Thesis (MSc)--University of Stellenbosch, 2006. / ENGLISH ABSTRACT: The Barremian to Albian siliciclastic deep-water deposits of the central
Bredasdorp Basin were investigated primarily in terms of their stratigraphic
evolution, depositional characteristics and facies distribution. Cores from the
deep-water deposits reveal that the facies successions are composed of
massive, ripple cross- to parallel-laminated sandstones, conglomerate,
massive claystone, alternating laminated to interbedded sandstone/siltstone
and claystone, laminated and clay-rich siltstone. These facies are grouped
into channel-fill, sheet-lobe, overbank and basin plain deposits, by inference.
The application of sequence stratigraphy, based on gamma ray and resistivity
log patterns, reveals that all 3rd-order depositional sequences comprise 4thorder
cycles. The latter are subdivided into three components (lowstand,
transgressive and highstand systems tracts), based on vertical facies changes
and internal stratigraphic key surfaces.
Taking the 13Amfs as the stratigraphic datum for each well, correlation was
possible on a regional basis. Lowstand deposits, comprising thick
amalgamated massive sandstones, were interpreted to represent channelfills.
Their vertical and horizontal stacking forms channel-fill complexes above
Type 1 unconformities. Adjacent thin-bedded intervals, comprising parallel- to
ripple cross-laminated sandstones, were interpreted as levee/overbank
deposits, whereas clay-rich intervals were interpreted to represent basin plain
deposits of hemipelagic origin. Facies associations and their distribution have revealed that channel-fills are
associated with overflow deposits and sheet sand units. These deposits, as
well as downdip sheet sands associated with small channel-fills within the 9A,
11A/12A, 13A Sequences and the 14A Sequence were interpreted to have
been deposited in a middle fan to upper fan setting. A similar association
occurs in the 10A Sequence, except that thick conglomerate units are present
at the base of proximal channel-fills. This led to interpret the 10A Sequence
as being deposited in a base-of-slope to upper fan setting.
The thickness of each sequence, as revealed by isochore maps, shows
sinuous axial flow path which corresponds to channel-fill conduit. The
continuous decrease of this sinuosity upward in the succession was
interpreted as being related to basin floor control along the main sand
fairways. Successive flows result in erosion-fill-spill processes, which locally
favour connectivity of reservoirs over large areas.
Recognition of higher-order sequences and key stratigraphic surfaces helps to
understand internal stratigraphic relationships and reveals a complex and
dynamic depositional history for 3rd-order sequences. However, sparse well
control and uneven distribution of boreholes, as well as lack of seismic and
other data, limited the models derived for this study. / AFRIKAANSE OPSOMMING: Die Barremiaanse tot Albiaanse silisiklastiese diepwater afsettings van die
sentrale Bredasdorp Kom is hoofsaaklik in terme van stratigrafiese evolusie,
afsettingskarakteristieke en fasies distribusie ondersoek. Kerne van die
diepwater afsettings toon dat die fasies opeenvolgings uit massiewe,
riffelkruis- tot parallel-gelamineerde sandstene, konglomerate, massiewe
kleistene, afwisselende gelamineerde tot intergelaagde sandstene/slikstene
en kleistene, sowel as gelamineerde en klei-ryke slikstene bestaan. Hierdie
fasies word onderverdeel in kanaalopvulsel, plaatlob, oewerwal en komvlakte
afsettings.
Die toepassing van opeenvolgingsstratigrafie gebaseer op gammastraal en
resistiwiteit log patrone toon dat alle 3de-orde afsettingsopeenvolgings uit 4deorde
siklusse bestaan. Laasgenoemde word onderverdeel in drie komponente
(lae-stand, transgressie en hoë-stand sisteemgedeeltes), gebaseer op
vertikale fasies veranderinge en interne stratigrafiese sleutel vlakke.
Korrelasie op ‘n regionale basis is moontlik gemaak deur die 13Amfs as die
stratigrafiese verwysing vir elke boorgat te neem. Lae-stand afsettings, wat uit
dik saamgevoegde massiewe sandstene bestaan, word geïnterpreteer as
kanaalopvulsels. Die vertikale en horisontale stapeling van die sandstene
vorm kanaalopvulsel komplekse bo Tipe 1 diskordansies. Naasliggende dungelaagde
intervalle, wat uit parallel- tot kruisgelaagde sandstene bestaan,
word geïnterpreteer as oewerwal afsettings, terwyl klei-ryke intervalle geïnterpreteer word as verteenwoordigend van komvlakte afsettings van
hemipelagiese oorsprong.
Fasies assosiasies en hul verspreiding toon dat kanaalvul geassosieër word
met oorvloei afsettings en plaatsand eenhede. Hierdie afsettings, sowel as
distale plaatsande geassosieër met klein kanaalopvulsels binne die 9A,
11A/12A, 13A en die 14A Opeenvolgings, word geïnterpreteer as afgeset in ‘n
middelwaaier tot bo-waaier omgewing. ‘n Soortgelyke assosiasie bestaan in
die 10A Opeenvolging, behalwe dat dik konglomeraat eenhede teenwoordig is
by die basis van proksimale kanaalopvullings. Dit het gelei tot die interpretasie
van die 10A Opeenvolging as afgeset in ‘n basis-van-helling tot bo-waaier
omgewing.
Die dikte van elke opeenvolging, soos verkry vanaf isochoor kaarte, toon ‘n
kronkelende aksiale vloeipad wat ooreenkom met ‘n kanaalopvulling
toevoerkanaal. Die aaneenlopende afname van hierdie kronkeling na bo in die
opeenvolging word geïnterpreteer as verwant aan komvloer-beheer langs die
hoof sand roetes. Opeenvolgende vloeie veroorsaak erosie-opvul-oorspoel
prosesse, wat lokaal die konnektiwiteit van reservoirs oor groot areas
bevoordeel.
Herkenning van hoër-orde opeenvolgings en sleutel stratigrafiese vlakke dra
by tot ‘n goeie begrip van die interne stratigrafiese verhoudings en ontbloot ‘n
komplekse en dinamiese afsettingsgeskiedenis vir 3de-orde opeenvolgings.
Beperkte boorgatbeheer en ‘n tekort aan seismiese en ander data het egter ‘n
beperkende rol gespeel in die daarstel van modelle vir hierdie studie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/17400 |
| Date | 12 1900 |
| Creators | Megner-Allogo, Alain-Cedrique |
| Contributors | Wickens, H. de V., Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xxiv, 178 leaves : ill., maps |
| Rights | Stellenbosch University |
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