Clay Minerals and Organic Matter from Deeply Buried Ordovician- Silurian Shale in Western Iraq: Implications for Maturity and Hydrocarbon Generation

The present work is conducted on the Paleozoic (Ordovician) Khabour and the (Silurian) Akkas shales in the Akkas-1 well of western Iraq. The study is aiming to determine the implications of clay mineral transformation, organic mineral distribution and maturity of hydrocarbon generation, using X-ray diffraction (XRD), scanning electron microscopy (SEM) in addition to organic matter concentrations. In the shale of the Khabour Formation, amorphous organic matter is common and includes various Tasmanite-type organic matter, vitrinite, inertinite, and bituminite. The main clay minerals observed include illite, chlorite, kaolinite, in addition to mixed-layer illite-smectite and rare smectite. In Silurian shale, high content of organic matter is recorded in addition to abundant vitrinite and low content of grainy organic matter (Tasmanites) and pyrite. Illite and kaolinite are commonly found in addition to chlorite and illite-smectite clay minerals. Conversion of smectite to mixed-layer illite-smectite (I-S) and an increase in vitrinite reflectance are commonly observed below 2500 m depth in the studied formations, which coincides with oil and gas generation. These results could be used as an indication of higher maturity and hydrocarbon generation in the deeply buried shale of the Khabour and Akkas formations in western Iraq.


Introduction
Clay minerals and organic matter usually coexist in clastic rocks and are used as a tool to identify potential hydrocarbon generation and expulsion due to their high susceptibility to temperature changes that control related mineral conversions and organic maturity [1][2][3][4][5]. Clay minerals and organic matter also have significant influences on the origin, preservation, and production of shale gas due to the substantial role of their nanoscale pores in the generation, storage, and seepage of shale gas [6].
To assess the thermal maturity of sedimentary rocks, several paleothermometers are used as thermal maturity indicators to reconstruct palaeotemperature histories; a technique that is widely applied in petroleum exploration [3,7,8]. Frequently used parameters include vitrinite reflectance (%Ro), maximum pyrolytic temperature (Tmax), fluid inclusion data, conodont alteration index, level of thermal maturity of organic matter (OM), and clay mineral transformation ratios [3,[9][10][11]. Coexisting clay minerals and organic matter in the reservoir sandstones and shale source rocks are sensitive to temperature changes that accompany hydrocarbon generation and expulsion processes [4][5].
After deposition of the source rocks in a sedimentary basin, oil and gas are generated and the rock is termed mature at a temperature of about 60°C and depth of 1.5 km [12]. The type, richness, and thermal maturity of these source rocks can be evaluated using the mineralogical and geochemical parameters as mentioned above. For example, the degree of ordering of illite-smectite (I-S) clay minerals is useful for studying hydrocarbon generation. A coincidence is commonly observed between the temperature for the conversion from random to ordered I-S and the temperature for the onset of peak oil generation [4,[13][14][15][16][17].
The Akkas-1 well in western Iraq (Figure-1) contains the most complete Ordovician and Silurian succession in Iraq and was designated as the subsurface reference section for the Ordovician Khabour and Silurian Akkas Formations, comprising an interbedded succession of sandstones and shale between 1463 m and the total depth of 4238 m [18][19]. Based on their economic and depositional significance, the shales in the Khabour and Akkas Formations were described in numerous papers, theses and reports [18][19][20][21][22][23][24][25]. Most of these works focus on the stratigraphy, depositional evolution, palynostratigraphy, organic geochemistry, hydrocarbon potential and exploration of Paleozoic prospects in western Iraq. Nevertheless, the relationship between clay mineral transformation and organic matter in hydrocarbon generation was not previously investigated.
Concerning source rock potential of the Paleozoic source rocks in Iraq, Aqrawi et al. (2010) [26] revealed that the lower Ordovician shales (assigned to Member 7 of the Khabour Formation, according to Al-Hadidy, 2007, [18]) and the lower Silurian ("hot" shale of the Akkas Formation) could be regarded as principal hydrocarbon source rocks in the Paleozoic sequence of western Iraq, similar to their role across the region in Jordan Syria, Libya, and Saudi Arabia [27,23,25]. The lower "hot" shale unit of the Silurian Akkas Formation, western Iraq, has an average-good source rocks (TOC 1.2-5.25 wt%, mean 2.2 wt%; S2 1.2-8.7 kg/tonne, mean 4.2 kg/tonne) [28]. Whereas, TOC values are 0.17-1.42wt% for the 2750-3000m interval and 0.5-1wt% for the 3570-3650m interval in the Khabour Shale of Akkas-1 well, as suggested by Al-Ameri (2010) [23]. Shales in the Khabour Formation were also identified as the source rocks for the gas found in sandstone reservoirs in the K1 to K4 members [23].

Al-Juboury et al.
Iraqi Journal of Science, 2020, Vol. 61, No. 11, pp: 3017-3034 3019 The present work correlates the transformation of clay minerals and changes in organic matter maturity in the shales of the Ordovician Khabour and Silurian Akkas Formations in western Iraq, aiming to determine their implications for organic matter maturity and hydrocarbon generation.  Buday and Jassim (1987) [35] showing the location of the Akkas-1 and Khleisia-1 wells. (B) Inset map showing countries neighboring Iraq.

Stratigraphy and Paleogeography
During the Palaeozoic, the Arabian Plate formed a part of the long and wide northern passive margin of Gondwana, bordering the Paleo-Tethys Ocean [29][30][31]. Iraq occupies the northeastern part of the Arabian Plate, which lies in higher southern latitudes with dominant clastic sedimentation [32][33][34], The stratigraphy of Iraq has been strongly affected by the structural position of the country within the main geostructural units of the Middle East region, since Iraq lies on the border between the Arabianpart of the African (Nubian-Arabian) shield and the Asian branches of the Alpine tectonic belt.
Western Iraq formed a part of the stable shelf of the Nubio-Arabian shield. Clastic deposition of alternating sandstones and shales was dominant in Iraq during the Early Paleozoic (Ordovician and Silurian [36]). These clastic units were deposited in shallow-marine epeiric seas over large areas of the Arabian Platform [37][38][39][40]19]. The areal extent of the seas changed in response to eustatic controls as the Paleozoic era advanced [30,41]. These epicontinental shallow epeiric seas regressed and transgressed over vast areas throughout the Paleozoic, resulting in variable bed thicknesses and lithotype associations [42][43]. In western Iraq, the thickness of the Paleozoic successions ranges between 3-4 km. North-south trending graben structures prevailed in some areas during the Infracambrian and Paleozoic, which resulted in thicker deposition in these grabens. These grabens indicate an extensional tectonic regime, but they were all aborted and never developed into fullfledged rifts [42].
In the Akkas-1 well Figures-(1 and 2), the alternating marine shales and sandstones in the Ordovician Khabour and the Silurian Akkas Formations are more than 2775 m thick [22][23]. There is large petroleum potential in these Paleozoic strata, where a series of shale source rocks have been identified [44,[18][19]. The oldest shale in the lower part of the Ordovician Khabour Formation comprises about 600 m of black fissile shale, while other shale units interbedded with sandstones are present higher in the Ordovician succession. All these shales are highly mature and organic matter-rich and are considered to be suitable source rocks for hydrocarbons.
The overlying Silurian Akkas Formation contains two important high gamma-radiation (hot) shales ( Figure-2). The basal hot shale is 39 m thick while the second unit (19 m) occurs about 60 m above it in the Akkas-1 well [20,22,45]. Silurian hot shales are believed to be the main Palaeozoic source rocks in the western and southwestern deserts of Iraq [27]. These hot shales Note that Ordovician Khabour Formation is divided into seven members (K1-K7) based on biostratigraphic data, whereas Akkas Formation is divided into two member namely (Hoseiba and Qaim) based on the gamma-ray log and organic matter content [18]. are black, fissile, calcareous, bituminous, pyrite-spotted throughout, and organic-rich with TOC values ranging between 0.96% and 16.6% in Akkas-1, and 0.94% and 9.94% in Khleisia-1 [27]. The Silurian hot shales could be over-maturing in the deeper areas of southwestern desert of Iraq, whereas in other shallower western areas they might be immature. The difference in maturation distribution between the deeper and shallower Silurian hot shales was complicated by an intense "Hercynian-age" horstgraben tectonic phase [22].

Materials and methods
Twenty one core samples of shale ( Figure-2) were studied to identify the main mineralogical composition using X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) techniques accompanied by backscattered electron imaging. A randomly oriented X-ray powder diffractogram was obtained using Spellman DF3 diffractometer. Quantitative determination was performed using the SIROQUANT V3 program developed by the CSIRO (Commonwealth Scientific and Industrial Research Organization), Australia. A JEOL JSM 6460A scanning electron microscope equipped with a backscattered electron detector and energy-dispersive system was used in the morphological and geochemical analysis of selected minerals. For organic matter typing of polished blocks of shale samples, the procedures of Falcon and Snyman (1986) [46] were used. All these analyses were carried out in the laboratories at the University of Wollongong, Australia.

Clay mineralogy
Illite, mixed-layer illite-smectite, kaolinite, and chlorite are the main clay minerals indicated by XRD analysis in the studied Ordovician shale (Table-1 Commonly in shale buried below this depth, the randomly interstratified illite-smectite changed to a regular or welldeveloped orderly interstratified (OI) form [47].
In deeper buried shales in the well, mica is commonly observed and these shales become more fissile with common irregular pyrite patches [19]. The enrichment with mica and the common presence of illite plates and thick hexagonal platy kaolinite (dickite) in these deeper shales is clearly observed (Figures-3 F and 5), while disc-shaped chlorite together with hexagonal kaolinite (Figure-5  B) is also present as pore fillings. This gradual conversion of smectite into mixed-layer I-S and mica with increasing burial depth and increasing temperature is common in shales deposited in subsiding basins [48]. This reaction starts with randomly distributed I-S layers that become gradually ordered and accompanied with chlorite, dickite, and the dissolution of potassium feldspars as depth increases [49,13]. This conversion was clearly seen within the studied Khabour shale.
In the overlying Silurian Akkas Formation shale, kaolinite is commonly present as hexagonal plates, some of which are degraded ( Figure-4B-C). Illite is commonly present as fibres and fine white flakes. There is good evidence that the fibrous illite grew from precursor kaolinite (illitization of kaolinite, see Figure-4B); this characteristically occurs during burial diagenesis. Quartz overgrowth and carbonates are also present.   Organic matter According to the Maturation Range Chart (Cook 1982) [50], the color and type of the preserved organic matter in the studied shales may be referred to as mature-type organic matter in the deeply buried Ordovician Khabour shale. Amorphous organic matter is common, in addition to the various types of vitrinite, inertinite, and the algal Tasmanites that were commonly recorded. These organic matter constituents show no fluorescence. In the basal shale of the Khabour Formation at a depth of more than 3500 m, bituminite, amorphous bituminite and many other types of organic matter are recorded. Pyrite and other mineralized matter are also observed. The groundmass is commonly brownblack Figures-(6 and 7).
In Silurian shale, many types of amorphous organic matter are recorded in addition to abundant vitrinite and pyrite as well as large fragments of vitrinite and some very grainy organic matter (Tasmanites) (Figure-7 F). According to the Maturation Range Chart of Cook (1982) [50], the color and type of the preserved organic matter in these shales may also be referred to as mature-type organic matter, similar to those of the more deeply buried Khabour shale.

Discussion
The Khabour Formation shales (Ordovician) are highly-mature, marine, organic-rich rocks with total organic carbon content (TOC) values of 0.9-5% by weight in the Akkas-1 well in western Iraq [20]. Whereas shales in the Silurian Akkas Formation are black, fissile, calcareous, bituminous, pyritespotted, and organic-rich with TOC values ranging between 1.0 -16.6% in the Akkas-1 well [20]. The "hot" shale units (Figure-2) are likely to form the principal hydrocarbon source rock in the Palaeozoic sequence of western Iraq and were deposited mostly under anoxic conditions [22,23,25,28].
Al-Haba et al. (1994) [20] suggested that hydrocarbons generated in Akkas field were derived from the Khabour shale source rocks while overlying shales in the formation could act as cap rocks for the older Khabour sandstone reservoirs. These uppermost Khabour sandstones are composed of silty quartz, as well as mica, pyrite, and glauconite [20].
The Paleozoic Khabour and Akkas Formations were continuously deposited over a wide geographic area in shallow subsiding epicontinental or epeiric seas in a homoclinal ramp setting [19].
In the present study, the clay mineral illite-smectite transformation in addition to organic matter content reveal that shale units in the Ordovician Khabour and Silurian Akkas Formations can be recognized as an organically mature oil shale, especially in the more deeply buried basinal areas. According to vitrinite reflectance data, the paleogeothermal gradient at the time of maximum burial in the study area was 55 ºC/km, giving an estimated maximum burial temperature of approximately 80-120 ºC in the Khabour Formation [47]. Majidee (1999) [45] used a quantitative technique to calculate the anomalous temperature gradient and heat generation across the "hot" shale at 2,208-2,327 m of Akkas-1 well. He attributed the higher temperature gradient (6.1˚C/100 m) to two factors: (1) the high uranium content in the "hot" shale of up to 33.8 ppm [28] which increases the rate of radiogenic heat generation to about 5.0 microWatt per cubic meter; and (2) the dominance of shale in the Akkas Formation which reduces the conductivity and increases the thermal gradient.
Commonly, temperature increases after deposition in subsiding basins coincide with the onset of the transformation of smectite to illite [51]. The gradual conversion of smectite into mixed-layer illitesmectite (I-S) with increasing burial depth and increasing temperature is common in shales deposited in subsiding basins [48]. This conversion is coincident with the generation of oil in sedimentary basins, as seen in the topmost oil bearing horizons in the US Gulf Coast Tertiary succession [48]. Fine-grained smectite layers may initially protect organic matter from oxidation and then catalyze its transformation into petroleum. Water released by the fixation of interlayer potassium may aid in flushing hydrocarbons out of source rocks and into reservoirs, and pore space resulting from the collapse of smectite layers could provide pathways through source rocks for petroleum migration [4,48,52].
This reaction first produces random distributions of I-S layers that gradually become ordered and accompanied by the authigenic development of chlorite and dickite, along with the dissolution of potassium feldspar [49,13]. This conversion and its mineral associations are seen in the Khabour Formation Figures- (3)(4)(5) and were also noted from intercalated sandstones in the Silurian Akkas Formation [53].
Source rocks with organic matter from higher plants have a hydrocarbon compound known as the vitrinite maceral. Various types of vitrinite, inertinite and bituminite, in addition to algae of Tasmanite-type, are recognized in the Khabour Formation Figures-(6 and 7). Vitrinite reflectance has been widely used to determine the maturity of hydrocarbon source rocks. Vitrinite reflectance percentages (%Ro) in the Khabour and lower Akkas Formations (Figure-8) commonly exceed the value of 1, which indicates a high level of organic maturity in the rocks [12,54].
According to the hydrocarbon generation potential scheme proposed by Al-Ameri (2010) [23] (Figure-8), it is evident that some levels within the Khabour Formation in Akkas-1 well generated condensates and wet and dry gas with kerogen type B. In the Akkas Formation, mixed kerogen types A and B and oil generation were shown by Al-Ameri (2010) [23]. The degradation of the organic matter down to a depth of 3000 m may result in abundant amorphous organic matter, including 70-75% amorphogen.
In the present study, algal-type Tasmanites and traces of vitrinite are common at depths below 3000 m in Akkas-1 Figures-(6 and 7). The marine algal phytoplankton Tasmanites is an important type of organic matter in the Ordovician to Devonian black shales in the Appalachian basin. They correspond to type I kerogens that appear to be derived from extensive bacterial reworking of lipid-rich algal debris [55]. The common amorphous and laminated organic matter at shallower depths may be the type II kerogen of Peters and Moldowan (1993) [55] which corresponds to the wet gas and condensates recorded by Thompson and Dembicki (1986) [56].
Clay mineral distribution (Figure-8) shows that the studied shales include illite, chlorite, kaoloinite, and illite-smectite mixed layers. The main change in clay mineralogy with depth is an increase of illite, dickite (kaolinite), and mica downwards.
Al-Juboury (1999) [47] showed that the change from random to regular mixed-layered illitesmectite due to burial diagenesis occurs at a depth of about 2400 m, at which vitrinite reflectance is about 0.6%. A specific relationship of vitrinite reflectance to I-S expandability and illite crystallinity is observed and is probably controlled by thermal history. This relationship is in accordance with the depletion in kaolinite content and disappearance of K-feldspar leading to an overall increase in amounts of illite and illite-smecite. It gives an indication of the exhaustion of K-feldspar as a source for the formation of I-S mixed-layer clays.
The depth of 2400 m also represents the contact between the shallower Silurian inner neritic environment and the deeper outer neritic Ordovician sequences [19,23]. These Paleozoic formations were rapidly deposited in grabens with elevated geothermal gradients, so that the diagenetic processes were controlled by thermal history (tempreture+time of residence) in these tectonic-environmentalsedimentologic settings.
The conversion of smectite into mixed-layer illite-smectite (I-S), and subsequently from random to ordered I-S, associated with increasing vitrinite reflectance, is commonly observed below 2500 m depth in Khabour Formation in Akkas-1 well (Figure-8). From a regional perspective, the current study deals with the relation between clay minerals distribution and diagenesis with organic matter typing in two promising formations including shalegas units [57]. This study can contribute to the regionally trending studies on unconventional shale gas around the world in North America, Latin America, Middle East, North Africa, Central Asia, Russia, and China [4,58,59], with the fact that clay-organic fabric controls the clay mineral effects on hydrocarbon generation. Figure 8-Hydrocarbon generation potential chart predicted from stratigraphic variations in kerogen types, paleoenvironments, maturation assessments, and total organic carbon (TOC) in the Akkas and Khabour Formations from Akkas-1 well (after Bujak et al. 1977, [60]). OM = organic matter, CM = clay minerals, TAI = thermal alteration index, VRo = vitrinite reflectance value (after Al-Ameri 2010, [23]). The present study data (right two columns) includes the organic matter types and distribution of clay mineral in the buried succession of the Akkas-1 well.

Conclusions
The Ordovician Khabour and Silurian Akkas shales are important potential source rocks in western Iraq. The recorded conversion of smectite into mixed-layer I-S and, subsequently, from random to ordered I-S with changes in illite-smectite crystallinity, associated with increasing vitrinite reflectance in the Akkas-1 well, are commonly associated with oil and gas generation (kerogen and dry gas). In the studied shales, these changes are associated with algal-type Tasmanites, various vitrinite/vitrinitelike macerals, and kerogen. Hence, the results from this study could be used as an indication of higher maturity and hydrocarbon generation in the deeply buried rocks of the Khabour and Akkas Formations in western Iraq. These results could also have a regional perspective, since they deal with the relation between clay minerals distribution and diagenesis with organic matter typing and can contribute to the regionally trending studies on unconventional shale gas around the world.