R. Olaru1*, C. Krézsek1, T. M. Rainer2, C. Ungureanu1, V. Turi1, G. Ionescu1 and G. Tari2

1 OMV PETROM, Exploration and Appraisal, Bucharest 013329, Romania.

2 OMV, Exploration and Appraisal, Austria Vienna 1020, Austria.

*Corresponding author, email: radu.olaru@petrom.com

Key words: Romania, Black Sea, Maikop Group, source rocks, black shales, Oligocene, Miocene, basin modelling, migration, petroleum system.

Hydrocarbon discoveries in the Western Black Sea have proved the presence of both thermogenic and biogenic petroleum systems. The presence of Tertiary biomarkers in oils from the Romanian part of the Western Black Sea sub-basin, and correlation with Oligocene to Lower Miocene black shales, suggests that the thermogenic petroleum system is sourced mainly by the Oligocene – Miocene Maikop Group. Older source rocks may also be present locally in other parts of the sub-basin, but their contribution is currently poorly understood.

This paper presents the results of 3D basin modelling which was intended to evaluate charge models for prospects in the Western Black Sea sourced by the Maikop Group shales. The model is built on the regional-scale interpretation of recently acquired, long-offset 2D reflection seismic data, and was calibrated with proprietary and published well, geochemical and temperature data. The sensitivity of the thermal models on source maturity was tested. The basin models investigated two end-member heat-flow scenarios, "hot" and "cold". Whereas the "hot" model more successfully reproduces the field and well data in shelfal areas of the Western Black Sea, the "cold" model is considered to be more valid for deeper-water areas. Hydrocarbon expulsion maps were calculated for both scenarios at key stratigraphic levels, with preferential migration routes identified.

The results of the basin modelling suggest that the most likely source rocks for the oils in accumulations offshore Romania are located in the mid-Maikop Group (Upper Rupelian? to Chattian). Core data from offshore wells indicate that the source rocks consist of black shales with fair to good oil generation potential (TOC ~ 0.5 to 4.5%, HI <600 mg/g TOC, and mixed Type II/III kerogen). At the present day, these shales are in the early oil window offshore Romania to the SE of the producing fields, and in the wet gas window further to the east. Hydrocarbon expulsion from the mid-Maikop interval began during the Middle Miocene, but significant volumes of liquids were generated only in the Late Miocene with the peak of expulsion not yet reached. Charging the accumulations on the Romanian Shelf requires lateral migration along the base-Oligocene unconformity over distances of about 20-50 km. In addition, hydrocarbons have charged underlying Eocene and Cretaceous reservoir sections by lateral downward migration, filling structural traps and spilling over to higher structural levels. The results highlight the underexplored potential associated with the Maikop Group in the Western Black Sea.

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