The studied samples were collected from the Palaeozoic outcrops, Khabour, Pirispiki, Kaista, Ora, Harur, and Chia Zairi formations in Nazdur Anticline, Amadia about 457 km due north of Baghdad, Iraq's capital city. Qualitative assessments of 29 samples were conducted by using textural microscopy in determining the percentage of amorphous organic matter for palynofacies type belonging to different types of kerogen. This organic matter contains acritarchs, spores, and algae deposited in the dysoxic-anoxic environment. The organic matters are from mature to post mature and have a thermal alteration index of-3 to +3 by Pearson's and Staplin's scales, respectively, with palynomorphs of orange to dark brown colors on the palynomorphs. These ranges of colors indicate mature organic matters.
The present study focuses on source rock evaluation of the Sargelu Formation by using core chips of rocks collected from well Atrush-2, Duhok, Kurdistan Region-Iraq. The Rock-Eval pyrolysis and vitrinite reflectance were executed. Subsequently, the selected parameters were used for source rock evaluation and 1-D Basin Modelling calibration. The upper part of the formation mainly comprises argillaceous limestone with low content of organic matter (0.64%-1% TOC). By contrast, the lower part is dominated with shale interval and contains high amounts of TOC values (>4% for 1272-1278 m) reveling good to very good quality source rock. Accordingly, good to very good hydrocarbon generation potential is suggested for this formation. Organic matter of the Sargelu Formation contains type II and mixed-type II-III kerogen. The values of Tmax and vitrinite reflectance (Ro%) demonstrate that the formation is thermally mature and in the oil zone. In order to construct a thermal history of the formation and determine the timing of hydrocarbon maturation and generation, the 1-D basin modelling PetroMod 2019.1 was used in this study. Based on the 1-D Basin modelling simulation and its outputs, about 3500 m of overburden have been eroded at the study area. The present-day heat flow was found to be 30 mW/m 2. The organic matter of Sargelu Formation entered the early oil zone in 64 Ma and reached the main oil zone ca. 5 Ma. The formation is still in the main oil zone at present-day. In well Atrush-2, the highest rate of oil generation for the Sargelu Formation was in the 8.5 Ma, the onset of oil expulsion from Sargelu Formation was in 50 Ma and the expulsion mass has been reached 0.5 Mtons at present-day.
This study indicates the source rock evaluation and reservoir properties of the Middle-Jurassic Sargelu Formation in two selected sections (well Atrush-3 and Warte outcrop) northern Iraq. The well log data display that the shale includes 30% of the upper part of the Sargelu Formation. This extraordinary ratio of shale content has negative impacts on the reservoir properties in this part of the formation. The lower part of the formation has a good porosity ratio that reaches 15% as calculated from the log data. The porosity is obtained from analyzed core samples support the same conclusion that obtained in the log data analysis and it is measured at 9-17.5% for interval 1282-1301 m in Atrush-3 well and 5-7.9% in the interval 54-75 m Warte section. The formation has very good organic matter quality for black shales as the average total organic carbon (TOC) measured from the pyrolysis assessments is about 2.61 wt% in both the well and the section. The low oxygen index (average 26 mg CO2/g TOC) and high hydrogen index (average 423 mg HC/g TOC) indicate the variation of the kerogen class from type II in the Atrush-3 to type II-III in the Warte section. These two forms of kerogen can produce oil and condensate gas in a suitable state. Pristine/phytane (Pr/Ph) ratios are less than one (0.67-0.83) and this reveals the anoxic condition for the formation. The colors of the palynomorphs in the examined samples under the microscope changes from yellowish brown to brown (TAI=+2, -3), and the average Tmax and vitrinite reflectance are 444 oC and 0.80, respectively. Sargelu Formation is located within the mature and oil window. This is also supported by the carbon preference index which is around one (average 0.98).
Petrophysical properties of the Early Cretaceous Sarmord and Garagu formations from the Atrush and Sarsang Blocks in the Kurdistan Region are studied. These formations are generally composed of limestones and dolomitic limestones interbedded with thin to medium layers of yellowish-gray marl (calcareous mudstone). The current study shows that the average shale volume in the Sarmord and Garage formations is between 16% and 20%. In Atrush-1 Well, the average porosity ratio of the Sarmord and Garagu formations is fair to good (15% and 11%, respectively). However, in Mangesh-1 Well, the porosity value is poor; it is around 4% on average. Generally, most of the hydrocarbons that have been observed within the pore spaces are residual oil type in Atrush-1 Well and movable hydrocarbon type in Mangesh-1 Well. In Atrush-1 Well, out of 362 m thickness of both formations, only 180 m is considered to be a pay zone; whereas, the pay zone is just around 8.0 m thick out of 347 m of the total thickness. According to the calculated porosity values, the Garagu and Sarmord formations are not considered as good reservoirs in the studied wells, with the exception of Atrush-1 Well where the Sarmord Formation has fair potential reservoir characteristics.
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