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Facie, Provenance and Reservoir Quality of the Valanginian Age Sandstone of the Bredasdorp Basin, South Africa

Fadipe, Oluwaseun A.; Carey, Paul F.; Donker, Jan Van B.
Earth Science Department, University of the Western Cape, Cape Town, South Africa.

The Bredasdorp Basin, offshore South Africa, formed as a result of Gondwana break-up and rifting during the middle-late Jurassic period. Dextral trans-tensional stress which was produced by the breakup of Gondwanaland which occurs to the east between the Falkland Plateau and the Mozambique Ridge, initiated normal faulting north of the Agulhas-Falkland fracture Zone. The F-O tract is located in the eastern part of Block 9 on the northern flank of the Bredasdorp Basin, a sub-basin of the Outeniqua basin on the southern continental shelf, offshore South Africa.

This work examines sandstone samples from F-O2 well and attempt to clarify facie associations, sediment provenance and post depositional diagenetic modification of these potential reservoir sands through a combination of analytical techniques including thin section petrographic characterization, X-ray difractometry (XRD), X-ray fluorescence (XRF) analyses (major element analysis after Al-Harbi and Khan, 2008). The data generated from these analytical techniques have been utilized to interpret the diagenetic and geochemical variation and development of the Valanginian sands of this well as a characteristic of potential play.

The principal Valanginian age reservoirs are tight, highly faulted shallow marine sandstones beneath the drift-onset unconformity. Five lithofacies were interpreted from the cored section of the F-O2 well (mudstone, heterolithic sandstones, massive sandstone, silty-stone with foreset mud-drapes, sandy centimetric heteroliths). The sandstones have detrital mineralogy dominated by quartz, K-feldspar, stylolite, and ranges from greywacke - litharenite. Diagenetic modifications include feldspar and lithic fragment dissolution, compaction and reduction of the depositional porosity through grain rearrangements, rotation and fragmentation of grains and cementation (primarily quartz and carbonate). Relatively early formation of smectite and kaolinite with authigenic chlorite, followed quartz cementation occurred in an environment where the movement of formation waters was relatively unrestricted.


AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012