Wajid Sandstone Research Articles

Hydrochemical and Isotopic Investigation of the Groundwater from Wajid Aquifer in Wadi Al-Dawasir, Southern Saudi Arabia

The Wajid aquifer is considered the main source of water for drinking and irrigation in Wadi Al-Dawasir and Najran, the southern region of Saudi Arabia. This aquifer has been used since the 1960s, and due to the expansion in agricultural activities, the aquifer has been overexploited. The study aims to understand the origin, hydrochemical processes of the groundwater in the shallow unconfined, deep unconfined, and confined parts of the Wajid aquifer in the Wadi Al-Dawasir area. In-situ hydrochemical parameters (pH, temperature, EC, and TDS) were measured in the field, and groundwater samples were collected for major ions and stable isotopes (2H and 18O) measurements in the laboratory. The results show that the groundwater in shallow unconfined, and confined aquifers are of two types; Cl.SO4-Ca. Na and Cl.SO4-Na. Ca; however, groundwater in deep, unconfined aquifers is characterized as HCO3-Ca. Na, and Cl. HCO3-Ca. Na; types of groundwater. The isotopic analysis results reveal that all groundwater samples have values of δ18Oand δ2Hclose to the local and global meteoric water lines, indicating the meteoric origin of Wajid groundwater. Three major hydrochemical processes, including rock weathering, ion exchange, and evaporation, have been identified as key controls on the chemical composition of water in the studied aquifer. The evaporation and ion exchange processes have more influence on the chemical composition of groundwater in the shallow unconfined and confined aquifers. On the contrary, weathering of carbonate minerals affected more the chemistry of groundwater in a deep unconfined aquifer. The unconfined section of the Wajid aquifer shows a reverse pattern of salinity with higher salinity in the recharge area, which is most probably related to the return irrigation water and leaching of salty soil. The open fractures in the upper part of Wajid sandstone most likely act as conduits to percolated saline water to the Wajid aquifer.

Impact of authigenic iron oxides, clay content and grain size on the aquifer quality properties of the Cambrian-Ordovician Wajid Sandstone, southwest Saudi Arabia

The Wajid Sandstone is one of the most prolific aquifer sequences in southwestern Saudi Arabia. The petrophysical properties of the Wajid Sandstone in Gebel Al-Habala, including bulk and grain densities (ρb, ρg), water and helium porosities (∅w, ∅He), gas permeability (k), true formation resistivity factor (FRT), and the Archie's parameters (a, m) were measured. The aquifer quality parameters were also estimated including the aquifer assessment index (AAI), the flow zone indicator (FZI), the aquifer potentiality index (AQPI), and discrete rock type (DRT).The Wajid Sandstone can be split into two units: the lower red unit (LRU) and upper grey unit (UGU). These are composed of four rock types (RRTs) and six hydraulic flow units (HFUs). The most prolific rock type is coarse-grained slightly ferruginated and calcareous sandstones ‘RRT1’ (average ∅He = 21.72%, ∅W = 12.58%, k = 88.9 md, R35 = 4.86 μm, AAI = 0.59 μm, FZI = 2.10 μm, AQPI = 1.5, DRT = 12.0, and average FRT = 262.0) which form the top of the LRU (HFU-2 unit) and UGU (HFU-6 unit). HFU-2 unit is characterized by the best aquifer quality parameters and is the thickest unit (27.5 m). It forms approximately 72.3% of the flow capacity of the entire Wajid Sandstone sequence and is the most prolific flow unit. The aquifer quality properties of the Wajid Formation are mainly controlled by grain size, which varies from coarse-grained sandstone (RRT1) to siltstone (RRT4). The authigenic clay and iron oxide content are additional diagenetic factors that greatly reduce the aquifer quality. The lowest quality is assigned to HFU-1 unit which is composed of highly ferruginated and compacted siltstone RRT4 (average ∅He = 13.77%, ∅W = 8.44%, k = 0.02 md, R35 = 0.058 μm, AAI = 0.012 μm, FZI = 0.08 μm, AQPI = 0.0, DRT = 6.0, and average FRT = 202.9). The free and forced-regression porosity exponent (m, m') was found to increase with aquifer quality in the studied samples.

Behavior of uranium series in groundwater of the Wajid Formation, Wadi AdDawasir, Saudi Arabia.

The inventories and the possible mechanisms behind the relative deficiency of both radium and uranium release processes within an elevated gamma-anomalous rock were investigated. A field survey was performed on the highest radioactive anomalous zone that was recorded at Jabal Al Alam (20° 13' 10.06″ N and 44° 14' 32.13″), with the ferruginous sandstone, iron oxide band, and iron concretions (with uranium content and reaching up to1500 ppm). The chemical analyses and the laboratory's gamma-ray spectrometric measurements demonstrated high uranium levels in the analyzed rock samples of the Wajid Sandstone (up to 1000 ppm). The borehole geophysical logs further confirmed that the radioactive anomalies are attributed to the sandstone sequence of the Wajid Formation that is often found associated with elevated concentrations of uranium. The groundwater samples taken from the wells tapping the Wajid aquifer showed uranium concentrations ranging from 0.01 to 5.5 ppb (μg/L). The average 226Ra in groundwater samples was 0.2 Bq L-1. The majority of the 226Ra and 228Ra activities were below the lower limit of detection (LLD). The radiochemical analyses of water samples from the Wajid aquifer display low concentrations of both uranium and 226Ra, with relation to uranium content in host rocks. This was attributed to the fact that uranium is susceptible to form iron oxide complexes, causing them to precipitate in a more stable form. Furthermore, iron oxides coat the sand grains of the Wajid Formation and accordingly might act as a foundation for re-adsorption for both uranium and radium, resulting in their relative deficiency in the surrounding water. The coating might also act as a physical barrier resulting in hindrance of the recoil nuclei due to its significant thickness (several orders of magnitude) compared with that of the average (120 nm) whole alpha-recoil track (ART). The coating layer thickness was determined via scanning electron microscopy (SEM) and was found to be up to 180 μm.

Facies, petrography, reservoir heterogeneity and quality of the late Carboniferous-Permian Juwayl Member, Wajid Sandstone, SW Saudi Arabia

The Paleozoic Wajid Sandstone in southwestern Saudi Arabia comprises four members known as Dibsiyah, Sanamah, Khusayyayn and Juwayl. The study characterizes the sedimentary facies of the Juwayl Member from five outcrop localities in Wadi Ad-Dawasir area. The objective is to reveal the influence of facies heterogeneity on the reservoir distribution and quality. The field investigations involve facies description, sampling and architectural elements identification. The second step involves the interpretation of facies associations to predict the depositional environments and controls. The petrographic analysis, of thin section, SEM and XRD, helped to define the diagenetic controls that affected the quality and heterogeneity of rock properties such as porosity and permeability. The facies analysis revealed ten lithofacies corresponding to glacio-fluvial and braided-fluvial depositional environments. The glacio-fluvial deposits consist of pebbly to conglomeratic sandstone lithofacies that suggest high to medium energy conditions within a proximal fluvial setting. The braided-fluvial deposits are of medium to coarse-grained, trough to planner cross-bedded sandstone lithofacies. These lithofacies indicate a lower energy distal condition of laterally switching channels. The remaining lithofacies include overbank deposits of fine-grained argillaceous-ferruginous sandstone lithofacies, observed at the top of fining upward sequences. The study classified Juwayl sandstone as mineralogically mature quartz arenite with good to very good porosity and permeability. It dominates by (90%) quartz, and other components of calcite, clay and iron oxides with subordinate K-feldspars. The diagenetic controls including cementation, grain-size reduction, quartz overgrowth and clay minerals are affecting the Juwayl sandstone porosity and permeability. The depositional environments of Juwayl sandstone indicate a system switching from high energy glacio-fluvial to lower energy braided-fluvial setting. The thickness and width of the sandstone bodies tend to vary in this model implying vertical and lateral sandstone heterogeneity. Clay, hematite and calcite cements contributed to porosity reduction, while grain dissolution, clay coats and rims contributed to porosity enhancement. Macro-scale outcrop heterogeneity in terms of facies, paleonvironments and stratigraphic architecture and microscopic scale in terms of texture, composition and diagenesis, all might provide guides for subsurface reservoir assessment, comparison and correlations.

Geochemical evolution of geothermal fluids around the western Red Sea and East African Rift geothermal provinces

The geothermal provinces of the western Red Sea coast and East African countries have evolved synchronous with the evolution of the landmasses around the Red Sea. The Red Sea opening was initiated subsequent to the opening of the Aden Gulf that was triggered by the Afar plume. During the Late Oligocene, the Red Sea rift started extending from south towards north culminating into a failed arm near the Suez Gulf. The geothermal systems evolved due to shallow mantle below the Danakil depression and circulating seawater. Besides the volcanic flows, the regional sedimentary aquifers like the Wajid Sandstone and Amron limestone, that occur in Ethiopia, Egypt, Eritrea and Djibouti, that were deposited prior to the younger tectonic events that started at 31 Ma are the main geothermal reservoirs in these countries. In contrast, rain surface and evaporated lake water are the main sources for the Kenyan geothermal systems. Although these geothermal fluids show variation in their chemical components due to different recharging sources, all the geothermal systems are of high temperature, capable of generating electricity.

Implications of increasing the ferruginous cement on the physical and mechanical properties of the Cambro-Ordovician Wajid Sandstone in southwest Saudi Arabia: applications for construction purposes

The main target of the present study is to characterize the physical and mechanical properties of the Wajid Sandstones in SW Saudi Arabia and to evaluate its economic value for construction purposes. A number of samples were collected representatively for the upper Gray ‘UGU’ and lower red ‘LRU’ units of the Wajid Sandstones from three sites in the area of study. The studied physical properties include porosity ‘∅’, capillary water absorption ‘Sw’ and bulk specific gravity ‘SPG’, while the mechanical properties include the uniaxial compressive strength ‘UCS’, the ultimate tensile strength ‘UTS’, the Schmidt hammer number ‘RN’ and degree of Abrasion ‘Ab’. Petrographically, the studied Wajid samples are composed of fine to coarse quartz grains cemented by siliceous cement with some ferruginated cement in the ‘LRU’ unit. In addition, some samples were studied using the ‘SEM’ and ‘XRD’ analyses to reveal their mineral composition and homogeneity of their internal fabrics and cement type. The ‘XRD’ and SEM studies indicate that the ‘LRU’ samples are cemented by a ferruginated siliceous silica cement (8.12 ≤ hematite ≤ 13.96%) in the form of iron spherules attached to the pore and grain surfaces. Both the physical and mechanical values of the studied samples have given weights and ratings, and their quality index ‘QI’ was then calculated. The ‘QI’ values indicate that the sandstone samples of the ‘UGU’ unit are fair to poor (19.8 ≤ QI ≤ 22.5) at the different sites and not suitable for construction purposes. On the other hand, the ‘QI’ values of the ‘LRU’ samples are good (32.5 ≤ QI ≤ 37.6) and are suitable for construction purposes. A model has been introduced to predict in precise the ‘UCS’, ‘UTS’, ‘∅’ and ‘SPG’ of the studied Wajid Sandstones during further quarrying. Statistical equations of the proposed model are characterized by very high reliability (0.876 ≤ R2 ≤ 0.975). Finally, the areal extension of the studied fluvial silici-clastic deposits of the Wajid Sandstones have been traced through their regional extensions in the Arabian Peninsula and it has been stated that they are comparable with the Nubia sandstone sequences in NW Africa.

The Wajid Sandstone around Abha/Khamis Mushayt and in its type area: lithostratigraphic architecture and correlations

The Wajid Sandstone of southeastern Saudi Arabia is a prolific aquifer and a recent target for hydrocarbon exploration, but a sedimentologic model and the stratigraphic architecture of the sediments have only recently been presented for the type area of the Wajid Group around Wadi Dawasir. Farther to the west, the Wajid Sandstone was also recognized, but stratigraphic architecture and sedimentology are poorly known. This paper presents the preliminary results of investigations targeted at the outcrops west and south of the type area, covering the area between Wadi Dawasir, Najran in the south, and Abha in the west. Two successions are recognized, a lower red one and an upper beige one. Although the lower one sedimentologically shares several features with the lowermost unit in the type area, the marine Dibsiyah Formation, a correlation remains doubtful. The red succession lacks the Skolithos fauna of the Dibsiyah Formation, and its facies associations point to a fluvial depositional environment. From its stratigraphic position and from its lithology, the red succession is similar to the Siq Formation of the northern Kingdom and it will be discussed whether the red succession might be a yet unrecognized equivalent of the Siq Formation. The lower red unit is bounded by a major unconformity, separating it from the underlying basement. This pan-African unconformity developed during a latest Neoproterozoic–Cambrian episode of intensive weathering and peneplanation. It is characterized by a thick weathering zone and an overlying coarse but thin quartz pebble breccia to conglomerate, which together represent a regolith. The beige succession is definitely correlated to the Khusayyayn Formation. Both successions are characterized by macro-scale to giant 2D and 3D submarine dunes and share many other phenomena. The basal unconformity also shows regolith development with a quartz pebble conglomerate, whose clasts seem to have been reworked from the pan-African regolith.

Chemostratigraphic and sedimentologic evolution of Wajid Group (Wajid Sandstone): An outcrop analog study from the Cambrian to Permian, SW Saudi Arabia

The Paleozoic age succession in Saudi Arabia represents one of the most prolific petroleum producing systems in the Arabian Peninsula. This succession is also considered important for unconventional tight gas and shale gas reservoirs. The Wajid Group (Wajid Sandstone) in SW Saudi Arabia consists of four formations, namely, Dibsiyah (Lower and Upper), Sanamah, Khusayyayn and Juwayl from bottom to top. This study investigates the major oxides, trace and rare earth elements for the Wajid Group formations in southwestern Saudi Arabia. We characterize and compare the sandstone types, provenance, tectonic setting, and climate. Moreover, we applied the chemostratigraphic technique for stratigraphic differentiation. Concentrations of certain elements indicate that Wajid Group was deposited in a passive continental margin. The geochemical analysis reveals that Wajid Group sediments were likely derived from the upper and bulk continental crust and mafic igneous provenance. The elemental geochemical data has been applied in this study to improve the stratigraphic subdivision and correlation. Using selected elements, geochemical vertical profiles, binary, and ternary diagrams allow clearly distinguishing between Wajid Group formations. Thus supports the established formation boundaries that constructed using lithostratigraphy and sedimentology. The geochemical elements variation between formations can be related to differences in rock-forming minerals, facies change, climate, and provenance. The results of this study may help in constraining and correlating complex facies strata and can be used as a guide for stratigraphic correlations in the subsurface within the Wajid basin and other equivalent stratigraphic successions within Saudi Arabia.

Assessment of physicomechanical parameters in the Lower Red Unit (LRU) of the Wajid Sandstone, Bir Askar, Najran, Saudi Arabia

The aim of this study is to determine the mechanical and petrophysical properties of sandstone rocks and using P-wave to correlate among these properties as well as the influence of cement which are intrinsic factors controlling the mechanical properties of clastic rocks. The field study included the description of the Lower Red Unit of the Wajid Sandstone outcrop analog section described previously, lithofacies description, identification, and rock sample collection from the subsurface section. Both X-ray diffraction, SEM and petrographic analyses indicate that the red sandstones are composed of predominantly K-feldspar and quartz grains and an illite matrix. In the laboratory work; porosity (n%), permeability (K md), apparent density g/cc, water absorption (Ab%), void index (e%), Schmidt hammer rebound hardness (SHR), uniaxial compressive strength (σc MPa)), Point load index (Pl) and ultrasonic pulse tests (Vp) were carried out. Then, the results were analyzed statistically to find the correlation coefficient between these measured parameters. The results of Vp with the mechanical and physical properties of the rocks were analyzed using the method of least squares regression. In all cases, the weakly to moderately correlation coefficient (R2) was determined for each regression. Relatively high correlation values were seen between Vp index and porosity of R2 0.60. The sandstone in the study can be classified according to the UCS classification for intact rock into very low to low strength (1–20.39MPa) and very low velocity of sound velocity classification.

Lithofacies, depositional environments and stratigraphic architecture of the Wajid Group outcrops in southern Saudi Arabia

ABSTRACT The Wajid Group is a Palaeozoic siliciclastic succession of southern Saudi Arabia. In the outcrop belt it is ca. 500 m thick, whereas in the subsurface, the thickness increases to more than 4,500 m. The siliciclastic sediments have great reservoir potential for hydrocarbons and for groundwater. Although they represent one of the most important aquifers of the Arabian Peninsula, neither their sedimentologic, lithostratigraphic, nor their reservoir characteristics are satisfactorily known. In this study, a detailed description of lithology and sedimentology is given and the Wajid Group sediments are interpreted in terms of depositional environment and facies architecture. Thirteen lithofacies (LF 1 to LF 13) have been recognised, most of them composed of different subfacies. These lithofacies are grouped into 9 lithofacies associations (LF-A1 through LF-A9). LF-A1 through LF-A3 and LF-A7 represent shallow-marine siliciclastic environments. The remaining lithofacies associations describe periglacial environments of the Hirnantian (Late Ordovician) and Permian Gondwana glaciations. Except for a few pro-glacial fluvial deposits, fluvial successions and aeolian sediments are absent in the outcrops of the Wajid Sandstone. Five formations are recognised in the Wajid Group: the Dibsiyah, Sanamah, Qalibah, Khusayyayn, and Juwayl formations. They are all separated by major unconformities. The Dibsiyah Formation represents a vast sand-sheet complex with core and margin facies formed under shallow-marine conditions. These marine conditions enabled an abundant fauna to proliferate and leave its traces in the form of Skolithos piperock and Cruziana sp. A late Cambrian to Early Ordovician age is inferred for these deposits from regional considerations. The Sanamah Formation records the Late Ordovician Hirnantian glaciation with coarse sandstones and conglomerates. A variety of glacier-induced sedimentary structures are present. The internal succession is composed of three major sediment packages reflecting three ice advance-retreat cycles. The latest of these cycles is overlain by a few metres of marginal-marine sediments of the Qalibah Formation. The Khusayyayn Formation was deposited probably during Early Devonian times. It also represents a sand-sheet environment characterised by the dominance of mega-scale and giant cross beds and bed sets. A marine depositional environment is assumed from scarce Skolithos sp., and because nearly all indicators of a braided river system are absent. The Juwayl Formation of Permian age was deposited at the interface of the Late Palaeozoic Gondwana ice shield with a large lake that may have covered most of southern Arabia and adjacent areas. Proglacial sandstones and conglomerates were deposited close to the glaciers, whereas fine-grained sediment with dropstones, boulder pavements and a wide spectrum of soft-sediment deformation are characteristic of the lake environment. While the two glacial successions and the Khusayyayn Formation can rather confidently be attributed to the geological time scale, either through seismic correlation or biostratigraphically, the Dibsiyah Formation has not yet been biostratigraphically well dated.

Matrix versus fracture permeability in a regional sandstone aquifer (Wajid sandstone, SW Saudi Arabia)

Sandstones are often characterized as fractured aquifers. We present a case study of the Wajid sandstone, which forms a regional aquifer system in SW Saudi Arabia, where matrix, fracture, and large-scale hydraulic conductivities are coincident. The measurements deal with different scales and methods and are based on porosity and permeability measurements in the laboratory, as well as pumping tests in the field. Porosities of the sandstone samples in general are high and range between less than 5 % and more than 45 %. Gas permeabilities for strongly cemented samples are < 1 mD, whereas most samples range in between 500 and 5,000 mD. There is only a weak anisotropy with preference of the horizontal x-, y-directions. Hydraulic conductivities of the matrix samples (5.5 · 10−6 m/s and 1.1 · 10−5 m/s for the Upper and Lower Wajid sandstone, respectively) were in the same order of magnitude compared to hydraulic conductivities derived from pumping tests (8.3 · 10−5 m/s and 2.2 · 10−5 m/s for the Upper and Lower Wajid sandstone, respectively).

Effect of tectonic prominence and growth of the Arabian shield on Paleozoic sandstone successions in Saudi Arabia

Paleozoic successions in Saudi Arabia are exposed around and bordering the south, north, and northeastern edge of the Arabian shield. They are represented by the Wajid group in the south and by the Taymah, Tabuk, Qalibah, Huj, and Buraydah groups in the north and northeast. The Wajid group includes Dibsiyah, Sanamah, Khusayyan, and Juwayl formations. The Taymah group includes Siq, Amai’er, Quweira, Saq, and Qasim formations. The Tabuk group includes Zarqa, Sara, and Hawban formations. The Qalibah group includes Baq’a, Qusaiba, and Sharawra formations. The Huj group includes Tawil, Jauf, and Jubah formations. The Buraydah group includes Berwath, Unayzah, and Khuff formations. The Wajid group form one block in the south and the other groups form another block in the north, and they can be correlated. There are similarities between the northern belt which consists of the Cambro-Ordovician formations of the Tayma and Tabuk groups and the southern belt which consists of the Dibsiyah and Sanama formations of the Wajid group. Similarities include sandstone composition, sedimentary environment, paleocurrent directions, unconformities, tectonic events, and influence of Gondwana glaciations. These formations and probably some or all the rest of the Paleozoic formations used to form one block but later separated after erosion caused by gradual tectonic growth, uplift, and prominence of the Arabian shield. During early Paleozoic time, the process started by poststabilization then sedimentation and at a later stage the growth and uplift of the shield occurred gradually. Growth of shields is a fact and it is the only way to explain the exposure of the Wajid sandstone on top of the highest mountain of the shield which exceeds 3,000 m in As Sawdah in Asir area in southwestern Saudi Arabia. The sandstone sediments of these outcrops were deposited on a low lying basin before been raised to this elevation.

Palaeozoic glacial depositional environments of SW Saudi Arabia: process and product

Abstract Saudi Arabia is one of the few places in Gondwana where deposits of two Palaeozoic glaciations are well preserved. They were formed under similar conditions in a tectonically stable shelf setting. We studied the sedimentary facies and architectural geometries of Upper Ordovician and Permo-Carboniferous glaciogenic deposits in the Wajid Sandstone and developed a genetic model for both units. Common features and of both glacial episodes are (a) incision of glacial (tunnel?) valleys, (b) subsequent valley fill by 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 deformation due to glacial surge during deglaciation, and (e) marine transgression most probably due to eustatic sea-level rise following deglaciation. The general patterns and observations fit well with recently published observations on Upper Ordovician deposits in northern Africa, pointing to closely coupled glaciological processes of the North African–Arabian ice shield. Although the same general patterns apply for the Permo-Carboniferous glaciation, sedimentary styles and petrographical properties differ, probably due to a more heterogeneous ice-flow pattern controlled by a more pronounced topography after the Hercynian tectonic event.

DISTRIBUTION OF PETROPHYSICAL PARAMETERS IN THE CAMBRO‐ORDOVICIAN DIBSIYAH MEMBER OF THE WAJID SANDSTONE, SW SAUDI ARABIA

The Cambrian‐Ordovician Dibsiyah Member is the lowermost unit of the Wajid Sandstone, a major aquifer in SW Saudi Arabia. The Dibsiyah is present throughout most of south‐central Arabia including the Rub'Al‐Khali Basin where it is a potential exploration target and a reservoir for gas. The Member can be divided into Upper and Lower Units based on depositional facies. The Lower Unit is interpreted to have been deposited in a fluvial setting, whereas the Upper Unit, which displays abundant bioturbation, was deposited in shallow‐marine conditions.Patterns of porosity and permeability distribution in the Dibsiyah Member were analysed statistically. In general, the Member exhibits a limited range of porosity values and a wide range of permeability values. Histograms show that the porosity distribution of both Lower and Upper Units are approximately normal, and that their corresponding vertical and horizontal permeability distributions are positively skewed.The coefficient of variation for porosity in both the Lower and Upper Units is less than 0.5 indicating that the porosity range is small. However, the horizontal and vertical permeability values are more variable due to the spatial distribution of architectural elements and sedimentary structures within both units. The dissolution of unstable minerals, together with inhomogenous grain packing and poor connectivity, has resulted in a variable permeability distribution.

Modelo de flujo de fractura basado en imágenes satelitales de Wajid Sandstone, Saudi Arabia

Large-scale geological features have been identified by satellite imagery and global positioning system data in the Wajid Sandstone in Saudi Arabia. The main objective is to evaluate the importance of fractures for the overall flow behaviour in this fractured rock aquifer and to estimate in-situ hydraulic apertures. Data on fractures and lineaments were available for three outcrops. By applying a “cut-out” routine on the fracture endpoint data of these fracture trace windows, three deterministic discrete fracture networks (DFN), with an area of 100 m × 100 m, could be generated. These were used to simulate the fracture flow and to determine the hydraulic conductivity tensors. Using additional data on hydraulic pumping tests and matrix conductivities, in-situ hydraulic apertures could be determined. Average in-situ hydraulic apertures range from 1,300 to 1,700 µm. Observations from the field support these results. In addition, a hydraulic conductivity ratio between the matrix and fracture system was used to identify the contribution of the DFN to the overall fluid transport. A ratio of 10.4 was determined, which indicates that the effective flow behaviour in the Wajid Sandstone aquifer is not entirely dominated by the fracture system, though evidently strongly controlled by it.

Petrography and geochemistry of the Cambro-Ordovician Wajid Sandstone, southwest Saudi Arabia: Implications for provenance and tectonic setting

The Wajid Sandstone, of probable Cambro-Ordovician age, rests unconformably on peneplained Precambrian crystalline rocks on the southeastern margin of the Arabian Shield. The sandstone is composed of varicolored siliciclastic rocks, mainly coarse-to fine-grained sandstones with granule conglomerate and siltstone interbeds, displaying diverse sedimentary primary structures such as planar- and trough-cross bedding, and flat bedding. They form repeated fining-upward cycles, typical of deposition from braided streams. Current indicators indicate that flow was from SW to NNE. Petrographically, all samples of the Wajid Sandstone are of quartz arenite type, highly enriched in quartz, but poor in heavy minerals, feldspar and lithic fragments. The provenance and tectonic setting of the Wajid Sandstone have been assessed using integrated petrographic and geochemical studies. Petrographic analysis reveals that mono-and poly-crystalline quartz grains and heavy minerals from metamorphic and igneous rocks of a craton interior setting were the dominant sources. Chemically, major and trace element concentrations in the rocks of the Wajid Sandstone indicate deposition in a passive continental margin setting. Petrographic and geochemical data suggest that the sediments were derived from metamorphic and igneous rocks forming the adjacent Precambrian basement rocks of the Arabian Shield, and were deposited on a passive continental margin. Deposition took place after stabilization of the Arabian Shield following the Late Precambrian Pan-African Orogeny. The Wajid Sandstone can be correlated with similar deposits elsewhere in Arabia and North Africa which were deposited on a low-lying landmass forming the stable continental margin of the Arabian-Nubian Shield, flanking the southern margin of the Paleo-Tethys Ocean.

Heavy minerals in the Wajid Sandstone from Abha-Khamis Mushayt area, southwestern Saudi Arabia: implications on provenance and regional tectonic setting

ABSTRACTThe Wajid Sandstone (Ordovician-Permian) as exposed along the road-cut sections of the Abha and Khamis Mushayt areas in southwestern Saudi Arabia, is a mediun to coarse-grained, mineralogically mature quartz arenite with an average quartz content of over 95%. Monocrystalline quartz is the dominant framework grain followed by polycrystalline quartz, feldspar and micas. The non-opaque heavy mineral assemblage of the sandstone is dominated by zircon, tourmaline and rutile (ZTR). Additional heavy minerals, constituting a very minor fraction of the heavies, include epidote, hornblende, and kyanite. Statistical analysis showed significant correlations between zircon, tourmaline, rutile, epidote and hornblende. Principal component R-mode varimax factor analysis of the heavy mineral distribution data shows two strong associations: (1) tourmaline, zircon, rutile, and (2) epidote and hornblende suggesting several likely provenances including igneous, recycled sedimentary and metamorphic rocks. However, an abundance of the ZTR minerals favors a recycled sedimentary source over other possibilities.Mineralogical maturity coupled with characteristic heavy mineral associations, consistent north-directed paleoflow evidence, and the tectonic evolutionary history of the region indicate a provenance south of the study area. The most likely provenances of the lower part (Dibsiyah and Khusayyan members) of the Wajid Sandstone are the Neoproterozoic Afif, Abas, Al-Bayda, Al-Mahfid, and Al-Mukalla terranes, and older recycled sediments of the infra-Cambrian Ghabar Group in Yemen to the south. Because Neoproterozic (650-542 Ma) rocks are not widespread in Somalia, Eritrea and Ethiopia, a significant source further to the south is not likely. The dominance of the ultrastable minerals zircon, tourmaline and rutile and apparent absence of metastable, labile minerals in the heavy mineral suite preclude the exposed arc-derived oceanic terrains of the Arabian Shield in the west and north as a significant contributor of the sandstone. An abundance of finer-grained siliciclastic sequences of the same age in the north, is consistent with a northerly transport direction and the existence of a deeper basin (Tabuk Basin?) to the north. The tectonic and depositional model presented in this paper differs from the existing model that envisages sediment transportation and gradual basin filling from west to east during the Paleozoic.

Abstract: Provenance of the Lower Paleozoic Wajid Sandstone, Southeastern Margin of the Arabian Shield: A Geochemical and Petrographic Approach

Abstract: Provenance of the Lower Paleozoic Wajid Sandstone, Southeastern Margin of the Arabian Shield: A Geochemical and Petrographic Approach

The stratigraphy, depositional environments and periods of deformation of the Wajid outcrop belt, southwestern Saudi Arabia

The Wajid outcrop belt in southwestern Saudi Arabia is an area of over 22,000 square kilometers where strata of Middle(?) Cambrian through to Jurassic crop out. The Early Palaeozoic sediments in southwestern Arabia have been placed in the Wajid sandstone. This paper elevates this unit to group status and recognizes its former members as formations. The remaining Palaeozoic successions are placed in the Qalibah and Khuff formations. The Middle(?) Cambrian through to Early(?) Ordovician Dibsiyah Formation, Wajid Group, correlates with the Saq sandstone of central and northern Saudi Arabia. The ‘lower’ Dibsiyah buries a topography developed on the Arabian shield during the Late Precambrian Pan-African and/or the late Early Cambrian periods of uplift. The late Early Cambrian period of deformation is documented in southwestern Saudi Arabia and adjacent Oman by an angular unconformity which separates the Dibsiyah Formation correlative from the underlying Late Precambrian-Early Cambrian Lower Haima Group. The ‘lower’ Dibsiyah was deposited in a northward prograding fluvial apron. The ‘upper’ Dibsiyah was deposited in intertidal to shallow subtidal marine settings during a period of coastal onlap. The Late(?) Ordovician Sanamah Formation overlies the Dibsiyah Formation in angular and/or erosional unconformity. This unconformity was developed during a period of regional uplift which began near the close of the Middle Ordovician. The Sanamah Formation was deposited in broad valley complexes excavated in fluvial settings. The Early Silurian Qusaiba Member of the Qalibah Formation unconformably overlies the Sanamah. The Qalibah Formation was deposited in shallow marine to mid-shelf settings during a major period of coastal onlap. The angular unconformity to disconformity, which separates the Qalibah Formation from underlying units, resulted from diastrophism of Late Ordovician to earliest Silurian age. The Early Devonian to Middle(?) Carboniferous Khusayyayn Formation unconformably overlies the Qalibah Formation. The Khusayyayn Formation was deposited in shallow marine, lagoonal, eolian and fluvio-deltaic settings. The unconformity separating the Khusayyayn Formation from the Qalibah Formation documents a period of regional diastrophism of late Early Silurian to late Early Devonian age. Unconformably overlying the Khusayyayn is the Middle Carboniferous through Early Permian Juwayl Formation. This unit was deposited in broad valley complexes excavated in fluvial settings. The base Juwayl unconformity was produced by the Hercynian orogeny. The Permian Khuff Formation unconformably overlies the Juwayl and the former deposited in shallow marine settings. The base Khuff Formation unconformity resulted from a period of major coastal onlap. The Early Triassic Sudair shale succeeds the Khuff and the former was deposited during a period of coastal offlap in lagoonal to coastal plain settings. Near the end of the Early Triassic the Arabian Peninsula underwent another period of tectonism. In central and southwest Saudi Arabia this event is reflected by the erosion of the Sudair shale and older units from the higher portions of reactivated fault blocks and the syndepositional thickening of the Jilh Formation on the down thrown sides of these fault blocks.

Lithofacies and petrography of Wajid sandstone (Cambrian-Ordovician) Saudi Arabia

Six lithofacies are recognized in the Wajid sandstone. These litofacies are identified as: silty and argillaceous sandstone; fine-grained sandstones; coarse-grained sandstones; pebbley to very coarse-grained sandstones; conglomerates and massive sandstones. Mineralogic composition suggests that the Wajid sandstone is considered clean sands for it consists 95% of quartz grains 5% heavy minerals, mica, potash feldspar, clay matrix, ferruginous cement and traces of carbonate cement. Quartz grain extinction types and inclusions plus heavy mineral types and shapes suggest nearby igneous and metamorphic sources for these deposits, i.e. the adjacent Arabian Shield rocks, in addition to repeated recycling of these sediments as indicated by the appearance of a small number of rounded to very rounded grains of heavy minerals. Furthermore, perhaps most of the secondary silica present in the Wajid was developed from solution and from silica reprecipitated later during periods of aeolian reworking of these fluvial sediments. Moreover, the ferruginous material in Wajid was very likely derived either from siderite or came in solution from igneous and metamorphic provenance or generally came from both sources. The carbonate material in the sediments might either be derived from post-Wajid carbonate formations or formed from solution of carbonate shells trapped in these sediments. X-ray diffraction analysis shows that kaolinite is the only clay mineral present in the Wajid deposits. On the other hand, spectroscopic analysis reveals the presence of Si, Mg, Al and Ca elements as major constituents in the Wajid sandstone. Moreover, according to facies characteristics, the Wajid sediments were deposited under conditions of fluvial environments.