vahid tavakoli

Efforts have been dedicated to correlating pore throat sizes with petrophysical and geological parameters. Log data is helpful in examining and analyzing the degree of pore type, the presence of clay minerals as well as porosity and the density of the reservoir. This study aims to establish a relationship between pore throat sizes and well log data through regression analysis. Well-logging data are routinely accessible and can be compared with the core data.The research uses mercury injection capillary pressure, core samples, and well log data from three wells within a field in the central Persian Gulf region. Equations were developed to link data from various well logs and their combinations to pore throat sizes in the reservoir. To address with challenges, core plugs were categorized into more homogeneous groups using Winland and flow zone indicator rock typing methods. The Winland method revealed that equations within each rock type exhibited low R2 values due to significant porosity variations. Conversely, integrating data from all seven rock types yielded better fits as the diverse porosity values counterbalanced each other's effects. However, the flow zone indicator rock typing approach did not enhance results as it was designed for circular and cylindrical capillary tubes, making it less effective for developing complex equations in carbonate reservoirs. The findings underscore the significance of defining homogeneous units accurately, as this step is crucial for enhancing results and establishing robust relationships between pore throat sizes and well log data. By integrating data from various rock types and refining the approach to defining homogeneous units, the study demonstrates the potential for improving the accuracy and applicability of pore throat size predictions in carbonate reservoirs.

The Middle-Late Triassic sequence in the Zagros and Persian Gulf, known as the Dashtak Formation, holds significant importance as the cap rock for the reservoir formations of the Dehram Group. Additionally, the lower sections of this formation serve as a gas reservoir in certain fields. However, due to the lack of sufficient data on this formation, limited studies have been conducted. This research investigates the facies (and microfacies), depositional conditions, and diagenesis of this formation in an onshore field (Field Y) and an offshore field (Field X) in the coastal Fars region. The data includes petrographic analysis of 548 thin sections prepared from drill cuttings from 15 wells in Field Y, along with well-logging data from 2 wells in Field X. The study results identify 15 microfacies within 5 facies belts. The findings indicate that diagenetic processes such as mechanical and chemical compaction, dissolution, dolomitization, and anhydritization, cementation (including calcite cements of syntaxial, mosaic, pervasive, and coarse-crystalline twin forms), and occasionally fracturing, significantly influence the distribution of pore systems within the various microfacies based on their primary textural characteristics. Comparison of gamma logs from the wells in Field Y and Field X reveals that shale volume in different zones is lower in Field Y compared to the wells studied in Field X. Additionally, carbonate minerals (limestone and dolomite) are more abundant in the wells of Field X. The results also show that Well Y has a higher volume of anhydrite compared to the wells in the offshore field. Overall, the results suggest that the Dashtak Formation in the studied onshore well was deposited at a shallower depth. In other words, the water depth during deposition decreased from the current south to north in the central Persian Gulf region.

One of the most important carbonate gas reservoirs in the Zagros and Persian Gulf sedimentary basins is the Dalan Formation. This research investigates the microfacies, lithostratigraphy, and sequences of carbonate-evaporite deposits in the Lower member of the Dalan and the Nar member belonging to the Guadalupian series with a total thickness of 2402 meters in four wells namely A, D, E, and G located in the eastern part of the Qatar-Persian arc in the Persian Gulf. The study of 340 thin sections prepared from the drilling cutting pieces with 2-meter intervals of well E, and a review of documents related to other wells led to the identification of 20 species belonging to 14 genera of foraminifera and the foraminiferal assemblages consisting of 65 species belonging to 25 genera. A foraminiferal biozone was recognized in well E, assigned to the Wordian? - Capitanian: Pachyphloia- Globivalvulina- Hemigordius, Assemblage Zone.  Additionally, 11 microfacies were determined, mainly distributed in five environments (sabkha, intertidal, lagoon, shoal, and shallow marine). Based on the stacking patterns facies and the gamma-ray log trends, two 3rd-order transgressive–regressive (T-R) sequences were determined. These sequences correspond to the sequences of the reservoir unit’s equivalent formations in the southern and eastern Tethyan basin including Saudi Arabia, UAE, Oman, Iraq and South China.

Understanding the variations in electrical conductivity in porous media is relevant in various fields, including geology, petroleum engineering, and chemistry. Changes in rock electrical resistivity can indicate alterations in their geomechanical properties. Electrical conductivity in rocks reflects the geometric relationships of pores within rock samples, which are influenced by the depositional environment, microfacies, and diagenetic processes. Thus, a thorough study of geological and petrophysical characteristics and their connection to changes in electrical resistivity is crucial for identifying and developing hydrocarbon fields. This research, for the first time, explores the impact of the depositional environment, microfacies, diagenetic processes, and petrophysical factors on the variations in the electrical resistivity of rocks. The dataset for this study comprises 293 meters of core samples, 720 porosity and permeability data points, 921 thin section images, and 251 meters of various well log data, collected from an exploratory well in the central Persian Gulf. The study involved identifying and determining microfacies and depositional environments, examining diagenetic processes through microscopic and macroscopic analysis, and evaluating electrical conductivity in these environments and microfacies using the formation's deep electrical resistivity log. Results show that samples from lagoon environments have lower electrical resistivity and higher permeability, while those from open marine environments exhibit higher electrical resistivity and lower permeability. The sabkha environment, due to extensive anhydrite cementation, has the highest electrical resistivity. Diagenetic processes like dolomitization and isopachous calcite cementation reduce electrical resistivity by improving pore connectivity and preserving primary porosity. Conversely, anhydrite cement and blocky calcite cement increase electrical resistivity by blocking pores and pore throats.

The potential vulnerability of primitive societies to natural disasters, such as droughts, floods, and famines caused by climate change, is an important issue that requires careful study. The main aim of this research is to investigate the possible effects of ancient environmental and climatic changes on Bronze Age settlements in southeastern Iran, as well as the main dynasties that ruled Iran based on archaeological and historical evidence of territorial boundaries, economic and political prosperity. Adaptation of climatic and cultural changes in the southeast of Iran can provide valuable information for researchers. In this regard, this article aims to answer the question of whether climate change has affected the ancient societies of Jiroft, and to what extent climate change has affected the economic prosperity and political influence of the ruling dynasties that have affected Iran. In the present study, using a combination of geochemical and pollinological indicators, we examine paleoclimatic changes of the southeastern plateau of Iran during the past 4000 years. Significant agricultural activities existed between 3900 and 3700 years ago in the southeast of Iran during moderate climatic conditions. Dry conditions with increased dust prevailed over the region from 3300 to 2900 years ago. Wet conditions from about 2900 to 2300 years ago facilitated extensive agriculture and coincided with the flourishing of regional governments such as the Medes, Urartos, and Mannas in the western Iran, and after that the Achaemenid Empire throughout Greater Iran. The decline of the Achaemenid Empire coincided with the beginning of a dry period that made agriculture less prosperous in Jiroft for nearly 200 years. Southeast Iran experienced humid conditions between 1550 and 1300 years ago, which coincided with the economic prosperity of the middle to late Sassanid Empire.

Evaluating the characteristics of carbonate reservoirs, given their significant heterogeneity, is always accompanied by challenges and high uncertainties. Water saturation is a crucial parameter in assessing these reservoirs, and the Archie equation is commonly used for water saturation estimation. Moreover, the effect of water saturation on the mechanical behavior of rock is recognized as an important phenomenon in geotechnical engineering. The accuracy of water saturation calculated through the Archie equation depends on the precision of its parameters, including cementation exponent, saturation exponent, and tortuosity exponent. The heterogeneity of carbonate reservoirs significantly affects the Archie equation coefficients and, consequently, water saturation calculations. In this study, various methods, including the electrical efficiency, current zone indicator, and Winland method, were employed to manage reservoir heterogeneity. Subsequently, Archie parameters were calculated for each category, and water saturation was determined and compared with Dean-Stark water saturation. Furthermore, the influential parameters on the accuracy of water saturation were discussed and examined. To achieve the study objectives, 157 Dean-Stark water saturation data, 57 core plug samples for formation resistivity factor (FRF) determination, 20 core plug samples for measuring formation resistivity index (FRI), 1114 porosity and permeability measurements from core plug samples and 1368 thin sections were utilized from an exploration well in the western Gulf of Persian. Our findings highlight the significance of exploring electrical behavior characteristics and pore throat radii as crucial elements influencing the precision of water saturation calculations. As per the results, employing constant Archie parameters leads to an overestimation of water saturation and, consequently, an underestimation of hydrocarbon reserves. Our analysis illustrates that effectively managing reservoir heterogeneity through the electrical efficiency method significantly improves the accuracy of predicted water saturation compared to other approaches. Conversely, the Winland method exhibits the highest uncertainty in predicting water saturation.

This study investigated the effect of various factors on the pore-throats, in a well in the upper Dalan and Kangan Formations in the central part of the Persian Gulf. Microfacies, sedimentary environments, dominant diagenetic processes on microfacies and the general effect of primary and secondary processes on the dispersion of porosity and permeability data, was investigated. Petrographic studies led to the identification of 12 microfacies in four facies belts in an epeiric carbonate platform environment. The grain dominant texture has higher values of porosity and permeability compared to the mud dominated texture. The most prevalent diagenetic processes are dolomitization, dissolution, cementation and compaction. The moldic, vuggy, intercrystalline and intergranular porosities are the most important identified pores. The study of the effects of each diagenetic process separately indicates the effect of the process of dissolution and dolomitization on increased the pore-throats and reservoir quality. Results showed that cementation and compaction alone do not have a reducing role and its greater effect is due to the combination of both factors. In order to investigate the effects of facies and diagenesis on the pore-throat, porosity and permeability data were plotted on the Winland diagram and seven rock types with different pore-throats were identified. Due to the effects of different diagenetic factors on the microfacies, the pore-throat sizes vary between same microfacies. The grain dominant facies (packstone and grainstone) belonging to the shoal facies belt, have the highest pore-throats and the best rock type determined by the Winland method. Plotting the porosity and permeability data in the Winland diagram has let to detection of 7 zones with different pore throat size, that samples with higher R35 are affected by dissolution and dolomitization and samples with lower R35 are affected by cementation and compaction.

One of the key and influential factors that affect reservoir quality is the occurrence of effective diagenetic processes, including dolomitization, anhydritization, and dissolution (moldic and vuggy porosities). This research focuses on studying these processes in the Upper Dalan Member and Kangan carbonate formation located in the Central Persian Gulf. Initially, a comprehensive analysis of petrography was conducted to examine the effects of diagenesis at different depths within core samples from key well. Subsequently, well logs such as NDS, Vp/Vs, RHOB, VDL, and SPI were utilized to create electrofacies related to dolomitization, anhydritization, and dissolution using MRGC method. By applying the geostatistical kriging method to the data of electrofacies from twelve wells, three-dimensional models of dolomitization, anhydritic cementation, and dissolution processes were constructed at a field scale. Based on the sedimentary model of ramp carbonate depositional environment in the Permian-Triassic and the dolomitization model of seepage-reflux, it has been determined that in horizontal sections, the major dolomitized and anhydritic portions are predominantly observed in the southern and middle parts of the field. In the vertical sections, based on the minimum sea-level surfaces (middle K4, lower K3, upper K2, middle K2, and upper K1), the major dolomitized portions are observed in Upper K1, Upper K2, and Lower K3. The major anhydritic portions are found in Upper K1, Middle K2, and Lower K3 to the top of the boundary with K4. Considering that dissolution predominantly occurs in areas exposed to water and that most moldic and vuggy porosities are formed as a result of dissolution, dissolution is more prevalent in the southern to middle parts of horizontal sections. Consequently, there is a higher concentration of moldic and vuggy porosities in these southern to middle parts of horizontal sections. In vertical sections as well, moldic and vuggy porosities align with those seen in upper K1, upper K2, and lower K3.

Quantitative heterogeneity presentation is very important to compare the amount of heterogeneity in different reservoir characteristics in carbonate reservoirs. The aim of the present study is to quantify the heterogeneity by calculating the heterogeneity log and its application in investigating the heterogeneity of porosity and permeability in the Late Barremian-Early Aptian Darian carbonate Formation in the central part of the Persian Gulf. In order to calculate the heterogeneity logs, the well log data of gamma ray, bulk density, neutron porosity, acoustic and deep resistivity and the statistical methods of Lorenz coefficient and coefficient of variation have been used. Results show that the calculation of the heterogeneity log provides the possibility of quantitative comparison of the heterogeneity between different parts of the reservoir for different purposes. Heterogeneity calculation with both methods is efficient for calculating heterogeneity, but it should be noted that the coefficient of variation for the maximum heterogeneity does not have a specific range like the Lorenz coefficient. Also, increase in porosity is associated with a decrease in the amount of heterogeneity in the heterogeneity logs of deep resistivity, acoustic, neutron porosity and density. Unlike other logs, gamma ray heterogeneity log has little direct relationship with porosity in Darian Formation. Two types of neutron porosity heterogeneity and deep resistivity are the most used in determining the reservoir properties in this formation. Permeability shows a weak relationship with heterogeneity logs. In bioclast wackestone microfacies of carbonated units with high porosity, less heterogeneity is observed. Also, units with higher porosity are thicker and therefore exert stronger mean effects on heterogeneous areas. High variability in porosity and permeability causes higher heterogeneity. This is also the case about lithology and clay minerals. Heterogeneity changes at facies change boundaries. Therefore, heterogeneity log can be used to investigate geological heterogeneity, porosity, permeability and reservoir zoning.

Carbonate reservoirs are very complex and heterogeneous. Overcoming heterogeneity is important and necessary for accurate reservoir characterization. Dalan-Kangan formations, as the largest non-associated gas reservoir of the world, are heterogeneous and complex due to the influence of the sedimentary environment and diagenesis processes. Carbonate reservoirs are routinely studied using laboratory data. The use of well logging instead of using laboratory methods is very cost-effective in reducing time and cost. To overcome the heterogeneity by acoustic log, samples of Dalan-Kangan carbonate formations were prepared. A total of 87 limestone thin sections were evaluated by petrography, routine core analysis, and sonic velocity. Porosity, permeability, sedimentary textures, pore types, and diagenetic processes were determined precisely. From the studied well, well logging and acoustic logs were also available. After data quality control, the acoustic log was converted to velocity. Velocity-porosity model was constructed based on the differential effective medium (DEM) approach for different values of equivalent pore aspect ratio (EPAR). The results show that moldic, vuggy, interparticle, and microporosity pores have the largest aspect ratio, respectively. Due to their spherical shape and high aspect ratio at the given porosity, moldic and vuggy pores have higher velocity than flat and narrow pores. Through the geometrical shape and pore type and by using the acoustic log, the rock types were determined. These rock types clearly showed the porosity evolution, permeability changes and diagenetic processes that have been occurred in Dalan-Kangan reservoirs.

In the present study, in order to reconstruct the paleoenvironmental and climatic changes of Jiroft during the last 4000 years, several evidences of sedimentology and biogeochemistry on a sedimentary core have been investigated. Around. 3950 cal yr BP, low values of Ti/Al, Si/Al, C/N and CPI along with high values of δ13COM, and Paq indicate a wet period in Jiroft. evidence indicates a relative decrease in humidity between about 3900 and 3293 cal yr BP. Between 3293 and 2897 cal yr BP, Jiroft was dry and dusty. The results show very dry conditions with a significant increase in the amount of dust around 3200 cal yr BP. A long-wet period from about 2897 to 2302 cal yr BP can be recognized with high Paq values. The highest Ti/Al values along with the lowest δ13COM values indicate increased wind activity and dry conditions between 2100 and 1650 cal yr BP. Jiroft experienced wet conditions between 1540 and 1315 cal yr BP. With the relative decrease of rainfall, a semi-humid climate prevailed in Jiroft between 1315 and 854 cal yr BP.

The Lachinag synformal anticline is located to the northwest of the Mil anticline and the western end of the Dochah Fault in northern central Iran. This synform includes the terminal members of the Qom Formation (members E and G) and the Upper Red and Pliocene conglomerates. Additionally, with an axial surface plunge from the northwest to the southeast, this fold trends toward the southeast, resulting in its asymmetric geometry. In this synform, the deformable marl and gypsum layers of the E and G members of the Qom Formation have contact with the competent conglomerate and sandstone layers from the Upper Red Formation. The juxtaposition of these layers and the occurrence of deformation phases resulted in the migration of ductile layers as well as a significant increase in the thickness of the marl and gypsum deposits of the Qom Formation (particularly in the E member). Due to the migration of these layers towards low pressure areas and their substantial thickening at the hinge of the Lachinag synformal anticline, the layers as well as the limbs of this fold were overturned. Finally, a synformal box fold was formed. Structural investigations have revealed that this fold initially formed as a result of the right-lateral strike-slip shear parallel to the Dochah Fault. This process occurred during the post-Miocene under the influence of counterclockwise left-lateral strike-slip shear forces around a pole axis at approximately 135 degrees from the north. Consequently, the fold acquired a synformal geometry. The left-lateral shear force may be due to the clockwise rotation of the South Caspian basin and the application of left-lateral shear forces on the northern parts of Central Iran, similar to what has been observed in the Kushk-e Nosrat Fault.

Paleohighs play an important role in increasing the quality and sedimentological characteristics of the reservoirs. The Upper Dalan Formation with Late Permian age in the center of the Persian Gulf Basin is known as a giant gas reservoir. In this paper, 60 meters of the Upper Dalan Formation have been studied in two fields in the center of the Persian Gulf Basin. Petrographic studies of the two fields led to the identification of seven microfacies in field A and six microfacies in field B in the form of four facies belts in the carbonated ramp environment. In order to evaluate the heterogeneity of the reservoir, the determination of rock types was performed by using four standard methods. Examination of the average porosity and permeability shows that the dissolution in field B is more severe than in field A. Study of diagenetic processes, the type of dolomites formed and the average porosity and permeability in the samples of the two fields indicate that the location of field B on the Qatar arc leads to the formation of dolomicrites, increased dissolution and finally higher reservoir quality in microfacies groups in this field in comparison with field A.

This study focuses on reservoir heterogeneities of the Sarvak Formation in the Abadan Plain. Coefficient of variation (CV) and Dykstra-Parsons (VDP) approaches are used for quantification of reservoir heterogeneities. To define the hydraulic flow units, rock types, and reservoir zonation, flow zone indicator (FZI), Winland R35, and Lorenz (SMLP) methods are adopted. Heterogeneities of porosity and permeability data are quantified in each rock type, HFU and reservoir zone. Then, results of sedimentological studies are integrated with petrophysical data to analyze their scale, origins, and predictability in sequence stratigraphic framework. Depositional and diagenetic heterogeneities are differentiated. Facies variations and changes in textural characteristics and sedimentary structures provided small scale heterogeneities. Meteoric dissolution beneath the Cenomanian–Turonian palaeoexposure surface formed the best reservoir zone of the formation. Cementation and compaction, mostly related to burial diagenesis, reduced the reservoir quality. Results of this study indicate that large scale heterogeneities of the Sarvak Formation are predictable in the framework of third-order sequences. Meteorically dissolved rudist dominated facies provided the best productive zones in the regressive systems tract (RST) of the Cenomanian and Turonian sequences. However, small scale heterogeneities are not easily predictable in third-order depositional sequences and systems tracts.

The aim of the present study is to investigate the permeability changes and the ratio of horizontal to vertical permeability in Kangan and Upper Dalan carbonate reservoirs. The effect of geological parameters such as lithology, texture, facies and porosity has been investigated on the amount of horizontal and vertical permeabilities and their changes as well as the ratio of horizontal to vertical permeability in these formations in the central part of Persian Gulf. For this purpose, statistical methods have been used on the data obtained from the study of plugs and microscopic thin sections prepared from the 402 meters of cores. The results showed that the mean value of horizontal permeability is closed to the mean value of vertical permeability in the two types of dolomite and limestone lithology as well as crystalline carbonate and mudstone textures. By increasing porosity without considering other geological factors, permeabilities increase and the ratio of horizontal to vertical permeability decreases. Also, the two factors of heterogeneity and increase in the amount of grains (from mudstone to grainstone texture) are more effective in comparison with micrite and cement in the changes of permeabilities. The presence of grains increases heterogeneity. Horizontal permeability will be higher than vertical permeability in energetic sedimentary environments and in grain-supported facies. It was also observed that dolomitization increases the ratio of horizontal to vertical permeability due to increased mineralogical heterogeneity. The ratio decreases in very high amounts of this mineral (more than 90 percent) due to the homogeneity of rock texture. Therefore, this ratio depends on the degree of heterogeneity of rock, the amount of grain, and lithology. Increasing porosity and permeability, homogeneity of the sample both in terms of the type of mineral forming the rock and in terms of texture, causes closer horizontal and vertical permeabilities.

Qom region is one the significant area insight of geological features in Central Iran. Several researches have studied about the Cenozoic strata in terms of sedimentology, Stratigraphy and paleontology but, few structural detail data are available from this area. The most important exposure of the rock unites at the west of the Qom city is related to the Eocene volcanics, Lower Red, Qom and Upper Red Formations. Major structures at this area are Kamar Kuh and Mil anticlines, Yazdan syncline, Dochah and Sefid Kuh faults. Dochah Fault with E-W trending and ~70° dipping to the northward placed at the northwest termination of Qom-Zefreh Fault as a recent sinistral strike slip fault. This fault with ~15 km length separate Mil anticline from Yazdan syncline and eliminates the southern limb of Dochah overturned anticline. In this study, we focused on the Dochah Fault damaged zone in order to paleostress analysis using geometric and kinematic characteristics of fault slip data, which is achieved from the deformed Qom and Upper Red Formations. For this purpose, 100 fault slip data with precise and accurate geometric and kinematic characteristics have been measured in the field and analyzed with software Dasiy and Rotax methods. In order to determine the sense of shearing of the faults, the criteria of Petit (1987) and Doblas (1998) have been used. While the trend of the major structures is east-west but, most of slip data is related the transverse oblique slip faults, because the Dochah Fault passes through the soft materials of Lower Red Formation and consequently it is not possible or too hard to find the slicken line. Meanwhile, our results indicate the magnitudes of the axes of the maximum and minimum principal stress (σ1, σ3) as 030/05 and 285/05, Geometric and kinetic structural analysis related to the dochah fault and according to the spatial arrangement of the main stress axes indicate the readiness of the left-hand section on the right-hand section, especially in the western parts of the region (Caspian) attributed. oblate shape of field stress ellipsoid shape (R~0.7). Based on the field stress ellipsoid shape and the rotation of the fault data regarding the Anderson's theory for the compressive stress regime, the stress transition trajectory map has been prepared. The arrangement of maximum stress trajectories is consistent with the general stress regime in the Iranian crust and is consistent with the activity of the Dochah Fault. Different criteria have been proposed to evaluate the activity of a fault in terms of seismicity. In experimental studies, there are various estimates for selection of the part of the fault that the movement rediscovers for each tectonic seismic zone. Here, the possibility of moving Dochah Fault has been estimated by the method of Lee et al. (1997). In this method, the angular relationship between maximum principal stress axis (σ1) and the pole of the fault plane considered in order to evaluate the Fault Movement Potential (FMP) based on equation “FMP=f (G, σ)”. The angle between maximum principal stress axis (σ1) and the pole of Dochah Fault (θ) is equal to ~40° and so FMP=0.33 based on equation FMP= (θ-30°) ⁄ (30°) if θ∈[30°,60°]. This value of FMP indicates the low seismic potential of Dochah fault for movement and creating earthquakes.

Porosity and permeability of reservoir rocks increase due to the significant pressure drop at surface condition, accordingly this leads to overestimation of volume of hydrocarbons in place. In this study, the effect of overburden pressures on the porosity changes of Kangan and Dalan carbonate formations in the central part of the Persian Gulf has been investigated. Porosity and permeability of 111 plug samples were measured at pressures of 14.7, 2200, 3700, 5000 and 5700 psi. Porosity decreases up to 2 % by increasing up to maximum pressure. Samples react to pressure changes differently; and therefore, they justified into various groups. To evaluate the effect of heterogeneity and complete understanding of these reactions, the most acceptable methods of Winland, flow zone indicator, Lucia, porosity and permeability ranges have been used. The changes in porosity against pressure were plotted for each sample and the equation of decreasing the porosity by increasing pressure was defined. The purpose of this study is considering the changes in porosity by increasing the pressures; and therefore, the resulting line slope was considered as an index of changes, and samples with the same slope were considered as similar samples. In other words, if the rate of porosity change versus pressure is similar, the samples have been assumed as similar ones. Afterwards, the coefficient of variation was calculated as the index of line slopes’ dispersion. The results of various rock typing methods indicated that the best methods for evaluating the heterogeneity of porosity change with pressure, are flow zone index, permeability range, Winland and Lucia, respectively.

The upper Permian–Lower Triassic Dalan and Kangan formations comprise two of the most important hydrocarbon reservoirs in the Persian Gulf. The goal of this study is to analyze sedimentary microfacies and establishing a conceptual depositional model and the sequence stratigraphy framework of the upper Dalan and the Kangan formations in central and eastern oil fields Persian Gulf. According to petrographical studies carried out on 3651 thin sections, fifteen carbonate microfacies associated with tidal flat, lagoon, shoals and open marine depositional settings were identified. Microfacies analysis and their vertical and lateral changes, gamma-ray response as well as comparison with the ancient and modern analogues suggest a gentle carbonate ramp as the depositional model for the studied successions. Comparison of the microfacies abundances in the studied oil fields revealed dominance of the shallower depositional setting in the central field studied well compared to those of the eastern oil fields which indicates a northeast-ward deepening paleo-dip of the sedimentary basin. Five third-order depositional sequences were established; three sequences labeled here as USD1, USD2 and USD3 in the upper Dalan Formation and two sequences in the Kangan Formation namely KGS1 and KSG2. The sequences comprise TST and HST system tracts, separated by sequence boundaries and maximum flooding surfaces. While the sequence boundaries were identified based on the sedimentary successions or microfacies indicative of relative sea level fall, the maximum flooding surfaces were recognized based on occurrence of deepest microfacies in the sequences. Correlation of the established sequences in the studied wells represents an overall shallowing upward depositional pattern.

Common methods for measuring permeability include well test data and core samples. In the laboratory, due to the low cost and high operating speed, the absolute gas permeability of plug samples is often measured. The permeability of the gas in the porous media depends on the average injection pressure and the type of used gas. It is obvious that by changing these parameters, different results are obtained. Therefore, it is necessary to measure the liquid permeability or determine the equivalent permeability of the liquid (Klinkenberg permeability). In this study, a total of 1245 porosity and permeability data were obtained from core analysis of Kangan and Dalan carbonate formations. The permeabilities of 111 samples were measured at 4 different pressures. The pressures were plotted versus permeabilities and the fluid equivalent permeability was calculated. Then, in this diagram, the slope of the fitted line was calculated for each sample, and the coefficient of variations was calculated for the slope of the samples in each rock type to find out the scatter of the samples. Results showed that determination of rock types by the used methods (Winland, flow zone indicator and Lorenz) has little effect on improving the results of this experiment (determination of the Klinkenberg coefficient). Among the methods used to determine the rock types, flow zone indicator parameters (porosity, permeability and matrix) have direct effects on the slope of the line (permeability-pressure regression line). This method also yields the highest coefficient of determination. It can also be concluded that it is not possible to obtain the equivalent liquid permeability of all samples using a constant correction factor. Comparison of the liquid permeability results with laboratory measured values shows that the method could be used for increasing the accuracy of the predicted permeabilities.

Abstract
This study investigates the effect of facies and diagenesis on the pore-throats in 110 m of carbonate rocks of the Dariyan Formation from one well in the central Persian Gulf. A total of 390 thin sections and 160 core porosity and permeability data were investigated. Petrographic studies led to the identification of nine microfacies in five sub-environments deposited in a carbonate ramp platform. The most important identified diagenetic processes include micritization, cementation, compaction, fracturing and neomorphism that affected the sediments of the Dariyan Formation during different stages of marine, meteoric and burial diagenetic environments. The Winland number has been calculated and compared with measured pore throats. Comparing the Winland rock types with microfacies, it is concluded that two microfacies of laminated mudstone and orbitolina wackestone have the smallest pore-throats (R35< 0.1 µµ). This is due to mud-dominated texture, presence of micrites, presence of clay minerals blocking the pores, and compaction and cementation. In contrast, bioclast peloid packstone to grainstone of shoal environment, due to the grain-dominated texture, low micrite abundance and open fractures, have the highest pore-throats (0.5<R35< 1 µ) among the total microfacies. In some samples, cementation and compaction have reduced the pore-throats and transferred them to units with a value of R35 between 0.1 to 0.5 µ. The upper Dariyan and lower Dariyan contains the microfacies with large (R35> 0.5 µ) and medium (R35> 0.5> 0.1 µ) pore-throats and have the best reservoir quality. In contrast, the Hawar Member corresponds to microfacies with a smaller pore-throats (R35> 0.1 µ) and has almost no reservoir quality.
Keywords: Dariyan Formation, Microfacies, Diagenetic, Pore-throats, Winland diagram
 
 
Porosity and permeability are two important parameters in identifying and understanding the behavior of an oil reservoir. These two parameters show many changes in the shallow carbonate reservoirs (Moore 2001; Lucia 2007). Rock types divide the reservoir rocks into separate units based on similar properties such as permeability and porosity. There are different ways to classify the rock types. The Winland method (Winland 1972) is one of the most widely used approaches for determining these rock types. The method is based on the Winland empirical equation which expresses the relationship between porosity, permeability, and pore-throat sizes at 35% mercury saturation in mercury injection capillary pressure (MICP) test. The diversity of microfacies and the effects of diagenesis, determined the pore-throat sizes and resulted Winland number. Therefore, it is important to study the geology of carbonate reservoirs and the impact of facies and diagenesis on the pore-throat sizes. This study, after examining facies and diagenesis, determines the impact of these two factors on the pore-throat size distribution in the Dariyan Formation. The results could show the most important factors affecting the distribution and changes of the pore-throat sizes in the carbonate rocks of this formation.
 
Materials & In this study, 110 meters of carbonate rocks of the Dariyan Formation in a well from a field in the central part of the Persian Gulf have been studied. At first, petrographic studies were performed using a polarizing microscope (on 390 thin sections) and the facies were named after the Dunham classification method (Dunham 1962). Flügel classification (Flügel 2010) was used to determine the sedimentary environments of the microfacies. Then, the diagenetic processes were investigated. Porosity and permeability of 160 plug samples were measured using Boyle and Darcy methods, respectively. Winland formula has been used to calculate and classify the pore-throat sizes of the samples. At last, the effects of microfacies and diagenetic processes on pore-throat sizes have been investigated through porosity-permeability diagrams and statistical calculations. To check the pore throats using the Winland R35 method, it is necessary to ensure a good correlation between the values of R35 calculated through the experimental formula of Winland using the porosity-permeability values and the actual values of R35 determined by the mercury curves.
 
Discussion of Results & Facies analysis led to the identification of nine microfacies in five sub-environments deposited in a carbonate ramp platform. Micriticization, bioturbation, cementation, especially calcite cement, compaction (physical and chemical), neomorphism and fracturing are observed in the studied formation. These diagenetic processes affected the sediments of the Dariyan Formation during different stages of marine, meteoric and burial diagenesis. Petrographic studies do not show high porosity, but core tests show considerable porosity in these samples. This confirms the presence of microporosity as the main type of porosity in the samples. To investigate the effects of facies and diagenesis on the pore-throat sizes, porosity and permeability data were plotted on the Winland diagram. According to the Winland method, three zones with different pore-throats (R35< 0.1 µ, 0.1 <R35< 0/5 µ, 0.5<R35< 1 µ) were identified. The Winland number has been calculated and compared with measured pore throats. The results show an acceptable correlation between the measured pore throat values and the calculated pore throats. Zone 3 (0.5<R35< 1 µ) shows the best reservoir quality. Comparing the Winland rock types with microfacies, it could be concluded that two microfacies of laminated mudstone (MF1) and orbitolina wackestone (MF3) have the smallest pore-throats (R35< 0.1 µ). This is due to mud-dominated texture, the presence of micrites, the presence of clay minerals that block the pores, compaction and cementation. Bioclast wackestone (MF2) microfacies deposited in lagoon environment have the highest frequency in units with a value of 0.1> R35 µ. Two microfacies of MF5 and MF8 with average pore-throats have the highest frequency in units with a value of 0.5<R35< 1 µ. In contrast, bioclast peloid packstone to grainstone of shoal environment, due to the grain-dominated texture, low micrite abundance and open fractures, have the largest pore-throats (0.5<R35< 1 µ) among the total microfacies. Cementation and compaction have reduced the pore-throats of some samples and moved them to units with a value of R35 between 0.1 to 0.5 µ. The upper Dariyan and lower Dariyan contain the microfacies with large (R35> 0.5 µs) and medium (R35> 0.5> 0.1 µs) pore-throats and have the best reservoir quality. In contrast, the Hawar Member corresponds to microfacies with smaller pore-throats (R35> 0.1 µs) and has almost no reservoir quality. MF6 and MF9 have various pore-throat sizes due to the effect of different diagenetic processes and could be observed in all three zones.

Upper Dalan and Kangan carbonate formations with Late Permian–Early Triassic ages are considered the main gas reservoir rocks in some Persian Gulf fields. In this study, to investigate the factors affecting the distribution of reservoir rock types, microfacies and diagenetic processes in the upper part of Dalan and Kangan formations in one of the Persian Gulf fields have been investigated. A total of 1110 thin sections and 840 porosity and permeability data (RCAL data) were used. In the petrographic studies, 15 microfacies were classified into four facies belts of the tidal zone, lagoon, shoal, and open marine in an epeiric carbonate platform environment. Studies showed that diagenetic imprints such as calcite cementation, dolomitization, anhydrite nodule formation, dissolution, and compaction had been occurred in three marine, meteoric, and burial diagenetic environments. To investigate and group the reservoir rock types on the Lucia petrophysical diagram, the porosity and permeability data were plotted on this diagram. Depending on the type of microfacies and diagenetic processes, porosity and permeability data are included in all parts of the diagram. This study shows that the reservoir rock of the studied field is very heterogeneous in terms of reservoir properties and is affected by sedimentary and diagenetic processes. Accordingly, grain supported samples have been moved from classes one and two to the lower parts of the diagram, in the non-reservoir section, due to calcite and anhydrite cement. In contrast, in some cases, dissolution and dolomitization in mud-supported microfacies led to data placement from class three of Lucia to classes one and two. Dissolution and dolomitization in grain-supported microfacies have resulted in the placement of these samples in the upper parts of classes one and two.

Fluid permeability is one of the most important parameters in reservoir characterization. In this study, permeability calculated from different models and compared with the laboratory measured permeability in a hydrocarbon field in the central part of the Persian Gulf. The permeability models used in this study include Winland, Swanson, Pittman, and Dastidar. In this analysis, 50 mercury injection experiments from Dalan and Kangan formations were used. Conclusions indicate that Swanson and Winland permeability models are the best reservoir permeability prediction models for the Kangan and Dalan carbonate formations, respectively. Swanson’s model unlike other models considers the effects of pore throats in mercury injection curve as the main factor, which it has a key role in permeability prediction. In carbonate environments, there is not specific relation between porosity and permeability. Therefore, models that considered porosity as a factor for permeability prediction show less accuracy. The reservoir’s lithology (carbonate or clastic), because of the different facies and lithology, have various petrophysical features. In this manner, the models that have been calibrated based on carbonates lead to a better prediction in the carbonates in comparison with models which calibrated based on clastic or both.

Calculating water saturation is one of the most important reservoir parameters for determining the volume of hydrocarbons in place, which it must be calculated with high accuracy. In this study, after the necessary corrections were made on the Water-base-core in Kangan and Dalan formations, the core water saturation was calculated by the Dean-Stark test method. After electrical measurements and determination of Archie coefficients on the core, water saturation values were calculated from Archie electrical models, Waxman Smith, Dual Water, and Archie-Dual Water in Geolog7 software. To generalize water saturation to other wells in this field, three facies models were produced by Multi-Resolution Graph-based Clustering (MRGC) and applied in one of the fields in the Central Persian Gulf. Afterwards, in the whole well, the difference between the average water saturation of the Dean-Stark test method and the electrical models was measured in 3 facies models. The results showed that in all three facies models created, the average values of water saturation calculated from the equations were higher than the water saturation calculated from the Dean-Stark test method. By making a comparison between facies models, it is obvious that the facies models based on input logs, including electrical resistance, sonic, neutron, and density, showed the least difference between the average water saturation of the Dean-Stark test and the electrical models. Comparing the average water saturation in the Dean-Stark method and the electrical models between the electrical facies of a model based on its input logs, it was concluded that the petrophysical parameters including porosity, sonic wave transit time, shale volume and density, as opposed to electrical resistance, are directly related to the reduction of the average water saturation difference. But lithology has less effect on the difference in average water saturation between the Dean-Stark method and electrical models. Therefore, to select the sample for determining the coefficients of Archie, it is appropriate to use this method of determining the electrical facies.

Organic geochemical investigations using thin layer chromatography-flame ionization detection (TLC-FID), Gas Chromatography (GC), Gas Chromatography-Mass Spectrometry (GC-MS), API gravimetry, elemental analysis, and isotope-ratio mass spectrometry (IR-MS) were carried out on eleven oil samples from the Sarvak reservoir in the Abadan Plain (SW Iran). Oil chemical composition, source, thermal maturity, age, lithology, and depositional environment of these oils’ source rock were determined in this study. Moreover, Sarvak oils are mainly naphthenic and paraffinic type. Their API degree between 16.2 and 20.14 and about 4.6% sulfur content indicate heavy and sulfur-rich oils. The results of the study of biomarkers, stable carbon isotope composition, trace elements, aromatic and sulfur content indicated that all oil samples are related to a marine-carbonate source rock with strongly anoxic conditions. The absence of oleanane in all oil samples, the variation of Pr/Ph versus δ13C of the whole oil, and C28/C29 steranes versus geological age proved that these oils had been produced earlier than the Late Cretaceous. Furthermore, the distribution of n-paraffins, calculation of Rc (%) from aromatic compounds, CPI (Carbon Preference Index) from gas chromatograms, and biomarker maturity indices indicated that the Sarvak oils are mature. Although the Sarvak oils are heavy, they show approximately maturity of peak oil-generative window, which represents a challenge in this study. It is guessed that the high sulfur content and low API gravity in the Sarvak reservoir oils are due to the presence of sulfur-rich organic matter (type IIS kerogen) in the source rock.

In this study, 110 m of carbonate rocks from one well of the Dariyan Formation in the central part of the Persian Gulf were investigated by using 390 thin-sections and gamma ray data. In order to determine the depositional sequences of this formation, facies analysis led to the identification of nine microfacies in five sub-environments deposited in a carbonate ramp platform. The sea-level changes at the time of deposition of the Dariyan Formation have resulted in formation of three third-order complete depositional sequences. To determine the boundaries by gamma deviation method, each gamma ray data was subtracted from the average data in the whole well and then the obtained values were added together to calculate cumulative deviation from the mean value. Three units and the associated main boundaries were identified by the first level analysis. The second level analysis, which was done separately on all three units, other sequence boundaries (except the boundary at a depth of 1127.2 m) were identified. Comparing the determined boundaries resulting from this method with sequence stratigraphic studies based on the core analysis results indicates that this tool is valuable in determining sequence stratigraphic boundaries in carbonate formations.