Geochemical-and-petrographic-characterization-of-Late-Jurassic-Early-Cretaceous-Chia-Gara-Formation

Geochemical and petrographic characterization of Late Jurassic e Early Cretaceous Chia Gara Formation in Northern Iraq:Palaeoenvironment and oil-generation potential

Ibrahim M.J.Mohialdeen a ,Mohammed Hail Hakimi b ,*,Fawzi M.Al-Beyati c

Department of Geology,School of Science,University of Sulaimani,Kurdistan,Iraq b

Geology Department,Faculty of Applied Science,Taiz University,6803Taiz,Yemen c

Technical College,University of Kirkuk,Iraq

a r t i c l e i n f o

Article history:

Received 24September 2012Received in revised form 31January 2013

Accepted 4February 2013

Available online 14March 2013Keywords:

Chia Gara Formation Organic-rich sediments Biomarkers

Marine reducing environment Northern Iraq

a b s t r a c t

The 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.

1.Introduction

Organic 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 ?uences 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 Gara

sediments 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 ?ectance data.In addition,various biomarkers were used to establish the maturity of the organic matter and to help identify the depositional conditions.2.Stratigraphic setting

The 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@https://www.360docs.net/doc/5b10023328.html, (M.H.

Hakimi).

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Marine and Petroleum Geology 43(2013)166e 177

Barsarin 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,ostracodes

and

Figure 1.Paleofacies map of the Tithonian e Berriasian age of Iraq,showing location map of the study area including studied

wells.

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 177167

tintinnids (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 ?rst appearance of brown to dark shales or argillaceous limestones above the stromatolitic limestone beds is de ?ned 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 ?ed 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 ?ssility 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 ?ects 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 methods

A 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 performed

on 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 ?ectance (%Ro)measurements were performed on polished whole rock blocks using a microscope with white re ?ected 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 ?ed 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.Petrography

Thin 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,calpionellids

and

Figure 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 177

168

calcisphers,and some unidenti?ed 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 relationship

There 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 https://www.360docs.net/doc/5b10023328.html,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 kerogens

4.3.1.TOC and Rock Eval pyrolysis data

Source rock properties of the Chia Gara Formation were inves-tigated in this paper for the purpose of characterizing the organic

Table1

Bulk geochemical results of Rock-Eval pyrolysis and elemental data with calculated parameters and vitrinite re?ectance(%Ro)data of the Chia Gara sediments.

Wells Samples Depth

(m)Lithology Rock-Eval pyrolysis R o(%)Elemental analysis

(mg/g)

T max

( C)

S2/S3HI OI PI TOC

wt%

S/C

ratio

BJ-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 1.75

B102175Limestone0.20 2.240.60434 3.73195520.080.65 1.150.05 3.74 3.25

B92191Limestone0.469.01 1.054268.58352410.050.51 2.560.06 3.75 1.46

B82213Limestone0.8012.880.8242915.7497320.060.56 2.590.06 4.32 1.67

B72233Calcareous shale 1.2315.000.8042818.7554300.080.54 2.710.06 4.20 1.55

B62245Calcareous shale0.597.460.824309.10278300.070.50 2.680.05 3.89 1.45

B52251Limestone 1.2113.460.7143118.9464240.080.60 2.900.05 3.04 1.05

B4a2261Limestone0.42 6.340.4843313.2382290.060.63 1.660.05 3.36 2.02

B42277Calcareous shale0.6110.51 1.334347.90312390.050.65 3.370.07 2.700.80

B32289Limestone 1.5422.760.8343527.4573210.060.67 3.970.080.740.19

B22295Calcareous shale0.9633.21 1.8143518.3452250.030.677.350.10 2.400.33

Arithmetic means0.7011.450.8643212.5371350.070.7 2.770.06 3.05 1.1

TK-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 1.67

T72798Limestone 2.289.550.7343713.1393300.190.70 2.430.04 1.940.80

T62818Limestone 1.348.570.7144012.1374310.140.76 2.290.06 2.47 1.08

T52828Limestone 2.0913.540.5843923.3445190.130.74 3.040.06 2.640.87

T42838Limestone 1.6714.320.6543722.0439200.100.70 3.260.09 3.43 1.05

T32858Calcareous shale 2.1015.00 1.1043513.6417310.120.67 3.600.19 3.140.87

T22866Limestone 2.9635.65 1.5643922.9480210.080.747.420.25 3.140.42

T12886Limestone 1.9017.780.9643918.5462250.100.74 3.850.08 1.630.42

Arithmetic 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 2.010.100.830.41

H103120Limestone0.19 3.120.78442 4.00181450.060.79 1.720.090.740.43

H93140Limestone0.29 3.90 1.10440 3.55242680.070.76 1.610.100.750.47

H83160Limestone0.23 3.53 1.23434 2.87201700.060.65 1.760.090.750.43

H73165Limestone0.18 3.23 1.11431 2.91203700.050.60 1.590.090.700.44

H63185Limestone0.18 3.01 1.13434 2.66197740.060.65 1.530.080.760.50

H53210Limestone0.23 2.70 1.02432 2.65172650.080.62 1.570.090.670.43

H43230Limestone0.25 4.23 1.44432 2.94246840.060.62 1.720.090.780.45

H33245Limestone0.25 3.50 1.08430 3.24215660.070.50 1.630.080.680.42

H23280Limestone0.29 4.65 1.49431 3.12306980.060.60 1.520.080.590.39

Arithmetic means0.23 3.56 1.12435 3.27215680.060.23 1.660.090.690.42

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.

OI:Oxygen Index?S3?100/TOC,mg CO2/g TOC.

T max:Temperature at maximum of S2peak.

PI:Production Index?S1/(S1tS2).

TOC:Organic Carbon,wt%.

N:Nitrogen,wt%.

S:Surfer,wt%.

I.M.J.Mohialdeen et al./Marine and Petroleum Geology43(2013)166e177169

richness,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 ?ed 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 ?ect on maturity but may also be in ?uenced by kerogen type (Hunt,1996),thus the de ?ned 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 ?ed 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.

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170

4.3.2.Visual kerogen

Organic-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 identi?ed 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 classi?ed into three types as follows:(A)Red brown amorphous organic matter,associated with small pyrite framboids;(B)Light brown and?ne 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 Gara

sediments,showing kerogen quality and thermal maturity

stage.

Figure 6.Pyrolysis S2versus total organic carbon(TOC)plot showing generative

source rock potential for the Chia Gara sediments in the study

area.

Figure8.Photomicrographs of three types of amorphous organic matter in the marine

sediments of the Chia Gara Formation under white transmitted light(a)Red brown

amorphous organic matter,associated with small pyrite framboids,type A.;(b)Light

brown and?ne amorphous organic matter,type B;(c)Brown to dark amorphous

organic matter,type C.(For interpretation of the references to colour in this?gure

legend,the reader is referred to the web version of this article.)

I.M.J.Mohialdeen et al./Marine and Petroleum Geology43(2013)166e177171

Table 2

Summary of biomarker parameters of the Chia Gara extracts from northern Iraq.Samples n -alkane and isoprenoids

Triterpanes and terpanes (m /z 191)

Steranes and diasteranes (m /z 217)

Sterane/hopane

Pr/Ph Pr/C 17Ph/C 18CPI

C 3222S/(22S t22R)C 29/C 30Ts/Tm Tm/Ts MC 30/HC 30Hopane index C 2920S/(20S t20R)Diasteranes/steranes

C 29/C 27Regular steranes C 27C 28C 29T70.350.370.980.890.57

0.930.61 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 3.390.080.090.430.080.9240.322.737.00.13H110.710.250.440.920.46

1.14

0.34

2.98

0.36

0.12

0.30

0.05

1.09

30.3

36.5

33.2

0.97

Pr:pristane.Ph:phytane.

CPI:carbon preference index (2[C 23tC 25tC 27tC 29]/[C 22t2{C 24tC 26tC 28}tC 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 conditions

Biomarker 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 isoprenoids

The 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 signi?cant waxy alkanes(tn-C25)in some samples thus gave moderate CPI values(Table2),suggesting a signi?cant 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 signi?cant 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 steranes

The 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 in?uenced 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 contain

a 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/(20St20R)(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 classi?cation of Huang and Meinschein(1979)related C27steranes to strong algal in?uence and C29steranes to strong higher plant in?uence.The Chia Gara extracts are composed of C27e C29regular steranes

which Figure10.Phytane to n-C18alkane(Ph/n-C18)versus pristane to n-C17alkane(Pr/n-C17),showing depositional conditions of Chia Gara extracts.Modi?ed after Shanmugam,1985.

I.M.J.Mohialdeen et al./Marine and Petroleum Geology43(2013)166e177173

is an indicator of the mixed marine/terrigenous origin (e.g.plank-tonic-bacterial land plant)as indicated by regular sterane ratio ternary diagram (Fig.12;Huang and Meinschein,1979).This is consistent with the presence of C 30steranes (Fig.11)and low C 29/C 27sterane ratio (Table 2).The lower diasterane/sterane ratios in the analysed samples (Table 2)correspond to the lower clay con-tents in the former as suggested by predominance of C 29norhopane in the m /z 191mass fragmentograms (Fig.11).

4.5.Origin,preservation and diagenesis of organic matter

The diagenesis of marine organic matter is complex and known to depend on several factors;these include sedimentation rates,productivity and water column oxygenation levels (Hofmann et al.,2000)and their interactions to each other (Tyson,1995).

The Chia Gara sediments have been interpreted to have been deposited in a principally marine environment under low oxygen-de ?cient conditions.Sulphur contents >0.58wt%are within the range observed in a marine environment (e.g.Berner and Raiswell,1983)(Fig.5).However,small amounts of pyrite associated with organic matter,commonly in framboidal form,suggest that the organic matter of Chia Gara sediments was preserved under anoxic conditions,as pyrite formation is related to the reduction of sul-phate in saline water by anaerobic bacteria (Leventhal,1987;Rantitsch,2007).This is as indicated by S/C ration stated above (Table 1)and most of the samples plot along a line with a positive intercept on the sulphur axis (Fig.5).The ?ne-grained mineral matrix and low-oxygen conditions favoured the preservation of organic matter,as indicated by high TOC contents (TOC up to 7wt%)and the high proportion of AOM (Fig.8).It is further noteworthy that the Chia Gara Formation has high sedimentation rates during Middle Tithonian e Berriasian,which can be indicated by a greater thickness (110e 230m;Fig.2).Higher sedimentation rates imply shorter residence times at the sediments/water interface,and

Figure 11.The m /z 191mass fragmentograms (right)and m /z 217mass fragmentograms (left)of saturated hydrocarbon fractions of some studied Chia Gara extracts.

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faster burial rate of organic matter through the post-depositional diagenetic stage.Therefore,they are favourable to protect the organic matter from decomposition,thereby enhance the overall preservation potential (Hofmann et al.,2000).In summary,the preservation of organic-rich sediments in the Chia Gara Formation was mainly controlled by water column oxygenation levels,prob-ably with the help of the sedimentation rate.4.6.Oil generation potential

The geochemical and petrographic results indicate that the marine sediments of the Chia Gara Formation have good petro-leum generative potential.The type of organic matter and what

hydrocarbons might be generated from Chia Gara sediments were characterized by Rock-Eval pyrolysis data.Several studies have been indicated that there is a direct correlation between pyrolysis data and petroleum generative potential (e.g.Bordenave et al.,1993;Hunt,1996).Samples that contain a Type III vitrinitic kerogen would be expected to generate gas with hydrogen index <200mg HC/g TOC whereas samples with hydrogen index values higher than 200mg HC/g TOC can generate oil although their main generation products are gas and condensate.Moreover,samples with hydrogen index greater than 300mg HC/g TOC that contain a signi ?cant amount of type II kerogen can generate oil (Bordenave et al.,1993;Hunt,1996).In this respect,the Late Jurassic e Early Cretaceous Chia Gara sediments are likely the most proli ?c petroleum sources where abundant oils to condensate might be expected to generate.This is suggested by abundance of Type II and mixed Type II e III kerogens (Fig.7)and supported by high hydrogen index up to 300mg HC/g TOC (Table 1).The ana-lysed samples with HI values lower than 200mg HC/g TOC,in contrast,can generate gas if they are subjected to suf ?cient burial and heating.

The thermal maturity of dispersed organic matter governs,in part,the character of the organic matter and therefore may in ?u-ence interpretation of hydrocarbon generation.For this reason,when assessing a hydrocarbon generation,one has to be aware of the effects of maturation on the organic matter and to take this effect into consideration.A variety of maturity indicators have been used to evaluate the level of thermal maturity of the Chia Gara sediments;these include mean vitrinite re ?ectance (%Ro),Rock-Eval T max and production index (PI)data.Rock-Eval T max and the production index (PI)indicate that most of the analysed Chia Gara samples are within the main stage of hydrocarbon generation (Fig.13).The PI values are also internally consistent with the HCs being indigenous to a mature source rock (Fig.13).The T max and PI values are consistent with the mean vitrinite re ?ectance (%Ro)values that range from 0.50to 0.80%Ro (Table 1),indicating that the Chia Gara sediments are thermally mature and have entered the early-mature to peak-mature oil window.On the basis of these data,the Chia Gara sediments under investigation are an effective source rock for signi ?cant oil and limited gas-generation potential (Fig.14).

Table 3

Peak assignments for alkane hydrocarbons in the gas chromatograms of aliphatic fractions in the m /z 191(I)and 217(II)mass fragmentograms.

Compound abbreviation

(I)Peak no.Ts 18a (H),22,29,30-trisnorneohopane Ts Tm 17a (H),22,29,30-trisnorhopane Tm

2917a ,21b (H)-nor-hopane C 29hop 3017a ,21b (H)-hopane Hopane 3M 17b ,21a (H)-Moretane

C 30Mor 31S 17a ,21b (H)-homohopane (22S)C 31(22S)31R 17a ,21b (H)-homohopane (22R)C 31(22R)32S 17a ,21b (H)-homohopane (22S)C 32(22S)32R 17a ,21b (H)-homohopane (22R)C 32(22R)33S 17a ,21b (H)-homohopane (22S)C 33(22S)33R 17a ,21b (H)-homohopane (22R)C 33(22R)34S 17a ,21b (H)-homohopane (22S)C 34(22S)34R 17a ,21b (H)-homohopane (22R)C 34(22R)35S 17a ,21b (H)-homohopane (22S)C 35(22S)35R

17a ,21b (H)-homohopane (22R)C 35(22R)(II)Peak no.a 13b ,17a (H)-diasteranes 20S Diasteranes b 13b ,17a (H)-diasteranes 20R Diasteranes c 13a ,17b (H)-diasteranes 20S Diasteranes d 13a ,17b (H)-diasteranes 20R Diasteranes e 5a ,14a (H),17a (H)-steranes 20S aaa 20S f 5a ,14b (H),17b (H)-steranes 20R abb 20R g 5a ,14b (H),17b (H)-steranes 20S abb 20S h

5a ,14a (H),17a (H)-steranes 20R

aaa

20R

Figure 12.Ternary diagram of regular steranes (C 27e C 29)showing the relationship between sterane compositions and organic matter input,showing that the analysed Chia Gara extracts are composed of mixed marine/terrigenous organic matter.Modi-?ed after Huang and Meinschein,1979

Figure 13.Plot of pyrolysis T max versus production index (PI),showing the maturation and nature of the hydrocarbon products of the Chia Gara samples.

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5.Conclusions

Investigation of sediments from Chia Gara Formation in the northern Iraq by organic geochemical and petrological methods has revealed that the Chia Gara sediments are rich-amorphous organic matter with a high TOC values in the range of 1.05e 7.42%.The high total organic matter content (TOC)is mainly due to good preser-vation of organic matter in a marine environment under reducing conditions,as supported by high sulphur/organic-carbon ratio and biomarkers that are indicator for the depositional environment.The periodic reducing conditions are also evidenced by the pres-ence of pyrite associated with organic matter of the Chia Gara sediments.

The n -alkane distributions are characterized by an even-over-odd predominance in most of the Chia Gara extracts,indicate a high decay of marine derived organic matter that was preserved under reducing conditions.In some of the Chia Gara extracts,long-chain n -alkanes indicate the presence of minor terrigenous organic matter contribution.This has also been achieved from acyclic iso-prenoids,terpane and sterane biomarkers.

The hydrocarbon generation potential of the Chia Gara sedi-ments was deduced basically from Rock-Eval pyrolysis.The Chia Gara sediments are likely the most proli ?c petroleum sources,which their main generation products are oils with limited gas.This conclusion is suggested by abundance of Type II grading into Type II e III kerogens with minor Type III kerogen.Acknowledgements

The authors thank the Kirkuk Oil Company,north Iraq for sup-plying the samples for this study.The authors are most grateful to the University of Sulaimani and the Kurdistan Regional Govern-ment,North Iraq for ?nancial support to complete this research.Special thanks are offered to Drs.Volker Thiel and Lorenz Schwark for their comments on an earlier version of the manuscript.We would like to sincerely thank a reviewer Dr.Van Loon A.J.(Tom)for his critical comments and advice that signi ?cantly improved the revised manuscript.References

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