Petrological and geochemical characterization of Candiota coal seams, Brazil — Implication for coal facies interpretations and coal rank

International Journal of Coal Geology 64 (2005) 217 – 238 www.elsevier.com/locate/ijcoalgeo

Petrological and geochemical characterization of Candiota coal seams, Brazil — Implication for coal facies interpretations and coal rank M.B. Silva, W. Kalkreuth* Laborato´rio de Ana´lises de Carva˜o e de Rochas Geradoras de Petro´leo, Instituto de Geocieˆncias, UFRGS, Av. Bento Gonc¸alves, 9500, CEP.: 91501-970, Porto Alegre, RS, Brazil Received 5 January 2005; accepted 27 April 2005 Available online 14 June 2005

Abstract Seven coal seams, from cores of the borehole M-IV-C-101, Candiota Coalfield located in Rio Grande do Sul State, southern part of the Parana´ Basin were analyzed by coal petrographical and organic geochemical methods. The seams occur in the Rio Bonito Formation, Guata´ Group, Tubara˜o Supergroup of the Parana´ Basin and are of Permian (Artinskian–Kungurian) age. The composition, rank, coal facies and depositional environment characterization were made based on lithotype, maceral and microlithotype analyses. The data were then used in the definition of different types of facies-critical petrographical indices and were displayed in diagrams of coal facies. In addition, coal seams samples from Candiota Mine were analyzed by proximate, total moisture and gross calorific values and were classified using the United Nations-Economic Commission for Europe (UN-ECE) classification. In addition, Rock-Eval Pyrolysis, Liquid Chromatography, GC and GC–MS analyses were made to obtain independent parameters on coal rank and organic matter composition and environment of deposition. The results suggest deposition in open mires, with elevated contents of detrital macerals and clay minerals and also limnotelmatic coal facies. Coal-bearing sequence of coal seams represent a back-barrier setting. According to the International Classification (UN-ECE), Candiota coals are in the subbituminous (or low rank coal A) stage. Results from geochemical analyses showed that terrestrial organic matter is predominant as indicated by hydrogen and oxygen indices, carbon preference indices and the sterane distribution. Geochemical maturity parameters such as the 20 S / (20 S + 20 R) and ahh / (ahh + aaa) ratios for C29 regular steranes support the low level of rank (maturity) as indicated by the vitrinite reflectance measurements (0.41–0.45%) and gross calorific values (b 24 MJ/kg, m, af). D 2005 Elsevier B.V. All rights reserved. Keywords: Parana´ Basin; Permian coal; Coal facies; Coal rank; UN-ECE classification

* Corresponding author. Tel./fax: +55 51 3316 6355. E-mail address: [email protected] (W. Kalkreuth). 0166-5162/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2005.04.003

218

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

Marques-Toigo, 2000; Cazzulo-Klepzig, 2001) suggests the predominance of pteridophitic flora. Other palynomorph assemblages include Gymnosperma (Coniferales and Cordaitales) and pteridospermales, including Glossopteris (Meyer, 1999), but quantitatively constitute only a minor proportion. The palynological data suggest (Cazzulo-Klepzig, 2001) that lycopods (herbaceous and arborescent) dominated in the coastal mires, whereas Glossopteris, Cordaitales and Coniferales occupied the more distant and drained areas. Studies integrating results from coal petrography and organic geochemistry on Brazilian coals are rare (Henz, 1986; Arau´jo et al., 1995; Correˆa da Silva et al., 1985, 1987, 1996) and have been focussing on the coalfields in the northeastern part of Rio Grande do Sul and Santa Catarina. The only geochemical study in the Candiota Coalfield is that of Miranda et al. (1999), describing in detail the aliphatic and aromatic hydrocarbon composition of the I4 and I2 coal seams. The study revealed a new series of saturated and aromatic bicyclic compounds, possibly related to a conifer resin-derived tricyclic diterpenoid. The C29– C32 hopanes determined in that study had ha and hh stereochemistries confirming the low degree of rank (maturity) of Candiota coal (%Rr = 0.4; Silva, 1994). The aim of the present paper is to characterize coal seams from the Malha IV area of Candiota mine

1. Introduction The Candiota Coalfield is located in the southern part of Rio Grande do Sul, Brazil, and covers an area of approximately 2000 km2. The coal occurrences are assigned to the Rio Bonito Formation (Schneider et al., 1974), a fluvial to marine sandstone and shaleprone unit of Early Permian age (Artinskian–Kungurian) (Fig. 1). In general, the Candiota coal seams are characterized by high ash yields in excess of 50 wt.%. The organic matter is intimately mixed with the mineral matter and a number of seams are characterized by relatively high liptinite and/or inertinite contents (Silva, 1994, 1999; Holz and Kalkreuth, 2004). Studies by Correˆa da Silva (1981) suggest that the coal seams have petrographical characteristics indicative of an origin in limno-telmatic mires, where arborescent and herbaceous plant material accumulated after some transport. More recently, Holz et al. (2002) and Holz and Kalkreuth (2004), focusing on sequence stratigraphic interpretation of the coal-bearing strata, showed that the precursor mires of the coal seams developed in a lagoonal/back-barrier depositional setting, and that coal seam properties were found to change in respect to their stratigraphic position. The distribution of palynomorph assemblages in the Candiota coal seams (Meyer, 1999; Meyer and South America

Rio Grande do Sul State

Stage 0

Serra Geral Plateau (volcanic rocks)

1200 km

BRAZIL

Lithostratigraphy Passa Dois Group

KUNGURIAN

Paraná Basin

Guatá Group

Palermo Formation Rio Bonito Formation

Itararé Group

Rio do Sul Formation

ARTINSKIAN

Rio Grande do Sul shield Paleozoic-Mesozoic sedimentary rocks

SAKMARIAN

Irati Formation

lithostratigraphy after Schneider et al. (1974) 0

200 Km

Porto Alegre

Fig. 1. Location of study area and stratigraphic position of the coal-bearing Rio Bonito Formation.

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

(seams Inferior 4 to 1, seam Camada Candiota Inferior, seam Camada Candiota Superior and the Banco Louco seam), based on coal petrographical and organic geochemical analyses. Coal petrological studies were performed with focus on coal composition and evaluation of paleoenvironmental deposition based on lithotype, maceral and microlithotype analyses. Organic geochemical analyses were carried out to characterize chemical composition of the organic matter to further enhance our understanding of organic matter type, depositional environment and rank (maturity). Another objective of the present paper is to demonstrate the applicability of the International Classification of in-seam coals (UN-ECE, 1998) for the Candiota coal seams.

2. Sampling, area description and methods The Candiota Mine area has been subdivided by the Companhia Rio-Grandense de Minerac¸a˜o (CRM) in 15 areas, which are named Malha I, II etc. In the area of Malha IV (Fig. 2), which is subject of this study, seven coal seams are present ranging from 0.1 to 3.2 m in thickness. The seams are named from the base to the top: Camada Inferior 4 (I4), Camada Inferior 3 (I3), Camada Inferior 2 (I2), Camada Inferior 1 (I1), Camada Candiota Inferior (CCI), Camada Candiota Superior (CCS), and Camada Banco Louco (BL). In the area of Malha IV, the coal seams are associated with sandstone, siltstone and mudstone layers. The seams were collected as core samples from borehole M-IV-C-101, drilled in the Malha IV area, sector C (UTM 6501701.05 N/ 242 486. 65 E) (Fig. 2). Coal lithotype analyses followed the Australian system (Diessel, 1965), which describes the lithotypes in terms of bright and dull proportions (bright, banded bright, banded coal, banded dull, dull, fibrous). Maceral, microlithotype analyses and random reflectance measurements on vitrinites (%Rr) were performed according Bustin et al. (1983). In addition, the maceral sporinite was subdivided in the maceral varieties microsporinite and macrosporinite. Reference in text to sporinite contents includes both varieties. Note that maceral groups and maceral contents are discussed on a mineral matter-free basis. Mineral matter content determined by maceral analysis is reported in volume

219

percent, whereas mineral matter (ash) determined by proximate analysis is reported in weight percent. The classification of Candiota coals were made according to the International Classification of inseam coals according to UN-ECE (1998). This classification takes into account rank (calorific value, vitrinite reflectance), petrographic composition, and grade (mineral matter content). Candiota coal facies interpretations are based on facies-critical petrographical indices and coal facies diagrams (Hacquebard and Donaldson, 1969; Diessel, 1982, 1986; Marchioni and Kalkreuth, 1991; Calder et al., 1991). Organic geochemical analyses included total organic carbon (TOC) determination, Rock-Eval pyrolysis, bitumen extraction and Liquid Chromatography, Gas Chromatography, and Gas Chromatography/Mass Spectrometry. TOC was analyzed using a WR-12 LECO Instruments carbon elemental analyzer, in which samples are heated at 1200 8C for 2 min under a constant oxygen stream. TOC was measured by a thermal conductivity detector and the result was reported as atomic weight (%). Rock-Eval pyrolysis was carried out according to Espitalie´ et al. (1977) procedures. Bitumen was extracted in a Soxhlet apparatus with dichloromethane as solvent, and the organic extracts were separated via liquid-column chromatography. The GC analyses of aliphatics were performed on a Varian 3700 chromatograph equipped with a split injector and fitted to a 25 m
0.33 mm SE-54 column using hydrogen as carrier gas, with a temperature program of 40–280 8C/min. The GC–MS analyses were performed using a Varian 3700 chromatograph, A Varian 112-5 mass spectrometer coupled with a Varian 55-100-MS data acquisition system.

3. Results and discussion 3.1. Petrology of Candiota coal 3.1.1. Lithotype distribution Dull and banded dull coal lithotypes dominate in most coal seams (Fig. 3), whereas bright lithotypes are rare. The coal seams are often interbedded with continuous bands of coaly or carbonaceous mudstone. Dull coal is the most important lithotype in

220

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

53°45’ 240

242

53°41’14.7” 248

246

244

31°25’ Bagé 6520

Rio Grande do Sul State B R

6518

29 3

Serra Geral Plateau (volcanic rocks)

Seival 6516

Rio Grande do Sul shield Paleozoic-Mesozoic sedimentary rocks

Pelotas

6514

Airport 6512 Porto Alegre

6510 Barrage 6508

NM

CRM 6506

NG

NQ

TUPI SILVEIRA

Coal Powerplant

20 6504

Candiota Village II

45 15 23

PEDRAS PINHEIRO ALTAS MACHADO

8°39’ 1°22’36”

29 37 36 42

HULHA NEGRA SEIVAL TORRINHAS

Candiota Village I

6502

PASSO PASSO CERRO SÃO DA CHATO DIOGO ARREIA

Airport 101

6500

Highway Secondary Road City Candiota Power Plant Coal exploration borehole studied by Silva (1994) Coal exploration borehole this study

MalhaIV

6498

39°36’49” 1 : 250000 0

2

4

6 Km

Fig. 2. Detailed map of the Candiota area, showing borehole location, extension of Mine IV (see insert) and the borehole locations studied earlier (Silva, 1994) and location of the Candiota Power Plant.

Lithotype 1.20

Coastal Plain High Stand

3.59

6.85 7.82

Rr % 0 .45

Vitrinite Liptinite Inertinite Mineral Matter 0.5 0 10 20 30 40 50 60 1 0 1 5 2 0 2 5 3 0 35 10 20 30 40 50 60 70 5 10 15 20 25

0.97 BANCO LOUCO (BL)

9.45 9.65 2.24 CAMADA CANDIOTA SUPERIOR

(CCS)

11.89 12.46

Depth (m)

15.25 15.43 15.70 Transgressive

Lagoon/barrier island

2.79 CAMADA CANDIOTA INFERIOR (CCI)

18.59 19.05

sandstone siltstone mudstone

21.10

coal

1.48 INFERIOR 1 (I1) 22.58

Fluvial

Low Stand

24.15 24.62 25.30 25.53

0.68 INFERIOR 2 (I2)

26.75 27.18

0.43 INFERIOR 3 (I3)

28.59 28.71 29.18 29.99

--

bright coal banded bright coal banded coal dull banded coal dull coal carbonaceous rock

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

4.93 5.06 6.00

0.4

seam thickness (m)

Facies Systems Tracts

0.81 INFERIOR 4 (I4)

Fig. 3. Lithological column of borehole M-IV-C-101, showing coal seam distribution and thickness, lithotypes, vitrinite reflectance values, maceral groups (vol.%, mineral matter-free basis), and mineral content (vol.%). Depositional setting of Candiota coal-bearing strata as suggested by Holz (1998) and Holz et al. (2002). 221

222

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

I3 (Fig. 3), whereas I1 and BL show dominance either of dull coal or carbonaceous mudstone. I4 and I2 vary from banded to banded bright coal (Fig. 3), and CCI and CCS are dominated by banded dull and banded coal lithotypes. In general, coal brightness decreases from the base to the top of the coal-bearing sequence (Fig. 3). 3.1.2. Maceral and mineral matter distribution Maceral and mineral matter distribution in the Candiota coal seams collected from the Malha IV area are similar to those described in earlier studies (Correˆa da Silva and Marques-Toigo, 1985; Ade, 1993; Silva, 1994, 1999; Holz and Kalkreuth, 2004). Seams I4 and I2 are characterized by the highest contents of vitrinite group macerals and the lowest contents of mineral matter (Fig. 3). Seams I3 and I1 have the lowest vitrinite contents, whereas seam BL is extremely rich in inertinite (Fig. 3). CCI and CCS seams are characterized by roughly equal amounts of vitrinite and inertinite with only minor contribution from macerals of the liptinite group (Fig. 3). Coal brightness and/or dullness is controlled by petrographic composition (Fig. 3), with the degree of dullness related to mineral matter and inertinite contents. Microscopical results show that Candiota coals are rich in mineral matter (Fig. 3), which consists mostly of clay minerals, with minor contributions of quartz and pyrite. The majority of clay minerals occurs as organic matter-free layers, but occurs sometimes also in form of finely disseminated particles in vitrinite. Detrovitrinite, mainly in form of collodetrinite, dominates in the vitrinite group (Table 1) in all coal seams, and decreases from the base to the top in the coal-bearing sequence. Sporinite and liptodetrinite are the most important liptinite macerals (Table 1), and liptodetrinite content is highest at the top of the coal sequence (BL seam). Alginite also occurs in the liptinite group in form of Botryococcus and Portalites. Inertinite content increases from the base to the top of the coal-bearing sequence (Fig. 3). Fusinite, semifusinite and inertodetrinite are most common, with inertodetrinite content being particularly high in I3, I1, and BL coal seams (Table 1). BL shows the highest content of inertinite, mainly in the form of fusinite and semifusinite (Table 1). A transition from collotelinite to semifusinite (Fig. 4) is also observed, e.g. in I3 seam. Seams I1, I3 and BL are mineral matter-rich

Table 1 Macerals and mineral matter content of Candiota coal samples from borehole M-IV-C-101, Candiota Mine, RS, Brazil Seam Maceral groups (vol.%) Vitrinite

BL CCS CCI I1 I2 I3 I4

Liptinite

Inertinite

Mineral matter

Ct Cd Vd Ms Sp Li Cu Re Al Fu Sf In A

P

2 21 17 12 21 13 22

– – – – – 1 –

3 26 24 19 27 21 31

– 1 2 4 1 2 1

– 1 1 3 5 4 4

15 11 – 9 4 – 5 4 1 17 6 – 20 5 1 11 7 1 15 5 3

– – – 1 – 1 –

1 – – 1 – – –

36 19 22 14 9 15 10

19 9 14 10 6 12 7

13 10 10 13 5 13 2

25 23 22 22 5 17 7

BL — Camada Banco Louco; CCS — Camada Candiota Superior; CCI — Camada Candiota Inferior. Camadas Inferiores (I1, I2, I3 I4). Ct — collotelinite; Cd — collodetrinite; Vd — vitrodetrinite; Ms — macrosporinite; Sp — microsporinite; Li — liptodetrinite; Cu — cutinite; Re — resinite; Al — alginite; Fu — fusinite; Sf — semifusinite; In — inertodetrinite; A — clay minerals; P — pyrite.

coals (Fig. 3) associated with the highest contents of detrital macerals (vitrodetrinite, liptodetrinite, inertodetrinite; Table 1). 3.1.3. Microlithotype distribution Results from microlithotypes analyses show that carbargilite is the most common microlithotype in all seams except in I4 and I2 (40–49 vol.%). Vitriniterich clarite (Clarite V) dominates in seams I4 and I2, whereas durite shows the highest content in seam BL (Table 2). The dominance of carbargilite associated with vitrinite as described by Correˆa da Silva and Marques-Toigo (1985) for seams CCI and CCS has also been confirmed in this study. 3.2. Vitrinite reflectance In the Malha IV area, random vitrinite reflectance measurements range from 0.41% to 0.45% (Table 3), similar to vitrinite reflectances of 0.37–0.44% reported in an earlier study (Silva, 1994). Slightly higher reflectance values (0.51%) have been reported for other parts of the Candiota Mine (CIENTEC, 1980). 3.3. International classification of in-seam coal Prior to the classification of in-seam coal presented in this paper, a number of rank classifications for

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

223

Fig. 4. Photomicrographs of representative macerals in the Candiota coals under reflected white light and oil immersion (Magnification = length of X-axis approx. 200 microns): (a) transition from collotelinite to semifusinite, seam I3 (CT=collotelinite, SF=semifusinite); (b) collotelinite associated with enrichment of microsporinite and cutinite, seam I2; (c) funginite (?), microsporinite, macrinite, and inertodetrinite in collodetrinite, seam Camada Candiota Superior (CCS); (d) abundant detrital macerals in mineral ground mass, seam I1.

Candiota were proposed. This included the study of Nahuys and Caˆmara (1972) that classified beneficiated Candiota coal as a Flambantsec according to the French classification system. Ferreira (1976) classified Candiota coal as a high volatile C bituminous according to ASTM using calorific value and volatile matter. Correˆa da Silva and Wolf (1980), Correˆa da Silva and Marques-Toigo (1985), and Correˆa da Silva (1989) have used random vitrinite reflectance, total

organic carbon, moisture, and volatile matter content of vitrain to classify the Candiota coals as a subbituminous A/high volatile C bituminous coal. Following the same system and analyzing samples of several coal seams from different locations, Silva (1994) suggested a subbituminous A coal rank for Candiota coal. The difficulty in correlating chemical and physical rank parameters of the Candiota coals was pointed out by Correˆa da Silva and Wolf (1980) and Correˆa da

224

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

Table 2 Microlithotype distribution (vol.%) in Candiota coal samples from borehole M-V-C-101, Candiota Mine, RS, Brazil Seam

V

LI

IN

CLV

CLL

BL CCS CCI I1 I2 I3 I4

– 12 17 5 8 10 12

2 – – – 8 3 3

16 10 8 9 8 9 12

– 2 1 7 23 3 30

– – 1 8 1 3

VV

VI

DUL

DUI

DC

CD

VTL

CA

M

9 8 2 2 1 2

1 4 6 1 2 1 3

4 2

26 10 3 9 4 10 7

2 8 3 4 8 8 10

3 4 2 1 4 5 5

– – – – 3 1 1

37 34 35 49 10 40 11

9 5 17 9 4 4 0

3 8 4 1

V — vitrite; LI — liptite; IN — inertite; CLV — clarite V; CLL — clarite L; VV — vitrinertite V. VI — vitrinertite I; DUL — durite L; DUI — durite I; DC — duroclarite; CD — clarodurite; VTL — vitrinertoliptite; CA — carbargilite; M — minerite. BL — Camada Banco Louco; CCS — Camada Candiota Superior; CCI — Camada Candiota Inferior. Camadas Inferiores (I1, I2, I3, I4).

Silva (1989). According to these studies the impregnation of vitrinite by lipoid substances during the process of coalification may result in a slight suppression of vitrinite reflectance values in Candiota coals. The mean random vitrinite reflectance values determined in the present study range from 0.41% to 0.45%, suggesting a subbituminous C/B coal rank. According to Silva (1994), these random values, possibly lowered by the high mineral matter content of the samples, may indicate a somewhat lower rank of the coals when compared to results from chemical rank parameters. Kalkreuth et al. (2004), combining vitrinite reflectance (VR) and fluorescence alteration of multiple macerals (FAMM) analyses, provided insights into the chemical nature of vitrinites (i.e. perhydrous vs.

orthohydrous vs. subhydrous composition) in Permian Gondwana coals of the Parana´ Basin, Brazil. In the Candiota Coalfield, the differences between FAMMderived equivalent VR (EqVR) and directly measured VR range from 0.08–0.13% indicating a subhydrous nature of the coals. The causes for VR suppression are most likely related to a combination of factors such as liptinite and mineral matter contents (Kalkreuth et al., 2004). Using the VR (EqVR) values obtained by the FAMM method a subbituminous A rank for the Candiota coals was suggested. In the present study the CCI and CCI seams of the Candiota Coalfield are classified according to the International Classification of in-seam coals (UNECE, 1998), which takes into account rank, petrographic composition, and grade (ash) (Fig. 5 A,B).

Table 3 Vitrinite random reflectance range in Candiota coal, Candiota Mine, RS, Brazil SEAM BL CCS + CCI CCS CCI I1 I2 I3 I4 1

%Rr1

%Rr2

%Rr3

0.47 F 0.05 0.51 F 0.05 0.46 F 0.04

0.40 F 0.04 0.40 F 0.04

%Rr4

%Rr5

0.37–0.42 0.37–0.51 0.41–0.43 0.40–0.43 0.40–0.43 0.40–0.43 0.38–0.41 0.39–0.43

0.44 F 0.04 0.41 F 0.03 0.41 F 0.05 0.41 F 0.03 0.45 F 0.03 0.42 F 0.04 0.43 F 0.04

Nahuys and Caˆmara (1972) 2 CIENTEC (1980) 3 Correˆa da Silva and Marques-Toigo (1985) 4 Silva (1994) — range of random reflectance This study. BL — Camada Banco Louco; CCS + CCI — Camada Candiota Superior + Camada Candiota Inferior; CCS — Camada Candiota Superior; CCI — Camada Candiota Inferior. I1, I2, I3, and I4 — Camadas Inferiores; %Rr — random reflectance. 5

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

225

A RANK GRADE

LOW B

A

D

C

B

HIGH A

C

B

A

L%

V%

I

High grade coal

Camada Candiota Inferior V

Ortho Meta-

LIGNITE

SUB-BIT

Low grade coal

Very low grade coal

PETROGRAPHIC COMPOSITION L

Sapropelic Coal

Carboonaceous Rock

Rock

Banded coal (mainly humic)

Oil Shale

Medium grade coal

Non-banded coal

C

MEDIUM

Para-Ortho Meta- per- Para- Ortho Meta-

BITUMINOUS

Domain of the Classification

I%

ANTHRACITE

Not to be included in the Classification

Ash (HT) mass%,db Rr % 100

80

50

30 20 10

0

0.6 20

15

1.0

1.4 2.0

3.0

4.0

GCV (MJ/kg, m.af)

24

B RANK GRADE

LOW B

A

D

C

B

HIGH A

C

B

A

L%

L

V%

I

High grade coal

Camada Candiota Superior

V

Ortho Meta-

LIGNITE

SUB-BIT

Very low grade coal

Low grade coal

Sapropelic Coal

Carboonaceous Rock

Rock

Banded coal (mainly humic)

Oil Shale

Medium grade coal

Non-banded coal

C

MEDIUM

Para-Ortho Meta- per- Para- Ortho Meta-

BITUMINOUS

I%

ANTHRACITE

Domain of the Classification Not to be included in the Classification

Ash (HT) mass%,db Rr % 100

80

50

30 20 10

0

0.6 15

20

24

1.0

1.4 2.0

3.0

4.0

GCV (MJ/kg, m.af)

Fig. 5. Classification of seams A) Camada Candiota Inferior (CCI) and B) Camada Candiota Superior (CCS) according to the International Classification of in-seam coals (UN-ECE, 1998).

226

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

! Gross Calorific Value (moist, ash free) b24 MJ/ kg, provided that Rr = 0.6%.

The objective of this classification is to create an instrument that permits the classification of coals and to contribute to the characterization of the coal deposits. The classification is not intended to be used for commercial or trade purposes. In this classification, the first step is to identify coal rank based on calorific value and vitrinite reflectance using the following thresholds:

Petrographic composition is shown in terms of maceral groups (mineral matter free) and grade is reported as ash yield. Following this concept (Fig. 5 A,B), the CCI and CCS seams are low rank coals (subbituminous or Low Rank A coal) because their gross calorific values (moist, ash free basis) are lower than 24 MJ/kg and their random vitrinite reflectance values are lower than 0.6% (Figs. 5 and 6). According to the amount of impurities (grade), CCI is classified as a carbonaceous rock and CCS is a very low grade coal (Fig. 5 A,B). In terms of petrographic composition, CCI seam is characterized by 38 vol.% vitrinite, 55 vol.% inertinite, 7 vol.% liptinite (on a mineral

1) Low rank coals are coals with: ! Gross Calorific Value (moist, ash free basis = m.a.f.) b 24 MJ/kg and Rr b 0.6% 2) Higher rank coals i.e. medium- and high-rank coals, are coals with: ! Gross Calorific Value (moist, ash free) z 24 MJ/ kg or

TREE DENSITY (+)

(-)

LIMNO-TELMATIC

TELMATIC

100.0 50.0

10.0 5.0

LIMNIC

GELIFICATION INDEX

Lower Delta Plain Marsh Upper Delta Plain

Transgressive

Wet Forest Swamp

I4 I2

Back Barrier

I1

1.0

CCS I3 CCI

Regressive

Piedmont Plain

Dry Forest Swamp

0.5

(TPI= 3.6)

Terrestrial 0.5

1.0

1.5

2.0

> BL

2.5

3.0

TISSUE PRESERVATION INDEX Facies Boundaries of Diessel (1985) Field of Cretaceous Strand-Plain coals of Kalkreuth and Leckie (1989) BL - Camada Banco Louco CCS - Camada Candiota Superior CCI - Camada Candiota Inferior I1, I2... - Camadas Inferiores 1, 2...

Fig. 6. Coal facies diagram as developed by Diessel (1986) and Kalkreuth and Leckie (1989) and position of Candiota coals analyzed in this study.

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

matter-free basis), and 39 vol.% mineral matter. The CCS seam is characterized by 39 vol.% vitrinite, 44 vol.% inertinite and 17 vol.% liptinite, and 16 vol.% mineral matter. 3.4. Coal facies Coal facies refers to the primary genetic types of coal, which are dependent on the depositional environment under which the peats originate. Maceral content, maceral ratios, microlithotypes and lithotype distribution, and mineral matter content and type have been used in numerous studies to evaluate the palaeoenvironments of the precursor mires (amongst others Hacquebard and Donaldson, 1969; Teichmu¨ller and Teichmu¨ller, 1975, 1982; Correˆa da Silva, 1981; Diessel, 1982, 1986; Matos, 1984; Kalkreuth and Leckie, 1989; Calder et al., 1991; Lamberson et al., 1991; Arau´jo et al., 1995; Silva, 1994, 1999). 3.4.1. TPI–GI concepts and coal facies diagram The concept, originally proposed by Diessel (1982, 1986), is based on the following maceral indices: Tissue Preservation IndexðTPIÞ ¼

telovitrinite þ semifusinite þ fusinite detrovitrinite þ macrinite þ vinertodetrinite ð1Þ

Gelification Index ðGIÞ ¼

vitrinite þ macrinite semifusinite þ fusinite þ inertodetrinite

ð2Þ

The TPI index is the ratio of structured to unstructured macerals of the vitrinite and inertinite groups. Coal with a high TPI contains abundant well preserved plant tissue. GI is the ratio of gelified to nongelified material and may therefore indicate relative humidity or oxidation in the mire. In the original Diessel diagram, eight different depositional environments are recognized: limno-telmatic, telmatic, marsh, fen, limnic, wet forest swamp, dry forest swamp, and terrestrial. TPI–GI values for the Candiota coals suggest for most coal seams a back-barrier depositional setting on the original Diessel TPI–GI diagram (Fig. 6), confirming earlier studies (Ade, 1993; Silva, 1994).

227

These results are also supported by sequence stratigraphic interpretations of the coal-bearing strata (Alves, 1994; Alves and Ade, 1996; Holz, 1996, 1998). These studies suggest that the Rio Bonito Formation forms part of a 3rd-order sequence, with major coal development in a transgressive system tract (Fig. 3), with peat accumulation in a backbarrier setting. It appears that the Diessel model is not applicable for the BL seam, enriched in structured inertinite (TPI = 3.6, Fig. 6). Petrographic results show that Candiota coal is inertinite-rich, suggesting a shallow or variable ground water table. Alternatively, the high inertinite content may reflect oxidation caused by oxygenated water in the mire and also wildfire. Fusinite is common in Candiota coal, indicating that forest fires in the vicinity of the mire occurred during peat accumulation. Studies on the origin of fusain and fusain-derived fusinite suggest that most, if not all, fusinite is derived from wildfire (Scott, 1989; Scott and Jones, 1994). I3 and I1 seams are characterized by high inertodetrinite and mineral matter contents, suggesting a higher energy level and organic matter circulation at the time of peat accumulation, with frequent inundations of the mire. 3.4.2. Ternary microlithotype coal facies diagram A facies diagram originally proposed by Hacquebard and Donaldson (1969) is based on four groups of microlithotype associations and suggests six coal facies environments (Fig. 7). The ternary diagram is actually subdivided into two parts: in the upper part, the dull component (D) constitutes less than 20% of the lithotype associations and is incorporated with component A at one corner of the triangle. In the lower diagram this component constitutes more than 20%, and when enriched in major concentrations are considered to indicate open-water moor facies. Correˆa da Silva and Marques-Toigo (1975) and Correˆa da Silva and Marques-Toigo (1985) applied this concept to interpret CCI and CCS coal facies, and together with observed maceral associations, recognized four coal-forming mire types: open water (limnic), limno-telmatic, telmatic, and terrestrial. The distribution of the microlithotype associations determined in the present study are shown in Fig. 7, modified slightly from the original facies diagram of Hacquebard and Donaldson (1969).

228

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238 Terrestrial Forest Moor

B

Telmatic Forest

Telmatic Reed Moor

Moor

C

A+D < 20 % of dull components (D) in coal 20 % of dull components (D) in coal

B+C

A I4

I2

Limno-telmatic Forest Moor

Limno-telmatic Reed Moor CCI CCS I3 I1 BL

A= clarite L + duroclarite + vitrinertoliptite + liptite B= inertite + vitrinertite I C= clarite V + vitrite + vitrinertite V D= clarodurite + durite + carbominerite + minerite BL- Camada Banco Louco CCS- Camada Candiota Superior CCI- Camada Candiota Inferior Camadas Inferiores (I1, I2, I3, I4)

D Open-water Moor

Fig. 7. Four-component facies diagram for the coal samples from borehole M-IV-C-101, Candiota Mine, RS, Brazil (modified from Hacquebard and Donaldson, 1969).

According to this figure open-water moor conditions are indicated for seams I3, I1, and BL, whereas CCS and CCI seams are located at the transition zone of an open-water/limno-telmatic forest moor. I4 and I2 seams have a microlithotype association indicating somewhat drier conditions within the limno-telmatic facies zone (Fig. 7), characterized by the highest clarite contents and the lowest volumes of carbominerite and minerite. The changes in microlithotype associations do not sug-

gest a change in vegetation contributing to the ancient mire, but are considered to relate to differences in the preservation of the organic material. 3.4.3. T–D–F coal facies diagram This coal facies diagram originally proposed by Diessel (1982), later modified by Marchioni and Kalkreuth (1991), is shown in Fig. 8. These were modified in the present study by addition of detrital resinite and cutinite to D, be-

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

D

through the addition of liptinite macerals and mineral matter contents to the maceral ratios: VI¼ ðtelovitrinite þ semifusinite þ fusinite

I1 I2

þ suberinite þ resiniteÞ=ðdetrovitrinite þ alginite þ liptodetrinite þ sporinite

I3 I4 CS

229

CI

þ alginite þ cutinite þ inertodetriniteÞ

BL

ð3Þ

GWI ¼ ðdetrovitrinite þ gelovitrinite þ mineral matterÞ=telovitrinite

T

F BL - Camada Banco Louco CCS - Camada Candiota Superior CCI - Camada Candiota Inferior I1,I2.. - CamadasInferiores 1, 2...

Fig. 8. T–F–D coal facies diagram in the position of Candiota coals, modified from Marchioni and Kalkreuth (1991). T = telovitrinite; F = semifusinite + fusinite; D = detrovitrinite + sporinite + liptodetrinite + resinite (detrital) + cutinite + alginite + inertodetrinite + detrital minerals (clay minerals + quartz).

ð4Þ

According to this concept the lower seams (I4, I3, I2, and I1) and CCI and CCS seams were likely deposited in a mire characterized by a rheotrophic hydrologic regime (Fig. 9) with higher supply of ions under conditions of both higher pH and groundwater level (Fig. 9). GWI values over 5 for the BL seam (Fig. 11) suggest a waterlogged environment (inundated marsh) as also indicated by a later study Groundwater Index (GWI) 100

cause liptinite macerals such as sporinite and alginite occur intimately associated with detrital mineral matter, suggesting a deposition in a marginal or subaquatic environment (open moor facies): T = telovitrinite; F = semifusinite + fusinite; D = detrovitrinite + sporinite + liptodetrinite + resinite (detrital) + cutinite + alginite + inertodetrinite + detrital minerals (clay minerals + quartz). According to Marchioni and Kalkreuth (1991), high values of D suggest a dominance of open moor peat facies, meanwhile higher values of F indicate terrestrial, and high T values suggest a telmatic coal facies. Applying this model to the Candiota coal seams the relatively high D values (Fig. 8) would suggest a subaquatic deposition of peat formation, characterized by a high content of detrital macerals and mineral matter. 3.4.4. GWI–VI coal facies diagram In order to characterize the hydrological regimes under which the precursor mires accumulated, a GWI–VI (Groundwater Index–Vegetation Index) diagram (Calder et al., 1991) was used. This diagram is a modification of the original Diessel TPI / GI ratios

50

BL 10 5

Inundated Marsh

Inundated Forest

I1 I3

Reotrophic

CI

Limnic

Swamp

I4 I2

Swamp Forest

CS

Fen Mesotrophic

1 0.5

Ombrotrophic Bog Forest

Bog

0.1 1 marginal aquatic/ herbaceous

2

3

4

5

6

7

treed

Vegetation Index

BL - Camada Banco Louco CCS - Camada Candiota Superior CCI - Camada Candiota Inferior I1,I2, I3, and I4- Camadas Inferiores

Fig. 9. Coal facies diagram after Calder et al. (1991) in the position of Candiota coals.

230

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

by Diessel and Gammidge (1998), in which the groundwater influence index was discussed. 3.5. Organic geochemistry 3.5.1. TOC and Rock-Eval pyrolysis TOC contents and Rock-Eval pyrolysis data are presented in Table 4. In general, TOC values are relatively low, ranging from 28 to 45 wt.%. T max values range from 428 to 436 8C suggesting a low thermal maturity and correspond to the low rank as indicated by random reflectance measurements (Table 4). Fig. 10 shows the relationship between hydrogen index, T max, and kerogen types. According to this figure, a kerogen type III is indicated for the Candiota coals with relatively low HI values (16–147 mg HC/g TOC). Fig. 11 suggests that HI values are related to petrographic composition with decreasing HI related to increasing inertinite contents (correlation coefficient r = 0.7546). 3.5.2. Bitumen extraction and liquid chromatography Solvent extract results (ppm) and the saturated, aromatic, and polar fractions of the solvent extracts of the coal samples are presented in Table 5. In general, the bitumen extract content is low, ranging from 1222.8 to 4647.8 ppm. The lowest content of bitumen extract occurs in I3 seam, whereas the highest contents were identified in I4 and I2 coal samples. The I4 and I2 seams also have the highest contents in liptinite and vitrinite contents, as well as the lowest mineral matter contents. For the sample set analyzed

there exists a trend of increasing extract yields with increasing vitrinite and liptinite contents (Fig. 12A, correlation coefficient r = 0.7258), and also a trend of increasing extract yields in relation to decreasing mineral matter contents (Fig. 12B, correlation coefficient r = 0.7583). It is less related to the coal rank, since all Candiota coal samples are of approximately the same low rank (0.41–0.45% Rr). At this level, the coalification process could not have affected the bitumen extract content. The extract data reported here are in general lower than those shown for the Santa Terezinha Coalfield to the north (maximum 9260 ppm) related to the higher bituminization level of the coal seams at Santa Terezinha (Arau´jo et al., 1995). The hetero (NSO) compounds predominate in the extracts of all samples, ranging from 50% to 62% (Table 5). This is in agreement with studies by Correˆa da Silva et al. (1985), Henz (1986) and Arau´jo et al. (1995) showing the predominance of NSO compounds in extracts from coal seams in the northeastern coalfields of Rio Grande do Sul (Charqueadas, Faxinal, Santa Terezinha). However, in coal samples from the Barro Branco seam, Santa Catarina Coalfield and in the extract of seam CL6, Chico Loma˜ Coalfield (Correˆa da Silva et al., 1985) a predominance of the aromatic fraction was described. 3.5.3. GC analysis The n-alkanes of Candiota coal samples are dominated by long-chain n-alkanes (N n-C21) with an odd over even predominance (CPI from 2.17 to 4.21,

Table 4 Total Organic Carbon (TOC), Rock-Eval results, maceral groups, mineral matter, and reflectance values of Candiota coal from borehole M-IVC-101, Candiota Mine, RS, Brazil Seam

TOC (wt.%)

T max (8C)

S2 (mg HC/ g rock)

HI (mg HC/ g TOC)

OI (mg/CO2/ g TOC)

Vitrinite (vol.%) mmf

Liptinite (vol.%) mmf

Inertinite (vol.%) mmf

Mineral matter (vol.%)

Rr (%)

SD

BL CCS CCI I1 I2 I3 I4

31 29 30 28 40 32 45

428 435 436 433 433 429 431

5 12 15 16 55 46 61

16 41 50 57 138 147 135

– 31 41 11 33 16 18

5 48 43 35 48 36 54

27 14 11 28 32 24 27

68 38 46 37 20 40 19

25 23 22 22 5 18 7

0.44 0.41 0.41 0.41 0.45 0.42 0.42

0.04 0.03 0.03 0.03 0.03 0.04 0.04

BL — Camada Banco Louco; CCS — Camada Candiota Superior; CCI — Camada Candiota Inferior Camadas Inferiores (I1, I2, I3, I4). TOC — total organic carbon; wt — weight; HI — hydrogen index; OI — oxygen index; mmf — mineral matter-free basis. Rr — random reflectance; SD — standard deviation.

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

0. 5% R

o

II 600

Ro

400

1.

35

%

Hydrogen Index (mg HC/g TOC)

800

200 150

I3 I2 I4

III

100 50 0

400

I1 CCI BL CCS 430 450 465

500

550

Tmax (°C) Immature

I Mature I

Postmature

BL - Camada Banco Louco CCS - Camada Candiota Superior CCI - Camada Candiota Inferior Camadas Inferiores (I1, I2, I3, I4)

Fig. 10. Diagram showing Hydrogen and T max indices for Candiota coals (modified from Mukhopadhayay et al., 1995).

Table 5), with the most prominent peaks at n-C25 in I3, I1, CCI, and CCS samples (Fig. 13) and at n-C27 in samples I4 and I2. The BL sample shows maximum intensities at n-C25 and n-C27 (Fig. 13). The predominance of odd-carbon-numbered compounds is a characteristic feature of organic matter derived from higher terrestrial plant material and the long-chain n-alkanes (C21–C34) can be related to macerals of the liptinite group such as sporinite, resinite and cutinite (Tissot and Welte, 1984). The CPI values determined for the Candiota samples (range = 2.17–4.21) are considerably higher than those reported by Arau´jo et al. (1995) for time equivalent coals of the Santa Terezinha Coalfield (range = 0.96–1.15). Coals of the Santa Terezinha Coalfield are of significantly higher rank (high volatile A bituminous) than the Candiota coals (subbitu-

minous) and show the well known effect of decreasing CPI values with increase in rank (Bray and Evans, 1965). In this study sporinite and liptodetrinite contents are predominant in the liptinite group (Table 4) and thus likely contributing to the occurrence of the longchain n-alkanes in the Candiota coal samples. A secondary n-alkane population occurs in the shorterchain region (b C20), particular in the chromatograms of samples CCS, BL and sample I3, suggesting a contribution of algae and microorganisms to the precursor peat. Alternatively, this population may also reflect a biodegradation of long-chain n-alkanes during peat accumulation (Peters and Moldowan, 1993). The contribution of algae to the n-alkane distribution remains uncertain. The C14–C20 n-alkane homologues region, typical for an indication of algal contribution, is present in some samples (BL, CCS, CCI, I3), but is minor in I1, I2 and I4 (Fig. 13). The new family of bicyclic alkanes as described by Miranda et al. (1999) were also detected in relatively high abundance in the isoalkane/cycloalkane fractions of samples I4 and I2 seams. The position of these isoalkane/cycloalkane fraction is shown for seam I4 in Fig. 13. These bicyclic hydrocarbons have been interpreted to relate to a common biological precursor, possibly a conifer resin-derived tricyclic diterpenoid (Miranda et al., 1999). Fleck et al. (2001) observed similar compounds in Westphalian-age coals of the Lorraine Basin, France, and interpreted that there presence indicated degradation of plant material insitu with sufficient time for bacterial degradation before burial. 80

HI (mgHC/g COT)

I

1000

231

70 60

BL

50 40

y=-0.0097 x + 57.542 r = 0.7546 CCI I3

CCS

I1

30 20

I4 I2

10 0 0

50

100

150

200

Inertinite (vol%)

Fig. 11. Diagram showing the relation of Hydrogen Index (HI) vs. inertinite for Candiota coal samples. BL = Camada Banco Louco, CCS = Camada Candiota Superior, CCI = Camada Candiota Inferior, I1 = Camada Inferior 1, I3 = Camada Inferior 3, I4 = Camada Inferior 4.

Liptinite + Vitrinite (vol %)

A 90 80 70 60 50 40 30 20 10 0

I4

I2 I1

I3

CCS CCI y = 0.0099x + 32.715 r = 0.7258 BL

0

1000

2000

3000

4000

5000

Extract (ppm)

B Mineral Matter (vol %)

Rr — Random reflectance; SD — standard deviation; CPI — carbon preference index; Pr — pristane; Ph — phytane; TRI / HOP = tricyclic triterpanes to pentacyclics ratio.

– 1.1 – 0.65 0.54 – – – 47.83 – 49.06 43.9 – – – 22.19 – 19.95 22.1 – – – 29.98 – 30.99 34.01 – – – 1.4 – 0.37 0.29 – – – 0.49 – 0.39 0.43 – – – 0.2 – 0.11 0.15 – – – 0.65 – 1.29 1.03 – – 2.17 2.20 4.21 3.31 3.09 2.69 3.23 0.44 0.41 0.41 0.41 0.45 0.42 0.42 BL CCS CCI I1 I2 I3 I4

0.04 0.03 0.03 0.03 0.03 0.04 0.04

2163.8 3190.7 2365.4 2528.1 4347.7 1222.8 4647.2

– 18 19 33 15 22 18

– 30 22 14 23 28 24

– 52 59 53 62 50 58

2.18 1.54 4.47 2.85 1.49 2.10 1.89

1.15 1.04 2.04 1.23 1.08 1.47 2.10

Hopane / Sterane Rr (%) Seam

SD

Extract (ppm)

Saturates (%)

Aromatics (%)

NSO (%)

Pr / Ph

Pr / C17

CPI

20 S / 20 S + 20 R Sterane

ahh / ahh + aaa Sterane

TRI / HOP

% C27 Sterane

% C28 Sterane

% C29 Sterane

Diasterane / Sterane

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238 Table 5 Liquid chromatography, gas chromatography, and gas chromatography/mass spectrometry results of Candiota coal samples from borehole M-C-V-101, Candiota Mine, RS, Brazil

232

30 BL CCI I 1

25

CCS

20 I3

15

y= - 0.005x + 32.141 r = 0.7583

10

I4

5

I2

0 0

1000

2000

3000

4000

5000

Extract (ppm)

Fig. 12. Diagrams showing the relation of (A) liptinite + vitrinite and (B) mineral matter vs. extract for Candiota coal samples. BL = Camada Banco Louco, CCS = Camada Candiota Superior, CCI = Camada Candiota Inferior, I1 = Camada Inferior 1, I3 = Camada Inferior 3, I4 = Camada Inferior 4.

The pristane (Pr) / phytane (Ph)-ratio is considered to be an indicator of relative oxic or reducing environments during early diagenesis (Tissot and Welte, 1984). Phytol (C20), derived from the side chain of chlorophyll may transform to pristane (C19) due to oxidation within a suboxic or oxic environment or to phytane (C20) within a highly reducing environment (Tissot and Welte, 1984). Suboxic to oxic environments are characterized by Pr / Ph-ratios N 1 (Peters and Moldowan, 1993). Candiota coal samples show Pr / Ph-ratios ranging from 1.49 to 4.47 (Table 5) thus indicating a certain level of oxygen access to the site of peat accumulation. These Pr / Ph-ratios confirm the results of maceral analyses, where a high inertinite content (37–68 vol.%, Table 4) was identified for almost all samples, except in I4 and I2 seams (20 vol.%, Table 4). The diagram of Pr / Ph-ratios vs. inertinite contents (Fig. 14A) shows a weak positive trend between Pr / Ph-ratios and inertinite for Candiota samples (correlation coefficient = 0.3397). An inverse trend (r = 0.4048) is indicated for the relationship between Pr / Ph-ratio and Gelification Index (Fig.

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

BL

233

I1 C25

C27 C25 C27

PRI

C18 PHY PRIC 17

C18PHY C17

I3

CCS

C25

C25

C27

C23 PHY C18 PRI C17

C18 PHY PRI C17

I4

CCI C25

1- 5*

C23 C27 C25

PHY C18 PRIC17

C18PHY PRI C17

Fig. 13. Gas chromatograms of saturated hydrocarbons of Candiota coal extracts. *New series of aliphatic and aromatic bicyclic compounds (Miranda et al., 1999). BL = Camada Banco Louco, CCS = Camada Candiota Superior, CCI = Camada Candiota Inferior, I1 = Camada Inferior 1, I3 = Camada Inferior 3, I4 = Camada Inferior 4.

234

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

A 80

BL

Inertinite (mmf, vol%)

70 60 50

I3

CCS

40

CCI I1

30

I2

20

I4

y = 5.4421x + 25.442 r = 0.3397

10

0 0

1

2

3

4

5

Pr/Ph

B

3

I4

y = -0.3697x + 2.2012 r = 0.4048

I2

GI

2

CCS 1

I3

CCI

I1

BL 0

0

1

2

3

4

5

Pr/Ph

Fig. 14. Diagram showing the relation of A) pristane/phytane (Pr/ Ph) vs. inertinite; B) pristane/phytane (Pr/Ph) vs. Gelification Index (GI). BL = Camada Banco Louco, CCS = Camada Candiota Superior, CCI = Camada Candiota Inferior, I1 = Camada Inferior 1, I3 = Camada Inferior 3, I4 = Camada Inferior 4.

are referred to as norhopanes. The steranes are derived from sterols that are found in most higher plants and algae (Waples and Machihara, 1991). Steranes are in general considered to be indicators for photosynthetic biota, whereas triterpanes are thought to reflect more the depositional and diagenetic conditions (Waples and Machihara, 1991). In Candiota coals the hopane / sterane ratio is 0.65 for CCS sample, whereas samples I2 and I1 show ratios of 1.03 and 1.29, respectively. The interpretation of the hopane / sterane ratio is difficult, since the distribution of these biomarkers is both related to variations in input of organic matter (terrestrial vs. lacustrine/marine) and depositional environment. Regular steranes are in general biomarkers indicating organic matter sources that contributed to the organic portion of the sediment. A predominance of C29 steranes is related to a strong terrestrial contribution, whereas a predominance of C27 and C28 indicate a predominance of marine phytoplankton and lacustrine algae, respectively (Waples and Machihara, 1991). The relationships are complicated by the known fact that the C29 sterane also occurs in certain brown and green algae. Table 5 and Fig. 15 show the proportion for C27– C28–C29 regular steranes for the Candiota samples. The proportions of C29 steranes (44–49%) are highest, followed by C27 steranes (30–34%) and C28 steranes %C28

Green algae and diatoms

14B), the latter considered to indicate a relatively high water level during peat accumulation preventing higher oxidation rates. 50

3.5.4. GC/MS analysis Table 5 shows biomarker parameters, which are frequently used to indicate organic facies, possible sources of the organic matter and maturity for I2, I1 and CCS seams are shown in Table 5.

50

I2

Red algae and zooplankton

3.5.4.1. Facies and sources of organic matter. Triterpanes are cyclic saturated hydrocarbons and their source are probably triterpenoids derived from bacteria and certain higher land plants (Philp, 1985). The molecule containing 30 carbon atoms is simply called hopane, whereas C27 and C29 homologues

CCS I1

%C27 B)

Higher plants and brown and green algae

50

%C29

Fig. 15. Triangular diagram showing the distribution of C27, C28, and C29 regular steranes and interpreted sources, modified from Hunt et al., 1965. BL = Camada Banco Louco, CCS = Camada Candiota Superior, CCI = Camada Candiota Inferior, I1 = Camada Inferior 1, I3 = Camada Inferior 3, I4 = Camada Inferior 4.

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

(20–22%). Although the C29 steranes are predominant, even a higher contribution had been expected since typically humic and waxy coals display a strong dominance of C29 steranes (Waples and Machihara, 1991). The relatively high C27 and C28 sterane proportions suggest a contribution of algae, bacteria and possibly fungi to the precursor peat. The C29 sterol chain may also have been degraded by bacteria (Piedad-Sa´nchez et al., 2004), which is supported by the relatively high contents of degraded collodetrinite in the Candiota coal seams. To a lesser extent, an input of algae to the precursor peat is suggested by the CGtrace of CCS and BL samples, and also by the alginite volumes determined petrographically in BL and I1 samples. The relative amount of diasteranes compared with regular steranes appears to be dependent on both lithology and maturity (e.g. Waples and Machihara, 1991). According to these authors, diasteranes seem to form most readily in clastic sediments, where clay minerals may act as catalysts in their formation from other steranes. This relationship is also seen in the samples analyzed from the Candiota Coalfield, where the diasteranes/regular steranes ratios increase with increasing clay mineral contents. 3.5.4.2. Organic matter maturity. Biomarker ratios such as the 20 S / (20 S + 20 R) and ahh / (ahh + aaa) for C29 regular steranes and the ratio of tricyclic triterpanes to pentacyclics (TRI / HOP) are used in petroleum exploration to evaluate maturity levels of the organic matter and are used here to confirm the low rank of Candiota coals as determined by vitrinite reflectance measurements. In the aa steranes the proportions of the two epimeric forms (20 R and 20 S) change with increasing maturity. The proportion of 20 S increases as some of the 20 R molecules, depending on the level of maturity, change their configuration. In the Candiota coals the low values of the 20 S / (20 S + 20 R) ratios (b0.20; Table 5) confirm the low maturity (low rank) of the organic matter estimated by vitrinite reflectance (0.41–0.45%). Another maturity parameter derived from C29 regular steranes is the proportion of hh and aa forms expressed as the ratio aaˆaˆ / (ahh + aaa) sterane. As the maturity increases, the aa form, which is produced biologically, converts gradually into a mixture

235

of aa and hh. The low values of this ratio (0.39–0.49; Table 5) confirms also the low maturity of the Candiota coals. In addition, the ratio tricyclic triterpanes to pentacyclics (TRI / HOP) is used as a qualitative indicator of maturity on Candiota samples. According to Snowdon et al. (1987), tricyclics seem to be more stable than pentacyclics, resulting in the increase of the tricyclic / pentacyclic ratio at maturity levels corresponding to mature and slightly overmature maturity levels. In Candiota samples these ratios show a wide spread ranging from 0.29–1.4 (Table 5), suggesting that this ratio does not serve as a maturity indicator in Candiota coals since the seams are of same rank.

4. Conclusions On the basis of coal petrological and geochemical data the following conclusions can be drawn for the Candiota coal seams: 1. Lithotypes. Candiota coal lithotypes dull upwards, paralleled by an increase in inertinite and mineral matter contents, with a vitrinite decrease. 2. Maceral composition and mineral matter. The organic matter in the Candiota coals is dominated by macerals of the vitrinite and inertinite groups. Despite a significant scatter in the data points, a general increase of inertinite is indicated from the base to the top of the coal-bearing sequence, whereas vitrinite contents decrease. Liptinite contents vary from 11 to 32 vol.%, with microsporinite and liptodetrinite being the most abundant liptinite macerals. Mineral matter ranges from 5 to 25 vol.% and is dominated by clay minerals. 3. Microlithotypes. Carbargilite dominates in all coal seams (range 34–49 vol.%) except in I4 and I2, confirming the high clay mineral content. In seams I4 and I2, characterized by relatively low mineral matter contents and higher vitrinite-rich contents, clarite rich in vitrinite (Clarite V) is the most common microlithotype (30 and 23 vol.%, respectively). In seam BL, characterized by a very high inertinite content, the microlithotype durite rich in inertinite (Durite I) is abundant (26 vol.%). 4. In-seam coal classification. According to UN-ECE classification, the CCI and CCS seams are subbi-

236

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

tuminous coals or low rank A coals. With an ash yield of over 53 wt.% the CCI seam is classified as a carbonaceous rock, whereas the CCS seam is classified as a very low grade coal (39 wt.% ash). In terms of petrographic composition the CCI seam consists of 38 vol.% vitrinite, 7 vol.% liptinite, and 55 vol.% inertinite, whereas the CCS seam consists of 39 vol.% vitrinite, 17 vol.% liptinite, and 44 vol.% inertinite. 5. Coal facies interpretations. Coal facies models based on microlithotype distribution, Groundwater and Vegetation Indices, and TDF indices suggest that the Candiota peat accumulated under high water tables under a reothrophic regime (transition to open mire). Some of these models incorporate detrital mineral matter and liptinite macerals in the evaluation of the environment of deposition. For example, the occurrence of clay minerals associated with liptinite macerals results in higher D and GWI indices. A somewhat different depositional environment for the Candiota coal seams is indicated by TPI and GI values based on vitrinite and inertinite maceral distribution, suggesting a limno-telmatic peat accumulation in a back-barrier/strandplain setting consistent with the geological model of coal formation in the area. Contrasting the two facies interpretations (transition to open mire vs. limno-telmatic) we believe that the incorporation of detrital mineral matter and liptinite macerals (Hacquebard and Donaldson, 1969; Calder et al., 1991) is essential in the coal facies interpretation of the mineral matter rich coals at Candiota. In conclusion, detrital components, mostly clay minerals, are likely the most important parameters to evaluate the coal facies of Candiota coal seams. 6. Geochemical analyses. Results from Rock-Eval Analyses, CPI and biomarker distribution (C27, C28, C29) indicate a predominance of terrestrial organic matter, although the presence of lower chain alkanes (b C20) and the proportion of C27 steranes suggest also some contribution of algae or bacterial source organic matter. The Pr / Ph ratios are relatively high, suggesting a suboxic/oxic environment of deposition. Biomarker ratios such as the 20 S / (20 S + 20 R) and ahh / (ahh + aaa) for C29 regular steranes

confirm the low maturity of the organic matter as determined by vitrinite reflectance. (0.41–0.45%) and gross calorific values (b24 MJ/kg, m, af).

Acknowledgements This paper was written during a research study granted by CNPq to M.B. Silva. Z. Correˆa da Silva is thanked for critically reading early drafts of the manuscript. L. A. Trindade is thanked for supporting the geochemical analyses at CENPES/PETROBRAS. M. J. Lemos de Sousa (Porto University, Portugal) and his research group are acknowledged for taking part in the UN-ECE coal classification analyses and discussions. N. Franco, UFRGS is thanked for critical reading of the geochemical part of the paper. G. Sima˜o, K. Freire and V. Engelke, all UFRGS are thanked for technical assistance in preparation of the final figures. CRM is thanked for providing logistical support during the collection of the coal samples. The manuscript benefited from the critical comments of two anonymous reviewers.

References Ade, M., 1993. Caracterizac¸a˜o dos Sistemas Deposicionais e das Camadas de Carva˜o no Pacote Sedimentar da Malha IV, Jazida de Candiota, RS. M.Sc. thesis, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil, 98 pp. Alves, R., 1994. Correlac¸a˜o estratigra´fica de alta resoluc¸a˜o aplicada ao Permiano Inferior da Bacia do Parana´, na regia˜o de Candiota, Rio Grande do Sul. Porto Alegre. M.Sc. thesis, Instituto de Geocieˆncias, Universidade Federal do Rio do Rio Grande do Sul. 120 pp. Alves, R., Ade, M., 1996. Sequence stratigraphy and coal petrography applied to the Candiota Coalfield, Rio Grande do Sul, Brazil: a depositional model. International Journal of Coal Geology 30, 231 – 248. Arau´jo, C.V., Correˆa da Silva, Z.C., Rodrigues, R., 1995. Petrographic and geochemical study of coals from Santa Terezinha Coalfield, Rio Grande do Sul, Brazil. Revista Latino-Americana de Geoquı´mica Orgaˆnica 1, 28 – 45. Bray, E.E., Evans, E.D., 1965. Hydrocarbons in non-reservoir-rock source beds. American Association of Petroleum Geologists Bulletin 49, 248 – 257. Bustin, R.M., Cameron, A.R., Gireve, D.A., Kalkreuth, W.D., 1983. Coal petrology: its principles, methods, and applications, vol. 3. Geological Association of Canada, Victoria, British Columbia. 248 pp.

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238 Calder, J., Gibling, M., Mukhopadhyay, P., 1991. Peat formation in a Westphalian B piedmont setting, Cumberland Basin, Nova Scotia. Bulletin de la Socie´te´ ge´ologique de France 162/ 2, 283 – 298. Cazzulo-Klepzig, M., 2001. Palinologia aplicada a` reconstituic¸a˜o das unidades de paisagem e dinaˆmicas das turfeiras formadoras dos carvo˜es permianos do Rio Grande do Sul. Ph.D. thesis, Curso de Po´s-Graduac¸a˜o em Geocieˆncias, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil, (1, 2), unpublished. 350 pp. CIENTEC, 1980. Carvo˜es Minerais do Brasil: Caracterı´sticas de Carvo˜es Brutos do Rio Grande do Sul. Porto Alegre. Centro de informac¸o˜es sobre carva˜o. 50 pp. Correˆa da Silva, Z.C., 1981. Origin and facies of some South Brazilian coal seams. International Conference on Coal Science, Du¨sseldorf 1981, Proceedings, pp. 38 – 44. Correˆa da Silva, Z., 1989. The rank evaluation of South Brazilian Gondwana coals on the basis of different chemical and physical parameters. International Journal of Coal Geology 13, 21 – 39. Correˆa da Silva, Z., Marques-Toigo, M., 1975. Carva˜o no Brasil: Mina de Candiota, Rio Grande do Sul. II Congresso Ibero-Americano de Geologia Econoˆmica, Buenos Aires, Actas, vol. 1, pp. 263 – 285. Correˆa da Silva, Z., Wolf, M., 1980. O poder refletor como paraˆmetro para determinac¸a˜o do grau de carbonificac¸a˜o dos carvo˜es gonduanicos do Brasil. Pesquisas, vo. 13. Instituto de Geocieˆncias UFRGS, Porto Alegre, pp. 35 – 42. Correˆa da Silva, Z., Marques-Toigo, M., 1985. Considerac¸o˜es petrolo´gicas e palinolo´gicas sobre a camada Candiota, Jazida Carbonı´fera de Candiota, Rio Grande do Sul. II Simpo´sio Sul-Brasileiro de Geologia, Floriano´polis, SC, Outubro, Anais, pp. 432 – 448. Correˆa da Silva, Z.C., Hagemann, H.W., Pu¨ttmann, W., Wolf, M., 1985. Caracterı´sticas petrogra´ficas e geoquı´micas de alguns carvo˜es gonduaˆncios do sul do Brasil. Simpo´sio Sul-Brasileiro de Geologia, Anais, vol. 2, pp. 419 – 431. Correˆa da Silva, Z.C., Pu¨ttmann, W., Wolf, W.M., 1987. Studies on the rank determination of South Brazilian Gondwana coals. International Conference on Coal Science, Proceedings, pp. 165 – 168. Correˆa da Silva, Z.C., Arau´jo, C.V., Cazzulo-Klepzig, M., MarquesToigo, M., 1996. Fa´cies orgaˆnica, maturac¸a˜o e ambiente deposicional do carva˜o coletado na sondagem 2-AO-01-RS, Rio Grande do Sul, Brasil. Revista Latino-Americana de Geoquı´mica Orgaˆncia 2, 30 – 39. Diessel, C.F.K., 1965. Petrographic features of N.S.W. coals. BHP Technologic Bulletin 9, 2 – 8. Diessel, C.F.K., 1982. An appraisal of coal facies based on maceral characteristics. In: Mallett, C.W. (Ed.), Australian Coal Geology, vol. 4 (2), pp. 474 – 484. Diessel, C.F.K., 1986. On the correlation between coal facies and depositional environments. Advances in the study of the Sydney Basin, 20, Newcastle, Australia, Proceedings, pp. 19 – 22. Diessel, C.F.K., Gammidge, L., 1998. Isometamorphic variations in the reflectance and fluorescence of vitrinite — a key to depositional environment. International Journal of Coal Geology 3, 167 – 222.

237

Espitalie´, J., Laporte, J.L., Madec, M., Marquis, F., Leplat, P., Paulet, J., Boutefeu, F., 1977. Me´thode rapide de caracte´risation des roches meres, de leur potentiel pe´trolier et de leur degre´ d´e´volution. Revue Institute du Pe´trole 32, 23 – 42. Ferreira, J.A.F., 1976. Estudo do grau de evoluc¸a˜o dos carvo˜es do Rio Grande do Sul. Congresso Brasileiro de Geologia 29, Ouro Preto, MG, Anais, vol. 2, pp. 273 – 282. Fleck, F., Michels, R., Izart, A., Elie, M., Landais, P., 2001. Palaeoenvironmental assessment of Westphalian fluvio-lacustrine deposits of Lorraine (France) using a combination of organic geochemistry and sedimentology. International Journal of Coal Geology 48, 65 – 88. Hacquebard, P.A., Donaldson, R., 1969. Carboniferous coal deposition associated with flood-plain and limnic environments in Nova Scotia. In: Dapples, E.C., Hopkins, M.E. (Eds.), Environments of Coal Deposition, Geol. Soc. Am., vol. 114, pp. 143 – 191. Special Paper. Henz, G.I., 1986. Organo-petrografia e organo-geoquı´mica dos carvo˜es da Jazida Faxinal, Municı´pio de Arroio dos Ratos. Rio Grande do Sul, Brasil. M.Sc. thesis. Curso de Po´s-Graduac¸a˜o em Geocieˆncias, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil, unpublished, 243 pp. Holz, M., 1996. The Eo-Permian coal seams of the Parana´ Basin in southernmost Brazil: an analysis of the depositional conditions using sequence stratigraphy concepts. International Journal of Coal Geology 36, 141 – 163. Holz, M., 1998. The Eo-Permian coal seams of the Parana´ Basin in southernmost Brazil: An analysis of the depositional conditions using sequence stratigraphic concepts. International Journal of Coal Geology 36, 141 – 163. Holz, M., Kalkreuth, W., 2004. Sequence stratigraphy and coal petrology applied to the Early Permian coal-bearing Rio Bonito Formation, Parana´ Basin, Brazil. In: Pashin, J., Gastaldo, R. (Eds.), Sequence Stratigraphy, Paleoclimate, and Tectonics of Coal-bearing Strata, AAPG Studies in Geology, vol. N8 51, pp. 147 – 167. Holz, M., Kalkreuth, W., Banerjee, I., 2002. Sequence stratigraphy of coal-bearing strata: an overview. International Journal of Coal Geology 48, 147 – 179. Hunt,, Bray, E.E., Evans, E.D., 1965. Hydrocarbons in non-reservoir-rock source beds. American Association of Petroleum Geologists Bulletin 49, 248 – 257. Kalkreuth, W., Leckie, D.A., 1989. Sedimentological and petrographical characteristics of Cretaceous strandplain coals: a model for coal accumulation from the North American Western Interior Seaway. International Journal of Coal Geology 12, 381 – 424. Kalkreuth, W., Sherwood, N., Cioccari, G., Correa da Silva, Z., Silva, M.B., Zhong, N., Zufa, L., 2004. The application of FAMM (Fluorescence Alteration of Multiple Macerals) analyses for evaluating rank of Parana´ Basin coals, Brazil. International Journal of Coal Geology 57, 167 – 186. Lamberson, M.N., Bustin, R.M., Kalkreuth, W., 1991. Lithotype (maceral) composition and variation as correlated with paleowetland environments, Gates Formation, northeastern British Columbia, Canada. International Journal of Coal Geology 18, 87 – 124.

238

M.B. Silva, W. Kalkreuth / International Journal of Coal Geology 64 (2005) 217–238

Marchioni, D.L., Kalkreuth, W.D., 1991. Coal facies interpretations based on lithotype and maceral variations in Lower Cretaceous (Gates Formation) coals of western Canada. International Journal of Coal Geology 18, 125 – 162. Matos, L.B.B., 1984. Contribuic¸a˜o ao estudo petrogra´fico e palinolo´gico dos carvo˜es da Jazida de Chico Loma˜-RS. Porto Alegre. Dissertac¸a˜o de Mestrado em GeocieˆnciasInstituto de Geocieˆncias, Universidade Federal do Rio Grande do Sul. 218 pp. Meyer, K., 1999. Caracterizac¸a˜o palinolo´gica das camadas de carva˜o da Malha IV na Mina Candiota, RS, Permiano da Bacia do Parana´. M.Sc. thesis. Curso de Po´s-Graduac¸a˜o em Geocieˆncias, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil, unpublished, 120 pp. Meyer, K., Marques-Toigo, 2000. O significado paleoambiental da microflora dos carvo˜es da Malha IV, Mina de Candiota, RS, permiano da Bacia do Parana´, Brasil. Revista Universidade de Guarullhos, Geocieˆncias (no especial), 17–20. Miranda, A.C.M.L., Loureiro, M.R.B., Cardoso, J.N., 1999. Aliphatic and aromatic hydrocarbons in Candiota coals samples: novel series of bicyclic compounds. Organic Geochemistry 30, 1027 – 1038. Mukhopadhayay, P., Wade, J., Kruge, A., 1995. Organic facies and maturation of Jurassic–Cretaceous rocks, and possible oil-source rock correlation based on pyrolysis of asphaltenes, Scotian Basin, Canada. Organic Geochemistry 22, 85 – 104. Nahuys, J., Caˆmara, M.R.S., 1972. Carva˜o de Candiota: suas caracterı´sticas quı´micas e petrogra´ficas. Boletim do Instituto Tecnolo´gico do Rio Grande do Sul 59, 1 – 46. Peters, K.E., Moldowan, J.M., 1993. The biomarker guide. Interpreting Molecular Fossils in Petroleum and Ancient SedimentsPrentice Hall, New Jersey. 363 pp. Philp, R.P., 1985. Fossil fuel biomarkers. Applications and SpectraElsevier, Amsterdan. 294 pp. Piedad-Sa´nchez, N., Sua´rez-Ruiz, I., Martı´nez, L., Izart, A., Elie, M., Keravis, D., 2004. Organic petrology and geochemistry of the Carboniferous coal seams from the Central Asturian Coal Basin (NW Spain). International Journal of Coal Geology 57, 211 – 242. Schneider, R.L., Mu¨hlmann, H., Tommasi, E., Medeiros, R.A., Daemon, R.F., Nogueira, A., 1974. Revisa˜o estratigra´fica da

Bacia do Parana´. Congresso Brasileiro de Geologia 28, Porto Alegre, Anais, vol. 1, pp. 41 – 66. Scott, A.C., 1989. Observations on the nature and origin of fusain. International Journal of Coal Geology 12, 443 – 475. Scott, A.C., Jones, T., 1994. The nature and influence of fire in Carboniferous ecosystems. Palaeogeography, Palaeoclimatology, Palaeoecology 106, 91 – 112. Silva, M.B., 1994. Caracterizac¸a˜o petrogra´fica das camadas de carva˜o da Malha IV, Jazida de Candiota, RS. M. Sc. thesis, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil, 106 pp. Silva, M.B., 1999. Petrolologia de carvo˜es nas jazidas do Lea˜o e Candiota, Permiano Inferior da Bacia do Parana´, RS, Brasil. PhD Thesis, Instituto de Geocieˆncias, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil. Snowdon, L.R., Brooks, P.W., Williams, G.K., Goodarzi, F., 1987. Correlation of the Canol Formation source rock with oil from Norman Wells. Organic Geochemistry 11, 529 – 548. Teichmu¨ller, M., Teichmu¨ller, R., 1975. Fundamentals of coal petrology. In: Stach, E., Mackowsky, M.Th., Teichmu¨ller, M., Taylor, G.H., Chandra, D., Teichmu¨ller, R. (Eds.), Stach’s Textbook of Coal Petrology, 2nd ed. Gebru¨der-Borntraeger, Berlin, pp. 5 – 35. Teichmu¨ller, M., Teichmu¨ller, R., 1982. Fundamentals of coal petrology. In: Stach, E., Mackowsky, M.Th., Teichmu¨ller, M., Taylor, G.H., Chandra, D., Teichmu¨ller, R. (Eds.), Stach’s Textbook of Coal Petrology, 3rd ed. Gebru¨der-Borntraeger, Berlin, pp. 5 – 86. Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence, 2nd ed. Springer-Verlag, Berlin. 535 pp. UN-ECE, 1998. International classification of in-seam coals. Economic Commission for Europe, Committee on Sustainable EnergyUnited Nations, New York. Document ENERGY/1998/19, 41 pp. Waples, D.W., Machihara, T., 1991. Biomarkers for Geologists — a practical guide to the application of steranes and triterpanes in Petroleum Geology. Tulsa, Oklahoma, AAPG Methods in Exploration Series No. 9The American Association of Petroleum Geologists. 91 pp.