The Lower Silurian (Hot Shale) Source Rocks of Western Iraq: Depositional Model and Stratigraphic Distribution

Abstract


1.Introduction
The Akkas-1 exploration deep well penetrating the Paleozoic near Iraqi border close to with Syria (Fig. 1), was drilled in 1993, whereas, the Khleisia-1 well was drilled in1959and Qaim-1 well at the end of 2002 (Gaddo and Parker, 1959;FWR final well reports, 1959FWR final well reports, , 2002)).The Akkas field is located on the Rutba Arch in the Western Desert of Iraq.Hot Shale existed in two beds as the main source rocks to the lower Paleozoic as in Akkas-1 well (Fig. 2), with light and sweet oil as tested in deep reservoir sandstone Ordovician Khabour Formation in the exploration of three wells.The hot shale does not exist as surface exposure but only in subsurface exploration wells (Van Bellen et al., 1959).Al-Hadidy (2007) introduces the names for the Hoseiba and Qaim Members in the Akkas Formation.The upper and lower rock unit of Hot Shale have high uranium content with high radioactive nature in hot shale (up to 380 API GR (within two hot shales) (Buday and Jassim, 1987;Jassim and Goff, 2006;Al-Husseini,1990, 1991).The palaeogeographic reconstruction of Iraq as a part of the Arabian Plate was in the south pole (Vaslet, 1990) (Fig. 3.) The previous studies on the hot shales have focused on their distribution (Klemme and Ulmishek, 1991), subdivision of the Akkas Formation into two horizons "upper and lower Units" (Hot shale) (OEC, 1994), Their maturation as source (Aqrawi, 1998;lüning 2000, 2000a;Al-Sharhan and Christopher, 2021).The study aims to describe hot shale, in terms of their facies distribution, thickness, and predictive depositional model correlated with the Arabian plate in addition to focusing on the hydrocarbon potential of the hot shale basin and sector in Iraq.(Vaslet, 1990)

Geological Setting
The Akkas Formation was named by Al-Juboury et al. (1997), and correlated with neighboring countries (Fig. 4).The hot shale beds are two organic-rich, black fissile, pyritic with graptolites with high-gamma uranium radiation (Figs. 2 and 5).The upper hot shale, in Akkas-1 is 24 m (68 ft) thick, and the lower is 41 m (134 ft) thick.two genetically related glaciation-deglaciation depositional cycles that occurred following the Mid-Ordovician unconformity.This unconformity also coincides with the first glaciation event in Gondwana that covered the Arabian plate in North Africa and South America, (Fig. 3).The typical basal Silurian hot shale North Gondwana is dated as early Liandovery (Lüning et al. 2000) and is too have been deposited during a large-scale anoxic event associated with the initial phase of the latest Ordovician-early Silurian transgression (Lüning et al. 2000a and2005).Hot Shale in the Akkas Formation (Hoseiba Member) is termed so because the high gamma may exceed that of 300+API radiation content (Fig. 3).The quality and quantity of organic matter in hot shale control natural gas potential, as the Silurian two hot shale thickness (65, 64, 20) m.In the Akkas-1, Qaim -1and Khiesia-1 wells respectively which are reduced in well Kh-1 to north direction, (Table -1) (Figs. 3,5 and  6).The Akkas Formation's maximum thickness in Akk-1 is (864) m. reduced to North Iraq (152) m. in well Kh-1 (Table-1).

Materials and Methods
The study is based on • Lithofacies core and cutting from the Akkas-1 well.
• The logs ray -gamma data and analysis of TOC (Total Organic Carbon).• Association of facies and distribution and hot shale depositional model.

Stratigraphy of Hot Shale
Stratigraphically in the well Ak-1, separate intervals one in the lower beds Rhuddanian (lower Llandovery) ascensus-acuminatus graptolite Biozone and the other at upper beds, in the middle Rhuddanian estimated.(OEC, 1994).As equivalents of Hoseba Member.The basal Qusaiba Member in central and south Arabia accumulated in a trough that subsided from Late Ordovician through Early Devonian time.Abundant graptolites in the Qusaiba Member indicate an Early to Middle Llandovery age for the lower Qusaiba and Uranium content is high in these weathered levels, the stratigraphical extent of the "hot shale" interval into the middle Rhuddanian (Mc Gillivary, 1999) (Fig. 4).The underlying hot shale sharp contact sandstone reservoir (Ordovician Ashgillan of the Khabour Formation which taken by high gamma ray of hot shale (source rocks and cap rocks) above Quartz arenite reservoir facies.Lower Paleozoic reconstructions, Silurian organic-rich shale hot shale is passive margin of Gondwana and Pangea (Mc Gillivary, 1999).

Silurian Subsidence
Silurian basin configuration affected lateral variation in thickness in three studied wells.The Global climate change glaciation on the Arabian plate margin of Gondwana South Pole drifted (Heron and Craig, 2004).Silurian source rocks are distributed from North, West, and South-West Iraq as parts of the Arabian plate which effect by paleo-Tethyan tectonic.Major transgression was rapid during the early Llandovery then phase sea level regression (Al-Husseini, 1991), Silurian Hot shale is primarily the main Paleozoic source rock (Al-Haba, 1991).Silurian basin in Iraq is an epeiric basin located on the Arabia large Paleozoic platform (Al-Laboun, 1986) and its setting is as intracratonic basins (Abd Al-Wahab, 2013).Hail -Arch elements affect the thickness and distribution of Hot shale due to phases of Tacnian movements, to create a sub-basin (Figs. 8, 9, and 10).

Ora sub-basin, north Iraq
This sub-basin location in extreme Northern Iraq thrust zone (Ora Village).During this time three episodes of tectonism are demonstrated by vertical deformation associated with volcanic rock intrusions (Chalki volcanic with pirispeki Formations).This orogeny caused, uplift and erosion during the Toconian phase (Caledonian orogenic events) with Chalki volcanic and named after the Chalki Village (Van Bellen et al. 1959).This event is associated with the uplift of the region and directly occurs upon the Caledonian unconformity.All Silurain (Hot Shale) is absent in Ora North Iraq (Fig. 10) as Unconformity Pirispiki formation local volcanic impact effect on Silurain Akkas formation deposition in Ora Basin.

Al-Jeizra sub-basin, NW Iraq
This sub-basin is located nearly to trend NW in Al-Jezira zone as Silurian thining as depocenter to south Akkas Sub-basin.The thickness of the Akkas formation reaches about 1000 m. a maximum in Adepo center in the Akkas sub-basin as the Silurian section (Jordan) thickness reduced towards northern Iraq, Khlesia well (Gaddo and Parker, 1959) is located in the northern part of Al-jezira subzone (Buday and Jassim, 1987) within this sub-basin (Fig. 10).25

Akkas sub-basin, West Iraq
The secondary basin is a very good potential source rock that extends from the Jordanian border to the Al-Rutba Highland and the Akkas field (Fig. 10), due to the effect of the rise of the Hail-Rutbah arc its extension to the Risha Basin (Jordan) in western Iraq, and thus led to a difference in the hydrocarbon efficiency of the Akkas and Qaim fields.The study area and the emergence of basins of the Horst & Graban type in the West desert and the reasons for these differences is the effect of the structural elements in different phases in the Paleozoic.

Widyan-Al-Nakhib sub-basin
The main body of Paleozoic rocks occurs within this basin.The Widyan Basin lies on Ha'il-Rutbah Arch effect this basin (Al-Laboun, 1986) and continued correlation of early Silurian deep water (graptolitic) to southwest Iraq during early Paleozoic of is separated from the (Tabuk Basin) Hail -Rutba Arc (Loydell et al., 2009) (Fig. 10).

Hot shale hydrocarbon potential
The play of the future in Iraq and the world's reserves are generated by The hot shale (Silurian) which is one of the six intervals of 9% of source rocks (Klemme and Ulmishek, 1991), (Rahman, et al., 2022).The fundamental to any petroleum system are source rocks.Hot Shale source rocks and hydrocarbon generation and the amount of organic matter study can generate and release enough hydrocarbons to form a commercial accumulation of oil or gas.Hot shale two layers (Upper and lower) thickness variation in Akkas-1 (TOC of 1.0-16.6%)Qaim-1 and Khleisia-1 (65, 64, and 30) respectively reduced to northern Iraq.
Two kerogen black hot shale units have been studied in detail across the west.NW Iraq by means of good logs, and facies, is crucial for depositional models and more sophisticated evaluations of the regional source rock potential.a hydrocarbon yield of 49 kg (kilograms) per ton of rock, one of the highest yields in the world (Al-Gailani, 1996).The average total organic carbon content (TOC) is about 6% in the Akkas hot shales and ranges from 0.96% to 16.2%.According to Majidee, (1999) the Akkas hot shale in Akkas-1 is in the oil generation window and has a thermal maturation index (TTI) that ranges from 75 to 93.Silurian hot shales contain 1-9.94% TOC in well Kh-1 and 0.95-16.62%TOC in well Akk-1 (Al-Haba, 1991 and1994), kerogen extracted from these shales reveals a marine origin highest total organic carbon.However, the distribution of Silurian source rocks was modified by erosion at the intra-Devonian and Upper Carboniferous (Hercynian) unconformities (Aoudeh and Al-Hajri, 1995).All scenario Hot shale exploration of gas and light oil (Younger age) downwards first migration to reservoir older sandstone (Ordovician Khabour formation as styling Block Horst and Graben block faulting in the west desert. In Iraq, along the eastern flank of the Akkas field, Al-Habba and Abdullah (1989) concluded that hydrocarbons migrated downward from the Lower Silurian source into the Ordovician upper Khabour fractured sandstone reservoir.

Discussion and Conclusions
The lower Paleozoic limited data of the study through mainly well Akkas-1 and (correlated with wells (Qa-1 and Kh-1) in Iraq also with Arabian plate hot shale controlled by (thickness and type) get that the in the west and south extended of widyian basin in Iraq promising zone as the potential for gas reservoirs.These distributions as intra-cratonic Silurian basin is subdivided into subbasin: • Akkas and Widyan-Al-Nakhab subbasins is excellent and future production gas in Iraq's lower Silurian 'Hot Shales' are thick (quantity) and quality (Fig. 11).• Al-Jezera subbasin is moderately potential as Hot shale wedged out in Kh-1 well and absent in Ora Northern Iraq.• Hot Shale, Hoseiba member lower Silurian Marine are the Major and important source rock for in unconventional gas and condensate deeper tight reservoirs Upper and Middle Ordovician Members Khabour formation hydrocarbons discovered in the Iraqi desert.

Fig. 1 .
Fig.1.Location map of the lower Paleozoic three wells (Akkas-1, Qaim -1 and Khleisia-1) and Paleozoic outcrops of north Iraq.Iraq is located North East of the Arabian Plate.The structural provinces are afterBuday and Jassim (1987).

Fig. 4 .
Fig. 4. Regional correlation of the Arabian Plate showing the Hot shale, Hoseiba Member, Akkas Formation with neighboring countries.

Fig. 5 .
Fig. 5.The lithological section with natural gamma-ray of the Akkas Formation at Akkas-1 well

Fig. 8 .
Fig.8.Ice cap paleo latitude and transgression NE (Iraq), Arabian plate and old world which cause deposition of the Hot Shale in the world,(Casey et al., 2015)

Fig. 11 ,
Fig. 11, Hot shale distribution of Iraq is considered to be prospective in three regions.The northern area (Yellow ( is considered to have poor to moderate potential absent hot shale, the central area (Green ( is considered to have good potentials demonstrated by the Akkas field and Khleisia-1 well.The southern area (Brown ( may also have good potential but there are no deep wells in this vast region.