Sedimentology and Timing of Hydrocarbon-seepage (Lower Eocene, Varna, Bulgaria)

Eva De Boever¹, Rudy Swennen², and Lyubomir Dimitrov^3
1K.U. Leuven, 3001 Leuven, Belgium
²K.U.Leuven, 3001 Heverlee, Belgium
³Institute of Oceanology, 9000 Varna, Bulgaria

In the Pobiti Kamani area (Varna, NE Bulgaria), Lower Eocene sandy sediments contain several clusters of up to 8m high calcite-cemented chimney structures. δ¹³C values as low as -43‰ V-PDB indicate a hydrocarbon-seepage related origin. The depositional sequence of the shallow marine platform sediments is characterized by several cemented stratal surfaces which are cross cut by chimney structures. In this contribution, the origin of the cemented surfaces is addressed based on sedimentological, petrographical and stable isotope geochemical data and the implications with respect to the timing of hydrocarbon seepage are evaluated. Grain size measurements in two continuous vertical sections allow to distinguish two depositional sequences. Transgressive (TS) and maximum flooding (MFS) surfaces are characterized by extensive calcite cementation, thus indicating a sequence stratigraphical control on cementation. Different cement-types have been recognized. The bulk stable isotope signature of these cements indicates precipitation from Lower Eocene marine pore fluids, affected by later meteoric resetting. δ¹³C depletions of the dominant pore cementing “mosaic” cement as low as -20.6‰ V-PDB however supports also a pre-compactional influence of hydrocarbon-seepage which decreases within m-distance from chimney clusters. The MFS near the top of the Dikilitash Formation is partly cemented by transparent poikilotopic calcite in keystone-type vugs and in interparticular porosity. Its very early diagenetic origin and δ¹³C depletion (-16‰ V-PDB) suggest that hydrocarbon-bearing fluids percolated through the sandy sediments near the δ seafloor at the end of the Upper Ypresian. Other coarse-grained,¹³C depleted (-26‰ V-PDB) concretionary horizons likely resulted from post-sedimentary lateral migration of seepage fluids.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece