TU Darmstadt / ULB / TUbiblio

Sedimentology, stratigraphy and reservoir quality of the Paleozoic Wajid Sandstone in SW Saudi Arabia

Ajmi, Hussain al (2013)
Sedimentology, stratigraphy and reservoir quality of the Paleozoic Wajid Sandstone in SW Saudi Arabia.
Buch, Erstveröffentlichung

Kurzbeschreibung (Abstract)

This PhD thesis was embedded in a regional groundwater study in the Paleozoic Wajid Sandstone of southwest Saudi Arabia. The Wajid Sandstone holds important fossil groundwater resources in southwest Saudi Arabia which became increasingly overused due to extensive irrigation for agricultural purpose. This thesis contributed to this groundwater project by evaluating the hydraulic properties of aquifer rocks. This involved also fundamental questions about the sedimentary facies, depositional environments, stratigraphy, architecture, age, and petrology of the Wajid Sandstone Group. The fundamental concept of this PhD thesis is based on the principle of out-crop-analogue studies, i.e. properties of reservoir rocks in the subsurface are investigated in outcrops at the surface. This concept is applicable for the Wajid Sandstone because of a well exposed outcrop belt and extractions wells whose aquifer rocks are exposed only few kilometres from the well location.The outcrop-analogue studies included all basic parameters of reservoir characterization. The major focus was on 1D standard sections where samples were preferentially taken for further analysis. In particular for heterogenous glacigenic deposits also 2D wall panels were composed and analysed. Field surveys lasted altogether around 4 months. The extensive sample collection was sent to Germany and all laboratory and microscopic work was done at the Institut für Angewandte Geowissenschaften of TU Darmstadt. This included measurements and analysis of gamma-ray, porosity, permeability, and thin sections as well as stratigraphic work applying palynology. The multiple data were merged into one data base and analysed by multivariate statistics and principal component analysis.

The study area is located in the south-western part of the Kingdom of Saudi Arabia and comprises the entire outcrop belt of the Wajid Group (ca. 44,000 km2). Correlation of sections is mainly based on unconformities that subdivide the succession into distinct stratigraphic packages. In its outcrop belt, the Wajid Group can be subdivided into 5 formations: the Dibsiyah Formation, Sanamah Formation, Qalibah Formation, Khusayyayn Formation, and Juwayl Formation. Thirteen lithofacies have been distinguished (LF1 through LF 13), which cover the entire spectrum of siliciclastic grain size classifications. Shales and siltstones are relatively rare in the succession, whereas sandstones, especially medium-grained to coarse-grained sandstones are abundant. Conglomerates are locally abundant in the Sanamah Formation and in the Juwayl Formation. A second order descriptor is bioturbation, which is very common in the Dibsiyah Formation, but rare to absent in the other units. The 13 lithofacies have been combined in 9 lithofacies associations (LF-A1 through LF-A9). By stacking the different sections a new standard lithostratigraphic log is presented in this thesis. Sediments of the Wajid Group were deposited during approximately 200 Ma to 260 Ma, depending on the definite depositional age of the Dibsiyah Formation. Extrapolations of sedimentation rates to depositional time show that within the Wajid Group and under the assumption of very conservative sedimentation rates, 62 Ma years are represented in the sediments: 20 Ma in the Dibsiyah Formation, 2 Ma (the length of the Hirnantian) in the Sanamah Formation, 15 Ma in the Khusayyayn Formation, and 25 Ma in the Juwayl Formation. Most likely, the time represented is much less. This means that the sediments preserved do not even cover ¼ of the Palaeozoic era. This is compatible with field observations of abundant unconformities and sedimentary breaks in the starved successions. This indicates that throughout the Palaeozoic, southern Saudi Arabia was located in an epicratonal setting, in which tectonic subsidence and relative sea level changes exerted only minor control.

A specific aim of this study was to systematically explore the heterogeneous glacial and proglacial deposits, develop a genetic depositional model and compare the findings with other regions at the northern rim of Gondwana. Saudi Arabia is one of the few places where both glaciations can be studied in well-preserved sedimentary succession, which formed under similar boundary conditions. Among these are (a) incision of subglacial tunnel valleys according to the ice-loading model, (b) subsequent valley-fill by subaqueous to subaerial proglacial deposits in front of an oscillating, polythermal ice-shield, (c) intra-formational erosional events through repeated ice advance, (d) widespread and large-scale soft sediment deformation due to glacial surge during deglaciation, (e) marine transgression as a consequence of eustatic sea-level rise following deglaciation. The general pattern fits well with observations from northern Africa, pointing to closely coupled glaciological processes along the northern margin of the Gondwana ice-shield during Upper Ordovician glaciation. Although the same general patterns apply for the Permo-Carboniferous glaciation, styles and petrographical properties differ most probably due to a more complex ice-flow pattern with a rougher topography after the Hercynian event and contrasting weathering.

This thesis presents the first statistical analysis of porosities and permeabilities over the entire Wajid Sandstone Group in the outcrop belt and links these values with lithofacies and microfacies studies in order to identify controlling factors. Furthermore, for the first time a combined approach of standard mini permemeter and sophisticated column permeameter measurements were carried out, the latter enables to convert gas in water permeabilities. Although the data of this thesis confirms the overall good reservoir quality, also wide scatter is obvious. This is particularly the case for permeabilities. High porosities do not guarantee high permeabilities and correlation of both is weak, although a positive trend exists. This means that porosities cannot be used to predict permeabilities accurately. Medians of porosities range from 23 to 27% and 15 to 28% for formations and lithofacies types respectively. Different permeability measurements all show highest medians of 1500 to 2000 mD for the Khusayyayn Formation and lowest medians of 300 to 1400 mD for the Dibsiyah Formation. The variability is highest for the Sanamah Formation and the Juwayl Formation. To analyse these heterogeneities and identifying controlling factors, two approaches were applied: (i) lithofacies types, which represent grain texture and sedimentological structures at the mesoscopic scale in the field, and (ii) microfacies analysis using thin sections and raster electron microscopy of selected samples. Referring to lithofacies, the following trends have been identified. Siltstone and fine sandstones exhibit the highest porosities, but permeability is low for silt-dominated samples. Pebbly sandstones show reduced porosities and permeabilities, most probably because of general poorer mixing. Highest porosities and permeabilities were found for cross-bedded sandstones. Thick massive sandstones in the Juwayl Formation, which are expected to have very good reservoir properties show surprisingly high variability. Anisotropy is generally low except for siltstones, and bioturbated sandstones. Porosites and permeabilities are most closely related to each other for sandy to pebbly, cross bedded lithofacies types. Bioturbated samples and massive sandstones show a relatively high permeability compared to their porosity, which may be interpreted as homogenization of the grain fabric and/or secondary porosity by leaching. Finer grained samples show the expected opposite trend, but some exceptions exist.

Referring to microfacies, the following trends have been identified. High permeabilities can be linked with open pore space, pore connectivity, and rounding, but less with sorting. Low permeabilities are linked with increased proportions of pseudo matrix and cementation by iron oxides, calcite, and/or quartz. The Khusayyayn Formation has higher feldspar content. This leads to higher secondary porosity, proportions of pseudo matrix, and some calcite cement. As destruction and formation of secondary porosity prevails the highest average permeability in the entire Wajid Sandstone Group is observed. Biomodal grain distribution in the Juwayl Formation leads to strongly reduced permeability despite the favourable main grain texture. Secondary leaching in the Juwayl Formation is more heterogeneous and cemented or matrix rich patches are common. Furthermore, the Juwayl Formation exhibits lower porosities at the same permeabilities, which is interpreted as a more efficient connectivity due to secondary leaching. Iron cementation is strongest in the lower Wajid Sandstone Group, in particular in the Dibsiyah Formation. This seems to be the major reason for reduced permeabilities there. Likely, the iron originates from continental weathering of shield areas during the warm Cambro-Ordovician period and was remobilized during burial. The Qusaiba Shale presumably hindered the circulating pore waters to penetrate the upper Wajid Sandstone equally.

When comparing water peremabilities of matrix samples with pumping tests, both are surprisingly close and point out the high relevance of matrix permeabilities for groundwater storage and groundwater flow in the Wajid Sandstone. Pumping tests yield a slightly higher hydraulic conductivity for the Lower Wajid Aquifer. Matrix permeabilities for water show the same trend but differences are even lower. Whereas hydraulic permeabilities for matrix and pumping tests are very close in the Lower Wajid Aquifer, matrix permeabilities are almost one magnitude lower in the Upper Wajid Aquifer. In conclusion, this makes matrix permeabilities of prime importance in the Wajid Aquifers. Hence, the investigation of sedimentary heterogeneities at the outcrop and microscopic scale is of specific relevance for the prediction of reservoir quality in the Wajid Sandstone Group.

Typ des Eintrags: Buch
Erschienen: 2013
Autor(en): Ajmi, Hussain al
Art des Eintrags: Erstveröffentlichung
Titel: Sedimentology, stratigraphy and reservoir quality of the Paleozoic Wajid Sandstone in SW Saudi Arabia
Sprache: Englisch
Referenten: Hinderer, Prof. Matthias ; Schüth, Prof. Christoph ; Kempe, Prof. Stephan ; Urban, Prof. Wilhelm
Publikationsjahr: Februar 2013
Datum der mündlichen Prüfung: 15 März 2013
URL / URN: http://tuprints.ulb.tu-darmstadt.de/3581
Kurzbeschreibung (Abstract):

This PhD thesis was embedded in a regional groundwater study in the Paleozoic Wajid Sandstone of southwest Saudi Arabia. The Wajid Sandstone holds important fossil groundwater resources in southwest Saudi Arabia which became increasingly overused due to extensive irrigation for agricultural purpose. This thesis contributed to this groundwater project by evaluating the hydraulic properties of aquifer rocks. This involved also fundamental questions about the sedimentary facies, depositional environments, stratigraphy, architecture, age, and petrology of the Wajid Sandstone Group. The fundamental concept of this PhD thesis is based on the principle of out-crop-analogue studies, i.e. properties of reservoir rocks in the subsurface are investigated in outcrops at the surface. This concept is applicable for the Wajid Sandstone because of a well exposed outcrop belt and extractions wells whose aquifer rocks are exposed only few kilometres from the well location.The outcrop-analogue studies included all basic parameters of reservoir characterization. The major focus was on 1D standard sections where samples were preferentially taken for further analysis. In particular for heterogenous glacigenic deposits also 2D wall panels were composed and analysed. Field surveys lasted altogether around 4 months. The extensive sample collection was sent to Germany and all laboratory and microscopic work was done at the Institut für Angewandte Geowissenschaften of TU Darmstadt. This included measurements and analysis of gamma-ray, porosity, permeability, and thin sections as well as stratigraphic work applying palynology. The multiple data were merged into one data base and analysed by multivariate statistics and principal component analysis.

The study area is located in the south-western part of the Kingdom of Saudi Arabia and comprises the entire outcrop belt of the Wajid Group (ca. 44,000 km2). Correlation of sections is mainly based on unconformities that subdivide the succession into distinct stratigraphic packages. In its outcrop belt, the Wajid Group can be subdivided into 5 formations: the Dibsiyah Formation, Sanamah Formation, Qalibah Formation, Khusayyayn Formation, and Juwayl Formation. Thirteen lithofacies have been distinguished (LF1 through LF 13), which cover the entire spectrum of siliciclastic grain size classifications. Shales and siltstones are relatively rare in the succession, whereas sandstones, especially medium-grained to coarse-grained sandstones are abundant. Conglomerates are locally abundant in the Sanamah Formation and in the Juwayl Formation. A second order descriptor is bioturbation, which is very common in the Dibsiyah Formation, but rare to absent in the other units. The 13 lithofacies have been combined in 9 lithofacies associations (LF-A1 through LF-A9). By stacking the different sections a new standard lithostratigraphic log is presented in this thesis. Sediments of the Wajid Group were deposited during approximately 200 Ma to 260 Ma, depending on the definite depositional age of the Dibsiyah Formation. Extrapolations of sedimentation rates to depositional time show that within the Wajid Group and under the assumption of very conservative sedimentation rates, 62 Ma years are represented in the sediments: 20 Ma in the Dibsiyah Formation, 2 Ma (the length of the Hirnantian) in the Sanamah Formation, 15 Ma in the Khusayyayn Formation, and 25 Ma in the Juwayl Formation. Most likely, the time represented is much less. This means that the sediments preserved do not even cover ¼ of the Palaeozoic era. This is compatible with field observations of abundant unconformities and sedimentary breaks in the starved successions. This indicates that throughout the Palaeozoic, southern Saudi Arabia was located in an epicratonal setting, in which tectonic subsidence and relative sea level changes exerted only minor control.

A specific aim of this study was to systematically explore the heterogeneous glacial and proglacial deposits, develop a genetic depositional model and compare the findings with other regions at the northern rim of Gondwana. Saudi Arabia is one of the few places where both glaciations can be studied in well-preserved sedimentary succession, which formed under similar boundary conditions. Among these are (a) incision of subglacial tunnel valleys according to the ice-loading model, (b) subsequent valley-fill by subaqueous to subaerial proglacial deposits in front of an oscillating, polythermal ice-shield, (c) intra-formational erosional events through repeated ice advance, (d) widespread and large-scale soft sediment deformation due to glacial surge during deglaciation, (e) marine transgression as a consequence of eustatic sea-level rise following deglaciation. The general pattern fits well with observations from northern Africa, pointing to closely coupled glaciological processes along the northern margin of the Gondwana ice-shield during Upper Ordovician glaciation. Although the same general patterns apply for the Permo-Carboniferous glaciation, styles and petrographical properties differ most probably due to a more complex ice-flow pattern with a rougher topography after the Hercynian event and contrasting weathering.

This thesis presents the first statistical analysis of porosities and permeabilities over the entire Wajid Sandstone Group in the outcrop belt and links these values with lithofacies and microfacies studies in order to identify controlling factors. Furthermore, for the first time a combined approach of standard mini permemeter and sophisticated column permeameter measurements were carried out, the latter enables to convert gas in water permeabilities. Although the data of this thesis confirms the overall good reservoir quality, also wide scatter is obvious. This is particularly the case for permeabilities. High porosities do not guarantee high permeabilities and correlation of both is weak, although a positive trend exists. This means that porosities cannot be used to predict permeabilities accurately. Medians of porosities range from 23 to 27% and 15 to 28% for formations and lithofacies types respectively. Different permeability measurements all show highest medians of 1500 to 2000 mD for the Khusayyayn Formation and lowest medians of 300 to 1400 mD for the Dibsiyah Formation. The variability is highest for the Sanamah Formation and the Juwayl Formation. To analyse these heterogeneities and identifying controlling factors, two approaches were applied: (i) lithofacies types, which represent grain texture and sedimentological structures at the mesoscopic scale in the field, and (ii) microfacies analysis using thin sections and raster electron microscopy of selected samples. Referring to lithofacies, the following trends have been identified. Siltstone and fine sandstones exhibit the highest porosities, but permeability is low for silt-dominated samples. Pebbly sandstones show reduced porosities and permeabilities, most probably because of general poorer mixing. Highest porosities and permeabilities were found for cross-bedded sandstones. Thick massive sandstones in the Juwayl Formation, which are expected to have very good reservoir properties show surprisingly high variability. Anisotropy is generally low except for siltstones, and bioturbated sandstones. Porosites and permeabilities are most closely related to each other for sandy to pebbly, cross bedded lithofacies types. Bioturbated samples and massive sandstones show a relatively high permeability compared to their porosity, which may be interpreted as homogenization of the grain fabric and/or secondary porosity by leaching. Finer grained samples show the expected opposite trend, but some exceptions exist.

Referring to microfacies, the following trends have been identified. High permeabilities can be linked with open pore space, pore connectivity, and rounding, but less with sorting. Low permeabilities are linked with increased proportions of pseudo matrix and cementation by iron oxides, calcite, and/or quartz. The Khusayyayn Formation has higher feldspar content. This leads to higher secondary porosity, proportions of pseudo matrix, and some calcite cement. As destruction and formation of secondary porosity prevails the highest average permeability in the entire Wajid Sandstone Group is observed. Biomodal grain distribution in the Juwayl Formation leads to strongly reduced permeability despite the favourable main grain texture. Secondary leaching in the Juwayl Formation is more heterogeneous and cemented or matrix rich patches are common. Furthermore, the Juwayl Formation exhibits lower porosities at the same permeabilities, which is interpreted as a more efficient connectivity due to secondary leaching. Iron cementation is strongest in the lower Wajid Sandstone Group, in particular in the Dibsiyah Formation. This seems to be the major reason for reduced permeabilities there. Likely, the iron originates from continental weathering of shield areas during the warm Cambro-Ordovician period and was remobilized during burial. The Qusaiba Shale presumably hindered the circulating pore waters to penetrate the upper Wajid Sandstone equally.

When comparing water peremabilities of matrix samples with pumping tests, both are surprisingly close and point out the high relevance of matrix permeabilities for groundwater storage and groundwater flow in the Wajid Sandstone. Pumping tests yield a slightly higher hydraulic conductivity for the Lower Wajid Aquifer. Matrix permeabilities for water show the same trend but differences are even lower. Whereas hydraulic permeabilities for matrix and pumping tests are very close in the Lower Wajid Aquifer, matrix permeabilities are almost one magnitude lower in the Upper Wajid Aquifer. In conclusion, this makes matrix permeabilities of prime importance in the Wajid Aquifers. Hence, the investigation of sedimentary heterogeneities at the outcrop and microscopic scale is of specific relevance for the prediction of reservoir quality in the Wajid Sandstone Group.
Alternatives oder übersetztes Abstract:
Alternatives AbstractSprache

Diese Dissertation ist Teil einer regionalen Grundwasserstudie im paläozoischen Wajid-Sandstein von Saudi-Arabien. Der Wajid-Sandstein enthält wichtige, fossile Grundwasserressourcen im Südwesten Saudi-Arabiens, die durch umfangreiche landwirtschaftliche Bewässerung zunehmend ausgebeutet wurden. Diese Studie trägt zu diesem Grundwasserprojekt durch Evaluierung der hydraulischen Eigenschaften der Aquifergesteine bei. Dieses beinhaltet auch fundamentale Fragestellungen über die sedimentären Fazies, Ablagerungsbedingungen, Stratigraphie, Architektur, Alter und Petrologie der Wajid-Sandsteingruppe. Das zugrundeliegende Konzept dieser Dissertation basiert auf dem Prinzip von Aufschluss-Analog-Studien; z.B. wurden Eigenschaften von Reservoirgesteinen im Untergrund anhand von Oberflächenaufschlüssen untersucht. Dieses Konzept ist deshalb für den Wajid-Sandstein gut geeignet, da es sowohl einen Gürtel mit guten Aufschlüssen sowie Produktionsbrunnen, deren Aquifergesteine nur wenige Kilometer vom Brunnen entfernt aufgeschlossen sind, gibt. Die Aufschluss-Analog-Studien umfassten alle grundlegenden Parameter der Reservoir-Charakterisierung. Hauptaugenmerk lag hierbei beim Erstellen von 1-D Standardprofilen, aus denen Proben für weitere Analysen genommen wurden. Für besonders heterogene, glazigene Ablagerungen wurden 2-D Wandpaneele erstellt und analysiert. Die Geländearbeiten dauerten insgesamt ungefähr 4 Monate. Die umfangreiche Probensammlung wurde nach Deutschland geschickt, wo alle Labor- und Mikroskopierarbeiten am Institut für Angewandte Geowissenschaften der TU Darmstadt ausgeführt wurden. Diese beinhalten Messungen und Analyse von Gamma-Ray-Daten, Porosität, Permabilität und Dünnschliffen sowie stratigraphische Arbeiten mittels angewandter Palynologie. Die verschiedenen Daten wurden in eine einzelne Datenbank zusammengeführt und durch multivariante Statistik und Hauptkomponentenanalysen ausgewertet.

Das Untersuchungsgebiet liegt im südwestlichen Teil des Königreichs von Saudi-Arabien und umfasst den gesamten Aufschlussgürtel der Wajid Gruppe (ca. 44.000 km2). Die Korrelation der einzelnen Profile basiert hauptsächlich auf Diskordanzen, die jede Abfolge in distinkte stratigraphische Pakete unterteilt. Im Aufschlussgürtel kann die Wajid Gruppe in 5 Formationen unterteilt werden: die Dibsiyah Formation, Sanamah Formation, Qalibah Formation, Khusayyayn Formation und Juwayl Formation. Dreizehn Lithofaziestypen konnten unterschieden werden (LF1 bis LF13), welche das gesamte Spektrum der siliziklastischen Korngrößenklassifikation umfassen. Shales und Siltsteine sind in der Abfolge relativ selten, während Sandsteine, besonders mittelkörnige bis grobkörnige Sandsteine, sehr häufig vorkommen. Konglomerate kommen lokal häufig in der Sanamah Formation und Juwayl Formation vor. Ein Unterscheidungsmerkmal zweiter Ordnung ist Bioturbation, welche häufig in der Dibsiyah Formation auftritt, jedoch selten bis nicht in den anderen Einheiten vorkommt. Die 13 Lithofaziestypen wurden in 9 Lithofaziesassoziationen (LF-A1 bis LF-A9) zusammengefasst. Durch Stapeln der verschiedenen Profile wird in dieser Arbeit ein neues lithostratigraphisches Standardlog vorgestellt. Die Sedimente der Wajid Gruppe wurden, abhängig vom Ablagerungsalter der Dibsiyah Formation, vor ungefähr 200 Ma bis 260 Ma abgelagert. Extrapolationen von Sedimentationsraten zur Ablagerungszeit zeigen, dass unter Annahme sehr konservativer Sedimentationsgeschwindigkeiten, innerhalb der Wajid Gruppe 62 Ma im Sediment repräsentiert sind: 20 Ma in der Dibsiyah Formation, 2 Ma (die Dauer des Hirnatiums) in der Sanamah Formation, 15 Ma in der Khusayyayn Formation und 25 Ma in der Juwayl Formation. Es ist jedoch sehr wahrscheinlich, dass die repräsentierte Zeitdauer deutlich geringer ist. Das bedeutet, dass erhaltenen Sedimente nicht einmal ein Viertel des Phanerozoikums repräsentieren. Dies ist mit Geländebeobachtungen häufig vorkommender Diskordanzen und Sedimentationsschnitten in kondensierten Abfolgen vereinbar. Das weist darauf hin, dass sich durch das gesamte Paläozoikum hindurch das südliche Saudi-Arabien in einem epikontinentalen Umfeld befand, in welchem tektonische Subsidenz und Änderungen des relativen Meeresspiegels nur untergeordneten Einfluss hatten.

Ein besonderes Ziel dieser Studie war es, die heterogenen glazialen und proglazialen Ablagerungen systematisch zu erforschen, ein genetisches Ablagerungsmodell zu entwickeln und mit Funden in anderen Regionen am Nordrand Gondwanas zu vergleichen. Saudi-Arabien ist einer der wenigen Plätze, an dem beide Vereisungen in gut erhaltenen Sedimentabfolgen, welche unter ähnlichen Rahmenbedingungen gebildet wurden, beobachtet werden können. Unter diesen finden sich (a) einschneidende Tunneltäler nach dem ice-loading Modell, (b) darauffolgende Talfüllung durch subaquatische bis subäolische, proglaziale Ablagerungen vor einem oszillierenden, polythermalen Eisschild, (c) intraformationelle Erosionsereignisse durch wiederholte Eisvorstöße, (d) weitläufige und großräumige Deformation von weichem Sediment durch glaziale Fluten während des Eisrückzuges, (e) marine Transgression als Folge von eustatischem Meeresspiegelanstieg in Folge des Eisrückzuges. Das generelle Muster passt gut zu den Beobachtungen aus Nordafrika, was auf eng verbundene Vereisungsprozesse entlang des Nordrands des Eisschilds von Gondwana während der oberordovizischen Vereisung hindeutet. Obwohl die gleichen generellen Muster auch für die permokarbonische Vereisung zutreffen, unterscheiden sich Stil und petrographische Eigenschaften höchstwahrscheinlich durch ein komplexeres Eisflussmuster mit einer rauheren Topographie nach dem herzynischen Event und unterschiedliche Verwitterung.

Diese Studie präsentiert die erste statistische Analyse von Porositäts- und Permeabilitätswerten über die gesamte Wajid Sandsteingruppe im Aufschlussgürtel und verbindet diese mit Lithofazies- und Mikrofaziesstudien um die verantwortlichen Faktoren zu identifizieren. Des Weiteren wurde zum ersten Mal ein kombinierter Ansatz von standard Minipermeameter- und fortschirttlichen Säulenpermeametermessungen verfolgt, wobei letztere erlauben, Gas- in Wasserpermeabilitäten umzurechnen. Obwohl die Daten dieser Studie die grundlegend gute Reservoirqualität bestätigt ist eine weite Streuung der Daten offensichtlich. Dies trifft besonders auf Permeabilitätswerte zu. Hohe Porositätswerte garantieren nicht auch hohe Werte für die Permeabilität und die Korrelation zwischen beiden Faktoren ist schwach, obwohl ein positiver Trend existiert. Das bedeutet, dass Porositätswerte nicht benutzt werden können, um die Permeabilität akkurat zu berechnen. Mediane der Porosität reichen von 23 bis 27% und 15 bis 28% für Formationen und Lithofaziestypen, respektive. Verschiedene Permeabilitätsmessungen zeigen alle maximale Mediane von 1500 bis 2000 mD für die Khusayyayn Formation und minimale Mediane von 300 bis 1400 mD für die Dibsiyah Formation. Die Variabilität ist am größten für die Sanamah Formation und die Juwayl Formation. Um diese Heterogenitäten zu analysieren und die bestimmenden Faktoren zu identifizieren, kamen zwei Ansätze zur Anwendung: (i) Lithofaziestypen, welche Korntexturen und sedimentologische Strukturen im mesoskopischen Maßstab im Gelände repräsentieren sowie (ii) Mikrofaziesanalysen mittels Dunnschliffen und Rasterelektronenmikroskopie anhand ausgewählter Proben. In Bezug auf Lithofazies wurden folgende Trends identifiziert. Siltsteine und Feinsandsteine zeigen die höchsten Porositäten, aber mit geringen Permeabilitäten für siltdominierte Proben. Kiesige Sandsteine zeigen verringerte Porositäten und Permabilitäten, höchstwahrscheinlich durch generell schlechtere Durchmischung. Höchste Porositäten und Permeabilitäten wurden in kreuzgeschichteten Sandsteinen gefunden. Mächtige, massive Sandsteine der Juwayl Formation, von denen gute Reservoireigenschaften erwartet werden, zeigen eine überraschend hohe Variabilität. Die Anisotropie ist generell gering, außer für Siltsteine und bioturbierte Sandsteine. Porositäten und Permeabilitäten hängen am meisten zusammen für sandige bis kiesig, kreuzgeschichtete Lithofaziestypen. Proben mit Bioturbation und massive Sandsteine zeigen eine relativ hohe Permeabilität verglichen mit ihren Porositätswerten, was interpretiert werden kann als Homogenisierung des Korngefüges und/oder Bildung von Sekundärporosität durch Auswaschung. Feinköringere Proben zeigen den erwarteten gegenläufigen Trend, jedoch gibt es auch einige Ausnahmen.

Die folgenden Trends wurden in Bezug auf Mikrofazies beobachtet. Hohe Permeabilitäten können mit offenem Porenraum, Porenverbindungen und Rundung verbunden werden, weniger jedoch mit Sortierung. Geringe Permeabilitäten sind verbunden mit erhöhten Anteilen von Pseudomatrix und Zementation durch Eisenoxide, Kalzit und/oder Quarz. Die Khusayyayn Formation zeigt einen hohen Feldspatgehalt. Dies führt zu hoher sekundärer Porosität, hohen Anteilen an Pseudomatrix und einigem Kalzitzement. Die höchste mittlere Permeabilität in der gesamten Wajid Sandsteingruppe wird beobachtet, wo Zerstörung und Bildung von sekundärer Porosität vorherrschen. Die biomodale Kornverteilung in der Juwayl Formation führt zu einer stark reduzierten Permeabilität, entgegen der günstigen Textur der Hauptkornfraktion. Sekundäre Auswaschung in der Juwayl Formation ist heterogener und zementierte oder matrixreiche Stellen sind häufig. Des Weiteren zeigt die Juwayl Formation geringere Porositäten bei gleichen Permeabilitäten, was als effizientere Konnektivität durch sekundäre Auswaschung interpretiert wird. Eisenzementation tritt in der unteren Wajid Sandsteingruppe häufig auf, besonders in der Dibsiyah Formation. Dies erscheint als der Hauptgrund für die dortige reduzierte Permeabilität. Wahrscheinlich stammt das Eisen von der kontinentalen Verwitterung von Schildgebieten während der warmen kambroordovizischen Periode und wurde während der Versenkung remobilisiert. Vermutlich hinderte der Qusaiba Shale das zirkulierende Porenwasser daran, den oberen Wajid Sandstein gleichmäßig zu durchdringen.

Wenn man Wasserpermeabilitäten von Matrixproben mit Pumpversuchen vergleicht, sind beide überraschend ähnlich und weisen somit auf die hohe Relevanz von Matrixpermeabilitäten für die Grundwasserspeicherung und den Grundwasserfluss im Wajid Sandstein hin. Pumpversuche ergaben eine leicht erhöhte hydraulische Konduktivität für den unteren Wajid Aquifer. Matrixpermeabilitäten für Wasser zeigen den gleichen Trend, die Unterschiede sind jedoch noch geringer. Wohingegen hydraulische Matrixpermeabilitäten und Pumpversuche im unteren Wajid Aquifer eng beieinander liegen sind Matrixpermeabilitäten nahezu um eine Größenordnung geringer im oberen Wajid Aquifer. Daraus folgt die allerhöchste Bedeutung der Matrixpermeabilität in den Wajid Aquiferen. Dies verleiht der Untersuchung von sedimentären Heterogenitäten im Aufschluss und im mikroskopischen Maßstab besondere Relevanz für die Vorhersage der Reservoirqualität in der Wajid Sandsteingruppe.

Deutsch
URN: urn:nbn:de:tuda-tuprints-35815
Sachgruppe der Dewey Dezimalklassifikatin (DDC): 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
Fachbereich(e)/-gebiet(e): 11 Fachbereich Material- und Geowissenschaften
11 Fachbereich Material- und Geowissenschaften > Geowissenschaften
11 Fachbereich Material- und Geowissenschaften > Geowissenschaften > Fachgebiet Angewandte Sedimentgeologie
Hinterlegungsdatum: 25 Aug 2013 19:55
Letzte Änderung: 24 Jul 2024 11:20
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Referenten: Hinderer, Prof. Matthias ; Schüth, Prof. Christoph ; Kempe, Prof. Stephan ; Urban, Prof. Wilhelm
Datum der mündlichen Prüfung / Verteidigung / mdl. Prüfung: 15 März 2013
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