## Coal Quality and Trace Element Characteristics of Samples from the Sohagpur Coal Field, North-Central India
Poster

By Peter D. Warwick,1 Robert C. Milici,1 Edwin R. Landis,2 Abijit Mukhopadyay,3 Sudhansu Adhikari,4 and Paul C. Hackley1
1U.S. Geological Survey, Reston, VA, 20192.
2U.S. Geological Survey, Denver, CO 80225.
3oal Wing, Geological Survey of India, Kolkata 700-064, India.
4Reliance Industries Limited, Ahmedabad 382-330, India.

### INTRODUCTION

During the last several years, the Geological Survey of India (GSI) and the U.S. Geological Survey (USGS) have been engaged in a study of the coking coal deposits in the Sohagpur coal field, near Shahdol, Madhya Pradesh (fig. 1). The Coal Wing of the GSI has studied the Sohagpur area intensely since the early 1980s and has determined that the major occurrences of coking coal in the coal field are on the northern, downthrown side of the regional Bamhani-Chilpa fault. Depths of coking coal generally range from 100 to 500 m. The primary coking coal deposits (coal beds I-V) are within the Permian Barakar Formation (fig. 2) and comprise the lower coal measures of the Gondwana Supergroup. Equivalent coal beds on the southern side of the fault are generally noncoking and are currently being mined in open-cast and underground mines and used as fuel for electric power generation. In this paper, we describe the stratigraphy of the coal-bearing units in the Sohagpur coal field on the basisi of outcrop studies and extensive borehole data (see fig. 3) and integrate thermal signatures (vitrinite reflectance and volatile matter) of the Barakar and Raniganj coal beds with chemical and physical analytical coal data.

Figure 1. Major coal fields of India and location map of the Sohagpur coal field.

Figure 2. Generalized stratigraphic column for the Sohagpur coal field. The stratigraphic
terminology used herein follows the usage of the Geological Survey of India and is
summarized by Veevers and Tewari (1995).

Figure 3. Coal-bearing cores. The cyclic nature of
Raniganj deposition is readily apparent from studies of
cores. The dark-colored intervals contain coal and
associated mudstone; the light-colored intervals are
dominated by sandstone.

### THE SOHAGPUR COAL FIELD

The Sohagpur coal field mining district, both underground and surface, is ocated primarily on the southern upthrown side of the Bamhani-Chilpa Fault (fig. 1). Exploratory drilling has been close to the Bamhani-Chilpa Fault and is concentrated in two areas: the eastern and western part of the coal field. The eastern area is dominated by small-scale faults associated with the Bamhani-Chilpa fault (fig. 1). A large area of unexplored, and presumably deeper coals lies north of the Bamhani-Chilpa Fault. No coking coal is currently being produced in the Sohagpur basin because of the coal beds on the northern side of the fault are too deep.

### BARAKAR FORMATION

In general, the Barakar Formation can be divided into two informal members in the Sohagpur coal field (fig. 2). The lower member consists primarily of fine-grained sandstone beds, some interlaminated and rippled sandstone and shale, and relatively thin, discontinuous coal beds. The upper member consists almost entirely of coarse-grained sandstone, a much lower proportion of shale and siltstone than in the lower member, and five thick regionally distributed coal beds (seams I-V) (figs. 2, 4, 5). Seam I marks the boundary between the lower and upper members of the Barakar Formation. In some places, calcareous sandstone occurs as bands and lenses in the lowermost part of the lower member. Carbonate cement mixed with an argillaceous matrix, however, is almost invariably found throughout the Barakar. The thickness of the Barakar Formation is about 300 m in the Sohagpur coal field. Additional information on the coal geology of the Barakar Formation can be found in the following reports: Rao (1983), Adhikari and Hore (1990), Sinha and Hore, (1989), Hore and Chatterjee (1990), Roy and Das (1991), and Adhikari and Datta (1991).

Figure 4. Massive Barakar sandstone. The sandstone above rests on the top of a thin coal bed.

Figure 5. An example of a coal bed approximately 3 m thick in the Barakar Formation.

### CHARACTERISTICS OF BARAKAR AND RANIGANJ COALS

Average ash yield for coals occurring in the Sohagpur coal field ranges from approximately 22 to 39 percent (as received) (fig. 6). Average gross calorific value (dry, ash free) of major coal beds ranges from a low of approximately 7,600 kcal/kg in the Raniganj Formation to approximately 8,500 kcal/kg in beds IV and V of the Barakar Formation (fig. 7). Volatile matter (VM), an indicator of coking potential, varies systematically in Barakar Formation coal beds as a function of depth and distance from the Bamhani-Chilpa Fault (fig. 8). VM increase stratigraphically upwards and away from the fault (fig. 8). The decrease in VM downsection can be attributed to the effects of depth of burial. Even though the coal beds are increasing in elevation toward the fault, they are decreasing in VM, an indication that the fault may be a conduit for heat that increases the rank of the coal beds (Ghosh and others, 1993). Sedimentological studies by Mukhopadyay and others (2001) indicate that marine rocks are commonly associated with coal bed I of the Baraker Formation and the Raniganj Formation coal zones. The slightly higher average sulfur content of these coal beds may reflect a possible marine influence (fig. 9). When the vitrinite content of Gondwana Permian-age coals is compared with that of Carboniferous-age coals, it is generally less abundant (Taylor and others, 1998). In the Sohagpur coal field, the vitrinite content of the Barakar coals is as low as 25 percent, and the inertinite content is as great as 45 percent,; these conditions are generally not conducive to forming good coking coals (fig. 10). The variation in Romax and its relation to depth of burial and proximity to extrusive rock are shown for two example cores in figure 11. Figure 11 shows that Romax values generally increase with depth in core SPB-28, indicating the heat effects of depth of burial. In contrast, Romax for core SPT-23, is greatest near the overlying extrusive body and rapidly decreases down and away from the intrusive body. A plot of Romax data as a function of sample elevation is illustrated in figure 12. The widely scattered data points for higher Romax values suggest that increases in Romax are caused by the proximity of intrusive and extrusive rocks. Because of the wide scatter of data concentrated between 0.5 and 1 Romax values, it is difficult to plot a line that might show the observed increase in rank down hole. This scatter may be due to differences in the reflectance measurement techniques used by different operators.

Figure 6. Average ash yield (as-received - ar, whole coal weight percent basis) for the major coal
beds in the Sohagpur coal field. All data for coals in the Raniganj Formation have been combined.
Data are from this study and unpublished Geological Survey of India data. Number of samples = n.

Figure 7. Average gross calorific value (kcal/kg on dry, ash-free basis - daf) for major Baraker
Formation coal beds and all coal beds in the Raniganj Formation in the Sohagpur coal field. Data are
from this study and unpublished Geological Survey of India data. Number of samples = n.

Figure 8. Relation of volatile matter (VM) in Soghagpur coals to the Bamhani-Chilpa Fault. The inset in the top
figure shows the line of section of the samples illustrated in the bottom figure.

Figure 9. Average sulfur content (as-received - ar, whole coal basis) for the major coal
beds in the Sohagpur coal field. All data for coals in the Raniganj Formation have been
combined. Data are from this study and unpublished Geological Survey of India data.
Number of samples = n.

Figure 10. Petrographic composition of major coal beds (ash-free basis). The Y scale is in percent; n = number of
analyses. Inset microphotograph shows vitrinite in Barakar Formation coal.

Figure 11. Romax and depth and extrusive effect for two drill holes in the Sohagpur coal field. Scale in meters shows
elevation on right of diagram.

Figure 12. Plot of all Romax data and elevation. Average surface elevation of Sohagpur basin of about
500 m. Note that the number of samples (n) = 115 and includes both Raniganj and Barakar samples.

### TRACE-ELEMENT STUDIES

One hundred coal and rock samples were collected during this study and were analyzed for 36 elements using the standard USGS analytical suite for coal (table 1). A summary of the analytical results for elements that may have potential to harm the environment is presented in table 2. The environmentally sensitive element groupings used in table 2 were previously described by Finkelman and Gross (1999). Many trace-element concentrations were below detection levels for the analytical techniques used on the coal samples. Of the environmentally sensitive elements listed in table 2 (groups 1 and 2), all elements except chromium have means that are within the normal range for most coals of the world as outlined by Swaine (1990) and Bowen (1979). Many of the environmentally sensitive elements (group 1, table 2), however, have maximum values greater than the top of the normal range for those elements reported for world coals. These elements are arsenic, beryllium, chromium, cobalt, manganese, nickel, and uranium. In table 2 (group 2), most elements have maximum concentrations above the normal range for most coals. These elements are barium, copper, molybdenum, tin, thallium, thorium, and vanadium. Group 3 consists of the elements such as cesium, gallium, germanium, rubidium, and thorium that have greater concentrations than what is found in most coals of the world (table 3). It is interesting to note that, in a preliminary study of trace element concentrations in coal samples from the Sohagpur coalfield, Pareek (1987) also reported elevated concentrations of the following elements: cobalt, manganese, nickel, copper, and germanium. Gupta (1999) found that coal mining and coal combustion activities in the Pench Valley coal field, located in the south-central part of Madhya Pradesh (fig. 1), were mobilizing some of the environmentally sensitive elements and that this mobilization was contributing to the elevated concentrations of these elements found in surface and ground water. Gupta (1999) also suggested that human health and the environment are being adversely affected in the Pench Valley area. Given the elevated concentrations of some of these elements in Sohagpur coal samples, conditions similar to those reported by Gupta (1999) for the Pench Valley may exist in the Sohagpur area. Additional studies of ground and surface water in the Sohagpur area should be carried out to determine if trace elements are being mobilized by coal mining and coal combustion activities.

T able 1. Sources of elemental data used in this study [See Swanson and Huffman (1976), Golightly and Simon (1985), Briggs (1997), and Meier (1997) for a discussion on sampling methods and analytical procedures].

Table 2. Selected trace element average concentrations in Sohagpur basin coal zone samples (ppm, whole coal basis). [The environmentally sensitive elements shown on this table are described by Finkelman and Gross (1999); "Range of most coals" data from Swaine (1990) and Bowen (1979).]

*Denotes some samples were below the analytical detection limit and these were multiplied by 0.7 to approximate the low concentrations for averaging.

Table 3. Contents of trace elements in most coals. [Data from Bowen (1979), Swaine (1990), and Taylor and others (1998).]

Element | Parts per million
--- | ---
Antimony (Sb) | 0.05-10
Arsenic (As) | 0.5-80
Beryllium (Be) | 0.1-15
Boron (B) | 5-400
Cadmium (Cd) | 0.1-3
Cesium (Cs) | 0.3-5
Chlorine (Cl) | 50-20005
Chromium (Cr) | 0.5-60
Cobalt (Co) | 0.5-30
Copper (Cu) | 0.5-50
Fluorine (F) | 20-500
Gallium (Ga) | 1-0
Germanium (Ge) | 0.5-50
Gold (Au) | Up to 0.01
Hafnium (Hf) | 0.4-5
Lanthanum (La) | 1-40
Lead (Pb) | 2-80
Lithium (Li) | 1-80
Manganese (Mn) | 5-300
Mercury (Hg) | 0.02-1
Molybdenum (Mo) | 0.1-10
Nickel (Ni) | 0.5-50
Niobium (Nb) | 1-20
Phosphorus (P) | 10-3000
Rubidium (Rb) | 2-50
Scandium (Sc) | 1-10
Selenium (Se) | 0.2-10
Silver (Ag) | 0.02-2
Strontium (Sr) | 15- 500
Tantalum (Ta) | 0.1-1
Thallium (Tl) | <0.2-1
Thorium (Th) | 0.5-10
Tin (Sn) | 1-10
Titanium (Ti) | 10-2000
Tungsten (W) | 0.5-5
Uranium (U) | 0.5-10
Vanadium (V) | 2-100
Yttrium (Y) | 2-50
Zinc (Zn) | 5-300
Zirconium (Zr) | 5-200

### CONCLUSIONS

The geologic controls on coking coal deposits in the Sohagpur coal field include depth of burial, proximity to intrusive and extrusive rocks, and proximity to the Bamhani-Chilpa Fault. VM decreases toward the Bamhani-Chilpa Fault, a possible indication of paleo-heat flow along the fault zone. Maceral composition of the Sohagpur coals is variable between coal beds. Because high vitrinite content is related to the propensity of coals to form coke, areas in the coal field of increased vitrinite content need to be identified for potential coking coal resources. Ash yield of some coal beds may be related to their proximity to syndepositional faults. Raniganj coals and Barakar coal bed I have the greatest sulfur content (<1% as received), which may indicate some marine influence in those intervals. For the Sohagpur coal samples, the environmentally sensitive trace element average concentrations are generally within the range of most coals of the world. Many trace elements, however, have a maximum concentration that is greater than the range for most world coal.

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