2. New Coal
Resource and
Reserve Data in Illinois
Overview
The identified resource, demonstrated reserve base (DRB), and estimated recoverable reserves data for the State of Illinois were updated in a two-part cooperative agreement between EIA and the Illinois State Geological Survey (ISGS). The first part was completed in November 1995 for resources in those parts of the Illinois coalfield where new data were available that could be fully integrated during the initial 12-month project period.(1) Those data were incorporated in EIA's subsequent coal reserves report and database.(2) The final part of the cooperative agreement was completed in March 1997.(3) It incorporated additional new file data for which broader analysis was required--data which might demonstrate coal resources of low- to medium-sulfur content or resources in areas previously without reliable DRB estimates. Those data are incorporated via the present report and database.
The new DRB of coal in Illinois, calculated by the ISGS as of January 1, 1996, was 105.16 billion short tons (Table 1). This compares with 78.01 billion short tons prior to the ISGS project (remaining DRB, as of January 1, 1993(4)) and with 90.05 billion short tons following the first part of the update (remaining as of January 1, 1994).
Compared on the same remaining-year basis, the DRB estimate in Illinois increased by more than 35 percent, or 27.4 billion short tons. EIA adjusted the new Illinois DRB for depletion during 1996, resulting in a DRB of 105.07 billion short tons remaining as of January 1, 1997.
The new estimate includes resources revised using recent mapping in 16 counties, as well as adjustments throughout most of the 72 counties included in the study for depletion due to past mining. The new estimate for identified resources is 199 billion short tons, as compared with EIA's previous file data totaling estimate of 181 billion short tons. These coal resource estimates are neither ultimate nor definitive; they represent qualifying data obtainable at reasonable cost at the time of study for mapped coal resources in the areas and depths likely to be minable. As with all DRB estimates, they do not include hypothetical or inferred resources or coal deposits too thin, too deep, or too impure to mine.
The new resource estimates incorporate analyses of sulfur, heat content, and coal rank to characterize the remaining coal resources in Illinois. Relevant coal quality data were examined in conjunction with coal resource mapping. Coal analyses from exploration drill holes and channel samples at mines and outcrops were compiled and mapped, along with data on geologic trends, to allocate coal resources to ranges of sulfur and heat content and to rank group. The new allocations place almost 1 percent of the DRB of Illinois in the two lowest sulfur categories, as compared with none in EIA's 1993 reserve base. These allocations also place 89 percent of the DRB in the highest sulfur category, as compared with EIA's allocation of 69 percent in 1993.
By comparing depletion of reserves as calculated from maps of mined areas with depletion based on reported production and recovery rates, ISGS uncovered potential pitfalls in estimating depletion based on reported production alone, without adequate knowledge of mine operations. The issues include production data reported on the basis of tipple location rather than point of extraction and depletion by underground mining of reserves classified as surface-minable in the database. The destruction of unmined reserves by preferential mining of lower seams is also an important issue but it could not be estimated from the statistics.
Table
1. Estimates of the Demonstrated Reserve Base and Estimated Recoverable
Reserves of Bituminous Coal in Illinois by Btu/Sulfur Range and
Type of Mining (Million Short Tons Remaining as of January 1, 1996) |
||||||||
|---|---|---|---|---|---|---|---|---|
| Heat
content (million Btu per short ton) |
Sulfur
Content (pounds of sulfur per million Btu) |
Total
All Sulfur Categories |
||||||
| < 0.40 | 0.41-0.60 | 0.61-0.83 | 0.84-1.24 | 1.25-1.67 | 1.68-2.50 | > 2.50 | ||
| Demonstrated Reserve Base | ||||||||
| Minable from Surface | ||||||||
| 15-19.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 8.84 | 121.98 | 130.82 |
| 20-22.99 | 0.00 | 0.00 | 4.16 | 161.45 | 132.20 | 501.23 | 14,102.65 | 14,901.71 |
| 23-24.99 | 0.00 | 0.00 | 1.36 | 26.42 | 20.12 | 80.81 | 1,084.50 | 1,213.21 |
| 25-25.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 372.25 | 372.25 |
| Total | 0.00 | 0.00 | 5.52 | 187.87 | 152.33 | 590.88 | 15,681.39 | 16,617.99 |
| Minable Underground | ||||||||
| 20-22.99 | 199.42 | 683.74 | 1,089.46 | 1,660.13 | 1,220.92 | 2,439.41 | 48,028.67 | 55,321.75 |
| 23-24.99 | 2.44 | 34.08 | 255.55 | 874.93 | 650.75 | 1,509.48 | 28,712.89 | 32,040.12 |
| 25-25.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 1,167.04 | 1,167.04 |
| >25.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 9.45 | 9.45 |
| Total | 201.86 | 717.82 | 1,345.02 | 2,535.06 | 1,871.67 | 3,948.88 | 77,918.06 | 88,538.37 |
| Minable Total | ||||||||
| 15-19.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 8.84 | 121.98 | 130.82 |
| 20-22.99 | 199.42 | 683.74 | 1,093.63 | 1,821.58 | 1,353.12 | 2,940.64 | 62,131.33 | 70,223.46 |
| 23-24.99 | 2.44 | 34.08 | 256.91 | 901.35 | 670.87 | 1,590.28 | 29,797.40 | 33,253.33 |
| 25-25.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 1,539.30 | 1,539.30 |
| >25.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 9.45 | 9.45 |
| Total | 201.86 | 717.82 | 1,350.54 | 2,722.93 | 2,023.99 | 4,539.76 | 93,599.45 | 105,156.40 |
| Estimated Recoverable Reserves | ||||||||
| Recoverable from Surface | ||||||||
| 15-19.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 5.22 | 71.18 | 76.41 |
| 20-22.99 | 0.00 | 0.00 | 2.48 | 110.69 | 87.11 | 303.37 | 8,586.19 | 9,089.84 |
| 23-24.99 | 0.00 | 0.00 | 0.70 | 0.72 | 4.46 | 30.94 | 682.05 | 718.86 |
| 25-25.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 226.75 | 226.75 |
| Total | 0.00 | 0.00 | 3.18 | 111.41 | 91.57 | 339.53 | 9,566.17 | 10,111.86 |
| Recoverable Underground | ||||||||
| 20-22.99 | 45.58 | 176.75 | 325.61 | 450.29 | 356.33 | 696.70 | 15,180.30 | 17,234.57 |
| 23-24.99 | 0.54 | 8.29 | 80.18 | 316.55 | 226.24 | 554.23 | 9,435.98 | 10,622.01 |
| 25-25.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 266.76 | 266.76 |
| Total | 46.12 | 185.04 | 405.79 | 766.85 | 582.58 | 1,250.93 | 24,883.03 | 28,120.34 |
| Recoverable Total | ||||||||
| 15-19.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 5.22 | 71.18 | 76.41 |
| 20-22.99 | 45.58 | 176.75 | 328.09 | 560.99 | 443.45 | 1,000.07 | 23,766.48 | 26,321.41 |
| 23-24.99 | 0.54 | 8.29 | 80.88 | 317.28 | 230.70 | 585.17 | 10,118.03 | 11,340.88 |
| 25-25.99 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 493.51 | 493.51 |
| Total | 46.12 | 185.04 | 408.96 | 878.26 | 674.14 | 1,590.47 | 34,449.20 | 38,232.20 |
| Note:
Data may not equal sum of components due to independent rounding.
Source: Illinois State Geological Survey, Illinois Coal Reserve Assessment and Data Base Development: Final Report, Open-File Series 1997-4. |
||||||||
The January 1, 1996, accessible reserve base was estimated at 69.56 billion short tons. EIA's previous (January 1, 1992) accessible reserve base estimate of 56.49 billion tons excluded surface-minable coal under prime farm land, an exclusion that is no longer considered valid. The new estimate excludes: resources under towns, interstate highways, and public land; underground-minable resources less than 4 feet thick; resources in small, irregular blocks between mines; and coal allocated for barriers and small blocks left in future mines. For underground mining, the coal accessibility based on land use restrictions related to surface features ranges from 42 percent to 100 percent, varying by coalbed and county. These basic accessibility rates were further reduced to account for irregular blocks of coal and barriers expected to be left unmined, and to exclude coal less than 42 inches thick.(5) For surface mining, a net accessibility factor of 80 percent or less was applied to all coalbeds and counties. This was based on these extrapolations: (1) that on average, 15 percent of future minable coal will be inaccessible because of barriers between mine blocks and at property boundaries and because of irregularly shaped isolated resource blocks that cannot economically be accessed; and (2) that another 6 percent, on average, is expected to be restricted due to land use conflicts. In addition, specific accessibility-limiting restrictions were identified in several counties.
The ISGS is currently involved in a multiyear study supported by the USGS to assess the availability of coal for mining. When complete, the findings from the Coal Availability Studies are expected to lead to additional adjustments in the accessible reserve base.
Estimated recoverable reserves of 38.23 billion short tons as of January 1, 1996, were calculated using recovery factors of 50 percent for underground-minable reserves and 70 to 85 percent (depending on location and thickness) for surface-minable coal (Table 1). These rates are similar to the traditional 50 percent and 80 percent rates, respectively, used previously in DRB calculations. They were selected, however, based on data on reserves depletion and mine production from January 1979 to January 1996, rather than on the regional resource depletion factors applied to the DRB and traditionally assumed to account for inaccessible resources as well. The revised recovery rates account both for coal that would be lost in cleaning and handling and for coal left as pillars or barriers in mines.
The Illinois coalfield (Figure 1) is in the Interior Region of the United States and contains most of Illinois (Figure 3) as well as western portions of Indiana and Kentucky. Minable coal is found in the Pennsylvanian-age strata of the geologic basin. The rank of the coals is high volatile bituminous, ranging from the A rank group at the extreme southern margin of the basin to rank groups B and C in the southern, central, and northern portions of the basin. Illinois has the largest DRB of bituminous coal and the second largest total DRB of any State.
| Figure 3. Coal-Bearing Areas of Illinois |
|
| Source: United States Geological Survey, Coalfields of the United States, 1960. |
Since the development of modern surface mining equipment, coals as deep as 150 feet have commonly been mined by surface methods in Illinois. Large dragline and power shovel mining or small truck and shovel operations are the primary forms of surface mining. Augering is sometimes used to recover additional coal from the final cut of a surface mine.Shafts and slopes are the most common means of access to underground mines in this region, but some mines employ a drift entrance constructed at an abandoned surface mine highwall or a box cut.(6) Partial- and high-extraction room-and-pillar mining and longwall mining methods are used. During the past 20 years, production has shifted from entirely room and pillar to more than 40 percent from longwall operations.
In the early 1950s, the ISGS completed the first comprehensive survey of coal resources in the State.(7) The landmark report on the survey results, published in 1952, provided a framework and format generally followed in subsequent resource assessments. In particular, it established reliability categories that reflect the above average lateral continuity of most coals found in Illinois (Table 2). The 1952 report is also the only source of resource and reserve estimates for a few seams that have not attracted sufficient interest to warrant revised mapping.
The ISGS categories of reliability are comparable to those defined by the USGS. Because of the lateral continuity of most Illinois coals, however, the radii of influence assigned to each datum point are larger than those used by the USGS. The ISGS categories of class I-A, I-B, and II-A are considered equivalent in reliability to the USGS categories of measured, indicated, and inferred and had previously been accepted as such in EIA coal resource data.
Table
2. Reliability Classifications for Coal Resources in Illinois |
|||
|---|---|---|---|
| Class | Maximum
Distance from Datum Points* |
Accepted
Datum Points |
Remarks |
| I-A Proved (Measured) |
0.5 mile | Mined-out areas, Diamond drill holes, Outcrops, Coal test geophysical logs | Approximately equivalent to measured category of the U.S. Geological Survey |
| I-B Probable
(Indicated) |
2 miles | All points of Class I-A plus coal-test churn drill holes | Approximately equivalent to indicated category of the U.S. Geological Survey |
| II-A Strongly
Indicated (Inferred) |
4 miles | All points of Classes I-A and I-B plus churn drill holes drilled for oil or water with unusually good records, control rotary drill holes and oil-test geophysical logs | Approximately equivalent to inferred category of the U.S. Geological Survey |
|
* Distances modified in practice by geological considerations. Source: Adapted from ISGS Bulletin 78, 1952. |
|||
Although the DRB did not exist at the time of the 1952 report, the criteria used in the study are compatible with current DRB definitions and would indicate a DRB of 61 billion tons. Additional mapping since 1950 raised the DRB to 78 billion tons as of January 1, 1993.
The ISGS began computerizing its coal resource mapping in the 1960s. Computers expedite merging of coal thickness data with data on coal depth, sulfur, rank, heating value, and mined areas, and with other information such as calculated depletion, accessibility, and recoverability of reserves. Subsequent updates, revisions, and accessibility adjustments can also be made more efficiently with a digital database.
Many of the coal resource maps needed for this study were already in a digital format of some kind. For this study, all data were combined into a common digital map database to facilitate processing as well as to provide a suitable foundation for future updates and revisions. All remaining paper maps were digitized into the common database and numerous adjustments were made to edge match and correlate all data from earlier databases and base maps.
Resources and reserves were divided into two categories based on the mining method most likely to be used. The surface-minable category consists of coals that would be mined most likely by removing the overburden to expose and excavate the coal. The underground-minable category consists of resources expected to be extracted by underground methods such as room-and-pillar or longwall mining.
The factors that determine the method used to mine a particular deposit are primarily economic rather than technical. The main factors are: thickness of the coal, average stripping ratio of the mine block, nature of the overburden material, surface ownership and land use, proximity to other surface features, and the capital and previous mining experience of individual companies.
The ISGS found the 150-foot depth line to be the most representative average delimiter between surface-minable and underground-minable resources. In Illinois, the ISGS defines surface-minable resources as having a minimum thickness of 18 inches. Underground-minable resources are defined as having a minimum thickness of 28 inches. These minimum thicknesses are based on historical mining practice in the State. For economic reasons, seams less than 48 inches thick have not been extensively mined underground in Illinois for the past three decades or more; however, resources less than 48 inches thick were retained in the DRB for this study in order to provide compatibility with current DRB estimates of other midwestern States. As explained later, those seams are excluded from the accessible reserve base.
The categories of coal seam thickness and overburden thickness followed in Illinois are summarized in Tables 3 and 4. No maximum depth was established for underground-minable reserves. The deepest mapped resources in the State are slightly more than 1,500 feet below the surface. Interviews with representatives of mining companies indicated that this depth does not prevent mining.
Table
3. Categories of Coal Seam Thickness in Illinois |
|
|---|---|
| Thickness
Rangea (inches) |
Average
Thickness (feet) |
| 18-28 | b2 |
| 28-42 | 3 |
| 42-54 | 4 |
| 54-66 | 5 |
| 66-78 | 6 |
| 78-90 | 7 |
| 90-102 | 8 |
| 102-114 | 9 |
| 114 | 10 |
| aThickness
ranges expressed in even-inch values represent the isopach contour
lines mapped to define areas of average thickness. Isolated thicknesses
corresponding to the upper limit of a range are included in the next
thicker range. bSurface-minable coal only. Source: Illinois Coal Reserve Assessment and Data Base Development: Final Report, Open-File Series 1997-4, 1997. |
|
Table
4. Categories of Overburden Thickness |
|
|---|---|
| Underground
Mining (feet) |
Surface
Mining (feet) |
| 150-500 | 0-50 |
| 500-1,000 | 50-100 |
| 1,000-2,000 | 100-150 |
| Note: Thickness ranges expressed in even-foot values represent the isopach contour lines mapped to define areas of average thickness. Isolated thicknesses corresponding to the upper limit of a range are included in the next thicker range. Source: Illinois Coal Reserve Assessment and Data Base Development: Final Report, Open-File Series 1997-4, 1997. | |
The thicknesses of coal deposits were mapped using contour lines at one-foot intervals. Coal tonnages were calculated using a density factor of 1,800 tons per acre per foot of coal thickness (equivalent to 1.32 specific gravity). The mean value of the two contours defining an area was used for this calculation. For example, the area between the 5.5-foot and 6.5-foot isopachs was assumed to have an average thickness of 6 feet.
Cumulative depletion of the DRB was calculated and analyzed both using information on mined areas and using production data and recovery factors. The results were compiled by ISGS to update the DRB to January 1, 1996, and to provide comparative statistics on reported production and depletion of reserves. EIA depleted the DRB from January 1, 1996, to January 1, 1997, using EIA production data and standard depletion factors.
Resources were allocated to EIA coal quality categories for sulfur, rank, and calorific value. Face channel, column, composite bench, and drill core samples were used in this study (See "coal sampling" in Glossary). ISGS has on file more than 4,000 analyses of Illinois coal, the majority of which are of the face channel type. These samples were collected and analyzed by ISGS staff, the U.S. Bureau of Mines, or coal companies. All Illinois coals are high-volatile bituminous. Coal rank varies systematically with distribution and depth of the deposit in the coal field. Rank was determined by calculating the heating value of samples on a moist, mineral-matter-free basis, according to formulas of Standard D388, American Society for Testing and Materials.(8) The coal analyses were used to map million Btu per ton and pounds of sulfur per million Btu on an as-received basis and to assign corresponding coal resources to the Btu/Sulfur ranges used in EIA allocations.
Coal
Accessibility Adjustments
The accessible reserve base can be described as the portion of the DRB that can be mined at present, when local or regional mining practice and technologies and physical or geologic conditions are taken into account. For recent resource studies and revisions, EIA has broadened the concept of accessibility to include the limiting effects of certain technological and geologic conditions. In the past, such adjustments were made in the resource database, by the field investigator or within the DRB derivation, and were difficult to reassess. They can now be applied through computerized resource mapping systems and they enhance the comparability of EIA coal accessibility and USGS coal availability. The expanded definition has been incorporated into the Illinois study. The accessible reserve base includes that portion of USGS available resources that would meet DRB criteria.
At the end of the EIA cooperative agreement, the ISGS was in the third year of a multiyear USGS Coal Availability
Study (CAS). Five quadrangles had been evaluated, about 20 percent of the number needed to reliably assess availability of resources in Illinois. The amount of coal available for mining in the sample areas ranged from 18 percent to
61 percent of the original resources. Technical factors such as thickness of the coal and overlying bedrock, roof and
floor conditions, faults, and size of the mining block account for most of the restrictions on coal availability. Land use
restricts from less than 1 percent to 16 percent of the resources in the quadrangles studied. Although it was too early to apply most of the initial findings of this study, some preliminary observations were incorporated into this estimate of accessible resources. Final findings of the CAS are expected to significantly alter the accessible reserve base. The factors considered for estimating the accessible reserve base are listed in Table 5.
Table
5. Factors Considered and Applied in Illinois to Estimate the Accessible
Reserve Base
| ||
|---|---|---|
| Factor Considered | Applied? | Remarks |
| Technical | ||
| Prime farmland | No | There is no evidence that prime farmland restricts access. |
| Areas densely drilled for oil | No | The presence of wells does not raise costs enough to restrict access. |
| Barrier pillars and small blocks between mines | Yes | Tonnage of existing blocks and barriers was calculated from maps. Tonnage of blocks and barriers created by future mining was estimated to be 15 percent of reserves otherwise accessible. |
| Thin coal | Yes | Underground-minable reserves <48 inches thick were excluded. |
| Land Use | The tonnage of underground-minable reserves restricted by all land use categories was estimated from previous mapping; 6 percent of all surface-minable reserves was assumed to be inaccessible because of land use. | |
| Interstate highways | Yes | |
| Towns | Yes | |
| Cemeteries | Yes | |
| Public lands | Yes | |
| Source: Illinois Coal Reserve Assessment and Data Base Development: Final Report, Open-File Series 1997-4,1997. | ||
Almost 60 percent of the 36 million acres of land in Illinois are classified as prime farmland. EIA's estimate of accessible coal in Illinois had excluded surface-minable reserves in areas of prime farmland, but Illinois surface mine regulations do not preclude mining these areas. Further, surface mine operators in Illinois are successfully mining and reclaiming prime farmland, and in CAS interviews with operators, they did not consider it a limit to reserve accessibility. For these reasons, prime farmland was not a factor in estimating the new accessible reserve base.
Since 1952, the ISGS had excluded areas densely drilled for oil from its calculation of reserves. Coal mining experts interviewed by Treworgy and Bargh in 1981(9) confirmed this restriction and the amount of coal excluded (9.6 billion tons) was documented. The thinking was that safety considerations prevent mining coal in such areas. In the CAS, however, it was found that mining companies no longer regard closely spaced oil wells as an absolute barrier to mining. Although regulations require that a barrier pillar be left around wells, experienced mining companies have been allowed to reduce the size of the pillar. In many cases of abandoned wells, it has been feasible to plug the well to meet specifications and then mine through it. The decrease in the amount of coal recovered and/or the increase in the cost of mining is not severe enough to consider the reserves inaccessible.
The five CAS quadrangles completed by the end of the Coal Reserves Data Base (CRDB) study contain about 2 percent of the underground-minable resources in the State and include all the major seams. Six of the eight companies operating major underground mines in the State were interviewed. All six considered coal less than 42 inches thick as presently too thin to mine economically by underground methods. Because there are few natural outcrops, most underground mines require extensive exploratory and development drilling to obtain data for mine planning and permitting and for the construction of slopes and shafts for the movement of air, miners, materials, and coal. To recoup these expenses, mines must produce large tonnages of low-cost coal. Mining in thin seams requires more acreage and the mining costs are higher. For these reasons, underground-minable reserves less than 42 inches thick(10) have been excluded from accessibility in this update.
A significant portion of inaccessible resources consists of blocks of coal left as barrier pillars (the law requires 200 feet between mines) or simply left out because of the geometry of the mine plan, the early abandonment of a mine, inability to obtain land ownership or mineral rights, or unfavorable geology. Once surrounded by abandoned mines, these blocks are too small or irregular to be minable. The approximate area of coal rendered inaccessible by mining was calculated by creating a 200-foot buffer around each mine and adding to that any areas of coal considered to be unminable because of small mining-area size, convoluted geometry, or proximity to mined areas. The coal tonnage in the adjusted buffer areas was calculated and excluded from the accessible reserve base.
Additional blocks will become inaccessible as mining continues. The ISGS found that, on a county-by-county basis, the amount of inaccessible coal ranged from 6 percent to more than 40 percent of the original resources in mined areas, or roughly 20 percent on a Statewide basis. Since some of this coal may have been left because of surface features, which are accounted for separately, it was estimated that 15 percent of the coal otherwise qualified for the accessible reserve base will be rendered inaccessible by future mining.
Earlier investigations have identified land uses such as interstate highways, railroads, cemeteries, towns, and public lands as factors that limit the accessibility of coal. However, changes in mining practice and findings from CAS indicate that land use restrictions have changed. For this assessment, an interim 94-percent rate of accessibility was applied to the surface-minable DRB in all counties to account for land use restrictions. This figure was chosen because it coincides with the state- wide average for underground-minable reserves and is midrange for the surface-minable resources in the CAS quadrangles studied to date.
Other factors that restrict the accessibility of reserves include insufficient thickness of bedrock overburden, insufficient thickness of or incompetent interburden, and unfavorable roof and floor conditions. The impacts of these factors are not fully studied at this time, but at the completion of the Illinois CAS, the accessible reserve base may be adjusted to include as many of these factors as practical. The January 1, 1994, accessible reserve base is considered more accurate than the previous data even though based partly on preliminary CAS data.
EIA provided data on reported recovery rates from individual mines in Illinois for the years of 1991 to 1993. These data were compared with regional recovery rates calculated from depletion (measured from resource maps) and production data (compiled from State reports).
Recovery rates for underground reserves were calculated by comparing cumulative depletion of underground reserves with reported production. For those counties where a valid comparison could be made, recovery rates for the period 1979 through 1993 ranged from 40 percent to 58 percent on a county and seam basis, averaging 48 percent for all seams and counties combined. This agrees with EIA's data for 1991 through 1993, which show a weighted average recovery rate of 50 percent for all underground mines. Based on these statistics, a factor of 50 percent was used to calculate remaining recoverable underground-minable reserves.
Consideration was given to using a higher recovery rate in counties where longwall mining is being practiced; however, EIA data did not show consistently higher recovery rates at mines operating longwalls. This is probably due to the influence of such factors as geology, amount of coal preparation, and development stage of individual mines. A valid comparison between depletion and production could not be made in several counties where reported production included production from outside the county, or underground-minable reserves were depleted by surface mining, or production was too limited to measure depletion at the scale of mapping.
EIA data for individual surface mines for 1991 through 1993 showed recovery rates ranging from 60 to 90 percent, with a weighted average of 75 percent. These figures compare favorably with recovery rates in selected counties, as calculated from cumulative depletion of surface-minable resources and reported cumulative production from surface mines. Both the EIA data and the ISGS cumulative depletion data from base year of mapping to 1994 indicate that recovery rates are lower for thinner seams or seams with many impurities. For example, the Herrin Coal in the northwestern part of the State commonly contains impurities--a widespread parting known as the "blue band" and prevalent occurrences of "white top" and clay dikes. Based on these data, a recovery rate of 70 percent was used in this study to calculate surface-minable reserves for seams less than 42 inches thick or for the Herrin Coal in northwestern Illinois. A recovery rate of 85 percent was used for all other surface-minable reserves.
Endnotes
1. Illinois State Geological Survey, Illinois Coal Reserve Assessment and Data Base Development: Final Report for Part 1, Open-File Series, 1995-11, published report prepared for the Energy Information Administration (Champaign, IL, 1995).
2. Energy Information Administration, U.S. Coal Reserves: A Review and Update, DOE/EIA-0529(95) (Washington, DC, August 1996).
3. Illinois State Geological Survey, Illinois Coal Reserve Assessment and Data Base Development: Final Report, Open-File Series, 1997-4, published report prepared for the Energy Information Administration (Champaign, IL, 1997).
4. Energy Information Administration, Coal Production 1992, DOE/EIA-0116(92) (Washington, DC, October 1993).
5. About 5 percent of the coal in the less than 42-inch category was mapped using prior criteria and includes beds as thick as 47 inches; hence it is identified in the study both as less than 42 inches and as less than 4 feet thick.
6. A drift mine typically enters the coalbed directly at a location where the coal and overlying rocks are exposed, such as a natural hillside. In Illinois, drift mines sometimes access the coal seam via an artificial exposure, such as a "final cut" high wall, where the overburden becomes too thick for further surface mining, or in a box-like pit excavated specifically to create an exposure surface in the coal seam.
7. Illinois State Geological Survey, Minable Coal Reserves of Illinois, Bulletin 78 (Champaign, IL, 1952) 138 pp.
8. American Society for Testing and Materials, Annual Book of ASTM Standards, Section 5, Petroleum Products, Lubricants, and Fossil Fuels, Volume 5.05: Gaseous Fuels, Coal and Coke (Philadelphia, PA, 1990).
9. Illinois State Geological Survey, Deep-Minable Coal Resources of Illinois, Circular 504 (Champaign, IL, 1982).
10. About 5 percent of the coal in the less than 42-inch category was mapped using prior criteria and includes beds as thick as 47 inches.