AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Turbiditic facies distribution and BSR's relationships: A case study for the Western Black Sea

Abstract

Danube deep-sea fan is extended more than 100 km downslope the present-day shelf break, in an area with a wide shelf extension that covers offshore Bulgaria, Romania and Ukraine. This sedimentary system evolution from Neogene to Holocene have been largely studied by various teams, considering that the Western Black Sea Basin has been a prolific hydrocarbon region in the last decades, with some of the main Mio-Pliocene biogenic gas fields in Romania associated to the main Danube fairway (Ana&Doina, Pelican South, Domino, etc.). Multiple bottom-simulating reflectors (BSR’s) associated with free gas occur in several areas of the Black Sea. This study is focused on the distribution of those seismic features in relationship with the architecture of a single channel-levee complex branched from the Proto-Danube Plio-Pleistocene deep sea fan. The interpretation of the shallow section of a 3D seismic cube in the Han Asparuh Block (offshore Bulgaria) helps understanding the different seismic facies and the vertical and lateral relationships within the system. The stratigraphic distribution and sedimentological evolution of channel-levee complexes plays an important role in the entrapment of shallow gas, as demonstrated by some authors in different areas of the Western Black Sea.. In this case study seismic attributes (acoustic impedance, signal envelope, coherence, frequency, AVO…) have been used in order to enhance structural and stratigraphic features, BSR’s and possible fluid presence. BSR’s appear semi-parallel to the low-angle outer levees, roughly following the seabed geometry and enhancing some amplitudes in alternating porous layers. In the channel axis they mark the boundary between High Amplitude Reflectors (HAR’s) -caused by coarse-grained sediments and the overlying non-permeable hemipelagic deposits. This interpretation and methodology provide a practical study case of stratigraphic analogues for main depositional sub-environments within a deep sea fan system. In addition to that, BSR’s distribution and their association with free gas trapped within the channel-levee complexes plays an important role in detecting shallow hazards prior to offshore drilling campaigns. The approach presented in this study can imply its application on geohazards assessment of different exploratory areas worldwide, including analogues from the Oligo-Miocene turbiditic paleochannels in the West-African and Brazilian Atlantic margins. The relationship of shallow gas-related structures (BSR’s, chimneys, pockmarks…) and channel-levee complexes presented in this study have been also studied from the geochemical point of view of shallow hydrocarbon generation and migration in offshore areas of the Congo Basin.