Abstract
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).
Highlights
The Middle-Late Jurassic Megasequence was deposited during a period of isolation of the main intra-shelf basin of Mesopotamia from the neo-Tethyan ocean probably due to renewedIraqi Geological JournalMamaseni (2D), 2020: 77-93 rifting along the NE margin of the Arabian plate
The formation in well Atrush-3 is discovered in depth interval 1261-1301 m between Alan and Barsarin formations, while the formation contacted with Neokelekan and Sekhanian formations in Warte outcrop section
As in age-equivalent units throughout the Arabain Shield, the Sargelu Formation is comparable to parts of the Cudi Unit of SE Turkey (Altinli, 1966); in central Syria, it is equivalent to the black shale of the uppermost portion of the Dolaa Group (Dubertret, 1966); in both Saudi and southwest Iran, it is equivalent to Dhruma and the lower part of the Sarmeh formations respectively
Summary
The Middle-Late Jurassic Megasequence was deposited during a period of isolation of the main intra-shelf basin of Mesopotamia from the neo-Tethyan ocean probably due to renewedIraqi Geological JournalMamaseni (2D), 2020: 77-93 rifting along the NE margin of the Arabian plate. Deposition within the basin occurred in a restricted, relatively deep-water environment during Middle Jurassic (Jassim and Goff, 2006). Middle Jurassic Sargelu is the most significant formation in northern Iraq. Sargelu Formation was acknowledged and pronounced by Wetzel at Surdash anticline in northern Iraq's High Folded Zone (Bellen et al, 1959). The thickness of the formation in the zone of outcrops at the High Folded, Imbricated and northern Thrust Zones varies from 20–125m (Jassim and Goff, 2006); the thickness in the current area is 40 m in well Atrush-3, and 75m in the outcrop (Warte). Mamaseni et al, 2019 mentioned that the organic matter content in the source formations of northern Iraq is typing II ≥ I and II kerogen, which are highly oil prone. The main aims of this study are determined petrophysical characteristics of the formation based on well log data and core samples, in addition to potentiality of Sargelu Formation to hydrocarbon generation
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