SIMEONOVA, ANELIA, RS Group Ltd, Bulgaria; GEORGES GORIN, University of Geneva; and WERNER LEU, Geoform Ltd, Geneva

Abstract: Depositional Environments and Lithofacies Prediction - Deep Offshore, Northeast Bulgaria

This study presents an example of integrated application of seismic facies analysis and computer stratigraphic modeling to unraveling sedimentary evolution of the deep offshore, Northeast Bulgaria. It aims at predicting depositional paleoenvironments, main lithologies and facies patterns of the Late Neogene-Quaternary deposits in the present-day outer shelf, slope and basin floor.

The study area lies within the frontier Western Black Sea Deep Basin, which has undergone a different geological evolution, compared to the surrounding thrust belts (Fig. 1). The compressive deformations have affected and modified only the marginal areas of the basin. The stratigraphic record onshore and on the shelf is therefore only of limited use for predicting lithostratigraphy in the deep-water. There is no direct information on the nature of the late Neogene slope and basin floor deposits within the study area. The closest wells Nanevo-1 and Elizabetino-1 (for location see Fig. 1) are not very useful for calibrating the seismic data. Both are drilled on the present-day shelf, where only a thin uppermost Neogene section is present.

Seismic facies analysis is an essential tool for better understanding the stratigraphic evolution of the deep Bulgarian offshore, and may contribute to extend the petroleum exploration seawards. In the present study, seismic stratigraphic analysis has been applied to define sequence stratigraphic systems tracts and to predict depositional environment, associated lithologies and facies distribution. Two prominent unconformities are documented on seismic in this region. We assign both truncation surfaces to type I sequence boundaries, associated with major sea level falls within the West Black Sea Basin - in the late Middle Miocene (10.5Ma) and in latest Miocene (5.5Ma). Six seismo-stratigraphic units have been recognized along the regional seismic line E92BG01-09, based on reflection configuration patterns, amplitude, frequency, continuity and terminations (Fig. 2). These units are interpreted as the seismic expression of sequence stratigraphic systems tracts. Lateral seismic facies changes within each seismo-stratigraphic unit are carefully examined and interpreted as lithofacies variations from proximal to distal areas.

Computer stratigraphic modeling (PHIL ^TM ) has been performed to support the stratigraphic and lithofacies predictions. The seismic stratigraphic observations and interpretations have been used to model a WNW-ENE trending cross-section, coinciding with the studied regional seismic line. Basin subsidence, sea-level history (short term, Haq. et al., 1987) and sedimentary influx were the main key variables at the modeling input. Though poorly constrained (no calibration wells) the generated model provides insights into the basin fill history, paleoenvironments and facies distribution in the deep offshore section, throughout the time interval studied.

Both seismic facies analysis and the computer generated model indicate the presence of basin floor and slope fan complexes in the deep-water domain. Conceptual models (Posamentier & Vail, 1988, Mitchum et al., 1993) suggest sand-rich features associated with the deposition of lowstand fans. Consequently, coarse-grained siliciclastics with subordinate intercalations of finer-grained to muddy lithologies may be expected in the deep offshore of Bulgaria. This sand-prone facies may represent attractive petroleum targets for future exploration..The authors are grateful to Enterprise Oil (UK) for giving the permission to present data from the seismic line E92BG01-09.