Osman Abdullatif

Channel-fill and sheet-flood facies sequences in the ephemeral terminal River Gash, Kassala, Sudan

Abstract Detailed facies analysis of the River Gash fluvial sediments, on two-dimensional profiles across and along the main channel, has revealed a number of facies and two distinct types of sequences: channel-fill and sheet-flood sequences. It seems that each sequence type is produced by specific depositional event, and both sequences represent two-end members for suite of mixed sequences. The facies and sequences show rapid lateral variability, and they merge and interfinger with each other. They also show partial to complete modification due to subsequent floods and stage variation. It seems that the thick sediments of the River Gash in the Kassala area were produced in two phases. First by braided channel aggradation and lateral migration. Second by both channelized and unchannelized sheet-flood deposition. Deposits of these two phases contribute to the sediments of the present-day Gash fluvial basin in the Kassala area. The Gash sediments show some similarities and differences to those of ephemeral and low-sinuosity braided streams. The predominance of sheet-flood deposits is consistent with the ephemeral flashy high flow regime in the River Gash. The non-conformity of the River Gash depositional style with the braided model of fluvial deposition may be due to this semi-arid terminal flashy character which has control on the depositional processes and patterns.

Groundwater resources in a semi-arid area: a case study from central Sudan

Abstract Hydrogeological investigations were carried out to evaluate the groundwater resources in the northern part of the Khartoum Basin, central Sudan. The hydrogeological system consists of two aquifers. The upper aquifer includes mainly the Upper Gezira Formation, the upper part of the Lower Gezira Formation in the area between the two Niles, and the uppermost parts of the Lower Omdurman Formation in other areas of the study region. The lowef aquifer is developed mainly in the other deeper sedimentary sequences. The sedimentological characteristics of the host formations appear to have affected the physico-chemical properties of the two aquifers in different ways. The grain size and pumping test analyses indicate the higher productivity of the lower aquifer, where the water storage is approximately eight times as much as the upper one. Except in a few parts of the upper aquifer, groundwater in the study area is free from objectionable properties in relation to drinking and irrigation demands.

Integration of facies architecture, ooid granulometry and morphology for prediction of reservoir quality, Lower Triassic Khuff Formation, Saudi Arabia

Prediction of the reservoir quality of oolitic carbonates is one of the main challenges for petroleum geology due to the inherited heterogeneity in pore systems. This study investigated and characterized the sedimentological, stratigraphic, morphological and granulometric data of oolitic outcrop strata of the Khartam Member of the Khuff Formation in Saudi Arabia, including the giant hydrocarbon reservoirs (Khuff reservoirs), and the results have implications for exploration and development. Linking ooid morphology to sequence stratigraphy revealed that the distribution of the oolitic beds and their pore systems are mainly controlled by stratigraphic position, morphology (controlled by primary mineralogy) and grain size. The systematic distribution of ooids with distinct original mineral compositions allows for the prediction of the porosity distribution and occurrence within analogous reservoirs. The ooids in the studied interval were aragonitic (lower interval), bimineralic (middle interval) and aragonitic (upper interval). Linking ooid grain size with depositional environments indicated that the ooid grain size increases with water energy and follows the upwards-shallowing pattern of the studied succession. These results can be used to improve the understanding and prediction of the reservoir quality of oolitic carbonates.

High-resolution facies and porosity models of the upper Jurassic Arab-D carbonate reservoir using an outcrop analogue, central Saudi Arabia

Subsurface models of hydrocarbon reservoirs are coarse and of low resolution when compared with the actual geologic characteristics. Therefore, the understanding of the three-dimensional architecture of reservoir units is often incomplete. Outcrop analogues are commonly used to understand the spatial continuity of reservoir units. In this study, a Late Jurassic outcrop analogue for the Arab-D reservoir of central Saudi Arabia was used to build a high-resolution model that captures fine geologic details. Subsurface reservoir lithofacies were matched with those from the studied outcrop, and porosity values derived from published core and well log data from the Ain Dar, Uthmanyah, and Shudgum areas of the Ghawar Field, eastern Saudi Arabia, were then applied to the equivalent lithofacies in the outcrop. Maximum, minimum, and average subsurface porosity for each lithofacies were distributed in the facies model using a geostatistical algorithm to produce nine porosity models for the field data. Several realisations were run to visualise the variability in each model and to quantitatively measure the uncertainty associated with the models. The results indicated that potential reservoir zones were associated with grainstone, packstone, and some wackestone layers. Semivariogram analysis of the lithofacies showed good continuity in the N-S direction and less continuity in the E-W direction. The high-resolution lithofacies models detected permeability barriers and isolated low porosity bodies within the potential reservoir zones. This model revealed the porosity distribution in areas smaller than one cell in the subsurface model and highlighted the uncertainty associated with several aspects of the model.

Sedimentological, mineralogical, and geochemical characterization of sand dunes in Saudi Arabia

This paper investigates the morphology, texture, composition, mineralogy, and geochemistry, and provenance of sand dunes from 10 locations in Saudi Arabia. Morphologically, these sand dunes include linear, parallel to subparallel ridge, parabolic, barchans, and star sand dunes. Sixty-seven samples were collected from these different sand dune types. Generally, sands from dunes in all locations were characterized by fine to coarse mean grain size, were moderately sorted, and had near symmetrical skewness with mesokurtic distribution. Skewness and mesokurtic distribution characterize sand dunes in most locations except the Red Sea, Qassim, central Arabia, and the Eastern Province where sand dunes all show of subangular grains. The sand dunes are composed of quartz, feldspar, calcite, and mica. Quartz dominates the mineralogy of all sand dunes, although significant amounts of feldspars and mica are found in Najran, the Red Sea, and Central Arabia. While calcite is present in sand dunes at Sakaka and NW Rub’ Al-Khali. Basement-related sand dunes at Najran (N1), central Arabia (C5), and the Red Sea are mineralogically submature. However, nonbasement sand dunes at other locations are mature. Both petrographic and geochemical analyses of sand dunes indicated that most sand dunes are classified as quartz arenite, except in the basement-related sand dunes at Najran (N1), central Arabia (C5), and the Red Sea, where they range from subarkose to litharenite. Moreover, major, trace, and rare earth elements indicated an active continental margin tectonic setting for sand dunes from the Red Sea, basement-related Najran (N1), and central Arabia (C5) sand dunes, and passive continental margins for the other locations.

Porosity evolution within high-resolution sequence stratigraphy and diagenesis framework: outcrop analog of the upper Jurassic Arab-D reservoir, Central Saudi Arabia

The upper Jurassic Arab-D reservoir is considered as the most prolific reservoir in the Ghawar field in Saudi Arabia. Exposed strata equivalent to the Arab-D reservoir was investigated and evaluated to establish a relationship between lithofacies, sequence stratigraphy, diagenesis, and porosity evolution within the Arab-D reservoir analog. The study revealed eight lithofacies which interpreted to have been deposited in an open-marine lower-slope and upper-slope of a ramp platform, ramp-crest, distal to proximal lagoon and tidal flat environments. The diagenetic and paragenetic analysis showed early marine dissolution and cementation, followed by replacement of aragonite and high Mg calcite to low Mg calcite. Porosity enhancement started with shoaling-up of the system. This produces most of the moldic and intraporosity in the study area. Although the outcrop section showed a degree of burial compaction, there is no pronounced effect of compaction on porosity reduction. With extensive progradation of the proximal lagoon and inner ramp along with meteoric realm, near-surface dolomitization was formed and resulted in porosity increase. This was very clear at the top of each of the high-frequency sequences of the lower part of the outcrop. Later, the whole system had undergone fracturing which enhanced porosity dramatically. Although, the outcrop has little or no macro porosity due to sub aerial exposure and recent meteoric cementation, the paragenesis study provides a predictive porosity distribution model within a high-resolution sequence stratigraphy framework and its associated diagenetic events. This model could provide better understanding of porosity evolution and valuable guide for subsurface exploration.

Integration of spectral gamma-ray and geochemical analyses for the characterization of the upper Jurassic Arab-D carbonate reservoir: outcrop analogue approach, central Saudi Arabia

Investigation by spectral gamma-ray (SGR) and inductively coupled plasma-mass spectrometry (ICP-MS) geochemical analyses of the Upper Jurassic Arab-D reservoir analogue (central Saudi Arabia) revealed a strong correlation between the SGR response of the outcrop lithofacies and their elemental content. The two units of the reservoir (the Upper Jubaila Member of the Jubaila Formation and the Arab-D Member of the Arab Formation) showed distinctive SGR log profiles controlled mainly by their lithofacies associations. The geochemical analysis revealed four groups of chemical associations. Group 1 includes SiO2, Al3O2, Fe2O3, K2O, TiO2, Zr and Zn. This group has a strong relationship with the radioactive elements U, K and Th (Group 4). The reservoir facies exhibit high concentrations of chemicals from these two clusters. Group 2 includes CaO and Sr. A high concentration of chemicals from this group indicates a tendency towards pure carbonate facies and fewer siliciclastic impurities. A high concentration of MgO, the only chemical included in Group 3, marks dolomitic zones. Group 4 contains the radioactive elements. The boundary between the Upper Jubaila Member and the Arab-D Member is clearly defined from vertical SGR log profiles, vertical geochemical data logging and cross-plots of Group 1 chemicals with the radioactive elements in Group 4. The geochemical data for the Upper Jubaila Member show a very low concentration of U, K and Th. Consequently, the SGR response of the lithofacies was very low. All of the reservoir lithofacies showed high concentration of Group 1 and Group 4 components compared with the non-reservoir lithofacies. The Th/U ratio indicates a general shoaling upwards following the same trend of the outcrop lithofacies. A high Th/U ratio characterizes reservoir lithofacies, whereas a low ratio characterizes non-reservoir lithofacies. The lithofacies and the SGR log motifs were related in the measured sections. This study provides a method for predicting lithofacies from SGR log motifs within a high-resolution stratigraphic framework integrated with the geochemical data analysis. Here, we introduce an exploration guide for subsurface reservoir zonation and the identification of formation tops.

High-resolution lithofacies and porosity modeling of the mixed siliciclastic–carbonate deposits of the Burdigalian Dam Formation, Eastern Saudi Arabia

Conventional subsurface models of reservoir bodies are limited with respect to the good understanding of small-scale heterogeneities at inter-well spacing. Uncertainties associated with data limitation of such large-scale models can be enhanced by information from small-scale modeling. These high-resolution models may provide a realistic three-dimensional insight into their relevant subsurface reservoir setting. In this context, outcrop studies are frequently used to produce high-resolution models. In this paper, the Burdigalian Dam Formation outcrop in eastern Saudi Arabia was targeted to construct high-resolution models of lithofacies and porosity variations. These models were used to identify small-scale heterogeneities in lithofacies and porosity distribution using geostatistical modeling. Ten different lithofacies were identified and modeled. Indicator semivariogram analysis of the lithofacies showed good continuity in NW–SE direction but less continuity in NE–SW. The porosity model showed differing porosity distributions in different carbonate lithofacies. Several realizations of lithofacies and porosity models were generated and ranked against the input data set. Furthermore, the porosity variation was investigated through detailed petrography and SEM analyses for each of the studied lithofacies. The models demonstrated the presence of patterns of lithofacies and porosity variation at a small scale that cannot be obtained from conventional subsurface models. The study indicated the importance of such models in reducing the uncertainty associated with subsurface modeling as a result of data limitation.

Types and nature of fracture associated with Late Ordovician paleochannels of glaciofluvial Sarah Formation, Qasim region, Central Saudi Arabia

This study evaluates the Late Ordovician glaciofluvial deposits of the Sarah Formation and equivalent outcroppings in north, central, and southwestern Saudi Arabia. The Sarah Formation also covers a wide area in the subsurface and is considered as an important target for unconventional tight gas reservoir. Defining the fracture types, nature, and distribution in outcrop scale might help to establish a successful fracture simulation model and behavior for the Sarah tight gas reservoir in the subsurface. This study investigates fracture characteristics for the Sarah Formation at Sarah paleochannel outcrops. The study revealed three sets of fractures, which have EW, NS, and SE-NW directions, and these fractures vary from open, resistive, and filled to resistive fractures. The closed fractures are filled with ferruginated iron oxides and gypsum. The filled fractures (the thrust boundary) are found in the study area at the SE-NW strike fracture set, while open and resistive fractures are found mainly at S-N and E-W fracture sets, respectively. The syn-depositional filled fractures (iron oxides) are considered as the younger fracture sets while the open and resistive fractures are post-depositional fractures which may have resulted from uplift or tectonic movement. A general model representing the fracture pattern and the thrusting boundaries due to glacial movement was constructed. It has been noticed that the systematic occurrence of filled fractures (thrust boundaries) described the boundaries between different glacial events, which act as a fluid barrier (filled fractures) and decrease the reservoir quality. The finding of this study might be utilized as a guide and lead for exploration in the subsurface Sarah glacial deposits. It will also help to understand and speculate the nature pattern and distribution of fractures with the Sarah Formation.

Workflow of integration of digital outcrop modeling and sedimentology of the Early Triassic Upper Khartam Member of Khuff Formation, central Saudi Arabia

The Permian–Triassic Khuff Formation is considered to be the most prolific for the natural gas production in the world. The Khuff reservoirs exhibit vertical and lateral heterogeneities which cannot be fully captured within the interwell spacing from the subsurface data. The Upper Khartam Member in central Saudi Arabia represents an excellent outcrop analog for the upper parts of Khuff-B and the whole Khuff-A reservoirs. The main objective of this study is to reconstruct digital outcrop model of the Upper Khartam Member. This study included sedimentological data and digital laser scanning modeling. Four stratigraphic sections were measured to cover the whole outcrop of Upper Khartam, and a composite section was constructed. The investigations revealed that the studied outcrop of Upper Khartam comprises six lithofacies associations deposited in three depositional settings: offshore (deep–shallow subtidal zone), foreshoal, and shoal complex. The terrestrial laser scanner was used to scan the outcrop from four different scan positions. The Polyworks software was used for the processing and to produce the photorealistic model for Upper Khartam. ArcGIS combined with the Geo Analysis Tool was utilized to interpret the digital models. The digital modeling allows accurate structural and statistical measurements of lithofacies and integrated with the traditional field data on it and examined the lateral continuity and the architecture of the strata in the outcrop. Also, the model revealed the architecture and the continuity of the reservoir units as well as the barriers or the non-reservoir units within the studied outcrop. The integration of the results of this work with subsurface data might help to provide better understanding and prediction of the quality of Khuff reservoirs in the subsurface.