Thesis (PhD)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: This study investigates the crustal structure, and assesses the qualitative and quantitative impacts of crust-mantle dynamics on subsidence pattern, past and present-day thermal field and petroleum
system evolution at the southern South African continental margin through the application of a
multi-disciplinary and multi-scale geo-modelling procedure involving both conceptual and
numerical approaches. The modelling procedure becomes particularly important as this margin
documents a complex interaction of extension and strike-slip tectonics during its Mesozoic
continental rifting processes. Located on the southern shelf of South Africa, the Western
Bredasdorp Basin (WBB) constitutes the focus of this study and represents the western section of
the larger Bredasdorp sub-basin, which is the westernmost of the southern offshore sub-basins. To
understand the margin with respect to its present-day structure, isostatic state and thermal field, a
combined approach of isostatic, 3D gravity and 3D thermal modelling was performed by integrating
potential field, seismic and well data. Complimenting the resulting configuration and thermal field
of the latter by measured present-day temperature, vitrinite reflectance and source potential data,
basin-scale burial and thermal history and timing of source rock maturation, petroleum generation,
expulsion, migration and accumulation were forwardly simulated using a 3D basin modelling
technique. This hierarchical modelling workflow enables geologic assumptions and their associated
uncertainties to be well constrained and better quantified, particularly in three dimensions.
At present-day, the deep crust of the WBB is characterised by a tripartite density structure (i.e. prerift
metasediments underlain by upper and lower crustal domains) depicting a strong thinning that is
restricted to a narrow E-W striking zone. The configuration of the radiogenic crystalline crust as
well as the conductivity contrasts between the deep crust and the shallow sedimentary cover
significantly control the present-day thermal field of the study area. In all respects, this present-day
configuration reflects typical characteristics of basin evolution in a strike-slip setting. For instance,
the orientations of the deep crust and fault-controlled basin-fill are spatially inconsistent, thereby
indicating different extension kinematics typical of transtensional pull-apart mechanisms. As such,
syn-rift subsidence is quite rapid and short-lived, and isostatic equilibrium is not achieved,
particularly at the Moho level.
Accompanied syn-rift rapid subsidence and a heat flow peak led to petroleum preservation in the
basin since the Early Cretaceous. Two additional post-rift thermal anomalies related to the Late
Cretaceous hotspot mechanism and Miocene margin uplift in Southern Africa succeeded the syn-rift
control on maturation. This thermal maturity of the five mature source rocks culminated in four
main generation and three main accumulation phases which characterise the total petroleum systems
of the WBB. The Campanian, Eocene and Miocene uplift scenarios episodically halted source
maturation and caused tertiary migration of previously trapped petroleum. Petroleum loss related to
the spill point of each trap configuration additionally occurs during the Late Cretaceous-Paleocene
and Oligocene-Early Miocene. The timing and extent of migration dynamics are most sensitive to
the geological scenario that combined faulting, intrusive seal bypass system and facies
heterogeneity. In fact, for models that do not incorporate facies heterogeneity, predicted past and
present-day seafloor leakage of petroleum is largely underestimated. This complex interplay of
generation and migration mechanisms has significant implications for charging of petroleum
accumulations by multiple source rocks. Due to early maturation and late stage tertiary migration,
the syn-rift source rocks particularly Mid Hauterivian and Late Hauterivian source intervals
significantly control the extent of petroleum accumulation and loss in the basin.
Lastly, the modelled 3D crustal configuration and Mezosoic to Cenozoic thermal regime of the
WBB dispute classic uniform lithospheric stretching for the southern South African continental
margin. Rather, this PhD thesis confirms that differential thinning of the lithosphere related to a
transtensional pull-apart mechanism is the most appropriate for accurately predicting the evolution
of basin and petroleum systems of the margin. Also, the presented 3D models currently represent
the most advanced insights, and thus have clear implications for assessing associated risks in basin
and prospect evaluation of the margin as well as other similar continental margins around the world. / AFRIKAANSE OPSOMMING: Hierdie studie ondersoek die korsstruktuur en evalueer die kwalitatiewe en kwantitatiewe impakte
van kors-mantel-dinamika op insinkingspatroon, die termiese veld en petroleumstels evolusie aan
die suidelike Suid-Afrikaanse kontinentale grens, in die hede en die verlede, deur die toepassing van
’n multidissiplinêre en multiskaal-geomodelleringsprosedure wat beide konseptuele en numeriese
benaderings behels. Die modelleringsprosedure veral is belangrik aangesien hierdie kontinentale
grens ’n komplekse interaksie van uitbreidings- en strekkingsparallelle tektoniek gedurende die
Mesosoïese vastelandskeurprosesse daarvan dokumenteer. Omdat dit op die suidelike platvorm van
Suid-Afrika geleë is, maak die Westelike Bredasdorp Kom (WBK) die fokus van hierdie studie uit,
en verteenwoordig dit die westelike deel van die groter Bredasdrop-subkom, wat die verste wes is
van die suidelike aflandige subkomme. Om die grens met betrekking tot sy huidige struktuur,
isostatiese staat en termiese veld te verstaan, is ’n kombinasie benadering bestaande uit isostatiese,
3D-gravitasie- en 3D- termiese modellering gebruik deur potensiëleveld-, seismiese en boorgatdata
te integreer Ondersteunend totot die gevolglike konfigurasie en termiese veld van die laasgenoemde
deur middel van hedendaagse temperatuur, soos gemeet, vitriniet-refleksiekoëffisiënt en bronpotensiaal
data, komskaal-begrawing en termiese geskiedenis en tydsberekening van
brongesteentematurasie, is petroleumgenerasie, -uitwerping, -migrasie en -akkumulasie in die
toekoms gesimuleer deur gebruik te maak van ’n 3D-kommodelleringstegniek. Hierdie hierargiese
modelleringswerkvloei maak dit moontlik om geologiese aannames en hulle geassosieerde
onsekerhede goed aan bande te lê en beter te kwantifiseer, veral in drie dimensies.
In die hede word die diep kors van die WBK gekarakteriseer deur ’n drieledige digtheidstruktuur
(met ander woorde voorrift-metasedimente onderlê deur bo- en benedekors domeine) wat dui op ’n
baie wesenlike verdunning, beperk tot ’n dun O-W-strekkingsone. Die konfigurasie van die
radiogeniese kristallyne kors, sowel as die konduktiwiteitskontraste tussen die diep kors en die vlak
sedimentêre dekking, beheer grotendeels die hedendaagse termiese veld van die studiearea. Hierdie
hedendaagse konfigurasie weerspieël in alle opsigte tipiese eienskappe van kom-evolusie in ’n
skuifskeur omgewing. Byvoorbeeld, Die oriëntasies van die diep kors en verskuiwingbeheerde
komsedimentasie byvoorbeeld is ruimtelik inkonsekwent en dui daardeur op verskillende
ekstensiekinematika, tipies van transtensionale tensiemeganisme. As sulks, is sin-rift-versakking
taamlik vinnig en kortstondig, en word isostatiese ekwilibrium nie by die Moho-vlak, in die
besonder, bereik nie.
Samehangende sin-rift vinnige versakking en hittevloeihoogtepunt het gelei tot petroleum behoud in die kom sedert die vroeë Kryt. Twee bykomende post-rift termiese anomalieë wat verband hou met
die laat Kryt-“hotspot” meganisme en die Mioseense kontinentale grensopheffing in Suidelike
Afrika het die sin-rift-beheer met maturasie opgevolg. Hierdie termiese maturiteit van die vyf
gematureerde brongesteentes het in vier hoofgenerasie- en drie hoofakkumulasie fases, wat die
totaliteit van die petroleumstelsels van die WBK karakteriseer, gekulmineer. Die Campaniese,
Eoseense en Mioseense opheffings senarios het episodies bronmaturasie gestop en tersiêre migrasie
van petroleum wat vroeër opgevang was veroorsaak. Addisioneel vind petroleumverlies gekoppel
aan die spilpunt van elke opvanggebiedkonfigurasie tydens die laat Kryt-Paleoseen en Oligoseenvroeë
Mioseen plaas. Die tydstelling en omvang van migrasiedinamika is die sensitiefste vir die
geologiese scenario wat verskuiwing, seëlomseilingstelsel en fasiesheterogeniteit kombineer.
Trouens, vir modelle wat nie fasiesheterogeniteit inkorporeer nie, is voorspellings van vroeëre en
huidige seebodemlekkasie van petroleum grotendeels onderskattings. Hierdie komplekse
wisselwerking van generasie- en migrasiemeganismes het beduidende implikasies vir die laai van
petroleumakkumulasies deur veelvoudige brongesteentes. Vanweë vroeë maturasie en laatstadiumtersiêre
migrasie, oefen die sin-rift-brongesteentes, veral middel Hauterivium- en laat Hauteriviumbronintervalle,
beduidende beheer oor die omvang van petroleumakkumulasie en -verlies in die
kom uit.
Laastens weerspreek die gemodelleerde 3D-korskonfigurasie en Mesosoïese-tot-Senosoïesetermiese
regime van die WBK ’n klassieke uniforme litosferiese rekking vir die suidelike Suid-
Afrikaanse kontinentale grens. Inteendeel, hierdie PhD-proefskrif bevestig dat ’n differensiële
verdunning van die litosfeer, gekoppel aan ’n transtensiemeganisme, die beste geskik is om ’n
akkurate voorspelling oor die evolusie van kom- en petroleumstelsels van die kontinentale grens
mee te maak. Verder, verteenwoordig die 3D-modelle, wat hier aangebied word, tans die mees
gevorderde insigte, en het hierdie modelle dus duidelike implikasies vir die assessering van
verwante risiko’s in kom- en petroleum teikene valuering van die kontinentale grens, so wel as van
ander soortgelyke kontinentale grense regoor die wêreld.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96832 |
| Date | 04 1900 |
| Creators | Sonibare, Wasiu Adedayo |
| Contributors | Mikes, Daniel, Rozendaal, Abraham, Scheck-Wenderoth, Magdalena, Di Primio, Rolando, Anka, Zahie, Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : Stellenbosch University, Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 248 pages : illustrations, maps |
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