The lateral and vertical reflectance and petrological variation of a heat-affected bituminous coal seam from southeastern British Columbia, Canada

International Journal of Coal Geology, 15 (1990) 317-339

317

Elsevier Science Publishers B.V., A m s t e r d a m - - Printed in The Netherlands

The lateral and vertical reflectance and petrological variation of a heat-affected bituminous coal seam from southeastern British Columbia, Canada 1

Fariborz G o o d a r z i a a n d T h o m a s Gentzis b aInstitute of Sedimentary and Petroleum Geology, Geological Survey of Canada, 3303 - 33rd Street N. W., Calgary, Alta., T2L 2A 7, Canada bAlberta Research Council, Coal Research Centre Devon, 1 Oil Patch Drive, Devon, Alta., TOC lEO, Canada (Received May 17, 1989; revised and accepted November 20, 1989)

ABSTRACT Goodarzi, F. and Gentzis, T., 1990. The lateral and vertical reflectance and petrological variation of a heat-affected bituminous coal seam from southeastern British Colombia, Canada. Int. J. Coal Geol., 15: 317-339. Thermally altered pods of coal of very high rank have been observed in a high-volatile-bituminous coal seam in the eastern side of Eagle Mountain, Elk Valley Coalfield, British Columbia. Rank changes have been measured over a strike distance of 7.5 m from 1.24% to 7.1% Ro m~, corresponding to a rank gradient of 0.78% Ro m - i. Petrologically, unaltered to extremely altered vitrinite showing nongranular (basic) anisotropy, mosaic-textured liptinite and pyrolytic carbon are the most abundant components. The limited presence of mosaic on vitrinite is an indication that the coal seam may have been weathered prior to being heat-affected. Evidence points to localized temperatures as high as 1,000 °C, which could have been caused by a lightning strike. The eastern side of Eagle Mountain has experienced higher temperatures than the western side, and it appears that the heat 'front' and zone of alteration have an irregular pattern, pointing to saturation of parts of the coal seam by water. Four types of pyrolytic carbon having distinct morphology, anisotropy and optical path with increasing temperature were observed. Reflectance of pyrolytic carbon falls within the zone of heataffected coals, whereas the optical path of heat-affected Seam 15 samples is different from that of fresh coal with increasing rank. Finally, the reflectance of vitrinite in heat-affected coal is higher than the reflectance of vitrinite in carbonaceous shale in the Seam 15 section. ' Geological Survey of Canada, Contribution # 14589.

0166-5162/90/$03.50

© 1990 Elsevier Science Publishers B.V.

18

t (IO(ll)ARZI ~ , N I ) I (IENIZtS

INTROI)U('TION

The petrology of heat-affected coals has been studied extensively by numerous workers (Clegg, 1955; Dutcher et al., 1966; Berkowitz, 1967; Jones and Creaney, 1977; Bostick, 1979; Creaney, 1986; Raymond and Murchison, 1989 ). Naturally heat-affected coals are frequently found in coal-bearing strata in western Canada (Pearson and Creaney, 1980; Bustin and Mathews, 1982, 1985: Goodarzi, 1987: and Goodarzi et al., 1988a). The heat-affected coals in western Canada are formed ( 1 ) due to intrusive implacement (Goodarzi and Cameron, 1989 ), (2) by in-situ ignition and continued burning of a coal seam due to forest fires (Bustin and Mathews, 1982, 1985; Goodarzi, 1987; Goodarzi et al., 1988a), ( 3 ) by lightning strike (Pearson and Creaney, 1980), and (4) by spontaneous combustion (Gentzis and Goodarzi, in press ). The coal-coke transformation in all above studies is directional and the coal became progressively transformed to coke with decreasing distance towards the source of the heat. The Fording coal property in British Columbia (Fig. 1 ) contains two different types of heat-affected coal: ( 1 ) a coal seam, which at present is being combusted and transformed partially to coke in the Aldridge Creek area (Bustin and Mathews, 1982, 1985; Goodarzi et al., 1988a, b) and (2) a highvolatile-bituminous coal seam (% Ro m a x = 0 . 8 4 ) in the Eagle Mountain section (Fig. 1 ), referred to as Seam 15, which underwent combustion due to a lightning strike (Pearson and Creaney, 1980 ). Pearson and Creaney (1980) examined the heat-affected coal seam from the eastern side of Eagle Mountain and pointed out that a 'lightning-strike' hypothesis can be substantiated on the basis of mineralogy of low-temperature coal ashes, the petrology of the seam, the absence of an igneous intrusion and localization of alteration. A thinner overlying coal seam has been reduced to ash and a two-coloured zone can be recognized in overlying and surrounding siltstone beds (Pearson and Creaney, 1980). The present study is an extension of the work of Pearson and Creaney (1980) and deals with the study of heat-affected coal from both eastern and western sides of Eagle Mountain as well as the contact, the extent, and zones of alteration of the coal seam over a wider area. EXPERIMENTAL

Seventy-two samples were taken from twenty-one channels cut perpendicular to the seam's strike (Fig. 2 ). Samples were collected between the major partings in each channel. Petrographic analysis was carried out on polished blocks cut perpendicular to bedding and were prepared following the method of Mackowsky ( 1982 ). Fifty reflectance measurements in oil (~/oi~= 1.518 ) were determined using

319

REFLECTANCEAND PETROLOGICALVARIATIONOF A HEAT-AFFECTEDCOALSEAM TABLE 1 Maximum, minimum reflectance and thickness of channel samples used in this study Sample

Thickness

Ro m a x

Ro min

Sample

Section A A

130

1.37

1.27

Section K Ki

95

1.68

1.31

K2 K3

40 95

1.80 4.36

1.58 3.05

Section B

Thickness

Ro m a x

Ro rain

Bi B2 B3 B4

15 30 28 58

1.55 1.47 1.54 1.64

1.45 1.35 1.40 1.34

Section L Ll L2

80 28

1.90 3.42

1.67 3.29

Section C Ct C2 C3

23 40 70

1.78 1.95 3.39

1.55 1.62 2.73

Section M Ml M2 M3

75 90 58

2.90 3.43 4.90

2.21 3.17 4.47

Dl D2 D3

25 17 30

1.56 1.70 1.72

1.29 1.42 1.55

M4 M5

40 60

5.0 4.78

3.84 3.79

D4

30

1.90

1.57

Section N N

20

0.84

0.70

D5 D6

38 50

1.98 1.94

1.67 1.58

Section 0 O

25

0.90

0.75

Section E El E2 E3 E4 E5 E6 E7 Es

15 20 15 21 5 18 32 20

1.71 1.61 2.14 1.52 1.52 1.29 0.58 1.39

1.53 1.39 1.74 1.35 1.41 1.10 0.49 1.15

Section P P Sample Q

19

0.96

0.81

Qi Q2 Q3 Q4 Q5

20 12 45 48 70

1.75 1.68 2.75 2.84 2.89

1.67 1.53 2.51 2.70 2.58

Section F E9 Eto Ell

20 25 53

4.19 4.37 4.26

3.96 3.99 3.77

Section G Gl G2 G3

6 8 46

1.39 1.24 2.36

1.16 1.13 2.16

Section R RI R2 R3 R4 R5 R6

60 30 20 6 40 23

2.25 2.52 2.48 2.65 2.58 2.58

2.05 2.30 2.08 2.49 2.45 2.44

Section H HI H2 H3 H4

16 15 24 224

6.89 6.90 7.02 6.96

6.16 6.39 5.74 6.14

S~ $2 $3 $4 Section T

19 25 64 43

3.95 5.25 5.42 5.37

3.06 4.73 4.74 4.85

Section I I

8

2.75

2.17

T~ T2

60 245

2.0 2.01

1.69 1.71

43 30 50 40 60 15

1.72 1.69 2.36 2.78 2.73 2.96

1.42 1.53 1.91 2.50 2.64 2.74

Section U Ui U2

70 110

1.92 2.07

1.68 1.88

Section D

Section J

Jl J2 J3 J4 J5 J6

Section S

320

F. GOODARZI AND I (iENTZ1S

a Zeiss MPM II microscope connected to a Zonax microcomputer and printer. Bireflectance (Ro m a x - - R o min ) was calculated for both nongranular and granular components of the samples. All reflectance results are summarized in Table 1. RESULTS

The fresh coal and heat-affected samples are grouped according to their degree of metamorphism, geographical location, their reflectance and morphology into the following groups: (A) Unaltered coals (% Ro max=0.84--0.95). These coals are in the highvolatile-bituminous range which is the normal rank for Seam 15 (Goodarzi, 1986). All three maceral groups are present (Plate 1,a). Unaltered coals have been encountered in channels N, O and P (Fig. 2 ). (B) Altered coals (% Ro max= 1.2-2.0). Coals are altered to medium-lowvolatile bituminous range and, petrologically, consist of vitrinite and inertinite groups macerals. However, liptinite macerals are identified under crossed polars showing fine-grained mosaic texture (Plate 1,b). They occur in channels A, B, C, D, E, G, J, K, L, Q, R, T and U (Fig. 2). (C) Moderately altered coals (% Ro max= 2.0-3.5 ). These coals are altered to semianthracite-anthracite; they are anisotropic (nongranular) and often contain granular anisotropic liptinite (Plate 1,c) and pyrolytic carbon (Plate 2,d). They occur in channels C, E, G, I, J, L, M, Q, R, S, T and U (Fig. 2). (D) Strongly altered coals (% Ro max= 3.5-5.5 ). These samples are in the anthracite meta-anthracite range, are strongly anisotropic, contain pyrolytic carbon (Plate 2,g) and fine- to medium-grained mosaic on vitrinite (Plate 1,d). They are encountered in channels E, K, and S (Fig. 2 ). (E) Severely altered coals (% Ro max= 5.5-7.0). They are strongly anisotropic (nongranular), and contain pyrolytic carbon with granular anisotropy (Plate 2,b) and spherulitic form showing cross extinction (Plate l,h). They occur in channel H only (Plate 2,c and d). Groups B to E often contain pyrolytic carbon. Pyrolytic carbon has the following morphology: (a) Granular form (Plate 2,a and b ), which shows granular anisotropy and fine- to medium-grained mosaic texture and has a reflectance of% Ro max= 2.26.7. (b) Simple spherulitic form which shows a cross extinction pattern, has a reflectance of % Ro m a x 7.8--11.2 and occasionally may develop vesiculation (Plate 2,c and d). (c) Conical laminated form (% Ro max= 5.5-9.2 ), often filling cavities and cracks among macerals (Plate 2e ) or (d) Fining the boundaries of organic fragments (Plate 2,f) or occurring

L VARIATION OF A HEAT-AFFECTED COAL SEAM

~~~LK V COA ......rVea~~ CcR

oo~u....

2:'

~

BRITISH

c

[

\,, b

a

:,~

. .~.;',~.,;:-

; ~

.~,.

.

:.

Z ~."

. ....

,..

,~.': - . , . . ~ . .

~

_,--:

~

"

.

~r~'~'~"

,~ . . . . ~

EAGLE

~~/"

~

MOON~A,N

.

.

. ..'o ::-',:'i~$.;'~"!~ ";-'~'''~

~

~

,

J

.

. ~

.

,n,erO,~O,O~eOoen,, (S I c i c l a s t i c s )

W

F. G O O D A R Z I A N D

~orizontal 5 m k--4"

scale

scree

sealments .¢~h++mati~

~limnrmm

nf

~mm

1~

L=mnl~ IUInllntain

3pplies ,n 1 5

Lie

to

REFLECTANCE AND PETROLOGICAL VARIATION OF A HEAT-AFFECTED COAL SEAM

325

PLATE1

Morphology of heat-affected coals. Oil immersion, partially crossed polars except la, long axis of each photomicrograph is 240 am. a. Fresh unaltered coal showing vitrinite (V), inertinite (I) and large resinite body (R); reflectance (% Rom~) of this sample (N) is 0.84%. b. Fine-grained mosaic texture developed on former liptinite maceral, possibly folded cutinite (C). Note the presence of devolatilization vacuoles in the altered coal; sample C2 (% Ro max= 1.95). c. Fine-grained mosaic texture developed on formerliptinitemaceral, possiblyresinite (R) in a moderately altered coal sample. Sporinite (S) has not developed granular anisotropy, possibly due to its weathering; sample Ll (% Ro ~ = 1.90). d. Vitrinite particle showing medium-grained mosaic texture (M m ); sample J4 (% Romax= 2.78 ). w i t h i n vitrinite f r a g m e n t s (Plate 2,g). Reflectance ranges b e t w e e n % Ro 8.7 a n d 12.0.

max

DISCUSSION

T h e v a r i a t i o n o f the petrology a n d reflectance o f heat-affected coals f r o m S e a m 15 will be discussed in relation to: ( 1 ) the n a t u r e o f the coal seam; (2) the t e m p e r a t u r e o f heat-affected coal a n d residues; a n d (c) the presence a n d f o r m s o f pyrolytic carbon.

Z L~

D

N

7~

REFLECTANCEANDPETROLOGICALVARIATIONOF A HEAT-AFFECTEDCOALSEAM

327

Nature of coal seam Most coal samples from Seam 15 consist of vitrinite and inertinite and have optical properties of semianthracite to meta-anthracite in that they show basic anisotropy. The presence of nongranular, anisotropic vitrinite indicates that the coal may have been weathered before being thermally altered. Basic anisotropy can only develop in reactive macerals (i.e. vitrinite), after their oxidation and subsequent carbonization (Goodarzi and Murchison, 1976; Goodarzi and Marsh, 1980). Therefore the Seam 15 coal must have been very close to or at outcrop position before thermal alteration since coal outcrops are often weathered to various degrees. An oxidized coal of bituminous rank does not go through the plastic phase of carbonization and often keeps its original morphology (Goodarzi and Murchison, 1976). As a result, the morphology of a weathered coal seam may remain almost unchanged during natural combustion. This is the opposite of an unoxidized bituminous coal, where the thermal energy produced by the combustion of coal results in melting of the roof and transformation of coal into a cohesive, porous coke (Goodarzi et al., 1988a,b). The absence of coke, combustion channel and vents, and the nonporous and compact nature of the residues indicate that the transfer of heat through Seam 15 must have been slow.

Temperature of heat-affected coal and residues General It is possible to estimate the temperature of carbonization and combustion using the initial and terminal reflectance of vitrinite of the Seam 15 coal. Thus, PLATE2 Types of pyrolytic carbon in heat-affected coals. Partially crossed polars, magnification same as Plate 1. a. Pyrolytic carbon (PC) showing fine-grained mosaic filling cracks in a moderately altered coal sample; sample M~ (% R . . . . =2.90). b. Strongly anisotropic pyrolytic carbon (PC) present in a severely altered coal sample; sample H3 (% R . . . . =7.02). c. and d. Anisotropic pyrolytic carbon (PC) showing a simple cross extinction (c) in carbonized matrix of a severely altered coal sample; in d, the microscope's stage is rotated 45 ° to show the changes in extinction cross; sample H2 (% Ro max= 6.90). e. Conical-laminated pyrolytic carbon (PC) in a carbonized matrix; sample Eto (%R . . . . =4.37). f. Thin pyrolytic carbon (PC) lining the boundaries of an organic particle; sample H~ ( % R . . . . =6.89). g. Pyrolytic carbon (PC) filling cracks in vitrinite of a strongly altered coal sample; sample K3 (% R . . . . = 4.36).

F GOODARZI AND I. (;EN'IZIS

328

when plotting maximum versus minimum reflectance on Goodarzi's ( 1975 ) laboratory-produced curves for a coal of similar rank (% Ro max = 1.07 ) ( F i g . 3 ). The altered coal has been heated to about 500 ° C, the moderately altered coal to 570°C, the strongly altered to about 750 ° C, the severely altered coal to a temperature in excess of 750°C. Figure 3 shows that a meta-anthracite of % Ro max 7.0 developed during combustion at a temperature of approximately 775 °C and rate of heating of 1 ° C m i n - 1. The coal close to the combustion 'front' in self-burning coal seams, experiences a higher rate of heating, and the rate of heating decreases with an increase in the distance from the source of the heat (Goodarzi, 1987; Goodarzi et al., 1988a). However, such a change in the rate of heating is not evident in Seam 15 (Fig. 3 ), possibly due to total combustion and devolatilization of the coal in the combustion 'front' which was facilitated by high

15.0 --

, \

10.0 _ ae N

.E 0)

\

\

..y..< .... , ,."'" E

•°o•• °°

t

~ 4.0 E

%%,

~ ,~800

~600

-% : \ : % I

• East

i50

I~ 3 " O I c

o West

2"0 B ~ ' -

l/min

500

After Goodarzi

_1 1.0

t

......... lO/min

( 1975)

~ °

0

0o

" ~ .

"-L

1.0

\

2.0

L

1

1

3.0

4.0

5.0

Minimum r e f l e c t a n c e

60/min

__

1 10.0

in oi1%

Fig. 3. Reflectance of residues compared to those of laboratory-prepared vitrinite carbonized at different rates of heating (modified after Goodarzi, 19 7 5 ). A = unaltered coal; B = altered coal; C = moderately altered coal; D = strongly altered coal; E = severely altered coal.

R E F L E C T A N C E A N D P E T R O L O G I C A L V A R I A T I O N O F A H E A T - A F F E C T E D COAL SEAM

329

temperatures (27,000°C) (Uman, 1969 ) and an unknown but very fast rate of heating, in the order of 800°C min-1, or more. Samples from channel H are the nearest to the source of heat with the coal having a maximum reflectance of 7.0%. The extreme thermal gradient (0.78% Ro m - l ) calculated for the eastern side of Eagle Mountain further indicates the high rate of heating. When plotting the maximum versus minimum reflectance of pyrolytic carbon and vitrinite with basic anisotropy on Goodarzi and Norford's (1985 ) graph (Fig. 4), the following observations are made: ( 1 ) The oxidized vitrinite trend is almost linear and increases continuously from low to high reflectance values, thus deviating from the coalification curve for fresh coal and confirming the observations that the coal was weathered prior to being thermally altered. (2) Pyrolytic carbon follows a distinct path, and it is within the zone of unaltered coals.

Pyrolytic carbon Pyrolytic carbon forms from the volatile matter during thermal cracking of carbonaceous material and in naturally altered coal and coke. Natural phe18.0

16.0

14.0

coal 0 12.0 z .1

Fresh coal

~Z 10.0 < I¢J .J .-J 8 . 0

/

I ~

6.0

oO/

/"~Oxidized

,~

X <

I

coal

1.4

4.0

2.0

010

2.0

4.0

6.0

8.0

10.0

M I N I M U M R E F L E C T A N C E IN O I L ( % l

Fig. 4. Reflectance trends of heat-affected residues: oxidized ( • ) and pyrolytic carbons ( © ) from Seam 15 compared to the trends of fresh coal and heat-affected coal (modified after Goodarzi and Norford, 1985).

330

F. G O O D A R Z I AND ] G E N T Z I S

nomena, such as lightning strikes, peat and coal fires result in localized increased temperatures and thermal cracking of coal (Brown et al., 1966; Shibaoka et al., 1984; Goodarzi, 1985 ). Chandra and Taylor (1975) state that pyrolytic carbon found in thermally altered coals is formed from cracking of volatile matter. The laminated pyrolytic carbon in the samples has probably formed over an extended period of time, as successive volatile-matter generations deposited layer after layer of pyrolytic carbon (Goodarzi, 1985 ). On the contrary, the formation of well-developed spherulitic pyrolytic carbon indicates that the process was rapid. Pyrolytic carbon in Seam 15 occurs in all heat-affected samples. The type of pyrolytic carbon is related to the temperature of its formation. The weakly anisotropic to fine-granular type (Plate 2,a) indicates a low temperature, by contrast the coarse-grained, strongly granular type (Plate 2,b) is formed at much higher temperatures (Fig. 5 ). This is evident by the vitrinite matrix 12.0 l

d

Type thin

lining

11.0

•

j.i

pc~li " f

f-

I

• .J

10.0 1 /

•f

•

/

9.0-

/ /f

Type b spherulitic

,,

pc

,,"

•

f'

/ o //

/o

/

/ .o

../"/

8.0-

~""

.9 7.07, -8

ype c //

laminated

pc

/7

• 0" /'

>" 8 . 0 -

0///0 O

x

"i

5.0-

el

n-

4.0

Type a l e granular

03

3.0

o

2.0

~c

1.0 0.O

oo

110 210 a10 410 8'.0 810

,i0

%Ro(max)Vitrinite q

T°C decreases

Fig. 5. Optical paths of the four types of pyrolytic carbon identified in Seam 15 samples.

REFLECTANCE AND PETROLOGICAL VARIATION OF A HEAT-AFFECTED COAL SEAM

3 31

reflectance, where the low-temperature pyrolytic carbon ( % Ro max= 2.0-6.5 ) often occurs in chars with an Ro max of 1.5-3.5%, and the strongly anisotropic type (% Ro max= 8.7--11.7) is found in chars with reflectance of 7.0% Ro max. This indicates that the genesis of pyrolytic carbon formed in situ, is related to the temperature phase and that its parent material may have migrated variable distances to where the pyrolytic carbon is formed. All types of pyrolytic carbon observed in the Seam 15 coal are anisotropic and show distinct morphologies and optical paths. The mosaic size of the granular pyrolytic carbon (type a) increases with increasing reflectance and temperature and appears to be related to the spherulitic form (type b) at higher temperatures (Fig. 5 ). The trends of the laminated (type c) and the thin lining (type d) forms of pyrolytic carbon are quite distinct from the granular to spherulitic types (Fig. 5 ). The present results indicate that it is possible to distinguish the various types of pyrolytic carbon in heat-affected coals, group them according to morphology and reflectance and get an estimation of the range of temperatures responsible for their genesis (Fig.5).

Comparison of coals between the eastern and western sides The field observations indicate that there are distinct differences between the eastern and western sides of the heat-affected coal seam. These two heataffected areas are separated by an interval of about 60 m of coal seam which has not been affected at all by the combustion. Based on estimated temperatures, it is evident that the eastern side experienced much higher temperatures than the western with a gap in between (channels N, O and P, see Fig. 6 ). In the eastern side of Eagle Mountain the authors noticed two concentric zones of colour change overlying and surrounding Seam 15 (Figs. 1 and 2 ), which consist of a burgundy-colored zone and a yellow-colored zone. Similar observations have been made in areas where coal seams have undergone in situ combustion (Pearson and Creaney, 1980; Bustin and Mathews, 1982; Goodarzi et al., 1988a) and are indicative of heat effects. No such zones were observed in the western side of Eagle Mountain. Seam 15 in Eagle Mountain besides being accessible with great difficulty, also has an irregular roof to overlying sediment contact due to combustion (Fig. 7 ). In addition, the floor of the seam could not be located in every channel because of the scree sediments covering it. Figures 6 and 7 indicate that channel H in the eastern side of Eagle Mountain must have been closer to the source of heat, while the area west of the adits in the locality of channels N, O and P was not affected at all. Figure 7 shows the correlation of intervals, which can be traced along the strike of the seam, based on their reflectance. Despite

East

1.0 ¸

2.0

3.0 ¸

4.0

5.0

/, ~

~ ~

max (low)

eRo

(high)

max

0Ro

~ &

h 5m

;

j Adit

~L-~

,',', C

Adit H

A

N

N

E

L

S

Water Saturated

( ce~

Fig. 6, Section along strike of Seam 15 showing the alteration zones and distance from source of heat.

ae

#

x

6.0

7.0

8.0

of

Seam

west

Z

t S

x Projection 5m

e

•

/

Points

8

Adit C

Adit H

A

N

N

E

L

S

Water Saturated

(Ice?)

•

•

|

IL

TOp of S e a m

I

Fig. 7. Section along strike of Seam 15 showing isoreflectance lines between the eastern and western parts of Eagle Mountain. Vertical scale is based on the reflectance variation only; true thickness of seam is relatively constant.

1.0-

of ~

sea~../

3 . 0 - Top

4.0-

E 5.o-

6.0-

7.0-

8.0-

L~

m

m

©

z

t"'

o _e

0

I"11

z

m

z

r-

.],34

l: ( i ! ) ( ) I ) & . R X I . t N [ )

[ (;I~NqZI%

the irregularity in the shape of the seam's rook isoreflectance lines correspond very well between the eastern and western sides of the mountain. Figure 7 also shows the extremely high reflectance gradients, which are one hundred thousand times greater than the normal for this area of the Rocky Mountains ( Pearson and Creaney, 1980 ). These gradients could not have been produced by anything other than an igneous intrusion or a lightning strike but since no igneous rocks are present in the vicinity of Eagle Mountain, the only plausible explanation may be that heat generated due to lightning was responsible for combusting parts of Seam 15. The discussion could be incomplete without a brief expansion on the lightning hypothesis.

Lightning-strike hypothesis Pearson and Creaney (1980) excluded the possibility that heat generated from the spontaneous combustion of a 1-m-thick coal seam, located about 7 m above Seam 15, could have been adequate to produce dramatic rank changes. They concluded that lightning strikes have caused these rapid changes along strike of the seam resulting in very high gradients ( ~ 0.78% Ro m - ~). The main question to be answered remains. Was the heat responsible for thermal alteration of the Seam 15 coal generated by a lightning strike, or possibly by numerous strikes? 'Fossil' lightning strikes approximately 250 million years old have been reported in the literature (Harland and Hacker, 1966 ). The structures produced are known as fulgurites, tubes formed in sand as a result of fusion by a lightning strike. The above authors indicate that the local air temperature is raised to 30,000 ° C, giving luminosity for 100 microsecs after the strike. U m a n ( 1969 ) has measured a temperature of 27,000 °C in a lightning bolt, but it is unlikely that all of this energy would be transferred to the strike surface (Pearson and Creaney, 1980). However, if one accepts the possibility that a lightning strike has impacted Seam 15, would it be possible to have as many as 5 or 6 strikes in the same seam. Harland and Hacker ( 1966 ) state that expansion of gas causes thunder and since an ionized path has formed by current movements in both directions several strikes may occur in quick succession. The present study shows that the rank gradient in the eastern side of Eagle Mountain may be as high as 0.78% Ro m - l , ten times higher than that reported by Pearson and Creaney ( 1980 ). The rank gradient in the western side of Eagle Mountain is 0.20% Ro m -t , approximately four times lower than the rank gradient responsible in the eastern side. If the temperature was 1,000 °C as estimated at a rate of heating of 1-2 ° C m i n - 1, then the seam could have been combusted in a matter of a few days. The shape of the curves in Figures 6 and 7 indicates that the heat 'front' whether a result of a lightning strike or not - must have been irregular. The occurrence of unaltered coal in channels N, O and P indicates that for area-

REFLECTANCE AND PETROLOGICAL VARIATION OF A HEAT-AFFECTED COAL SEAM

335

son not clearly understood yet, burning of Seam 15 was halted in that vicinity. The evidence is inconclusive but the presence of a localized water saturation or ice sheet may explain this sudden drop in reflectance. If the coal seam happened to be water saturated, then it is unlikely that heat could be transferred through the water-saturated part of the seam. This agrees with Ashton and Bonney (1896) who state that fulgurite tubes are rarely formed in water-saturated rocks.

Variation of reflectance in coal partings Variations in reflectance of organic matter in association with different lithologies have been reported previously for unaltered coals (i.e., Teichmiiller and Teichmiiller, 1968; Jones et al., 1971; Bostick and Foster, 1975; Timofeev and Bogolyubova, 1975; Goodarzi et al., 1988a,b). The above studies show that reflectance of organic matter in shale is lower than that in coal. By contrast, organic matter in carbonates has higher reflectance than in coal. The vertical vitrinite-reflectance variation is determined for channel E (Fig. 8 ). Measurements clearly show consistent differences in the reflectance obtained in accordance with the hosting matrix. Channel E contains numerous carbonaceous shale intervals interbedded with coal. Organic matter in coke has the highest (% Ro max> 4.0) and the heat-affected shale-hosted organic matter has the lowest reflectance (% Ro max< 2.0, see Fig. 8). The present results confirm that this relationship is maintained when both coal and carbonaceous shale are subjected to almost similar thermal stress.

Comparison with freshly burning coals in western Canada When comparing Seam 15 with that at Aldridge Creek described by Goodarzi et al. (1988a), the following similarities and differences become apparent: ( 1 ) The coal at Aldridge Creek is medium-volatile bituminous (% Ro max I. 1 ) whereas in Seam 15 it is high-volatile bituminous (% Ro max 0.83). (2) Aldridge coal is transformed in the distillation zone into a cohesive, porous coke and coal-tar pitch because it is fresh. In contrast, the absence of cohesive coke, similar to commercial coke in Seam 15 is attributed to its oxidation prior to being heat effected. ( 3 ) Morphology (internal layering of coal) is destroyed or distorted in the Aldridge coal seam, by contrast, it is almost intact in Seam 15. (4) Carbonization of the Aldridge coal was rapid (rate of heating 10-60 ° C

~: h ) ( ) D ~ R Z I A N D I (;ENTZIS

336

m i n - ~) by contrast, carbonization in Seam 15 with slow (rate of heating of l-2°Cmin-~). When comparing Seam 15 with that at Coalspur described by Goodarzi

3.0

roof

platform

sediments o

o %11

o o o OoO oo o oe o c

° °°° ° °10 o°o °

o°ooo

°° ° °° o ooO°Oo

9

2.0 8

E

7

(/)

6

(/) nl

z 5

0I m 3: :-----------------.

1.0 " _ : --~

JA---- 2-_~ 3

Z-_----_--_--_~

li

4

2

. o

0.O

1.0

' 1.0

2:0

3 .' 0

4i0

5 .' 0

% Roil V I T R I N I T E high temp char

~

low temp. char

~

red clay

Fig. 8. Variation of reflectance of vitrinite residues in channel E.

carbonaceous shale

REFLECTANCE AND PETROLOGICAL VARIATION OF A HEAT-AFFECTED COAL SEAM

337

(1987), the following similarities and differences become apparent: ( 1 ) There is a rank difference (% Ro max 0.70 for Coalspur and % Ro m a x 0.84 for Seam 15 coals). Heat generated due to combustion in both cases altered the coal to a nonporous, consolidated char and did not produce cohesive, porous coke. (2) The morphology of coal seams in both Coalspur and Seam 15 remained almost intact, perhaps due to low rank of Coalspur coal and weathering of Seam 15. (3) The maximum temperature of combustion is estimated at 700 °C for the Coalspur coal, approximately 400 °C lower than the temperature in Seam 15. (4) Combustion of coal in Coalspur is limited to the area of the seam in direct contact with air and the coal became oxidized prior to being combusted. No such oxidation zone is detected in the Seam 15 coal and combustion affects the upper 1 m of the 5-m-thick seam only. CONCLUSION

Thermally altered coals of very high rank (% Ro m a x 4.0-7.1 ) have been found within a high-volatile bituminous coal seam on Eagle Mountain, British Columbia. A detailed petrological study of these coals indicates the presence of mosaic-textured bodies such as cutinite and resinite within a nongranular vitrinitic matrix, and pyrolytic carbon. The limited presence of mosaic on vitrinite is an indication that the coal seam was probably weathered prior to being heat affected. Four types of pyrolytic carbon having distinct morphology, anisotropy and optical paths with increasing temperature are observed. The optical path of heat-affected coals from Seam 15 is different than that of fresh coal with increasing rank. By contrast, the pyrolytic carbons fall within the zone of heat-affected coals. There is evidence that the coal was subjected to localized temperatures as high as 1,000°C. The extremely high rank gradients (up to 0.78% Ro m-~ ) point to a lightning strike but this is only a hypothesis at present. Whatever the cause of the high rank, the heat 'front' was irregular pointing to water saturation of parts of Seam 15 and the eastern side of the mountain experienced higher temperatures than the western side. Finally, the reflectance of low-temperature char is higher than fragmentary char in shale in the heat-affected section. A similar relationship is observed in coal-bearing strata between coal and carbonaceous shale (Goodarzi et al., 1986b).

338

t: G O O D A R Z I A N D ' I { i E N q Z I S

ACKNOWLEDGEMENTS

The authors would like to thank Fording Coal for allowing access to Eagle Mountain and particularly Mr. P. Daignault, Mine Geologist, for assisting in sampling Seam 15. Dr. A.R. Cameron of the Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, critically reviewed the manuscript. In addition, the authors would like to thank Mrs. R. Schultz and Ms. J. Moir for typing various drafts of the manuscript, and Mrs. M. Goodarzi for technical assistance.

REFERENCES Ashton, E. and Bonney, T.G., 1896. On an Alpine nickel bearing serpentine with fulgurites. Q. J. Geol. Soc. London, 52: 452-460. Berkowitz, N., 1967. The coal-carbon transformation: basic mechanisms. Symposium on the Science and Technology of Coal, Ottawa, Department of Energy Mines and Resources, pp. 149-155. Bostick, N.H., 1979. Microscope measurement of the level of catagenesis of solid organic matter in sedimentary rocks to aid exploration for petroleum and to determine former burial temperatures - A review. In: P.A. Scholle and P.A. Schluger (Editors), Aspects of Diagenesis. Soc. Econ. Palaontol. Mineral., Spec. Publ., 26:17-43. Bostick, N.H. and Foster, J.N., 1975. Comparison of vitrinite reflectance in coal seams and in kerogen of sandstones, shales and limestones in the same part of sedimentary section. In: B. Alpern (Editor), Colioque International, P6trographie de la mati6re organique des s6diments. Paris, pp. 13-25. Brown, H.R., Hesp, W.R. and Taylor, G.H., 1966. Carbons obtained by thermal and catalytic cracking of coal tars. Carbon, 4:193-194. Bustin, R.M. and Mathews, W.H., 1982. In situ gasification of coal, a natural example: history, petrology and mechanics of combustion. Can. J. Earth Sci., 19:514-523. Bustin, R.M. and Mathews, W.H., 1985. In situ gasification of coal, a natural example: additional data on the Aldridge Creek coal fire, southeastern British Columbia. Can. J. Earth Sci., 22: 1858-1864. Chandra, D. and Taylor, G.H., 1975. Thermally altered coals. In: Stach's Textbook of Coal Petrology, In: E. Stach, M.-Th. Mackowsky, M. Teichmiiller, G.H. Taylor, D. Chandra and R. Teichmiiller (Editors), Gebruder Borntraeger, Berlin-Stuttgart, p. 165. Chatterjee, N.N., Chandra, D. and Ghosh, T.K., 1964. Reflectance of Poniati seam affected by mica periodotite dyke. J. Mines Met. Fuels, 12: 346-348. Clegg, K.E., 1955. Metamorphism of coal by periodotite dikes in southern Illinois. I11. State Geol. Surv. Rep. Invest., 178, 18 pp. Creaney, S., 1980. Petrographic texture and vitrinite reflectance variation on the Alston Block, Northeast England. Proc. Yorks. Geol. Soc., 42(32): 533-580. Dutcher, R.R., Campbell, D.L. and Thornton, C.P., 1966. Coal metamorphism and igneous intrusions in Colorado. In: R.F. Gould (Editors), Coal Science, Advances in Chemistry. Am. Chem. Soc., 55: 708-723. Gentzis, T. and Goodarzi, F., 1989. Organic petrology of a self-burning coal wastepile from Coleman, Alberta, Canada. Int. J. Coal Geol., 11: 257-273. Goodarzi, F., 1975. Some factors affecting the carbonization behaviour of coal macerals. Ph.D. Thesis, Univ. of Newcastle-upon-Tyne, UK, 347 pp.

REFLECTANCE AND PETROLOGICAL VARIATION OF A HEAT-AFFECTED COAL SEAM

339

Goodarzi, F., 1985. Characteristics of pyrolytic carbon in Canadian coals. Fuel., 65: 1672-1676. Goodarzi, F., 1987. Reflectance and petrology of a burning bituminous coal seam. Fuel, 66: 1073-1078. Goodarzi, F. and Cameron, A.R., 1989. Organic petrology of thermally altered coals from Telkwa. British Columbia. Contrib. Can. Coal Geosci., Geol. Surv. Can. Pap., 89-8: 96-103. Goodarzi, F. and Marsh, H., 1980. Retention of plant cell structures in carbons (HTT 873K) from pre-oxidized vitrinite carbonized under pressure of about 200 MPa. Fuel, 59:269-271. Goodarzi, F. and Murchison, D.G., 1976. Petrography and anisotropy of carbonized pre-oxidized coals. Fuel, 55: 141-147. Goodarzi, F. and Norford, B., 1985. Graptolites as indication of the temperature history of rocks. J. Geol. Soc. London, 142: 1089-1099. Goodarzi, F., Gentzis, T. and Bustin, R.M., 1988a. Reflectance and petrology profile of a partially combusted and coked bituminous coal seam from British Columbia. Fuel, 67: 12181222. Goodarzi, F., Gentzis, T., Feinstein, S. and Snowdon, L.R., 1988b. Effect of maceral subtypes and mineral matrix on measured reflectance of subbituminous coals and dispersed organic matter. Int. J. Coal Geol., 10: 383-398. Harland, W.B. and Hacker, J.L.F., 1966. Fossil lightning strikes 250 million years ago. Adv. Sci., April 1966:663-671. Jones, J.M. and Creaney, S., 1977. Optical character of thermally metamorphosed coals of Northern England. J. Microsc., 109( 1 ): 105-118. JOnes, J.M., Murchison, D.G. and Saleh, S.A., 1971. Variation of vitrinite reflectivity in relation to lithology. In: H.W. Gaertner and H. Wehner (Editors), Advances in Organic Geochemistry. Pergamon Press, New York, NY, pp. 601-612. Mackowsky, M.-Th., 1982. Methods and tools of examination. In: E. Stach, M.-Th. Mackowsky, M. Teichmiiller, G.H. Taylor, D. Chandra and R. Teichmiiller (Editors), Stach's Textbook of Coal Petrology, Gebruder Borntraeger, berlin-Stuttgart, p. 295. Pearson, D.E. and Creaney, S., 1980. Spontaneous carbonization of oxidized high-volatile coal by a lightning strike. Can. J. Earth Sci., 17: 36-42. Raymond, A.C. and Murchison, D.G., 1989. Organic maturation and its timing in a Carboniferous sequence in the central Midland Valley of Scotland: Comparisons with northern England. Fuel, 68: 328-335. Shibaoka, M., Foster, N.R., Okada, K. and Clark, K.N., 1984. Formation of pyrolytic carbon in a continuous reactor for coal hydrogenation. Fuel, 67:169-173. Teichmiiller, M. and Teichmiiller, R., 1968. Geological aspects of coal metamorphism. In: D. Murchison and T.S. Westoll (Editors), Coal and Coal-bearing strata. Oliver and Boyd, Edinburgh, pp. 233-267. Timofeev, P.P. and Bogolyubova, L.I., 1975. Relation of changes of organic and clay substances in deposits of the recent peat accumulation areas. In: B. Alpern (Editors), Colloque Internatinal P6trographie de la mati~re organique des s6diments, Paris, 1973, pp. 153-172. Uman, M.A., 1969. Lightning. McGraw-Hill, New York, NY.