Petroleum generation characteristics of heterogeneous source rock from Chia Gara formation in the Kurdistan region, northern Iraq as inferred by bulk and quantitative pyrolysis techniques

Marine and Petroleum Geology 71 (2016) 260e270

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Research paper

Petroleum generation characteristics of heterogeneous source rock from Chia Gara formation in the Kurdistan region, northern Iraq as inferred by bulk and quantitative pyrolysis techniques Mohammed Hail Hakimi a, *, Wan Hasiah Abdullah b, Ibrahim M.J. Mohialdeen c, Yousif M. Makeen b, Khairul Azlan Mustapha b a b c

Geology Department, Faculty of Applied Science, Taiz University, 6803 Taiz, Yemen Department of Geology, University of Malaya, 50603, Kuala Lumpur, Malaysia Department of Geology, School of Science, University of Sulaimani, Kurdistan, Iraq

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 October 2015 Received in revised form 14 December 2015 Accepted 4 January 2016 Available online 6 January 2016

This study is the first attempt which provides information regarding the bulk and quantitative pyrolysis results of the Chia Gara Formation from the Kurdistan region, northern Iraq. Ten representative earlymature to mature samples from the Chia Gara Formation were investigated for TOC contents, Rock Eval pyrolysis, pyrolysis-GC and bulk kinetic parameters. These analyses were used to characterize the petroleum generated during thermal maturation of the Chia Gara source rock and to clarify the quantity of the organic matter and its effect on the timing of petroleum generation. Pyrolysis HI data identified two organic facies with different petroleum generation characteristics; Type IIeIII kerogen with HI values of >250 mg HC/g TOC, and Type III kerogen with HI values < 100 mg HC/g TOC. These types of kerogen can generate liquid HCs and gas. This is supported by the products of pyrolysisegas chromatography (PyeGC) analysis of the extracted rock samples. Pyrolysis products show a dominance of a marine organic matter with variable contributions from terrestrial organic matter (Types IIeIII and III kerogen), and produces mainly paraffinic-naphthenic-aromatic low wax oils with condensate and gas. Bulk kinetic analysis of the Chia Gara source rock indicates a heterogeneous organic matter assemblage, typical of restricted marine environments in general. The activation energy distributions reveal relatively broad and high values, ranging from 40 to 64 kcal/mol with pre-exponential factors varying from 2.2835 Eþ12/sec to 4.0920 Eþ13/sec. The predicted petroleum formation temperature of onset (TR 10%) temperatures ranges from 110 to 135  C, and peak generation temperatures (geological Tmax) between 137  C and 152  C. The peak generation temperatures reach a transformation ratio in the range of 42e50% TR, thus the Chia Gara source rock could have generated and expelled significant quantities of petroleum hydrocarbons in the Kurdistan of Iraq. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Chia Gara source rock Pyrolysis Bulk kinetics Petroleum characteristics Kurdistan region Northern Iraq

1. Introduction The Late Jurassic-Early Cretaceous Chia Gara Formation is widespread and well exposed in northern Iraq. In the Kurdistan region, the Chia Gara sediments have thicknesses in the range 110e225 m (Mohialdeen, 2008; Mohialdeen et al., 2013). Organicrich limestone and calcareous shale sediments in the Chia Gara

* Corresponding author. E-mail address: [email protected] (M.H. Hakimi). http://dx.doi.org/10.1016/j.marpetgeo.2016.01.003 0264-8172/© 2016 Elsevier Ltd. All rights reserved.

Formation are important and prolific oil- and gas-prone source rocks in the Kurdistan region, northern Iraq (Al-Ameri and Zumberge, 2012; Mohialdeen et al., 2013, 2015). Most of the oil accumulated in the Kurdistan region is considered to be sourced from the organic-rich limestone and calcareous shale of the Chia Gara Formation (Mohialdeen et al., 2015). The organofacies recognised within the Chia Gara source rock, indicate a marine Type II facies and a mixed marine Type II/III facies (e.g., Mohialdeen et al., 2013). These facies strongly affects the petroleum type and the timing of petroleum generation. However, the kerogen characteristics and its relevance to petroleum generation studies of Chia Gara

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source rock from Kurdistan have not been investigated yet. In this respect, the main objective of this study is to determine the organic matter in the Chia Gara Formation and the petroleum type that is generated from the Chia Gara source rock. These data are then used as input for kinetic modeling to predict temperatures for the onset (TR 10%) and peak (maximum geological temperature) of petroleum generation. The petroleum characteristics of organic matter within the Chia Gara source rock and its relevance to predict HC generation have been approached using bulk kinetic and quantitative pyrolysis techniques. 2. Geological setting The Kurdistan region is an oil-rich area and has become the largest oil-producing part of northern Iraq (Fig. 1a). Kurdistan oil fields in northern Iraq are located in the Zagros Fold Belt, which extends throughout the region, primarily in the elongated area folded zone between the thrust zone in the triple junction boundary with Iran and Turkey (Fig. 1a). The Zagros Fold Belt in the northeastern Arabian Plate formed close to the Tethys paleo-ocean,

261

which includes a NW-trending Zagros Fault system through a series of composite faults oriented between N35 W and N30 W (AlSharhan and Nairn, 1997; Aqrawi et al., 2010). The main stratigraphic succession of the Kurdistan region is presented in Fig. 2, and is dominated by a thick Mesozoic succession that consists of Jurassic and Cretaceous strata. The sedimentary rocks of the Iraqi Kurdistan region are composed of carbonates, shales and anhydrites (Fig. 2), which were deposited under marine and subordinate lagoonal environments (Buday, 1980). The Chia Gara Formation is one of the formations in the sedimentary sub-cycle, which extended from Jurassic to Early Cretaceous. It is underlain by the Barsarin Formation and overlain by the Lower Sarmord Formation (Fig. 2). The Chia Gara Formation is mainly composed of organic-rich limestone and shale sediments with rich Ammonite faunas and diverse foraminifera, radiolarian, ostracode and tintinnid species (Mohialdeen, 2008). The type section of the Chia Gara Formation is located at the Chia Gara anticline, south of Amadiya town in the strongly folded zone in northern Iraq (Bellen et al., 1959). The Chia Gara Formation has been studied by many authors who have discussed the

Fig. 1. (a) Location map for the northeast Arabian Peninsula in Iraq, which shows the Zagros Fold Belt with oil and gas field locations in the Iraqi Kurdistan (compiled and modified using the map from Al-Ameri and Zumberge, 2012) (b) Location map of the studied section from Banik Village in the Kurdistan Region (c) Satellite image of area around Banik Village (modified after Edilbi, 2010).

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Fig. 2. Lithostratigraphic section for the Kurdistan region in northeast Iraq (Al-Ameri and Zumberge, 2012).

stratigraphy, sedimentology, paleontology, depositional environments and petroleum geology in the Kurdistan region (e.g. Buday, 1980; Al-Qayim and Saadalla, 1992; Salae, 2001; Sharland et al., 2001; Jassim and Goff, 2006; Mohialdeen, 2007; Mohialdeen and Al Beyati, 2007; Mohialdeen, 2008; Mohialdeen et al., 2013, 2015; Hakimi et al., 2015b). Tectonically, the Chia Gara Formation can be considered as a part of the tectono-stratigraphic megasequence AP8 (149-49 Ma) (Sharland et al., 2001) The Chia Gara sediment lithofacies and fauna reflects a marine environment (Buday, 1980; Jassim and Goff, 2006; Mohialdeen et al., 2013). The marine depositional setting of the Chia Gara Formation is deep outer shelf to carbonate slope environments (Mohialdeen, 2008). The fossil contents of the Chia Gara Formation generally indicates that the Chia Gara sediments were deposited during the Tithoniane Berriasian age (Bellen et al., 1959). In the Kurdistan sections, Spath (1950) reported rich ammonite faunas within the Chia Gara sediments, suggesting Middle Tithonian age.

3. Samples and methods A total of ten outcrop samples representing limestone and calcareous shale sediments within the Chia Gara Formation were collected from the Banik Village in the Kurdistan region (Fig. 1bec). These samples were used to investigate the petroleum generation characteristics of the source rock beds within the Chia Gara Formation. The Chia Gara Formation is believed to be the major source rock in the Kurdistan region, northern Iraq (Al-Ameri and Zumberge, 2012; Mohialdeen et al., 2013, 2015). The collected samples were crushed into powder and analysed using a Rock-Eval 6 instrument. However, the limestone and calcareous shale samples were pre-treated several times with concentrated HCl to remove the carbonate before analysis as the carbonate facies strongly affects the total organic carbon (TOC) contents. The pyrolysis was performed with 100 mg of crushed samples, which were heated to 600  C in a helium atmosphere. Several parameters such as TOC, S1, S2, S3 and the temperature of

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Table 1 Total organic carbon content (wt%), and pyrolysis results of the Chia Gara source rock from Kurdistan region. Section

Banik Village Kurdistan Region

Sample ID

BP-6 BP-7 BP-8 BP-9 BP-10 BP-11 BP-12 BP-13 BP-14 BP-15

Lithology

Limestone Limestone Limestone Limestone Calcareous shale Limestone Limestone Limestone Limestone Calcareous shale

TOC wt.%

0.75 1.10 0.36 1.23 1.10 1.16 1.01 1.00 0.55 2.00

Pyrolysis data S1 (mg/g)

S2 (mg/g)

S3 (mg/g)

S2/S3 (mg/g)

Tmax (oC)

HI (mg/g)

OI (mg/g)

0.08 0.14 0.02 0.27 0.53 0.10 0.08 0.43 0.03 0.23

1.92 3.29 0.33 4.55 4.2 0.99 3.78 3.89 0.4 5.63

0.30 0.59 0.68 0.51 0.34 1.36 0.66 0.23 0.86 0.94

6.40 5.58 0.49 8.92 12.35 0.73 5.73 16.91 0.47 5.99

439 442 445 439 441 446 434 441 444 446

256 299 92 370 382 85 374 389 73 282

39 48 148 36 26 87 65 19 108 42

TOCeTotal organic carbon (wt%). S1eVolatile hydrocarbon (HC) content, mg HC/g rock. S2eRemaining HC generative potential, mg HC/g rock. S3eCarbon dioxide yield, mg CO2/g rock. TmaxeTemperature at maximum of S2 peak. HIeHydrogen Index ¼ S2x 100/TOC, mg HC/g TOC. OIeOxygen Index ¼ S3x 100/TOC, mg CO2/g TOC.

maximum pyrolysis yield (Tmax) were measured (Table 1). This analysis was used to assess the petroleum generative potential and thermal maturity of the Chia Gara Formation. The powdered samples were also extracted with a mixture of dichloromethane (DCM) and methanol (CH3OH) (93:7 v/v %) for 72 h using a Soxhlet apparatus. The extracted samples were then analysed by pyrolysisegas chromatography (PyeGC). Pyrolysisegas chromatography (PyeGC) of kerogen was developed in the late sixties as a tool to elucidate the structural features of kerogens (Giraud, 1970; Larter and Douglas, 1980; Dembicki et al., 1983). This method is used to provide a direct indicator of the generative part of the kerogen composition, and to indicated the type of petroleum that can be generated by the kerogen during the maturation process (Larter and Douglas, 1980; Dembicki et al., 1983; Larter and Senftle, 1985; Horsfield, 1989; Eglinton et al., 1990; Horsfield and Dueppenbecker, 1991). Open system pyrolysis gas chromatography (PyeGC) was applied to provide compositional and structural characteristics of the kerogen. An HP-Ultra1, 50 m  32 mm i.d., dimethylpolyesiloxane-coated column (0.52 mm film thickness) was fitted into an Agilent GC chromatograph equipped with a pyrolysis unit and flame ionisation detector. Pyrolysis products were released over the range 300e600  C (25  C/min) and collected in a nitrogencooled trap. Identification of peaks based on reference chromatograms was done manually with Agilent ChemStation software and comparison with published data (e.g. Dembicki et al., 1983; Harry, 2008). Eight extracted samples were subsequently selected for bulk kinetic pyrolysis with online FID using a Weatherford Source Rock Analyzer™ (SRA). The extracted samples (10e20 mg) were heated at different heating rates of 1, 5, 10, 25 and 50  C/min. A steady flow of helium carrier gas transported pyrolysis products (S2) to the FID for continuous measurement of bulk formation rates. The FID electronic response was calibrated with a hydrocarbon standard from a quantitative by determined S2 yield and true temperature (tT) (Jarvie et al., 1996). The bulk petroleum formation curves measured at five heating rates served as input for the kinetic model. Activation energy distribution (Ea) and frequency factor (A) were evaluated using Kinetics 2000 and KMOD™ software. The kinetic analysis used the rigorous mathematical model that developed by the Lawrence Livermore National Laboratory (Burnham et al., 1987, 1988).

4. Results and discussion 4.1. Bulk kerogen characteristics Total organic carbon (TOC, wt%) content and pyrolysis data are shown in Table 1 and were used to identify the source generative potential, kerogen type and thermal maturity of organic matter in the Chia Gara sediments. The total organic carbon content (TOC, wt %) and hydrocarbon yield (S2) generated during pyrolysis were used to determine the amount of organic matter and to evaluate the generative potential (Peters, 1986; Bordenave, 1993). The Chia Gara samples have TOC contents in the range of 0.36e2.00 wt% (Table 1), suggesting generally fair to good source generative potential (Peters and Cassa, 1994; Hunt, 1995). This also concurs with the finding based on hydrocarbon yield (S2) generated during pyrolysis (Fig. 3). The pyrolysis hydrogen index (HI) of the Chia Gara samples ranged from 73 to 389 mg HC/g TOC (Table 1), revealed that the organic matter contain of mainly Types IIeIII kerogen grading

Fig. 3. Plot of total organic carbon (TOC, wt%) content versus remaining hydrocarbon potential (S2, mg HC/g rock), showing that the Chia Gara samples have fair to good source rock generative potential at a maturity of 434e446  C.

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through III kerogen (Fig. 4; Mukhopadhyay et al., 1995). The type of organic matter was also characterized based on pyrolysis yields, i.e. a (S2) versus TOC plot (Fig. 5) as suggested by Langford and BlancValleron (1990). This plot is in agreement with the previous plot and consistent with mainly mixed Type IIeIII kerogens and Type III kerogen (Fig. 5). The organic matter type is consistent with good oil- and gas-source rock generative potential. The hydrocarbon generation potential can also be established from potential yield (S2/S3), whereby most of the sample plot in an area indicating good

Fig. 6. Plot of total organic carbon (TOC) content versus (S2/S3), showing potential hydrocarbon generative and type. Most of the samples plot in an area indicating good potential source rock for oil and limited gas potential.

oil- and gas-prone source rock potential (Fig. 6). The level of thermal maturity of organic matter can be evaluated using pyrolysis , 1986; Peters and Cassa, 1994). The Chia Tmax values (e.g., Espitalie Gara samples have Tmax values in the range of 434e446  C (Table 1), suggesting that all the Chia Gara sediments investigated in this study are thermally mature and have entered early to peak oil window maturity (Fig. 4).

4.2. Pyrolysis-gas chromatography (PyeGC) data and molecular composition

Fig. 4. Cross plot of pyrolysis data Hydrogen index versus Tmax for Chia Gara source rock samples in Kurdistan region, northern Iraq, showing the kerogen type and thermal maturity of the sample set.

Fig. 5. Distribution of the Chia Gara samples into pyrolysis S2 yields versus total organic carbon (TOC) plot, showing kerogen type.

4.2.1. Normal alkanes/alkenes and aromatic hydrocarbon distributions Selected chromatograms of the Chia Gara source rock samples are shown in Fig. 7. The pyrolysate distributions of the studied samples indicate heterogeneous organic matter assemblages and two organofacies forms within the Chia Gara source rocks (Keym et al., 2006; Abbassi et al., 2014). These different organofacies have strong impact on the kerogen compositions and petroleum type. Generally, the two organofacies are composed of sulphur-lean organic matter, as indicated by low concentrations of thiophenic  et al., 1988; compounds in the pyrolysates (e.g., Sinninghe Damste  et al., 1998). Most of the Saiz-Jimenez, 1995; Sinninghe Damste studied samples display a Type IIeIII organofacies (>250 mg HC/g TOC) at a maturity of the range 434e446  C, and are dominated by a homologous series of n-alkene/alkane doublets, reaching a maximum chain length of >30 carbon atoms with some light aromatic compounds (Fig. 7aeb). This pyrolysate distributions support the assumption that these source rock samples were deposited under marine conditions with prevalent contribution of aquatic organic matter and only minor land plants input (Bordenave, 1993; Horsfield, 1997; Hartwig et al., 2012), where suboxic conditions were likely favourable for the preservation of organic matter. This interpretation of organic matter and paleoenvironment conditions has been confirmed by biomarker components as recently reported by Hakimi et al. (2015b). In contrast, three kerogen samples from Chia Gara source rocks with relatively low HI values (<100 mg HC/g TOC) are characterized by relatively low homologous series of nalkene/alkane doublets with a significant amount of aromatic compounds (Fig. 7c), indicating a more terrigenous organic matter

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Fig. 7. PyrolysiseGC pyrograms of selected Chia Gara source rock samples showing n-alkene/alkane doublets and labelled peaks used as kerogen type proxies.

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assemblage in these samples. However, the most abundant aromatic hydrocarbons in all the pyrolysates are toluene and xylene components (Fig. 7). These aromatic compounds are commonly found within Type IIeIII organic matter are generated in similar amounts to closely eluting n-alkenes (Keym et al., 2006). The source of aromatic hydrocarbons within pyrolysis products is suggested to be either aromatic moieties from the kerogen itself or aromatization products from cross-linked or alicyclic moieties (Muscio and Horsfield, 1996). 4.2.2. Kerogen composition and type of petroleums Insight inter petroleum generated during the maturation process can interpreted by the pyrolysis products using two different ternary diagrams as shown in Figs. 8 and 9. The relative percentages of three pyrolysate components 2,3-dimethylthiophene, orthoxylene and n-non-1-ene were used to define kerogen type using the diagram (Fig. 8) defined by Eglinton et al. (1990). These three pyrolysate components represent the aliphatic structures, aromatic, and organic sulphur within the macromolecular organic matter, which was generated from the source rocks (Keym et al., 2006; Hakimi et al., 2015a). These three pyrolysate components can be directly related to different organofacies types (I, II, III and IIS) (e.g., Keym et al., 2006). Based on these pyrolysate components, the kerogen is characterized by predominantly Type IIeIII kerogens and minor amount of Type III kerogen (Fig. 8). This kerogen characterization is consistent with the hydrogen index values (73e389 mg HC/g TOC) of the samples (Fig. 4). The second ternary diagram, originally defined by Horsfield (1989) can be used to assess the type of petroleum that can be generated by the kerogen during the maturation process using the total resolved C1 to C5 hydrocarbons, the sum of the n-alkenes/nalkanes in the C6 to C14 range, and the sum of the n-alkenes/n-alkanes in the Cþ15 range (Fig. 9). The majority of the analysed Chia Gara source rock samples fall within the field of the paraffinicenaphthenicearomatic oil field (low wax), consistent with their hydrocarbon-generating potential (Fig. 9). This is in good agreement with the commonly reported mixed type II/III kerogen which derived primarily from mixed organic matter that produces mainly low waxy oil (e.g. Keym et al., 2006). The low wax paraffinicenaphthenicearomatic (PNA) oils also indicate a restricted marine environment receiving high contributions of aquatic

Fig. 8. Ternary diagram to distinguish the type of organic matter from Py-GC technique by means of the compounds 2,3-dimethylthiophene, oxylene (1,2dimethylbenzene) and n-non-1-ene (n-C9:1), adapted after Eglinton et al. (1990).

Fig. 9. Ternary diagram based on n-alkyl chain length distribution of pyrolysis products from the Chia Gara source rock samples, predicting the generation of a PNA low wax oils with condensate and gas (adapted after Horsfield, 1989).

organic matter and only a minor terrigenous organic matter (Mohialdeen et al., 2013). In contrast, the samples with a more terrigenous organic matter assemblage are plotted within the field of gas and condensate (Fig. 9), consistent with their Type III kerogen (Fig. 8). Overall, there is good correlation between the PyeGC and the bulk pyrolysis results, and both are in accordance with the liquid HCs and gaseprone nature of the Chia Gara source rock in the Kurdistan region of northern Iraq. 4.3. Bulk kinetics and prediction for geological conditions The bulk kinetics of the kerogen was evaluated on eight samples representative of the organofacies from the Chia Gara source rock. The calculated activation energy distributions and frequency factors are illustrated in Fig. 10, and the data are shown in Table 2. The activation energy distributions and frequency factors will be depended mainly on the type of organic matter (Dieckmann, 2005). The activation energy distributions of the Chia Gara source samples are characterised by a relatively broad and high activation energy distributions, ranging from 40 to 64 kcal/mol with pre-exponential factors varying from 2.2835 Eþ12/sec to 4.0920 Eþ13/sec (Tables 2 and 3). The characteristics of these samples indicate a heterogeneous organic matter assemblage in the Chia Gara source rock (Keym et al., 2006; Abbassi et al., 2014), typical of marine type IIeIII kerogen in general (Burnham et al., 1987; Espitalie et al., 1988; Wei et al., 1994; Reynolds and Burnham, 1995; Moretti and Deacon, 1995; Dessort et al., 1997; Hakimi et al., 2015a). The BP-6,7,9, 12,13 and 15 samples with relatively high HI values of 256e389 mg HC/g TOC are characterised by an AE of 40e61 kcal/ mol and frequency factors of around 2.2835 Eþ12/sec to 2.2965 Eþ13/sec (Fig. 10a). The remaining BP-11 and 14 samples with HI values of 73e85 mg HC/g TOC are characterised by relatively higher mean activation energies and cover a range of activation energies from 40 to 64 kcal/mol, reflecting more heterogeneous organic matter (Fig. 10b). However, the kinetic properties of the individual samples are also evident in the heterogeneous organic matter within the studied samples from the Chia Gara source rock.

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267

Fig. 10. Activation energy distributions of selected Chia Gara source rock samples from the Kurdistan region, northern Iraq.

The derived kinetic properties were also used to estimate the temperature and timing of bulk petroleum generation using a linear geological heating rate of 3.3  C/My. This heating rate corresponds to an average geological heating rate in most sedimentary basins (Schenk et al., 1997). The results in terms of onset (10% TR) and peak generation (geol. Tmax) temperatures for the Chia Gara source rock samples in the study area are presented in Table 3. The estimated predictions of the petroleum generation for the Chia Gara source rock samples show a large variation in terms of onset and peak generation temperatures. This is attributed to the significant differences in the distribution of activation energies (Fig. 10). The Chia Gara source rock samples reach the petroleum generation window (TR 10%) at variable temperatures ranging from the lower 110  C to higher 135  C (Table 3). The peak generation temperatures (geological Tmax) are also variable and range from lowest (geol. Tmax: 137  C) to highest (geol. Tmax: 152  C) (Table 3). The range in peak generation temperatures is consistent with temperatures corresponding to the late oil-window to gas-window. However, the geol. Tmax temperatures and HI values are inversely correlated (Fig. 11). The Chia Gara source rock samples with the highest geol. Tmax of 150e152  C, are composed mainly of gas prone kerogen (Fig. 11) and is characterized by a relatively low HI of 73e85 mg HC/ g TOC, and the lowest TOC contents of 0.55e1.16 wt.% (Table 3). In contrast, samples with the highest HIs and TOC contents of the 256e389 mg HC/g TOC and 0.75e2.00 wt.%, respectively, are characterized by a relatively low geol. Tmax values of 137e147  C and are composed mainly of oileprone kerogen (Fig. 11). Therefore, the different organic facies have an influence on the type of petroleum generated (Keym et al., 2006; Abbassi et al., 2014; Hakimi et al., 2015a). 4.4. Petroleum generation characteristics In this study, petroleum generation of the Chia Gara source rock was characterized based on bulk offer and also analytical pyrolysis

methods. The Chia Gara source rock contains mixed Type IIeIII kerogens with Type III kerogen (Figs. 4 and 5). This kerogen type dominated by Type IIeIII was deposited under a marine environment with a high contribution of aquatic organic matter and minor terrigenous organic matter input as reported by Hakimi et al. (2015b). This type of organic matter can mainly generate liquid hydrocarbons (oil and condensate) and gas if subjected to sufficient burial and heating. This interpretation was confirmed by kerogen composition examination and type of petroleum generated during the maturation process using pyrolysis products of pyrolysisegas chromatography (PyeGC), see Section 4.2.2. The pyrolysis products reflect that the Chia Gara source rock samples contain Type IIeIII and III kerogen that produces mainly paraffinicenaphthenicearomatic low wax oils with condensate and gas (Figs. 8 and 9). The kinetic model of kerogen types in the Chia Gara source rock has been used to compute the temperature of petroleum generation (see Section 4.3). Based on the temperature of petroleum generation, the transformation ratios of the kerogen types in the source rock have also been computed to predict the petroleum generation and expulsion (Fig. 12). The red lines represent predictions for type IIeIII kerogen (Liquid HC), while the blue lines represent those of Type III facies (gas HCs). The peak generation temperature values of the Type IIeIII kerogen reach significant transformation ratio (42e50% transformation) (Table 3). This transformation ratio is indicates that the Chia Gara source rock potential could have been realized, and mature liquid HCs expelled at temperatures covering a range of 110e147  C at Table 3. This findings are confirmed by the large amount of oil produced from the Kurdistan region (Fig. 1a), in which most of the oil accumulated in the Kurdistan oilfields is considered to be contributed by the marine carbonates and shales of the Chia Gara Formation (Mohialdeen et al., 2015). In contrast, the transformation ratio values of the Type III kerogen are between 42% and 45%, indicating that limited HC gas has been generated but not till, this gas is yet not expelled.

268

Table 2 Activation energy distributions (kcal/mol) for the Chia Gara source rock samples from Kurdistan region, North Iraq. Organic facies

Samples/EA (kcal/mol) 37 0.13 0.36 e e e 0.09 e e

38

39

40

41

42

43

44

45

46

47

0.24 e e 0.41 e e 0.07 e

0.41 0.31 e e e 0.01 0.13 e

0.57 0.04 0.10 0.38 e 0.04 0.15 0.22

0.40 0.16 e 0.45 e e 0.24 0.14

0.47 e 0.09 0.38 e 0.03 0.71 0.56

1.09 0.31 0.24 0.86 0.49 0.04 0.73 0.05

0.59 0.13 0.22 1.02 0.05 e 1.25 0.64

1.06 0.37 e 1.90 e 0.58 2.42 0.72

1.39 0.59 0.79 1.73 0.20 0.27 4.22 0.86

1.69 1.95 1.26 0.24 e e 3.70 4.50 0.89 0.12 e e 6.94 12.14 0.90 1.09

48

49

50

51

52

53

4.89 e e 11.51 0.34 e 18.14 2.75

e 19.77 e 21.87 0.20 36.32 15.62 5.25

18.25 32.61 e 19.12 1.15 31.59 10.67 8.40

28.67 19.91 47.69 11.14 12.68 13.08 7.12 15.63

20.55 8.77 6.18 0.31 8.53 5.22 4.38 e 21.39 20.20 2.50 4.48 6.24 4.70 3.89 1.92 28.60 26.30 15.70 5.07 4.83 4.45 2.48 e 7.77 3.87 2.07 2.84 17.54 11.80 12.60 5.59

54

55

56

57

58

59

60

61

62

63

64

e 3.24 e 1.12 3.51 3.35 0.42 5.64

e 0.56 1.49 1.93 1.42 0.17 0.59 2.73

e e e e 2.11 e e 2.25

e e 0.27 e 0.39 e 1.69 e

2.39 2.00 0.46 e e 2.67 e 2.7

e e e 1.24 e e e 0.04

e e 0.10 e e e e e

e e e e 0.74 e e 1.97

Table 3 Pre-exponential factors (A [I/S]) by predicted onset (TR 10%) and peak generation temperatures, and computed transformation ratios (TR%) for the analysed Chia Gara source rock samples set at a linear heating rate of 3.3  C/My. Organic facies

Sample ID

Lithology

A [1/S]

Type IIeIII kerogen

BP-6 BP-7 BP-9 BP-12 BP-13 BP-15 BP-11 BP-14

Limestone Limestone Limestone Limestone Limestone Calcareous shale Limestone Limestone

1.7291 7.7940 2.2965 6.2188 2.2835 5.8184 4.0920 2.2740

Type III kerogen

Ae pre-exponential factors. HIeHydrogen Index ¼ S2x 100/TOC, mg HC/g TOC. Onset (oC): onset (TR 10%) temperature. Peak (oC): peak generation (geological Tmax) temperature. Peak TR(%)e Transformation ratios of peak generation geological Tmax) temperature.

Eþ13/sec Eþ12/sec Eþ13/sec Eþ12/sec Eþ12/sec Eþ12/sec Eþ13/sec Eþ13/sec

TOC

HI

Onset (oC)

Peak (oC)

Peak TR (%)

0.75 1.10 1.23 1.00 1.01 2.00 1.16 0.55

256 299 370 389 374 282 85 73

113 127 134 110 110 128 135 123

146 143 147 139 137 141 152 150

50 46 50 40 42 44 45 42

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Type IIeIII kerogen BP-6 BP-7 BP-9 BP-12 BP-13 BP-15 Type III kerogen BP-11 BP-14

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Fig. 11. Plot showing the relationship between HI values and calculated geological maximum temperature values for two types of kerogen in the Chia Gara source rock samples from the Kurdistan region, northern Iraq.

269

(1) The studied Chia Gara source rock has good petroleum generation potential, in which two kerogen types with different petroleum generation characteristics are identified; Type IIeIII kerogen with HI values of >250 mg HC/g TOC and Type III kerogen with HI values < 100 mg HC/g TOC. (2) PyrolysiseGC data confirmed that the Chia Gara source rock samples are of type IIeIII and III kerogens that can be generated mainly low wax PeNeA oils with condensate and gas. (3) Bulk kinetics demonstrates that the organic matter in the Chia Gara source rocks was derived from heterogeneous organic matter, have a relatively broad and high activation energies of 40e64 kcal/mol, and peak generation temperatures (geological Tmax) between 137  C and 152  C, consistent with temperatures for the late oil-window to the gaswindow. (4) Generation temperature as high as 50% TR is required to release petroleum from the organic matter. This is supported by oil produced from the Kurdistan oilfields and this oil is considered to be contributed by the Chia Gara source rock (Mohialdeen et al., 2015).

Fig. 12. The results in terms of temperature vs. transformation ratio curves for the organic facies of the Chia Gara source rock samples, illustrating the onset (10% TR) and peak generation (geol. Tmax) temperatures.

5. Conclusions

Acknowledgements

Bulk and quantitative pyrolysis investigations of Chia Gara source rock samples from Kurdistan, northern Iraq have enabled a better understanding of the variability of the organic matter and their effect on the timing of petroleum generation and characterisation. The following conclusions were suggested:

The authors are grateful to the Departments of Geology in the University Malaya for providing organic geochemistry facilities to complete this study. The authors also would like to sincerely thank an Associate Editor Dr. Janice Kenney and anonymous reviewers for their careful and useful comments that improved the revised manuscript.

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