ABSTRACT:

Oils and asphalts (gilsonite) from the Dead Sea area, Israel, were characterized by bulk composition, specific C15+ hydrocarbons (n-alkanes, isoprenoids, steranes, triterpanes), and aromatics including sulfur compounds and light hydrocarbons (C2 to C8 molecular range). These indicators, complemented by literature data (sulfur content, sulfur isotope composition and distribution of petroporphyrins) define the oils and asphalts as belonging to a single geochemical province. The geochemical characteristics suggest a source material of bacterially reworked algae and a source rock poor in clay minerals, probably a calcareous rock.

Calcareous bituminous rocks of the Campanian-Maestrichtian Ghareb Formation are exposed on the margin of the Dead Sea graben, on downwarped blocks, and they probably extend under the thick graben fill. The agreement of the geochemical indicators suggests that these bituminous rocks are a potential source of the asphalts and oils in the Dead Sea area.

Differences between the various types of hydrocarbon occurrences are due to a combination of several effects such as variations in source material, the thermal history of the different downwarped blocks, water washing, and biodegradation. The distribution characteristics of hydrocarbons (C6 to C35 and C2 to C8) is sensitive to biodegradation and/or water washing. The aromatic fractions (except very soluble compounds like benzene, toluene, etc) are less affected by these processes and record the variability due to source and possibly thermal history. The interpretation of the triterpane and sterane distribution corroborates that of other hydrocarbons with respect to maturation and type of source rock. Differences in maturation are pronounced: the bituminous rocks and asphalts are immature, and the oils are derived from source rocks in an initial stage of petroleum generation.

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