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Marine and Petroleum Geology 48 (2013) 31e46

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Marine and Petroleum Geology

journal homepage: www.elsevier .com/locate/marpetgeo

Organic geochemical and petrographic characteristics of Tertiarycoals in the northwest Sarawak, Malaysia: Implications forpalaeoenvironmental conditions and hydrocarbon generationpotential

Mohammed Hail Hakimi a,*, Wan Hasiah Abdullah b, Say-Gee Sia b, Yousif M. Makeen b

aGeology Department, Faculty of Applied Science, Taiz University, 6803 Taiz, YemenbDepartment of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 15 May 2013Received in revised form12 July 2013Accepted 13 July 2013Available online 24 July 2013

Keywords:Tertiary coalfieldPetrologyCoal rankPetroleum potentialMalaysia

* Corresponding author. Geology Department, FTaiz University, 6803 Taiz, Yemen. Tel.: þ967 73 6517

E-mail address: ibnalhakimi@yahoo.com (M.H. Ha

0264-8172/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.marpetgeo.2013.07.009

a b s t r a c t

Tertiary coals fromMukah and Balingian coalfields in the northwest Sarawak, Malaysia were investigatedto evaluate their regional rank variation, petroleum generative potential and to reconstruct the palae-oenvironment conditions during peat accumulation. The Tertiary coals are characterized by high totalorganic carbon contents (TOC) and yield of bitumen extraction ranging from 31.3 to 55.9 wt. % and25,724.9-92,143.7 ppm, respectively, meet the standard as a good source rock potential. The Mukah andBalingian coals were generally plotted in an area of Type III kerogen and mixed Type II/III kerogens withHI values between 90 and 289 mg HC/g TOC, whereby the coals were derived from plant materials ofterrigenous origin. This shows that the Balingian coals are dominated by Type III terrigenous kerogenwhile Mukah coals are dominated by Type II/III kerogens, and are thus considered to be generate mainlygas-prone and limited oil-prone. This is also supported by macerals composition and open system py-rolysis gas chromatography (Py-GC). The Mukah and Balingian coals are thermally immature in rangfrom lignite to sub-bituminous C rank, possessing huminite reflectance in the range of 0.26%e0.39%. Thisimmaturity has a considerable influence on the proximate analysis, particularly on relatively highmoisture and volatile matter contents and relatively low fixed carbon content.

Petrographically, it was observed that the Mukah and Balingian coals are dominated by huminite, withlow to high amounts of liptinite and relatively low amounts of inertinite, pointing to predominantlyanaerobic deposition conditions in the paleomires, with limited thermal and oxidative tissue destruction.The palaeoenvironment conditions of the coals are generally interpreted as a lower deltaic plain wetpeat-swamp depositional setting, which are generally characterised by low TPI and high GI values, andare plotted on the marsh field of the Diessel’s diagram. This is usually consistent with generate relativelyhigh ash yield as is the case of the Mukah and Balingian coals.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Coals in Malaysia are present in the Tertiary basins in all threegeographical provinces, viz. Sarawak, Sabah and PeninsularMalaysia (Fig. 1a). However, most of the coal resources are locatedin the states of Sabah and Sarawak (EIA, 2012). As at the end of 2011,total coal resources in Malaysia amounted at 1819 Mt, of which1468 Mt or 80.7% were located in Sarawak, 334 Mt or 18.3% inSabah, and the remaining 1% in Peninsular Malaysia (EIA, 2012).

aculty of Applied Science,422.kimi).

All rights reserved.

Most of the coals are thermal coal and ranging from sub-bituminous to anthracite; nonetheless coals with coking proper-ties exist in the Bintulu, Silantek, Slimponpon and Maliau coalfields(Fig. 1a). Several studies concerning sedimentology, geochemicaland organic petrographic characteristics have been performed onTertiary coals in the Sarawak and Sabah Basins (e.g., Wan Hasiah,1997, 1999, 2003; Zulkifli et al., 2008; Sia and Abdullah, 2011,2012; Alias et al., 2012; Hakimi and Abdullah, 2013; Mustaphaand Abdullah, 2013).

The study area is located onshore in the northwest part of Sar-awak which is situated in the low-lying coastal plain between theMukah and the Balingian Rivers of Sarawak, Malaysia (Fig. 1b).Offshore area is currently active petroleum exploration area andthus an evaluation of source rock quality in the onshore in the

Figure 1. (a) Location map showing the coal bearing Tertiary basins in Malaysia. (b) Geological map of Balingian and Mukah coalfields and surrounding areas, Sarawak. Study area shown by box. (c) Map showing distribution of theBalingian and Liang Formations around Balingian and Mukah coalfields, northwest Sarawak, Malaysia, and location of sampling points.

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M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e46 33

Sarawak will contribute in such exploration activity. The currentstudy is focused on the Late Miocene to Late Pliocene coals in theMukah and Balingian coalfields (Fig. 1b and c). Despite the impor-tance of these coalfields, little is known about the coal. Sia andAbdullah (2011) worked on the concentration and association ofminor and trace elements in Mukah coal, with emphasis on thepotentially hazardous trace elements. Sia and Abdullah (2012)published on geochemical and petrographic characteristics oflow-rank Balingian coals and their implications on depositionalconditions and thermal maturity.

The characterization of the coal using the integration of con-ventional geochemical and coal petrological approaches is themainobjective of this study with the purpose to investigate in detail thehydrocarbon generation potential of the Tertiary coals fromnorthwest Sarawak and their depositional conditions. The organicpetrological and organic geochemical data involved total organiccarbon (TOC) content, pyrolysis yields, proximate data, bitumenextraction, maceral composition and vitrinite reflectancemeasurements.

2. Geologic setting

The Mukah and Balingian coalfields are located in the low-lyingcoastal plain between the Mukah and the Balingian Rivers of Sar-awak, covering an area of approximately 710 km2 in Sarawak(Fig. 1bec). The Mukah coalfield is underlain by the Balingian

Figure 2. Generalized stratigraphic column of the Tertiary coal-bearing Ba

Formation of Early Miocene age, which is in turn unconformablyoverlain by the Begrih Formation of Early Pliocene age (Fig. 2a). Thecontact between these two formations is marked by a wedge ofbasal conglomerate, called Begrih Conglomerate (Wolfenden,1960). The Balingian Formation is composed of mudstones,shales, siltstones, sandstones, tuffite, and coals. Foraminiferaidentified by Visser and Crew (unpublished), such as Ammodiscussp., Glomospira sp., Haplophragmoides sp., and Trochammina sp.,suggest a brackish-water depositional environment. The coalfieldcontains 13 coal seams, comprising 5 major (well developed, witheconomic potential) and 8minor seams (Fig. 2a). The coal seams areusually between 1 and 2 m thick, with a cumulative coal thicknessup to 16 m. The coal is characterized by a high amount of huminite(89e99%), low to moderate amounts of liptinite (<1e9%), and traceto low amounts of inertinite (<5%), on a mineral matter-free basis.The coal is of lignite rank, with high total moisture, and low totalsulphur content, and ash yield (Sia and Dorani, unpublished). Incontrast, the accumulation of the Balingian coal took place in theLiang Formation during Upper Pliocene. The Liang Formation isunconformably underlain by the Lower Pliocene Begrih Formationin the north and by the Eocene Belaga Formation in the south(Figs. 1c and 2b) (Hutchison, 2005). The Liang Formation has athickness of approximately 950 m (Fig. 2b), and is made up of thickand massive clays, sands, tuffite, coal seams and gravel lenses. Thefauna identified in the coal zone include Ammodiscus sp., Glomo-spira sp., Haplophragmoides sp., and Trochammina sp., which

lingian and Formations in the Mukah (a) and Balingian (b) coalfields.

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e4634

suggest a brackish-water environment of deposition. Outside thecoal zone, however, the sediments were deposited in a veryshallow, near-shore type of marine deposition (Hutchison, 2005).Four coal seams with an accumulated thickness of 20.42 m havebeen identified in this study (Fig. 2b). The coal has been classified asSubbituminous C (Sia and Abdullah, 2012) and represents the onlyminable lignite deposit reported in the country.

Table 1Results of pyrolysis analysis with calculated parameters and proximate analysis (as-rece

Coalfields Samplingsites

Depth(m)

SampleID

Lithology Pyrolysis data

S1(mg/g)

S2(mg/g)

Balingiancoalfield

BO1 Mine face BO1-1 Coal 7.8 75.9Mine face BO1-2 Coal 5.5 99.9Mine face BO1-3 Coal 7.5 68.0Mine face BO1-4 Coal 8.3 71.3Mine face BO1-5 Coal 10.8 83.0Mine face BO1-6 Coal 5.9 53.0Mine face BO1-7 Coal 5.9 60.5

BO2 Mine face BO2-1 Coal 4.8 54.2Mine face BO2-4 Coal 7.8 60.7Mine face BO2-5 Coal 14.6 79.2

BO3 Mine face BO3-1 Coal 9.5 79.1Mine face BO3-2 Coal 10.5 74.8Mine face BO3-2A Coal 9.2 73.7Mine face BO3-3 Coal 9.0 75.7Mine face BO3-4 Coal 14.8 85.1Mine face BO3-5 Coaly

sediments2.0 24.4

Mine face BO3-6 Coal 13.5 80.4Mine face BO3-9 Coal 19.7 95.7

ML46 12.11 m ML43A-3 Coal 7.2 76.314.11 m ML46A-1 Coal 13.0 104.713.11 m ML46A-2 Coal 7.8 77.311.11 m ML46A-4 Coal 10.8 79.010.11 m ML46A-5 Coal 8.9 71.638.3 m ML46B-1 Coal 5.7 71.437.30 m ML46B-2 Coal 4.7 63.7

Mukahcoalfield

MO1 Mine face MO1-1 Coal 3.6 58.5Mine face MO1-P Coaly

sediments2.9 45.6

MO2 Mine face MO2-1 Coal 3.6 81.0Mine face MO2-2 Coal 3.9 75.4Mine face MO2-3 Coal 4.7 93.0

MO3 Mine face MO3-2 Coal 2.3 49.4Mine face MO3-3 Coal 2.5 64.4

MO4 Mine face MO4-2 Coalysediments

0.5 17.3

Mine face MO4-3 Coal 2.2 70.3MO5 Mine face MO5-1 Coal 5.6 83.5

Mine face MO5-2 Coal 3.6 57.2Mine face MO5-3 Coal 6.5 90.0

O37 Outcrop O37-1 Coal 13.6 161.3Outcrop O37-2 Coal 2.9 69.3

O38 Outcrop O38-1 Coal 4.7 89.8O39 Outcrop O39-1 Coal 4.2 79.1O43 Outcrop O43-B Coal 1.8 28.9

Outcrop O43C Coal 2.9 64.3O45 Outcrop O45 Coal 5.6 113.2O46 Outcrop O46-B Coal 4.2 87.3

S1: Volatile hydrocarbon (HC) content, mg HC/g rock.S2: Remaining HC generative potential, mg HC/g rock.S3: Carbon dioxide yield, mg CO2/g rock.HI: Hydrogen Index ¼ S2 � 100/TOC, mg HC/g.OI: Oxygen Index ¼ S3 � 100/TOC, mg CO2/g TOC.Tmax: Temperature at maximum of S2 peak.PI: Production Index ¼ S1/(S1 þ S2).TOC: Organic Carbon, wt %.MI: Total moisture content, wt % (as-received basis).VM: Volatile Matter content, wt % (as-received basis).FC: Fixed Carbon, wt % (as-received basis).AS: Ash content, wt % (as-received basis).

The Begrih Formation consists of conglomerates, conglomeraticsandstones, mudstones, shales, tuffite and also a coal seam. Theformation contains mixed marine and brackish-water fauna south,suggesting depositional conditions that were probably predomi-nantly littoral (Hutchison, 2005). However, towards the north andeast, pure marine fossil such as Ammobaculites sp., Bolivia spp.,Cibicides sp., Elphidium sp., Frondicularia sp., Rotalia sp., Textularia

ived basis) of the Mukah and Balingian coals in the northwest Sarawak.

TOCwt%

Proximate analysis

S3(mg/g)

Tmax

(�C)S2/S3 HI OI PI MI

wt%VMwt%

FCwt%

ASwt%

9.4 417 8.1 183 23 0.09 41.4 25.0 36.3 33.5 5.27.2 417 13.9 274 20 0.05 36.4 17.4 41.1 40.1 1.4

12.0 413 5.7 159 28 0.10 42.8 15.3 49.3 32.3 3.18.7 412 8.2 163 20 0.10 43.7 25.2 38.0 34.2 2.68.3 409 10.0 186 19 0.11 44.6 23.8 36.5 35.8 3.9

11.8 413 4.5 151 34 0.10 35.0 25.4 35.4 34.4 4.811.5 413 5.3 170 32 0.09 35.5 e e e e

11.9 411 4.6 153 33 0.08 35.5 19.8 38.7 40.4 1.112.0 402 5.1 169 34 0.11 35.9 22.8 37.6 36.6 3.012.0 381 6.6 177 27 0.16 44.7 18.9 40.8 37.2 3.19.9 406 7.9 182 23 0.11 43.4 25.1 38.7 34.7 1.59.7 407 7.7 175 23 0.12 42.7 16.4 44.2 37.2 2.2

10.3 403 7.2 175 24 0.11 42.1 e e e e

9.8 405 7.7 163 21 0.11 46.4 15.4 41.5 40.1 3.014.1 399 6.0 190 31 0.15 44.7 18.5 39.8 37.4 4.34.6 423 5.3 195 37 0.08 12.5 19.9 42.3 35.7 e

14.4 402 5.6 188 34 0.14 42.7 28.2 37.8 30.3 3.713.9 398 6.9 208 30 0.17 46.1 21.9 41.0 34.6 2.59.4 407 8.1 164 20 0.09 46.5 27.1 37.0 33.0 2.9

12.0 407 8.7 232 27 0.11 45 31.7 35.6 30.0 2.712.3 408 6.3 180 29 0.09 42.9 32.5 36.9 27.5 3.19.0 406 8.8 182 21 0.12 43.3 31.8 33.9 29.4 4.9

13.8 402 5.2 200 39 0.11 35.9 29.9 37.6 30.3 2.29.4 419 7.6 181 24 0.07 39.4 26.7 37.4 31.0 4.99.2 414 6.9 161 23 0.07 39.5 30.5 31.1 32.4 6.08.7 421 6.7 237 35 0.06 24.7 20.6 25.9 20.4 33.14.2 428 10.9 257 23 0.06 17.7 23.7 19.0 9.1 48.2

9.5 417 8.5 191 22 0.04 42.4 20.5 33.8 42.6 3.18.0 420 9.4 221 23 0.05 34.1 11.4 27.9 27.4 33.38.9 415 10.4 236 23 0.05 39.4 11.4 33.7 34.2 20.75.6 424 8.8 183 21 0.05 27.0 12.5 20.1 20.9 46.56.7 420 9.6 206 21 0.04 31.3 17.5 27.0 30.9 24.62.9 427 6.0 141 24 0.03 12.3 7.4 14.5 8.7 69.4

9.9 412 7.1 152 21 0.03 46.2 17.8 36.1 40.7 5.410.7 417 7.8 229 29 0.06 36.5 23.2 35.9 35.7 5.24.6 426 12.4 235 19 0.06 24.4 e e e e

11.6 418 7.8 206 26 0.07 43.8 21.3 36.9 37.3 4.57.3 402 22.1 289 13 0.08 55.9 15.4 41.0 36.4 7.26.4 418 10.8 271 25 0.04 25.6 13.8 26.4 22.0 37.87.8 418 11.5 233 20 0.05 38.6 12.0 32.6 35.4 20.08.1 411 9.8 186 19 0.05 42.5 18.1 34.6 44.1 3.2

11.8 415 2.4 90 37 0.06 31.9 12.7 37.5 43.4 6.410.2 415 6.3 166 26 0.04 38.7 24.7 32.6 40.2 2.58.5 408 13.3 247 19 0.05 45.8 12.9 37.6 44.3 5.28.6 414 10.2 232 23 0.05 37.7 18.1 33.5 35.1 13.3

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e46 35

sp., and Triloculina sp., replaces the mixed fauna, which indicates anincreasing marine influence (Visser and Crew, unpublished). TheBelaga Formation is a highly deformed deep-water turbidity de-posit of the Upper Cretaceous to the Middle Eocene age (Hutchison,2005).

3. Sampling and methodology

A total of 45 samples were picked up from fifteen samplingsites (eleven sampling sites in the Mukah coalfield and foursampling sites from Balingian coalfield; see Table 1 and Fig. 2),applying the bench-by-bench channel sampling method. Thesamples were collected from coal seams within the Balingian andLiang Formations (Fig. 2). The sampling interval was decided onthe basis of changes in coal lithotypes, with each sample repre-senting a single bench sample with a bench thickness of not morethan 1 m.

3.1. Petrographic analysis

For maceral analyses, the coal samples were crushed to amaximum particle size of 1 mm, mounted in epoxy resin and pol-ished. The maceral analyses were performed on a Leica DM6000Mmicroscope in monochromatic and UV light illumination on 1000points. The maceral description used in this study follows the ter-minology developed by the International Committee for Coal and

Table 2Random huminite reflectance (R%), maceral composition (mineral free, %), and petrograp

Coalfields Samplingsites

SampleID

R% Huminite (%) Liptinite

Tx U Dh Ch Total Sp Cu

Balingiancoalfield

BO1 BO1-1 0.30 0 47 38 1 86 0 3BO1-2 0.36 47 19 1 28 95 0 2BO1-3 0.29 0 93 2 0 95 1 1BO1-5 0.32 0 24 53 1 78 0 0BO1-6 0.26 0 44 46 0 90 0 0

BO2 BO2-1 0.35 0 2 87 5 94 0 0BO2-5 0.35 0 5 74 1 80 0 2BO2-4 0.32 0 1 83 1 85 0 1

BO3 BO3-1 0.30 1 27 40 21 89 0 1BO3-2 0.30 0 8 66 1 75 0 0BO3-3 0.27 1 74 17 2 94 0 0BO3-4 0.35 0 9 52 4 65 0 0BO3-6 0.28 0 18 74 0 92 0 0BO3-9 0.32 1 60 31 1 93 0 1

ML46 ML46A-4 0.32 0 5 77 6 88 0 2ML46A-5 0.34 0 10 74 2 86 0 3ML46A-7 0.34 0 11 64 1 76 0 1ML46B-1 0.32 0 4 68 1 73 0 1ML46B-2 0.33 0 10 52 4 66 0 0

Mukahcoalfield

MO2 MO1-1 0.38 0 14 76 1 91 0 0MO2 MO2-3 0.37 4 33 44 10 91 0 2MO3 MO3-2 0.38 19 27 42 1 89 0 2

MO3-3 0.40 0 55 25 3 83 0 1MO5 MO5-1 0.36 0 7 82 0 89 0 1

MO5-3 0.37 0 41 52 0 93 0 0O37 O37-1 0.38 0 18 29 18 65 0 0

O37-2 0.39 1 8 79 6 94 0 0O38 O38-1 0.39 0 14 69 1 84 0 2O39 O39-1 0.38 0 15 71 3 89 0 2O43 O43C 0.36 0 15 67 3 85 1 3O46 O46B 0.38 0 2 75 0 77 0 12

Mean 0.34 3 23 55 4 85 1 2

R%: Huminite reflectance.Huminite e Tx: Textinite; U: Ulminite; Dh: Detrohuminite; Ch: Corpohuminite.Liptinite e Sp: Sporinite; Cu: Cutinite; Rs: Resinite; Ld: Liptodetrinite; Sub: Suberinite; EInertinite e Fg: Funginite; Idt: Inertodetrinite; F: Fusinite; Sf: Semifusinite; Ma: MacriniTPI ¼ (telohuminite þ semifusinite)/(detrohuminite þ macrinite þ inertodetrinite).GI ¼ huminite/inertinite.

Organic Petrology for huminite (Sykorova et al., 2005), liptinite(Taylor et al., 1998) and inertinite (ICCP, 2001) nomenclature. Thereflectance measurements were performed under a mono-chromatic light of 546 nm using a Leica DM6000Mmicroscope andan optical sapphire glass standard having a reflectance of 0.589% inoil immersion, following the procedures outlined by Taylor et al.(1998). The rank was determined by measuring the randomreflectance on huminite and the values reported were an average ofat least 100 measurements per sample.

3.2. Organic geochemical analyses

Geochemical analyses include proximate and pyrolysis analyses,as well as bitumen extraction, were performed at the GeochemistryLaboratories of the Department of Geology in the University ofMalaya.

Part of the samples was ground to <150 mm and analysed on aDiamond ThermogravimetriceDifferential Thermal Analyser (TGeDTA). Proximate analysis carried out to determine moisture, ash,volatile matter and fixed carbon contents. Proximate analysis wasfollowed ASTM standard (1990).

All of the samples were screened by Source Rock Analyzer (SRA).The collected samples were crushed into fine powder (<150 mm)and analysed using (SRA-Weatherford)-TOC/TPH instrument(equivalent to Rock-Eval equipment). Parametersmeasured are TOCcontent and S1, S2, S3 pyrolysis yields and temperature of maximum

hic facies indices of the studied Mukah and Balingian coals.

(%) Inertinite (%) TPI GI

Rs Ld Sub Ex Total Fg Idt F Sf Ma Total

3 2 3 0 11 0 1 1 1 0 3 1.23 28.70 0 1 0 3 0 1 0 0 1 2 33.50 96.01 0 1 0 4 0 1 0 0 0 1 31.00 95.00 2 14 0 16 1 1 2 3 0 6 0.50 13.01 1 1 0 3 0 3 2 2 0 7 0.94 12.92 1 1 0 4 1 0 0 1 0 2 0.04 47.03 1 12 0 18 1 0 0 1 0 2 0.08 40.01 1 9 0 12 0 1 0 2 0 3 0.04 28.30 5 2 0 8 0 1 1 1 0 3 0.71 29.72 0 22 0 24 0 0 1 0 0 1 0.12 75.01 1 3 0 5 0 0 1 0 0 1 4.41 94.00 1 32 0 33 0 0 1 1 0 2 0.19 32.51 1 5 0 7 0 0 0 1 0 1 0.26 92.00 0 4 0 5 1 0 0 1 0 2 2.00 46.51 1 6 0 10 0 1 0 1 0 2 0.08 44.03 1 4 0 11 1 2 0 0 0 3 0.13 28.71 1 18 0 21 1 1 0 1 0 3 0.18 25.3

19 1 3 0 24 1 1 1 0 0 3 0.06 24.30 1 30 0 31 1 0 1 1 0 3 0.21 22.06 0 1 0 7 0 0 1 1 0 2 0.20 45.51 1 2 0 6 1 0 2 0 0 3 0.84 30.32 1 0 0 5 0 3 2 1 0 6 1.04 14.81 1 6 0 9 1 0 4 3 0 8 2.32 10.42 0 0 0 3 3 2 1 2 0 8 0.11 11.10 0 3 0 3 2 1 1 0 0 4 0.77 23.31 1 31 0 33 0 2 0 0 0 2 0.58 32.52 2 1 0 5 0 1 0 0 0 1 0.11 94.03 1 3 0 9 1 2 1 3 0 7 0.24 12.00 1 6 0 9 1 1 0 0 0 2 0.21 44.53 1 3 0 11 1 1 0 2 0 4 0.25 21.38 1 0 0 21 0 1 1 0 0 2 0.03 38.52 1 7 0 12 1 1 1 1 0 4 2.66 40.4

x: Exsudatinite.te; TPI: Tissue Preservation Index; GI: Gelification Index.

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e4636

S2 pyrolysis yield (Tmax). Hydrogen (HI), oxygen (OI) and production(PI) Indies were calculated (Table 1).

The soluble organic matter (bitumen) was extracted from pul-verized coal samples (<150 mm) using a Soxhlet apparatus for 72 husing an azeotropic mixture of dichloromethane (DCM) andmethanol (CH3OH) (93:7 v/v). The fractions of the soluble organicmatter were separated into saturated hydrocarbons, aromatic hy-drocarbons and NSO compounds by liquid column chromatographyover silica gel and aluminium oxide.

4. Results and discussion

4.1. Petrographic characteristics of coal

4.1.1. Huminite reflectance measurementsThe huminite reflectance measurements of ulminite are given in

Table 2. The mean random reflectance measurement of ulminitemaceral of the studied coals varied from 0.26 to 0.39%, indicatingthe immature nature of the materials.

Figure 3. Photomicrograph of (a) ulminite (U) associated with resinite (Rs) and funginite (F(Sp) and funginite (Fg); (c) the presence of textinite (Tx), ulminite (U), phlobaphinite (Ph)phlobaphinite (Ph) of the corpohuminite maceral associated with resinite; (e) funginite (Fg(Ex) in the textinite (Tx). All photos were taken under reflected light examination.

4.1.2. Maceral compositionThe maceral content of the analysed coals is reported in Table 2

(vol. %, mineral matter free) and illustrated in Figures 3 and 4. Thecoal samples are classified as humic coal (Fig. 5) and dominated byhuminite (65e95%), with low to high amounts of liptinite (3e33%)and low amounts of inertinite (1e8%).

Huminite group is the most abundant macerals in the studiedcoals and range from 65 to 96%, in total three sets of samples andwere occurred in approximately equal proportions of telohuminite(ulminite/textinite) and detrohuminite (Table 2). Generally, indi-vidual samples are dominated by telohuminite and detrohuminiteas subgroup of Huminite group or the other, indicative of therelative degree of degradation and decomposition of the originalpeat material (Hackley et al., 2007). The detrohuminite (Fig. 3e)content varied greatly in the studied coal, ranging from 1 to 87%with an average of 55% (Table 2). The ulminite content is also variedgreatly in the studied coals (Fig. 3a, b and d), ranging from 1 to 93%(Table 2). Textinite (Fig. 3c and f) was present in low concentrationsin most of the studied samples (1%), with the exceptions of only

g), inertodetrinite (Idt) and semifusinite (Sf); (b) ulminite (U) associated with sporiniteof the corpohuminite maceral and porigelinite (Pg); (d) the presence of ulminite (U),) in the matrix of attrinite of the detrohuminite (De) and ulminite (U); (f) exsudatinite

Figure 4. Photomicrograph of (a) yellow to orange fluorescing suberinite (Sb) associated with sporinite; (b) Bright yellow fluorescing resinite (Rs) associated with cutinite (Cu) andliptodetrinite (Ld); (c) yellow to orange fluorescing cutinite (Cu); (d) yellow fluorescing cutinite (Cu) associated with sporinite (Sp) and greenish fluorescing fluorinite (Fi); (e) yellowto greenish fluorescing fluorinite (Fi) associated with sporinite (Sp) and cutinite (Cu); (f) yellow to greenish fluorescing fluorinite mega-sporinite (Sp) associated with liptodetrinite(Ld). All photos were taken under reflected UV light examination. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of thisarticle.)

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e46 37

samples MO2-3, BO1-2 and MO3-2 having a relatively high contentof textinite, at 4%, 19% and 48%, respectively (Table 2). The coalscontained low amounts of corpohuminite, usually below 6%(Table 2), except for MO2-3, O37-1, BO3-1 and BO1-2 samples,which contained 10%, 18%, 21% and 28% of corpohuminite respec-tively (Table 2). Corpohuminite occurred mainly as cell fillings intextinite (Fig. 3c and d).

Liptinite group is containing significant amounts of terrestrialliptinite macerals and ranging from 3 to 33% (Table 2). The liptinitemacerals were identified by their different nature of fluorescence inultraviolet light (Fig. 4). The common liptinite macerals in the coalsare sporinite, cutinite, resinite, suberinite and liptodetrinite, whilethe unstructured liptinites included fluorinite and exsudatinite.Higher liptinite content (>10%) was recorded in samples BO1-1,BO1-5, BO2-5, BO2-4, BO3-2, BO3-4, ML46A-5, ML46A-7, ML46B-1, ML46B-2, O37-1, O38-1, 046B, and O43C, with liptinite contentas high as 31e33% (Table 2). High liptinite content in these samples

was due mainly to the presence of high amounts of suberinite (upto 33%), and also resinite (Table 2). Suberinite content varied greatlyfrom non-existent to 32% (Table 2), which it occurred as cell walltissues and was characterized by a yellow to brownish-yellowfluorescence (Fig. 4a). Resinite appeared mostly as an isolatedglobular bodies (Fig. 4b), but there were also some cell-filling intextinite (Sia and Abdullah, 2012). The maceral exists as infillings oftextinite cells and points towards the peat contribution fromconiferous vegetation (Stefanova et al., 2013). The contents ofcutinite and liptodetrinite were low, ranging from 0 to 12% and 0 to5%, respectively (Fig. 4bed and Table 2). Liptodetrinite in thestudied coal was often present was as finely dispersed in agroundmass of attrinite (Fig. 4b). Other liptinite macerals such assporinite, exsudatinite and fluorinite had also been observed in thestudied coal, but they were either absent or were present in traceamounts (Table 2). Exsudatinite in the studied coal appeared ascrack fillings and was yellow in fluorescent light (Fig. 3f). Fluorinite

Figure 5. Ternary diagram of maceral group composition (huminiteeliptiniteeiner-tinite) for analysed Mukah and Balingian coals.

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e4638

in the studied samples was always associated with cutinite andcharacterized by a greenish-yellow in fluorescent light (Fig. 4d ande). Sporinite in the studied coal was appeared as micro and mega-sporinite and associated with cutinite (Fig. 4def).

Inertinite group was also widely detected in the studied coalswith low amount ranging from 1 to 8% (Table 2). The inertinitemacerals identified were semifusinite, fusinite, inertodetrinite,macrinite and funginite. The inertodetrinite, semifusinite andfunginite are representing major macerals of the inertinite (1e3%),whereas fusinite and macrinite are representing minor macerals(<2% and <1%, respectively) as shown in Table 2. Fungus, seen asthemaceral funginite (Fig. 3a, b and e), is known to play a role in thedevelopment of degraded maceral forms (Hower et al., 2010, 2011;O’Keefe and Hower, 2011).

4.2. Geochemical characteristics of coal

4.2.1. Molecular compositionThe amounts of Extractable Organic Matter (EOM) together with

the relative proportions of saturated and aromatic hydrocarbonsand NSO (nitrogen/sulphur/oxygen) compounds have been

Table 3Bulk geochemical results of soluble organic matter (EOM) yields (ppm), relative proportioof selected Mukah and Balingian coals.

Coalfields SamplesID

Bitumen extraction and chromatographicfractions (ppm of whole rocks)

EOM Sat. Aro. NSO HC

Balingiancoalfield

BO1-5 47405.3 4724.00 3283.18 39398.2 8007.BO1-6 45202.5 1459.32 4377.96 39365.2 5837.BO3-9 92143.7 1813.14 2629.05 87701.5 4442.ML46B-2 31857.7 1954.5 5863.4 24039.8 7817.

Mukahcoalfield

MO1-1 25724.9 1590.76 7226.62 16907.6 8817.MO3-3 29333.3 2120.48 2456.22 24756.6 4576.O38-1 47260.8 2297.00 4881.13 40082.7 7178.O46B 47663.7 2008.30 6761.28 38894.1 8769.

EOM ¼ Extractable organic matter (Bitumen extraction).Sat ¼ Saturated hydrocarbons.Aro ¼ Aromatic hydrocarbons.NSO ¼ Nitrogen, Sulphur, Oxygen components.HC ¼ Hydrocarbon fractions (Saturated þ Aromatic).

calculated and tabulated (Table 3). The EOM yields from the coals ofthe Mukah and Balingian coalfields range from 25724.9 to92143.7 ppm. All the extracts are mainly composed of hetero-compounds (NSO) in the range of 65.7e95.1% (Table 3), which iscommon for coals. In most of the extracts of analysed coals, aro-matic hydrocarbon (aromatics; 2.9%e28.1%) are more abundantthan saturated hydrocarbons (alkanes; 2.0%e10.0%). The variationin the alkane content has been related to the origin of plant ma-terial and to the intensity of biochemical degradation of the plantmaterial (�Zivoti�c et al., 2008). The saturated and aromatic fractionstogether create the petroleum-like hydrocarbon fraction; thus, thesum of these two fractions is referred as hydrocarbons (HCs). Sincethe hydrocarbon portion of the bitumen extracted from sediment isthe petroleum-like portion, it is used as an important parameter inthe source-rock evaluation (Philippi, 1957; Baker, 1972). In thisrespect, the Mukah and Balingian coals are likely to be prolificpetroleum sources where abundant gas with limited oils may beexpected to generate. This is suggested by relatively high hydro-carbon fractions (4.9%e34.3%; see Table 3) and moderately satu-rated hydrocarbon proportions (2.0%e10.0%). The hydrocarbongenerative potential of a source rock can also be estimated fromplots of TOC content versus extractable organic matter (EOM) andhydrocarbon yields (Fig. 6). The plots show that the studied coalsamples are viable source rocks for significant gas and limited oil-generation potential (Fig. 6) as supported by the TOC content andpyrolysis S2/S3 yields (Fig. 7).

4.2.2. Organic geochemical analysesTotal organic carbon (TOC) content and pyrolysis data were

performed to character the organic content, hydrocarbon potentialof the organic matter and its thermal maturity level. Total organiccarbon (TOC) analysis demonstrated high TOC values of the coalsand coaly sediments from Mukah and Balingian coalfield (Table 1).Samples from Mukah and Balingian coalfield are characterized byvery similar TOC values with slightly higher TOC values in theMukah samples (Table 1). The content of hydrocarbon yield (S2)generated during pyrolysis is a useful parameter to evaluate thegeneration potential of source rocks (Peters, 1986; Bordenave,1993). The Mukah and Balingian samples were characterised bypyrolysis S2 yield values in the range of 17.3e161.3 mg HC/g rock(Table 1). These TOC contents and pyrolysis S2 yield values meet theaccepted standards of a source with very good to excellent sourcerock potential (Fig. 8) as classified by Peters and Cassa (1994).Hydrogen index (HI) and oxygen index (OI) of the studied sampleswere calculated and determined in the range of 90e289 mg HC/g

ns of saturated and aromatic hydrocarbons and NSO compounds of the EOM (inwt %)

Chromatographic fractions ofBitumen extraction (EOM wt%)

TOC(wt.%)

HC/TOC(mg/g TOC)

Sat./EOM Aro./EOM NSO/EOM HC

18 10.0 10.9 79.1 20.9 44.6 179.529 3.2 9.7 87.1 12.9 35.0 166.819 2.0 2.9 95.1 4.9 46.1 96.490 6.1 18.4 75.5 24.5 39.5 197.938 6.2 28.1 65.7 34.3 24.7 356.970 7.2 8.4 84.4 15.6 31.3 146.213 4.9 10.3 84.8 15.2 38.6 185.958 4.2 14.2 81.6 18.4 37.7 232.6

Figure 6. Plots of TOC content versus bitumen extractions and hydrocarbon yields, showing source potential rating and hydrocarbon source-rock richness for the selected coals.

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e46 39

TOC and 13e39 mg CO2/g TOC, respectively (Table 1). In addition,Tmax value which represents the temperature at the point where S2peak is the maximum is also determined (Espitalié et al., 1977),whereby Tmax values ranging from 381 to 428 �C (Table 1), indi-cating the immature organic matter as agreement with huminitereflectance values (Fig. 9) and supported by hydrocarbon extractionyields (Fig. 10). A diagram was constructed based of pyrolysis data,kerogen classification and thermal maturity using the HI versusTmax data as carried out by above cited authors. In general, the re-sults show that the Balingian coals are dominated by Type IIIkerogen while Mukah coals are dominated by Type II/III kerogens(Fig. 11). In this respect, all the analysed samples are generallyplotted in the immature organic matter of Type III kerogen andmixed Type II/III kerogens (Fig. 11). In comparison to the othercoalfields, Mukah coals are more capable to generate oil comparing

to Balingian coals. This is most probably due to the higher pyrolysisS2/S3 yields of the Mukah coals (Fig. 7).

4.2.3. Proximate analysis and coal rankProximate geochemical analysis is carried out to determine the

rank and suitability of coals for various industries uses (ASTM,1989) and to appreciate thermal maturity of the coal bearing se-quences and individual coal seams (Ward, 2002). Results fromproximate analysis indicated that the coals from Mukah and Bal-ingian coalfields generally are characterized by similar volatilematter and fixed carbon contents with higher values in the Bal-ingian coal samples (Table 1). The volatile matter and fixed carboncontents indicate that the coals could be consider as lignite ortransitional to sub-bituminous C coals according to ASTM classifi-cation (Stach et al., 1982). This interval is also confirmed by

Figure 7. A plot of total organic carbon (TOC) versus S2/S3 yields, showing potentialhydrocarbon generative and type of the studied coals.

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e4640

huminite reflectance values, which vary in the range of 0.26 and0.39%.

The ash content of the analysed coal samples from the Balingianand Mukah coalfields (Sarawak basin) ranges between 1.1 wt% to69.4 wt% (Table 1). Generally, the ash contents were low to mod-erate for the Balingian coals, ranging from 1.1 to 6.0 wt%, whereastheMukah coals are characterized by high ash contents in the rangeof 2.5e69.4 wt. % (Table 1). In the literature, the presence of highash content is explained by different phenomenon. One is the peatdepositional environment where conditions are a periodic floodingof the paleomire during deposition (�Zivoti�c et al., 2013). Moreover,Siavalas et al. (2009) reported the high ash contents from the Ter-tiary and Quaternary coal deposits in the Megalopolis Basin andrelated to the high inorganic input during peat accumulation. Thehigh ash content of Mukah coals is explained by the high peatinorganic input during peat accumulation as indicated by Sia andAbdullah (2011). In addition, TOC values show good negative cor-relationwith the ash contents (Fig.12; R2¼ 0.75) indicating that thechanges in TOC contents are due to the increased contribution of

Figure 8. Pyrolysis S2 versus total organic carbon (TOC) plot showing generativesource rock potential for the Tertiary coals from northwest Sarawak.

inorganic matter, rather than to coalification degree (e.g., Stefanovaet al., 2013).

The total moisture contents obtained from the coal samples inBalingian and Mukah coalfields are given in Table 1. In comparisonto other coalfields, coal samples from the Mukah coalfield arecharacterized low total moisture contents, in the range of 7.4e24.7 wt. %, whereas the Balingian coals are characterized by hightotal moisture contents, in the range of 15.3e32.5 wt. %. The lowertotal moisture content in the Mukah coals is explained by maturityof the Mukah coals more mature comparing to the Balingian coalsas indicated by huminite reflectance and pyrolysis Tmax data(Fig. 13). The main explanation could be the depth of burial and ageof the analysed coals. The Mukah coals are older than Balingiancoals as concluded from stratigraphic sections (see Section 2).

4.3. Hydrocarbon generation potential

The type of the organic matter and content of hydrocarbons thatmight be generated were appreciated based on organic geochem-ical and optical data. Different types of kerogen will producedifferent type of hydrocarbons. Generally, Type I and Type IIkerogen characterizing marine and lacustrine type are consideredas the best kerogen capable to generate liquid hydrocarbons. TypeIII kerogens that are mostly characterized by terrigenous organicmatter are considered as gas-prone. Several studies indicate thatthere is a direct correlation between pyrolysis data and petroleumgeneration potential (Bordenave et al., 1993; Hunt, 1996). Thethresholds for the source rock quality are based on HI valuesdefined by Peters and Cassa (1994): samples that contain a Type IIIvitrinitic kerogenwould be expected to generate gas with hydrogenindex <200 mg HC/g TOC whereas samples with hydrogen indexvalues higher than 200 mg HC/g TOC can generate oil althoughtheir main generation products are gas and condensate. Moreover,samples characterized by HI higher than 300 mg HC/g TOC cangenerate oil (Bordenave et al., 1993; Hunt, 1996). In this study, forthe coal samples are determined HI values in the range of 90e289 mg HC/g TOC, indicating that the coals can generate oilalthough their main generation products are gas. This is supportedby the characteristics of the extracted bitumen and hydrocarbonyields.

Petrographic observations were also performed and the dataobtained were compared with the results from pyrolysis. Micro-scopic observations of the studied coal samples indicate that thecoals are characterized by low to high amounts of liptinite (Table 2).

The significant representation of liptinite macerals are sporinite,suberinite, resinite, liptodetrinite and cutinite. Suberinite, a mac-eral that is almost exclusively found in Tertiary coals and only in afew Mesozoic coals (Teichmüller and Teichmüller, 1982) was rec-ognised as the most common liptinite maceral in this study(Table 2). This suberinitemaceral shows a close associationwith themacerals sporinite, cutinite, exsudatinite and liptodetrinite. It ismost apparent that the oil-prone nature of the analysed coals ispredominantly attributed to the common occurrence of suberiniteand its associated macerals such as liptodetrinite, resinite, cutinite,bituminite, sporinite and exsudatinite. Suberinite and cutinite areabundant waxy oils might be the expected products (Stasiuk et al.,2006), and where resinite and other liptinite macerals (e.g., lipto-detrinite and sporinite) are likely the most productive sources inthe coals thus where the former is abundant naphthenic oil tocondensate might be expected (Petersen et al., 2013).

The relatively higher value of HI and liptinite content are alsosupported by the presence of n-alkene/n-alkane doublets pre-dominant in the open system pyrolysis gas chromatography (Py-GC) for a coal samples (e.g., Wan Hasiah,1999; Petersen et al., 2001;Alias et al., 2012; Hakimi and Abdullah, 2013). The open system

Figure 9. A plot of Tmax values versus huminite reflectance (%Ro) values, showing good agreement between Tmax and huminite reflectance (%Ro) data and generally an immaturestage for the Mukah and Balingian coals.

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e46 41

pyrolysis gas chromatography (Py-GC) of the analysed coals(Fig. 14) depicts mixed kerogen fingerprints of predominantly n-alkane/n-alkene doublets and aromatic compounds. In the Mukahcoal samples where the liptinite macerals content is higher than35% and relatively high hydrogen index values (289 mg HC/g TOC)are determined, the Py-GC is dominated by n-alkane/n-alkenedoublets extended beyond C30 are indicative for aliphatic-rich, oil-prone nature of these macerals (Fig. 14a) (Petersen et al., 2001). Incontrast, in the Balingian coal samples characterized by relativelylow liptinite macerals content and low hydrogen index values, thePy-GC also is displayed by n-alkane/n-alkene doublets extendedbeyond C30. They are indicative for aromatic-rich, and with signif-icant aliphatic compounds, suggest oil although their main gener-ation products are gas (Fig. 14b).

Figure 10. A plot of extract yield (mg HC/g TOC) versus percent of hydrocarbon in the total e1978).

4.4. Palaeoenvironmental conditions of peat-forming

The maceral composition of coals reflects the organic sourcematerials which contributed to the accumulation of peat andfurther provides information about the conditions during deposi-tion (Kalkreuth et al., 1991). Maceral analysis measures the relativeproportions and interrelationships of various maceral groups(Table 2). The diagnostic macerals and petrographic facies indicesderived from this analysis have been used as an indicator for thepalaeoenvironment of the coal-forming peat.

The high amount of huminite macerals with a general pre-dominance of detrohuminite (Table 2), indicates overall oxygen-deficient depositional conditions in the peat-forming mires anddeposition in waterlogged conditions (wet forest) (Flores, 2002;

xtracts (HC%), showing source rock potential and maturity level (modified after Powell,

Figure 11. Plot of Hydrogen Index (HI) versus pyrolysis Tmax, showing kerogen quality and thermal maturity stages of the analysed coal samples in the northwest Sarawak.

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e4642

Petersen et al., 2009; Erik, 2011). Detrohuminite has also beenconsidered to be derived from herbaceous, cellulose-rich wood(Teichmüller, 1989) and from poor preserved big woody plantsdue to prolonged humification in slowly subsiding paleomires(Diessel, 1992; Petersen et al., 2009; Súarez-Ruiz et al., 2012). This

Figure 12. A plot of total organic carbon (TOC) versus ash conte

suggests that the resulting maceral composition is also influencedby the degradational conditions in the mires as indicated by thepercent of funginite maceral (Fig. 3a, b and e) (e.g., Hower et al.,2010, 2011; O’Keefe and Hower, 2011). In contrast, the lowamount of detrohuminite in the studied coals was always

nts of the analysed coal samples in the northwest Sarawak.

Figure 13. Scatter plot showing the relationship between moisture and vitrinite/huminite reflectance and pyrolysis Tmax.

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e46 43

accompanied by increase of ulminite and/or textinite and corpo-huminite, indicating either increasing forest type mires or a lowerdegree of gelification under relatively dry conditions (Sia andAbdullah, 2012).

The relatively low inertinite content in the Balingian andMukahcoals assumes low levels of peat fire and/or oxidation occurred inthese mires (e.g. Scott and Glasspool, 2007). The presence of highamounts of liptinite groupmacerals like suberinite, resinite and thepresence of cutinite also suggests an accumulation in a forest typeswamp (Ratanasthien et al., 1999; Erik, 2011).

The palaeoenvironment of the coal-forming peat has also beeninterpreted using petrographic facies. In petrographic facies, thepetrographic composition of coal seams and petrography-basedfacies indicators (gelification index (GI) and tissue preservationindex (TPI)) have been used to track the evolution of peat-forming depositional environments (Calder et al., 1991; Diessel,1986, 1992; Kalkreuth et al., 1991; Siavalas et al., 2009; Jasperet al., 2010; Koukouzas et al., 2010; �Zivoti�c et al., 2013 and

many others). The GI-TPI diagram was firstly proposed by(Diessel, 1986) for high-rank Australian Permian coals. For lowrank Miocene and Jurassic coals, these indices have been modi-fied by Kalkreuth et al. (1991). The GI and TPI are used in thepresent study as they were modified by Sia and Abdullah (2012)for low-rank Tertiary coals (Table 2). The petrographic data fromthe present study indicate significant fragmentation of theorganic matter in the Mukah and Balingian coals (Tissue Preser-vation Index, TPI < 1) and high gelification (Gelification Index,GI > 1) for most of the coal samples (Fig. 15). The fragmentationof the organic matter is mainly due to the herbaceous peat-forming plants, and hence the accumulation mostly of soft tis-sues and trees were rare (Koukouzas et al., 2010 Jasper et al.,2010). This is also due to a hydrodynamic level that favouredthe mechanical destruction of tissues during short-term trans-portation (Koukouzas et al., 2010).

For the coals with low TPI values (<1; Table 2) could beassumed a high large scale destruction of wood in forested

Figure 14. Open system pyrolysis gas chromatograms for representative samples from the: (a) Balingian coalfield (BO3-9) and (b) Mukah (O37-1) coalfields of the northwestSarawak.

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e4644

swamps (Amijaya and Littke, 2005; Diessel, 1992) Respectively,the organic matter preservation was low to extremely low(Siavalas et al., 2009). Moreover, the low TPI values (TPI < 1;Table 2) could imply significant contribution gymnosperm vege-tation is more resistant to decay because of high content ofresinous compounds. They protect tissue from microbial attack(Mandic et al., 2008). In contrast, high TPI value (TPI > 1) wasdetermined for samples BO1-1, BO1-2, BO1-3, BO1-2, BO3-3 andBO3-9, suggesting well-preserved plant tissues (textinite andulminite) (�Zivoti�c et al., 2013). On the other hand, it was suggestedthat tissue preservation depends mostly on the water level andthe climatic conditions during peat accumulation, rather than onthe botanical properties of the vegetation (�Zivoti�c et al., 2013).According to this study, higher GI values (>10), it could beassumed that during peatification water column level was mod-erate to high (e.g., �Zivoti�c et al., 2013) and represent predomi-nantly topogenous mire conditions (Jasper et al., 2010). Moreover,the high values of GI in the studied coals further suggest

gelification of plant tissues in continuous wet forest swamp(Diessel, 1992; Sia and Abdullah, 2012) and could imply pro-nounced microbial activity (�Zivoti�c et al., 2013).

TPI vs. GI diagram is shown in Figure 15 shows data of all thesamples. From it could be surmise that palaeomire was created byherbaceous plants able to thrive in a marsh-wet forest swampenvironment. Nevertheless, marsh and forested swamp areconsidered as a kind of minerotrophic mires (Amijaya and Littke,2005). Coals originating from both of these sources usuallygenerate relatively high ash yield (Amijaya and Littke, 2005;Diessel, 1992), which is consistent with the case for the studiedcoals with ash contents in the range of 1.1e69.4 wt.%. In the presentcase, the low TPI (TPI < 1) which is accompanied by relatively highash contents of the studied coal, could also be related to sedi-mentary environment with clastic contribution such as a lowerdeltaic plain (Escobar and Martínez; 1997). This shows that theinterpretation as suggested by the Diessel’s diagram is valid for thestudied coals (Fig. 15).

Figure 15. Diagram of TPI versus GI showing the paleodepositional environment of the Balingian and Mukah coals facies.

M.H. Hakimi et al. / Marine and Petroleum Geology 48 (2013) 31e46 45

5. Conclusions

Organic petrographic and geochemical analyses were per-formed on the coal seams within the Balingian and Liang Forma-tions in Mukah and Balingian coalfields of northwestern Sarawak.Coal rank and petroleum generation potential as well as palae-oenvironment conditions using critical petrographic facies andmaceral compositions were studied. The data gave ground toformulate the following conclusions:

(1) The organic matter is classified on pyrolysis HI versus Tmaxdiagram the OM was determined as predominantly Type IIIkerogen (gas-prone) grading into mixed Type IIeIII kerogens(oil and gas-prone) as indicated by hydrogen index values (90e289 mg HC/g TOC). This assumption is also supported bymacerals compositions, dominated by huminite, with low tomoderate amounts of liptinite.

(2) The geochemical classification of thermal maturity (coal rank)based on proximate and huminite reflectance values suggestthat the Mukah and Balingian coals are generally thermallyimmature for hydrocarbon generation potential and range fromlignite to sub-bituminous C rank. In addition, the pyrolysis Tmaxdata and hydrocarbon extraction yields confirm this attainedthermal maturity level.

(3) The studied coals are dominated by huminite (detrohuminiteand ulminite) with low amounts of inertinite, suggesting pre-dominantly herbaceous plants in the paleomires preservedunder anaerobic deposition conditions with limited thermaland oxidative tissues destruction.

(4) Most of the studied coal samples are characterized by low TPIand high GI, suggesting a lower deltaic plain wet peat-swampdepositional setting. They are also plotted on the marsh fieldof the Diessel’s diagram, which usually are characterised by

relatively high ash contents (Amijaya and Littke, 2005; Diessel,1992), consistent with the observation for the studied coals.

Acknowledgements

The authors would like to sincerely thank Mr. Dorani J. and Mr.Bakar J. of Sarawak Coal Resources Sdn. Bhd., who provided theopportunity to access the sampling sites. The authors are mostgrateful to the Department of Geology, University Malaya forproviding facilities to complete this research. The authors alsowould like to express their gratitude to Mr. Wong, V.C. of theMinerals and Geoscience Department Malaysia (Sabah) for fieldassistance and Ms. Jacinta John for arranging field transportation.We would like to sincerely thank an associate Editor MassimoZecchin and an anonymous reviewer for their careful and usefulcomments that improved the revised manuscript. The studyreceived financial support from the University of Malaya researchgrants (Nos. PS438-2010A, RP002C-13AFR and RG145-11AFR).

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