Salih Saner

Lithological and Zonal Porosity-Permeability Distributions in the Arab-D Reservoir, Uthmaniyah Field, Saudi Arabia

Permeability vs. porosity plots for the Arab-D carbonate reservoir show a relatively poor correlation, so permeability estimation still remains an issue for discussion. In this study, we integrated basic core measurements with stratigraphic and petrographic analyses to determine porosity and permeability distributions and to identify the association of high permeability, in Darcy range, with lithology and defined reservoir zones. Carbonate rock types in the Arab-D reservoir have been grouped into three lithofacies: granular, muddy-granular, and muddy. The granular lithofacies is the most porous and permeable, and all permeability measurements in Darcy range are associated with it. The muddy facies is represented by low porosity and permeability values. Porosity and permeability distributions of the muddy-granular facies spread over the two regions representative of the muddy and the granular facies. Porosity and permeability distribution patterns of the Arab-D zones are controlled by both type and the arrangement of different lithologies. Zones made up of granular facies demonstrate uniformly high porosity and permeability distributions and reveal the highest permeability intervals. Zones represented by muddy-granular and muddy facies indicate more heterogeneous and mostly bimodal porosity and permeability distributions.

Diagenesis of the Safaniya Sandstone Member (mid-Cretaceous) in Saudi Arabia

Abstract Safaniya Member of the mid-Cretaceous Wasia Formation was deposited as fluvio-marine deltaic sediments in northeastern Saudi Arabia and the northwestern Arabian Gulf. After its deposition and initial burial, the mid-Cretaceous sequence was uplifted during Late Cretaceous structural movements. Structural traps in the region also started forming at this time, but structural growth continued during the Eocene, and new structures developed during the Plio-Pleistocene. The diagenetic mineral assemblage in the Safaniya sandstones includes kaolinite, illite, glauconite, pyrite, quartz and carbonate cements. Kaolinite and illite occur both as authigenic and detrital minerals. Authigenic kaolinite exists as skeletal and euhedral plates forming vermicules and aggregates of booklets infilling the pore spaces. Authigenic illite occurs as overgrowths of projecting laths. Quartz cement is present as syntaxial overgrowths and as microcrystalline quartz. Carbonates occur as patches and show replacement textures. Early diagenetic events include the formation of calcite, siderite, glauconite and pyrite, at or close to the sediment/water interface. Vermicular kaolinite and microcrystalline quartz probably formed by meteoric-water diagenesis during the Late Cretaceous uplift. Quartz overgrowths and the bulk of the euhedral kaolinite may have formed during the Late Cretaceous-Early Tertiary burial. Illite and ankerite were precipitated during deep burial, probably from fluids expelled from the basinal Kazhdumi shales to the east and northeast. The precipitation of authigenic minerals in the clean quartz sandstones has reduced the primary interparticle porosity by an average of about 9% of the rocks. Because the clays occur mainly as pore filling cements, their precipitation has caused only a small reduction in the permeability of these rocks.

Use of tortuosity for discriminating electro-facies to interpret the electrical parameters of carbonate reservoir rocks

Abstract The electrical parameters of 80 various carbonate rock plugs from a Saudi Arabian reservoir were measured. The results were first analyzed without considering the lithology, and then subsequently lithology was considered. Tortuosity distribution and correlations with other petrophysical parameters were used in differentiating samples of similar electrical properties, called “electro-facies”. Tortuosity distribution indicated two populations of data points one of which corresponded to the oolitic and skeletal-intraclastic lithofacies, referred to as “granular electro-facies”; and the other to the muddy and the dolomitic lithofacies, named “dolomitic-muddy electro-facies”. Three methods were applied to interpret the cementation factor: (1) the Archie; (2) the best fit; and (3) the cementation factor as a function of porosity. Error analysis showed that the Archie cementation factor was not an appropriate method for the studied reservoir rocks. The best fit and the cementation factor as a function of porosity methods yielded more reasonable results and they are recommended over the Archie method. A comparison of the water saturations obtained using the electrical parameters of the overall samples with the water saturations obtained using the parameters of each electro-facies showed significant errors especially in low water saturation zones. This indicates the importance of litho- or electro-facies discrimination in heterogeneous reservoirs.

Experimental Determination of the Biot Elastic Constant: Applications in Formation Evaluation (Sonic Porosity, Rock Strength, Earth Stresses, and Sanding Predictions)

An experimental method has been developed to obtain the Biot elastic constant of rocks from laboratory acoustic measurements. To date, the Biot elastic constant has never been determined experimentally. It has always been calculated using various empirical equations. The experimental determination of the Biot elastic constant is very important to engineering problems associated with sand control, hydraulic fracturing, wellbore stability, earth stresses. sonic porosity and estimation of compressional (P) and shear (S) wave velocity. Both the dynamic and static moduli of actual reservoir sandstone core samples. jacketed and mounted in a triaxial cell under high vacuum (< 0.15 mbar), were measured at various confining and overburden pressures. The vacuum was obtained in-situ and maintained till the end of the experiment. This vacuum-dry rock condition represents the dry skeletal frame of the rock. The bulk modulus of the dry skeletal frame of the rock (K sk ) was calculated using the measured P- and S-wave velocities of the vacuum-dry rock. The rock matrix bulk modulus (K s ) was estimated by averaging the harmonic and arithmetic means of the constituent pure mineral moduli. The results obtained show that the Biot elastic constant is a complex function of porosity, permeability, clay content. pore-size distribution and overburden and confining pressure. The experimentally determined Biot constant values may be significantly different from those calculated using empirical equations. This experimental method suggests a new approach to quantifying the Biot elastic constant in the laboratory. This approach gave a better estimate of rock strength, earth stresses. sanding predictions, P- and S- wave velocities, porosity, and pore-fluid from sonic and seismic data.

Formation resistivity response to loading and unloading confining pressure

Abstract Electrical resistivity measurements of sandstone cores were conducted under both laboratory and overburden conditions. At the conclusion of laboratory condition measurements a cementation factor of 1.619 and a saturation exponent of 1.343 were determined. When the measurements were conducted under overburden conditions, cementation factor increased to 1.851 and the saturation exponent increased to 2.144. Lower cementation factors for well cemented samples at laboratory conditions than values observed for poorly consolidated samples denoted that the Archie cementation factor was not indicative of the degree of cementation for the samples studied. The effect of confining pressure was investigated by measuring resistivities of five fully brine-saturated sandstone samples, while the confining pressure was increased from 250 to 2300 psi in a stepwise manner (loading) and than decreased to 500 psi (unloading). The formation resistivity factor increased as confining pressure increased and decreased as confining pressure decreased, but it did not return to its initial value. In all cases the value of the formation factor at the end of the unloading cycle was higher. This is evidence of hysteresis in formation factor in the loading and unloading cycles. The mean values of the formation and cementation factors at overburden conditions were 45% and 13.9% higher than those obtained at laboratory conditions. Low porosity samples were affected to a greater degree in terms of changes in formation factor and cementation factor when confining pressure was increased. This implied that the laboratory condition electrical parameters would lead to underestimation in water saturation calculations. The error in water saturation calculation is more significant in oil zones as water saturation decreases. The error also increases as porosity decreases.

Lithostratigraphy and Depositional Environments of the Upper Jurassic Arab-C Carbonate and Associated Evaporites in the Abqaiq Field, Eastern Saudi Arabia

Lithostratigraphy and depositional environment of the Arab-C carbonate in eastern Saudi Arabia were studied using cores and well logs from the Abqaiq oil field. A 275-m Upper Jurassic evaporite sequence occurs between the Sulaiy-Yamama (Thamama Group) and the Tuwaiq-Hanifa-Arab-D (Tuwaiq Group) carbonate sequences. Dolomite and limestone interbeds of 1-30 m in thickness within the evaporite sequence are continuous over long distances and have uniform thicknesses and log characteristics indicating an aggradational type of deposition. The lithologically heterogeneous Arab-C carbonate is the thickest interbed, reaching 26-30 m in thickness. Lithology types recognized in the Arab-C carbonate are (1) ooid bioclast grainstone, (2) oolite grainstone, (3) pellet grainstone, (4) p llet packstone, (5) skeletal wackestone, (6) mudstone, (7) anhydritic limestone, and (8) dolomite. The Arab-C carbonate has been divided into seven layers based on lithology and porosity variations. Four layers (1, 3, 5, 7) are porous calcarenitic carbonates and the other three layers are nonporous. A basinal (subaqueous) rather than a sabkha-type deposition is interpreted for the evaporite sequence. Lithological and sedimentological successions in the Arab-C carbonate indicate abrupt storm-type deposition and an open-marine connection with the restricted evaporite basin. This sequence was followed by a subtidal carbonate deposition and then a gradual upward return to an evaporitic environment. As a consequence of depositional environments, a massive, very porous interval developed in the lower Arab-C carbonate sequence, and alternating porous and nonporous layers developed in the upper Arab-C.

Electrical resistivity behavior of high salinity brine suspensions

Abstract Resistivities of various suspensions, prepared by adding incremental amounts of rock powder materials ( μ m) in NaCl brines of different salinities ranging from 10 to 200 kppm, were measured and compared. Results indicated a linearly increasing suspension resistivity with increasing volume of solid particles, on a semilogarithmic scale. Except for Wyoming bentonite, the relationships between the resistivity and volume of particles were similar for all materials measured at different brine salinities. An Archie-type linear relationship was observed between the suspension resistivity factor, which is the ratio of the suspension resistivity to the brine resistivity, and the fractional brine volume of the suspension. The slopes m of the linear relationships on log-log plots were comparable for all samples. The average slope was 1.708. The Wyoming bentonite exhibited different characteristics. Elimination of surface conductivity by using high salinity brines and increase of tortuosity due to the association of clay particles probably account for the higher slope ( m = 2.852) for Wyoming bentonite.