Sedimentology and stratigraphy of deep-water reservoirs in the 9A to 14A Sequences of the central Bredasdorp Basin, offshore South Africa

[摘要] ENGLISH ABSTRACT: The Barremian to Albian siliciclastic deep-water deposits of the centralBredasdorp Basin were investigated primarily in terms of their stratigraphicevolution, depositional characteristics and facies distribution. Cores from thedeep-water deposits reveal that the facies successions are composed ofmassive, ripple cross- to parallel-laminated sandstones, conglomerate,massive claystone, alternating laminated to interbedded sandstone/siltstoneand claystone, laminated and clay-rich siltstone. These facies are groupedinto channel-fill, sheet-lobe, overbank and basin plain deposits, by inference.The application of sequence stratigraphy, based on gamma ray and resistivitylog patterns, reveals that all 3rd-order depositional sequences comprise 4thordercycles. The latter are subdivided into three components (lowstand,transgressive and highstand systems tracts), based on vertical facies changesand internal stratigraphic key surfaces.Taking the 13Amfs as the stratigraphic datum for each well, correlation waspossible on a regional basis. Lowstand deposits, comprising thickamalgamated massive sandstones, were interpreted to represent channelfills.Their vertical and horizontal stacking forms channel-fill complexes aboveType 1 unconformities. Adjacent thin-bedded intervals, comprising parallel- toripple cross-laminated sandstones, were interpreted as levee/overbankdeposits, whereas clay-rich intervals were interpreted to represent basin plaindeposits of hemipelagic origin. Facies associations and their distribution have revealed that channel-fills areassociated with overflow deposits and sheet sand units. These deposits, aswell as downdip sheet sands associated with small channel-fills within the 9A,11A/12A, 13A Sequences and the 14A Sequence were interpreted to havebeen deposited in a middle fan to upper fan setting. A similar associationoccurs in the 10A Sequence, except that thick conglomerate units are presentat the base of proximal channel-fills. This led to interpret the 10A Sequenceas being deposited in a base-of-slope to upper fan setting.The thickness of each sequence, as revealed by isochore maps, showssinuous axial flow path which corresponds to channel-fill conduit. Thecontinuous decrease of this sinuosity upward in the succession wasinterpreted as being related to basin floor control along the main sandfairways. Successive flows result in erosion-fill-spill processes, which locallyfavour connectivity of reservoirs over large areas.Recognition of higher-order sequences and key stratigraphic surfaces helps tounderstand internal stratigraphic relationships and reveals a complex anddynamic depositional history for 3rd-order sequences. However, sparse wellcontrol and uneven distribution of boreholes, as well as lack of seismic andother data, limited the models derived for this study.

[发布机构] Stellenbosch University