AAPG Geoscience Technology Workshop

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Oil Genetic Groups in the Levant Basin - East Mediterranean Sea and Onshore Israel

Abstract

Oil genetic groups classification is among the fundamental elements required for identification of effective source rocks and oil systems in a basin and evaluation of its generative potential. In this presentation we review detailed composition (bulk, molecular, isotopic) data obtained for 17 oil shows during few decades of exploration activities at the East Mediterranean Levant Basin inland and offshore Israel and North Sinai (Egypt) shelf and their classification into genetic groups. Saturated and aromatic hydrocarbons (HCs) molecular characteristics and bulk 13C and compound specific 34S data obtained for the oil samples show clear clustering into five different genetic groups. The genetic group clustering is concordant with the geographic distribution of the oils, namely, the inland Dead Sea, Meged and Coastal Plain groups, and the continental margin Yam and Mango groups. Some differences are distinct in the HCs composition between samples within some of the identified groups. These differences are mainly confined to the light-end n-alkane and monoaromatic compounds whereas heavier acyclic, polycyclic alkanes and aromatic parameters show consistent correlations. Hence, the differences observed are attributed to selective post-generation modification, mainly by biodegradation and perhaps some water washing. On the other hand, the classification to genetic groups relies on biomarkers which are more durable to secondary modifications and thus retain better the genetic source. To support the genetic classification, a study employing a new method of oil-oil and oil-source rock correlation in 4 different oil samples from the Dead Sea petroleum system was carried out (Rosenberg et al., 2017). The method is based on compound specific sulfur isotope analysis (CSSIA) in alkyl benzotiophene and dibenzotiophen compounds present in the aromatic fraction of the oils. Despite post-generation weathering effect on the oils from the Dead Sea petroleum system, the results obtained clearly corroborates the classification of the samples into the same genetic group, as previously inferred based on biomarkers analysis. Moreover, it also corroborate the origin of the oils in this group from the Upper Cretaceous Ghareb Fm. type IIS kerogen, as the CSSIA of artificially maturatured Ghareb sample resembled closely that of the natural oils (Rosenberg et al., 2017). Such CSSIA studies on other oil groups in Israel may help in their classification into distinct genetic groups. The clear distinction by stable molecular "fingerprints" between samples from different groups and consistent similarity within each group, along with similar thermal maturity level of samples within each group, suggest that each of the genetic groups represents origin from a sole source rock and generation "kitchen". Furthermore, differences in biomarkers characteristics and 13C in the saturated and aromatic fractions provide indications as to the depositional environment and type of the oil's source rock. In general, this suggests that the Dead Sea, Meged and Coastal Plain oil samples were all generated from different marine carbonate-rich source rocks whereas the Mango oil was generated from a clastic source rock, probably with an input of some terrestrial organic matter. Biomakers and aromatic compound ratios of the continental margin Yam oil reveal distinctly different and more mature oil, but the source rock and depositional environment signals are somewhat ambiguous. The study of the oil shows suggests that there had been considerable generation of mature oil from at least five different active source rocks in different sub-terrains of the Levant Basin at the east Mediterranean Sea. The Upper Cretaceous Ghareb Fm. was identified with a fair confidence as the source rock of the Dead Sea group oil. However, there are still doubts regarding the source rocks for the other 4 genetic oil groups.