--> --> 3-D Architecture of Cretaceous Channel Systems in the Northern Orange Basin, Offshore South Africa

2018 AAPG International Conference and Exhibition

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3-D Architecture of Cretaceous Channel Systems in the Northern Orange Basin, Offshore South Africa

Abstract

The integration of seismic, well and core data provided the basis for characterizing the depositional framework and geomorphology of Cretaceous fluvial and marine channel systems offshore the northern Orange Basin. In this study, the morphologies of the depositional systems were investigated using time, variance and RMS amplitude seismic attributes on high-resolution 3D seismic. Lithological information and depositional environments were obtained from well logs and core data of exploration wells drilled on the shelf. These environments range from braided and fluvial-meandering channel systems to fluvio-deltaic systems from the Aptian to Cenomanian. The depositional geometries identified within the systems include flood plains, splays, point bars, levees and cut bank deposits. Channel widths vary between 50m to 2km. In the Aptian-Albian interval in the north eastern part of the Orange basin, channels are controlled by curvilinear faults dipping towards the southwest. Dominant faults visible within this sequence dissipate towards the west. Wider channel systems within the Cenomanian sequence are intersected by extensive northwest-southeast trending faults. These faults disrupted channel deposition and would compartmentalise potential reservoir sands if they are sealing. Distinct thin sinuous channels occur throughout the Cretaceous and are laterally extensive from north to south. Channel system patterns in the Aptian-Albian sequence are disorganised while those in the Cenomanian are organised. The fill ratios of the channel systems are higher in the Aptian-Albian than in the Cenomanian. The paleo current direction during the mid Aptian through Early Albian switched between east and southeast. During the Cenomanian the paleo current direction shifted between the northeast and southeast. This suggests a strong influence by the paleo topography. Further offshore, these systems progress from a slope environment with large scale marine channels into a deep marine environment with basin and slope floor fan features recognised on seismic. Potential reservoirs include pro-delta/sheet-like turbidites, deltaic and fluvial sands, slope channel complexes and basin floor fan systems. Source rocks charging these Cretaceous reservoirs include the proven Barremian-Aptian restricted marine oil and gas prone source as well as a postulated Cenomanian-Turonian aged source rock.