Oil And Gas Generation History Based On Burial History Reconstruction And Thermal Maturity Modeling Of Petroleum Systems In Northern Iraq

Burial history, thermal maturity, and timing of hydrocarbon generation were modeled for five key source-rock horizons at five locations in Northern Iraq. Constructed burial-history locations from east to west in the region are: Taq Taq-1; Qara Chugh-2; Zab-1; Guwair-2; and Shaikhan-2 wells. Generally, the thermal maturity status of the burial history sites based on increasing thermal maturity is Shaikhan-2 < Zab-1 < Guwair-2 < Qara Chugh-2 < Taq Taq-1. In well Qara Chugh-2, oil generation from Type-IIS kerogen in Geli Khana Formation started in the Late Cretaceous. Gas generation occurred at Qara Chugh-2 from Geli Khana Formation in the Late Miocene. The Kurra Chine Formation entered oil generation window at Guwair-2 and Shaikhan-2 at 64 Ma and 46 Ma, respectively. At Zab-1, the Baluti Formation started to generate gas at 120 Ma. The Butmah /Sarki reached peak oil generation at 45 Ma at Taq Taq-1. The main source rock in the area, Sargelu Formation started to generate oil at 47, 51, 33, 28, and 28 Ma at Taq Taq-1, Guwair-2, Shaikhan-2, Qara Chugh-2, and Zab-1, respectively. The results of the models demonstrated that peak petroleum generation from the Jurassic oiland gas-prone source rocks in the most profound parts of the studied area occurred from Late Cretaceous to Middle Oligocene. At all localities, the Sargelu Formation is still within the oil window apart from Taq Taq-1 and Qara Chugh-2 where it is in the oil cracking and gas generation phase. No.29(12) 2020 Journal of Petroleum Research & Studies (JPRS) E96


Introduction:
A petroleum system as defined by Magoon and Dow [1] is "a pod of active source rock and all genetically related oil and gas accumulations. It includes all of the geologic elements and processes that are essential if an oil and gas accumulation is to exist". The petroleum system is a combining conception that includes all of the different elements and processes of petroleum geology, including: the necessary elements (source, reservoir, seal, and overburden) and processes (trap formation, generation, migration, and accumulation).
This study abridges the burial history, thermal maturity, and timing of petroleum generation at five locations for five key petroleum system source rock horizons in  E98 combined and the Upper Jurassic Naokelekan is present only at the burial-history locations that are on the Shaikhan-2 and Taq Taq-1 wells (Fig. 1). The age equivalent unit to Naokelekan Formation, in the Low Folded Zone, is the Najmah Formation [9,10,11]. The Naokelekan Formation is transitional to the south with the neritic and restricted lagoonal Najmah Formation [9]. Thus, the Najmah Formation occurs in the Qara Chugh-2, Zab-1, and Guwair-2. The Cretaceous stratigraphy and nomenclature is complex within the region and these units do not comprise main source rocks. The Lower and Middle Cretaceous Group in the region comprise of numerous formations whose names differ from site to site. The rock units of the Cretaceous Group that are above the Jurassic unconformity at the base of the limestone are alluded to as the lower part of the Garagu/ Yamama Formation. The interval below the unconformity is alluded to as the upper part of the Jurassic formations.
There is no proof for this unconformity at Shaikhan-2 Well [12]; accordingly, the whole interim is alluded to as the Lower-Middle Cretaceous formations combined and named informally inferable from multifaceted interfingering relationships of the beds.
Each of Qamchuqa, Maudud, and Jawan rocks is joined. The nomenclature differs for the Upper Cretaceous sequence that includes the Bekhme, Shiranish, Hartha, Sa'adi, and Tanuma within the region. Lower Tertiary units are additionally joined as a result of the complex interfingering relationship among the Aalaiji, Kolosh, Sinjar, Khurmala, Gercus, Jadala, Pilaspi, and Avanah across the region. To simplify tables and figures in this review, we allude to this interval as "Aalaiji (and reciprocals)".
The unconformity within the Triassic between Kurra Chine and Baluti Formations [3] probably does not represent a significant amount of erosion in the region that would affect the thermal maturity of the non-penetrated underlying formation source rocks.

It is included an erosional event between the Middle Jurassic Sargelu and Upper
Jurassic Najmah Formations and between Najmah and Garagu Formations at Qara Chugh-2, Guwair-2, and Zab-1 wells to burial-history [9]. Elsewhere in the Shaikhan-2 and Taq Taq-1, these two unconformities do not exist because the contact between Sargelu and Naokelekan Formations is gradational with no unconformity [9,13]. In the same way, the gap between Lower Sarmord Formation and underlying Chia Gara Formation does not exist [3] at the Taq Taq-1 Well, but the gap exists between Chia Gara and Garagu Formations at Shaikhan- 2 Well. No.29-(12) 2020 Journal of Petroleum Research & Studies (JPRS) E99 More than one hiatus exists within the Lower Cretaceous formations [3,14]. A hiatus for the unconformity at the base of Qamchuqa, Dokan, and Gulneri Formations at Taq Taq-1 Well is applied [3,9]. At well Zab-1, the Upper Qamchuqa (Maudud) without any depositional break [3,9,15].
In the latter well, the Jadala Formation which underlies Oligocene rocks Miocene Lower Fars (Fatha) Formation is present [17,18]. The geology of areas adjoining the Shaikhan region is used to estimate the post-Eocene sedimentation and erosion due to the absence of post-Eocene rocks preservation. Accordingly, the amount of post-Eocene deposition is estimated at 2900m [12]. In the same way, the amount of post-Oligocene deposition is estimated at 2500m for Qara Chugh-2 locality [16].

Methodology:
The studied wells are modeled for burial history using one-dimensional modeling software and thermal maturity are performed on five well locations ( The bottom-hole temperatures are acquired from well-log headers of the particular wells used in modeling.

The T max values and their mathematically calculated %R o equivalent, determined by
Rock-Eval pyrolysis, are used for calibration. The T max values are reported by Abdula [5] and Abdula [6]. It is plausible to assume that heat flow varied through time, but it is not required to make assumptions about the time of heat flow changes because using a constant heat flow through time resulted in a suitable context of designed %R o values, as decided by EASY%R o [7] with the measured %R o values. The mean surface temperature for the entire burial-history sites is assumed to be 21 °C according to recorded data by Iraqi Meteorological Organization and Seismology-Kirkuk Station for years 2010-2015 and the results from this study. This mean surface temperature is 4 °C less than the data that were used by Pitman et al. [8] for southern Iraq.

Results: Burial History
The present depth (Table 1) is less than the calculated maximum depth of burial.
Consequently, intended temperatures at extreme depth for the source-rock prospects

Taq Taq-1
This well is located on the crest of the Taq Taq anticline, which is about 13km southeast of Koi Sanjak Figure (1). The well was completed in 1978 [17]. The Taq Taq structure is a slightly asymmetrical anticline some 29km long and about 11km wide with a general northwest-southeast trend. This structure is situated immediately southwest of the Kurdistan Orogenic Mountain Zone [17]. Only minor faults are recorded at the surface. Dips are generally slightly steeper on the southwest flank.
Closure on top of the Pilaspi with the Baba Bawi structure of the main orogenic belt is in the order of 610m [17]. The structural trap occurred by Early Eocene ( Fig. 2A) during Alpine Orogeny. The Alpine Orogeny started in the Late Mesozoic after deposition of the main Middle and Upper Jurassic source rocks in the region.
During Jurassic, the sediment accumulation rate appears to be fairly constant and continuous; a total of 1554m of sediments of this age was deposited in this area. This amount of sediment is roughly the same amount that was preserved at Guwair-2 site, but more than what was preserved at Qara Chugh-2 and Shaikhan-2 locations and less than what was preserved at Zab-1 area (  (Fig. 3). The Qara Chugh-2 Well is located 20km to the northwest of Makhmour City (Fig. 1).
The ground level elevation is 793m above sea level. The well was completed in 1979.
The Qara Chugh structure is 23km long and 2km wide. This structure is a long, twisting structure with a minimum of three peaks on top of Tertiary rocks. The well is located on the southern dome [16]. Neogene uplift of Qara Chugh relative to neighboring structures (Paleogene rocks are exposed in its core) suggests that it is an inverted graben [19]. The anticlinal trap in this site started to exist after deposition of Sargelu Formation by Upper Cretaceous (Fig. 2B).

Zab-1
The second deep-basin burial-history setting, Zab-1 Well is situated nearly 500m to the west of Zeigawra Well and about 10km to the northeast of Guwair Town near Great Zab River in the Shamamik Plain area of the southern Erbil Governorate (Fig.   1). The ground level is 247m. The well was drilled in 1983 [20]. In the same way, the structural trap occurred here due to an effect of Alpine Orogeny in the Middle Miocene (Fig. 2C). The expelled and migrated oil was preserved in Cretaceous and Tertiary reservoirs. The source rocks younger than Jurassic are still not mature in this area [21].
The history of Zab-1 is unlike that at Qara Chugh-2 in terms of the rapid rates of subsidence/sedimentation during Jurassic when 1930m of deposits were preserved with one hiatus, the same as the break in Qara Chugh-2 at the base of Najmah.
Throughout the Cretaceous, the sedimentation rate was a lot slower with five breaks but during Tertiary, 62 Ma to 8.8 Ma the sedimentation rate increased dramatically and a total of 1586m was preserved. The Maximum burial 6250m was occurred around 9 Ma (Fig. 5), after which time uplift and erosion caused the removal of approximately 1100m of the section in this area. Figure 1.

Guwair-2
Guwair Well is located 25km to the southwest of Erbil and 10km to the southwest of Zab-1 Well (Fig. 1). The well was completed in 1981.
Guwair structure is an elongated surface feature trending NW-SE with the Lower Fars (Fatha) exposed. Both flanks are gently dipping and not affected by faulting. Deepseated faulting is probably responsible for the major changes in the axis below the Lower Cretaceous [18]. Reverse fault was detected in the Lower Qamchuqa Formation, which could be interpreted as a folded gravity fault [18].
All petroleum system elements have existed by Middle Miocene (Fig. 2D) before the end of Alpine Orogeny in Pliocene. The Middle Miocene Lower Fars (Fatha) Formation which is a seal rock covered the reservoir rocks before oil started to migrate.
The burial history here (Fig. 6) is very similar to that at Zab-1 Well, except that the rate of sediment accumulation was not as rapid during deposition of the Lower Cretaceous formations as at Guwair-2 where this sequence is 200m thicker than the sequence at Zab-1. Although the Tertiary sequence appears to be the same, the thickness of the sequence is about 700m more at Zab-1 due to the presence of Upper Fars Formation at Zab-1 with a thickness of 746m. We estimate that in the last 11.6 m.y., more than 900m of rock has been removed from these areas as Upper Fars  Figure 1.

Shaikan-2
The Shaikhan block is located around 85km northwest of Erbil covering an area of 283km² and is one of the onshore developments. Shaikan-2 Well was drilled nine kilometers southeast of the Shaikan-1 Discovery Well (Fig. 1). The anticlinal trap was formed by Early Eocene (Fig. 2E). The Lower Fars (Fatha) Formation which is a regional cap rock does not exist in this area. Due to a decrease in tectonic impact the oils were preserved in Lower Jurassic Mus Formation (under Alan Formation) and in

Middle Jurassic Sargelu Formation's limestone rock (under Barsarin Formation). The
Sargelu Formation in this area is a source and reservoir rock at the same time [12].
The Shaikhan-2 represents the shallow depth at which the source rocks were buried (Fig. 7).
The burial history is similar to that of Qara Chugh-2 except for the lack of evidence

Petroleum-Generation History:
Timing of oil and gas generation was determined for the five petroleum source rocks at the five burial-history locations. These source rocks are nominated in table 2 as gas-or oil-prone conferring to accessible geochemical rock data and oil-to-source relations [5,22,23].

Oil-prone source rocks
According to kerogen's organic sulfur content, the oil-prone source rocks are categorized into two categories. Oil-prone source rocks which produce high-sulfur oils like Najmah, Naokelekan, Sargelu, and Kurra Chine Formations characteristically contain marine carbonate-or chert -controlled rocks and encompass Type-IIS kerogen (Os in Table 2). In contrast, oil-prone source rocks which produce low sulfur oil like Geli Khana Formation normally comprise of marine detrital source rocks and contain Type-II kerogen (O in Table 2).
The inconsistency between Type-II and Type-IIS kerogen is imperative in describing the timing and range of oil generation from the source rock. Source rocks containing Type-IIS kerogen generate oil at lower thermal maturities and an earlier stage of maturation than regular Type-II kerogen [24,25]. modeling studies [27]. Throughout the oil generation phase, the gas to oil ratios do not reach 28.3m 3 /barrel. Likewise, this ratio starts to escalate considerably to initiate the previously generated oil cracking to gas once the thermal stress becomes high enough (≥1.17) [28].
The mass of generated gas from source rocks with Type-II and Type-IIS kerogen is significantly smaller than gas generated from the cracking of oil and according to Lewan and Henry [28] the mass is about 3 to 7 times less. Therefore, only gas generation from cracking of produced oil to gas was displayed. Hydrous-pyrolysis kinetic constraints utilized to regulate the timing of gas generation from oil cracking are from Lewan and Ruble [26] and Tsuzuki et al. [29] and are shown in table 3.
Immature source rocks have a genetic potential of less than 0.01, and source rocks that have compatibly passed oil generation window and have terminated cracking to gas have a genetic potential of more than 0.99 [30].  [33]. Consequently, EASY %R o was used to calculate gas generation from Type-III kerogen, with the start and end of gas generation arising at 0.5 and 2.0 %R o , correspondingly. Throughout this assessment, the conferred generated gas signifies solitary to thermogenic gas.

Maturation History
The measured and calculated vitrinite reflectances (%Ro) are used to determine the thermal maturity's history for each burial-history location. The range of 0.5 to 0.8 %R o denotes the recommended start to peak of gas generation from Type III kerogen.
The %R o values of 1.10 for peak and 2.0 for the end of gas generation are determined.
Only Baluti Formation is considered gas-prone source rock (mostly composed of Type III kerogen) and so vitrinite reflectance (%R o ) may be used to estimate the extent of gas generation from this formation (Fig. 8).

Petroleum Generation History from Source Rocks
The timing and amount of petroleum generation from the known source rocks at the five burial-history locations are listed in table 4 and summarized in figure 8. With the exception of the Qara Chugh-2 and Shaikhan-2 locations, the burial-history curves represent relatively deep parts of the region. As a result, they represent the earliest timing and greatest extent of petroleum generation in the identified source-rock intervals of the region. Toward the west and southwest of these sites, is the time at which petroleum generation arises will be far ahead and the amount of petroleum generation will be scarcer. The place of the Guwair-2 burial-history location in the central part of the south (Fig. 1) and the thickness of overburden over the main source rock of the area, Sargelu Formation which is 2130m, suggest that the timing and extent of petroleum generation will become later and less toward northwest and west due to decrease of overburden thickness in the area and earlier and greater toward the east and southeast. Table 4 shows that Sargelu Formation has entered oil generation at all localities, but entered oil cracking to gas only in Taq Taq-1 Taq Taq-1 have not consumed 100% of its organic matter potentiality to generate hydrocarbon mainly during six to four million years ago in the Miocene time as stated by Al-Ameri et al. [34].
Rates of oil generation, oil cracking to gas, and gas generation were significantly reduced at low mature areas such as Shaikhan-2, Zab-1, and Guwair-2. The Sargelu Formation in Jabal Kand-1, which is located nearby to Shaikhan-2 has consumed only 70% of its potentiality because of its shallow depth [34] and had passed the -oil window in the Late Eocene to Late Miocene [35]. Location is shown in Figure (1). The red line shows prediction from model scenario [6]. A) Taq Taq

Oil Cracking to Gas
The start, peak, and end of oil cracking to gas timing (genetic potential of 0.01, 0.50, and 0.99, correspondingly) is displayed in figures 3, 4, 5, 6, and 7. After oil generation ends, the oil cracking to gas does not start immediately and there will be a This discrepancy is related to the thermal-burial history settings. This break is larger for source rocks with Type-IIS kerogen (that is, Sargelu Formation) than for those with Type-II kerogen (that is, Geli Khana Formation). This discrepancy is due to a property of chemical bonds. The oil generation from Type-IIS kerogen takes place prior and at lower thermal maturities than regular Type-II kerogen [24,25].
Nonetheless, the oils from both kerogen forms have equal kinetic constraints for cracking to gas. This break is also confirmed in the vitrinite reflectance values.
Termination of oil generation for Sargelu oil is at nearly 0.73 %R o , and for the other oil-prone source rocks it is between 1.12 and 1.18 %R o . The initial cracking of oil to gas for both source-rock types is between 1.67 and 1.77 %R o ( Table 4).
The Zab-1, Guwair-2, and Shaikhan-2 locations have the smallest potential for gas from oil cracking because oils from Sargelu source rock has not been cracked to gas yet. Generally, the order of increasing potentiality of the burial-history sites for gas generation from the cracking of oil is Shaikhan-2< Zab-1< Guwair-2 < Qara Chugh-2 < Taq Taq-1.

Gas generation from Source Rocks
The range of gas production from gas-prone source rocks throughout this investigation is directly connected to vitrinite reflectance values based on experimental remarks and not a definite kinetic model. As a result, the timing of gas generation is directly related to vitrinite reflectance values (that is, initiate = 0.5 %R o , peak = 0.8 %R o , and termination = 2.0 %R o ), and the burial-history curves for vitrinite reflectance (Fig. 8) compare gas generation from gas-prone source rocks. Table 4 displays the time of gas production for every source rock that is gas prone. In the Zab-1 Well the gas generation occurred from the gas-prone source rock, Baluti formation (Fig. 8C).

Conclusions:
-Results for the base of the Sargelu Formation, a source rock containing Type-IIS kerogen, designate the timing for the initiate of oil generation arose within a wide range, from 51 to 28 Ma, and oil generation ended at all sites by 7 Ma.
-The timing and extent of petroleum generation in the studied area will occur later and less toward northwest and west due to the decrease of overburden thickness and earlier and greater toward the east and southeast.
-At Qara Chugh-2 cracking of oil occurred from Geli Khana Formation, and it is still in progress. In the same way, at Shaikhan-2 and Guwair-2 cracking of oil occurred also from Kurra Chine Formation and it has not ended yet.
-In the deepest parts of the region, Taq Taq-1, the production of gas from the cracking of oil initiated at about 26 Ma for Butmah/ Sarki Formation and 11 Ma for Sargelu Formation. Gas generation from oil cracking terminated at these deep sites by nearly 9 Ma for Butmah/ Sarki Formation and has not ended yet for Sargelu Formation.
-The Zab-1, Guwair-2, and Shaikhan-2 locations have the smallest potential for gas from oil cracking because oils from Sargelu source rock have not been cracked to gas yet.