The application of fluid inclusions to migration of oil and diagenesis in petroleum reservoirs

Fluid inclusions can be used to interpret thermal history and petroleum maturation and migration relative to burial history. Temperature, pressure and composition data collected from fluid inclusions are used to determine the environment of diagenesis and the timing of cementation and migration. Cements in petroleum reservoirs contain both oil and aqueous fluid inclusions. Fluorescence spectroscopy is used to identify oil inclusions and to determine the maturity of entrapped oil. The lifetime of fluorescence induced by a pulsed laser is related to the API gravity of entrapped oil. Interpretation of fluid inclusion data depends on knowing the origin of fluid inclusions and the probability that they survive in the burial environment. Those aspects of fluid inclusion study are investigated by synthesizing oil and aqueous inclusions in calcite crystals in laboratory experiments. Examples of how fluid inclusions are used to determine the physico-chemical environments of diagenesis in petroleum reservoirs and the timing of cementation and migration are given for the Wealden Basin, England, the Mishrif Formation, Dubai, the Smackover Formation, Gulf Coast, U.S.A. and Jurassic sandstones, offshore Norway. In the Wealden Basin, temperature data from fluid inclusions are used to determine that oil migration occurred in the Cretaceous and that the reservoir rocks have been uplifted to varying degrees at a time after migration. Distribution of oil inclusions indicates that generation and migration of oil was principally in the western part of the basin. The geochemistry of oil inclusions in calcite cements from the Mishrif Formation, Dubai, are used to determine the type and maturity of entrapped oil. Temperature data from oil and water inclusions are used to relate reservoir diagenesis to burial history and the migration of oil. In deep Smackover reservoirs oil contains H₂S. The origin of the H₂S is examined by study of fluid inclusions containing H₂S. In Jurassic sandstones, offshore Norway, fluid inclusion studies show that silica cementation is related to burial depth whereas a later calcite cementation originated from invasion of a hot fluid.     

Geographic analysis of manufacturing industry in Saudi Arabia

The study focuses on the analysis of manufacturing industry in Saudi Arabia by examining development, structure and spatial distribution. The analysis has shown that Saudi manufacturing is basically of recent development and is highly capital- and energy-intensive, particularly the hydrocarbon-based groups. Perhaps the most conspicuous weakness of Saudi industrialization is the almost total dependence on expatriate labour. Foreign capital and expertise share in the building and development of the industrial sector by providing technical, managerial, and marketing skills. Saudi industry is heavily subsidized by the government, and the public sector plays a commanding role in industrial development. Large-scale basic industries are owned and run by the government, while light manufacturing of the import-substitution type is dominated by the private sector. One of the main characteristics of manufacturing industry is that it is scantily diversified, modestly concentrated and remarkably linked geographically but not functionally or structurally. Although the authorities are exerting tremendous efforts to achieve a regional balance in industrial development, Saudi manufacturing industry is significantly polarized, urban-oriented and highly associated with the major axes of population scatter. Like all developing countries beginning to industrialize, Saudi manufacturing industry faces certain problems which have to be resolved so as to lay the foundations for a sound and prosperous industrial future. The paper goes on to indicate problems and to suggest solutions.     

The Jinadriyah anticlines: a surface model for oil fields in eastern Saudi Arabia

Mesozoic oil in Saudi Arabia exists in north/south-oriented anticlines. Such anticlines are usually studied using subsurface data. The present study introduces, for the first time in Saudi Arabia, a surface analog for these anticlines. The study covers two northerly oriented anticlines located in the Jinadriyah area at 15 km to the northeast of the Riyadh city. They are named herein the North and South Jinadriyah anticlines. The outcrops in both anticlines belong to the Lower Cretaceous Yamama Formation which consists of limestone in its lower part and limestone with shale in its upper part. The study included initially detailed interpretation of Google Earth and Landsat TM images to map the structural pattern of the anticlines. Detailed field mapping confirmed the satellite image interpretation and helped describe the geometry of the two anticlines in detail. The 3.5-km-long South Jinadriyah anticline is an open doubly plunging asymmetric anticline. The western flank is dissected by 13 minor reverse faults of north–south orientation. The North Jinadriyah anticline is about 5.5 km long and is relatively more complex than the South Jinadriyah anticline. It consists of northern, central, and southern segments that differ from each others in orientation and style. The anticline is dissected by 18 minor faults of different orientations and sense of displacement. Two perpendicular fracture sets with one being parallel to the anticline axes were recorded in the two anticlines. Both anticlines are interpreted as fault-propagation folds that were formed during the Late Cretaceous first Alpine orogeny. The mid-Late Tertiary second Alpine orogeny and Late Tertiary eastward tilting of the Arabian Plate increased the degree of folding and faulting.     

Optimal aquifers and reservoirs for CCS and EOR in the Kingdom of Saudi Arabia: an overview

An overview on the tectono-stratigraphic framework of the Arabian plate indicates obvious differences between two distinct areas: the hydrocarbon-prolific sector and non-hydrocarbon-prolific sector. These differences resulted from the interplay of a variety of factors; some of which are related to the paleo-geographic configuration (eustatic sea level fluctuations, climatic conditions, and salt Basins), others to differential subsidence (burial) and structural inversions. During the Paleozoic, the regional compression was caused by far field effects of the Hercynian orogeny. This led to major folded structures in central and eastern Saudi Arabia (e.g. Ghawar anticline). During the Mesozoic, the most important tectonic factor was the stretching of the crust (extension), accompanied with the increase in temperature, resulting in an increase of the accommodation space, and thicker sedimentary successions. Regional unconformities are mostly found where folded structures are dominant, and they acted as a carrier systems for the accumulation of hydrocarbon and groundwater. A good understanding of the stratigraphy and tectonic evolution is, thus, required to develop carbon capture and storage (CCS) and to design efficiently enhanced oil recovery (EOR) in both sectors. Oil and gas reservoirs offer geologic storage potential as well as the economic opportunity of better production through CO₂-EOR. The world greatest hydrocarbon reservoirs mainly consist of Jurassic carbonate rocks, and are located around the Arabian Basin (including the eastern KSA and the Arabian Gulf). The Cretaceous reservoirs, which mainly consist of calcarenite and dolomite, are located around the Gotnia salt Basin (northeast of KSA). Depleted oil and gas fields, which generally have proven as geologic traps, reservoirs and seals, are ideal sites for storage of injected CO₂. Each potential site for CO₂-EOR or CCS should be evaluated for its potential storage with respect to the containment properties, and to ensure that conditions for safe and effective long term storage are present. The secured deep underground storage of CO₂ implies appropriate geologic rock formations with suitable reservoir rocks, traps, and impermeable caprocks. Proposed targets for CCS, in the non-hydrocarbon-prolific sector, are Kharij super-aquifer (Triassic), Az-Zulfi aquifer (Middle Jurassic), Layla aquifer (Late Jurassic), and Wasia aquifer (Middle Cretaceous). Proposed targets for EOR are Safaniya oil field (Middle Cretaceous) (Safaniya, Wara and Khafji reservoirs), Manifa oil field (Las, Safaniya and Khafji reservoirs) (Late Jurassic), and Khuff reservoir (Late Permian-Early Triassic) in central to eastern KSA.     

The Effect of finite strain and deformation history of HALABAN area,eastern Arabian shield,Saudi Arabia

The present study aims to evaluate a relationship between the mineralogy and structural analysis in the Halaban area and to document the tectonic evolution of Halaban and Al Amar faults. The collected samples were taken from deformed granitiods rocks (such as granite, gneisses and tonalite), metasedimentary, metavolcanic, metagabbro and carbonate rocks are trend to NE-SW with low dip angle in the Halaban area. These samples were 8 from granite, 14 metagabbro, 6 metavolcanics, 5 tonalite, 6 metasedimentary, 10 gneisses and 8 carbonate rocks. Our results are described for the different axial ratios of deformed rocks as the following: XZ sections range from 1.10 to 4.60 in the Fry method and range from 1.70 to 2.71 in the Rf/? method. YZ sections range from 1.10 to 3.34 in the Fry method and range from 1.62 to 2.63 in the Rf/Phi method. In addition, XY sections range from 1 to 3.51 in the Fry method and range from 1 to 1.27 in the Rf/? method for deformed granite rocks, metasedimentry rocks, and metagabbro. The stretch axes for measured samples in the X direction axes (S_X) variety from 1.06 to 2.53 in the Fry method and vary from 1.20 to 1.45 in the Rf/? method. The values of the Y direction axes (S_Y) vary from 0.72 to 1.43 in the Fry method, which indicates contraction and extension in this direction and vary from 1.13 to 1.37 in the Rf/? method which indicates extension in this direction. Furthermore, the Z direction axes (SZ) varies from 0.09 to 0.89 in the Fry method and from 0.52 to 0.71 in the Rf/? method. The stretches axes in the Z direction (SZ) show a vertical shortening about 11% to 91% in the Fry method and show vertical shortening about 29% to 48% in the Rf/? method. The studied rock units are generally affected by brittle-ductile shear zones, which are sub-parallel to parallel NW or NNW trend. It assumed that different rock types of have similar deformation behavior. Based on these results, it is concluded that the finite strain is accumulated during the metamorphism after that was started the deformation by thrusting activity. The contacts between the different rock types were deformed during thrusting under semi-brittle to ductile deformation conditions by simple shear. A component of vertical shortening is also involved causing subhorizontal foliation in the Halaban area.     

Trace analysis of hydrocarbons in coral cores from Saudi Arabia

As part of a long-term environmental assessment of the impact of the 1991 Gulf War on coral reefs, the Australian Institute of Marine Science (AIMS) conducted growth and chemistry studies on coral cores from the Arabian Gulf. Twenty-eight cores were collected from four coral reefs located offshore from Saudi Arabia.Annual coral growth bands surrounding the 1991 oil spill were analysed in selected cores. Additionally, in cores that extended to the early 1980s, annual layers from 1980 to 1986 were analysed for possible residues from the 1983–4 oil spill caused by the Nowruz oil field blowout during the Iraq–Iran war. Both major spill events were targeted to provide additional confidence in relating oil concentrations to specific pollution events. We detected petroleum biomarkers in several coral annual bands related to the major pollution events. However, the oil remaining in these cores has been altered over time and the biomarker ratios found in these oil residues differed from Gulf crude oils.The concentrations of hydrocarbons were compared with the growth parameters of the coral cores. Only one sample may have recorded an “impact” of oil exposure. There was no other correspondence between slight changes in growth parameters between years and the trace chemistry. Therefore, chemical analysis enabled detection of the exposure incident, but provided no insight into the amount of oil to which the coral had been exposed. The chemical data can only infer biological impact if growth was significantly decreased.The average coral growth characteristics of Porites from the four sites in the Arabian Gulf were normal, as predicted from the average water temperatures of the region. There was an indication of an overall decline in growth over time, which should be monitored in the future.     

Geological history and petroleum system of the Mittelplate oil field, Northern Germany

The geological history of Germany’s largest and most productive petroleum accumulation, the Mittelplate oil field in Schleswig-Holstein (Northern Germany), is reconstructed by simulating the structural and thermal evolution along a 2D cross-section. The Mittelplate field is located at the western flank of the Büsum salt dome at the transition from the Schleswig-Holstein mainland to the German North Sea Sector. Organic geochemical data confirm the Lower Jurassic Posidonia Shale to be the predominant oil source rock in the Schleswig-Holstein area. The studied section is characterized by salt walls and salt domes built up by Permian evaporites. Reconstruction of the structural and thermal evolution of the Mittelplate field by means of basin modelling reveals the dominating influence of salt dynamics on the entire petroleum system: The development of secondary rim-synclines during salt rise provided accommodation space for the deposition of the Posidonia Shale as well as the deltaic Middle Jurassic reservoir sandstones. The rise of the nearby Oldenswort salt wall controlled the timing of maturation and petroleum generation during Cenozoic times. Hydrocarbon migration from the Posidonia Shale into the reservoirs occurred up-dip from the deeper subsiding rim-syncline into the structural trap with the Middle Jurassic reservoir sandstones pinching out at the flank of the Büsum salt dome. Along the modelled 2D section the field’s recent temperature field and its complex reservoir architecture are reconstructed.     

Fluid flow evolution in petroleum reservoirs with a complex diagenetic history: An example from Veracruz, Mexico

This paper discusses the fluid flow evolution in the Veracruz petroleum province of eastern Mexico based on results of an integrated diagenetic, sedimentological and structural analysis. The area progressively changed from passive foreland towards an active fold-and-thrust belt into a passive belt with infill of the foreland basin and finally underwent transpressional deformation. This varied kinematic evolution resulted in numerous fluid flow events, among which were four different petroleum migration pulses.     

Strength and compressibility of Abqaiq marl, Saudi Arabia

The increasing number of construction projects and the lack of good quality soil for use in these projects in eastern Saudi Arabia necessitate the use of available marginal soils. Most, if not all, engineering projects use the fine-grained calcareous sediments, known locally as marl, either as a foundation material or as a fill material. The lack of published information on their engineering characteristics and behavior under ambient conditions has resulted in many construction and post-construction problems. This investigation is one of a series to evaluate the geotechnical properties of selected marl soils obtained from different areas in eastern Saudi Arabia. Marl samples, obtained from Abqaiq, were prepared in the laboratory under controlled conditions and subjected to different tests including CBR (soaked and unsoaked), triaxial compression, unconfined compression and one-dimensional compression tests. Samples were prepared at different moisture contents on both the wet and dry sides of optimum. X-ray diffraction analysis indicates that this marl contains high percentages of dolomite and calcite, in addition to other materials. The molding water content significantly affects the engineering properties of this material. Compaction on the dry side of optimum resulted in samples that have some swelling potential and highly compressible characteristics when subjected to a load after inundation.     

Hydrogeology and hydrochemistry of a shallow alluvial aquifer,western Saudi Arabia

A hydrogeological and hydrochemical study was conducted on a shallow alluvial aquifer, Wadi Wajj, in western Saudi Arabia to assess the influence of protection measures on groundwater quality. The hydrochemistry was assessed up-gradient and down-gradient from potential contamination sources in the main city in dry and wet seasons prior to and after the installation of major drainage and wastewater facilities. Wadi Wajj is an unconfined aquifer where water is stored and transmitted through fractured and weathered bedrock and the overlying alluvial sediments. Natural recharge to the aquifer is about 5% of rainfall-runoff. Hydrochemistry of the aquifer shows temporal and seasonal changes as influenced by protection measures and rainfall runoff. Both groundwater and runoff showed similar chemical signature, which is mostly of chloride-sulfate-bicarbonate and sodium-calcium type. Groundwater downstream of the city, though of poorer quality than upstream, showed significant improvement after the installation of a concrete runoff tunnel and a wastewater treatment plant. Concentrations of many of the groundwater quality indicators (e.g., TDS, coliform bacteria, and nitrate) exceed US Environmental Protection Agency drinking-water standards. Heavy metal content is, however, within allowable limits by local and international standards. The chemical analyses also suggest the strong influence of stream runoff and sewage water on the groundwater quality.

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Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.

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Résumé Une étude hydrogéologique et géochimique a été menée sur l’aquifère phréatique alluviale Wadi Wajj dans l’Ouest de l’Arabie Saoudite afin d’évaluer l’influence de mesures de protection sur la qualité de l’eau souterraine. L’hydrogéochimie de l’eau a été étudiée en amont et en aval de sources potentielles de contamination dans la ville principale, pendant les saisons sèches et humides, avant et après l’installation de réseaux majeurs de drainage et d’eaux usées. L’aquifère Wadi Wajj est libre, l’eau est stockée et s’écoule dans les sédiments alluviaux et dans le socle fracturé et altéré sous-jacent. La recharge naturelle de l’aquifère représente 5% des eaux de pluie et de ruissellement. L’hydrogéochimie de l’eau de l’aquifère montre que les changements saisonniers et temporaires sont influencés par les mesures de protection et par le ruissellement des eaux pluviales. L’eau souterraine et l’eau de ruissellement ont présenté le même faciès chimique, de type bicarbonatée-sulfatée-chlorée et calco-sodique. En aval de la ville, l’eau souterraine, bien que de moins bonne qualité qu’en amont, a présenté une nette amélioration après l’installation d’un système de récupération et d’une station de traitement des eaux usées. Les concentrations de plusieurs paramètres indicateurs de la qualité de l’eau (tels que la charge totale dissoute, les coliformes, et les nitrates) dépassent les normes de potabilité de l’eau de consommation de l’agence américaine de la protection de l’environnement. Les teneurs en métaux lourds en revanche n’excèdent pas les normes locales et internationales. Les analyses chimiques indiquent aussi l’influence importante de l’écoulement par ruissellement et des eaux usées sur la qualité de l’eau souterraine.

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Resumen Un estudio hidrogeológico e hidroquímico fue hecho en un acuífero somero de Wadi Wajj, en Arabia Saudi oeste para evaluar la influecia de medidas de protección en la calidad del agua subterránea. La hidroquímica fue evaluada gradiente-arriba y gradiente-abajo de las fuentes potenciales de contaminación de la ciudad principal durante las estaciones seca y lluviosa, antes y después de la instalación de sistemas principales de drenaje y aguas servidas. Wadi Wajj es un acuífero no-confinado donde el agua es almacenada y transmitida a través de roca fracturada y meteorizada, y los sedimentos aluviales que le sobreyacen. La recarga natural del acuífero es de cerca del 5% de la precipitación-escorrentía. La hidroquímica del acuífero muestra cambios temporales y estacionales influenciados por las medidas de protección y la escorrentía de precipitación. Ambas, agua subterránea y escorrentía mostraron composición química similar, siendo mayoritariamente de tipos cloruro-sulfato-bicarbonato y sodio-calcio. El agua subterránea aguas arriba de la ciudad, aunque de calidad más pobre que aguas abajo, mostró significante mejoría después de la instalación de un tunel de concreto para escorrentía y una planta de tratamiento de aguas servidas. Las concentraciones de muchos de los indicadores de calidad de agua subterránea (e.g., STD, coliformes, y nitrato) exceden los estándares de la Agencia de Protección Ambiental USA para agua potable. El contenido de metales pesados está, sin embargo, dentro de los límites permisibles de los estándares locales e internacionales. Los análisis químicos también sugieren la fuerte influencia de la escorrentía y aguas residuales en la calidad del agua subterránea.

     

Earth Fissures in Wadi Najran,Kingdom of Saudi Arabia

The formation of earth fissures due to groundwater depletion has been reported in many places in North America, Europe, and Asia. Najran Basin is in the southern part of the Kingdom of Saudi Arabia, and agricultural activities and other groundwater uses have caused significant groundwater depletion there. The basin recently experienced a sudden appearance of numerous earth fissures. An interdisciplinary study consisting of an evaluation of land-use changes, and hydrological, hydrogeological, and geophysical investigations was conducted to determine the reason for the formation of the earth fissures. The hydrological analysis strongly revealed that the groundwater level is decreasing with time. Groundwater depletion would lead to the accumulation of subsurface stress, causing soil hydro-consolidation which creates the ideal condition for the formation of earth fissures. Electrical resistivity, data indicated that there are anomalies in the profiles, which are most probably due to the presence of subsurface topography, another key factor for the formation of the earth fissures.     

Estimation of groundwater recharge in Wadi Al-Yammaniyah,Saudi Arabia

Groundwater recharge by natural replenishment for the unconsolidated alluvial aquifer in Wadi Al-Yammaniyah is estimated on a daily basis instead of the conventional monthly basis The study reveals that during the two-year period (1978 and 1979), the estimated recharge in the area is about 40% of the total average annual rainfall of 155 mm Subsurface underflow estimated at 36×10⁻⁶ m³/yr from the Wadi Al-Yammaniyah aquifer occurs in the vicinity of Wadi Ash-Shamiyah A comparison of the recharge and extracted volumes of water from the aquifer indicates that there is a net increase of 10 million m³ and 38 million m³ of water in the storage for 1978 and 1979, respectively     

Statistical analysis of different chemical elements in groundwater of northwestern Saudi Arabia

A total of 72 water samples were collected from the sub-surface aquifer system in the Midyan basin and analyzed for 24 major, minor and trace elements. Histograms and normal quantile plots were used to delineate the sub-populations of the chemical constituents in the studied groundwater samples. Some of the elements such as Al, K, Ca, Mg, Na and Cl have concentrations that could be linked to the weathering of the surface rock strata. On the other hand, the elements like As, Pb and Sb have concentration, that can be linked to agricultural practices in the area. The use of simple statistical analysis, frequency histograms, and Q-Q plots were useful for the detection and evaluation of elemental constituents in the groundwater of the study area.     

Estimation of potential evapotranspiration in Al-Hassa oasis,Saudi Arabia

An accurate estimation of the potential evapotranspiration can be achieved ba applying empirical evapotranspiration coefficients which are determined under Al-Hassa prevailing climatic conditions. Correlation coefficient results indicate that most of the climatological variables have a strong relationship with potential evapotranspiration except wind speed and precipition variables which have a weak relationship. Multiple regression results also indicate that there is a significant effect of some of the climatologiecal variables in the potential evatranspiration and there is no significant effect of the others at the significant level of 0.05. The results of the stepwise regression indicate that evaporation class “A” pan, air temperature, sunshine hours, radiation, relative humidity, and wind speed variables have a significant effect at the significant level of 0.05 on the overall mean monthly potential evapotranspiration of the Blaney-Criddle, Pan Evaporation, and Jensen-Haise, therefore, the equation which has developed for estimating potential evapotranspiration in Al-Hassa is based on these six variables (evaporation class “A” pan, air temperature, sunshine hours, radiation, relative humidity, and wind speed).     

Finite Strain Analysis of the Wadi Fatima Shear Zone in Western Arabia,Saudi Arabia

Neoproterozoic rocks, Oligocene to Neogene sediments and Tertiary Red Sea rift-related volcanics (Harrat) are three dominant major groups exposed in the Jeddah tectonic terrane in Western Arabia. The basement complex comprises amphibolites, schists, and older and younger granites unconformably overlain by a post-amalgamation volcanosedimentary sequence (Fatima Group) exhibiting post-accretionary thrusting and thrust-related structures. The older granites and/or the amphibolites and schists display mylonitization and shearing in some outcrops, and the observed kinematic indicators indicate dextral monoclinic symmetry along the impressive Wadi Fatima Shear Zone. Finite strain analysis of the mylonitized lithologies is used to interpret the deformation history of the Wadi Fatima Shear Zone. The measured finite strain data demonstrate that the amphibolites, schists, and older granites are mildly to moderately deformed, where XZ (axial ratios in XZ direction) vary from 2.76 to 4.22 and from 2.04 to 3.90 for the R_f/φ and Fry method respectively. The shortening axes (Z) have subvertical attitude and are associated with subhorizontal foliation. The data show oblate strain ellipsoids in the different rocks in the studied area and indication bulk flattening strain. We assume that the different rock types have similar deformation behavior. In the deformed granite, the strain data are identical in magnitude with those obtained in the Fatima Group volcanosedimentary sequence. Finite strain accumulated without any significant volume change contemporaneously with syn-accretionary transpressive structures. It is concluded that a simple-shear deformation with constant-volume plane strain exists, where displacement is strictly parallel to the shear plane. Furthermore, the contacts between various lithological units in the Wadi Fatima Shear Zone were formed under brittle to semi-ductile deformation conditions.     

Hydrogeochemical characterization and groundwater quality assessment in Al-Hasa,Saudi Arabia

Fifty groundwater samples were collected from Al-Hasa to analyze the pH, electrical conductivity (EC, dS m^?1), total dissolved solids (TDS), major anions (HCO₃^?, CO₃^2?, Cl^?, SO₄^2?, and NO₃^?), major cations (Ca²⁺, Mg²⁺, Na⁺, and K⁺), and total hardness. The analyzed data plotted in the Piper, Gibbs, and Durov diagrams, and water quality index (WQI) were calculated to evaluate the groundwater geochemistry and its water quality. The results reveal that most of the investigated samples are Ca²⁺, Mg²⁺, SO₄^2?, Cl^? and Na⁺, and HCO₃^? water types using the Piper diagram. Na⁺?>?Ca²⁺?>?Mg²⁺ are the dominant cations, while Cl^??>?HCO₃^??>?SO₄^2??>?CO₃^2? are the dominant anions. Sodium adsorption ratio (SAR) values varied from 0.79 to 10; however, the Kelly ratio (KR) ranged between 0.1 and 2.2. The permeability index (PI) showed that well water is suitable for irrigation purposes with 75% or more of maximum permeability. The US salinity diagram revealed that the water quality classes of studied waters were CIII-SI, CIII-SII, and CIV-SII, representing height hazards of salinity and medium- to low-sodium hazard. The water quality index (WQI) results indicated that total dissolved solids are out of the drinking water standard limits in Saudi Arabia. The WQI revealed that 38% of the studied wells were considered as poor water (class III), 52% are found as very poor water class (IV), and 10% are unsuitable water for drinking class (V).     

Geological Controls of Variograms in a Complex Carbonate Reservoir, Eastern Province, Saudi Arabia

To derive meaningful results from a geostatistical study, it is extremely important to establish the relationship between geology and variograms. However, it is not always easy to establish such a relationship, mainly because of the inadequacy of quantitative or qualitative information. Large amounts of reliable well-log porosity data and the detailed geological information from a complex carbonate reservoir located in the Eastern Province of Saudi Arabia provided an ideal case study of the relationships between geology and variograms. The reservoir under consideration is in the form of a gently dipping, elongated anticline and consists of three productive zones which have been subdivided into several lithologically distinct layers. Results, which indicate an excellent match between geology and porosity variograms, are summarized as follows: (1) use of stratigraphic distances resulted in considerable improvement in the behavior of variograms, (2) structurally controlled geometrical anisotropy is the most obvious feature of variograms, and (3) layers consisting of more complex lithologic framework provided variograms with relatively greater nugget variances and shorter ranges. These results provide further insight into geology and form a meaningful basis for estimation and simulation of the reservoir. They also could be used as a general guide in the geostatistical study of similar carbonate reservoirs.