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Abstract

Located in the northern part of Arabian Peninsula, Syria is one of the Middle East oil countries. The most petroliferous province in Syria is the Euphrates Graben system in the eastern part of the country. Oil and gas have been discovered in this graben in the mid 1980's by Shell E&P and its partners. Since then no comprehensive study has been performed to investigate the origin of crude oils produced from more than 60 oil fields in the area. This study deals with this issue from a petroleum geochemistry perspective and tries to answer open questions regarding the source of light and heavy oils produced over the Euphrates Graben. Eighty two oil samples in addition to 37 rock samples have been analysed geochemically in order to investigate the molecular composition of hydrocarbons and the maturation degree of their associated source rocks. Routine geochemical analysis in addition to stable isotopes and diamondoid analyses were carried out for 30 oil samples. Based on gross composition, biomarker and non-biomarker characteristics, oil-oil correlation identified three oil families in the study area: Family 1, Family 2A and Family 2B. Crude oils of Family 1 have been found to be generated from a marine, clay-rich and highly mature source rock. The related source rock is older than Jurassic in age based on age-related biomarker parameters. Maturity-related parameters (aliphatic biomarkers) and non-biomarkers (like diamondoids) imply that a highly mature source rock is responsible for generating Family 1 crude oils. These features fit very well to Palaeozoic Tanf Formation (Abba group) which is equivalent to Lower Silurian Hot Shales found elsewhere in the Middle East and North Africa. However, the Upper Cretaceous R'mah Formation and Shiranish Formation were found to be responsible for generating the remaining crude oils studied here. Compositional and molecular differences between families 2A and 2B were attributed to facies and subtle maturation variations. Geochemical oil-source rock correlation supported the classification of oil families that Family 2A was most likely generated from the Shiranish Formation, while the R'mah Formation was the source rock for Family 2B oils. According to the very complex tectonic situation of this rift basin and, additionally, the lack of geological data, it was not possible to definitely retrace the migration pathways for oils from source rocks to reservoirs. However, an attempt to figure out the potential migration fairways is presented by concepts for trap configurations for specific areas especially for crude oils found in shallow Miocene reservoirs. To predict to which extent these oil families could mix with each other, oil mixing mathematical models have been applied for crude oils, which have different signatures from different source. The results of the theoretical mixing were promising and showed that some oils in the southeastern part of the graben generated principally from the Upper Cretaceous R'mah Formation, and have got significant contribution from a Silurian source rock. These findings about petroleum mixtures could support the attempts to find more hydrocarbon plays in the Palaeozoic section in south- and northeastern part of the graben by retracing possible oil migration routes. Secondary alteration processes inuenced the petroleum composition particularly in shallow reservoirs. Geochemical investigations for crude oils in the northwestern part of the graben showed that biodegradation took place resulting in lower API gravities and poorer light ends.