Positive factors dominate negatives for Illinois basin coalbed methane

March 31, 2003
The Illinois basin is an elliptical, intracratonic basin that covers 34 million acres in Illinois, southwestern Indiana, and western Kentucky (Fig. 1).

Positive factors dominate negatives for Illinois basin coalbed methane

The Illinois basin is an elliptical, intracratonic basin that covers 34 million acres in Illinois, southwestern Indiana, and western Kentucky (Fig. 1).

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All coals of economic interest are found in the Pennsylvanian age strata in which 75 individual seams have been identified and of which 20 have been actively mined since the 1800s. The coals that have potential for gas production are found in the Middle and Upper Lower Pennsylvanian rocks of the Carbondale and Spoon formations.

The coals vary in thickness from a few inches to more than 15 ft. They vary in rank from high volatile A in the southern part of the basin grading to high volatile bituminous C on the eastern, northern, and western flanks.

Mining is generally limited to areas where the coal is less than 600 ft deep.

Coalbed methane exploration and exploitation has been sporadic since the mid-1980s. One coalbed methane project in Indiana has been reported to be producing since 2000 but is not considered economic, and two other pilots in central and southern Illinois are presently in the dewatering stage. Other pilots have either been temporarily abandoned or have failed because of operator problems or poor reservoir characteristics.

The focus of coalbed methane production in the basin for the past 15 years has been gas production from numerous abandoned mines around the rim of the basin. Gas content data for many of the coals varies from 10 to over 250 scf/ton with the main seams varying between 50 and 150 scf/ton. Adsorption values range from 100 to 225 scf/ton.

The coals and associated shales in the Illinois basin are an attractive potential gas resource because of their location in relation to major gas markets in St. Louis, Chicago, and Indianapolis. The road network, large agricultural base, existing petroleum infrastructure, favorable regulatory bodies, almost 100% fee acreage, and present lack of environmental opposition are factors that favor coalbed methane exploitation in the Illinois basin.

Illinois geology

The Illinois basin began as a rift complex that failed and evolved into a rapidly subsiding Paleozoic cratonic embayment that was structurally closed by major tectonic events in post-Pennsylvanian time.1

The basin was once part of an extensive continentalwide Pennsylvanian deltaic system that included but was not limited to the Western Interior, Warrior, Michigan, and Appalachian basins.

The Illinois basin is bounded by several major arches that have separated it from adjacent basins. It contains several large anticlinal features that have produced over 4 billion bbl of oil from generally less than 2,500 ft from Mississippian, Devonian, Silurian, and Ordovician reservoirs since the late 1800s.

All significant coals in the basin are found in the Pennsylvanian rocks that are concealed by a thin veneer of Pleistocene-age sediments. The present maximum thickness of the Pennsylvanian strata is approximately 2,500 ft.

The Pennsylvanian sediments were deposited unconformably on older rocks following a major period of uplift and erosion at the end of the Mississippian.2 The Pennsylvanian rocks were generally deposited in a marine to upper deltaic environment on a shallow cratonic platform.

Within the Pennsylvanian-age rocks are over 51 identified cyclothems in the Illinois basin. A classic Pennsylvanian cyclothem is represented by the development of major peat swamps on a low relief topographic surface, followed by drowning, deepening of the basin, which is represented by the overlying marine black shales. This is followed by a slow shallowing of water depth as represented by gray-green, gray shales, and thin limestones.

The cyclothem is culminated with the deposition of nearshore marine and deltaic clastic sediments. As the delta returns the sequence is started over again by the formation of a new peat swamp. Fluvial, tidal, and marine sandstone deposits are found throughout the section.

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The coals that can be exploited for coalbed methane potential are confined to the Carbondale and Spoon formations of Desmoinesian age (Fig. 2). Other coals present in overlying and underlying formations have limited lateral extent and seem to be barren of gas.

The main coal seams that are actively mined or have potential for coalbed methane are the Danville (No. 7), Jamestown, the Herrin (No. 6), the Briar Hill (No. 5A), the Springfield (No. 5), Houchin Creek (No. 4), Colchester (No. 2), and Seelyville and its equivalents such as the Davis and Dekoven coals.

These coals are laterally extensive covering a vast majority of the basin and have been extensively mapped. These coals represent a time of major swamp development and peat accumulation in the Illinois basin. Coal thickness varies from a few inches to over 15 ft.

Major distributary channel systems have been mapped in the No. 5 and No. 6 coal seams because of the extensive mining involving these coals. The present available data indicate that these distributary channels are restricted to the basin flanks and are absent in the basin center.

No distributary significant channel systems have been mapped at present in association with the Danville (No. 7), Jamestown, Briar Hill (No. 5A), Houchin Creek (No. 4), Colchester (No. 2), and Seelyville coals. Below the Seelyville and its equivalents the Dekoven and Davis coal, very few coals seem to be laterally extensive.

A major issue in exploiting coalbed methane in the Illinois basin is the lack of density-neutron logs that cover the coal section in oil and gas wells. Most wells drilled for conventional petroleum targets in the Illinois basin may have a lateralog or dual induction log run from total depth to surface. Identifying the major coal seams on these logs is difficult at best, and the thickness values are suspect.

Major black shales in the Illinois basin that overlie some of the coals are the Excello shale, which overlies the Houchin Creek (No. 4) coal and is equivalent to the Excello shale in Kansas and Missouri; the Dykersburg shale, which is equivalent to the Little Osage shale in Kansas and Missouri; and the Anna shale that overlies the Herrin (No. 6) coal and is equivalent to the Anna shale in Kansas and Missouri.

The No. 4 coal is equivalent to Mulky coal in Kansas and Missouri. The Excello, Little Osage, and Anna shales have historically produced gas in Kansas and Missouri.

Reservoir, gas parameters

The coals in the Illinois basin range from high volatile A from the depositional center in southern Illinois and western Kentucky trending toward high volatile C on the fringes of the basin to the north, east, and west.

An expected aggregate net thickness of coals in the Carbondale and Spoon formations in the Illinois basin is 15-45 ft except in areas of outcrop or subcrop. The BTU content of the coal varies from 11,000 to 15,500 DMMF (dry mineral matter free). Moisture content varies from 5% to 19%; ash content varies from 1% to 25%; and volatile matter content varies from 28% to 41%.

Gas content data for the Illinois basin have been reported previously reported by Archer,3 Archer and Kirn,4 Smath,5 Damberger,6 and Demir and Damberger7 and generally vary from a low of 5 to 6 scf/ton along the shallow areas of the basin to 80 to 150 scf/ton in the center of the basin to 40 to 230 scf/ton in the areas of higher rank coals in the southern part of the basin.

Adsorption data generally range from 150 to 225 scf/ton. Gas content indicates the coals are undersaturated based on present methods of analysis. The coals in the central part of the basin give off gas within a few days of production, whereas the coals on the flanks take weeks to months to dewater before gas is produced.

It has been reported that some areas in southern Illinois have low water production and begin producing gas immediately. These wells have not been placed on production since they were drilled over 4 years ago. Permeability measurements to date have been estimated via injection fall-off tests and indicate in the higher scf per ton areas in southern Illinois and western Kentucky that permeability is generally in the microdarcies to less than 10 millidarcies range. Permeability data in the central part of the basin varies from the single digits to over 50 millidarcies.

Gas quality data are limited. Recently collected data indicate 85% to 98% methane, 1% to 6% C2 through C4, with the remainder being nitrogen and trace amounts of carbon dioxide. BTU content has been reported to be as low as 450 (gob or mine gas) to as high as 1,050 BTU. The gas, based on isotopic analysis, seems to be predominantly biogenic with a minor thermogenic composition.

Exploitation activity

One nonmine or nongob well has produced from the virgin coals prior to 1995. That well produced gas from unidentified coals and provided lighting for Hidalgo, Jasper County.

The well reportedly was completed in the early 1920s in a coal seam(s) and was used for street lights for a town of 800 people. The well was abandoned in the early 1930s. No underground mining has occurred within 30 miles of the town.

There have been indications of gas flows or shows from coals in Illinois while drilling along the La Salle anticline in eastern Illinois and flows of gas when drilling exploration holes for coal in Daviess County, Ind.

Illinois has historically not been a gas productive basin, and the coals and black shales have not been exploited for gas. Gas associated with almost all of the conventional oil fields was flared, vented, or used locally. The few gas fields that exist are on the flanks of the basin and typically are productive from pre-Pennsylvanian rocks.

Presently, one project produces from virgin coals that consists of over 25 wells in Vigo and Sullivan counties and is operated by BPI Industries of Vancouver, BC, and Pulse Energy of Evansville, Ind.

The wells produce from the Seelyville coal seam at 300-500 ft. The Seelyville coal is 3 to 7 ft thick in this area. Water rates are reported to be 10 to 300 b/d with gas production beginning several weeks after production is started. Gas rates have been reported at 10 to 150 Mcfd.

No scf per ton data are available, and it is reported that the project has been under way for some time. The state holds production data confidential. However, the project is not considered to be economic.

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Most of the past and present activity is concentrated in areas of historical coal mining in southern Illinois in Franklin, Saline, Gallatin, Williamson, Hamilton, and White counties (Fig. 3). Several operators, Berry Petroleum Co., Bakersfield, Calif., Dorado Gas Resources LLC, Englewood, Colo., DeMier Oil, Mount Vernon, Ill., BPI Industries, Peabody Coal Co., St. Louis, and CH4, Harrisburg, Ill., continue to attempt moving projects into the pilot stage or from the pilot stage to full production.

Several wells in the basin have been drilled and completed in abandoned mines. Flow rates have varied from less than 100 Mcfd to over 14 MMcfd.

The gas quality ranges from 30% to 90% methane with the remainder typically nitrogen. The gas is either sold to existing pipelines or local end-users. Flow rates typically are strong initially followed by rapid decline.

Drilling, completion

The focus in the basin is presently dominated by multiple zone completions of several coals. Several coals can be grouped into packages such as the No. 5 through the No. 7 coal; and the Seelyville, Davis, Dekoven, and Colchester coals.

In some areas the strata between the coals can be significant whereby it will require individual seam completion. Some coals, such as the Houchin Creek No. 4, are typically isolated within the geologic section, may not be considered economic, and may have to remain behind pipe.

Gob wells require drilling into an existing pillar or a few feet above the mine, set pipe, and then drill or frac into a dry part of the mine and produce.

Water issues

To date the water production from the coalbed methane wells in Saline, Williamson, Franklin, and White counties, Illinois, has been saline.

Water from wells in unmined coal seams in Illinois is slightly saline and requires subsurface disposal. Produced water from the wells in Indiana has been reported to be potable and is being disposed on the surface. Several potential disposal zones include Pennsylvanian sandstones in the lower part section and several carbonates or sandstones in Mississippian strata.

Land, environmental issues

The vast majority of the land in the Illinois basin is held in fee. Invariably in areas where oil and gas are produced, split minerals are present.

In the southern and western parts of the basin many coal mining companies own individual seams through conveyances via coal severance in the areas that are presently or have historically been mined. In the central part of the basin these types of severances do not exist.

These severances are a major roadblock to developing coalbed methane resources in areas where the No. 5 and No. 6 coals lie along the shallower parts of the basin. Land use in the Illinois basin is predominately farming. Population density is high in certain areas due to the proximity to major interstate highways and cities such as St. Louis, Peoria, Effingham, and Springfield.

Reserves, resources

Estimates of potential coal gas resources for the Illinois basin have been identified to be up to 21 tcf by the former Gas Research Institute, now Gas Technology Institute.

This evaluation of gas in place was based on estimates using coal rank, depth, gas content, seam thickness, and lateral continuity of the basin. Based on new data being collected in the basin, the resource seems to be considerably less but is still significant.

Potential reserves on a per well basis are unknown. Based on existing gas standard cubic feet per ton and using 15 to 30 ft of net coal in the Carbondale and Spoon formations, expected reserves will probably range from 1.5 to 5 bcf/sq mile depending on recovery factor, location in the basin, and depth. The potential for the basin seems to be 3 to 11 tcf of gas in place.

Summary

The present negatives for the Illinois basin are low gas content, high inert contents in the gas in certain areas, and coal mining associated coal severances.

The positives for the Illinois basin are multiple coal seams from 100 to 1,700 ft, net coal of 15 to 35 ft in the Carbondale and Spoon formations, strong local and regional gas markets that are dominated by unlimited end-users and several gas pipeline purchasers, the land is predominantly in fee, and pro-industry regulators and minimal environmental opposition.

References

1. Leighton, Morris W., Kolata, Dennis R., Oltz, Donald F., and Eidel, J.J., "Interior Cratonic Basins," AAPG Memoir 51, 1990, 819 pp.

2. Willman, H.B., et al., "Handbook of Illinois Stratigraphy," Illinois State Geological Survey Bull. 95, 1975, 261 pp.

3. Archer, P.L., "Illinois Basin Report, Pennsylvania Geology and Coal and Coalbed Methane Resources of the Illinois Basin, Illinois, Indiana and Kentucky," TRW Energy Systems Group, Contract No. DE-AC21-78MC08089, 1979.

4. Archer, P.L., and Kerr, N., "Pennsylvanian Geology, Coal, and Coalbed Methane Resources of the Illinois Basin, Illinois, Indiana, and Kentucky," in "Coalbed Methane Resources of the United States," AAPG Studies in Geology Series #17, Craig Rightmire et al., eds., 1984, pp. 105-134.

5. Smath, Richard A., Williams, David A., Cobb, James C., and Fisher, Barry W., "Feasibility Assessment of Unconventional Gas in Kentucky, Phase II–Western Kentucky Coal-Bed Methane Study," Kentucky Geological Survey, Grant No. 201-65-8W220-S5276, 1985, 451 pp.

6. Damberger, Heinz H., "Miscellaneous Information on Coals, Coalbed Methane and Coal Mine Methane in the Illinois Basin," Illinois State Geological Survey, 1999, 30 pp.

7. Demir, I., and Damberger, H.H., "Coalbed and Coal Mine Methane Potential of Illinois Basin," Rocky Mountain Association of Geologists, 2000 Coalbed Methane in the Rocky Mountains Symposium, Denver.

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The author
Steven A. Tedesco (stedesco @atoka.com) is owner of Atoka Coal Labs and manager of geology for Dorado Gas Resources LLC. He is a coal geologist by training and has worked in the coal mining industry for 23 years and the oil and gas industry for 20 years. He has been active in coalbed methane in the Rockies, Midcontinent, and Appalachians since the late 1980s. He has a BS in geology from Northeastern University and an MS in geology from Southern Illinois University.