Hydrocarbon Generation Potential of Chia Gara Formation in Three Selected Wells, Northern Iraq

Wrya J. Mamaseni; Srood F Naqshabandi; Falah Kh. Al-Jaboury

doi:10.1515/geo-2019-0007

Article Open Access

Hydrocarbon Generation Potential of Chia Gara Formation in Three Selected Wells, Northern Iraq

Wrya J. Mamaseni , Srood F Naqshabandi and Falah Kh. Al-Jaboury

Published/Copyright: March 22, 2019

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From the journal Open Geosciences Volume 11 Issue 1

Abstract

In this study collected samples of Chia Gara Formation in Atrush, Shaikhan and Sarsang oilfields are used to geochemical characteristics of organic matter in this formation. This determination was based on Rock-Eval pyrolysis and Biomarker analyses. The Chia Gara Formation can be considered as good to excellent source rock; it’s TOC content ranges from 1.14-8.5wt% with an average of 1.85%, 3.91%, and 6.94% in Atush-1, Mangesh-1 and Shaikhan-8 wells respectively. The samples of Chia Gara Formation contain kerogen type II. These properties are considered optimal for oil generation. The low oxygen index (OI) and pristane/phytane (Pr/Ph) ratios (Average 20.73, 0.61 respectively) and high hydrogen index (HI) (average 637.6) indicate that the formation was deposited under anoxic condition. According to regular sterane (C₂₇%, C₂₈%, C₂₉%) and terpanes ratios (C₂₉/C₃₀, C₃₁/C₃₀ hopane), the formation was deposited in marine environment.

The average value of the Carbon Preference Index (CPI) is one with Tmax values of more than 430 ºC; these indicate peak oil window for the selected samples. Overall, the 20S/(20S+20R), ββ/(ββ+αα)C₂₉ steranes and 22R/(22R+22S)C₃₂homohopane, with Ts/ (Ts+Tm), and moretane/ hopane ratios point to a mature organic matter and to the ability of the formation to generate oil.

Keywords: Chia Gara Formation, Anoxic condition, Biomarker assemblage; Type II kerogen; Mature organic matter; Oil-prone

1 Introduction

Northern Iraq is one of the most suitable locations for deposition of carbonate rocks during Mesozoic Era. As global temperature raised good source rocks in Jurassic and Cretaceous periods are formed, Chia Gara Formation is the most important potential source rocks at these intervals [1]. Chia Gara is the most typical and widespread formation of the Tithonian-Berriasian subcycle [2]. This formation was first introduced by Wetzel in 1950 at its type locality in the Gara anticline, northern Iraq [3]. Lithologically, it consists of thin bedded limestone, argillaceous limestone and fissile black shale. This formation overlies the Barsarin Formation and underlies the Garagu Formation. To determine hydrocarbon potentiality of Chia Gara Formation, three wells which are Atrush-1, Shaikhan-8 and Mangesh-1 in three different blocks in Duhok basin are selected. The studied blocks include of Atrush, Shaikhan and Sarsang as shown in Figure 1 (a, b & c). The thickness of the formation is 196 m (depth from 932 m to 1128 m) in Atrush-1 well, 160 m (depth from 1320 m to 1480 m) in Shaikhan-8 well and 100m(depth from 2180 m to 2280 m) in Mangesh-1 well. For this study, some organic geochemistry analyses are carried out in groups in order to identify redox conditions, type of organic matter, thermal maturity and hydrocarbon generation potential.

Figure 1

Location map for the northeast Arabian Platform in Iraq, which shows Zagros Thrust-Fold Belt with oil and gas field locations, including studied wells

2 Geological Setting

The Zagros Fold Belt is a world-class hydrocarbon province in Iraq, which extends from the Iranian borders in the east to the Turkish borders in the north [1]. The northwestern segment of this belt became a region of interest for hydrocarbon generation and exploration. The studied boreholes are located within the High Folded Zone (Fig. 1a) which is a part of the northwestern segment of the Zagros Fold Belt that covers the deformed portion of the northeastern Arabian platform due to Cenozoic convergence between Eurasia and Arabia. All significant hydrocarbon discoveries in this region occur in compressional structures formed during this convergence. The subsidence in the developing Zagros foredeep induced a major phase of source rock evaluation and hydrocarbon expulsion [4]. The main source rock in Zagros Fold Belt during the Late Jurassic to Early Cretaceous period is Chia Gara Formation [5].

Atrush block is located within the Zagros sedimentary basin, 25 km. to the east of the Duhok city. Atrush structure is a fault-related fold that developed along a shallow thrust that is oriented in the east-west direction with the Zagros Orogenic Belt trend. The topography of Atrush block is similar to that of the Shaikhan block, the last one is located 60 km. to the southeast of Duhok city. Shaikhan structure is an anticline at the northwestern terminus of the Zagros Fold Belt and is affected by several faults. It lies in the highly prospective and productive oil province in northern Iraq. Sarsang block is located on the northern limb of the Gara anticline, 50 km to the northeast of Duhok city. Structurally, this block is more complex than the Atrush and Shaikhan blocks. The Sarsang block is characterized by several faults that have horst and graben structures.

3 Material and method

Thirty cutting samples from Chia Gara Formation were selected and analyzed by Rock-Eval pyrolysis-6 in both Kurdistan Institution for Strategic Studies and Scientific Research/Sulaimaniya Governorate and Scientific Research Center-Soran University/ Erbil Governorate.

The samples were powdered and subjected to Rock-Eval pyrolysis to determine their TOC, volatile hydrocarbon (S1), remaining hydrocarbon generation potential (S2), and T_max value. This technique uses temperature programmed heating of a small amount of rock sample (50 mg) in an inert atmosphere (nitrogen). The samples were heated from 300 ^∘C for 3min to 650 ^∘C at a rate of 25 ^∘C/min; the instrument is equipped with two ovens for pyrolysis and combustion processes respectively. The hydrocarbons generated during a Rock-Eval analysis are monitored by a Flame Ionization Detector (FID) whereas the non-hydrocarbons compounds like CO₂ and CO released during pyrolysis and oxidation stages are monitored by an infra-red detector.

Nine rock samples out of total thirty samples were analyzed by gas chromatography mass spectrometer (GC/MS) instrument. The analyses were performed at Organic Geochemistry Laboratory in Department of Geology and Petroleum Geology at University of Aberdeen/Scotland-UK.

Organic matter extraction (soxhlet extraction)

30 g of the crushed samples were weighed and extracted for forty eight hours (48 hrs.) with 93:7 dichloromethane/methanol (DCM/MeOH) via soxhlet-extraction. Then, the extracts were fractionated into saturated, aromatic and polar compounds using silica column chromatography. Saturated fractions were dissolved in certain amount of hexane and finally detected and analyzed by GC-MS apparatus. GC-MS analysis of the saturate fraction was performed using an Agilent 6890N GC. Relative abundances of n-alkanes (m/z 85), triterpanes (m/z 191) and steranes (m/z 217) were calculated by measuring peak heights.

Column Gas Chromatography (CC)

The extracted organic matter from soxhlet was fractionated by Column chromatography (CC). A short pipette stopped with a little clean cotton bud was loaded with clean silica gel (1/2 to 1/3 of column length). The silica gel about 60-120 mesh laboratory reagent was activated at 120 ^∘C in oven for 24 hrs prior to use.The saturated fraction was eluted with 3 mL of n-hexane, the aromatic fraction with 3 mL mixture of n-hexane and DCM (3:1) v/v and polar fraction with 3 mL mixture of DCM and methanol (2:1) v/v. The eluted fractions were evaporated under a stream of dry nitrogen gas to remove the solvent. After removing the solvent the glass vials of each fraction were reweighed determine masses.

Gas chromatography-mass spectrometry (GC-MS) procedure

Gas chromatography-mass spectrometry (GC-MS) was performed on Agilent Technologies 6890N Network GC system fitted with a 30.0 × 250.0 μm i.d; film thickness 0.25 μm fused silica DB-5 column coupled to an AT 5975 quadrupole mass selector detector for identification (electron input energy 70 eV, source temperature 250 ^∘ C) with helium as carrier gas, normal initial flow: 1.3mL min-1. The saturated hydrocarbon fractions were analysed using an oven program at 60 ^∘C (2.0 min) to 120 ^∘C at 20 ^∘C/min to 290 ^∘C at 4 ^∘C/min and held at 290 ^∘C for 23 min. Data acquisition was controlled by CHEMSTATION software in ion selection monitoring (SIM) for saturated.

4 Results

4.1 Total Organic Carbon (TOC) and Rock-Eval Pyrolysis

The TOC and Rock-Eval analyses were conducted on thirty samples of Chia Gara Formation in Duhok basin. TOC values increase gradationally from 1.14-3.73 wt% in Atrush-1 well to 3.97-4.39 wt% in Mangesh-1 well and 4.85-8.5 wt% in Shaikhan-8well (Table 1).Hydrogen index (HI) values in the three studied boreholes are too high ranging between 468-713 mg HC/g TOC. In contrast, values of oxygen index (OI) are too low (8-51 mg CO2/g TOC). The average T_max value of Chia Gara Formation is 434.7 ^∘C. The average potential product value (GP) decreases from 49.81 mg HC/g TOC in Shaikhan-8 well to 27.92 and 11.0 mg HC/g TOC in Mangesh-1 and Atrush- 1wells respectively with the average production index (PI) value is 0.06. Production index values indicate the oil generative potential, but at low level to early mature stages.

Table 1

Pyrolysis results of Chia Gara Formation in the studied wells

Wells	Depth (m)	TOC (Wt %)	S₁(mg/g)	S₂(mg/g)	S₃(mg/g)	Tmax (°C)	HI (S₂/TOC*100)	OI (S₃/TOC*100)	GP (s₁+s₂)	PI s₁/(s₁+s₂)
AT-1	945	1.26	0.39	7.09	0.34	431	563	27	7.48	0.05
	955	1.14	0.10	6.58	0.58	433	577	51	6.68	0.01
	975	1.24	0.10	5.89	0.39	434	468	31	5.99	0.02
	985	1.63	0.49	10.02	0.34	434	615	21	10.51	0.05
	1000	2.51	0.81	14.52	0.37	436	578	15	15.33	0.05
	1015	1.50	0.76	9.37	0.38	435	625	25	10.13	0.08
	1035	1.50	0.84	10.01	0.36	433	667	24	10.85	0.08
	1060	2.11	0.83	11.61	0.40	436	550	19	12.44	0.07
	1100	3.73	1.32	18.29	0.45	437	544	12	19.61	0.07
Sh-8	1333	6.81	2.76	44.92	1.00	434	660	15	47.68	0.06
	1345	6.94	2.66	46.92	0.92	435	676	13	49.58	0.05
	1360	7.04	2.65	49.40	1.07	436	702	15	52.05	0.05
	1372	6.24	2.19	43.71	1.11	436	700	18	45.90	0.05
	1384	7.47	2.53	52.58	0.83	437	704	11	55.11	0.05
	1396	7.37	2.68	52.40	0.87	436	711	12	55.08	0.05
	1408	8.50	2.91	60.00	0.84	436	706	10	62.91	0.05
	1420	7.11	2.55	43.33	2.15	431	609	30	45.88	0.06
	1432	4.89	2.44	29.22	1.16	431	598	24	31.66	0.08
	1444	4.85	2.12	34.29	0.57	437	707	12	36.41	0.06
	1456	8.23	2.48	56.21	0.69	435	683	8	58.69	0.04
	1468	8.28	2.40	56.58	0.75	437	683	9	58.98	0.04
	1480	6.54	2.15	45.46	0.56	437	695	9	47.61	0.05
Mang-1	2186	3.97	4.00	18.44	1.75	436	464	44	22.45	0.18
	2198	3.91	2.38	19.44	1.95	439	497	50	21.82	0.11
	2213	3.84	2.23	27.27	0.64	434	710	17	29.50	0.08
	2225	4.00	2.63	28.28	0.81	433	707	20	30.91	0.09
	2240	3.95	2.23	26.28	0.89	433	665	23	28.51	0.08
	2255	3.79	2.21	27.01	0.73	432	713	19	29.22	0.08
	2270	4.39	2.48	30.00	0.72	435	683	16	32.48	0.08
	2280	3.94	2.24	26.30	0.88	432	668	22	28.54	0.08

TOC: Total Organic Carbon, wt.%; S₁: Volatile hydrocarbon (HC) content, mg HC/g rock; S₂: Remaining HC generative potential, mg HC/g rock; S₃: Carbon dioxide content, mg CO₂ / g rock, Tmax: Maximum Temperature. HI: Hydrogen Index = S₂ /TOC*100 mgHC/g TOC; OI: Oxygen Index= S₃/TOC*100, mgCO₂/g TOC; GP: Genetic Potential= (S₁+S₂); PI: Production Index= [S₁/(S₁+S₂)]

4.2 Molecular composition and biomarker assemblages

4.2.1 Isoprenoids / n-alkanes (m/z 85) distribution

According to GC/MS analysis results, n-alkanes range from C₁₄ to C₃₅ in the analyzed samples. The distribution of n-alkanes of studied samples contains a much higher concentration of short molecular weight components than the long molecular weight n-alkanes (Fig. 2). The long-chain homologues are known to be derived from higher plant waxes [6]. Carbon preference index (CPI) is obtained by dividing the sum of the odd carbon-numbered alkanes (C₂₅-C₃₃) by the sum of the even carbon numbered alkanes (C₂₄-C₃₄) [7], and the observed values are in between 0.98-1.09, except in one sample,which is 0.89 and this is low as compared to the other samples. The isoprenoid hydrocarbons pristane (Pr) and phytane (Ph) are present in all analyzed samples. Generally Chia Gara Formation has low Pr/Ph ratios ranging from 0.39 to 0.96. Pr/n-C₁₇ and Ph/n-C₁₈ ratios range from 0.48 to 1.13 and 0.76 to 1.59 respectively (Table 2). These values are used to determine the type of organic matter and deposition conditions for the studied samples.

Figure 2

Molecular compositions, Isoprenoids/n-alkanes (m/z 85) and Biomarker assemblages, Steranes (m/z 217) and Terpanes (m/z 191) distribution of Chia Gara Formation in the studied wells

Table 2

Gas Chromatography/ mass spectrometer results of Chia Gara Formation in the studied wells

Isoprenoid /n-alkane						Sterane					Hopane

Wells	Depths (m)	Pr/Ph	Pr/n-C₁₇	Pr/n-C₁₈	CPI	C₂₇%	C₂₈%	C₂₉%	20S/(20S+20R) C₂₉	ββ/(ββ+αα)C₂₉	C_3122R/ C_{30 Hopane}	C_29norhopane/C_{30 Hopane}	Ts / (Ts+Tm)	Mortane index C₂₉	GI	22S/(22R + 22S) C₃₂
AT-1	955	0.45	0.48	1.04	0.89	43	19	38	0.48	0.61	0.49	0.65	0.39	0.07	0.09	0.61
	985	0.73	0.67	0.76	0.99	37	18	45	0.42	0.59	0.43	1.15	0.36	0.08	0.12	0.59

	1060	0.77	0.95	1.59	1.00	35	17	48	0.39	0.60	0.47	1.05	0.37	0.07	0.12	0.58
	1100	0.42	0.64	1.35	0.98	43	17	40	0.45	0.60	0.47	0.60	0.41	0.08	0.11	0.59
	1345	0.47	0.75	1.30	1.01	35	17	48	0.40	0.55	0.36	1.06	0.33	0.06	0.11	0.60
Sh-8	1408	0.39	0.82	0.92	1.03	37	15	48	0.41	0.57	0.38	1.06	0.34	0.06	0.12	0.59
Sh-8	1456	0.63	0.69	0.79	1.02	33	17	50	0.41	0.55	0.31	1.00	0.40	0.05	0.16	0.60
Mang-1	2198	0.70	1.13	1.12	1.08	26	18	56	0.32	0.56	0.38	1.24	0.42	0.12	0.18	0.59
Mang-1	2255	0.96	0.61	0.94	1.09	27	19	54	0.37	0.55	0.43	1.25	0.43	0.11	0.19	0.58

Pr: Pristane
Ph: Phytane
CPI: Carbon preference index= ½(C₂₅:C₃₃/C₂₄:C₃₂) + (C₂₅:C₃₃/C₂₆:C₃₄)
GI: Gammacerane index = (Gammacerane/Gammacerane+ C₃₀ αβ)
Moretane Index=C₃₀ moretane/C₃₀ hopane
Ts: 18α(Η),21β(H)-22,29,30-trisnorneohopane, Ts, (C₂₇)
Tm:17α(Η),21β(H)-22,29,30-trisnorhopane, Tm, (C₂₇)
C₃₂ 17α(H),21β(H)-30,31-bishomohopane (22S)
C₃₂ 17α(H),21β(H)-30,31-bishomohopane (22R)
C₂₉ 5α(H),14α(H),17α(H) sterane (20S)
C₂₉ 5α(H),14β(H),17β(H) sterane (20R)
C₂₉ 5α(H),14β(H),17β(H) sterane (20S)
C₂₉ 5α(H),14α(H),17α(H) sterane (20R)

4.2.2 Steranes (m/z 217) distribution

Steranes are a class of tetracyclic, saturated biomarkers constructed from six isoprene subunits (nearly C₃₀) [8]. They originate from sterols, which are important membrane and hormone components in eukaryotic organisms. The most commonly used steranes are in the range of C₂₆-C₃₀ and are detected using mass/charge 217 mass chromatograms [8]. The regular steranes (C₂₇, C₂₈ and C₂₉) are found in all samples. The samples have high percentage value of C₂₉ and C₂₇, especially in Atrush-1 and Shaikhan-8 wells and their abundances are ordered as C₂₉ >C₂₇>C₂₈(Fig. 2). The dominance of C₂₉ and C₂₇ homologs indicates an open marine organic matter input [9, 10, 11].The values of 20S/(20S+20R) C₂₉ and ββ/ (ββ+αα) C₂₉ vary from 0.32 to 0.48 and 0.55-0.61 with an average value of 0.40 and 0.58 respectively; these two parameters are used to determine maturation level of Chia Gara Formation.

4.2.3 Terpanes (m/z 191) distribution

The detectable hopanes ranged from C₂₇ to C₃₅ in all of the hopane bearing samples. The C₂₉17α,21β(H)-30-norhopane and C₃₀17α,21β(H)-hopane were the most abundant. The biomarkers parameters such as high C₂₉/C₃₀ 17α (H) hopane ratio can be considered as the evidence of carbonate-rich source rocks [12].In this study the relative C₃₀ nor-hopane abundance is typically more than C₃₀ hopane (Fig. 2). The values of C₃₅/C₃₄ and C₂₉/C₃₀ hopane are more than 0.8 and 0.6 respectively. Most oils from marine carbonate source rocks show high C₃₅/C₃₄ hopane (>0.8) combined with high C₂₉/C₃₀ hopane (>0.6) (Table 2) [8]. The analyzed samples contain detectable amounts of C₃₁ hopanes. The gammacerane index is calculated using the peaks of gammacerance and C₃₀αβ hopane identified on m/z 191. This parameter is very specific for the hypersaline environment [13]. The values of gammacerane index ranges from 0.09 to 0.19 (low value). The ratio of 17β,21α(H)-moretanes to their corresponding 17α,21β(H)-hopanes decreases with increasing maturity from 0.8 in immature bitumens to <0.15 in mature source rocks and oils to a minimum of 0.05 [14, 15]. Biomarker maturity ratio 22S/(22S +22R) describing the conversion of the biological 22R to the geological 22S configuration of homohopane molecules was calculated. Typically, this ratio is calculated for the C₃₂ 17α-homohopanes,which varies from 0.58 to 0.61 (Table 2). The Ts/ (Ts+Tm) ratio (sometimes reported as Ts/Tm) is extensively applied in petroleum geochemistry as maturity parameter [16]. It is calculated for all samples in Chia Gara Formation which ranging between 0.330.43.

5 Discussions

5.1 Quantity of organic matter

The quantity of organic matter is usually expressed as Total Organic Carbon [13]. The Total Organic Carbon (TOC, wt%) of a sedimentary rock is defined as the weight percentage of organic carbon in the rock [17]. Typical oil-prone, marine source rocks contain 2-5 wt% of TOC. Chia Gara Formation in Duhok basin is organically rich showingTOC values ranging between 1.14-3.73 and 3.79-4.39 wt% in Atrush-1 and Mangesh-1 wells respectively. The high TOC content is mainly due to good preservation of organic matter in marine environment under reducing conditions. Similar TOC values within Chia Gara Formation have been recorded in Shorish-1well in the Erbil Governorate [18], but strata in the range of 5-20 % are known to occur in high yield petroleum system [19] which is the case in Shaikhan-8 well that shows values of TOC between 4.85-8.5 wt% (Fig. 3). Same high TOC content with values of more than 5% has been recorded in Hr-1, Bj-1 and Tk-3 wells in the Kirkuk and Tkrit Governorates in northern Iraq [20]. In Atrush-1 well, the values of S1, S2, and TOC are low compressive in contrast to the other sections, the lithological compositions were directly affected the organic matter richness of the formation, in Shaikhan-8 well it consists of alternation between very thin black shale and argillaceous limestone, whereas in Mangesh-1 well the formation is mainly composed of argillaceous limestone and little limestone, while in Atrush-1 well the lower part is argillaceous limestone and limestone increases towards the upper part of the formation (Fig. 3). In addition, the configuration of the basin can affect the distribution of the organic matter throughout the basin. It is thought that towards the center of the basin, the percentage of the TOC is possibly increases. This configuration of the basin may show that the organic productivity in the area around the Atrush-1 is low in comparison to the other two localities (The Atrush area might be away from the center of the basin in comparison to Shaikhan and Sarsang areas).

Figure 3

Total Organic Carbon (TOC wt%) of Chia Gara Formation in the studied wells

5.2 Quality of organic matter

The type of organic matters controls the type of the kerogen formed later, which in turn controls the type of hydrocarbon generated subsequently. This means some kerogens are only oil prone (type I), some are only gas prone (type III), and others are oil, gas-prone (type II) [21]. To determine the organic matter type of Chia Gara Formation S2 vs. TOC [22] and HI vs. T_max kerogen classification diagrams were manipulated [23]. In both diagrams, samples of Chia Gara Formation are plotted in type II kerogen field (Figs. 4 and 5). Type II kerogen is from algae, spores, pollen and steam cuticles of land plants. On the other hand, the Pr/n-C17 vs. Ph/n-C18 diagram shows that the samples derived from Chia Gara Formation are marine algal which represented by type II kerogen (Fig. 6). Regionally most of the recent literatures claimed this type of kerogen (type II) for Chia Gara Formation in different parts of Iraq, in Shorish-1 well Erbil province [18]; in Kirkuk-109 (K109) well Kirkuk province [24, 25], in outcrop at Banik area in Duhok province [26]. This type of kerogen can be type II-S kerogen [27], because the most of the Jurassic source rocks in Iraq are commonly comprised of carbonates rather than pure shaly sediments. As published in literature sulfur content in type II kerogen is generally higher than in type I or type III kerogens and may be particularly high in sulfur-rich rocks. Type II-S kerogens are derived from marine carbonate environments. The reason for this is that clay-poor carbonate source rocks contain insufficient iron to utilize the available sulfide,much of which becomes incorporated into the kerogen network. The C₂₉/C₃₀ hopane ratios of the studied samples are relatively more than one in most of the analyzed samples (except two samples in well Atrush-1) which can confirm that the kerogen type II-S (from a carbonate source rock).

Figure 4

TOC vs. S2 showing type II kerogen of Chia Gara Formation in the studied wells

Figure 5

T_max vs. HI showing type II kerogen and early mature-peak oil window of Chia Gara Formation in the studied wells

Figure 6

Phytane/n-C₁₈ vs. pristane/n-C₁₇ showing variation of organic matter type, thermal maturity and depositional environment of Chia Gara Formation in the studied wells

5.3 Depositional environment

The most widely used biomarker parameter for the assessment of redox (reducing and oxidizing) conditions during sediment accumulation is pristane/phytane ratio [28]. According to [8] Pr/Ph ratios substantially below unity can be taken as an indicator of petroleum origin and/or highly reducing depositional environments. Certain depositional environments and lithologies are associated with specific values of the Pr/Ph ratio [29]. Values less than 1 have been associated with marine carbonates, between 1 and 3 with marine shales, and larger than 3 with non-marine shales and coals [30]. In the current study the values of the Pr/Ph ratio are less than one (Table 2). This indicates that Chia Gara Formation was deposited under reducing environment and lithologically is associated with marine carbonate rocks [30]. The marine source of organic matter deposited under reducing conditions is also supported by the position of the samples in the plot of pristane/n-C₁₇ versus phytane/n-C₁₈ (Fig. 6). Although the low oxygen index (OI) values show that the formation was deposited under reducing conditions. Similar conclusions concerning the depositional environment of this formation have been recently reported by [20] in Hr-1, Bj-1 and Tk-3 wells northern Iraq. Regardless of thermal maturity, the values of carbon preference index (CPI) in all analyzed samples are around one and some of the values are less than this figure, this indicates marine and anoxic conditions. Samples of Chia Gara Formation are plotted in the open marine area on the C₂₇%, C₂₈%, and C₂₉% regular steranes ternary diagram especially in Atrush-1 and Shaikhan-8 wells with few slides of the samples in the Mangesh well (Fig. 7) [9]. Gammacerane index varies between 0.09-0.19, no hypersaline condition of deposition for the initial organic matter within the analyzed samples has been detected, it could be assessed that the formation is deposited in anoxic basin (Fig. 8) [31].

Figure 7

Ternary diagram of C₂₉%, C₂₈%, C₂₇% regular steranes showing marine depositional environment of Chia Gara Formation in the studied wells

Figure 8

Pristane/phytane (Pr/Ph) vs. Gammacerane Index (GI) showing anoxic condition of Chia Gara Formation in the studied wells

5.4 Thermal maturation

Maturity history of Chia Gara Formation is modeled by [32] in Hr-1 and Aj-12 wells, northern Iraq; the results indicate that the formation is thermally mature within the peak oil window. The thermal maturation level of Chia Gara Formation in the current study is deduced from the T_max values. T_max is the temperature at which the maximum amounts of S2 hydrocarbons will generate during rock-Eval pyrolysis. T_max depends on the type of kerogen that presents in the source rock. Values between 430-450 ^∘C indicate "oil window" condition (mature organic matter) [33]. In the studied oilfields, Chia Gara Formation is thermally mature source rock and ranges from early-mature to peak oil-generation window (T_max values ranges between 431-439 ^∘C) (Table 1, Fig. 5). In addition, Chia Gara Formation samples with T_max values up to 430 ^∘C have been detected

in outcrops in the northern Iraq [26, 34]. Also biomarker maturity parameters are used to determine the maturity level of the formation. The values of CPI in nine rock samples is close to one express mature sample [35]. Although, the concentration of the even carbon number is higher than odd carbon number, indicates that the organic matter in the samples of the formation is mature [36]. Isoprenoids/n-alkanes (Pr/n-C₁₇ and Ph/n-C₁₈) ratios provide valuable information on biodegradation, maturation and diagenetic conditions [8]. According to the values of the isoprenoid/ n-alkane, samples of the formation have not been affected by biodegradation process, and thermally they are mature (Fig. 6). This study determined the maturity by following the 20S/(20R +20S) C₂₉ and ββ/(ββ+αα) C₂₉ sterane values [37, 38]. According to these relationships, samples of Chia Gara Formation are located within the early mature to peak oil window (Fig. 9). The obtained data of the C₂₉ Moretane index (17β21α norhopane/17α21β norhopane) are in the range (0.05-0.12) (Table 2) indicating mature source rocks [12]. Increasing Ts relative to the Ts/ (Ts + Tm) hopane ratios together with the decrease in the C₂₉ Moretane index are shown in Figure 10. Equilibrium values for 20S/(20S+20R) C₂₉ sterane ratios are in the range of 0.32-0.48, whereas equilibrium values for ββ/(ββ+αα) C₂₉ sterane and 22S/(22S+ 22R) C₃₂ homohopane ratios are 0.55-0.61 and 0.58-0.61 [12, 31]. These values indicate that the analyzed samples have at least reached the early stage of thermal maturity and peak oil window (Fig. 11).

Figure 9

Cross plot of 20S/(20S+20R) vs. ββS/ (ββS+ ααR) C₂₉ steranes showing early mature- peak oil window of Chia Gara Formation in the studied wells

Figure 10

Cross plot of Ts/(Ts+Tm) vs. Moretane Index C₂₉ showing maturity of Chia Gara Formation in the studied wells

Figure 11

Cross plot of (20S/20S+20R) C₂₉ Sterane vs. 22S/(22S+22R) C₃₂ Hopane showing maturity and potentiality of Chai Gara Formation in the studied wells

5.5 Hydrocarbon generation potential

The most important factor controlling the generation of oil and gas is the hydrogen content of the organic matter (OM), Genetic potential represents the amount of petroleum (oil and gas) that kerogen able to generate [39]. If the value of genetic potential is more than six, it can be considered as a good source rocks for evaluation but if the value is less than two, it is a poor source rock and cannot be evaluated [40]. Generation potential of the studied samples derived from Chia Gara Formation is more than six (Table 1) and this is an indicator of very good to excellent generation potential; in addition to, samples with TOC content greater than 1% with hydrogen index more than 300mg HC/g TOC and containing a significant amount of type II kerogen can generate oil [41], these properties are present in all analyzed samples, indicates that the formation is suitable for commercial hydrocarbon generation. Moreover, rock-Eval T_max and the production index (PI) values indicate that most of the analysed samples are within the main stage of hydrocarbon generation (Fig. 12).

Figure 12

Plot of pyrolysis T_max vs. production index (PI), showing the maturation and nature of the hydrocarbon products of Chia Gara source rock in the studied wells

6 Conclusions

The hydrocarbon genetic potential of Chia Gara Formation seemed to be dependent on TOC wt% and maturity in the tested sections, accordingly, all sections have good-excellent potential to generate oil due to high enough maturity level reached and excellent marine organic matter content.

Acknowledgement

We would like to express our sincere thanks and deep gratitude to Dr. Kamal Odisho, Mr. Muhamad Perioui at Soran University and Dr. Polla Khanaqa, Mr. Diyar Abdulqadir from Sulaimaniyah Governorate for running Rock-Eval Pyrolysis analysis.

Special thank goes to Dr. Stephen Bowden from University of Aberdeen/ UK, and Dr. Ayad Nuri Faqi from Soran University for their help and analyzing the samples by GC/MS instrument.

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Received: 2018-04-04

Accepted: 2019-02-01

Published Online: 2019-03-22

This work is licensed under the Creative Commons Attribution 4.0 Public License.

Articles in the same Issue

https://doi.org/10.1515/geo-2019-0007

Keywords for this article

Chia Gara Formation, Anoxic condition, Biomarker assemblage; Type II kerogen; Mature organic matter; Oil-prone

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