--> --> Thermal history and hydrocarbon generation potential of the southern Western Black Sea Basin (Bulgaria)

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Thermal history and hydrocarbon generation potential of the southern Western Black Sea Basin (Bulgaria)

Abstract

The Western Black Sea Basin (WBSB) is a large basin filled with 15+ km thick Cretaceous syn-rift and Cenozoic post-rift sediments. During the last decades some 20 exploration wells have been drilled offshore Bulgaria, exclusively on the shelf, resulting in the discovery of 3 gas/condensate fields. The main source rock (SR) in the WBSB is the oil/gas-prone Oligocene Ruslar (Maykop) Formation. Additional potential is provided by the gas-prone Upper Eocene Avren Formation and oil/gas-prone Jurassic strata (e.g. Etropole Formation). The latter is thought to act as SR for the Tjulenovo oil/gas field, which extends along the Bulgarian coast, partly also offshore. A syn-rift Lower Cretaceous SR remains speculative at present. Well data show that the Cenozoic source rocks at the Bulgarian shelf are thermally immature. No break in coalification between Mesozoic rocks and overlying basin fill has been found so far. Jurassic shales are at the onset of the oil window. The thermal maturity of Carboniferous coals range from the oil to the dry gas window, possibly influenced by Permian volcanism. In order to unravel the thermal history and the timing of hydrocarbon generation in the undrilled deep water parts of the basin, a 3D basin model was created for the WBSB using PetroMod® modeling software. Paleo-Heat flow maps were created using new and published data. Heat flow at the shelf is in the range of 40–50 mW/m2, decreasing in the deep segments of the basin to <30 mW/m2. The modeling results show the oil window at ∼3800 m below sediment surface, the onset of the wet gas window is expected at ∼6500 m. At the slope the Oligocene Ruslar Formation and the Eocene Avren Formation are in the oil window. Main hydrocarbon generation occurred since Early Miocene. The speculative Lower Cretaceous syn-rift SR entered the oil window during Late Oligocene and is currently at the onset of the wet gas window. In the center of the WBSB the Oligocene SR reaches locally the onset of the wet gas window. Lower Cretaceous strata are in the dry gas window. The Messinian crisis had minor to no impact on the maturity, but might have influenced seal integrity. The 3D basin modeling result using thermogenic kinetics is clearly encouraging for the unexplored deepwater parts of the WBSB. The basinwide extent and significance of the biogenic petroleum system encountered in the Galata, Akcakoca fields and in the Domino-1 discovery require additional modeling as biogenic kinetics need to be applied.