Determination of contributors to coal seam structure in Sullivan County, Indiana

International Journal of Coal Geology, 8 (1987) 339-354 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

339

D e t e r m i n a t i o n of C o n t r i b u t o r s to Coal S e a m S t r u c t u r e in S u l l i v a n C o u n t y , I n d i a n a S.C. ADAMS~and G. KULLERUD2 IBateUe Memorial Institute, ONWI, 505 King Avenue, Columbus, OH 43201, U.S.A. 2Departrnent of Geosciences, Purdue University, West Lafayette, IN 47907, U.S.A. (Receivedand accepted for publication May 21, 1987)

ABSTRACT

Adams, S.C. and Kullerud, G., 1987. Determination of contributors to coal seam structure in Sullivan County, Indiana. Int. J. Coal Geol., 8: 339-354. The structure of coal seams in Sullivan County, Indiana is subject to local variability controlled by differential compaction. Underlying Silurian reefs contribute to the maximum vertical variation and the underlying Mississippian unconformity surface controls the distribution of the most extensiveand mappable structural features.The vertical magnitude, orientation, and lateral extent of the features should be considered during mine layout. Potential underlying Silurian reefs can be distinguished from other contributors to coal seam structure for the purposes of petroleum exploration.

INTRODUCTION Background Structural closures of shallow P e n n s y l v a n i a n coal seams of the Illinois Basin have been advocated as favorable indicators of underlying structure. Esarey (1929) and M o u l t o n and Bell (1929) n o t e d the association of positive coal structures with underlying oil traps. B ecker and Keller (1979), Bristol (1974), and Stevenson (1973) observed an affiliation of Pennsylvanian structures with underlying Silurian pinnacle reef structures. However, e x a m i n a t i o n of petroleum drilling maps d e m o n s t r a t e s t h a t m a n y ot her known coal structural highs are n o t underlain by productive structures. Only little effort has been expended regarding co n tr ibut or s to the resultant structures in u n d e r g r o u n d coal mines. T h e s e features are generally a t t r i b u t e d by miners to "anticlines" or "synclines" or to " s a n d bodies". St r uc t ur al features can have a significant impact upon mining practices and costs.

0166-5162/87/$03.50

© 1987 Elsevier Science Publishers B.V.

340

Objectives The present study was undertaken to determine the geological contributors responsible for the structures displayed by Pennsylvanian coal seams and to define criteria for their identification. The results might contribute to structural interpretation for the purpose of petroleum exploration. It would be desirable to develop fundamental understanding of various orders of controls of structure, in order to interpret point source drilling data, so that projections across a given area can be made to accurately anticipate actual coal-mining conditions. Various contributors were examined from the regional through the very local scale for areal extent, vertical magnitude, and slope gradient. Study area Sullivan County, Indiana (Fig. 1) was selected for study, because of the presence of a number of known petroleum-producing Silurian reefs. One underground gas storage facility has been established there. No underground mines currently are operating in Sullivan County, although many were in the past in the eastern half of the county. Some of the older underground mines and geological problems were described by Ashley (1899) and Wier (1970, 1954). Strip mines are operating along the seam outcrops in the eastern half of the county. Sullivan County has the largest county coal resource base in Indiana. Bedrock geology consists of southwesterly dipping Pennsylvanian sediments covered by glacial till and alluvial fill. The upper half of the Pennsylvanian section consists of semicontinuous horizons of coal and limestone separated by much thicker intervals of deltaic and marine clastic sediments (Fig. 2 ). The lower half of the Pennsylvanian section beneath the Seelyville Coal contains discontinuous clastics and unmappable lenses of limestone and coal. The Pennsylvanian sediments overlie an erosional unconformity on the top of Chesterian-Mississippian sediments. The new Albany Shale of Mississipplan-Devonian Age is underlain by scattered occurrences of Silurian pinnacle reefs. Some seismic lines have been run within the county. Dana (1980) attempted using a gravity survey over the Wilfred reef. METHODS

Coordinates of drill hole locations along with values and codes for elevation, depth, thickness, clastic ratio, and lithology were used to compile study files. The data were based upon measurements and interpretations made from electric resistivity and spontaneous potential logs. Core drilling information was available for interpretative control. Sorting and clastic evaluation subroutines were designed to compile, statistically analyze, and map subsurface structural

341 CITED HORIZONS ~ DRIFT WEST FRANKLINc> LIMESTONE

~ PENNSYLVANIAN CONTINUOUS HORIZONS

HOUGHIN CREEK COAL SEELYVILLE COALm

..... DISCONTINUOUS "":"-... .....!::; PENNSYLVANIAN ~ :!:T SEDIMENTS ....'~:.i!!~l CHESTERIAN

~ MISSISSIPPIAN UNCONFORMITY

AUXVASESm, SHALE

NOIS

I? u

VALMEYERAN SEDIMENTS VERTICAL SCALE

iiiiii!ili!i!i!!i!i~ N!!i!iii!i!i!iI::.I DIANA

~ ~ 1 ~

SEDIMENTS

SULLIVAN C°UNTY

5o_.F2oo ~-~oo 0 n-O

~ METERFEET S NEW ALBANY SHALE SILURIAN LIMESTONE

Fig.1.Sullivan County,Indiana location map.

Fig.2.Stratigraphic column.

and depositional aspects. Descriptive statisticsand correlation analysis were run using S P S S software.

RESULTS Coal s e a m s t r u c t u r e s

The structures of six coal seams and of additional Pennsylvanian limestone horizons were mapped. These horizons proved to be concordant with one another to a remarkable degree. Thus, the structure of any one of the mappable coal seams is reflective of the structure of the others. Figure 3 demonstrates the structure of the Houchin Creek Coal, which only has a very limited bedrock subcrop extent covered by unconsolidated Pleistocene sediments. Structural

342 R I IW

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~T~I -~2 MILES CONTOUR I N T E R V A L = 2 5 F E E T (7 G METERS)

Fig. 3. Structure map of the Houchin Creek Coal.

relief is comprised mainly by positive and negative vertical departures of 25 ft (7.6 m) from regional dip. On a plane view the structural contours of these undulating departures extend 2-3 miles (3.2-4.8 km) in nearly updip and downdip directions from the regional strike. Closures constitute a small percentage of the area. Coal s e a m concordance

Thickness variations of clastic intervals between coal seams have been proffered by field workers to explain coal structures ( Stanley, 1952; Brittain, 1975; Khawaja, 1975). Correlation significance levels (p) from the present study demonstrate that much of the difference in thickness within an individual clastic interval results from differential compaction between sand and clay rich areas. Differences in compacted thickness within an individual clastic interval are succeeded by depositional compensation within the subsequent clastic interval or intervals. Deposition of relatively incompressible channel sands tends to be followed by deposition of compressible clay-rich environments until

343 INTERVAL THICKNESS IN F E E T FROM W E S T F R A N K L I N 0 I II

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I 150

Fig. 4. Multiple interval thickness histograms measured downward from the West Franklin Limestone to various coal horizons.

a cumulative compacted thickness compensation is established. Figure 4 illustrates multiple interval thickness frequency histograms, measured downward from the West Franklin Limestone to four horizons. The central tendency of each multiple interval thickness population is a consequence of the parallelism between the beds. The centrality indicates that depositional compensation permits correlations to be made through thick multiple sediment intervals about as accurately as through adjacent short intervals. The centrality also results from some compactional compensation which occurs during the time span of deposition of a single clastic interval. Individual clastic interval thicknesses average 45 ft (13.7 m) and have a standard deviation of 11 ft (3.4 m ). When multiple interval thicknesses exceed 100 ft (30.5 m), standard deviations stabilize at 25-30 ft (7.6-9.1 m), even when multiple intervals include up to 500 ft (152 m) of sediment. Concomitantly, average relative sand content (by electric log resistivities) stabilizes at 37% for thicknesses greater than 100 ft (30.5 m ) , owing to depositional compensation. Therefore, probability values can be assigned to correlations within a geographic area by using plots such as that shown in Fig. 5. Probability plots might have application as a correlation method for extensive, bedded sediments in other regions. Local minor structures associated with varying sandshale ratios are frequently of much smaller magnitude than the actual sediment interval thickness variations, owing to overall sediment depositional compensation. The sediment intervals between coal seams would constitute a tertiary control of structure.

344 99.99

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Fig. 5. P r o b a b i l i t y g r a p h o f i n t e r v a l thicknesses measured downward from the West F r a n k l i n Limestone to v a r i o u s coal h o r i z o n s .

Dip of: beds The strongly parallel nature of the coal bed structures also is reflected in their similar county-wide dips (Table 1 ), which could be considered the primary or first order structure. The "lower" Pennsylvanian sediments below the Seelyville Coal demonstrate an overall updip convergence of t0 ft per mile (1.9 m per kilometer), which is superimposed upon and is normal to unconformity TABLE 1 G r a p h i c a l l y d e r i v e d regional dip Horizon name

West F r a n k l i n Danville Hymera-Jamestown Springfield Houchin Creek

Colchester Seelyville Average above a n d i n c l u d i n g Seelyville A u x Vases New Albany

Slope (ft./Mi.)

D i p direction

dip 0 ° 15' 35" 0 ° 15' 45" 0 ° 16'54" 0 ° 17' 21" 0 ° 14'45" 0 °18'54" 0 ° 14'45"

23.9 24.2 26.0 26.7 22.6 29.0 22.6

W W W W W W W

0 ° 16' 16"

25.0

W 12 ° 37' S

0°23'46 " 0 ° 30' 00"

36.5 45.9

W 33°40'S W 31 ° 40' S

Degree of

9 ° 50' S 12 ° 50' S 12 ° 20' S 13 ° 10' S 13 ° 3 0 ' S 14 ° 7 ' S 12°36'S

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Fig. 6. M a p of thicknesstrends of the Pennsylvanian sectionbeneath the SeelyvilleCoal.

valley infilling. Additional basin-side wedging of 10 ft per mile (1.9 m per kilometer) is present within the sediment interval between the Aux Vases Shale and the top of the New Albany Shale. The resultant, in-mine, slope gradient contributed by the regional dip would be 10 ft per mile (1.9 m per kilometer). On the scale of most mines this would be masked by other factors. The seam dip is the only component of structure in this area attributable to tectonic activity. The remaining components are related to differential compaction and are superimposed upon the regional dip component.

Lower Pennsylvanian sediments The "lower" Pennsylvanian sediments below the Seelyville Coal were evaluated as one composite unit within which continuous horizons were not found. The "lower" Pennsylvanian sediments are more sand-rich than those above the Seelyville - 60 versus 37 percent. Individual sand units tend to be much thicker, cleaner, and more localized. The conspicuous N E - S W orientation of thickness trends (Fig. 6) is the result of infilling over Mississippian uncon-

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Fig. 7. Map of estimated percentage of sand within the Pennsylvanian section beneath the Seelyville Coal.

fortuity valleys through about half of the thickness of this unit. Local relief on the unconformity reaches 300 ft (91.4 m ). A map of estimated percentages of sand (Fig. 7 ), based upon average measured resistivities and SP demonstrates that composite sand patterns are coincident with unconformity valley and ridge trends. While some valleys are sand-rich and contain thick multistory channel sands, others are clay-rich. Most of the broader-scaled coal structures are strongly affected by both the Mississippian unconformity surface and the local percentage of Pennsylvanian sand filling the unconformity valleys. The prominent linear coal feature designated L1 on Fig. 3 is related to both the linear unconformity features L2 demonstrated on Fig. 6 and the linear sand features L3 shown on Fig. 7. These features constitute the dominant second-order contributors to coal structures. Average in-mine grades attributable to the unconformity and resultant infilling would be about 12 ft per mile ( 2.3 m per kilometer) parallel to the paleovalley trends and 100 ft per mile (18.9 m per kilometer) normal to the valley trends. Maximum normal grades related to this component could extend to 150 ft per mile (28.4 m per kilometer). Some

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Fig. 8. Structuremap of the Aux Vases Shale. closure upon coal seams occurs over knobs along the paleoridges, but these are of a lesser magnitude than those associated with underlying reefs.

Structure of the Aux Vases Shale Comparison of coal structures (Fig. 3 ) with the structures of the Chesterian Aux Vases Shale (Fig. 8) established that many of the Pennsylvanian structures do not extend downward into the Mississippian section. Intervals from the West Franklin Limestone down the Mississippian unconformity, to the Aux Vases Shale, and to the Albany Shale (Figs. 10A, 10B, 10C ) demonstrate wide histogram ranges of interval thickness. The flattened plots are consequences of the convergence occurring below the Seelyville Coal, in contrast to the units above the Seelyville horizon (Figs. 4 and 5). The structure of the Aux Vases Shale is largely reflective of that of the underlying New Albany Shale, although less severely affected by the underlying reefs.

348

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Fig. 9. Structure map of the New Albany Shale. Known, under lying Silurian reef locations are designated by number.

Structure of the New Albany Shale The New Albany Shale structural map (Fig. 9) illustrates that the known Silurian reef locations constitute the only significant structural closures. Known individual reefs are labeled. The influence of the reefs (Table 2) diminishes upward. Reef effects upon the overlying coal seams are evident as small, steepsided structures. Related coal seam structures commonly have about 60 ft (18.3 m) of closure, compared with about 150 ft (46.7 m) of closure on the top of the New Albany Shale. The New Albany Shale thins about 5 ft (1.5 m) over the reefs. This indicates that some compaction around the reefs was occurring concurrently with New Albany Shale deposition. The reefs influence an area of about one square mile (259 hectares) of both the New Albany Shale and the coal seams. In-mine slope gradients contributed by the reefs would be as much as 225 ft per mile (42.5 m per kilometer) on the edges of the reefs; and 150-175 ft per mile (28.4-33.1 m per kilometer) would be common over a

349 TABLE 2 Apparent structural effects (in ft) over Silurian pinnacle reefs Field name Horizon name

Apparent effect

West Franklin

Departure Closure

Danville

Departure Closure

Wilford

Lewis

c

¢

Marts

Fairbanks

Siosi a

50 50

N.D.

50 -

50 50

50 25

25 25

N.D.

50 -

c

50 50

75 75

25 25

50 25

50 -

c

Hymera

Departure Closure

Springfield

Departure Closure

50 25

¢

75 75

75 50

50 25-50

50 50

50 -

Houchin Creek

Departure Closure

50 25

25 0-25

75 75

50 25

25 25

50 50

50 -

Colchester

Departure Closure

50 50

25 0-25

50 50

50 50

50 50

25 -

Seelyville

Departure Closure

50 50

50 25

50 25

75 50

25 25

25 25

50 -

Aux Vases

Departure Closure

75 75

50 50

75 50

125 75

~

~

Departure Closure

125 75

50 25

150 125

125 75

150 125

150 b 150 b

New Albany

0-25 0

Heien

25 25

0-25 25 0

Dodd's bridge

~

25 125 -

%overs holes within Sullivan Co. only. bafter Esarey and Brooks (1950). Cinterval absent.

length of a half mile (0.8 km). The influence of the Silurian reefs is characterized as a tertiary influence upon overlying coal seam structure because of the restricted areal extent. Coal seam thickness and clastic interval thickness were apparently unaffected by the presence of reefs. It can be concluded that most of the compaction associated with the closure of the coal seams occurred later than the Pennsylvanian. E X P L O R A T I O N AND P L A N N I N G P O T E N T I A L

Underlying components of structure are reflected by Pennsylvanian coal seams. Individual sources of draping by compaction of underlying sediments may be determined in order to minimize petroleum drilling costs. Table 3 summarizes diagnostic characteristics of the components of coal structures. Areas

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Fig. 10. Multiple interval thickness frequency histograms for units beneath the Pennsylvanian measured downward from the West Franklin Limestone.

with positive structural closures greater than 25 ft ( 7.6 m) should be considered suitable targets for further interpretation. After probability curves have been plotted for existing interval thicknesses of an area, a stratigraphic testing program would only have to penetrate 150 ft (45.7 m) of bedrock and have to pass through 2 coal or limestone horizons in order to define coal structures originating from deeper features. The probability curves would allow structural projections to be made to any coal horizon with definable limits of error. TABLE3 Summary of diagnostic characteristics of components of coal seam structure Structural component

Diagnostic characteristics Closure (ft)

Size (miles)

Shape

Orientation

Regional dip

none

regional

planar

W12 ° 37' S

Infilling over Mississippian unconformity surface

_+ 0-25

2-16

linear

W33 ° 40' S

" L o w e r " Pennsylvania sand-shale ratios

_+ 0-25

2- 6

linear

W33 °40'S

Silurian reefs

+ 25-75

1- 2

circular

none

Individual Pennsylvania clastic sand-shale ratios

+ 0-20

0-10

linear or dendritic

variable W0 ° S to W90 ° S

351 Positive structural features, related to the Mississippian unconformity and the associated sand-shale trends of the "lower" Pennsylvanian, can be defined by size, magnitude, and orientation. Many of these features considerably exceed the approximately one square mile (259 hectares) area of the reefs. Extensive mapping of paleotopography in the Illinois Basin was completed by Bristol and Howard (1971). The parallel orientation of the unconformity valleys, locally oriented 20 ° south of the regional dip direction of the coal seams, has resulted in structural necks on the coal seam structural contours in Sullivan County. In this particular area of the basin the differences, in orientation of the countywide coal dips and of the underlying paleovalley trends, give a distinctive "twist" to the local coal structures away from the county-wide dip direction. These oriented coal structural necks represent poor exploration targets for reef structures. Additional minor amounts of gas and oil might be found by drilling "lower" Pennsylvanian sand bodies which drape over the unconformity ridges. One of the mappable Mississippian horizons would make a more reliable indicator of underlying structures than would the overlying Pennsylvanian coals. Within this area the cost, of testing of a coal structural high not associated with a Silurian reef, could be roughly halved by halting drilling at the Aux Vases Shale, if greater than 50 ft (15.2 m) of closure is not present. A data density of about one per half square mile (129.5 hectares ) is necessary to define a reef. Earlier work by Esarey and Brooks (1950) was not able to detect the Fairbanks or Lewis fields even though data was available in adjacent sections. Definition by drilling and interpretive projection of the structural orientation from the surrounding area would be beneficial during property acquisition in order to incorporate natural trends in preliminary planning. In subsequent planning stages consideration of structural features could increase productivity by reducing wear on equipment and decreasing energy requirements by routing haulage parallel rather than normal to linear features. The structural orientation in some cases could affect optimum mine drainage and ventilation. Actually observed mine grades would be affected by the additive interaction of all of the above influences. Some structural components could cancel, while others could supplement each other. CONCLUSIONS In Sullivan County, Indiana, Pennsylvanian coal seam structural highs are associated with underlying Silurian pinnacle reefs. However, regional dip constitutes the first-order component of coal seam structure. Secondary and tertiary components of structure result from draping by differential compaction at various depths. In this particular county coal seams were deposited over a tectonically stable shelf. Consequently, compaction constitutes the only mechanism controlling structure on the local scale.

352

The criteria for recognition of the components contributing to the draping of Pennsylvanian coal seam structures are definable: (1) The erosional unconformity, extending across the top of the Mississippian units, forms parallel valleys with local relief up to 300 ft (91.4 m). These valleys are oriented 20 ° south of the county-wide dip direction of the overlying coal seams, giving a "twist" to the structures away from the dip direction. Some contour undulations form structural map lineaments extending across the entire county. The unconformity surface is associated with contour necks with minor coal seam closure, generally less than 25 ft ( 7.6 m). The composite thickness of the"lower" Pennsylvanian section below the Seelyville Coal has a secondary influence on the structural deviation of the overlying coal seams. Composite thickness locally varies by 50% because of infilling of the valleys. ( 2 ) The "lower" Pennsylvanian sediments deposited over the Mississippian unconformity contain varying overall clastic ratios. Some unconformity valleys are filled with multistory sands, while others are clay-rich. Contrasting clastic ratios within the same paleovalley produce discernible coal structures. Both variable thickness and variable sand content, associated with the "lower" Pennsylvanian, are responsible for oriented coal structures and constitute the secondary order of controls of structure. Both variables result in minor contour undulations of about 25 ft ( 7.6 m) in vertical magnitude and 2-3 miles ( 3.2-4.8 km) of lateral amplitude. These structures constitute the majority of deviations from the regional dip and result in comparatively large-scaled features which are locally referred to as "anticlines" and "synclines" by miners. The relationship to the known underlying unconformity surface could be utilized to project data and then to plan haulage directions, to locate shafts, and to plan drainage, so that productivity could be maximized and haulage costs could be minimized. Crossing the resultant oriented structures could result in grades of 12 ft per mile ( 2.3 m per kilometer), while paralleling the features would result in haulage grades of 100 ft per mile (18.9 m per kilometer). (3) Silurian pinnacle reefs have a minor areal influence on Pennsytvanian coal structures and, thus, constitute one of the third order controls upon coal seam structure. Although they only underlie areas of about one square mile (259 hectares), they are responsible for the greatest magnitude of structural closure and relief of all the contributors to differential compaction. Coal structures frequently have 50-75 ft (15.2-22.9 m) of closure over these features. Since the features are relatively steep-sided, grades could exceed 225 ft per mile (92.5 m per kilometer) over distances of half a mile (0.80 km ) or more. (4) Clastic units between Pennsylvanian coal and limestone horizons have only a limited range of variability in thickness and minor influence on coal structures for the purposes of petroleum exploration and, therefore, constitute another third order influence upon coal structure. The mean standard deviation of individual clastic unit thickness is only 11 ft (3.4 m). On multiple intervals of up to 500 ft (152 m ), standard deviation of thickness only reaches

353 25-30 ft (7.6-9.1 m). For multiple Pennsylvanian clastic intervals above the Seelyville Coal in excess of 100 ft ( 30.5 m) in thickness, almost all depositional and compactional irregularities were counterbalanced to yield an overall relative sand content of 37%. For the purposes of petroleum exploration the coals are parallel. Detection of structures, originating from these minor sand-shale ratio variations, can save a great deal of unnecessary deeper drilling effort. This can be accomplished by mapping the structure of a number of shallow seams, rather than relying upon a single seam. However, for the purposes of mine planning, variations in sand-shale ratios for the individual clastic unit underlying a coal seam constitute the greatest uncertainty related to coal structure. Optimistically, some of these features may be projected across a study area by mapping underlying sand-shale ratios; but many features will be too narrow or of such a small size that they will never be recognized even if intersected by drilling. Local mining problems will occur where very high gradients exist in the underlying sand-shale ratios. These rapid shifts in clastic content can be responsible for gradients of up to 40 vertical feet (12.2 m) per hundred horizontal feet (30.5 m), but for only very short lateral extents. Many variations in underlying sand-shale ratios result in minor undulations of the seams which constitute no significant mining obstacle. They also can be conceived of as third order features which are superimposed upon the larger-scaled features. Owing to the remarkable evenness of deposition and parallel compaction of upper Pennsylvanian sediments between coal and limestone horizons over a platform depositional situation, the individual coal and limestone horizons may be used economically to aid in petroleum exploration for deeper structures. Such a program would only require stratigraphic data from a rock interval of 150 ft (45.7 m). In most cases, this would be sufficient to intersect at least 2 horizons of coal or limestone. Structural projections could then be made to any other mappable horizon within the Pennsylvanian using probability graphs. If closures exceeding 25 ft (7.6 m) in magnitude are found on two horizons of limestone or coal over an area of not larger than 2 square miles ( 518 hectares), then deeper investigations by drilling or seismology would be warranted. Drilling could be terminated, if 50 ft (15.2 m) of closure or of relief could not be demonstrated on the upper-most mappable Mississippian horizon. Termination of drilling in the Chesterian sediments would save half of the footage charges for testing of non-reef associated coal structural highs. Knowing that the Mississippian unconformity has the most pronounced influence on a coal seam structure on a property wide scale, the orientation of the unconformity ridge and valley systems can be used to plan major haulage routes with the minimum grade. It should be recognized that multiple components of structure can have additive or compensational cumulative effects. It can be expected that the regional dip component will diminish the effects of positive components contributing to mine grades in the updip direction and steepen the effects in the downdip direction. Further research, to define the

354

resultant stress fields and cleat directions in relation to the various components of seam structure, might be useful as an additional aid in mine planning. Grade and stressfields both will have to be considered, when orienting mine haulage directions. ACKNOWLEDGEMENTS

This study was supported by National Science Foundation Grant 742401 (G.K.), NSF-MRL Program Grant DMR 80-20249 (G.K.), the U.S. Department of H.E.W. (S.C.A.), Texas Gas Transmission Corp. and the Indiana Mining and Minerals Resources Research Institute. Dr. John B. Patton, Mr. Leroy Becker, Dr. Donald Cart, Mr. Donald Eggert, Mr. William Hamm, and Mr. Stan Keller of the Indiana Geological Survey provided advice, and facilitated: file access, duplication, and log purchase. Dr Harold Gluskoter and Mr. Roger Nance, formerly of the Illinois Geological Survey, assisted with file access.

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