Geochemistry and Reservoir Properties of the Upper Jurassic Formations, in Selected Oilfield Central Iraq

Geochemical analysis of subsurface organic matter in the most productive source rocks, the Sargelu and Naokelekan formations, has provided insight into the potential of hydrocarbon production in the Jurassic strata of the selected oil field in central Iraq. The aim of this study is to conclude if the source rock can provide reservoir properties such as porosity and permeability. This attempt is made to support petroleum production from both reservoir and source rocks. The cause of this idea was adopted due to some of very good source rock were kept the produced hydrocarbons within the thin impermeable layers. Rock Eval pyrolysis returned high total organic carbon ranging from 0.55 to 34.82 wt%, Type II kerogen with a hydrogen index of up to 368 mg HC/gTOC, and a rock potential of 0.58 to 50.9 kghc/ton rock. Mature organic material was found in both formations, with a Tmax between 434 and 450 °C. This study of source rock properties shows that the rocks have good properties that need to be considered as the Sargelu and Naokelekan formations are the main sources of hydrocarbons in the studied area. The multi-story source-reservoir rocks are represented in this secession. The reservoir properties of the Naokelekan Formation are impressive in that it can draw crude oil from the underlying Sargelu Formation, which produces oil and releases it into the local petroleum system. The effective porosity in the Naokelekan Formation reached 14% in the main reservoir area, while permeability values mostly reached 4 mD. This indication potentially underscores the objective of this study to upgrade the oil-producing source rock into reservoir intervals.


Introduction
The multi-story intervals of source, reservoir, and seal are stunning stratigraphic traps [1].These traps, as well as the structural ones, are crucial due to their abundance.As a result, the goal of this research is to identify new traps using new concepts and to inspire academics and firms to expand the petroleum sector.Despite the absence of knowledge, the intervals of Sargelu, Naokelekan, and Gotnia formations in the studied oilfield are highly amazing local petroleum system to be studied comparing with southern Iraqi petroleum system [2].
The research region of the investigated Field is located around 30 kilometres northeast of Tikrit City in the low-folded zone of the Zagros Fold Belt in central Iraq.On average, the field area is 150-170 metres above sea level.Figure 1 illustrates that this field is structurally oriented NW-SE inside the northern section of the Zagros Fold Belt's low folded zone [3].

Figure 1.
Location map of the study area, modified after [4].
The Sargelu Formation extended throughout over all the Iraq lands and its surroundings.It may be found on the surface in a number of locations, including the Qulqula-Khwakurk and Balambo-Tanjero tectonic zones.The structure can also be seen in subterranean wells [5].The Sargelu Formation is located under the Sehkaniyan Formation.In most cases, the lower contact between the two formations is gradational and conformable in nature.The upper limit of the formation is contained with a thinbedded limestone layers, which is more difficult to recognise in the Northern complex zones.Sargelu Formation appears to be degraded and is overlain by Aptian Sarmord Formation.The age of the underlying Upper Sehkaniyan Formation cannot be calculated based on faunal evidence.The age of the Naokelekan Formation, which overlies the Sargelu, has been determined based on its fossil abundance.The bottom portion of the Sargelu Formation is gradational, conformable, and unidentified.There is no indication of nonconformity at the lower or higher bounds.A marine incursion led to a facies change at the commencement of the Late Toarcian megasequence.The thickness of the Sargelu Formation varies.In northwest Iraq, it is 20 meters thick, whereas in northeast Iran, it is 125 meters thick.The thickness varies from 75-83 m in Kuwait and 152-213 m in northern Iran [6].The thickness of the Sargelu Formation in the typical region, which was between 110 and 148 meters, could not be verified during the field study.The formation thicknesses depicted on the isopach map appear to differ from those found in recently drilled wells at the location.Dark bituminous limestones, limestone interbedded with dolomite, and black shales with thin black chert evidences make up the Sargelu interval.The formation's surface and underground portions are both abound in fossils.This formation is made up of thin-bedded intervals of the lithology described above that are 115 metres long.[7].On the other hand, the Naokelekan Formation originated in a euxinic environment in a basin that was slowly subsiding.The formation was formed in a shallow, open marine depositional environment in a brackish lagoon [8].Palynomorphs, principally dinoflagellate cysts, were examined to establish the Callovian to Upper Oxfordian age of the Naokelekan Formation in Northern Iraq.A separate and unique petroleum system from the Triassic period is shown by the Jurassic epoch.[7].In Iraq, the Late Toarcian-Early Tithonian Megasequence (AP7) is commonly referred to as belonging to the Jurassic period.During this time, many rock packages with a genetic connection were deposited.Condensed sedimentary successions of a euxinic environment in the bottom section (Naokelekan Formation) and lagoon evaporites in the upper part of the Balambo-Tanjero and Northern Thrust Zones and the Barsarin Formation dominate Iraq's High Folded Zone, which includes the Balambo-Tanjero and Northern Thrust Zones.Around the end of the Early Jurassic, the tectonic environment formed the Gotnia Basin as a huge intrashelf basin.The majority of Iraq's territory was covered by this basin.Along with the open ocean water of the Tethys, the primary cap rocks of the Jurassic era were located somewhat outside of this basin [9].

Materials and Methods
Twenty one rock samples were obtained from the Sargelu and Naokelekan formations in the studied wells (Aj-8 and Aj-12) are applied to Rock-Eval pyrolysis at USGS, Hustin USA.Each sample was weighted at 60 grams, which was then pulverised into a clay-sized powder.After that, S1 and S2 peaks were released from each sample by heating it to 300o C for 3 minutes and 650o C for 25 minutes, respectively, as illustrated in Table 1.The maturity level, kinds of organic matter, [10]and number of hydrocarbons that have been formed or may be created from the rock samples are determined using the parameters Tmax, S1, S2, TOC, and d erived values (petroleum potentiality PP and hydrogen index HI).The temperature of the bottom holebore (BHT) and heat conductivity of the rock units were utilized to derive the subsurface geothermal gradients, which were then used to calibrate the current heat-flow values using the PetroMod 1D (PM) tool.The Paleo-Depth of Water (PDW), the Temperature of Sediment-Water Interface (SWIT), and the Heat Flow are the three boundary conditions used in the fundamental models for the thermal variation and evolution of sedimentary basins.In this investigation, the paleodepth of water was modelled using a range of well-known values.In addition, a value of 0.0m has been taken into account for erosion or hiatus occurrences.The depth of water ranged between 20 to 30 meters, which included to the model for carbonate rock.Using the Wygrala (1989) methodology, the PM-1D application evaluates SWIT estimation throughout geological periods [11].This direction is determined by geological age, fluctuations in mean surface paleotemperature vs. latitude and geological period, and water depth during the duration of deposition.The rifting and formation of the basin incidents led to hypothesise that a paleo-heat flow shifted throughout geological time.The vitrinet's reflection versus depth plot is the valueable way to analyze degradation.If the VR points are equal or less than 0.3, there is no erosion; however, if they are greater than 0.4, that means the formation is within maturity intervals and/or some beds may be eroded.On the other hand, zero is a gap that cannot be estimated at all.The interactive petrophysics software (IP) is used to determine the reservoir characteristics of the formation [12].The GR, resistivity, and porosity logs from the LAS file that have a vertical resolution of 0.2 m.Using the conventional Schlumberger approach, corrective methods have been carried out for each GR, resistivity, CNL, and RHOB log that more affected by the bottom well condition [13].Following rectification, the volume of shale is estimated using the gamma-ray method, and the all parameters of the formation are determined using logs of porosity.Resistivity logs are then used to check for saturation in the dolomite beds [14] and [15].

Results and Discussion
The Upper Jurassic formations in the study area are of significant interest due to their potentiality as reservoirs for oil and gas.Understanding the geochemistry and reservoir properties of these formations is crucial for successful exploration and production activities.This study aims to provide comprehensive insights into the geochemical characteristics and reservoir properties of the Upper Jurassic formations in central Iraq, with the ultimate aim of supporting the understanding of these reservoirs and optimizing their development.The results of this study will contribute to the advancement of exploration and production strategies in the region, ultimately leading to increased efficiency and success in oil and gas extraction [16].

The Hydrocarbon Potential of the Study Area
The Tertiary rocks in the study area and in the whole of northern Iraq are distinguished by the fact that they contain the main reservoir of hydrocarbons, both types of oil and gas, because they contain the generating source rocks, reservoir rocks and cap or seal rocks that represent an integrated petroleum system.On this basis, the results of drilling exploratory wells, which depended on the results of the two-dimensional interpretation, were successful in reaching important hydrocarbon explorations [17].

Interpretation of Pyrolysis Parameters:
Pyrolysis is the decomposition of OM caused by heating in the absence of oxygen.Rock-Eval pyrolysis is the most frequently utilized technique for determining quick assessment and characterization of OM in sediment [18].The content of organic carbon cannot determine the presence of source rocks potentiality alone, due to constraints such as different OM types having different HC yields for the same TOC content, a more precisely measurements of SR ability to produce HC is needed for a more detailed evaluation.So that the HC source potential and quality can be identified by combining Rock-Eval pyrolysis data with TOC measurements, and kerogen type, source richness, quality, and thermal maturity can be determined, Table 1.

Hydrocarbon Generation Potential:
The Rock-Eval pyrolysis data set with derivatives adds to the evidence by directly estimating the freeevolving HC, which is referred to as S1.S2 is the HC that can be produced straight from kerogen cracking.As defined by, the numbers of S1+S2 indicate the overall hydrocarbon generation potential, often known as petroleum potential (PP) [19] that suggested that the yield PP less than two (kghc/ton rock) types little oil production with low gas potentiality.PP ranged between 2 to 6 values indicate moderate potential source rock, and above six suggest good to excellent potentiality.However, a PP greater than 2.5 could be considered necessary for identification as a possible oil SR an d to provide the minimal oil content required near the peak of the main stage of HC formation to satur ate the pore network and enable expulsion [20].The PP values in Sargelu Formation samples ranges in their averages from 0.58 to 50.9 kghc/ton rock referring to different hydrocarbon potentials.While the values of Naokelekan Formation ranged from 1.76 to 33.92 kghc/ton rock.
The beneficial cross plot between PP (kghc/ton rock) versus TOC wt% give a good indication of SR potential.As shown in Fig. 3, the main present values are located in the fair or moderate zone, except some of them are in the good SR zone.The threshold, which is agreed upon in advance equal to 2.5 kghc/ton rock is overtaken by many of the analyst samples.

S1 and S2 vs Total Organic Carbone (TOC)
It is possible to distinguish between indigenous and non-indigenous HC using a plot of S1 vs. TOC.When S1 levels are greater than TOC levels, which must be relatively low in the same sample, migratory or contaminant HC is suggested.The values above the slanted line indicate this situation as shown in Fig. 4.However, the cross plot shows all the samples are indigenous and reveal the majority of these samples in respect to Sargelu and Naokelekan formations.It is worthy to mention that kerogen types can also be determined by intersecting S2 mgHC/gTOC versus TOC wt% values in an indicative cross plot as seen in Fig. 5.

Thermal Maturity of Source Rocks
It has to do with how much sedimentary OM (i.e.source rock) is transformed by the temperature-timedriven interaction into oil, wet gas, dry gas, and finally pyrobitumen [21].The thermal change in OM is due to a combination of temperature and burial history.Different maturity indicators, such as Vitrinite reflectance (Ro), thermal alteration index (TAI), spore color index (SCI), and pyrolysis Tmax combined with other parameters like depth and production index, can be used to identify the level of maturity state of SR.The extended scale for calibrating this chemical and physical transition with several stages of hydrocarbon synthesis is Ro values, although other indicators can also be used.The indicated relationship of the hydrogen index and Tmax as illustrated in Fig. 6, refers to the type II kerogen in respect to the Sargelu Formation, while Naokelekan Formation indicated to be type III kerogen, which mainly can produce oil and/or gas.The same cross plot is showing the Ro values within 0.5 to 1.3 that indicate a mature Catagenesis stage that represented as the main stage of oil generation.

Transformation Ratio TR
The ratio of HC that has previously produced S1 to the grade of entire HC that can be acquired from kerogen S1+S2 or PP.It is also known as the Production Index or PI.PI is used to determine SR generation status, but it is also useful for comparing homogeneous SR of different ranks [22].Therefore, in the oil window, a PI of 0.1-0.4suggests SR, after which gas is the predominant HC phase product.The PI of Sargelu and Naokelekan formations mostly indicates the oil zone, while Naokelekan Formation is almost in the low level of conversion (Fig. 7).This result support the idea of the multistory between the sourcereservoir intervals of Sargelu and Naokelekan formations.

Petrophysical Properties
Porosity types tracks that divided into effective porosity (PHIE), total porosity (PHIT), and Primary porosity (PHIsonic), which represent a good reservoir unit in Naokelekan Formation, while Sargelu Formation shows poor reservoir characterization, Figure 9  As a result, reservoir unit track is divided into three types as good reservoir unit in yellow, medium reservoir unit in green and poor reservoir unit in brown [24].
Based on what was mentioned above, it can be said that the Sargelu Formation is a good indicators that it is a good source rock, while the Naokelekan Formation can be considered as a reservoir rock that can store the oil formed from the Sargelu Formation and trapped it by the upper impermeable zone of the Gotina Formation.

Neutron-Density and Neutron-Sonic Cross Plot
One of the earliest quantitative interpretation techniques is the neutron-density cross plot, which served as the main approach for defining the lithology of the formation.With the ability to account the clay before entering the plot, it is still widely used for matrix identification and calculating the formation porosity in gas-bearing formations.The fundamental illustration makes use of the neutron lithology effect that was previously observed as well as the variations in matrix density between the three common rock types.The most common evaporites (rock salt and anhydrite) are also easily recognized, making this cross plot significant and widely used.It offered adequate resolution for quartz, calcite, and dolomite.The cross plots for the Naokelekan and Sargelu Formations demonstrate that the point falls along the line of limestone, with a few points of dolomite and shale, as shown in Figs.11 and 12.

Conclusion
The multi-story of source, reservoir, and seal intervals is good for stratigraphic traps.Therefore, the concluded results from this study are the ability of some source rocks to produce significant amounts of oil and/or gas.The case studies that are described here are from the Upper Jurassic petroleum system represented by the succession of the Sargelu, Naokelekan, and Gotnia formations.The Sargelu behaves as a source rock where its reservoir properties are very poor.The Naokelekan Formation, however, acts as excellent reservoirs due to their better reservoir properties.The Gotnia Formation, on the other hand, acts as a cap rock, trapping the oil and gas from the Naokelekan Formation and preventing its migration to other areas of the system.This allows oil and gas to accumulate in the reservoirs and potentially be extracted.

Figure 3 .
Figure 3. Oil potential schem of the Sargelu and Naokelekan formations in The studied oilfieldshowing the limited of Petroleum potential in the two formations.

Figure 4 .
Figure 4. S1 vs. TOC identifies hydrocarbon migration or pollutants and demonstrates the limitations of this phenomenon in Sargelu and Naokelekan formations.

Figure 5 .
Figure 5. S2 vs TOC identifying good hydrogen index, and showing fair to good indication of source rock in Sargelu and Naokelekan formations.3.1.4.Thermal Maturity of Source RocksIt has to do with how much sedimentary OM (i.e.source rock) is transformed by the temperature-timedriven interaction into oil, wet gas, dry gas, and finally pyrobitumen[21].The thermal change in OM is due to a combination of temperature and burial history.Different maturity indicators, such as Vitrinite reflectance (Ro), thermal alteration index (TAI), spore color index (SCI), and pyrolysis Tmax combined with other parameters like depth and production index, can be used to identify the level of maturity state of SR.The extended scale for calibrating this chemical and physical transition with several stages of hydrocarbon synthesis is Ro values, although other indicators can also be used.The indicated relationship of the hydrogen index and Tmax as illustrated in Fig.6, refers to the type II kerogen in respect to the Sargelu Formation, while

Figure 6 .
Figure 6.The product zone, maturity, and kerogen types in terms of hydrogen index, Tmax, and Ro reveal Sargelu interval within oil prospectivity.3.1.5.Transformation Ratio TRThe ratio of HC that has previously produced S1 to the grade of entire HC that can be acquired from kerogen S1+S2 or PP.It is also known as the Production Index or PI.PI is used to determine SR generation status, but it is also useful for comparing homogeneous SR of different ranks[22].Therefore, in the oil window, a PI of 0.1-0.4suggests SR, after which gas is the predominant HC phase product.The PI of Sargelu and Naokelekan formations mostly indicates the oil zone, while Naokelekan Formation is almost in the low level of conversion (Fig.7).This result support the idea of the multistory between the sourcereservoir intervals of Sargelu and Naokelekan formations.

Figure 7 .
Figure 7.The Sargelu and Naokelekan formations have entered the oil window, according to the Transformation Ratio vs. estimated Ro, which corresponds to the start of generation and expulsion in those formations.

Figure 8 .
Figure 8. Represents the burial history curves, superimposed by the transformation ratio.
represents a Computer Processed Interpretation (C.P.I) of Aj-12 well[23].Porosity in Naokelekan Formation divided into two zones, upper zone (top of the formation to 3490 m depth) shows low amount of porosity with poor reservoir characterization, while the lower zone shows high amount of effective porosity that reach to 11%.Porosity in Sargelu Formation shows low amount with poor reservoir properties along the formation.Permeability track, which shows significant permeable unit in Naokelekan Formation to reach approximately to 3.7 mD, while negligible permeable unit in Sargelu Formation, Figure10A and B.

Figure 10 .
Figure 10.Illustrate the main reservoir properties in wells Aj-8 (A) and Aj-12 (B) as well as the intervals of interest in respect to good, medium, and poor reservoir properties.3.3.Neutron-Density and Neutron-Sonic Cross PlotOne of the earliest quantitative interpretation techniques is the neutron-density cross plot, which served as the main approach for defining the lithology of the formation.With the ability to account the clay before entering the plot, it is still widely used for matrix identification and calculating the formation porosity in gas-bearing formations.The fundamental illustration makes use of the neutron lithology effect that was previously observed as well as the variations in matrix density between the three common rock types.The most common evaporites (rock salt and anhydrite) are also easily recognized, making this cross plot significant and widely used.It offered adequate resolution for quartz, calcite, and dolomite.The cross plots for the Naokelekan and Sargelu Formations demonstrate