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New Insights into the Origin of Gas Based on the History of the Thermal Regime in the Levant Basin (Eastern Mediterranean)

Abstract

The Levant Basin is becoming a prolific hydrocarbon province. Significant gas discoveries have been made in the sandy subsalt Oligo-Miocene sections. Smaller discoveries have also been made in the Plio-Pleistocene post-salt sediments in the southern part of the basin. The origin of gas in the basin is widely debated. Although the biogenic source of the gas is commonly accepted, the thermogenic source cannot be excluded. In addition to the chemical and isotopic composition of gas, basin modelling can provide some insights into the origin of gas in the basin. Biogenic gas is generated in the sediments under specific conditions of temperature and burial history. Thermogenic gas requires higher temperatures and/or deeper burial of the source rocks. Since the origin of gas is largely determined by the temperature of the source rock, reconstructing the thermal regime in the basin is important for understanding the origin of gas. The Levant basin is characterized by low present-day heat flow ranging between 30 to 40 mW/m2. The relatively low thermal gradient could favour the generation of biogenic gas in Paleogene-Neogene formations. The deep burial of any Cretaceous and Jurassic source rocks may lead to thermogenic gas generation. In this study we reconstructed the thermal history of the Levant Basin and we evaluate its influence on the history of biogenic and thermogenic gas generation using basin modelling concepts. We used the tectonic history of the basin and the characteristics of the deep crustal structure to reconstruct the history of basal heat flow in the basin. The new thermal model takes into account variations in basal heat flow, radiogenic heat generation in the sediments, surface temperatures and paleo water depths. The results show significant variations in heat flow that reached in some geological times up to 80% of the present-day values. This could have important consequences for the generation of biogenic and themorgenic gas in the basin. The history of hydrocarbon generation was modelled for deep Mesozoic source rocks and for Tertiary formations. Special attention was paid to the generation of biogenic gas from Oligo-Miocene and Plio-Pliocene formations. Expulsion of generated gases and the presence of the Miocene salt seal were used to constrain the petroleum chart and to identify the critical moment of the biogenic and thermogenic gas systems. This helped to understand the contribution of each gas component to the total gas accumulation.