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POPESCU, I. S., D.TAMBREA*, and A. RAILEANU, Geological Exploration Research and Design Center, Bucharest; D. LUTAC, and M. SISMAN, Petromar Constanta; Romanian Oil Corporation Petrom S.A., Romania

Abstract: Integrated Seismic and Depositional Facies Analysis of Muddy Carbonate Turbidite Systems in Istria Depression, Black Sea, Romania. Implications on Hydrocarbon Generation

The studied area is located in the eastern part of Romanian Black Sea shelf, on the northern flank of Istria Depression, which is the North Dobrogea Orogene extension into the basin.

Geological setting determined in-place deposition of oil-prone turbidite siliciclastic bodies and randomly carbonate ones, with unexpected implications on hydrocarbon generation. Hence, the target of this paper was to define the distribution and evolution of such carbonate turbidite complex, muddy type, with high organic content.

To outline the depositional model, seismic lines and data from three wells have been used, achieving an integrated stratigraphical, sedimentological, geochemical and geophysical interpretation.

Istria Depression is characterised by a rift tectonic style developed in Cretaceous, the rifting functioning towards south on faulted blocks, in a parallel trend to the depression boundary.To the south, the depression is delimited by Peceneaga-Camena Fault. Upper Cretaceous-Middle Eocene Heracleea Plateau borders the depression to the north.

The considered area sets in a basin similar to the ponded ones, controlled by northwest-southeast trending faults. Incised valleys and canyons systems originated from these faults.

The basin was deep, narrow, sediment starved, accumulating mainly shelf muddy decollement material, pelagic and hemipelagic sediments alternating with turbidite sediment influx.

On the seismic lines, the carbonate turbidite complex has been recognized by dominantly subparallel, high amplitude reflections that downlap basinwards.There are lateral changes towards slight convergent reflections with moderate amplitude. Low velocity Oligocene basinal shales (2100-2200 m/s) cover high velocity Upper Eocene carbonate turbidite complex (3800-4000 m/s).The turbidite body top provides a strong impedance contrast. Using a zero phase pulse, a strong peak occurs on the top and a strong trough on the base. The variation of the turbidite complex thickness affects the seismic response quality along the basal reflection.

The Upper Cretaceous-Middle Eocene, muddy carbonate shelf margin of Heracleea Plateau was the line source for the sediments that flowed onto the gentle deep gradient slope at about 1000 in difference in level. Concurrently, intermittent submarine movements evolved proximally as mud flow and distally turbidity currents through canyons only in high-energy stages. Pelagic and hemipelagic rain prevailed in quiet stages.

The muddy carbonate turbidite complex was considered to be composed of superimposed turbidite systems developed in Upper Eocene. The complex is small, it has a northeast-southwest elongate trend, 10 km long, 5.5 km wide and 150 in thick, In the carbonate turbidite facies associations, recurrent depositional episodes were encountered on cores, each presenting the complete or incomplete Bouma Sequence type of distal turbidites.AB type divisions prevail.

The Histria well was drilled in a distal muddy carbonate turbidite system with rich in.organic matter content (0.571.43 % g TOC) in laminated argillaceous limestone. The geochemical index ratios showed the presence of a type II kerogen. In massive or graded skeletal packstone, low porosity was recorded due to the diagenetic effects. The burial history diagram shows that the Upper Eocene carbonate turbidite deposits are in the oil window. The main phase of generation began 1.3 MA ago.
 
 

Figure 1. Southwest - northeast seismic line across the turbidite system.

Figure 2. Depositional setting and geometry of turbidite system.

Figure 3. Burial history diagram.

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