., Geological Survey of Kansas: Kansas Geological Survey, Vol. 1, 1896.
- Haworth, E., Special report on oil and gas, Kansas Geological Survey, Vol. 9, 1908, 586 p.
- Moore, R.C., and J.C. Frye, Oil and Gas in Eastern Kansas, Bull. 57, University of Kansas Publications, 1945.
- Stoekinger, W.T., Kansas coalbed methane comes on stream, OGJ, June 4, 1990, p. 88.
- Nation, L., Southeastern Kansas becomes methane playground, AAPG Explorer, February 1991.
- Hewitt, J.L., Geologic overview, coal and coalbed methane resources of the Warrior Basin-Alabama and Mississippi, in Coalbed Methane Resources of the U.S., AAPG Studies in Geology Series No. 17, 1984, pp. 73-104.
- Morz, T.H., J.G. Ryan and C.W. Bryer, Methane recovery from coalbeds: a potential energy energy source, U.S. Department of Energy, Morgantown, 1983, 200 p.
- Kansas Corporation Commission, Prefiled testimony of Duncan Energy Co. and Farleigh Oil Properties, Docket No. 177, 932-C, C-26-223, 1991.
- OGJ, Nov. 18, 1991, p. 48.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
