FEINSTEIN, SHIMON, Ben Gurion University of the Negev, Beer Sheva, Israel, ZEEV AIZENSHTAT and IRENA MILOSLAVSKI, The Hebrew University, Jerusalem, Israel, JOOP SLAGER, Isramco Inc., Tel Aviv, Israel, PETER GERLING, Federal Institute for Geoscience and Natural Resources (BGR), Hannover, Germany, and LLOYD R. SNOWDON, The Institute of Sedimentary and Petroleum Geology, Calgary, Alberta, Canada

ABSTRACT: Migrational Stratification of Hydrocarbons in the Eastern Mediterranean Basin

The Yam l and Yam 2 wells drilled in the eastern Mediterranean encountered a sequence of gas shows in the Cretaceous and Upper Jurassic section and a liquid oil in the underlying Middle Jurassic.

The gases vary in the hydrocarbon composition with depth from dry to wet gas with condensates, in the stable carbon and deuterium isotopes, and in the nonhydrocarbon constituents. The data obtained reveal that despite the differences indicated, all the gas shows (including the dry gas) are oil associated and thermally derived at a maturity level between 0.7 to 0.9% Ro equivalent. Stable carbon and hydrogen isotope data further suggest an admixture with variable amounts of biogenic gas, mainly methane.

The concentration of the gas shows in the same vertical section, and the fact that, despite the differences indicated, they are all oil-associated suggests a common source, possibly the underlying oil. In this respect, the differences in hydrocarbon composition between the shows and their systematic distribution in the vertical section may be attributed to molecular partitioning and geochromatographic effects with time and distance via migration of the gas phase from the parent oil. However, differences in the maturity indicated by the stable isotope composition possibly suggest different expulsion times.

Marked differences in the concentration and of (delta)13C, the CO2 associated with the various shows, suggest different origins for the CO2. This implies that the presence of the CO2 is a local phenomenon not related to the major oil source nor to the biogenic gas.

The oil discovered in the Middle Jurassic section is light and contains relatively large proportions of low molecular weight fraction. The chemical composition and the stable carbon and deuterium isotopes reflect, in general, a high level of thermal maturity. Some of the characteristics suggest high maturity within the "liquid window" range (ca. 0.9% Ro equivalent), whereas others appear to indicate "super" maturation. The present-day temperature of the reservoir is about 180 degrees C. The composition of the oil, however, appears to represent no long term "equilibrium" with such high temperature, suggesting a short lived, dynamic state for the studied reservoir.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.