Asian Review of Environmental and Earth Sciences ISSN: 2313-8173 Vol. 2, No. 1, 1-8, 2015 http://www.asianonlinejournals.com/index.php/AREES * Corresponding Author 1 Organic Petrological and Geochemical Evaluation of Jurassic Source Rocks from North Iraq Mohamed M. El-Kammar 1* --- Fuad S. Hussein 2 --- Govand H. Sherwani 3 1 Geology Department, Faculty of Science, Cairo University, Egypt 2 Geology Department, Salahaddin University, Iraq 3 Ministry of Natural Resources, Kurdistan Region, Iraq Abstract This work is licensed under a Creative Commons Attribution 3.0 License Asian Online Journal Publishing Group Contents 1. Introduction ................................................................................................................................................................................. 2 2. Stratigraphy ................................................................................................................................................................................. 2 3. Methodology ................................................................................................................................................................................ 3 4. Results and Discussions .............................................................................................................................................................. 3 5. Conclusions .................................................................................................................................................................................. 8 References ........................................................................................................................................................................................ 8 Immature Jurassic oil shale is widely distributed and frequently outcropping in North Iraq. The organic-rich Jurassic sedimentary sequence, including prolific oil shale, was recorded in Banik area in Duhok Governorate of North Iraq. This sequence was systematically sampled from the geological formations; Sehkanyian, Sargelu and Naokelekan. The organic geochemical parameters were analyzed for 72 samples as well as one oil sample. A detailed study of petrologic properties was carried out for 12 samples. Based on TOC content, the Sargelu and Naokelekan formations can be considered as good to excellent source rocks, whereas Sehkanyian Formation has no potential since the TOC does not exceed 0.1 %. The samples of Sargelu and Naokelekan formations contain both kerogen types I and II indicating marine organic matter mainly derived from algae and phytoplankton organisms proposing typical oil prone source kerogen. This is further confirmed by the predominance of alginite and liptodetrinite macerals, where liptinite maceral group contribute more than 90% relative to other maceral contents. In general, Sargelu Formation samples have Production Index (PI), Tmax and fluorescence parameters (λmax and red/green quotient) suggesting immature to early mature stage of thermal maturity. The calculated ratios of Pr/Ph, Pr/nC17 and Ph/nC18for the extracted bitumen and the oil sample, suggest generation of bitumen from marine organic matter deposited under reducing conditions at an early thermal maturity stage. Keywords: Organic petrography, Geochemistry, Source rock, Jurassic, Kerogen, Bitumen, Iraq. http://creativecommons.org/licenses/by/3.0/ Asian Review of Environmental and Earth Sciences, 2015, 2(1): 1-8 2 1. Introduction The Jurassic sedimentary sequence in northern Iraq contains very significant source rocks owing to their high total organic carbon (TOC) content, especially in both Sargelu and Naokelekan formations that were deposited throughout the Jurassic basin that exists in these areas [1-3]. As far as the authors are aware, the published data on these source rocks are insufficient and the available information concerning the TOC content, thermal maturation, and burial history require better understanding [4]. The dominant organic matter in oil shale is derived from one or more primary sources, such as terrestrial plants, lacustrine algae and marine organisms. These include large lake basins of tectonic origin, bogs, small lakes, lagoons and shallow seas [5]. Banik section is situated in the far north of the Iraqi territory, about 25 km to the NE of Zakho town, (Duhok Governorate). The studied section lies on latitude 37°13′ 33.4" N and longitude 42° 58′ 2.6" E, to the west of Banik village which is also known among resident farmers as Banik Haji Ghazi. Several mountains surround the area such as; Kokharash to the north, Khamtur to the west and Shaban to the south (Fig. 1). The present study deals with the hydrocarbon potential of the oil shale of the Sargelu and Naokelekan formations and their economic importance. 2. Stratigraphy The Jurassic rocks are commonly exposed as isolated patches at some eroded cores and limbs of anticlines in the highly folded, imbricate and thrust zones of Northern Iraq. The sedimentary section in Banik area lies in a rugged terrain where the Jurassic rocks crop out in the southern limb of Kokharash anticline (mountain). The section is located close to the west of Banik village near the open coal mine (dug in rocks of Naokelekan Formation). The general trend of strata is E-W. The stratigraphic succession starts with Sarki and Sehkanyian formations (Early Jurassic) which underlies Sargelu Formation (Middle Jurassic). The latter is overlain by the younger Naokelekan and Barsarin formations (Late Jurassic), with Chia Gara Formation topping them (Fig. 1). The lithologic composition of the Sargelu Formation consists of thinly bedded black bituminous limestone, dolomitic limestone and black papery shale intercalated by thin black streaks of chert in its upper part. In Banik section, the thickness of Sargelu Formation is about 30 meters of which the limestone rocks are mostly dolomitized. In the subsurface sections, at the foothill and Mesopotamian zone of the unstable shelf, the thickness is considerably higher and varies between 250 and 500 m Jassim and Buday [6]. Fig-1. Location map and the stratigraphic column of Jurassic rocks crop-out in the Banik area, North Iraq indicating the collected samples. According to Numan [7], the depositional history of the Sargelu Formation initiated and terminated as deep quiet marine environment, interrupted by some relatively shallow intervals but still remaining in the basinal realm. The organic–rich black shale and limestone of this Formation indicate anaerobic to dysaerobic conditions [8]. Such conditions are suitable for preservation of organic matter that gets thermally mature to generate hydrocarbon in subsequent geologic times. In some other subsurface sections, the behavior of the Sargelu Formation as potential source rock is provided by being seated at great depths [9]. The Naokelekan Formation consists of laminated bituminous limestone, alternated with bituminous shale (coal horizon) and thin bedded, highly bituminous dolomitie and limestone. Its age is assigned as Upper Oxfordian–Lower Kimmeridgian according to the study of ammonites of the Upper Jurassic [10]. The depositional environment of the formation was interpreted as euxinic in a slow subsiding basin [11]. The boundaries of the formation are agreed by Asian Review of Environmental and Earth Sciences, 2015, 2(1): 1-8 3 many researchers to be conformable and gradational with underlying Sargelu Formation and overlying Barsarin Formation. 3. Methodology Seventy two samples were collected to represent the targeted sedimentary interval in Banik area as well as one oil sample from Tawke oil field. The samples are collected along a traverse perpendicular to the bedding plane with spacing range of 0.5 to 1m. The collected samples represent the different lithologies of the Sargelu, Naokelekan and Sehkanyian formations. Analyses were conducted at Strato Chem Services© Company in Cairo, Egypt. The forty one samples that contain 0.7% TOC, were analyzed by Rock-Eval pyrolysis. The measured parameters include; S1 (mg HC/g rock), S2 (mg HC/g rock), S3 (mg CO2/g rock), Tmax (°C), and TOC (wt. %) are quoted in Table 1. Several additional parameters such as Hydrogen Index (HI = S2/TOC*100), Oxygen Index (OI = S3/TOC*100) and Production Index (PI = S1/(S1+S2)) are calculated (Table 1). Table-1. The ranges of the organic geochemical parameters of the Jurassic studied samples are summarized in the following table. Formation TOC S1 S2 S3 Tmax HI OI S1/ TOC PI Average Average Average Average Average Average Average Average Average (Min:Max) (Min:Max) (Min:Max) (Min:Max) (Min:Max) (Min:Max) (Min:Max) (Min:Max) (Min:Max) U. Sargelu 4.12 0.21 37.17 1.53 443 366 39 4 0.02 (0.06:19.52) (0.03:1.05) (0.8 : 127.16) (0.08:4.89) (437 : 449) (96 : 697) (3:165) (1:16) (0.00:0.06) M.Sargelu 2.4 0.24 24.92 1.26 443 317 29 9 0.04 (0.16:18.12) (0.04:0.43) (0.78:95.07) (0.20:3.96) (439:447) (103:525) (8:66) (2:24) (0.00:0.13) L. Sargelu 8.82 0.72 62.62 1.83 443 446 32 6 0.02 (0.12:28.57) (0.04:2.45) (1.49:192.89) (0.45:3.78) (441:446) (57:690) (6:92) (2:12) (0.00:0.03) Naokelekan 17.77 3.34 98.94 2.2 441 524 15 18 0.03 (5.39:25.55) (0.78:6.42) (18.05:149.1) (1.41:3.70) (434:445) (335:635) (8:32) (13:25) (0.02:0.04) Sehkanyian 0.06 (0.03:0.09) Gas chromatography (GC) and Gas chromatography-mass spectrometry (GC-MS) were carried out to identify different hydrocarbon compound classes and biomarkers of the rock extracts and oil sample. Organic petrologic examination of 12 samples was done, in the Kurdistan Institution for Strategically Studies and Scientific Researches, using DM 6000 Leica Microscope. Identification of the maceral types was carried out under reflected and blue light illumination. The latter is necessary to recognize the fluorescing liptinite macerals. The classification schemes proposed for the interpretation of organic materials by Teichmüller and Ottenjann [12], Teichmüller [13], Borchan and Powell [14], Scott [15] were adopted in the present study. The change in the intensities of the fluorescence spectra of liptinite group was measured during ultraviolet irradiation over a wavelength range from 400 to 700 nm. 4. Results and Discussions Pyrolysis techniques were used to establish the hydrocarbon source and the likely hydrocarbon products or source type. 4.1. Organic Matter Richness The TOC content of the study sediments can be considered as straight expression of kerogen and bitumen abundance. According to the classification given by Peters and Cassa [16], the upper part of the Sargelu Formation can be rated as having an excellent hydrocarbon potential (0.06% to 19.52%, averaging 4.12%) and the middle part as good hydrocarbon potential (0.16% to 18.12%, averaging 2.40%), whereas the lower part can be rated as an excellent hydrocarbon potential (0.12% to 28.57%, averaging 8.82%, table 1). The TOC contents of the Naokelekan Formation range from 5.39% to 25.55 %, averaging 17.77% and can be considered as an excellent source rock. The Sehkanyian Formation has no source potential. 4.2. Organic Matter Types It is necessary to recognize, that the quantitative aspects of kerogen evolution vary from one type to the other as a result of differences in the original composition of kerogen and that these generalizations may not reflect the true nature of the chemistry that occurs during the maturation process [17]. So, the types of organic matter must be distinguished and identified because different types of organic matter have different hydrocarbon generation potential and products [5, 18, 19]. The data of Rock-Eval Pyrolysis (Tables 1) indicate that Sargelu Formation plots generally in the field of kerogen type I and II (Fig. 2) which suggest common lacustrine depositional environment. The organic matter is mostly derived from marine algae and phytoplankton organisms which can be considered as a typical "oil source" kerogen Dahl, et al. [20]. Only one sample lies in the field of kerogen Type III which is derived mainly from terrestrial organic matter. Naokelekan Formation belongs to kerogen type I and is considered as excellent oil prone. The upper part of Sargelu Formation is characterized by localized intervals with capacity for both oil and mixed oil/gas generation. The oil-prone source rock intervals are characterized by very high TOC (5.37%-19.52%) and excellent potential to generate oil as indicated by their high Hydrogen Indices (pyrolysis S2 yields from 32 to 127mg HC/g Rock; and HI mostly >500 mg HC /g TOC). The mixed-prone source rock intervals are characterized by low to very high TOC (0.70%-8.13%) and fair to very good potential to generate oil and gas as indicated by their high Hydrogen Indices (pyrolysis S2 yields 2.60-19.03.16 mg HC/g Rock; and HI mostly >200 mg HC /g TOC). Asian Review of Environmental and Earth Sciences, 2015, 2(1): 1-8 4 Fig-2. Van Krevelen diagram, showing Rock-Eval Hydrogen Index vs. Oxygen Index for the analyzed samples of the study area (adapted from Espitalié, et al. [21]) The middle part of the Sargelu Formation is characterized by having capacity for oil, mixed oil/gas and gas generation. The oil generating rocks are characterized by very high TOC and very good to excellent potential to generate oil as indicated by their high Hydrogen Indices (pyrolysis S2 yields from 17 to 95mg HC/g rock; HI 365- 525 mg HC/g TOC). The mixed oil/gas generating rocks are characterized by high TOC (1.71%) and fair potential to generate oil and gas as indicated by their high Hydrogen Indices (pyrolysis S2 yields 4.15 mg HC/g rock; HI 242 mg HC/g TOC). In addition the gas generating rocks are characterized by relatively high TOC (1.81%) and fair potential to generate oil and gas as indicated by their high Hydrogen Indices (pyrolysis S2 yields 2.95 mg HC/g rock; HI 163 mg HC /g TOC). Lower part of Sargelu Formation is characterized by having capacity for oil, mixed oil/gas and gas generation. The oil-prone source rocks are characterized by their very high TOC (2.34-28.57%) and very good to excellent potential for oil generation as indicated by their high Hydrogen Indices (pyrolysis S2 yields 14.75-192.89 mg HC/g rock; HI >400 mg HC /g TOC). The mixed oil/gas prone source rocks are characterized by very high TOC (4.29%) and good potential to generate oil and gas as indicated by their high Hydrogen Indices (pyrolysis S2 yields 8.83 mg HC/g rock; HI 206 mg HC /g TOC). In addition the gas-prone source rocks are characterized by very high TOC (3.28%) and fair potential to generate oil and gas as indicated by their high HI (pyrolysis S2 yields 3.50 mg HC/g Rock; HI 107 mg HC /g TOC). Naokelekan Formation is characterized by having capacity for both oil and gas generation. The oil-prone source rocks are characterized by very high TOC (5.39-25.55%) and very good to excellent potential to generate oil as indicated by their high Hydrogen Indices (pyrolysis S2 yields 18.05-149.14 mg HC/g rock; HI >500 mg HC /g TOC, Fig. 3). The plotting of S1 versus TOC can differentiate between the migrated and non-migrated hydrocarbons. The dividing line on the plot is where S1/TOC = 1.5. Values belonging to non-indigenous hydrocarbons plot above this line while indigenous hydrocarbon values emerge below it Hunt [22]. Thus all the samples analyzed of Sargelu and Naokelekan formations indicate indigenous hydrocarbons (Fig. 4). Fig-3. TOC wt% versus S2 plot of Sargelu and Naokelekan formations indicates their hydrocarbon potentialities. Asian Review of Environmental and Earth Sciences, 2015, 2(1): 1-8 5 Fig-4. SI versus TOC for Sargelu and Naokelekan formations in Banik section (modified after Hunt [22]) 4.3. Thermal Maturity The maturity levels for the oil window depend on the type of organic matter [23], and encompass a vitrinite reflectance (Ro) ranges from 0.5 to 1.3% and temperature at maximum rate of hydrocarbon generation during S2 evolution (Tmax) from 435 to 470°C. The PI parameter is another measure of maturity, with values ranging from 0.15 to 0.4 normally associated with oil generation. HI versus Tmax is commonly used to avoid influence of the OI for determining kerogen type [22]. Determining kerogen type using HI versus Tmax appears to be more accurate than OI versus HI. The difference between the results is expected because the Sargelu and Naokelekan formations are dominated by carbonates which affect the OI. The cross plot of HI versus Tmax indicates dominance of kerogen typesI, II and mixed type II – III in both Sargelu and Naokelekan formations (Fig. 5). The samples are thermally immature to early stage of maturation, where pyrolysis Tmax ranges from 437 - 449°C and PI is less than 0.15 [24]). Fig-5. Kerogon plots for HI vs. Tmax of the samples of Sargelu and Naokelekan formations of the Banik section. 4.4. Rock Extracts and Oil Gas-chromatography analysis was performed for the extract of 6 rock samples in addition to one oil sample (Table 2). Asian Review of Environmental and Earth Sciences, 2015, 2(1): 1-8 6 Table-2. Gas Chromatograph data of the 6 extract samples of Sargelu Formation as well as the oil sample S. No. Pri/Phy Pri/nC17 Phy/nC18 CPI Marzi 4 N Paraffin Isoprenoids Resolved unknowns Ext. 1 1.14 0.26 0.37 0.89 11.9 1.5 86.6 Ext. 12 1.33 0.39 0.39 0.91 12.7 2.0 85.3 Ext. 18 1.15 0.28 0.25 0.97 23.6 2.2 74.2 Ext. 21 0.91 0.24 0.29 0.87 25.3 2.9 71.7 Ext. 26 1.38 0.23 0.20 0.78 20.9 3.1 76.0 Ext. 52 0.70 0.28 0.44 0.95 23.5 3.6 72.9 oil 0.73 0.26 0.44 0.98 30.8 3.8 56.5 The gas chromatograms of the extracts show a rather unimodal distribution pattern of the normal alkane with maximum intensity light ones (