Geochemical-and-petrographic-characterization-of-Late-Jurassic-Early-Cretaceous-Chia-Gara-Formation
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Geochemical and petrographic characterization of Late Jurassic e Early Cretaceous Chia Gara Formation in Northern Iraq:Palaeoenvironment and oil-generation potentialIbrahim M.J.Mohialdeen a ,Mohammed Hail Hakimi b ,*,Fawzi M.Al-Beyati caDepartment of Geology,School of Science,University of Sulaimani,Kurdistan,Iraq bGeology Department,Faculty of Applied Science,Taiz University,6803Taiz,Yemen cTechnical College,University of Kirkuk,Iraqa r t i c l e i n f oArticle history:Received 24September 2012Received in revised form 31January 2013Accepted 4February 2013Available online 14March 2013Keywords:Chia Gara Formation Organic-rich sediments BiomarkersMarine reducing environment Northern Iraqa b s t r a c tThe marine Late Jurassic to Early Cretaceous Chia Gara Formation is well exposed in northern Iraq.It is built of organic-rich limestones and calcareous shales.The organic-rich sediments have been investi-gated to determine the type and origin of the organic matter as well as their petroleum-generation potential.Kerogen microscopy shows that these sediments are characterized by large amounts of predominantly amorphous organic matter with a Total Organic Carbon content of 7.42%.The large amounts of organic matter are mainly due to good preservation under anoxic conditions,as evidenced by numerous pyri-tized fragments and biomarkers that are diagnostic for the depositional environment.The investigated biomarkers are characterized by a dominance of low to medium molecular weight compounds,a low Pr/Ph ratio (<1.0),a composition of C 27e C 29regular steranes,and the presence of tricyclic terpanes,indi-cating a strong decay of marine organic matter preserved under reducing conditions.A small amount of terrigenous organic matter is,according to the n -alkane distribution,also present.The Chia Gara sediments thus have a high oil-but a low gas-generation potential due to the high content of hydrogen-rich Type II and mixed Type II e III kerogens with a minor contribution of Type III kerogen.Ó2013Elsevier Ltd.All rights reserved.1.IntroductionOrganic matter (OM)is the precursor of oil and gas generated in sedimentary basins and has been extensively studied by organic geochemistry with different methods (Hunt,1996;Tissot and Welte,1984;Peters et al.,2005).Source rock evaluation consists of assessing the hydrocarbon generating potential of sediments by evaluating their capacity for hydrocarbon generation,the type of organic matter,what hydrocarbons might be generated,thermal maturity and how it in fluences generation (Dembicki,2009).The area that forms the scope of this study lies in the Kurdistan area,northern Iraq,where it extends along the Tigris River and to the north eastern part of Iraq,between Iran and the Syrian border (Fig.1).The Kurdistan region is an important hydrocarbon province in the Iraq (Fig.1),but the origin of its hydrocarbons has not been investigated yet.The Late Jurassic e Early Cretaceous Chia Garasediments are widespread and occur in northern Iraq.Published data related to organic geochemical and petrographic characteris-tics are also very limited.In this regard,the main objective of this study is to discuss the amount and type of organic matter as a function of depositional environment and oil generation potential of the Late Jurassic e Early Cretaceous Chia Gara Formation in the north of Iraq.The present analyses of the Chia Gara Formation are based on the interpretation of organic petrological and organic geochemical data,including total organic carbon content (TOC),Rock-Eval pyrolysis,elemental analysis,palynofacies and vitrinite re flectance data.In addition,various biomarkers were used to establish the maturity of the organic matter and to help identify the depositional conditions.2.Stratigraphic settingThe Chia Gara Formation (Middle Tithonian e Berriasian)is one of the formations in the sedimentary subcycle which extended from the Jurassic to the Early Cretaceous.It is underlain by the*Corresponding author..E-mail address:ibnalhakimi@ (M.H.Hakimi).Contents lists available at SciVerse ScienceDirectMarine and Petroleum Geologyjournal ho mep age:www.elsevier.co m/lo cate/marpetgeo0264-8172/$e see front matter Ó2013Elsevier Ltd.All rights reserved./10.1016/j.marpetgeo.2013.02.010Marine and Petroleum Geology 43(2013)166e 177Barsarin Formation and overlain by the Lower Sarmord Formation (Fig.2).The type section of the Chia Gara Formation is located at the Chia Gara anticline,south of Amadiya town in the strongly folded zone of north Iraq (Bellen et al.,1959).The thickness of the formation at the type locality area reaches 232m (Bellen et al.,1959)and the formation consists entirely of a succession of thin beds of limestone and shales with rich ammonite faunas and diverse species of foraminifera,radiolarian,ostracodesandFigure 1.Paleofacies map of the Tithonian e Berriasian age of Iraq,showing location map of the study area including studiedwells.Figure 2.Stratigraphic column of the Chia Gara Formation in the studied wells,northern Iraq.I.M.J.Mohialdeen et al./Marine and Petroleum Geology 43(2013)166e 177167tintinnids (Fig.2).The lower most 21m are characterized by yellow marly limestones and shales (Bellen et al.,1959).Tectonically,the Chia Gara Formation can be considered as a part of the tectonos-tratigraphic megasequence AP8(149e 49Ma);it was deposited during both TST and HST stages of the systems tract (Sharland et al.,2001).The Chia Gara Formation has been studied by many authors who have discussed the stratigraphy,sedimentology,paleontology and depositional environments in the Kurdistan region (e.g.Buday,1980;Howarth,1992;Al-Qayim and Saadalla,1992;Salae,2001;Jassim and Goff,2006;Mohialdeen,2007;Mohialdeen and Al-Beyati,2007;Mohialdeen,2008).The sequence as a whole represents transgressive sediments deposited on top of the Barsarin and/or Gotnia Formations.The Chia Gara Formation is underlain by the Barsarin Formation (Mohialdeen,2008).The Barsarin Formation is composed of stro-matolitic limestones and evaporite layers (Bellen et al.,1959).The first appearance of brown to dark shales or argillaceous limestones above the stromatolitic limestone beds is de fined as the basis of the Chia Gara Formation (Mohialdeen,2007,2008).The contact is sharp and abrupt and is considered as conformable (Buday,1980).The Chia Gara Formation is composed of organic matter-rich limestone and shale sediments and might represent good source rocks (e.g.Odisho and Othman,1992;Al-Beyati,1998;Al-Ameri and Al-Obaidi,2004).The sequence is rich with radiolarian and pyrite in form framboidal and large crystals,where the radiolarians replaced by sparry calcite (Mohialdeen,2007,2008).The Chia Gara limestones are thin to medium bedded and grey to dark in colour.The lime-stones are also characterized by the radiolarian wackestone e packestone with distribution of other bioclasts,such as ammon-ites,ostracodes,foraminifera,calpionellids and calcisphers,and some unidenti fied broken bioclasts (Mohialdeen,2008).On the other hand,the shales of Chia Gara Formation are organic-rich calcareous,brown to dark in colour,with developed fissility in most cases,and dominant in the lower part of the Chia Gara section (Fig.2;Mohialdeen,2008).The lithofacies of the Chia Gara sedi-ments re flects marine environment (Buday,1980;Jassim and Goff,2006).The deep outer shelf to carbonate slope environments is possibly the depositional setting of Chia Gara Formation (Fig.3;Mohialdeen,2008).3.Samples and methodsA total of 32samples were collected from three exploration wells in northern Iraq (Table 1and Fig.1).The samples were selected from organic-rich limestone and shale intervals within the Chia Gara Formation (Fig.3).The collected samples were crushed and analysed using LECO 412and Rock-Eval II instruments.Pyrolysis analysis was performedon 100mg crushed whole rock sample,which was heated to 600 C in a helium atmosphere,using a Rock-Eval II unit.In the pyrolysis analysis,free hydrocarbons in the rock (S 1)and the amount of hy-drocarbons (S 2)and CO 2(S 3)expelled from pyrolysis of kerogen and temperature of maximum pyrolysis yield (T max )are measured and shown in Table 1.Hydrogen (HI),oxygen (OI)and production (PI)indices were calculated (Table 1).Elemental content was performed on approximately 15e 20mg pulverized whole sample with LECO and the CNS Analyzer to determine the organic carbon and inorganic contents.The per-centages of carbon,nitrogen and sulphur were calculated (Table 1).Microscopic examinations were performed on thin sections and polished blocks to study the petrographic characteristics of the Chia Gara sediments.Thin sections,as a routine work,made for the samples as standard procedure (Harwood,1988)and these thin sections were studied using polarizing microscope type Meiji.Mean vitrinite re flectance (%Ro)measurements were performed on polished whole rock blocks using a microscope with white re flected light source and oil immersion objective,based on an average of at least 25points for each sample.In addition,palynofacies analysis consisted of observations of isolated kerogen under transmitted light microscopy to establish kerogen type of organic matter (e.g.Steffen and Gorin,1993).For geochemical analyses,approximately 15e 20mg pulverized whole sample extracted for approximately 1h using dichloro-methane (DCM)(CH[2]Cl[2])in a Dionex ASE 200accelerated sol-vent extractor at 70 C and 5,000,000Pa.Asphaltenes were precipitated from n -hexane and separated by centrifugation.The fractions of the hexane-soluble organic matter were separated into saturated hydrocarbons,aromatic hydrocarbons and NSO (nitro-gen/sulphur/oxygen)compounds by medium-pressure liquid chromatography using a MPLC instrument.Saturated hydrocarbon fractions were analysed by gas chromatography coupled to mass spectrometry (GC/MS)(Selected Ion Monitoring).The GC temper-ature programmed from 70 C held isothermally for 2min,then heated at 10 C/min to 160 C/min and 30 C/min to 330 C.GC e MS analyses were performed on a HP 5890/5989A MS Engine with a gas chromatograph attached directly to the ion source (70eV ionization voltage,equipped with a HP-5MS column).For the analysis of biomarkers,the fragmentograms for steranes (m /z 217)and triterpanes (m /z 191)were recorded.Individual components were identi fied by comparison of their retention times and mass spectra with published data (Philp,1985;Peters et al.,2005).Relative abundances of triterpanes and steranes were calculated by measuring peak heights in the m /z 191and m /z 217fragmento-grams,respectively.4.Results and discussion 4.1.PetrographyThin section petrography shows that the Chia Gara Formation is composed of organic-rich limestone and calcareous shale sedi-ments (Fig.4a).The calcareous shales are dominant in the lower and upper parts of the section (Fig.2).These calcareous shales are very rich of pyrite and characterize by bitumen staining (Fig.4b).Pyrite grains as framboidals and large crystals are abundant along the section and pyritization of radiolarians is also presence (Fig.4c).The presence of the pyrite associated with organic matter,which implies preservation of organic matter in periodically low oxygen concentrations in pore water,where reactive iron was converted into pyrite (Leventhal,1987).The limestones are characterized by radiolarian-containing wackestone/packstones with distribution of other bioclasts,such as ammonites,ostracodes,foraminifera,calpionellidsandFigure 3.Schematic block diagram represents the palaeodepositional environment of the Late Jurassic e Early Cretaceous basin including Chia Gara Formation.I.M.J.Mohialdeen et al./Marine and Petroleum Geology 43(2013)166e 177168calcisphers,and some unidentified broken bioclasts(Fig.4).The radiolarian molds replaced by calcite as a consequence of changing in chemical properties,such as alkalinity and water temperature (Fig.4d).The calpionellids fossils,especially Calpionella alpine Lorenz and Crassicollaria parvula Remane(Fig.4e e h)are repre-sented and indicated Late Tithonian to Berriasian age(Haq,1980). Authigenic glauconite grains are recognized along the Chia Gara, which indicated marine environment(Boggs,1992).4.2.Elemental analysis and organic-carbon/sulphur relationshipThere are several elements that have good relations with organic matter in the analysed samples and can be determined using elemental analyzer(Table1).The organic-carbon and sulphur contents are plotted in anic-carbon versus sulphur content plot(Fig.5),suggesting that the depositional environment of Chia Gara sediments was dominantly marine environment (Berner and Raiswell,1983).The organic-carbon/sulphur(C/S) relationship is also a quick method that can be used to assess the oxygen level of bottom water(Hofmann et al.,2000).This method is based on the covariance of organic carbon and sulphur,which results from concomitant reduction of sulphate by sulphate reducing bacteria to form hydrogen sulphide(H2S)that reacts with iron to form pyrite in the sediments(Leventhal,1987).The sedi-ments deposited under oxic marine conditions generally have S/C ratios of0.36(Berner,1984),whilst anoxic environments are characterized by S/C ratios higher than0.36with positive intercepts on the S-axis in the S e C plots(Leventhal,1987).In this respect,the Chia Gara sediments considered to be deposited in a marine envi-ronment and preserved under anoxic conditions,where reactive iron was converted into pyrite formation(Leventhal,1987; Rantitsch,2007).This is suggested by the S/C ratios are more than 0.36(Table1)and most of the samples plot along a line with a positive intercept on the sulphur axis(Fig.5).These conclusions are also supported by the presence of pyrite grains along the section (Fig.2)and further by the biomarker environment indicators.4.3.Characteristics of bulk kerogens4.3.1.TOC and Rock Eval pyrolysis dataSource rock properties of the Chia Gara Formation were inves-tigated in this paper for the purpose of characterizing the organicTable1Bulk geochemical results of Rock-Eval pyrolysis and elemental data with calculated parameters and vitrinite reflectance(%Ro)data of the Chia Gara sediments.Wells Samples Depth(m)Lithology Rock-Eval pyrolysis R o(%)Elemental analysisS1(mg/g)S2(mg/g)S3(mg/g)T max( C)S2/S3HI OI PI TOCwt%Nwt%Swt%S/CratioBJ-1well B122147Limestone0.15 2.560.56441 4.57203440.060.78 1.260.05 2.57 2.04 B112161Limestone0.24 2.000.53433 3.77190500.110.63 1.050.03 1.84 1.75B102175Limestone0.20 2.240.60434 3.73195520.080.65 1.150.05 3.74 3.25B92191Limestone0.469.01 1.054268.58352410.050.51 2.560.06 3.75 1.46B82213Limestone0.8012.880.8242915.7497320.060.56 2.590.06 4.32 1.67B72233Calcareous shale 1.2315.000.8042818.7554300.080.54 2.710.06 4.20 1.55B62245Calcareous shale0.597.460.824309.10278300.070.50 2.680.05 3.89 1.45B52251Limestone 1.2113.460.7143118.9464240.080.60 2.900.05 3.04 1.05B4a2261Limestone0.42 6.340.4843313.2382290.060.63 1.660.05 3.36 2.02B42277Calcareous shale0.6110.51 1.334347.90312390.050.65 3.370.07 2.700.80B32289Limestone 1.5422.760.8343527.4573210.060.67 3.970.080.740.19B22295Calcareous shale0.9633.21 1.8143518.3452250.030.677.350.10 2.400.33Arithmetic means0.7011.450.8643212.5371350.070.7 2.770.06 3.05 1.1TK-3well T92782Limestone0.64 5.440.78442 6.97389560.100.79 1.400.05 3.46 2.47 T82790Limestone0.80 5.100.82437 6.22313500.140.70 1.630.04 2.73 1.67T72798Limestone 2.289.550.7343713.1393300.190.70 2.430.04 1.940.80T62818Limestone 1.348.570.7144012.1374310.140.76 2.290.06 2.47 1.08T52828Limestone 2.0913.540.5843923.3445190.130.74 3.040.06 2.640.87T42838Limestone 1.6714.320.6543722.0439200.100.70 3.260.09 3.43 1.05T32858Calcareous shale 2.1015.00 1.1043513.6417310.120.67 3.600.19 3.140.87T22866Limestone 2.9635.65 1.5643922.9480210.080.747.420.25 3.140.42T12886Limestone 1.9017.780.9643918.5462250.100.74 3.850.08 1.630.42Arithmetic means 1.7513.880.8843815.4412310.12 1.75 3.210.10 2.730.85 HR-1well H123085Limestone0.17 3.320.70442 4.74201420.050.79 1.650.090.390.24 H113110Limestone0.28 4.02 1.22440 3.30200610.070.76 2.010.100.830.41H103120Limestone0.19 3.120.78442 4.00181450.060.79 1.720.090.740.43H93140Limestone0.29 3.90 1.10440 3.55242680.070.76 1.610.100.750.47H83160Limestone0.23 3.53 1.23434 2.87201700.060.65 1.760.090.750.43H73165Limestone0.18 3.23 1.11431 2.91203700.050.60 1.590.090.700.44H63185Limestone0.18 3.01 1.13434 2.66197740.060.65 1.530.080.760.50H53210Limestone0.23 2.70 1.02432 2.65172650.080.62 1.570.090.670.43H43230Limestone0.25 4.23 1.44432 2.94246840.060.62 1.720.090.780.45H33245Limestone0.25 3.50 1.08430 3.24215660.070.50 1.630.080.680.42H23280Limestone0.29 4.65 1.49431 3.12306980.060.60 1.520.080.590.39Arithmetic means0.23 3.56 1.12435 3.27215680.060.23 1.660.090.690.42S1: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 Index¼S2Â100/TOC,mg HC/g.OI:Oxygen Index¼S3Â100/TOC,mg CO2/g TOC.T max:Temperature at maximum of S2peak.PI:Production Index¼S1/(S1þS2).TOC:Organic Carbon,wt%.N:Nitrogen,wt%.S:Surfer,wt%.I.M.J.Mohialdeen et al./Marine and Petroleum Geology43(2013)166e177169richness,hydrocarbon potential of the organic matter and its thermal maturity level.Total organic carbon (TOC)analysis showed generally high TOC values of the Chia Gara samples and ranging from 1.05to 7.42%(Table 1).In the Rock-Eval pyrolysis analysis,the amount of hydrocarbon yield (S 2)is a useful parameter to evaluate the generative potential of source rocks (Peters,1986;Bordenave,1993).The Chia Gara samples have pyrolysis S 2yield values in the range of 2.00e 35.65mg HC/g rock (Table 1),which TOC content and pyrolysis S 2yield values meet the accepted standards of a source with good to excellent hydrocarbon-generative potential as clas-si fied by Peters and Cassa (1994)(see Fig.6).Hydrogen index (HI)and oxygen index (OI)of the studied samples were calculated and ranging from 172to 573mg HC/g TOC and 19e 98mg CO 2/g TOC,respectively (Table 1).In addition,T max value which represents the temperature at the point where S 2peak is the maximum is also determined (Espitaliéet al.,1977).The Chia Gara sediments have T max values in the range of 426e 442 C,commonly re flect on maturity but may also be in fluenced by kerogen type (Hunt,1996),thus the de fined maturity windows are only approximate.A plot of hydrogen index (HI)and pyrolysis T max ,can be used to classify thermal maturity and type of organic matter (Mukhopadhyay et al.,1995),shows that the analysed samples generally plot in the early mature to mature zone of Type II and mixed Type II e III kerogens with minor Type III kerogen (Fig.7).Figure 5.Sulphur content plotted against TOC,suggesting that the depositional environment of Chia Gara sediments was dominantly marine environment.Modi fied after Berner and Raiswell,1983.Figure 4.Thin-section photomicrographs of Chia Gara sediments depicting (a)Rich-organic matter in bands form;(b)Rich-organic matter mudstone associated with bitumen staining and pyrite (Py);(c)Pyritized radiolarian shell;(d)Radiolarian packestone;(e)Calpionellid (Calpionella alpine Lorenz)in radiolarian wackestone;(f)Calpionellid in bioclastic wackestone;(g)Calpionellid (Crassicollaria parvula Remane)in radiolarian wackestone bearing calpionellids.I.M.J.Mohialdeen et al./Marine and Petroleum Geology 43(2013)166e 1771704.3.2.Visual kerogenOrganic-rich sediments with a high TOC content from Chia Gara Formation were selected for kerogen microscopy study.The high total organic matter content(TOC)of the Chia Gara sediments provides enough kerogen residues to be optically studied.The main palynofacies identified in the Chia Gara samples are structured organic matter(SOM)and structureless(amorphous)organic matter(AOM).All the examined samples are dominated by amor-phous organic matter(AOM),which generally represents more than95%in the total kerogen residues from the Chia Gara sedi-ments.The structured organic matter,i.e.,palynomorphs and phytoclasts are very rare,hence the focus was on the AOM(Fig.8). The amorphous organic matter in the Chia Gara sediments is classified into three types as follows:(A)Red brown amorphous organic matter,associated with small pyrite framboids;(B)Light brown andfine amorphous organic matter;(C)Brown to dark amorphous organic matter(Fig.8).Figure7.Plot of Hydrogen index(HI)versus pyrolysis T max for the analysed Chia Garasediments,showing kerogen quality and thermal maturitystage.Figure 6.Pyrolysis S2versus total organic carbon(TOC)plot showing generativesource rock potential for the Chia Gara sediments in the studyarea.Figure8.Photomicrographs of three types of amorphous organic matter in the marinesediments of the Chia Gara Formation under white transmitted light(a)Red brownamorphous organic matter,associated with small pyrite framboids,type A.;(b)Lightbrown andfine amorphous organic matter,type B;(c)Brown to dark amorphousorganic matter,type C.(For interpretation of the references to colour in thisfigurelegend,the reader is referred to the web version of this article.)I.M.J.Mohialdeen et al./Marine and Petroleum Geology43(2013)166e177171Table 2Summary of biomarker parameters of the Chia Gara extracts from northern Iraq.Samples n -alkane and isoprenoidsTriterpanes and terpanes (m /z 191)Steranes and diasteranes (m /z 217)Sterane/hopanePr/Ph Pr/C 17Ph/C 18CPIC 3222S/(22S þ22R)C 29/C 30Ts/Tm Tm/Ts MC 30/HC 30Hopane index C 2920S/(20S þ20R)Diasteranes/steranesC 29/C 27Regular steranes C 27C 28C 29T70.350.370.980.890.570.930.61 1.650.090.100.430.08 1.0537.523.239.30.23T50.450.330.700.960.57 1.110.62 1.610.090.090.460.100.9141.021.637.40.25T30.570.48 1.12 1.100.56 1.430.40 2.500.090.100.420.100.8839.825.235.00.15T10.560.250.46 1.020.55 1.510.31 3.260.090.100.450.110.8740.624.035.40.14B90.600.55 1.120.900.580.950.70 1.430.100.080.400.16 1.2833.423.643.00.29B70.340.55 1.390.830.580.790.34 2.930.080.100.430.160.7347.517.834.70.26B50.390.380.970.910.580.730.50 2.000.090.090.400.14 1.2135.122.342.60.23B40.580.250.510.970.580.910.41 2.420.090.100.400.14 1.0337.922.939.20.15B20.570.260.550.990.58 1.090.30 3.390.080.090.430.080.9240.322.737.00.13H110.710.250.440.920.461.140.342.980.360.120.300.051.0930.336.533.20.97Pr:pristane.Ph:phytane.CPI:carbon preference index (2[C 23þC 25þC 27þC 29]/[C 22þ2{C 24þC 26þC 28}þC 30]).Ts:(C 2718a (H)-22,29,30-trisnorneohopane).Tm:(C 2717a (H)-22,29,30-trisnorhopane).C 29/C 30:C 29norhopane/C 30hopane.MC 30/HC 30:C 30moretane/C 30hopane.Hopane Index:(C 35/(C 31ÀC 35)homohopane).Diasterane/sterane ratio:C 27e C 29diasteranes/C 27e C 29regular steranes.Figure 9.Gas chromatograms (GC)of saturated hydrocarbons of some studied Chia Gara extracts.4.4.Biomarker distributions and depositional environment conditionsBiomarker distributions may provide information about organic facies and depositional environment conditions(Waples and Machihara,1991;Hunt,1996;Peters et al.,2005).Depositional environment condition and organic matter input of the Chia Gara sediments were examined through the use of steranes and tri-terpanes distributions recorded based on m/z217and m/z191mass chromatograms,respectively(Waples and Machihara,1991)and n-alkane and isoprenoids(Volkman and Maxwell,1986)as well as parameters calculated from these distributions(Table2).4.4.1.Normal alkanes and isoprenoidsThe saturated gas chromatograms of the analysed Chia Gara extracts display a full suite of saturated hydrocarbons between C13e C40n-alkanes and isoprenoids pristane(Pr)and phytane(Ph) (Fig.9).The n-alkane distributions show a predominance of low to medium molecular weight compounds(n-C14e n-C24)with the presence of significant waxy alkanes(þn-C25)in some samples thus gave moderate CPI values(Table2),suggesting a significant high contribution of marine organic matter with minor terrigenous organic matter contribution(Brooks et al.,1969;Tissot et al.,1978;Ebukanson and Kinghorn,1986;Murray and Boreham,1992).Acyclic isoprenoids occur in a significant amount in all studied Chia Gara extracts(Fig.9)and diagnostic biomarker ratios are listed in Table2.The phytane being the most dominant isoprenoids in the saturated gas chromatograms of the analysed samples(Fig.9); phytane concentration is always higher than pristane concentration, thus giving distinctively low pristane/phytane ratios of0.34e0.71 (Table2).The pristane/phytane(Pr/Ph)ratio is one of the most commonly used geochemical parameters and has been widely invoked as an indicator of the redox conditions in the depositional environment and source of organic matter(Didyk et al.,1978;Tissot and Welte,1984;Chandra et al.,1994;Large and Gize,1996;Peters et al.,2005).Organic matter originating predominantly from terrestrial plants would be expected to contain high Pr/Ph ratio of>3 (oxidizing conditions),while low values of(Pr/Ph)ratio(<1)indi-cate anoxic conditions,and values between1and3suggest inter-mediate conditions(suboxic conditions)(Peters and Moldowan, 1993;Powell,1988).In this respect,the Chia Gara sediments are likely to be deposited under anoxic conditions.This is suggested by low Pr/Ph ratios of<1(Table2).Isoprenoids/n-alkanes ratios(i.e.Pr/ n-C17and Ph/n-C18)are calculated and giving distinctively low pristane/n-C17and relatively high phytane/n-C18ratios in the range of0.25e0.55and0.44e1.39,respectively,further suggest a marine organic matter deposited under reducing conditions(Fig.10).These periodic reducing conditions are also supported by the presence of pyrite associated with organic matter in the horizons of the Chia Gara(Fig.4b and c),particularly when accompanied by high por-phyrins and sulphur contents(Table1and Fig.5).4.4.2.Triterpanes and steranesThe distributions of steranes and triterpanes are commonly studied using GC e MS by monitoring the ions m/z191and m/z217 (Brooks et al.,1969;Peters et al.,2005).The assignment of the peaks of steranes and triterpanes labelled in Figure11and are listed in Table3.Overall,the observed distributions of triterpanes and ste-roidal saturate biomarkers are similar in the Chia Gara extracts (Fig.11).The m/z191mass fragmentograms of the saturated hydrocarbon fractions of all analysed samples exhibit high proportions of hopanes relative to tricyclic terpanes(Fig.11).The relative abun-dance of C29norhopane is generally higher than that of C30hopane in most of the studied samples(Fig.11),with C29/C3017a(H)hopane ratios in the range of0.61e1.51(Table2).The predomi-nance of C29norhopane is frequently associated with clay-poor source rocks,but this is not always the case(Waples and Machihara,1991)and the enhanced norhopane input may also be associated with land plant input(Rinna et al.,1996).Tm(C2717a(H)-22,29,30-trisnorhopane)and Ts(C2718a(H)-22,29,30-trisnorneo-hopane)are well known to be influenced by maturation,type of organic matter and lithology(e.g.Seifert and Moldowan,1979; Moldowan et al.,1985).The investigated samples possess similarly low Ts/Tm ratios(0.30e0.70)and relatively high Tm/Ts ratios(1.43e3.39)as show in Table2,indicating that the samples containa mixture of land and marine-derived organic matter.The homohopane distributions are dominated by the C31 homohopane and decrease with increasing carbon number(Fig.11). The distribution of the extended hopanes or homohopanes(C31e C35)(Fig.11)has been used to evaluate redox conditions based on homohopanes index(Peters et al.,2005).This,in turn,suggests that the Chia Gara extracts were deposited under anoxic conditions.In support,relatively higher homohopanes index were obtained for Chia Gara extracts(Table2).Relatively high C34and C35homo-hopane concentrations have also been reported in a highly reducing marine(Peters and Moldowan,1991;Peters et al.,2005)as is the case of the Chia Gara extracts(Fig.11).In addition,the presence of tricyclic terpanes in the m/z191chromatograms of the Chia Gara extracts(Fig.11),support the high contribution from marine organic matter(Zumberge,1987;Burwood et al.,1992; Hanson et al.,2000).The distributions of diasteranes and the steranes(C27e C29)are characterized by the m/z217ion chromatograms(Fig.11).Relative abundances of C27,C28and C29regular steranes are calculated and the results are given in Table2.The distributions of C27:C28:C29 regular steranes for the analysed samples are similar as are the ratios of diasterane/regular sterane and the thermal maturity parameter C2920S/(20Sþ20R)(Table2).It is agreed that the relative amounts of C27e C29steranes can be used to give indication of source differences(Seifert and Moldowan,1979).The relative distribution of C27,C28and C29steranes is graphically represented in the form of a regular steranes ternary diagram in Figure12 (Huang and Meinschein,1979).The original classification of Huang and Meinschein(1979)related C27steranes to strong algal influence and C29steranes to strong higher plant influence.The Chia Gara extracts are composed of C27e C29regular steraneswhich Figure10.Phytane to n-C18alkane(Ph/n-C18)versus pristane to n-C17alkane(Pr/n-C17),showing depositional conditions of Chia Gara extracts.Modified after Shanmugam,1985.I.M.J.Mohialdeen et al./Marine and Petroleum Geology43(2013)166e177173。
