Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: This study presents an outcrop characterisation and modelling of the excellently exposed
Permian Kookfontein Formation of the Ecca Group in the Tanqua-Karoo sub-Basin. The
sedimentary modelling (i.e. facies architecture and geometry) and petrophysical
characterisation followed a hierarchical and deterministic approach. Quantitative outcrop data
were based on the thirteen sedimentary cycles that characterise this stratigraphic succession at
the Pienaarsfontein se Berg locality; and these data were analysed using a combination of
detailed sedimentary log, gamma ray log and photopanel analysis, as well as petrographic
thin-section and grain size-based petrophysical analysis.
Based on texture and sedimentary structures, twelve depofacies are recognised which are
broadly grouped into four lithofacies associations i.e. sandstone facies, heterolithic facies,
mudstone facies and soft-sediment deformation facies; these depofacies and lithofacies form
the basic building blocks for the flooding surface-bounded facies succession (i.e. cycle). Also,
based on sediment stacking and cycle thickness patterns as well as relative position to the
shelf break, the succession is sub-divided into: (1) the lower Kookfontein member (i.e. cycles
1 to 5) exhibiting overall upward thickening and coarsening succession with progradational
stacking pattern; representing deposition of mid-slope to top-slope/shelf-margin succession,
and (2) the upper Kookfontein member (i.e. cycles 6 to 13) exhibiting overall upward
thickening and coarsening succession with aggradational stacking pattern; representing
deposition of top-slope/shelf-margin to outer shelf succession. Lateral juxtaposition of
observed vertical facies variations across each cycle in an inferably basinwards direction
exhibits upward change in features, i.e. decrease in gravity effects, increase in waves and
decrease in slope gradient of subsequent cycles. This systematic upward transition in features,
grading vertically from distal to proximal, with an overall upward thickening and coarsening
progradational to aggradational stacking pattern indicates a normal regressive prograding
delta. However, in detail, cycles 1-3 show some anomalies from a purely thickening and
coarsening upward succession.
Deposition of each cycle is believed to result from: (1) primary deposition by periodic
and probably sporadic mouthbar events governed by stream flow dynamics, and (2)
secondary remobilisation of sediments under gravity. The facies distribution, architecture and
geometry which governs the sedimentary heterogeneity within the deltaic succession is
therefore mainly a consequence of the series of mouthbar flooding events governed by
sediment supply and base-level changes. These series of flooding events resulted in the delineation of the studied stratigraphic interval into two main parasequence sets, i.e.
transgressive sequence set and the overlying regressive sequence set. This delineation was
aided through the identification of a maximum flooding surface (i.e. maximum landwards
shift in facies) above Cycle 3 in the field. The architecture and geometry of the ensuing deposystem
is interpreted to have been a river-dominated, gravitationally reworked and waveinfluenced
shelf edge Gilbert-type delta. Widespread distribution of soft-sediment
deformation structures, their growth-style and morphology within the studied succession are
empirically related to progradation of Gilbert-type mouthbars over the shelf break as well as
the slope gradients of the Kookfontein deltaic clinoformal geometry. Analysis of hypothetical
facies stacking and geometrical models suggests that the Kookfontein sedimentary cyclicity
might not be accommodation-driven but rather sediment supply-driven.
The workflow employed for petrophysical evaluation reveals that the distribution of
reservoir properties within the Kookfontein deltaic sandbody geometries is strongly
influenced both by depositional processes and by diagenetic factors, the latter being more
important with increased burial depth. The reservoir quality of the studied sandstones
decreases from proximal mouthbar sands, intermediate delta front to distal delta front facies.
The major diagenetic factors influencing the reservoir quality of the studied sandstones are
mechanical compaction, chemical compaction (pressure solution) and authigenic pore-filling
cements (quartz cement, feldspar alteration and replacement, calcite cement, chlorite and
illite). Mechanical compaction was a significant porosity reducing agent while cementation
by authigenic quartz and clay minerals (i.e. illite and chlorite) might play a major role in
permeability distribution. The porosity-permeability relationship trends obtained for the
studied sandstones show that there is a linear relationship between porosity and permeability.
The relative timing of diagenetic events as well as the percentages of porosity reduction by
compaction and cementation indicates that compaction is much more responsible for porosity
reduction than cementation.
The described internal heterogeneity in this work is below the resolution (i.e. mm-scale)
of most conventional well-logs, and therefore could supplement well-log data especially
where there is no borehole image and core data. The combination of ‗descriptive‘ facies
model and schematic geological model for this specific delta, and petrophysical
characterisation make the results of this study applicable to any other similar ancient deposystem
and particularly subsurface reservoir analogue. / AFRIKAANSE OPSOMMING: Hierdie studie bied ‘n dagsoomkarakterisering en -modellering van die duidelik blootgelegde
Permiese Kookfontein-formasie van die Ecca-groep in die Tankwa-Karoo-subkom. Wat die
sedimentêre modellering (d.w.s. fasiesargitektuur en -geometrie) en petrofisiese
karakterisering betref is ‘n hiërargiese en deterministiese benadering gevolg. Kwantitatiewe
dagsoomdata is gebaseer op dertien sedimentêre siklusse wat hierdie stratigrafiese
opeenvolging in die Pienaarsfontein se Berg-lokaliteit kenmerk; en die data is geanaliseer met
behulp van ‘n kombinasie van gedetailleerde sedimentêre seksie, gammastraal-profiel en
fotopaneelanalises, asook petrografiese slypplaatjie- en korrelgrootte-gebaseerde petrofisiese
analises.
Op grond van tekstuur en sedimentêre strukture is twaalf afsettingsfasies onderskei wat
rofweg in vier assosiasies van litofasies gegroepeer kan word: sandsteenfasies, heterolitiese
fasies, moddersteenfasies en sagtesediment-deformasiefasies. Hierdie afsettingsfasies en
litofasies vorm die basiese boustene vir die fasiesopeenvolging (d.w.s. siklus) wat
oorstromingsoppervlakgebonde is. Verder word die opeenvolging aan die hand van
sedimentstapeling en skilusdiktepatrone, asook relatiewe posisie tot die rakbreuk, in die
volgende onderverdeel: (1) die benede-Kookfontein-deel (d.w.s. siklus 1 tot 5), wat in die
geheel ‘n opwaartse verdikkings- en vergrowwingsopeenvolging met ‘n
progradasiestapelpatroon vertoon en die afsetting van middelhelling-tot-boonstehelling- of
rakrand-opeenvolging verteenwoordig, en (2) die benede-Kookfontein-deel (d.w.s. siklus 6
tot 13) wat in die geheel ‘n opwaartse verdikkings- en vergrowwingsopeenvolging met ‘n
aggradasiestapelpatroon vertoon en die afsetting van boonste helling- of rakrand-tot-buiterakopeenvolging
verteenwoordig. Die laterale jukstaposisie van waargenome vertikale
fasiesvariasies oor elke siklus heen, in ‘n afleibare komwaartse rigting, vertoon opwaartse
verandering wat kenmerke betref, naamlik afname in gravitasiegevolge, toename in golwe en
afname in die hellinggradiënt van daaropvolgende siklusse. Hierdie stelselmatige opwaartse
oorgang van kenmerke, wat vertikaal van distaal tot proksimaal gradiënteer en in die geheel
opwaartse verdikking en vergrowwing in ‘n progradasie-tot-aggradasie-stapelpatroon vertoon,
dui op ‘n normale regressiewe progradasiedelta. Van naby beskou, vertoon siklus 1-3 egter
bepaalde afwykings van ‘n suiwer opwaartse verdikkings- en vergrowwingsopeenvolging.
Die afsettings van elke siklus is vermoedelik die gevolg van: (1) primêre afsetting deur
periodieke en waarskynlik sporadiese mondversperringsgebeure wat deur
stroomvloeidinamika beheer word, en (2) sekondêre hermobilisering van sedimente deur gravitasie. Die fasiesverspreiding, -argitektuur en -geometrie wat die sedimentêre
heterogeniteit in die deltaïese opeenvolging beheer, is dus hoofsaaklik ‘n gevolg van die
reeks oorstromingsgebeure by die mondversperring, wat deur sedimentvoorsiening en
basisvlakveranderings beheer word. Hierdie reeks oorstromingsgebeure het gelei tot die
delineasie van die bestudeerde stratigrafiese interval volgens twee hoofparasekwensie stelle,
naamlik die transgressiewe opeenvolgings- en die oordekkende, regressiewe
opeenvolgingsgroep. Dié delineasie word ondersteun deur die feit dat ‘n maksimum
oorstromingsoppervlak (d.w.s. maksimum landwaartse verskuiwing in fasies) bo siklus 3 in
die veld uitgeken is. Die argitektuur en geometrie van die daaropvolgende afsettingstelsel
word geïnterpreteer as behorende tot ‘n Gilbert-rakranddelta wat deur ‘n rivier gedomineer,
deur gravitasie herbewerk en deur golfwerking beïnvloed is. Die wye verspreiding van
sagtesediment-deformasiestrukture, en die groeiwyse en morfologie daarvan binne die
bestudeerde opeenvolging, is empiries verwant aan die progradasie van Gilbertmondversperrings
oor die rakbreuk heen, asook aan die hellinggradiënte van die
Kookfontein-deltaïese, klinoformele geometrie. Die analise van hipotetiese fasiesstapeling
en geometriese modelle dui daarop dat die Kookfontein-sedimentêre siklisiteit dalk nie deur
akkommodasieruimte gedryf word nie, maar deur sedimentvoorsiening.
Die werkvloei wat vir petrofisiese evaluering gebruik is dui daarop dat die verspreiding
van reservoir-eienskappe in die Kookfontein- deltaïese sandliggaam geometries sterk
beïnvloed word deur afsettingsprosesse en diagenetiese faktore. Die diagenetiese faktore
word belangriker op groter begrawing diepte. Die reservoir-aard van die bestudeerde
sandgesteentes neem algaande af van proksimale mondversperring-sandsoorte tot
intermediêre deltafront tot distale deltafrontfasies. Die hoof-diagenetiese faktore wat die
reservoir-kenmerke van die bestudeerde sandsteensoorte beïnvloed is meganiese verdigting,
chemiese verdigting (oplossingsdruk) en outigeniese porievullingsement (kwartssement,
veldspaatomsetting en -vervanging, kalsietsement, chloriet en illiet). Meganiese verdigting
is ‘n beduidende poreusheidreduseermiddel, terwyl sementering deur outigeniese kwarts- en
kleiminerale (d.w.s. illiet en chloriet) moontlik ‘n belangrike rol by
permeabiliteitsverspreiding kan speel. Die poreusheid-permeabiliteit-verhoudingstendense
wat bekom is vir die bestudeerde sandsteensoorte dui daarop dat daar ‘n lineêre verhouding
tussen poreusheid en permeabiliteit bestaan. Die relatiewe tydberekening van diagenetiese
gebeure, asook die persentasie poreusheidvermindering deur verdigting en sementering, dui
daarop dat verdigting baie meer as sementering tot poreusheidvermindering bydra. Die interne heterogeniteit wat in hierdie werk beskryf word, is onder die resolusie (d.w.s.
mm-skaal) van die meeste konvensionele boorgatopnames, en kan dus boorgatopnamedata
aanvul, veral waar daar geen boorgatafbeelding en kerndata bestaan nie. Die kombinasie van
die 'deskriptiewe‘ fasiesmodel en skematiese geologiese model vir hierdie spesifieke delta,
asook petrofisiese karakterisering, beteken dat die resultate van hierdie studie op enige ander
soortgelyke antieke afsettingstelsels toegepas kan word, maar veral op
suboppervlakreservoir-analoogstelsels.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/17884 |
| Date | 12 1900 |
| Creators | Sonibare, Wasiu Adedayo |
| Contributors | Mikes, Daniel, Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 205 p. : ill. |
| Rights | Stellenbosch University |
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