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№ 11 (November 2011)

Three-Dimensional Geological Structure of Sedimentary Basins Based on 3D Seismic Data Analysis

   Over the last years c Oil Company has conducted a great number of 2D/3D seismic surveys in order to prepare for drilling the prospective targets located in the deepwater part of the Black Sea. The survey area is located in the Eastern Black Sea basin and is confined within the Shatsky Ridge and the Tuapse Trough. 

By Almendinger O.A., Mityukov A.V., Myasoyedov N.K., RN-Shelf-Yug Nikishin A.M., Department of Geology, Lomonosov Moscow State University, Hayduk V.V., Gubarev M.V., NK Rosneft-NTC The article was reprinted from the Scientific and Technical Bulletin of NK

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   The area is characterized by a complex structure of the target Maikop Sequence; the key tasks are to build a correct tectonic sketch of folds and associated faults, to discover traps in the underthrust part, to prospect for potential reservoir development zones. It is not possible to reliably solve such problems based on 2D seismic data in complex seismic and geological conditions; in this respect, 3D seismic surveys were carried out. Interpretation of new data has allowed thoroughly studying the basic features of sedimentation environment and suggesting a model of target sequence sedimentation.
Types of folds. At the turn of Eocene and Oligocene and in connection with the start of formation of the fold-mountain structure of the Greater Caucasus, a flexure trough – the Tuapse Trough – began to form within the eastern part of the Shatsky Ridge. The highly folded structures have occurred over the last 5 million years [1, 2]. The fold structures are associated with the formation of the system of subparallel upthrow/overthrust faults with the detachment surface in the lower shale part of the Maikop Sequence. The most ancient folds are located in the backarch part of the Tuapse Trough inside the zone of bordering with the southern shoulder if the Greater Caucasus. The most recent folds are located in the western forefront of the Tuapse Trough in the area of its jointing with the Shatsky Ridge. Analysis of 3D seismic data has allowed correlating tectonic faults and revealing the fact that two basic types of folds are formed during development of tectonic deformations: fault propagation folds (i.e. the folds associated with a growth of a fault) and detachment folds (Fig. 1, 2). At late formation stages, a part of folds are developing by pop-up mechanism with formation of bottomless folds (Fig. 1, 3).

   Understanding of the mechanism of fold formation and classification is important to reconstruct the history of sedimentation and to build a geological model. Fault propagation folds (Fig. 1, 2) occur when a detachment moving along the strike-oriented layer grades into an upthrow fault or an overthrust fault which shears the bedding (so called ramp, i.e. a slip fault which shears the bedding). Horizons with plastic clays are expected to be confined in the Maikop Sequence cross-section. A detachment or a system of detachments occurs in clays of low Maikop Sequence. The Maikop strata and recent deposits began to slip in the direction of the Shatsky Ridge over the detachment surface under the pressure associated with deformation development in the Caucasus. In these circumstances a series of upthrow faults and overthrust faults occurred and their growth resulted in formation of anticlinal folds over the fault.

   Detachment folds (Fig. 1) occur when there are lots of plastic rocks and lots of harder rocks above. If such a rock mass is compressed with a detachment at the bottom of the lower plastic strata, the plastic part will slip and appear to be a slipping surface while the upper part will collapse in folds without faults. The main difference between the detachment folds and fault propagation faults is the fact that the detachment folds are formed at the beginning while they can be additionally complexed by faults. During the fault propagation fold formation, the fold formation is associated with the presence of a fault. In fact, combined folds are frequently observed in the cross-section of the Tuapse Trough.
Most upthrown/overthrust structures of the Tuapse Trough are represented in today’s topography in the form of underwater ridges with a relative increase of seabed level of up to 400-600 meters (Fig. 4). This indicates that the fold growth is currently ongoing.

   Sedimentation Processes. Since its formation, the Tuapse Trough has been extensively filled with deepwater turbidite sediments. Within the limits of the Tuapse Trough the Maikop time (Oligocene – early Miocene) saw the formation of a sedimentary lens which is thinning both towards the Caucasus and in the direction of the Shatsky Ridge. This lens was filled with sediments that had been formed by erosion of rocks forming the growing orogenic belt of the Greater Caucasus. Based on geological knowledge on the structure of the region, results of 2D and 3D seismic data interpretation as well as the analysis of land well data and coastal outcrops, it was concluded that the study area cross-section may include elements of deepwater depositional systems – fans, lobes, channels. Sediment supply was controlled by a dendritic coastal system of paleorivers.
The acquired 3D seismic data allowed identifying and mapping the main parts of the fan complex within the study area – proximal part with feeder channels (Fig. 5), central part with axial channels (Fig. 6), and distal part presented with covers. Understanding of the fan structure is the key in mapping of reservoirs over the area. Depending on which part the trap is in – proximal, central or distal, the trap prospects are determined. This problem was one of the key tasks in the analysis of 3D seismic data.

   It was found that the change in the fan orientation in plan view is observed over time. For the intervals for lower and middle Oligocene, the fan orientation is characterized by northwest strike, i.e. parallel to the present-day position of the coastline (Fig. 5). This, in turn, allows relying upon dominating development of sedimentation carried by paleovalleys of the Mzymta and Bzyb Rivers. These rivers have their origins in the central Caucasus with the developed extensive outcrops of granitic masses which are the best source for formation of quartz sandstone observed in cross-sections of coastal outcrops and boreholes of Sochi-Adler land area. Intervals of Upper Oligocene – Lower Miocene (upper part of the Maikop series) are characterized by the change of fan complex direction. Body of fans and their lobes are oriented at an acute angle to the shoreline (Fig. 6). It is probably due to the formation of additional sources of drift in the North Caucasus.

   Conclusions. (1) The use of 3D seismic data to construct a geological model and to locate wells is a necessary condition in the areas with complex seismological structure. (2) As a result of the work, target intervals of the cross-section were analyzed in detail, the tectonic fault model was constructed, and facies analysis was carried out. (3) Intervals and zones of the most probable development of reservoirs were identified. (4) The completed work allowed to build the detailed geological model of the study area.  


Peferences.
Afanasenkov A.P., Nikishin A.M., Obukhov A.N. Geological Structure and Hydrocarbon Potential of the Eastern Black Sea. Moscow, Nauchny Mir, 2007, 172 pages.
Tugolesov D.A., Gorshkov A.S., Meisner L.B., Soloviev V.V., Khakhalev E.M., Akilova Yu.V, Akentieva G.P., Gabidulinf I.N., Kolomeitseva S.A., Kochneva T.Yu., Pereturina I.G., Plashikhina I.N. Meso-Cenozoic Tectonics of the Black Sea Basin Sediments Moscow, Nedra. 1985, 185 pages.

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