Central Persian Gulf Research Articles

Analyzing the impact of geological features on reservoir heterogeneity using heterogeneity logs: A case study of Permian reservoirs in the Persian Gulf

Carbonate reservoir heterogeneity has a major impact on fluid flow and hydrocarbon recovery. It is challenging to characterize due to the complex and heterogeneous nature of carbonate reservoirs. It is important to express heterogeneity as a quantitative parameter for comparing various reservoirs. It can also be used for investigating the influence of different factors on the degree of heterogeneity. Heterogeneity log is one of the effective methods for quantifying heterogeneity. The aim of the present study is to investigate the application of heterogeneity log to determine the geological features affecting heterogeneity in Upper Dalan Member (Late Permian) in the central Persian Gulf. The gamma ray, bulk density, neutron porosity, acoustic, and deep resistivity well logs data were used to calculate the heterogeneity logs. A total of 1093 thin sections were studied and primary depositional as well as secondary diagenetic changes were recorded. Porosity and permeability of the same number of samples were measured on the plugs. Heterogeneity logs were calculated by Lorenz coefficient method. The changes in heterogeneity logs were classified into different ranges and compared with frequency of geological features in corresponding intervals. Findings show that heterogeneity logs can help identify areas of the reservoir with distinct reservoir behavior (permeability and/or porosity). These areas can be targeted for increased oil and gas production. This can help reduce exploration and development costs. Dolomite samples are observed in both low and high heterogeneity values. Anhydrite lithology reduces porosity and increases heterogeneity. High heterogeneity values correspond with mudstone texture from supratidal, peritidal, and lagoon subenvironments with low porosity. Grainstone textures from shoal subenvironment and crystalline carbonates are associated with low values of heterogeneity. These microfacies have high porosity and permeability as well as a high rate of marine cementation. Also, interparticle and moldic pores along with stylolites and fractures are observed in low values of heterogeneity. The increase in the ratio of pore space to the solid component of samples has resulted in a reduction in the value of heterogeneity. By creating different subenvironments with different microfacies and average porosities, environmental energy controls the heterogeneity of the studied member. Therefore, the amount of heterogeneity is mainly controlled by two factors namely, porosity and pore type.

Deep Dive into the factors influencing acoustic velocity in the Dalan-Kangan formations, the central Persian Gulf

The propagation of acoustic waves in rocks is directly influenced by the rocks’ elastic properties. To evaluate petrophysical properties from acoustic data, it is essential to understand the factors that influence the variations in acoustic velocity of sedimentary rocks. Carbonate reservoirs are heterogeneous, and their evaluation is complicated. A closer look at the factors affecting the elastic properties of these rocks is essential because they reveal important information about the reservoir properties and their formation evaluation. Rock physics modeling relies on laboratory-measured acoustic data. This study, however, employs acoustic logs to elucidate the key factors influencing sonic velocity, offering a more comprehensive and field-applicable perspective. Samples were prepared from the Permian–Triassic Upper Dalan and Kangan formations. Porosity, permeability, sedimentary textures, mineralogy, and pore types were investigated. Furthermore, compressional, shear, and Stoneley well-logs data were recorded and converted to acoustic velocity in the sampled interval. Porosity-velocity modeling based on the differential effective medium (DEM) theory was performed for different values of equivalent pore aspect ratio (EPAR). The results show that there is a strong inverse relationship between acoustic velocities and porosity. Acoustic velocities also exhibit an inverse correlation with permeability, but the strength of these relationships is relatively weak. Acoustic velocities are not affected by carbonate textures. A specific texture may exhibit different velocities due to the influence of diagenetic processes. Mineralogy has only a minor influence on acoustic velocities. Dolostones have a slightly higher velocity than limestones, but the difference is insignificant. Stoneley log values are slightly different in limestone and dolostone, because this wave propagates along the mud-borehole interface. The velocities were affected by the variations in pore types within the studied formations. Therefore, in carbonate rocks, porosity, and the pore types play a significant role in controlling acoustic velocities. The value of EPAR = 0.15 can be considered as the boundary between stiff pores (moldic and vuggy) and soft pores (interparticle, intercrystalline, micro-intercrystalline, and microporosity). The lower EPAR value of interparticle pores in limestones compared to dolostones can be attributed to diagenetic processes such as dolomitization and compaction. The findings of this study can help to better understand what factors affect acoustic velocity in carbonate rocks. This study also shows that acoustic logs data can be effectively used in rock physics modeling and pore type identification. By identifying the pore types, it is possible to recognize diagenetic processes and determine the rock types to control the heterogeneity and estimate reservoir quality.

Integration of well logs and seismic attributes for prediction of thermal maturity and TOC content in the Kazhdumi Formation (central Persian Gulf basin)

This study proposes an indirect method for estimating source rock properties, specifically thermal maturity and Total Organic Carbon (TOC), using well logs and seismic data within the central Persian Gulf hydrocarbon fields. The primary focus is on the Kazhdumi Formation, an important source rock in the southern Iran. The study is carried out in two distinct steps, based on the employed data. In the first step, a Maturity Index (MI) is calculated utilizing well logs (such as neutron, density and water saturation) within the Kazhdumi Formation across six oil and gas fields. Then, the TOC content is determined using ∆Log R method across four wells. Geochemical data, including vitrinite reflectance and Rock-Eval pyrolysis data, used as validators of the calculated MI and TOC. In the second step, 2-dimensional post-stack seismic data analysis is conducted where the necessary data, including a full set of well logs are accessible. Multi-attribute regression is employed to extract seismic attributes and consequently used to predict the target parameters. MI and TOC logs are then predicted through Probabilistic Neural Network analysis (PNN) which is used for the indirect prediction of the target logs. Finally, two seismic volumes are obtained to provide a mapping of the predicted MI and TOC variations in the Kazhdumi Formation.One advantage of using this method is its efficiency and cost effective compared to the geochemical testing. Additionally, it provides a continuous prediction of the target parameter in an extensive area. Lastly, this method benefits from the availability of seismic data compared to the geochemical data in hydrocarbon fields that are typically sparse. This approach is proved to be applicable to source rock analysis in different areas for indirect estimation of thermal maturity and TOC.

Integrated reservoir characterization of the Permo-Triassic gas reservoirs in the Central Persian Gulf

The Upper Dalan and Kangan or Permian-Triassic carbonate formations in the central the Persian Gulf are considered as world’s giant gas reservoirs. The primary purpose of this research is to model and evaluate the relationship between hydraulic flow units (HFUs), electrofacies and microfacies with systems tracts of Permian-Triassic sequences. By integrating the results of core data, petrographic studies, and petrophysical logs of the studied formations, hydraulic flow units and electrofacies were identified. Based on the results of petrographic studies, twelve microfacies were identified in terms of textural and depositional characteristics. Based on depositional setting, sedimentary facies and INPEFA values obtained from gamma ray log and gamma deviation log (GDL) in the context of sequence stratigraphy, zonation of Dalan and Kangan reservoirs is carried out. The zonation boundaries correspond to the key stratal surfaces (sequence boundary and maximum flooding surface). Seven petrographic rock types (PRT) were identified for the upper Dalan-Kangan reservoirs based on sedimentary texture, diagenetic process and dominant pores. Using porosity and permeability data from the core analysis, five hydraulic flow units were identified based on the flow zone indicator (FZI) method. Using multi-resolution graph-based clustering (MRGC) four electrofacies were detected from petrophysical data (gamma, neutron, density and acoustic logs). Subsequently, the INPEFA, GDL and electrofacies were spatially modeled using the sequential indicator simulation (SIS) and sequential Gaussian simulation (SGS) geostatistical methods. Finally, a clear agreement was revealed between the reservoir zones and the stratigraphic sequence framework. It this regard, the microfacies belonging to the high-energy and grain-dominated settings (packstone, grainstone) of leeward shoal, shoal and seaward shoal belts have the best reservoir units due to the influence of dissolution and dolomitization. The best reservoir units in the Permian-Triassic deposits in the middle of the upper Dalan and lower Kangan are developed in UDS4, upper KS2 and middle KS1 units. On the other hand, mud-dominated facies (mudstone, wackestone) and anhydrite textures are mostly associated with the low-energy lagoonal environments, between tidal flat and Sabkha. Non-reservoir units have been formed in the upper Dalan/Kangan and in the transgressive systems tract of UDS3-a, KS2-a and the lower and upper part of KS1 transgressive-highstand systems tract.

Integration of geochemical data, well logs and seismic sections for source rock characterization of the Albian Kazhdumi Formation in the central Persian Gulf basin, South Iran

This study presents indirect methods of source rocks characterization by integrating geochemical data with well logs and seismic inversion results. Well logs and seismic data are commonly more available than scarce geochemical data in oil and gas fields. Combining high vertical resolution well logs with high lateral resolution seismic data generates some enhanced geochemical data on inter-well spaces where no well is drilled.Acoustic impedance was obtained and utilized for the TOC prediction and source rock detection because of its good correlation with geological parameters such as the lithology and fluid contents. A cross-plot of log-derived water saturation versus acoustic impedance successfully differentiates source from non-source rocks based on geological variations of lithology, porosity, formations density, and the TOC content. The impact of the TOC content on acoustic properties was elaborated in the next step. Acoustic impedance is obtained as the most important seismic attribute for the TOC prediction. Well logs and seismic data are integrated to predict the TOC logs from seismic data. First, seismic data were inverted to obtain the acoustic impedance. Then, the produced acoustic impedance was used as a seismic attribute for the TOC prediction. The outcome is used to illustrate and follow the TOC variation to distinguish source rocks in the oil and gas fields. The study is carried out in the Cretaceous succession in three hydrocarbon fields in the central part of the Persian Gulf to observe the TOC variations in the Albian-aged Kazhdumi Formation.

The role of textural parameters of industrial core CT scan images in detecting the petrophysical characteristics of carbonate reservoirs, Permian Dalan Formation, the central Persian Gulf

Digital petrophysics investigates the properties of rocks from images, especially core computed tomography (CT scan). The purpose of these investigations is to develop new methods for petrophysical evaluation of porous materials. In this article, textural parameters have been calculated from computed tomography images of the cores in Permian Dalan Formation in the central Persian Gulf. We tried to find more evidence that industrial CT scan images are useful in identifying the geological and petrophysical characteristics of hydrocarbon reservoirs. For this purpose, the textural parameters of the image, including contrast, correlation, homogeneity, energy and entropy were calculated and analyzed and a gray level co-occurrence matrix has been created. These parameters were compared with geological and petrophysical data obtained from laboratory measurements and thin section studies. Analysis of the thin sections under the microscope led to the identification of 12 microfacies. They range from mudstone to wackestone, packstone and grainstone. Some samples have been completely dolomitized and recognition of their primary microfacies is not possible. This group called crystalline carbonate. Results showed that various features in CT scan images can be differentiated using the arrangement of pixels, the gray intensity of each pixel and the relationship of each pixel to its neighboring pixels. Increasing porosity and permeability reduces the image heterogeneity. This is also the case for uniformly distributed pore types, including moldic and intercrystalline. Textural parameters were also compared with microfacies and diagenetic processes. Results indicate that they can increase heterogeneity with decreasing reservoir quality and vice versa. Finally, using numerical values of image textural parameters, three petrophysical classes were defined in the studied reservoir. As a result, various parts of the reservoir can be identified using CT scan images of the cores.

Geochemical characteristics and oil–oil correlation of the upper Cretaceous oils from the Iranian part of the Persian Gulf Basin

The Cenomanian Sarvak oil reservoirs are distributed over large areas of the Persian Gulf basin. The purpose of this study is analyzing the geochemical characteristics of the Sarvak oil reservoirs and their inter-relationships in the Persian Gulf, classification of the Sarvak oil samples and investigation of the possible causes of the genetic difference in oil families. In the previous studies, limited samples of Sarvak oil reservoir in scarce oilfields were studied individually and local interpretations are made accordingly. The current study employs a more complete set of geochemical from the Iranian part of Persian Gulf and regional interpretations are drawn. To achieve this goal, the geochemical data of 41 oil samples from 10 oilfields were collected and assessed based on gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), and stable carbon isotope analysis. It was demonstrated with the evaluations that the oils accumulated in the Upper Cretaceous Sarvak reservoirs in the Persian Gulf basin originate from different source rocks. The oil samples are genetically classified into four oil families based on the similarities and differences of parameters related to the depositional environment-dependent parameters using hierarchical cluster analysis (HCA), star diagram, and stable carbon isotope diagram. The source rocks were mainly deposited in anoxic marine carbonate environments. The thermal maturity of the oils was evaluated using steranes and trisnorhopanes. Oil families 2 and 4 (located at the center of the Persian Gulf) have the highest thermal maturity compared to the other samples; in contrast, oil family 1 (located at the west of the Persian Gulf) has the lowest thermal maturity. The possible source rocks of oil family 1 and family 3 (located at eastern Persian Gulf) using C28/C29 steranes are the upper Cretaceous successions; in contrast, the possible source rocks of oil families 2 and 4 are the upper Triassic and Lower Jurassic formations. Results of the study show a high similarity between the oils of families 2 and 4, which are located at the center of the Persian Gulf; these results also recognize the significant difference between these two oil families and oil families 1 and 3. The significant issue raised in this study is to find out the reason for this difference. The structural analysis of the central Persian Gulf shows that huge vertical faults created by salt domes intrusion provided the migration pathways for trapping oil in Sarvak reservoirs. The oils of the Surmeh, Fahliyan, and Dariyan reservoirs have migrated upwards through these faults to be trapped in the Sarvak Formation across the central Persian Gulf.

Spatial and seasonal distribution of particulate phosphorous and nitrogen in the Persian Gulf: Nitrogen enrichment ties to diazotroph bloom in stratified warm waters

Understanding the particulate organic and inorganic forms of nutrients and their elemental stoichiometry in suspended particulate matter of marginal seas has important implications in several areas. Here, we present the findings from two consecutive research expeditions conducted in September 2021 and January 2022 aboard the Persian Gulf Explorer research vessel, focused on the particulate P and N in the Persian Gulf and the Strait of Hormuz. The prominent features of spatial and seasonal distribution characteristics were nearshore-offshore decreasing gradients in particulate nutrients, the summer-to-winter declining trend in particulate nutrients more notably for particulate organic nitrogen, and higher biomass N:P ratio in summer than in winter with high spatial inhomogeneity. The trends were mainly governed by shallow remineralization of organic matter, resuspension of fine sediments, summer stratification/winter mixing of the water column, and inflow of the Indian Ocean Surface Waters from the Gulf of Oman. The contribution of inorganic N in the particulate pool of N (1.8% in summer and 1.3% in winter) was much less than the contribution of inorganic P in the particulate pool of P (23.0% in summer and 32.4% in winter), and hence the biomass N:P was higher than the corresponding bulk N:P by a factor of ∼1.5. Seasonal pattern in biomass N:P was similar to those observed at many sites being higher in the summer and lower in the winter. The formation of summer thermocline, characterized by high surface temperature (∼33 °C) and limited upward flux of remineralized inorganic nutrients, coincided with the proliferation of nitrogen-fixing cyanobacteria. Samples collected from the deep offshore region in summer showed significantly higher biomass N:P ratio, reaching up to 47, which is far beyond the classical Redfield ratio of 16. The central Persian Gulf experienced less seasonal variations than the deep region in the western Persian Gulf, the northwestern Arvand estuary, and the northwestern Gulf of Oman.

Linking sedimentary properties to mechanical characteristics of carbonate reservoir rock: An example from central Persian Gulf

A comprehensive understanding of mechanical properties of reservoir rocks can help in all stages of hydrocarbon and geothermal exploration, field development, and Carbon Capture Storage (CCS). In this study, the sedimentological properties of a carbonate reservior have been studied and the effect these properties have on mechanical parameters such as Uniaxial Compressive Strength (UCS), Young's modulus (E), Cohesion (C) and Internal friction angle (φ) of the rock at ambient (20 °C) and reservoir (90 °C) temperature conditions. These carbonates were grouped in three units (A, B, and C) based on a statistical analysis of petrophysical logs and core analysis. The results show that changes in mechanical properties are mainly influenced by diagenetic processes. The two main influencing overprints in the studied samples are dolomitization and anhydrite cementation, which strongly affect porosity, the dominant pore type and mineralogy which in turn are the most important influencing factors on mechanical properties. Although samples with 25–35% anhydrite (Unit A) show the highest strength, this parameter strongly reduces under high temperature conditions. The effect of dolomitization on rock texture and on porosity as the main strength-controlling factor is also very apparent. Therefore, samples with crystalline carbonate texture (Unit C) show the lowest UCS, E, C and φ values.

The effects of planar structures on reservoir quality of Triassic Kangan formation in the central Persian Gulf, an integrated approach

Planar structures including stylolites, solution seams, dense streaks, bedding and cross-bedding surfaces, and fractures of carbonate Early Triassic Kangan Formation were evaluated in the central Persian Gulf. Core X-ray computed tomography (CT), Fullbore Formation Micro-Imager (FMI) log, and cores were used to detect the structures. Furthermore, conventional borehole logging data, routine core analysis, and petrographic thin sections were employed to assess the reservoir properties. Stereographic projections demonstrated that the reservoir has two sets of planar structures. The first group contains shallowly-dipping structures, consisting of stylolites, solution seams, dense streaks, bedding and cross-bedding surfaces. These structures have been created by sedimentary and diagenetic processes. The other set includes deformation-related structures that are steeply-dipping and occurred scarcely, like fractures. Spearman correlation coefficients between the frequency of each planar structure and reservoir characteristics (porosity and permeability) were calculated to reveal the numerical relationship between the parameters. The significant correlation coefficients indicate that stylolites, solution seams, and filled tension gashes diminish reservoir quality while bedding and cross-bedding surfaces have enhanced reservoir characteristics. The strongest correlation was observed as a strong negative relationship between the frequency of stylolites and permeability in K2 reservoir unit. This study implies that sedimentary and diagenetic factors are more effective in controlling the reservoir quality than deformation factors in the studied formation. The present work also indicates that each used method has unique advantages and disadvantages. Therefore, simultaneous utilization of the methods to investigate planar structures would have the best efficiency.

Heterogeneity evaluation of pore types based on dipole shear sonic imager logs by means of statistical parameters, the central Persian Gulf

AbstractHeterogeneity analysis in carbonate reservoirs is of great importance for the interpretation of their characteristics. This study focuses on the evaluation of pore‐type heterogeneity by means of the sonic wave velocities in the carbonate–evaporite series of Permian–Triassic sequences in the Persian Gulf. Thin section petrography and laboratory measured the porosity and permeability of 1576 samples, as well as routine wire‐line logs, were employed. To manage heterogeneity, six electrofacies (EF) were determined based on an integrated analysis of compressional, shear and Stoneley waves as well as three routine logs (gamma, neutron and density). The frequency of pore types in each electrofacies was calculated. Then, the heterogeneity of pore‐type distribution in each electrofacies was measured by standard error and coefficient of variation. Sonic velocity has been compared with the geological properties of electrofacies. Results indicated that velocity is affected by factors other than the amount of porosity, facies or lithology. The standard error values in all electrofacies were close to zero. The coefficient of variation values ranges from 1.08 to 4.60. The lowest amount of coefficient of variation was observed in EF6 in the Khuff (K) 4 zone for moldic porosity. This value for intercrystalline porosity distribution in EF1 in the K1 zone was low, too. These results indicate the lowest standard deviation and the highest homogeneity in the distribution of intercrystalline and moldic porosities in K1 and K4. Also, comparing the mean values of sonic logs in each electrofacies with the total mean value in the whole sequence indicated that there is a relationship between sonic velocity logs and the type of porosity. High values of sonic velocity correspond to intercrystalline porosity in K1 and low values of velocity correspond to moldic porosity in K4. Results show that integrated pore‐type studies should be carried out to understand the sonic velocities in carbonate reservoir rocks.

A new approach to estimate and delineate the geothermal gradient of Iran

In spite of numerous deep wells drilled for oil, gas, deep water and geothermal purposes, and several local geothermal studies carried out using geochemistry analysis in Iran, lack of a comprehensive geothermal map is palpable. In this paper, nominated reliable surface and subsurface thermal data are gathered, examined and corrected to be involved in mapping process. In order to do so, 107 drilled wells accompanying with geothermal gradient data extracted from geothermometry analysis (available in hot springs) were nominated based on data quality and a proper dispersion. All selected points were evaluated and eventually, a real temperature gradient together with a temperature log were calculated in every location. Therefore, convergent interpolation technique was applied to estimate and distribute the temperature data points to the whole surface. According to the final proposed map, geothermal gradient is increased around geologically active areas such as quaternary volcanoes and active faults such that it is evidently possible to segregate high gradient areas into two major parts including volcano-related and active-fault related areas. Approximate average geothermal gradient in non-volcanic basins like Zagros, Kopet-dagh, central Persian Gulf, Qom, Moghan and Zabol regions are determined as 21 °C/km, 30 °C/km, 31.5 °C/km, 26.7 °C/km, 30 °C/km and 25 °C/km respectively and it reaches to about 149 °C/km around volcanic areas like Sabalan stratovolcano. Undertaken comprehensive survey will be of great assistance in future exploration of geothermal resources in Iran land.

Toward the standardization of heterogeneity evaluation in carbonate reservoirs: a case study of the central Persian Gulf

Toward the standardization of heterogeneity evaluation in carbonate reservoirs: a case study of the central Persian Gulf

How petrophysical heterogeneity controls the saturation calculations in carbonates, the Barremian–Aptian of the central Persian Gulf

Water saturation calculations are among the most important aspects of reservoir characterization. Despite this importance, the effects of these calculations on the resulted oil in place remain unclear. Extreme heterogeneity in most of the carbonate reservoirs strongly affects the coefficients of the Archie equation and resulted water saturation. Therefore, a practical method to evaluate and manage the heterogeneity of such reservoirs, greatly improves the reliability of the final results. In this study, porosity, permeability, thin sections data, well logs and saturation tests of Barremian–Aptian carbonate Dariyan Formation in the central Persian Gulf have been used. Both core and reservoir scale techniques including flow zone indicator, Winland R35, electrofacies and stratigraphically modified Lorenz plot were used to evaluate the heterogeneity. Archie coefficients were arranged in different rock types. Results show that flow zone indicator is the most accurate technique to manage the heterogeneity of the reservoir with higher R2 value between porosity and permeability. The cementation factor was measured for each unit and the amounts of water saturation was calculated using deterministic analysis. Water saturation also calculated with the constant coefficients, which are routinely used in formation evaluation. The average Sw amount for constant Archie coefficients method was determined as 0.62, whereas it is 0.73, 0.80, 0.70, and 0.77 in flow zone indicator method, Winland R35 technique, stratigraphically modified Lorenz plot and electrofacies determination, respectively. The results demonstrate that calculating water saturation with constant Archie coefficients leads to slightly lower water saturation values, resulting in higher oil saturations and overestimating the oil in place.

The effects of facies and diagenesis on the pore-throat sizes in Dariyan Formation, Central Persian Gulf

The effects of facies and diagenesis on the pore-throat sizes in Dariyan Formation, Central Persian Gulf

Integrating petrophysical attributes with saturation data in a geological framework, Permian–Triassic reservoirs of the central Persian Gulf

In this research, an integrated geological and petrophysical approach was applied to understanding the most critical factors affecting the water saturation exponent in the Permian–Triassic Kangan and Dalan carbonate formations in the central Persian Gulf in Iran. Economically, water saturation has a fundamental role in reservoir characterisation. A total of 8 sedimentary facies were recognised with multiple diagenetic phases. Diagenetic processes include marine cementation, moldic dissolution, early dolomitisation, recrystallisation, anhydrite cementation, compaction, and fracturing, which have influenced reservoir properties. Four third-order sequences were distinguished in the study area accompanied by nine fourth-order sequences. Petrographic analysis, scanning electron microscopy, petrophysical core analysis, wireline log data as well as mercury injection tests integrated through flow zone indicator, Lucia classes, and Lorenz plot approaches. Consequently, 4 integrated rock types (IRTs) were identified with particular depositional and diagenetic features. IRT1 with dominant interparticle pore type and strong porosity–permeability correlation (average 9.1% porosity and 27 mD permeability, R2 = 0.98), has a high saturation exponent (n = 2.2). IRT4 with grain-to mud-dominated facies, low porosities (average 1.4%), and a mean value of 4.54 mD permeability has the lowest saturation exponent (n = 1.07). Moldic dissolution with low pore connectivity is the dominant pore type in IRT2 and IRT3 which have a medium and relatively high saturation exponent, respectively (IRT2 n = 1.53 and IRT3 n = 1.78). Accordingly, a conceptual model was suggested for saturation exponent variations in a high-frequency sequence stratigraphic framework. In order to evaluate the heterogeneity of the studied formations and its impact on the reservoir properties, the characteristics of the determined rock units were studied. This study integrates microscopic to macroscopic geological and petrophysical factors to understand the saturation exponent distribution in a heterogeneous carbonate reservoir, leading to obtaining a more accurate water saturation estimation of such reservoirs.

Permeability's response to dolomitization, clues from Permian–Triassic reservoirs of the central Persian Gulf

Understanding the pattern of spatial permeability distribution in carbonate reservoirs strongly depends on its relationships to geological factors. Despite this importance, the relationship between dolomitization and permeability changes is poorly understood. This study represents the effects of dolomitization on permeability variations in carbonate Permian–Triassic strata of the central Persian Gulf. Porosity and permeability measured on 1389 plug samples using Boyle and Darcy lows, respectively. Petrographical studies under a polarizing microscope showed that various types of facies are recognizable including boundstone, mudstone, wackestone, packstone and grainstone. Dolomitization and dissolution are the main diagenetic processes. Permeability increases along with dolomite content in the samples with higher than 1 mD permeability. Dolomitization of the allochems increased the interparticle spaces and so improved the pore connectivity. High permeability is also due to the intercrystalline pores between dolomite crystals and vuggy porosities which have been created as the result of high degree of dolomitization. Classifying the dolomitic rocks based on dolomite crystal shapes indicates that nonplanar dolomites are more frequent in the samples with less than 0.001 mD permeability while samples with more than 1 mD permeability have more euhedral dolomites. The subhedral dolomites are more frequent in the sample with 0.1–1 mD permeabilities. Nonplanar dolomites are more abundant in K1 and K3 units which are non-reservoir parts. Also, pore spaces in coarse crystal dolomites are connected to each other more effectively, which mainly occurs in K2 and K4 reservoir units. Pore typing in the studied carbonates demonstrates that moderate amounts of dolomites (about 50%) connect the other pore types while they produce a contiguous pore network in higher quantities. In the samples with more than about 60% dolomites, the intercrystalline pores are combined together and touching vugs are created, which strongly increase the permeability of the samples.

Tectonic evolution of the Zagros foreland basin since Early Cretaceous, SW Iran: Regional tectonic implications from subsidence analysis

• Basement palaeo-highs exert major impact on the basin's subsidence history. • Architecture of the Zagros foreland dramatically changed during the Cretaceous. • NE-SW deep-seated basement faults were intensively re-activated during the Turonian . • N-S deep seated basement faults were re-activated during the last 20 Ma. Subsidence analysis of 42 exploration wells in the Zagros foreland basin indicates the tectonic evolution of the basin since Early Cretaceous. The impact of inherited basement structures on the basin evolution is documented in this study. Subsidence maps show that the architecture of the basin dramatically changed during the Cretaceous. In the Central Zagros, the Hercynian palaeo-high (i.e. Wadi Al Batin palaeo-high), which affected sedimentation until the Early Cretaceous time gradually disappeared and the South Pars palaeo-high within the central Persian Gulf started to show more impact on sedimentation. The palaeo-highs in the Zagros basement have different geometrical characteristics and origin. The Hercynian and South Pars are wide-wavelength palaeo-highs with a regional-scale impact on subsidence. In contrast, the Hendijan, Kharg-Mish and Kazerun palaeo-highs with different orientations and quite smaller wavelengths show different impact on the subsidence pattern. These kinds of palaeo-highs could be related to re-activation of pre-existing basement faults. Whereas South Pars and Hercynian paleo highs are possibly related to thermal expansion or contraction of the crust. The Hendijan and Kharg-Mish palaeo-highs with the NE-SW orientation were intensively re-activated during Turonian time. On the contrary, the Kazerun Fault with the N-S orientation became more active and considerably influenced sedimentation during the last 20 Ma. Selective re-activation of deep-seated faults might have taken place as a result of changes in the orientation of the far-field stress in the region. Understanding the complexities of inherited structures and their impact on reservoir characteristics can support future exploration activities in the Zagros foreland basin.

Changes in reef fish assemblages following multiple bleaching events in the world’s warmest sea (Kish Island, the Persian Gulf)

Climate change affects the ecology of reef fish through changing their habitat, community and their feeding and territorial behavior. The corals and their associated reef fish in the Persian Gulf as the world’s warmest sea suffer from the increasing frequency and severity of mass bleaching events. Yet, the effects of climate-induced coral bleaching events on reef fish in this sea are poorly understood. Using a time series fish count data, this study evaluated the response of reef fish to multiple severe coral bleaching events in Kish Island located in the central Persian Gulf. The results showed a decline in species richness and diversity of reef fish after bleaching events which reduced mean coral cover by 85% following the 2007 bleaching event and by 95% following the 2017 bleaching event. Besides a drop in the number of fish species, the total density, and density of trophic groups changed as the results of coral loss. The variation in density of different trophic groups indicated an overall increase in the abundance of benthic invertebrate feeders (21.2–136.3%), omnivores (32.4–163.4%) and plankton feeders (218.5–488%). In contrast, the abundance of coral polyp feeders (−6.9 to −50%) and piscivores (−4.9 to −38.9%) significantly decreased after bleaching events. The density of herbivorous fish significantly increased by 97.1% after the bleaching event in 2007 and decreased by 10.5% after the bleaching event in 2017. In the present study, we demonstrated how the impact of the multiple bleaching events on associated reef fish assemblages sustained for about 6 years after the disturbance. These results have significant implications for our understanding of the response of reef fish to global warming and highlight the growing concern about the future of coral reef ecosystems and their associated reef fish in the region as the warmest sea in the world.

Effects of progressive dolomitization on reservoir evolution: A case from the Permian–Triassic gas reservoirs of the Persian Gulf, offshore Iran

Dolostones are among the most important reservoir units within the Permian–Triassic Dalan and Kangan formations throughout the central Persian Gulf. In this study, petrographic and geochemical (carbonate carbon and oxygen isotopes) analyses led to the identification of several dolomitization phases from sabkha and hypersaline to burial diagenetic realms. An integration of the estimated temperatures of dolomites and their textural properties results in the differentiation of three thermal domains of dolomitization including low (LTD; < 30 °C), moderate (MDT; 30–50 °C), and high (HDT; > 50 °C) temperature dolomites. They are respectively associated with primary/fabric retentive and early micro-fabric destructive, late micro-fabric destructive, and fabric destructive (recrystallization) and saddle dolomites. Combination of petrographic, geochemical, and petrophysical results (core porosity and permeability, and pore-throat size distribution) shows a good correlation between the higher estimated temperatures of dolomites and progressive destruction of the primary depositional textures, larger crystal sizes, and higher porosity and permeability values. In general, the late micro-fabric destructive, wholly fabric destructive, and recrystallized dolomites led to the improvement of flow capacity in the studied intervals via increasing pore-throat size, decreasing tortuosity, and enlarging the crystal size.