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Abstract

The Shiranish Formation was deposited during the pre- to synrift phase of the Euphrates graben formation. The Shiranish sediments are rich in organic carbon and are potential source rocks in the Euphrates Graben. The subdivision into a lower and an upper part of the Shiranish Formation (USF, LSF) is overprinted by sedimentary cyclicity on high frequency (alternations of light-coloured and dark clay-rich intervals). According to the mineralogical composition the USF is composed of two submembers USF-1 and USF-2. The LSF and USF-2 resemble in higher kaolinite, ankerite contents and lower δ13Corg values, which indicate palaeo-methanogenesis. In contrast, USF-1 is characterised by intense organic matter and mixture of carbonate and clay deposition. A kerogen type II to II-III, and continuously increasing total organic carbon (TOC) contents and hydrogen index (HI) values characterizes the USF. In the central graben the USF is characterized by low diasterane indexes and Ts/ (Ts+Tm) ratios, indicating anoxic mix clay-carbonate source rocks. In contrast, the LSF is characterized by a kerogen type II-III, and lower TOC contents and HI values. The oxygen index (OI) values are markedly higher than in the LSF. Moreover, the LSF has relatively high diasterane index values and Ts/ (Ts+Tm) ratios, representing a fine-grained siliciclastic influence. A high sedimentation rate prevailed during deposition of the USF, and higher amounts of marine organic matter were incorporated in the sediments compared to LSF A strong input of terrestrial organic matter under shallow marine conditions prevailed during deposition of LSF sediments. During sedimentation of the USF extended bottom water anoxia prevailed. δ15N values increase from bottom to the top of the Shiranish Formation (from 3‰ to 7‰) and indicate a progressively higher palaeo-productivity and diagenesis influences. The LSF in the central graben shows a good potential for hydrocarbons, and a higher potential in the USF. Along the graben margins the LSF and USF-2 exhibit a good source rock potential, whereas USF-1 has a very good to excellent source rock potential. These differences are due to depositional conditions more suitable for accumulation and/or preservation of higher amounts of OM in the central graben. Maturity varies laterally from immature organic matter in the margin graben to oil window maturity towards the central graben. Open system pyrolysis reflects that the LSF and USF contain predominately marine kerogen with a strong aliphatic hydrocarbon signature and relatively small sulphur contents. They have the potential to generate paraffinic-naphthenic-aromatic low, high and high paraffinic wax petroleum type organofacies and range into the gas field for the LSF in the central and northern area. The changes of activation energy distributions from narrow for the USF in the central Graben, towards broader activation energy distributions at the margins of graben indicate that dominantly autochthonous marine organic matter was deposited in the central part of the basin, whereas at the margins mixed organic matter types (terrestrial and marine) were deposited and possibly also preservation conditions were not optimal. The calculated predictions for the timing of petroleum generation for the USF and LSF show a variation in the terms of the onset and peak generation temperatures. In the central area, the predicted petroleum formation began at 136 °C for the USF and at 144 °C for the LSF. This corresponds to ca. 360 m difference in burial depth for petroleum formation. The compositional kinetic model displays that gas shows occur in higher quantity in the LSF compared to the USF. Gas kicks during drilling the LSF can be interpreted by two scenarios. First one is less likely; Gas migrated upwards from the underlying R’mah Formation. The shaly intervals within the lower Shiranish Formation acted as trap for this gas. Second is more likely; Oil was generated and retained in the lower Shiranish Formation and cracked to gas when buried to greater depth.