organic geochemical characteristics and depositional environment

International Journal of Coal Geology 104 (2012) 9–21

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International Journal of Coal Geology

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Organic geochemical characteristics and depositional environment of the TertiaryTanjong Formation coals in the Pinangah area, onshore Sabah, Malaysia

Fatin Liyana Alias a, Wan Hasiah Abdullah a, Mohammed Hail Hakimi b,⁎,Mohd Harith Azhar a, Ralph L. Kugler a

a Department of Geology, Faculty of Science, University Malaya, 50603 Kuala Lumpur, Malaysiab Geology Department, Faculty of Applied Science, Taiz University, 6803 Taiz, Republic of Yemen

⁎ Corresponding author.E-mail address: [email protected] (M.H. Haki

0166-5162/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.coal.2012.09.005

a b s t r a c t
a r t i c l e i n f o
Article history:Received 6 July 2012Received in revised form 3 September 2012Accepted 11 September 2012Available online 19 September 2012

Keywords:Tertiary coalsTanjong FormationBiomarker distributionsLiquid hydrocarbonsPinangahMalaysia

The Tertiary Tanjong Formation coals exposed in the west middle block of the Pinangah Coalfield, central partof southern Sabah were analyzed, and their depositional environments were interpreted. The TertiaryTanjong coals are humic and generally dominated by vitrinite, with significant amounts of liptinite and lowamounts of inertinite macerals. Total organic carbon contents (TOC) of the coals range from 51.2 to77.7 wt.%, and yield of bitumen values ranging from 57,300 to 140,000 ppm, which meet the standard as asource rock with good hydrocarbon-generative potential. In support, good liquid hydrocarbons generationpotential can be expected from the Tanjong coals based on significant liptinitic content (>15%). This issupported by their high hydrogen index up to 300 mg HC/g TOC, consistent with Type II and mixed TypeII–III kerogens and Py–GC (S2) pyrograms with n-alkane/alkene doublets extending beyond C30. The coalsamples have vitrinite reflectance values in the range of 0.42%–0.66 Ro%, indicating immature to early maturestage for hydrocarbon generation. These vitrinite reflectance values also indicated that the Tanjong coals arewith sub-bituminous B–A and high volatile bituminous C rank. Tmax values ranging from 419 to 451 °C aregood agreement with the vitrinite reflectance data. This is supported by biomarker maturity parameters assuggested by the C32 homohopane. The saturated fraction of the Tanjong coals are characterized by dominantodd carbon numbered n-alkanes (n-C23 to n-C33), high Pr/Ph ratios (8–18), high Tm/Ts ratios (6–28), and pre-dominant of regular sterane C29. All biomarker parameters clearly indicate that the organic matter was de-rived from terrestrial inputs and deposited under oxic condition.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

In Malaysia, Cenozoic coals are very common and have been ana-lyzed based on organic geochemical and petrographic characteristics(e.g. Azlan et al., 2011; Hakimi and Abdullah, 2012; Wan Hasiah,1997, 1999, 2003; Zulkifli et al., 2008). The Tertiary coal occurrencesare found in Sabah and some areas around Sarawak and Labuan Islandof Malaysia. Sabah is one of the 16 states of Malaysian Federation lo-cated in Borneo Island (Fig. 1) which has emerged as one of the po-tential coal power in South East Asia. The area that forms the scopeof this study lies in the west Middle Block of Pinangah (Fig. 1) locatedin the central part of southern Sabah. Pinangah is one area undergoingcoal exploration and research since the Maliau Basin, which is locatedin the central part of southern Sabah, was proven to contain coal byCollenette in, 1965. A few organic geochemistry studies of the Tertiarycoals and coaly sediments were carried out in adjacent to the Pinangaharea (Zulkifli et al., 2008), however the organic geochemistry and

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petrographic investigations of the Pinangah coal are very limited. Theaim of this study is to evaluate the hydrocarbon generation potentialof the Tanjong Formation and interpret their depositional environmentsbased on organic petrographic method such as maceral analysis andvitrinite reflectance as well as organic geochemical method such asTOC, pyrolysis, and Py–GC. In addition, bitumen extraction and bio-marker distributions were used to further discern the assessment ofthermal maturity and depositional environment conditions. Outcropsamples were collected from twelve locations within the Tanjong For-mation of the Pinangah Coalfield (Fig. 1). These outcrops are locatedin the dense tropical rain forest regions and are yet to be explored indetail.

2. Geological setting

The central Sabah circular basins begin with the deep marine Oli-gocene Labang Formation on the western sides of the basins and inthe Miocene paralic condition and low energy deposition of theTanjong Formation developed towards the depocenter (Collenette,1965). The stratigraphic column of the Pinangah area of the southern

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Fig. 1. Location map of the Pinangah area in the central part of southern Sabah, Malaysia.

10 F.L. Alias et al. / International Journal of Coal Geology 104 (2012) 9–21

Sabah area is as shown in Fig. 2. The Pinangah area is underlain by theTanjong Formation as was described by Collenette (1965) and consists ofa sequence ofmudstones, siltstones, limestones, conglomerates and coals.Tanjong Formation is divided into two units as classified by Balaguru andNichols (2004). Unit I consists of amudstone and siltstone-dominated se-quence and Unit II consists of coarse grained sandstone, conglomerate,carbonaceous mudstone and coals seams (Balaguru and Nichols, 2004).The mudstone is mostly dark gray and soft and thick mudstone up to2 m thick is commonly observed. The sandstone, usually exposed at wa-terfalls or within the steep hills, is generally fine grained and gray tobrownish gray in color (Fig. 2). At least part of the Tanjong Formationwas deposited in the shallow water, partly in brackish and estuarine en-vironments, as shown by presence of carbonaceous material in the sand-stone, fossil Textualaria, and the coal seams (Collenette, 1965). Thedepositional environment of the remainder of the formation in uncertain,but neritic conditions is assumed by previous workers. The Tanjong For-mation is Early to Middle Miocene in the southern and eastern parts ofSabah Basin (Clennell, 1992; Collenette, 1965; Leong, 1974). The EarlytoMiddleMiocene age of the Tanjong Formation is recognized by severalresearchers using palynological and nanofossils studies (Balaguru, 1996,1997; Balaguru and Nichols, 2004).

3. Samples and methods

Thirteen outcrop samples were collected across the west MiddleBlock of the Pinangah Coalfield, central southern Sabah (Fig. 1). Thesamples were collected using channel sampling and handpicked fromnon-weathered exposures after removing the weathering surface andstored in closed bags. The locations of the samples are as shown in Fig 1.

The collected samples were crushed into fine powder and analyzedusing Source Rock Analyzer (SRA-Weatherford)-TOC/TPH instrument(equivalent to Rock-Eval equipment). Parameters measured are TOC,S1, S2, S3 and temperature ofmaximumpyrolysis yield (Tmax). Hydrogenindex (HI) and oxygen index (OI) were calculated as described byEspitalié et al. (1977) and Peters and Cassa (1994). Following pyrolysis

analysis, the samples were selected for further geochemical analysesand microscopic examinations.

Bitumen extractions were performed on the powdered samplesusing a Soxhlet apparatus for 72 h using an azeotropic mixture ofdichloromethane (DCM) and methanol (CH3OH) (93:7). The extractswere separated into saturated hydrocarbon, aromatic hydrocarbonand NSO compound fractions by liquid column chromatography. Achromatographic column (30×0.72 cm) was packed silica gel of60–120 mesh that was activated for 2 h at 120 °C and capped with afew cm of alumina. Only the saturated fraction was analyzed in thisstudy. The saturated fraction of all the analyzed samples were dissolvedin petroleum ether and analyzed using gas chromatography (GC) andgas chromatography–mass spectrometry (GC–MS). The GC was re-formed using HP-5MS column with a temperature programmed from40 to 300 °C at a rate of 4 °C/min, and then held for 30 min at 300 °C.The GC–MS analysis was performed on an Agilent 5975B inert MSDmass spectrometer with a gas chromatograph attached directly to theion source (70 eV ionization voltage, 100 mA filament emission cur-rent, 230 °C interface temperature). For the analysis of biomarkers,the fragmentograms for steranes (m/z 217) and triterpanes (m/z 191)were recorded. Individual components were identified by comparisonof their retention times andmass spectra with previously published lit-erature (e.g., Ahmed et al., 2009; Hakimi et al., 2010, 2011; Kashirtsev etal., 2010; Koeverden et al., 2011; Monika Fabianska and Kruszewska,2003; Peters and Moldowan, 1993; Wan Hasiah, 1999; Waples andMachihara, 1991) (see Appendix 1). Relative abundances of triterpanesand steranes were calculated bymeasuring peak heights in them/z 191and m/z 217 fragmentograms, respectively. Extracted samples weresubsequently analyzed by pyrolysis–gas chromatography (Py–GC)using GC-mounted furnace heated from 300 °C to 600 °C at 25 °C/min.

Samples for petrographic study were made by mounting wholerock fragments in slow-setting polyester (Serifix) resin mixed withresin hardener and allowed to set, then ground flat on a diamondlap and subsequently polished on silicon carbide paper of differentgrades (P800, P2400 and P4000) using water as a lubricant. Finally,

Fig. 2. Generalized stratigraphic column of the southern Sabah area based on field observations and biostratigraphic information (modified after Balaguru and Nichols, 2004). (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

11F.L. Alias et al. / International Journal of Coal Geology 104 (2012) 9–21

the samples were polished to a highly reflecting surface using progres-sively finer alumina powder (1 μm, 0.3 μm and 0.05 μm). Petrographicexaminationswere carried out under oil immersion in a plane polarizedreflected light, using a LEICADM6000Mmicroscope and LEICA CTR6000

Table 1Bulk geochemical results of pyrolysis analysis with calculated parameters and vitrinite refle

Samples ID Lithology TOC(wt.%)

Pyrolysis data (SRA)

S1 S2 S3

BDD 1 Carbargillite 64.7 9.5 246.4 0.BDD 2 Coal 62.8 3.5 167.6 7.BPC 2a Coal 67.0 5.7 199.0 6.BPC 2b Coal 67.2 14.5 308.2 0.BDD 3 Carbargillite 68.2 15.5 282.0 0.BDD 4 Coal 77.7 30.1 347.6 0.BDD 13 Coal 73.6 5.8 285.5 3.BDD 24 Carbargillite 57.3 7.7 281.6 3.BDD 28 Coal 56.3 3.5 151.9 9.BDD 30 Coal 67.1 14.4 309.6 2.BDD 31 Carbargillite 51.2 3.8 178.2 2.BDD 49 Coal 76.8 7.8 326.7 1.BDD 50 Coal 67.0 3.4 203.2 5.

SRA: Source rock analyzer.TOC: Total organic carbon, wt.%.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=S2×100/TOC, mg HC/g TOC.OI: Oxygen index=S3×100/TOC, mg CO2/g TOC.PI: Production index=S1/(S1+S2).PY: Potential yield=S1+S2.

photometry system equipped with fluorescence illuminators. The filtersystem consists of BP 340–380 excitation filters, a RKP 400 dichromaticmirror and a LP425 suppression filter. Maceral compositions, based on a1000 point count, have been determined under both normal reflected

ctance data of the analyzed coals and coaly sediments in the Pinangah area.

Ro%

Tmax

(°C)HI OI PI PY

65 424 381 1 0.04 255.9 0.4954 433 267 12 0.02 171.1 0.4703 429 297 9 0.03 204.8 0.5567 423 459 1 0.01 322.6 0.4968 451 414 1 0.05 297.5 0.5078 437 448 1 0.08 377.7 0.6668 430 388 5 0.02 291.3 0.5063 440 203 17 0.03 289.3 0.4897 428 270 5 0.03 119.5 0.4968 419 462 4 0.04 324.1 0.5105 423 348 4 0.02 181.9 0.4254 429 425 2 0.02 334.5 0.6236 434 303 8 0.02 206.6 0.62

Fig. 3. Pyrolysis S2 versus total organic carbon (TOC) plot showing generative sourcerock potential for the Tanjong coals and coaly sediments in the Pinangah area (modifiedafter Peters and Cassa, 1994).


‘white’ light and UV (ultraviolet) light. Measurements of mean randomvitrinite reflectance (Ro%) were carried out using a ×50 oil immersionobjective using the windows based DISKUS Fossil software package.

Fig. 4. Plot of hydrogen index (HI) versus pyrolysis Tmax for the analyzed coal and coaMukhopadhyay et al., 1995a, 1995b).

4. Results and discussion

4.1. Source rock properties

Source rock properties of the Tertiary Pinangah coalswere investigat-ed in this paper for the purpose of characterizing the organic richness,hydrocarbon potential of the organic matter and its thermal maturitylevel. Source rock generative potential was evaluated using total organiccarbon content (TOC wt.%) and pyrolysis S2 yields (Table 1). As expected,the coal samples contain high total organic carbon content (TOC) rangingfrom 51.2 to 77.7 wt.%. In the pyrolysis analysis, free hydrocarbons (S1)in the rock and the amount of hydrocarbons (S2) and CO2 (S3) expelledfrom pyrolysis of kerogen are measured and shown in Table 1. Theamount of hydrocarbon yield (S2) expelled during pyrolysis is a usefulmeasurement to evaluate the generative potential of source rocks(Bordenave, 1993; Peters, 1986). The samples have pyrolysis S2 yieldvalues in the range of 151.9–347.6 mg HC/g rock (Table 1). Such valuesmeet the accepted standards of a sourcewith excellent generative poten-tial (Peters and Cassa, 1994) (Fig. 3). Hydrogen index (HI) values rangefrom 203 to 462 mg HC/g TOC and oxygen index (OI) values are low(1–17 mg CO2/g TOC). A plot of hydrogen index (HI) and pyrolysisTmax, which can be used to classify thematurity and type of organicmat-ter (Mukhopadhyay et al., 1995a, 1995b), shows that the coals aremixedType II–III kerogens and grading to Type II kerogen (Fig. 4). Tmax values

ly sediments, showing kerogen quality and thermal maturity stages (modified after

Table 2Macerals and mineral matter content (%) of the Tanjong coals and coaly sediments from Pinangah area and GI and TPI values are also shown.

Macerals Sample no.

BDD 1 BDD 2 BPC 2b BDD 3 BDD 4 BDD 13 BDD 24 BDD 28 BDD 30 BDD 31 BDD 49 BDD 50

Tellocollinite 9 0 20 4 33 17 12 19 26 7 27 35Telinite 0 0 0 0 0 0 0 0 0 0 0 0Desmocollinite 30 81 53 46 47 50 23 60 51 40 51 40Corpocollinite 0 0 2 0 5 15 0 0 0 0 0 5Vitrinite group 39 81 75 50 85 82 35 79 77 47 78 80

Sporinite 4 5 1 2 0 1 0 1 1 6 4 2Cutinite 0 4 5 1 2 3 0 2 5 6 2 4Suberinite 0 0 9 0 1 7 0 7 4 4 1 0Liptodetrinite 9 7 1 7 6 2 0 2 0 0 6 5Resinite 0 0 0 2 0 1 0 0 0 3 0 0Exsudatinite 0 0 2 0 1 0 1 1 1 1 1 0Bituminite 8 0 2 4 1 0 2 0 1 0 0 0

Liptinite group 21 16 20 16 11 14 3 13 12 20 14 11Sclerotinite 1 1 1 1 0 1 0 1 2 4 1 0Fusinite 0 0 0 0 0 0 0 0 6 1 2 3Semifusinite 0 1 0 0 0 1 0 0 2 1 2 4Inertodetrinite 0 1 1 1 0 1 1 2 0 1 0 2Micrinite 0 0 1 0 1 1 0 4 1 0 0 0Macrinite 0 0 0 0 0 0 0 0 0 0 0 0

Inertinite group 1 3 3 2 1 4 1 7 11 7 5 9Mineral matter (MM) 39 0 2 32 3 0 61 1 0 26 3 0Total 100 100 100 100 100 100 100 100 100 100 100 100GI – 40.5 75.0 50.0 – 41.0 – 39.5 9.60 15.70 20.0 8.90TPI – 0.10 0.40 0.09 – 0.40 – 0.30 0.70 0.20 0.60 1.0

GI: Gelification Index.TPI: Tissue Preservation Index.


are fairly uniform, ranging from 419 to 451 °C, which commonly reflecton maturity but may also be influenced by kerogen type (Hunt, 1996)thus the defined maturity windows on this diagram are only approxi-mate. The Pinangah coal samples generally plot in the immature to ma-ture zone of Type II and mixed Type II–III kerogens (Fig. 4). The Tmax

values are in good agreement with the mean vitrinite reflectance data(0.42–0.66%; Table 1).

4.2. Maceral/kerogen assemblages

The proportions of the macerals in coals reflect the organic sourcematerials contributing to the accumulation of peat and the conditionsduring accumulation. Data related to the maceral composition of the

Fig. 5. Ternary diagram of maceral group composition (vitrinite–liptinite–inertinite)for analyzed Pinangah coals and coaly sediments.

Tanjong coals are shown in Table 2. The petrographic examinationsrevealed that two types of organic-rich sediments are identified: coal(MMb15%) and carbargillites (MM among 15%–65%). Most of the coalsamples are dominated by vitrinite up to 50% and classified as humicwith significant amounts of liptinite and inertinite macerals (Fig. 5).The vitrinite macerals are mainly tellocollinite and desmocollinite(Fig. 6). Inertinite contents are mainly fusinite, inertodetrinite andsclerotinite, and range from 1 to 11% (Table 2; and Fig. 6d–f). All ana-lyzed samples contain significant amounts of type II liptinitic macerals(3–21%; Table 2). Themost common liptinitic constituents are sporinite,suberinite, resinite, cutinite, bituminite and exsudatinite (Fig. 7). The sig-nificant representation of terrestrial liptinitic macerals indicated thatboth liquid hydrocarbons of waxy nature and naphthenic oils and con-densates could be generated by these coals (e.g. Hakimi and Abdullah,2012; Mukhopadhay and Hatcher, 1993; Ogala, 2011; Rongxi et al.,2008; Wan Hasiah, 1997, 1999, 2003; Zulkifli et al., 2008). A number ofpetrographic features that are also considered to indicate oil generationfrom the studied samples have been recognized. The most significantof these features is the occurrence of exsudatinite (Fig. 7d), a secondarymaceral commonly considered to represent the very beginning of oilgeneration in coal (e.g. Hakimi and Abdullah, 2012; Teichmuller, 1974;Wan Hasiah, 1997, 2003).

4.3. Bitumen analysis

The amount of extractable organic matter (EOM) yields from bitu-men extraction analysis is recorded in Table 3. Yields of EOM exceed130,000 ppm, indicating that the analyzed coal samples are viablesource rocks for hydrocarbon generation (Peters and Cassa, 1994) assupported by the TOC versus pyrolysis S2 yields plot (Fig. 3). Bitumen/TOC ratios generally ranged from 0.08 to 0.21 (Table 3).

4.4. Biomarker distributions

4.4.1. n-Alkanes and isoprenoidsThe gas chromatography (GC) and gas chromatography–mass

spectrometry (GC–MS) analyses were performed on the saturated

Fig. 6. Photomicrographs of macerals from Tertiary coals and coaly sediments in the Pinangah area, southern Sabah, under reflected white light examination. (a) Suberinite (Sb) andmicrinite (mc) associated with vitrinite (VR), sample BDD4; (b) veins exsudatinite (Ex) associated with vitrinite (VR), sample BPC2b; (c) telinite (Tl), sample BDD2; (d) semifusinite(SFu) associated with vitrinite (VR), sample BDD2; (e) vitrinite associated with sclerotinite (Sc), sample BDD2; and (f) fusinite (Fu) associated with vitrinite (VR), sample BDD49.


hydrocarbon fraction for the analyzed coal samples. Two representa-tive samples are shown in Fig. 8 and derived parameters are listed inTables 3 and 4. The saturated gas chromatograms of the analyzedsamples (Fig. 8a) display a unimodal n-alkanes distribution withn-C25 alkane maximum and a moderate waxy appearance. The distri-bution is depleted in the n-C10–n-C15 range and show odd predomi-nance of the heavier members (n-C25+) which gave moderate tohigh CPI values in the range of 1.15–1.41 (Table 3). The distributionssupport a significant terrestrial higher plant input for these sediments(e.g., Powell and McKirdy, 1973; Tissot et al., 1978). Acyclic isoprenoidsoccur in a significant amount (Fig. 8a). Isoprenoids, in particular pristane,occur in high relative concentrations, with pristane/phytane (Pr/Ph) ra-tios of >3.0 (8.3–18.0) which suggest that these coals and carbargillitesediments were deposited under oxic conditions (Chandra et al., 1994;Didyk et al., 1978; Large and Gize, 1996; Powell and McKirdy, 1973;Tissot and Welte, 1984). Pristane concentrations are generally higherthan the close eluting n-alkane (n-C17) in the analyzed samples, thus

giving distinctively high pristane/n-C17 and phytane/n-C18 ratios in therange of 7.40–20.1 and 0.42–1.22, respectively (Table 3).

4.4.2. Triterpanes and steranesThe distributions of steranes and triterpanes are commonly studied

using GC–MS by monitoring the ions m/z 217 and m/z 191 (Fig. 8b–c).There are also other important biomarker groups such as tricyclicterpanes (m/z 191), tetracyclic terpanes (m/z 191) and diasteranes(m/z 217) (Peters et al., 2005; Seifert andMoldowan, 1980). The assign-ments of the peaks of steranes and triterpanes are labeled in Fig. 8 andare as listed in Appendix 1.

All samples contain abundant pentacyclic triterpanes as shown bythe m/z 191 mass fragmentograms of the saturated hydrocarbon frac-tions (Fig. 8b). The relative abundance of C29 to C30 hopane is generallysimilar inmost of the studied samples and the C29/C30 ratios range from0.8 to 1.0. The Tm (C27 17α(H)-22,29,30-trisnorhopane) predominatesover Ts (C27 18α(H)-22,29,30-trisnorneohopane) with Tm/Ts ratios

Fig. 7. Photomicrographs of macerals from Tertiary coals and coaly sediments in the Pinangah area, southern Sabah, under reflected UV light examination. (a) Suberinite (Sb) associatedwith bright yellow fluorescing resinite, sample BDD13; (b) bright yellowfluorescingfluorinite (Fl) indicate rich hydrocarbon, sample BDD1; (c) cutinite (cu) that showyellow fluorescing,sample BDD30; (d) veins exsudatinite (Ex) associatedwith bright yellowfluorescing resinite (Rs), sample BPC2b; (e) Dull yellowfluorescing bituminite (Bi) associatedwith cutinite (cu),sample BDD30; and (f) yellow fluorescing sporinite (sporangium— Sp) associatedwith Resinte (Rs), sample BDD31. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)


ranging from 6 to 28 (Table 4). The studied samples display variableoleanane index (oleanane/C30 hopane) in the range of 0.15–0.66(Table 4). Extended hopanes are dominated by the C31 homohopaneand decreasing towards the C34 homohopane (Fig. 8b). The αβ-hopanesare more prominent than the βα-hopanes while the S-isomers aremore dominant than the R-isomers among the homohopane (C31–C34).Moretane/hopane ratios for the analyzed coal samples are 0.3–0.7.

The m/z 217 mass fragmentograms of all the analyzed samples aredominated by steranes over diasteranes with C29 sterane being the pre-dominant component (Fig. 8c). Relative abundances of C27, C28 and C29regular steranes are calculated and the results are given in Table 4. Thedistributions of C27:C28:C29 regular steranes for the analyzed samplesare very similar (Table 3). The C29/C27 sterane ratios and two differentsterane thermal maturity parameters, C29 20S/(20S+20R) and theC29 αββ (αββ+ααα), are calculated and listed in Table 4.

4.5. Thermal maturity

A variety of maturity indicators have been used to evaluate the levelof thermal maturity of the Pinangah coals; these include vitrinite reflec-tance (Ro%), pyrolysis Tmax data and biomarker maturity parameters(Peters and Moldowan, 1993; Peters et al., 2005). The mean vitrinitereflectance (Ro%) values are relatively low for all coals, in the range ofimmature to early mature (0.42–0.66%) that can be interpreted assub-bituminous A–B to high volatile C bituminous coal stage (Ward,1984). The pyrolysis Tmax values range from 415 to 451 °C, which arein good agreementwith vitrinite reflectance data. The biomarker param-eters are listed in Table 4 and are discussed in more detail below. In gaschromatography–mass spectrometry (GC–MS), biomarker maturationparameters such as C32 22S/(22S+22R)homohopane,moretane/hopaneand 20S/(20S+20R) and ββ/(ββ+αα) C29 sterane ratios, were used as

Table 3Bitumen extraction and normal alkane parameters of the studied Tanjong samples.

Samples ID TOC(wt.%)

Extractable organic matter n-Alkane and isoprenoids

Total extract(ppm)

Bitumen/TOC CPI Pr/Ph Pr/C17 Ph/C18

BDD 1 64.7 140,000 0.21 1.37 8.3 7.1 0.43BDD 2 62.8 58,400 0.10 1.34 17.5 9.3 0.53BPC 2b 67.2 66,700 0.09 1.19 13.1 13.1 0.64BDD 3 68.2 57,300 0.08 1.41 18.0 10.0 0.42BDD 4 77.7 70,400 0.09 1.15 12.3 8.2 0.44BDD 24 72.5 68,000 0.09 1.31 14.1 14.1 0.75BDD 28 56.3 60,300 0.11 1.41 18.0 10.0 0.42BDD 30 67.1 89,000 0.13 1.30 9.3 20.4 1.22BDD 31 51.2 86,600 0.17 1.27 12.3 14.0 0.62BDD 49 76.8 76,400 0.10 1.26 17.0 14.6 0.66BDD 50 67.0 74,400 0.11 1.22 13.9 13.9 0.70

Pr: pristane.Ph: phytane.CPI: CarbonPreference Index: (2[n-C23+n-C25+n-C27+n-C29]/[n-C22+2{n-C24+n-C26+n-C28}+n-C30]).


maturity indicators (Mackenzie et al., 1980; Waples and Machihara,1991). The investigated coals have C32 homohopane 22S/(22S+22R) ra-tios in the range of 0.55–0.60 (Table 2) and are considered at equilibrium.The 20S/(20S+20R) and ββ/(ββ+αα) C29 sterane ratios ranging from0.30 to 0.50, and 0.25–0.40, respectively (Table 2) and are not completelyequilibrated. Some of the variance in the extent of sterane isomerizationmay be attributed to other factors such as organic facies, environmentand lithology (Korkmaz and Kara Gülbay, 2007). These biomarker matu-ration parameters indicate that the investigated coals are at an early stageof oil maturity window (Fig. 9). The moretane/hopane ratio ranges from0.30 to 0.70, which indicates maturity less than the oil window, thoughthese values could be a consequence of high terrestrial organic mat-ter input (Waples and Machihara, 1991). The relationship betweenisoprenoids Pr/n-C17 and Ph/n-C18 ratios reflects the same interpreta-tion and is consistent with the observed biomarker maturity parame-ters (Fig. 10). The biomarker maturation parameters are consistentwith the observed vitrinite reflectance data.

4.6. Liquid hydrocarbon generation potential

Coals have long been recognized as a source for gas, primarilymethane and carbon dioxide, but its importance as a generator of eco-nomic accumulations of oil is difficult to prove as coals are ofteninterbedded with shales, which may be the source beds. However,coals are now known to be a potentially significant source of liquidhydrocarbons and there are some basins in the world where the evi-dence is very strong that coals are contributing to significant accumu-lations of liquid hydrocarbon (e.g. Hendrix et al., 1995; Hunt, 1991;Murchison, 1987; Obaje and Hamza, 2000; Wan Hasiah, 1997, 1999,2003). Pyrolysis data have revealed that the hydrocarbon richness ofthe samples is dependent on the amount and nature of liptinite andvitrinite macerals. Samples that contain a Type III vitrinitic kerogenwould be expected to generate gas with hydrogen indexb200 mg HC/gTOC. On the other hand, the samples contain a significant amount oftype II liptinitic macerals with hydrogen index>200 mg HC/g TOCwould be expected to generate gas and minor components of liquid hy-drocarbon (Curry, 1994; Hunt, 1996; Koeverden et al., 2011). The abun-dance of liptinite macerals is, therefore, the major criterion whenconsidering any sedimentary rock (including coal) as a potential sourcerock for liquid hydrocarbons (Hendrix et al., 1995; Mukhopadhay andHatcher, 1993; Stach et al., 1982). Aminimumof 15–20% liptinite content(by volume) of total macerals in sediments is considered as importantcriteria for a rock to be characterized as a potential oil source rock(Hunt, 1991; Thompson et al., 1985).

In the study area, liquid hydrocarbon generation is anticipatedfrom the Tanjong coals and coaly sediments based on significant of

terrestrial liptinitic constituents (Type II kerogen) and their apparentcapability to generate hydrocarbon as seen under the microscope.Most of the analyzed samples have liptinitic content exceeding 15%of the total macerals and two samples have 20% liptinite content(Table 2). The significant representation of terrestrial Type II liptiniticmacerals (sporinite, suberinite, resinite, bituminite and cutinite), in-dicates that liquid hydrocarbons could be generated by these Tanjongcoals based on previous studies (e.g. Fowler et al., 1991; Hunt, 1991;Mukhopadhyay et al., 1991; Wan Hasiah and Abolins, 1998). The coalsamples are suberinite-rich (Fig. 7a) can be expected to generateabundant waxy oil at a much lower maturation than do sporinite andcutinite (Teicmuller and Durand, 1983; Wan Hasiah, 1997). Pyrolysisanalyses (HI>300 mg HC/g TOC) are consistent with the observedmaceral distributions. In support of the organic petrographic and pyroly-sis data, the S2 Py–GC pyrograms of the analyzed coals (Fig. 11) displaymixed kerogen fingerprints of predominantly n-alkane/alkene doubletsand aromatic compounds. In the coal samples where the liptinitemaceral content is higher than 15%, with high hydrogen index values(>400 mgHC/g TOC), the pyrograms are dominated by n-alkane/alkenedoublets that extend beyond C30, indicative of the aliphatic-rich, oil-prone nature of thesemacerals (Fig. 11). In addition, coals are very variedand represent a complex group of source rocks, and the mechanisms ofliquid hydrocarbon generation and expulsion from coals are known tobe complex and depend on several factors that influence the timing ofoil generation (e.g.: Hunt, 1991; Scott and Fleet, 1994). The importantfactors that have been identified are maceral type, maceral association,coal micro-texture (e.g., exsudatinite crack network) and is also interre-latedwith the original precursormaterial, depositional environment,mi-crobial activity and mineral matter (e.g. Hunt, 1991; Littke et al., 1989;Scott and Fleet, 1994; Snowdon, 1995; Teichmuller, 1974; Teichmüller,1989; Wan Hasiah, 1997, 1999, 2003). As for the coals from Pinangaharea, the maceral type (its abundance and their association) and micro-texture (e.g. exsudatinite crack network) are among the important fac-tors influencing liquid hydrocarbon generation. The characteristic be-havior of exsudatinite that commonly formed crack networks (Fig. 7d),although not as extensive compared to the Tertiary coals of Sarawak(e.g. Hakimi and Abdullah, 2012; Wan Hasiah, 1999), suggests a poten-tial significant role in hydrocarbon expulsion into carrier beds. However,the generation of liquid hydrocarbon from liptinitic macerals such assuberinite, cutinite and sporinite (Fig. 7a, c, f) represents an earlymaturestage, which takes place at about 0.66 Ro%. This falls within the generallyconsidered “early oil window” that commonly occurs within 0.5–0.7%vitrinite reflectance values.

4.7. Depositional environment of coals

In this study, organic geochemical (biomarker distributions) andmaceral assemblages have been used to describe source input and condi-tion of depositional environment of the Pinangah coals. Depositional en-vironment condition and source inputwere examined through the use ofsterane and triterpane distributions recorded based on m/z 217 andm/z191 mass chromatograms, respectively, and parameters calculated fromthese distributions (Table 4). The distributions of n-alkanes with pre-dominance of odd carbon number alkanes to even carbon number al-kanes in the gas chromatograms of the coal samples indicate significantinput of terrestrial higher plants (e.g., Peters and Moldowan, 1993;Philp, 1985; Powell and Boreham, 1994). The pristane/phytane (Pr/Ph)ratio is one of the most commonly used geochemical parameters andhas been widely invoked as an indicator of the redox conditions in thedepositional environment and source of organic matter (Chandra et al.,1994; Didyk et al., 1978; Large and Gize, 1996; Powell and McKirdy,1973; Tissot andWelte, 1984). Organic matter originating predominant-ly from terrestrial plantswould be expected to contain high Pr/Ph ratio of>3.0 (oxidizing conditions), while low values of (Pr/Ph) ratio (b0.6) in-dicate anoxic conditions, and values between 1.0 and 3.0 suggest inter-mediate conditions (suboxic conditions) (Peters and Moldowan, 1993;

Fig. 8. (a) Gas chromatograms, (b) m/z 191 mass fragmentograms and (c) m/z 217 mass fragmentograms of saturated hydrocarbon fractions for rock extracts of Tanjong Formation.


Powell, 1988). Within the SE Asian region, for example, Wan Hasiah andAbolins (1998) and Zulkifli et al. (2008) showed that the coalswith Pr/Phratio exceeding 4 are typically deposited within a peat-swamp deposi-tional setting indicating the oxic condition, and this statement is alsoconsistent with the depositional settings of the analyzed Pinangah coalsamples which possess Pr/Ph ratios of >4 (Table 3) and supported byPr/n-C17 and Ph/n-C18 ratios (Fig. 10).

The relative distribution of C27, C28 and C29 regular steranes is graph-ically represented in the form of a ternary regular steranes diagram inFig. 12 (adapted after Huang and Meinschein, 1979). This diagram hasoften been employed to represent the relative proportions of thesethree steranes. The original classification of Huang and Meinschein(1979) related C27 regular steranes to strong algal influence and C29

regular steranes to strong higher plant influence. Based on this ternaryclassification (Fig. 12), the analyzed coal samples were shown to be de-posited in a terrestrial environment, consistent with their being humicand waxy coals which thus display a strong predominance of C29steranes.

Applying oleanane parameter to indicate angiosperm input in rocksof late Cretaceous or younger age to the coals of the Pinangah showsthat the investigated coals with measurable amounts of oleanane(Fig. 8) are a strong indicator of terrestrial angiosperm plant as initiallyreported by Ekweozor and Telnaes (1990). The presence of oleanane,however suggests probable marine-influence as indicated by earlierwork ofMurray et al. (1997). Tmand Ts arewell known to be influencedby both maturation and type of organic matter (e.g. Moldowan et al.,

Table 4Biomarker parameters for rock extracts calculated from m/z 191 and m/z 217 mass fragmentograms.

Samples ID Triterpanes and terpanes (m/z 191) Steranes (m/z 217)

C32 homohopane 22S/(22S+22R) C29/C30 Ts/Tm M30/C30 OI/C30 C29 20S/(20S+20R) C29ββ/(ββ+αα)

C29/C27 Regular steranes

C27 C28 C29

BDD 1 0.55 1.0 – 0.7 0.17 0.40 0.30 5.6 13 14 73BDD 2 0.56 0.8 26 0.7 0.15 – – 7.9 9 20 71BPC 2b 0.59 0.9 19 0.3 0.64 0.40 0.32 2.3 26 14 60BDD 3 0.58 1.0 17 0.5 0.41 0.50 0.31 5.6 14 7 79BDD 4 0.58 0.9 15 0.5 0.17 0.45 0.26 6.0 13 9 78BDD 24 0.58 0.9 28 0.4 0.25 0.40 0.25 5.3 15 5 80BDD 28 0.56 0.7 6 0.5 0.66 0.42 0.40 2.3 24 20 56BDD 30 0.60 0.8 21 0.4 0.57 0.30 0.30 3.0 21 16 63BDD 31 0.59 0.8 16 0.3 0.41 0.41 0.33 14.4 5 23 72BDD 49 0.57 0.9 24 0.5 0.15 0.43 0.35 12.4 7 6 87BDD 50 0.60 1.0 14 0.3 0.28 0.40 0.30 5.6 13 14 73

Ts: (C27 18α (H)-22,29,30-trisnorneohopane).Tm: (C27 17α (H)-22,29,30-trisnorhopane).C29/C30: C29 norhopane/C30 hopane.M30/C30: C30 moretane/C30 hopane.OI/C30: oleanane/C30 hopane.


1986; Seifert and Moldowan, 1978). Tm/Ts ratios do not appear to beappropriate for quantitative estimation of maturity, because facies ef-fects make this parameter somewhat imprecise (Cornford et al., 1983;Schou et al., 1985). Robinson (1987) noted the trend of Tm/Ts valuesin Indonesian oils that originated from terrestrial-derived organic mat-ter including coals. However, Moldowan et al. (1986) suggested thatredox potential is more important than lithology, with higher Tm/Ts ra-tios in oxic sediments than in anoxic ones. These statements are consis-tent with the depositional settings of the analyzed coal samples fromPinangah, which possess higher Tm/Ts ratios of >6 (Table 4).

The depositional environment of the Pinangah coals has also beeninterpreted using maceral assemblages and petrographic facies. Thepresence of terrestrial liptinite macerals (cutinite, sporinite, suberiniteand resinite; Fig. 7) further confirms they are coalfield‐originated fromterrestrial peats. The presence of cutinite, suberinite and resinite sug-gests an accumulation in a forest‐type swamp,while significant presenceof sporinite shows reed-marsh vegetation (Flores, 2002). Diessel (1986)developed a scheme to interpret the paleoenvironmental settings in re-lation to the type ofmirewith the help of two petrographic facies indicesderived from maceral analyses, namely the Tissue Preservation Index(TPI) and the Gelification Index (GI) as defined in Table 2, both ofwhich have been extensively used to interpret peat-forming conditionsof coal deposits. TPI is ameasure of tissue preservation versus destructivetissue breakdown and the proportion of woody plants in the originalpeat forming assemblages, whereas GI measures the relative dryness or

Fig. 9. Cross-plot of two biomarker parameters sensitive to thermal maturity of thePinangah sediments (modified from Peters and Moldowan, 1993).

wetness of autochthonous peat forming conditions (Alkande et al.,1992; Diessel, 1992; Kalkreuth et al., 2000; Lamberson et al., 1991;Silva and Kalkreuth, 2005). Most of the studied samples are character-ized by low TPI and high GI values, and are plotted on the marsh-lowerdeltaic fields of the Diessel's diagram (Fig. 13). Coals with low TPI valuescould also suggest large scale destruction of wood in forested swamps(Amijaya and Littke, 2005; Diessel, 1992). Nevertheless, marsh and for-ested swamp are considered as a kind of minerotrophic mires (Amijayaand Littke, 2005), coals originating from both of these sources usuallygenerate high ash yield (Amijaya and Littke, 2005; Diessel, 1992),which is consistent with the case for the studied coals that have ashyields in the range of 2.4–12.4 wt.%. This shows that the interpretationas suggested by the Diessel's diagram is valid for the studied coals. Over-all, the coals under investigation are believed to be derived from terres-trial origin anddeposited in a swampenvironment under oxic conditionswith probable minor marine influence.

5. Conclusions

The petrographic and geochemical analyses of the Tertiary Pinangahcoal and coaly sediments, suggest the following:

(1) The samples studied have a humic composition dominated byvitrinite macerals with significant liptinite up to around 20%of the whole rock.

Fig. 10. Log plot of the ratios of phytane to n-C18 alkane (Ph/n-C18) versus pristane ton-C17 alkane (Pr/n-C17) (modified after Shanmugam, 1985).

Fig. 11. Py–GC pyrograms of two analyzed coal (BPC 2b) and carbargillite (BDD 3) samples, which display kerogen type II (oil prone).


(2) The organic matter is classified on pyrolysis HI versus Tmax di-agram as predominantly mixed Type II–III kerogens (oil andgas prone) grading into Type II kerogen (oil prone) as indicatedby hydrogen index values (267–462 mg HC/g TOC). Liptinitic

Fig. 12. Ternary diagram of regular steranes (C27–C29) showing the relationship be-tween sterane compositions, source input, and depositional environment for the ana-lyzed samples (modified after Huang and Meinschein, 1979).

Type II kerogen was also identified based by petrographic anal-ysis and S2 Py–GC pyrograms.

(3) The Pinangah coals are at most marginally mature in conven-tional terms and have entered immature to early mature ofoil window based on mean vitrinite reflectance of 0.42–0.66Ro%. The pyrolysis Tmax and biomarker maturity ratios supportthis attained thermal maturity level.

(4) Overall, evaluation of the organic geochemical and petrograph-ic data implies that these Tertiary coals possess good potentialfor liquid hydrocarbon generation as suggested by the pres-ence of exsudatinite in a crack network form, which suggestsincipient migration even at this relatively low level of maturity.

(5) Saturated geochemistry of the Pinangah coals is characterized bypredominance of the heavier n-alkane (n-C25+) with moderateto high CPIs, high Pr/Ph (8.4–18.0), high Tm/Ts ratios (6–28)and dominance of C29 regular steranes, consistent with an originas terrestrial peats deposited in a swamp environment under oxicconditions. However, geochemical parameters for the Pinangahcoals demonstrate high‐terrestrial plantswith probableminorma-rine influence, as suggested by the presence of 18α(H)-oleananein the m/z 191 chromatograms of the Pinangah coal extracts.

Acknowledgments

The field assistance provided by staff of the Department of mineraland Geoscience sabah is much appreciated. The authors are grateful to

Fig. 13. Diagram of TPI versus GI showing the paleodepositional environment of the Pinangah coal facies (modified after Diessel, 1986).


the Department of Geology, University of Malaya for providing facilitiesto complete this research. This study is supported by University of Ma-laya research grant nos. PS363-2009c and RG145-11AFR. Special thanksare offered to Mr. Peter Abolins, anonymous reviewers and the Editor,Dr. Ozgen Karacan, for their careful and useful comments that improvedthe revised manuscript.

Appendix 1. Peak assignments for alkane hydrocarbons in the gaschromatograms of aliphatic fractions in the m/z 191 and 217mass fragmentograms

Compound Abbreviation

Peak no. (I)Ts 18α (H),22,29,30-trisnorneohopane TsTm 17α (H),22,29,30-trisnorhopane Tm29 17α,21β (H)-norhopane C29 hopOI 1817α (H)‐oleanane Oleanane index29M 17β (H),2lα (H)-30-norhopane (normoretane) Normoretane30 17α,21β (H)-hopane Hopane30M 17 β,21α (H)-moretane C30 Mor31S 17α,21β (H)-homohopane (22S) C31 (22S)31R 17α,21β (H)-homohopane (22R) C31 (22R)32S 17α,21β (H)-homohopane (22S) C32 (22S)32R 17α,21β (H)-homohopane (22R) C32 (22R)33S 17α,21β (H)-homohopane (22S) C33 (22S)33R 17α,21β (H)-homohopane (22R) C33 (22R)34S 17α,21β (H)-homohopane (22S) C34 (22S)34R 17α,21β (H)-homohopane (22R) C34 (22R)35S 17α,21β (H)-homohopane (22S) C35 (22S)35R 17α,21β (H)-homohopane (22R) C35 (22R)

Peak no. (II)a 13β,17α (H)-diasteranes 20S Diasteranesb 13β,17α (H)-diasteranes 20R Diasteranese 5α,14α (H), 17α (H)-steranes 20S ααα20Sf 5α,14β (H), 17β (H)-steranes 20R αββ20Rg 5α,14β (H), 17β (H)-steranes 20S αββ20Sh 5α,14α (H), 17α (H)-steranes 20R ααα20R

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