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

• Main Author: Al Ajmi, Hussain
• Format: Others
• Language: German; German; en
• Published: 2013
• Online Access: https://tuprints.ulb.tu-darmstadt.de/3416/1/PhD%20thesis%20Al-Ajmi.pdf; https://tuprints.ulb.tu-darmstadt.de/3416/7/Annex.zip; Al Ajmi, Hussain <http://tuprints.ulb.tu-darmstadt.de/view/person/Al_Ajmi=3AHussain=3A=3A.html> (2013): Sedimentology, stratigraphy and reservoir quality of the Paleozoic Wajid Sandstone in SW Saudi Arabia.Darmstadt, Technische Universität, [Ph.D. Thesis]

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.