Commercial coal gas play possible in Illinois basin

Timothy A. Gognat
Global GeoData LLC
Denver

Multiple seams of Pennsylvanian age coals exist in a structurally deformed area associated with the Cottage Grove fault zone (Fig. 1 [197,165 bytes]) in northeastern Williamson County, Ill. Their cumulative thickness is in excess of 30 ft.

BPI Industries has drilled preliminary coalbed methane (CBM) test wells in the vicinity of the Cottage Grove and Rend Lake fault zones in southern Illinois. Results from the wells drilled have been reported as "encouraging." New gas content information¹ with measurements exceeding 200 scf of methane/ton of coal indicate that the potential for a commercial CBM play in the Illinois basin is real.

Other indications of a CBM play include a history of underground coal mine explosions and a number of abandoned mines that have been tapped for domestic natural gas utilization.

Cleats are responsible for effective permeability in coals; therefore, cleat orientation plays an important role in the direction of movement of fluids through coal seams. Two dominant cleat trends (north-northeast and northwest) have been defined in the Herrin (No. 6) and Springfield (No. 5) coals in the study area and suggest directional trends for movement of fluids within the coals.

Previous studies

Earlier studies by Nelson et al.² and Nelson and Lumm³ addressed the dominant structural features associated with the Cottage Grove and associated fault systems, and Gognat⁴ and Gognat et al.⁵ characterized coal cleat, bedrock joint, and air photo linear trends in Williamson, Franklin, Jackson, Union, and Johnson counties (Fig. 2 [128,325 bytes]). Swenson et al.⁶ defined three dominant joint trends and related one of those trends to faulting in the fluorspar district in adjoining areas of southeastern Illinois and western Kentucky. They also interpreted in situ stress orientations and suggested that a relationship exists between in situ compressional stress and the orientation of a regional joint pattern.

Geologic complexities

Southern Illinois is a somewhat enigmatic area. It is composed of:

Largely undisturbed sedimentary rocks ranging in age from Cambrian to Eocene;
Zones of wrench faulting and associated structures;
A crypto-volcanic feature (Hicks dome);
A large laccolithic intrusive (Omaha dome); and
Scattered ultrabasic igneous dikes, sills, and diatremes of Permian age3 (Fig. 1).

Cretaceous, Paleocene, and Eocene strata of the Mississippi embayment unconformably cover Mississippian and older Paleozoic rocks in Illinois' southernmost tier of counties.⁷ Pleistocene glacial till and loess of the Illinoisian glacial period occur throughout the study area. Thickness ranges from a thin veneer to a maximum of 50 ft in valley fill strata.

The Cottage Grove fault zone is an east-west trending, right-lateral wrench fault system that extends for about 70 miles (Fig. 2). It reaches a width of 10 miles and contains individual faults that have as much as 200 ft of vertical displacement. It is thought to have been active during the Permian period.

The Rend Lake fault zone approaches the Cottage Grove system from the north at approximately right angles near the Williamson-Franklin county line. It is composed of a north-south trending series of high angle normal faults with a maximum measured displacement of 55 ft at the Herrin (No. 6) coal level. The Rend Lake system is thought to be the result of regional extension related to differential uplift and subsidence.²

Coal cleat trends

Measurements were taken with a Brunton compass at selected outcrops and in surface coal mines in William- son and Jackson counties. Coal face and butt cleat orientations were measured in equal numbers where possible.

Some 332 coal cleat directional measurements were obtained at six sites south of the Cottage Grove fault zone in Williamson and Jackson counties (Fig. 2). Most measurements were taken from the Herrin (No. 6) and Springfield (No. 5) coals. Also, 804 bedrock joints were measured at those six locations and at nine additional locations in Williamson, Union, and Johnson counties. Bedrock associated with the coals consisted of sandstones, siltstones, and shales with minor amounts of limestone.

To determine if bedrock fracture trends could be defined by air photo analysis, a total of 1,296 air photo linear trends were interpreted and mapped (Fig. 2). All bedrock fracture data were obtained south of the Cottage Grove master fault zone. Photo linears were also interpreted in Franklin County (north of the Cottage Grove fault zone) where surface subsidence caused by underground mines is a substantial problem.

Rose diagrams were generated to visually compare the orientation of directional data. At each collection site, orientation diagrams were drawn for coal cleats (where coal was present), bedrock joints, and air photo linears. Fig. 3 [47,235 bytes] shows a recognizable correlation between coal cleat trends and bedrock fracture trends in adjacent strata. The coal cleats display better consistency than bedrock joint trends. Bedrock joints in overlying and underlying strata exhibit some apparent rotation and have a larger degree of "scatter" in orientations. The two best-defined populations of coal cleats reside within the overall population of bedrock joints. A slight rotation of joint trends with succeeding stratigraphic position is not uncommon within sedimentary packages and may be a plausible explanation for the large degree of scatter observed in the bedrock joint data.

The best correlation of orientation data exists between the trend of dominant fault segments in the Cottage Grove and Rend Lake fault zones and the northwest-southeast trend of coal cleats found throughout the area of study (Fig. 3). This northwest-southeast trend is also followed by igneous dikes in Saline and Williamson counties (Fig. 4 [71,885 bytes]). These features are thought to be genetically linked; stresses that caused faulting along the Cottage Grove trend and allowed for the emplacement of igneous dikes most likely contributed to cleating orientations found in the coals.

What has been learned

Dominant coal cleat, bedrock joint, and air photo linear trends exhibit similarity in their orientation throughout the study area. En echelon segments of the Cottage Grove fault system and the northwest-southeast trend of coal cleats throughout the study area share identical orientations.

The general trend of coal cleats and bedrock fractures as viewed in rose diagrams appears to be reflected in the orientation of air photo linears throughout the study area. However, definition of fracture trends based on air photo linear analysis or adjacent bedrock joint orientations appears to provide only a good "first look" at gross cleat trends. Predicting cleat orientations and hence permeability directions by extrapolating from the trends of air photo linears and bedrock joints would provide only a limited degree of accuracy in this area.

References

Mitchell, C.A., George N. Mitchell Drilling Co. Inc., personal communication, 1999.
Nelson, W.J., Krause, H.F., and Bristol, H.M., The Cottage Grove fault system in southern Illinois, Illinois State Geological Survey, Circular 522, 1981, 65 p.
Nelson, W.J., and Lumm, D.K., Structural geology of southeastern Illinois and vicinity, Illinois State Geological Survey, Circular 538, 1987, 70 p.
Gognat, T.A., Fracture trace analysis in southern Illinois, M.S. thesis, Southern Illinois University at Carbondale, Ill., 1977, 71 p.
Gognat, T.A., McChesney, S.M., and May, B.I., Air photo linears, bedrock joints, and coal cleats: Their relationships in southern Illinois, abs., North Central Section, Geol. Soc. of America, Vol. 9, 1977, p. 598.
Swenson, A., Gognat, T.A., and Shurr, G.W., A comparison of structural features to linear trends observed on Landsat imagery, southern Illinois basin, abs., AAPG Bull., Vol. 74, 1990, p. 1311.
Willman, H.B., Atherton, E., Buschbach, T.C., Collinson, C., Frye, J.C., Hopkins, M.E., Lineback, J.A., and Simon, J.A., Handbook of Illinois stratigraphy, Illinois State Geological Survey, Bull. 95, 1975, 261 p.