https://jssr.ui.ac.ir/ Journal of Stratigraphy and Sedimentology Researches en daily 1 Sat, 21 Mar 2026 00:00:00 +0330 Sat, 21 Mar 2026 00:00:00 +0330 https://jssr.ui.ac.ir/article_30043.html https://jssr.ui.ac.ir/article_29706.html AbstractThis study investigates the impact of the Paleocene–Eocene Thermal Maximum (PETM) event on the organic petrographic characteristics of the base Pabdeh Formation in the Tang-e-Hati section, located at the Kuh-e-Gurpi Anticline, SW Iran. To delineate the Paleocene–Eocene boundary, nanofossil analyses were employed. Additionally, the collected samples from the studied section were investigated using organic petrographic methods under reflected white light. The nanofossil results indicate that the Paleocene–Eocene boundary is situated approximately 26.5 meters from the base of the Pabdeh Formation, specifically between subzones NP9a and NP9b, marked by the presence of the key species: Discoaster araneus, Rhomboaster cuspis, and R. spineus. Furthermore, the organic petrographic results suggest that the studied marls were deposited under oxidizing conditions. Results from this study are consistent with the presence of a grey marl with relatively darker color, lower fossil concentration, and higher organic matter concentrations compared to the lower and upper parts. Results from this study conclusively suggest that deposition of the grey marl was associated with a short-term relative sea-level fall, which in turn led to an increased sedimentation rate in the basin and a greater influx of terrestrial organic matter.Keywords: Purple shale, Organic petrography, Paleocene–Eocene boundary, PETM incident, Tang-e-Hati  IntroductionOrganic geochemistry, focusing on the study of organic compounds in sediments and their interactions with geological processes, plays a vital role in hydrocarbon exploration (Peters et al. 2005). One of the key analytical approaches in this field is organic petrography, which enables the identification of macerals, kerogen types, thermal maturity, and palaeo-depositional environments of potential source rocks (Hackley & Cardott 2016). The Paleocene–Eocene Thermal Maximum (PETM) was a short-lived but intense global warming event, associated with a 5–8 °C rise in global temperatures, ocean acidification, and major disruptions in the carbon cycle (Zachos et al. 2008).In southwestern Iran, the Pabdeh Formation, with its continuous marl and purple shale strata, provides an excellent opportunity to assess variations in organic matter preservation across the PETM (Motiei 1993). Although numerous studies have investigated the geochemical properties of the Pabdeh Formation (Alizadeh et al. 2012; Safaei-Farouji et al. 2022; Hosseiny et al. 2024), little is known about its organic petrographic characteristics, particularly across the Paleocene–Eocene boundary. This study aims to fill that gap by examining the organic matter variations and palaeo-depositional conditions during the PETM by means of organic petrography techniques. Material & Methods In this study, 24 samples were systematically collected from the base of the Pabdeh Formation at the Tang-e-Hati section (southern flank of the Kuh-e-Gurpi Anticline). Sampling intervals were generally less than 5 meters; however, in the vicinity of the Paleocene–Eocene boundary, the interval was reduced to approximately 1 meter or less to allow for a more detailed investigation of PETM-related changes. The collected samples, consisting of both consolidated and unconsolidated materials, were transferred to the laboratory for calcareous nannofossil and organic petrographic analyses. Calcareous nannofossils were prepared using the standard smear slide technique (Bown & Young, 1998), and examined under a polarized Olympus BX60 microscope at 1250× magnification. Species identification was carried out based on established references (Perch-Nielsen 1985; Agnini et al. 2014) and biozonation and boundary placement followed the schemes of Martini (1971), Romein (1971), and Aubry (1998). For organic petrographic studies, polished pellets were prepared from small fragments (approximately 1.5 × 1.5 cm) of the collected samples. These fragments were embedded in a 2:1 mixture of epoxy resin and hardener using standard protocols. After 24 hours, the samples were removed from the molds and polished according to the standard procedures (Bustin et al. 1985; Taylor et al. 1998). Petrographic observations were performed using a Zeiss Axioplan II microscope at 100× magnification under oil immersion. Discussion of Results & ConclusionsIn this study, the Paleocene–Eocene boundary was identified at approximately 26.5 meters above the base of the Pabdeh Formation, based on the recognition of nannofossil subzones NP9a and NP9b. Organic petrography revealed three distinct sections differing in organic matter content and color: i) a lower purple shale with low organic matter contents which was precipitated under oxidizing conditions, ii) a middle grey marl with higher organic content linked to the PETM event and increased water acidity, and iii) an upper purple shale with decreased organic content and a return to oxidizing conditions. Changes in color and organic matter content correspond to fluctuations in the relative sea level, sedimentation rate, and pH of the water. During deposition of the lower purple shale, higher sea levels and lower sedimentation rates favored good fossil preservation. During the PETM interval, sea level dropped, sedimentation rate increased, and water acidity increased, resulting in reduced fossil preservation and increased terrestrial organic matter input. After the PETM, sea level rose again, terrestrial organic matter input decreased, and fossil preservation improved. These results highlight the interplay of sea-level changes, sedimentation rates, and water pH in controlling the organic petrographic characteristics of the base Pabdeh Formation during the PETM, providing valuable insights for reconstructing palaeo-depositional environments in this part of the Zagros Basin. https://jssr.ui.ac.ir/article_29747.html AbstractOnline evaluation of gas in drilling mud provides valuable information about reservoir horizons and the type of fluid during drilling. This study investigates the productive horizons of the Fahliyan Formation reservoir in the Yadegaran Field located in the Abadan Plain using geochemical evaluation of hydrocarbon gases in the drilling mud. The recording of mud gas data was conducted by a gas chromatograph with a flame ionization detector, and the hydrocarbon ratios(Pixler, Wetness, Balance, and Character) were calculated for two wells in the Fahliyan reservoir. The results of these ratios indicated that the Fahliyan Formation has reservoir quality, and the fluid is light oil. The relationship between the Wetness and Balance ratios divides the Fahliyan reservoir into two reservoir horizons. The lower horizon contains light oil, where the difference between these two ratios is slight, , while the greater difference in the upper horizon is due to low production capacity accompanied by residual oil.Keywords: Mud gas, Fahlyian Formation, Yadavaran Field, Productive Zone, Reservoir continuity  IntroductionDuring drilling, valuable data is obtained that can be used to characterize the productive zones of hydrocarbon reservoirs. One of these data is the information on gas associated with drilling mud. By identifying the trends in gas composition and changes in their ratios, the petrographic changes and fluid content of the drilled formations can be examined (Farouk et al. 2014). The measurement of hydrocarbon gas amounts in the mud is performed by gas chromatography, which includes quality control of sampling at specified times and analysis of gas contents (Ferroni et al. 2012). The more accurate the identification of hydrocarbon gases and the broader the spectrum of hydrocarbons included, the higher the quality and clarity of formation evaluation during drilling, determination of reservoir fluid levels, and identification of the productive zone (Arief & Yang, 2020). The purpose of this research is to investigate the productive zones of the Fahliyan Formation in the Yadavaran Field located in the Abadan Plain, using geochemical evaluation of hydrocarbon gases in the drilling mud. Materials & MethodsThe record of mud gas information during drilling in two wells of the Yadavaran Field was carried out using a gas chromatograph equipped with a flame ionization detector. The gases are separated from the mud by gas trap motors and introduced into the gas chromatograph, where they are recorded based on the amount of gas and the time it takes to enter the detector. It is worth noting that the device is calibrated daily using standard samples. The quality of the recorded data is evaluated after the gas is analyzed by the device. For this purpose, the Gas Quality Ratio (GQR) index is used. This index is obtained from the ratio of the total gas amounts to the sum of hydrocarbon component amounts multiplied by their respective carbon atom numbers (Wiersberg & Erzinger 2007; Newton et al. 2014). A GQR value within the range of 0.8 to 1.2 indicates good data quality. The recorded results from the drilling mud in the two studied wells demonstrate the appropriate quality of these data. Discussion of Results & ConclusionUpon entering the Fahliyan Formation at a depth of 4050 meters, the concentration of hydrocarbon gases in the drilling mud, which was below 1000 ppm before this horizon, increases. This result is entierly consistent with the oil shows observed on the drilling cuttings. The ratio of methane to heavier gases such as ethane, propane, and butane can indicate gas, oil, and water intervals (Pixler 1969). A C1/C2 ratio between 2 and 15 indicates an oil zone, while a ratio between 15 and 65 indicates a gas zone. The higher this ratio, the richer the gas or the lower the hydrocarbon density. If the C1/C2 ratio is less than 2, it indicates residual oil, and if it is above 65, it signifies a non-productive zone (Pixler 1969). Interpretation of the Pixler C1/C2 ratio for the samples is between 2 and 15, indicating oil fluid in the Fahliyan reservoir, and the deviation of some data towards the gas suggests a higher API gravity of the oil. The Wetness ratio increases with increasing gas density; the Balance ratio is, in fact, a direct ratio between light and heavy hydrocarbons, used alongside the Wetness ratio for interpretation, and has an inverse relationship with it (Mode et al. 2014; Sahu 2018). Practically, a straightforwardrelationship between Wetness and Balance ratios is used to determine fluid type and fluid contact during drilling. If the Balance ratio is greater than the Wetness, it is predicted that gas exists in the layer, whereas if the Wetness ratio is greater than the Balance, oil is predicted in the layer. The closer the curves are to each other, the lighter the oil. The greater the distance between the curves, the heavier the oil or the presence of residual oil (Mode et al. 2014). Higher ratios of Wetness than Balance also confirm the oil fluid for the Fahliyan reservoir. The trend of these two ratios differs between the upper and lower horizons of the reservoir. These two ratios have little difference in the lower horizon, indicating a productive zone with light oil. In the upper horizon of the Fahliyan reservoir, the Wetness and Balance ratios differ more, which can be due to the presence of a heavier oil composition or a non-productive zone. Based on previous geological and petrophysical studies (Mohseni et al. 2016; Ramezani Akbari et al. 2017; Tavoosi Iraj et al. 2023), the reservoir quality of the upper horizon is low, and the hypothesis of low production capacity accompanied by residual oil seems more plausible. Moreover, the C1/C4+C5 ratio is used to determine the amounts of heavy hydrocarbons, and a high value of this ratio indicates low amounts of heavy hydrocarbons. The high value of this ratio indicates low amounts of heavy hydrocarbons in the Fahliyan reservoir. This ratio also divides the Fahliyan reservoir into two different reservoir horizons, with the lower horizon showing higher ratios. https://jssr.ui.ac.ir/article_29805.html .AbstractThe Permian–Triassic carbonates of the Kangan–Dalan formations in the central Persian Gulf represent one of the largest gas reservoirs in Iran and worldwide. Integrated petrophysical–core zonation, supported by multivariate cluster analysis, was applied to reduce reservoir heterogeneity and improve the understanding of reservoir properties. The results were compared with petrophysical logs, scanning electron microscopy, and pore-throat size distribution. Diagenetic processes were found to play a key role in reservoir quality. Fabric-destructive dolomitization in the upper K2 and lower K4 units generated micro-scale pathways connecting primary pores. Dissolution in the lower K2 and upper K4 units further enhanced pore connectivity, resulting in high permeability within these zones. In contrast, selective dolomitization and dissolution in K1 and K3 did not produce effective flow pathways. These processes led to the development of distinct reservoir zones, including high-porosity/low-permeability (Zone 2), high-permeability/low-porosity (Zone 3), and non-reservoir zones (Zones 4 and 5). Additionally, a thin interval with both high porosity and permeability (Zone 1) occurs as a transitional layer within the lower K4. Although diagenesis is the dominant control, primary depositional facies also influenced reservoir characteristics. All zones were successfully identified by the proposed algorithm.Keywords: Diagenesis, Petrophysical Zonation, Clustering, Permian–Triassic, Dolomitization  IntroductionReservoir quality is one of the most critical parameters influencing the performance of hydrocarbon reservoirs, with pore-throat size distribution acting as a key controlling factor. This distribution is governed by both primary depositional attributes and secondary diagenetic processes (Tucker and Bathurst 1990; Cerepi et al. 2003; Baron et al. 2008). Diagenesis plays an especially significant role in carbonate reservoirs, where processes such as dolomitization, cementation, and dissolution can substantially alter porosity and permeability (Anselmetti and Eberli 1999). Studies have shown that reservoir zones with similar petrophysical behavior often reflect comparable diagenetic histories (Ehrenberg 2006).In many cases, core data are limited, highlighting the need for well-log analysis as an alternative tool. Several log responses are sensitive to diagenetic alterations, making them useful for reservoir characterization when integrated with petrographic observations. Among these, sonic logs and derived velocity-deviation curves have proven effective in distinguishing pore types and depositional–diagenetic trends, thereby enhancing geological and petrophysical interpretations. Multivariate cluster analysis is among the most powerful approaches for reservoir zonation. While it has been widely applied for electrofacies classification in both clastic and carbonate settings (Gill et al. 1993; Ye and Rabiller 2000), its large-scale application for defining reservoir zones remains limited. Combining well-log responses with petrographic and petrophysical parameters provides a more robust basis for identifying reservoir units.This study evaluates the application of multivariate cluster analysis for integrated petrophysical zonation in the Kangan and Dalan formations of the central Persian Gulf. The proposed workflow aims to establish a practical framework for accurate reservoir characterization by linking log responses, diagenetic features, and pore system evolution. Materials and MethodsA key well with 420 m of continuous core from the Kangan and Dalan formations in the central Persian Gulf was selected. Core plugs were taken every 30 cm, cut at both ends, and had thin sections prepared. One-third of each section was stained with Alizarin Red-S (Dickson 1966) to distinguish calcite from dolomite. Thin sections were studied under a polarizing microscope to record depositional facies and diagenetic features. Twenty representative samples were selected for scanning electron microscopy (SEM) and pore-throat size distribution was determined by mercury injection up to 60,000 psi.Core plugs were cleaned, dried, and analyzed for porosity (Boyle’s law) and permeability (Darcy’s law). Petrophysical data from neutron porosity, bulk density, photoelectric factor, and sonic logs were integrated with petrographic porosity estimates to assess the effects of dolomitization and dissolution. For data analysis, customized programming in MATLAB was applied instead of conventional software, providing flexibility in clustering, optimization, and algorithm testing. Hierarchical clustering was employed due to its ability to handle diverse datasets and dynamically determine the number of clusters (Xu and Tian 2015). To further constrain diagenetic effects, velocity-deviation logs were calculated by comparing measured and predicted compressional velocities. Positive deviations indicate cementation or compaction, while negative values reflect enhanced porosity from dissolution or fracturing.This integrated workflow allowed for precise petrophysical zonation by linking log responses, petrographic features, and diagenetic alterations. Discussion of Results & ConclusionsThe integrated zonation results reveal that intervals with similar porosity and permeability values can be effectively distinguished, reflecting the diverse impact of pore types on flow properties. Each identified zone shows a characteristic porosity–permeability distribution controlled by diagenetic processes. Zone 1 exhibits both high porosity and permeability, largely associated with touching-vug porosity (Lucia 1995) where dissolution, fracturing, and fabric-destructive dolomitization enhanced pore connectivity. This thin interval acts as a transitional layer between the upper dissolution-dominated K4 and the lower dolomitized K4. Zone 2 displays slightly lower porosity but still retains effective reservoir quality, corresponding mainly to the lower K2 and upper K4 where dissolution was the dominant diagenetic process. Zone 3 includes the largest number of samples, with moderate porosity but significantly enhanced permeability due to well-connected pore systems created by fabric-destructive dolomitization. In contrast, Zone 4, representing parts of K1 and K3, shows reduced reservoir quality as porosity is partly occluded by anhydrite cement, leading to low permeability despite moderate porosity values. Zone 5 is non-reservoir, characterized by very low porosity and permeability due to pervasive anhydrite cementation and loss of primary pores.Velocity-deviation analysis confirms these patterns. Negative deviations in Zones 1 and 3 indicate dissolution-related microporosity and microfractures, producing higher-than-expected permeability. Zone 2 shows values close to zero with a slight negative trend, reflecting micropores with limited connectivity. In contrast, positive deviations in Zone 4 clearly point to cementation and compaction, while Zone 5 shows weakly negative trends but insufficient pore connectivity to sustain flow. Thus, negative deviations are reliable indicators of dissolution-enhanced reservoirs, whereas positive values reflect cementation-dominated intervals.Mercury injection capillary pressure (MICP) analysis further supports these findings. Dolomitized samples with preserved primary porosity show relatively uniform pore-throat distributions and moderate permeability, while fabric-destructive dolomitization and dissolution create wider throat-size spectra and higher permeability (Zones 1–3). In contrast, limestone samples with moldic pores sealed by anhydrite exhibit poor pore connectivity and reduced flow potential (Zones 4–5). SEM observations confirm these relationships, showing dissolution-enhanced pore networks in productive zones versus anhydrite-filled throats in non-reservoir intervals.Overall, the Permian–Triassic carbonates of the Kangan and Dalan formations exhibit a complex diagenetic history that strongly influences reservoir quality. Five reservoir zones were defined by hierarchical clustering of well-log data (NPHI, RHOB, Sonic, PEF) calibrated against petrographic and petrophysical observations. Among them, Zones 1–3 represent the productive units, with Zone 3 being the most significant due to widespread fabric-destructive dolomitization and pore connectivity. Zones 4 and 5, affected by anhydrite cementation, represent poor-quality or non-reservoir intervals. This integrated approach demonstrates that multivariate clustering, when combined with petrography, SEM, and MICP data, provides a robust framework for characterizing carbonate heterogeneity. The methodology is flexible, does not require prior training datasets, and can be applied to other fields. Importantly, zones with similar diagenetic histories and pore characteristics are shown to share comparable flow properties, offering a reliable basis for linking porosity, permeability, and reservoir performance. https://jssr.ui.ac.ir/article_29579.html AbstractBrachiopods, as one of the most important benthic fauna in the Late Ordovician, show a great abundance and diversity in many continents. In this study, a quantitative approach of multivariate analyses was conducted in order to study the palaeobiogeography of the brachiopods of Iran and their relationship with the brachiopods from other parts of the world, such as Baltica, Avalonia, Laurentia, South China, Kazakhstan, France, and Portugal, during the Late Ordovician (early Katian). Based on the results of the cluster analysis (CA), six main clusters and two sub-clusters were distinguished. The results obtained from the principal component analysis (PCA) method indicate the differentiation of seven main groups and show a high similarity with the main clusters obtained from the CA method. Based on the results of CA, the brachiopods of Bojnourd, Iran, are placed in a separate cluster together with the Avalonian brachiopods such as Shropshire, Powys (Wales), Anglesey (Wales), and Meath (Ireland). Based on the PCA scatter plot, the brachiopods of Bojnourd are placed in the same group with the Avalonian brachiopods. They are distinguished from the brachiopods of the Zagros and Anarak regions in central Iran, which form a separate group with the brachiopods of France, Portugal, and Morocco. The results of the PCA method indicate that the brachiopods from intracratonic Laurentian Basin such as New York, Manitoulin Island, Kentucky, and Indiana form a distinct group and are differentiated from brachiopods from continental margin such as the Appalachian Basin, Tyrone (Northern Ireland), and Girvan (Scotland).Keywords: Brachiopods, Late Ordovician, Palaeobiogeography, Multivariate analyses, Katian  IntroductionDuring the Late Ordovician, much of Laurentia was covered by a shallow epicontinental sea that created carbonate platforms in the intracontinental basins and pericratonic shelves (Finnegan et al. 2012). This marine transgression event and the creation of carbonate structures likely indicate a greenhouse warming episode in the Late Ordovician. In addition, some sedimentological and geochemical data support the hypothesis of a cooling episode in the Late Ordovician, leading to the cold and glacial climate of the Hirnantian (Page et al. 2007; Trotter et al. 2008; Buggisch et al. 2010). Interpretations of the climate change during the Late Ordovician are controversial and, like the Boda event in the late Katian, have been interpreted as both a warming and a cooling episode (Fortey & Cocks 2005; Cherns & Wheeley 2007).As one of the most important groups of marine invertebrates of the Paleozoic, brachiopods have a high diversity and abundance in the Ordovician and are therefore of great importance in palaeobiogeographic studies. According to Webby (2000), major fossil groups such as brachiopods show three global diversity maximum during the “Great Ordovician Diversification Event,”. Each palaeocontinent had different brachiopod diversification trajectories during the Ordovician. Based on Harper & Rong (2001), rhynchonelliform brachiopods diversified during the Dapingian and Darriwilian. The Ordovician brachiopods of Baltica show four diversity maxima in the mid-Darriwilian, late Darriwilian, late Sandbian, and late Katian and are different from the brachiopod diversity curves of Avalonia and Gondwana. The brachiopods of Gondwana had one diversity maxima during the late Sandbian (Hints & Harper 2001; Harper & Mac Nicaill 2002; Harper 2006). In South China, brachiopods had three diversity maxima in the early Floian, late Darriwilian, and late Katian (Zhan & Harper 2006).During the late Darriwilian, the earliest rhynchonellid brachiopods appeared in shallow marine environments of palaeotropical regions, including Laurentia, Siberia, and Kazakhstan (Jin 1996). During the Sandbian and early Katian, the total number of rhynchonellide genera increased from five to fifteen. By the late Katian, rhynchonellide brachiopods became widespread in the epicontinental seas of Laurentia and some of the genera such as Hiscobeccus was endemic to Laurentia (Sohrabi & Jin 2013a).The controlling mechanism for the major changes in biogeographic patterns is not well understood. Because of the importance of the Katian brachiopods in palaeogeographic interpretations, a quantitative approach was conducted to investigate the Katian brachiopods of Iran and other regions of the world. Measuring the faunal similarity of the Iranian brachiopods with those from other regions of the world can provide a comprehensive interpretation of the palaeoclimatic and palaeogeographic control on the brachiopods’ evolution and their changing biogeographic patterns. Material & Methods    In this study, the palaeobiogeography of the Late Ordovician brachiopods was investigated. The brachiopod data were compiled from various regions, including Baltica, Avalonia, Laurentia, Australia, Kazakhstan, France, South China, Portugal, Morocco, and Iran. Most of the brachiopod data in the present study are from the formations of the early Katian.The Laurentian brachiopods data were obtained from various regions of North America, including the Ottawa Valley, Lake Simcoe, northern Rocky Mountains, western Newfoundland, Manitoulin Island, New York, Kentucky, Indiana, Oklahoma, South Dakota, Nevada, California, Mississippi Valley, Champlain Valley, Hudson Valley, Appalachian Valley, Girvan (Scotland), and Tyrone (Northern Ireland). The brachiopods of Baltica are from the eastern Baltic (Estonia and Lithuania) of Keila and Oandu age (Rõõmusoks 2004; Hints 1998, 2010). The brachiopods of southern Norway are from the Oslo-Asker area, which are related to deep-water facies (Hansen 2008). The Late Ordovician brachiopods of Avalonia are from Waterford and Wexford (southeast Ireland), Meath (east Ireland), and Powys and Anglesey (Wales) (Cocks 2008). In Britain and Ireland, the late Sandbian–early Katian strata include Scotland, Shropshire, Wales, and Ireland. The brachiopods of Scotland are from the Caradoc aged formations in the Girvan area including the Craighead Limestone, Myoch Formation, Whitehouse Bay, and Albany Mudstone Formation. The brachiopods of the Shropshire region include the Caradoc aged formations such as the Acton Scott, Onny Shale, Cheney Longville, Spy Wood Grit, Horderley Sandstone, Whittery Shale, Hagley Shale, Whittery Volcanic, Hoar Edge Grit, Woolston, Smeathen Wood Beds, and Cheney Longville formations. The brachiopods of northwest Wales, Anglesey, include Sandbian–early Katian formations such as the Garn Formation, Llanbabo Formation, and Crewyn Formation. In northern Ireland, the brachiopods are from the Tyrone region including the Bardahessiagh Formation of the Burrellian age. The brachiopods of southern and southeastern Ireland are from Meath, Wexford, and Waterford and include the Burrellian age formations of the Duncannon Group, such as the Annestown Formation, Lower Tramore Volcanic Formation, Grange Hill Formation, Upper Tuffs and Shales of Grangegeeth Volcanic Series, Collon Formation, and Clashford House Formation.The early Katian brachiopods of Kazakhstan are related to the Chu–Ili, Ishim–Selety, and Boshchekul terrains and include the Anderken and Dolankara formations (Popov et al. 2002; Nikitin et al. 2006).In South China, the early Katian brachiopods include the Pagoda Formation (Zhan & Jin 2007; Bergstrӧm et al. 2009). The early Katian brachiopods of Portugal are related to the Cabeço do Peão and Ferradosa formations (Henry & Romano 1978; Cooper 1980; Romano 1980, 1982, 1991; Young 1985, 1988). In Morocco, the Late Ordovician brachiopods are from the Khabt-el-Hajar Formation (Fortey & Cocks 2005). Most of the Katian rhynchonelliform brachiopods of Gondwana are from the western Mediterranean regions such as France, Spain, and Portugal (Torsvik & Cocks 2011). In Iran, the Late Ordovician brachiopods include the Bojnourd region (Ghelli Formation), the Anarak region in Central Iran (Chah Gonbad Formation), and the Zagros region (Siahu Formation). Multivariate analyses: The data used in this study include a large number of genera in general, as well as a large number of endemic genera from different regions. In this study, multivariate analyses were conducted based on the early Katian brachiopods dataset to investigate the palaeobiogeography of the brachiopods of Iran and their relationships with the other brachiopods from Baltica, Avalonia, Laurentia, Australia, Kazakhstan, France, South China, Portugal, and Morocco.The dataset was generated based on binary data that includes 261 brachiopod genera of the early Katian from 30 geographical regions. In this dataset, the geographic regions were selected as locations and the brachiopod genera as variables (presence or absence). The dataset was subjected to multivariate analyses using PAST software (Hammer et al. 2001; Hammer & Harper 2006), which was developed for analyzing paleontological data.In order to distinguish the distribution patterns of the brachiopods in time and space, cluster analysis (CA) and principal component analysis (PCA) methods were employed. To perform cluster analysis (CA), a dendrogram algorithm was generated based on the paired group method by using the Raup-Crick similarity coefficient. The Raup-Crick similarity coefficient shows fully segregated clusters.The dataset was also subjected to PCA by using the variance-covariance algorithm in PAST software. The result was plotted in the PCA scatterplot, based on principal components 1 (X-axis) and 2 (Y-axis), which correspond to eigenvalues 1 and 2, respectively. Discussion of Results & Conclusions    In this study, the results of multivariate analyses of the Late Ordovician (early Katian) brachiopods from 30 geographical regions including Laurentia, Baltica, Avalonia, South China, Kazakhstan, France, Portugal, Morocco, and Iran indicate several palaeogeographic patterns. Based on the results of CA and PCA, several clusters and groups were recognized.The results of CA indicate several distinct clusters. Based on the CA dendrogram, five main clusters (A–F) and four subclusters (A1 and A2) were identified.Cluster A consists of two subgroups, clusters A1 and A2. Cluster A1 contains the brachiopods of Lithuania-North Estonia in the East Baltic, which indicate relatively shallow and warm-water carbonate environments during the early Katian. Cluster A2 comprises the brachiopods from Shropshire, Meath (Ireland), Anglesey and Powys in Wales, and Bojnourd in Iran. The cluster of Bojnourd brachiopods with the brachiopods of Shropshire, Meath (Ireland), and other Welsh regions indicates the close affinity of the Bojnourd brachiopod fauna with those of Avalonia during the early Katian. Cluster B includes western Newfoundland, New York, Kentucky, Indiana, Manitoulin Island, Lake Simcoe, British Columbia, Appalachian Basin, Great Basin, Tyrone (Northern Ireland), and Girvan (Scotland). Western Newfoundland, New York, Kentucky, Indiana, Manitoulin Island, and Lake Simcoe were located on tropical carbonate platforms at mid- to high-latitudes. The Appalachian Basin (from Pennsylvania and Tennessee to Alabama), British Columbia (Advanced Formation), Girvan (Scotland), and Tyrone (Northern Ireland) represent the continental margin of Laurentia during the Late Ordovician. Cluster C includes brachiopods from southeastern Ireland (Wexford and Waterford), South China, and southern Norway (Oslo-Osker), and corresponds to cluster D in the PCA plot. Cluster D in the CA analysis represents the brachiopods from France and Portugal. Cluster E in this dendrogram includes the brachiopods from the Zagros and Anarak regions of Iran.Cluster F in the dendrogram includes the brachiopod faunas of Kazakhstan terranes such as Chu-Il, Boshchekul, and Ishim-Selety and shows low similarity with the brachiopod faunas from other regions. The brachiopod faunas of Morocco are separated from other brachiopod faunas in this dendrogram, which indicates their low similarities with the brachiopod fauna from other areas.In the PCA method, the brachiopod data were scattered in the PCA plot based on principal components 1 (X-axis) and 2 (Y-axis). According to the PCA scatterplot, seven groups A1, A2, B, C, D, E, and F were distinguished, which are similar to the clusters identified in the CA dendrogram.Group A1 consists of the brachiopod faunas from the eastern Baltic, Lithuania, and northern Estonia, and shows consistency with cluster A1 in the CA plot. Group A2 includes the brachiopod faunas from Shropshire, Powys (Wales), Anglesey (Wales), Meath (Ireland), and Bojnourd (Iran). It is interesting to note that the brachiopod faunas of the Bojnourd region are located among the Avalonian brachiopods within cluster A2 in the CA diagram.Group B includes the brachiopod faunas of epicontinental Laurentian such as Lake Simcoe, Manitoulin Island, New York, Kentucky, and Indiana and are located in proximity to the Avalonian regions. The position of the brachiopod faunas of Bojnourd close to the Avalonian and epicontinental Laurentian brachiopod faunas indicates the similarity of the brachiopods from this region of Kopeh-Dagh in northwestern Iran with those of Laurentia and Avalonia during Katian time.With the onset of the marine transgression over Laurentia during the early Katian, the brachiopod faunas of North America began to show a distinction between pericratonic and intracratonic settings. Scoto-Appalachian brachiopod fauna on the southeastern margin of Laurentia were more closely related to the brachiopod fauna of Avalonian and deep-water Baltica facies than to the intracratonic Laurentian fauna. In contrast, the intracratonic (epicontinental) Laurentian brachiopod fauna was more similar to the Lithuanian-northern Estonian brachiopod fauna than to the Scoto-Appalachian brachiopod fauna on the cratonic Laurentian margin.Group C, in the right portion of the PCA plot, shows brachiopods from the Appalachian, Girvan (Scotland), and Tyrone (Northern Ireland) regions. This group, which is clearly separated from the fauna of other regions especially Laurentia, is consistent with the concept of the Scoto-Appalachian fauna that introduced by Jaanusson (1979) and Whittington and Williams (1955). Group D in the PCA diagram corresponds to brachiopod faunas from the Oslo-Oskar area (Norway), South China, and southeastern Ireland (Wexford-Waterford), which corresponds to cluster C in the CA diagram.Group E includes brachiopods from the Zagros and Anarak regions of Iran, Morocco, France, and Portugal. In the CA diagram, the brachiopod faunas from the Zagros and Anarak regions are grouped in cluster E, and the brachiopod faunas from France and Portugal are grouped in cluster D, which indicates the high similarity of the brachiopods of these regions.The group F in the PCA plot corresponds to cluster F in the CA diagram and is related to the brachiopods of the Chu-Il, Boshchekul, and Ishim-Selety regions of Kazakhstan. According to the results of the CA and PCA, the brachiopods of the Kazakh regions show low similarity to the brachiopods of Laurentia, Baltica, and Avalonia, which could be due to the presence of endemic species in the Kazakh regions and very limited faunal connection with the brachiopods of the other areas during the Katian.The brachiopod faunas of epicontinental Laurentia from the Ottawa Valley, Lake Simcoe, Ontario, Manitoulin Island, western Newfoundland, New York, Hudson Valley, Champlain Valley, Kentucky, Indiana, Mississippi Valley, and Oklahoma show a higher similarity to the brachiopod faunas of mainly Avalonian origin than to the Scoto-Appalachian brachiopods of pericratonic regions.The early Katian Scoto-Appalachian brachiopods show a higher affinity to the brachiopods from the western margin of Laurentia such as the northern Rocky Mountains in British Columbia and Great Basin.The differentiation of Laurentian pericratonic and intracratonic brachiopod fauna during the early Katian has been interpreted as a palaeobiogeographical pattern (Sohrabi & Jin 2013).Based on the results of this study, the brachiopods of the Bojnourd region of Iran show more similarity with the Avalonian brachiopods rather than with the brachiopod faunas of the Zagros and Central Iran regions, as shown in cluster A2 in CA and Group 2 in the PCA diagram. The high degree of faunal similarity between the brachiopods of the Bojnourd region and the Avalonian brachiopods could be related to similar environmental conditions of these brachiopods during the early Katian.The close faunal affinity of the brachiopod faunas of the Bojnourd and Kopeh-Dagh regions with those of Avalonia was more likely attributable to the position of Bojnourd region of Iran at relatively lower latitudes which had different environmental conditions than those of the Zagros and Central Iran regions during the early Katian. Also, the low degree of faunal similarity between the Bojnourd brachiopods and the brachiopods of the same age in the Zagros and Central Iran regions could be interpreted as the beginning of brachiopod endemism in the Bojnourd region during the Late Ordovician (early Katian).During the early Katian, the Kopeh-Dagh region of Iran was more likely part of the microplates (terranes) adjacent to the supercontinent of Gondwana at similar latitudes to Avalonia and separated from the Zagros and Central Iran regions. The close similarity of the brachiopod faunas of Zagros and Central Iran with the brachiopods of France and Portugal, which were parts of high-latitude Gondwana, indicates their Gondwana palaeogeographical affinity during the Katian.By collecting more Late Ordovician brachiopods from different regions of Iran and compiling a comprehensive dataset, a better interpretation of the palaeobiogeographic pattern of the brachiopods can be obtained, which could result in a more accurate palaeogeographic positioning of Iran during the Late Ordovician. https://jssr.ui.ac.ir/article_29947.html Abstract The Dalan Formation, a major carbonate-evaporite succession in the Persian Gulf Basin, is a significant gas-bearing reservoir in fields such as South Pars. This study integrates mud gas geochemistry and petrophysical evaluation to characterize hydrocarbon fluid types and assess reservoir quality in zones K3 and K4. Mud gas concentrations (C1–C8) were analyzed using Pixler diagrams and Haworth parameters (Wh, Bh, Ch), while petrophysical evaluation included gamma-ray, density, neutron, and resistivity logs to estimate effective porosity, water saturation, and hydrocarbon saturation. Results indicate that Zone K3 is a dry gas reservoir dominated by methane and ethane, with moderate to low porosity and hydrocarbon saturation, and limited economic potential. Zone K4 exhibits dual behavior, with a lower water-bearing interval of poor quality and an upper interval of moderate to good quality containing wet gas and light condensates. Integration of geochemical and petrophysical data provides accurate identification of hydrocarbon types, productive intervals, and fluid contacts, offering a robust scientific basis for reservoir development planning. This approach is particularly valuable in complex carbonate-evaporite reservoirs, reducing exploration risk and optimizing production strategies. Keywords: Dalan Formation, Mud gas analysis, Reservoir petrophysics, Haworth-Pixler gas ratios, South Pars     Introduction The Dalan Formation, one of the most significant carbonate-evaporite sequences in the Persian Gulf Basin, plays a crucial role in the country’s gas supply. Despite extensive petrophysical studies, integrated analyses combining mud gas geochemistry and petrophysical data have received limited attention. Identifying hydrocarbon fluid types and evaluating reservoir quality, particularly in zones K3 and K4, is essential for optimizing exploration and production strategies. This study aims to address this gap by integrating real-time mud gas measurements with well log and petrophysical data from a well in the South Pars gas field.   Materials & Methods Mud gas data, including hydrocarbon concentrations from methane (C1) to heavier hydrocarbons (up to C8), were recorded continuously during drilling using on-site mud logging systems equipped with gas chromatographs and flame ionization detectors. Quality control procedures were applied to remove background gas, drilling-induced artifacts, and unreliable measurements using the Gas QC index. Valid data were analyzed through Pixler diagrams and Haworth parameters (Wh, Bh, Ch) to identify hydrocarbon types and productive intervals. Petrophysical evaluation utilized gamma-ray, density, neutron, and resistivity logs, combined with lithological and mineralogical interpretation, to estimate effective porosity, water saturation, and hydrocarbon saturation. Probabilistic petrophysical models enabled correlation between fluid types and reservoir quality, providing a comprehensive understanding of the K3 and K4 intervals.   Discussion of Results & Conclusions Zone K3 (3262–3379 m) is predominantly a dry gas reservoir, dominated by methane and ethane. Petrophysical analysis indicates moderate to low effective porosity (3–20%) and hydrocarbon saturation (40–60%), within the lower interval (3300–3382 m) displaying poor reservoir quality due to high anhydrite content and elevated water saturation. Mud gas ratios (C1/C2–C1/C5) and Haworth indices consistently indicate a dry gas system with limited heavier hydrocarbons. Zone K4 (3382–3547 m) exhibits dual behavior: the lower interval (3385–3450 m) shows low porosity, high water saturation, and poor reservoir quality, whereas the upper interval (3450–3500 m) demonstrates moderate to good quality, with effective porosity of 5–20% and water saturation of 40–70%. Mud gas geochemistry and Haworth analysis reveal the presence of heavier hydrocarbons and condensates (wet gas), corroborated by increased resistivity and cross-over effects in density–neutron logs. Pixler diagrams indicate that while the majority of K4 remains in the gas domain, certain depths approach the gas/oil boundary, confirming the existence of mixed fluids. The integration of mud gas geochemistry and petrophysical data highlights significant heterogeneity within the Dalan Formation. Zone K3 is a mature dry gas reservoir with limited economic potential, while the upper interval of K4 contains condensates that enhance production prospects. Conventional methods such as well logging and reservoir testing, although valuable, may be insufficient in complex carbonate-evaporite settings due to lithological heterogeneity and operational limitations. In contrast, mud gas analysis provides a rapid, cost-effective, and complementary approach to assess hydrocarbon types, identify productive zones, and reduce exploration risk. The combined use of Pixler and Haworth methods allows precise differentiation between dry gas, wet gas, and light condensates, enhancing the accuracy of reservoir characterization. Data were collected from a single well, limiting the ability to directly generalize results across the entire South Pars field. Instrumental errors, depth alignment uncertainties, and variations in drilling conditions may influence the accuracy of gas geochemistry measurements. Nevertheless, the study provides a reliable scientific basis for understanding fluid distribution in the upper Dalan Formation. Conclusions of this study follows: Zone K3 is a dry gas reservoir dominated by methane and ethane, with low to moderate reservoir quality and limited production potential. Zone K4 consists of a lower poor-quality, water-bearing interval and an upper interval with moderate to good quality containing wet gas and light condensates, contrary to prior assumptions of exclusively dry gas. Integration of mud gas geochemistry and probabilistic petrophysics enables accurate identification of hydrocarbon types, productive intervals, and fluid contacts. The upper K4 interval represents a valuable target for condensate production and should be prioritized in reservoir development planning. This integrated approach provides critical insights for the South Pars gas field and similar complex carbonate-evaporite reservoirs, offering a scientific basis for optimized well placement, production strategy, and risk mitigation. https://jssr.ui.ac.ir/article_30228.html A micropaleontological analysis of the Miocene strata in the Qeshm and Minab regions of southern Iran has yielded a diverse foraminiferal assemblage, comprising twenty-five species belonging to fifteen genera. This fauna is predominantly reported for the first time from these studied outcrops. The identified taxa include: Asterorotalia dentata, A. pulchella, Triloculina tricarinata, T. terquemiana, T. trigonula, Trilobatus (Globigerinoides) trilobus, Globigerina bulloides, Quinqueloculina bogdanowiczi, Textularia agglutinans, Praeorbulina transitoria, Elphidium crispum, E. craticulatum, E. asiaticum, E. advenum limbatum, E. advenum macelliforme, E. advenum maorium, Poroeponides lateralis, Eponides repandus, Eponides isabellanus, Rotalinoides compressiuscula, Challengerella bradyi, Ammonia beccarii, and Bolivina spathulata. Biostratigraphic analysis, based on key index species, provides refined age constraints for the regional stratigraphy. Within the Mishan Formation on Qeshm Island, the presence of Praeorbulina transitoria and Quinqueloculina bogdanowiczi in the white sandy limestone of the Stars Valley section indicates a Langhian-Serravallian boundary age. The uppermost strata of the Mishan Formation on the island suggest a depositional range extending from the late Serravallian to the Mio-Pliocene boundary, potentially correlating with the interval of global planktonic foraminiferal zones N8-N9 to N19-N20. In the Minab region, the Gushi Marl of the Makran Basin is dated to the latest Miocene (Messinian) to early-middle Pliocene (Zanclean to Piacenzian boundary), corresponding to the time encompassed by biozones N19-N20. The paleobiogeographic distribution of the fauna is particularly significant. The co-occurrence of these species in the studied areas, coupled with the presence of Quinqueloculina bogdanowiczi-a taxon previously documented only from the Central Eastern Paratethys during the Serravallian to Tortonian-suggests the existence of a marine connection during the Late Miocene. This evidence supports a seaway linking the Iranian Gateway and the Iraqi Basin (represented by the Fatha Formation) with the marginal marine environments of the eastern Paratethys (Qom Basin), the Central Paratethys, the Indo-Pacific Ocean, and the proto-Mediterranean. https://jssr.ui.ac.ir/article_29992.html AbstractIn this study, the stratigraphic distribution of foraminifera and the environmental evolution of the Permian–Triassic boundary (PTB) successions in the central Persian Gulf (upper Dalan and the basal part of the Kangan formations) were investigated through the integration of micropaleontological, microfacies, and isotopic analyses (δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr). The results reveal three local composite biozones within the upper Dalan and lower Kangan formations, along with the abundance of the index taxon Paradagmarita, which shows biogeographic affinity with approximately coeval strata in Turkey, the Caucasus, Saudi Arabia, and Oman. Microfacies analysis identified three facies belts including lagoonal, subtidal shoal, and tidal flat, indicating a general shallowing trend from lagoonal toward peritidal settings. A simultaneous decrease in δ¹³C and δ¹⁸O values was observed near the boundary, coinciding with the biotic crisis and the development of oxygen-depleted conditions. The increase in ⁸⁷Sr/⁸⁶Sr ratios across the boundary suggests enhanced influx of continental materials and intensified chemical weathering. The results also indicate a moderate increase in ooid size and significant changes in foraminiferal assemblages, reflecting a relatively shallow, warm, stressed, and oxygen-deficient environment. The novelty of this study lies in the combined application of three independent datasets, including sedimentological, geochemical, and paleontological, from multiple subsurface sections, enabling a more precise reconstruction of the environmental evolution and biotic events at the onset of the Triassic.Keywords: Foraminifera, Microfacies, Ooids, Carbon and oxygen isotopes, ⁸⁷Sr/⁸⁶Sr, Permian–Triassic boundary, Persian Gulf  IntroductionThe Permian–Triassic transition marks the most profound biotic crisis in Earth’s history, with over 90% of marine species becoming extinct. This event significantly transformed carbonate platforms and their sedimentary environments. The Dalan and Kangan formations, representing the Late Permian to Early Triassic succession in the Zagros and Persian Gulf regions, record these changes in detail. Previous works have discussed the sedimentological and geochemical evolution of this interval (Abdolmaleki and Tavakoli 2016; Rafiei et al. 2016; Tavakoli et al. 2018; Haghighat et al. 2020). However, few studies have combined micropaleontological, isotopic, and microfacies evidence from multiple subsurface sections to elucidate the precise environmental evolution during this critical interval. The present study provides a comprehensive reconstruction of the Permian–Triassic boundary (PTB) based on integrated data from four wells in the central Persian Gulf. Materials & MethodsThe study is based on petrographic and geochemical analyses of 2,500 thin sections from the Dalan and Kangan formations in four wells (A, B, E, and F). Samples were taken at 30 cm intervals to ensure high stratigraphic resolution. Foraminiferal assemblages were identified under transmitted and polarized light microscopes. Microfacies were described and classified following Folk (1959), Dunham (1962), and Embry & Klovan (1971). Stable isotopes (δ¹³C and δ¹⁸O) were analyzed on micritic matrix samples to minimize diagenetic alteration. Samples with micritic fabrics (wackestones and fine packstones) were preferred due to their low permeability and better preservation of primary isotopic composition. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) were determined from wells B, E, and F. Petrographic and geochemical screening confirmed the absence of secondary dolomitization or recrystallization features in the selected samples, following the procedures described in Tavakoli (2015) and Abdolmaleki and Tavakoli (2016). Discussion of Results & ConclusionsForaminiferal Assemblages: Micropaleontological analysis revealed 51 species (41 genera) of benthic foraminifera dominated by Globivalvulina, Dagmarita, and Paradagmarita. Two major biozones were defined: the Glomomidiellopsis–Paradagmarita Assemblage Zone (Changhsingian) and the Microconchus–Ammodiscus kalhori Assemblage Zone (Griesbachian). These assemblages correlate well with equivalent zones in Saudi Arabia, Oman, Turkey, and the Caucasus (Haghighat et al. 2020). The disappearance of Late Permian taxa and the dominance of opportunistic forms (Ammodiscus kalhori, Microconchus phlyctaena) above the boundary indicate severe environmental stress and low-oxygen conditions.Microfacies Evolution: Nine microfacies types were recognized, ranging from ooid and bioclastic grainstones to thrombolitic boundstones and dolomitic mudstones. The upward transition from subtidal grainstones to peritidal thrombolitic facies indicates progressive shallowing. Ooid grainstones in the uppermost Dalan suggest deposition in high-energy shoal settings. In contrast, microbial boundstones in the lowermost Kangan represent early recovery of carbonate production under oxygen-depleted, restricted conditions. This pattern matches earlier models for the PTB carbonates in the Persian Gulf (Tavakoli et al. 2018; Davoodi et al. 2024).Isotopic Trends: The isotopic curves show a pronounced negative excursion in δ¹³C (from +4‰ to −1‰) and δ¹⁸O, synchronous with a rise in ⁸⁷Sr/⁸⁶Sr ratios across the PTB. The δ¹³C drop corresponds to global disturbances in the carbon cycle, potentially related to methane release, enhanced continental weathering, and volcanogenic CO₂ emissions (Tavakoli and Rahimpour-Bonab 2012). The gradual depletion of δ¹⁸O toward the boundary likely reflects rising seawater temperature and meteoric diagenesis under greenhouse conditions (Abdolmaleki and Tavakoli 2016; Naderi-Khujin et al. 2016). Elevated ⁸⁷Sr/⁸⁶Sr ratios across the boundary further support intensified continental weathering and influx of radiogenic strontium during the end-Permian climatic crisis.Integrated Interpretation: Integration of faunal, sedimentological, and isotopic data indicates a shift from a well-oxygenated, open-marine platform during the Late Permian to a restricted, shallow, and stressed lagoonal system in the Early Triassic. The decline in biodiversity, increase in microbial facies, and negative isotopic excursions reflect the combined effects of eustatic fall, climatic warming, and oceanic anoxia. These changes represent the regional expression of global end-Permian perturbations. The integrated multiproxy approach employed here refines previous reconstructions (Haghighat et al. 2020; Nazemi et al. 2021; Davoodi et al. 2024; Rezvannia et al. 2025; Shahkaram et al. 2025) and provides a more complete picture of the environmental transition in the central Persian Gulf. https://jssr.ui.ac.ir/article_30258.html Abstract The purpose of this study was to investigate the mineralogy, geochemistry, and provenance of clastic strata of the Aghajari Formation (Upper Miocene–Pliocene) in the north of Hoseynieh and Andimeshk. Petrographic studies and modal analyses of sandstones indicate that quartz, feldspar of igneous origin, and lithic fragments (igneous, metamorphic, and sedimentary) are the main constituents of the sandstones. It was also revealed that the sandstone intervals fall within the recycled or transitional recycled orogenic provenance field, which accumulated during the late Miocene–Pliocene under warm and semi-arid climates. The clay mineral content of these sediments is mainly chlorite and illite, of detrital origin. Th/Co versus La/Sc cross-plots indicated a siliceous source rock, and Ti/Zr versus La/Sc and La/Th versus Hf suggest andesitic source rocks for the Aghajari Formation. Furthermore, Th/Sc versus Zr/Sc showed a first-order sedimentation cycle. The orogenic events during Miocene–Pliocene (the Savian and Strian tectonic phases) in the Folded Zagros led to the erosion of a mixture of igneous–ophiolitic rocks from the Neotethyan oceanic crust and metamorphic rocks exhumed in northern Lorestan and Kermanshah regions, along with the sedimentary successions of the folded Zagros (the Amiran, Talehzang, Kashkan, Shahbazan and Asmari formations). These formations supplied the clastic sediments of the Aghajari Formation within the Hoseynieh and Andimeshk syncline. Keywords: Petrography, Geochemistry, Aghajari Formation, Provenance, Zagros Basin     Introduction Sedimentary rocks are the main source of information about past conditions. Based on the chemical composition of sediments and siliciclastic sedimentary rocks, the provenance and other depositional processes, such as weathering, transportation, and diagenesis, can be evaluated (Dickinson and Suczek 1979; Sharafi et al. 2018; Asiedu et al. 2019; Salehi et al., 2018; Zamanian et al. 2019; Peng et al. 2020). Among the various controlling factors, the tectonic setting and lithology of the source area are the most important parameters governing the formation of siliciclastic deposits (Yan et al. 2012; Salehi et al., 2014; Khazaei et al. 2018). The tectonic setting of siliciclastic rocks can be interpreted using petrographic and geochemical data through discrimination diagrams, as these reflect sediment-generation processes, source-area morphology, and paleoclimatic conditions (Sabbagh et al. 2018; Pourdivanbeigi Moghaddam et al., 2020). The Aghajari Formation, the youngest unit of the Fars Group, was formally introduced by James and Wynd (1965) and consists of a thick succession of syn-orogenic red molasse deposits widely distributed throughout the Zagros region. Its age ranges from Middle Miocene to Pliocene and varies spatially across the basin (Motiee 2003). In this study, mineralogical and geochemical evidence are used to investigate the provenance of the Aghajari siliciclastic strata. Since sedimentary rocks represent the main remnants of eroded ancient crust (Condie et al. 2001; Basu 2003), provenance analysis provides key insights into the Miocene–Pliocene tectonic evolution of the Zagros Fold-Thrust Belt. Material & Methods In this study, three stratigraphic sections of the Aghajari Formation, including Paalam (600 m), Khoshab (550 m), and Piravali (520 m), were investigated in the Hosseinieh area of the Andimeshk region. Sampling in each section was carried out based on facies variations and, in some cases, systematically. A total of 180 rock samples (60 hand specimens from each section) were collected from different facies. To investigate the provenance of the Aghajari siliciclastic strata, 60 sandstone samples (mostly coarse-grained) were selected for thin-section preparation. From these, 17 thin sections (7 from the Aghajari Formation and 10 from the Lahbari Member) were selected for modal analysis. The percentages of cement and matrix in each sample were calculated relative to the total rock framework. To determine major and trace elements, identify lithological characteristics, and evaluate the tectonic setting, 20 mudstone samples were analyzed using ICP-MS at the Zarazma Laboratory (Tehran). These samples were selected to represent the entire stratigraphic succession under study. Additionally, 10 mudstone samples were analyzed by X-ray diffraction (XRD) at the Central Laboratory of Lorestan University to identify the mineralogical composition of fine-grained deposits. Modal analysis of sandstones was performed by counting 300–500 points per thin section following the Gazzi–Dickinson method (Dickinson 1970; Ingersoll et al. 1984), and sandstone classification was carried out according to Folk (1980). Petrographic and geochemical data, together with established discrimination diagrams and the Chemical Index of Alteration (CIA) (Nesbitt & Young, 1984), were used to infer the tectonic setting, palaeoclimate, and paleocurrent patterns.   Discussion of Results & Conclusions The study area is located at the beginning of the Zagros folded zone. The Aghajari Formation is the main outcropping rock unit in the study area and consists mainly of sandstone, siltstone, claystone, mudstone, and shale. The investigated sandstones are mainly composed of sedimentary rock fragments, such as chert and carbonate fragments. The studied sandstones are generally poorly sorted, as seen in the thin sections. Petrographic studies revealed that sandstones of the Aghajari Formation are mainly lithic-arenite (sed-arenite), which is mostly composed of chertarenite and calclithite with calcite cement. The origin of the cement in these facies can be attributed to the dissolution of unstable carbonate grains, with an average composition of (Q38R60F2) in the upper part (Lehbari Member) and (Q33R66F1) in the lower part of the Aghajari Formation. The clay mineral content of these sediments is mainly chlorite and illite, of detrital origin.The grain size and particle geometry, as well as the degree of sorting of the studied sandstones, indicate that they are texturally immature in terms of textural maturity. Th/Co versus La/Sc cross-plots indicated a siliceous source rock, and Ti/Zr versus La/Sc and La/Th versus Hf suggest mixed acidic–basic and andesitic source signatures in the studied samples for the Aghajari Formation. Furthermore, Th/Sc versus Zr/Sc showed a first-order sedimentary cycle. The petrographic and geochemical studies of sandstones in the Aghajari Formation indicate that the investigated sediments have a transitional recycled origin, with recycling in an orogenic zone. Moreover, the results of the modal analysis indicate that the climatic conditions were semi-arid during the deposition of this formation. According to geochemical analyses, the investigated sandstones originated from intermediate or andesitic rocks. Furthermore, geochemical diagrams indicate that the studied sandstones formed in an oceanic arc-island field setting. The geochemical data in the A-CN-K triangular diagram indicate moderately weathered conditions in the source area of the Aghajari Formation. https://jssr.ui.ac.ir/article_30351.html The Asmari Formation, one of the most important carbonate reservoirs in the Zagros Basin, was studied in the Chahar Bishe oil field to identify its facies, depositional environments, and sequence stratigraphic framework. Examination of 316 meters of core and cutting samples of a Well from Chahar Bishe oilfield led to the recognition of fourteen carbonate facies deposited within four main facies belts: tidal flat, semi-restricted lagoon, reefal barrier, and open marine. The vertical succession of facies indicates a general shallowing-upward trend from open-marine to nearshore environments, suggesting deposition on a low-angle homoclinal carbonate ramp. Sequence stratigraphic analysis resulted in the identification of four third-order depositional sequences, each composed of a Transgressive Systems Tract (TST) and a Highstand Systems Tract (HST). Type-2 sequence boundaries and Maximum Flooding Surfaces (MFS) were clearly recognized, reflecting relative sea-level fluctuations during the Miocene. The results of this study define the Asmari Formation as a homoclinal carbonate ramp in which facies distribution was mainly controlled by sea-level oscillations and variations in depositional energy. https://jssr.ui.ac.ir/article_30376.html In this study, the depositional and post depositional history of carbonate rocks of the Asmari Formation in the Lorestan region was examined. Three stratigraphic sections were selected for this purpose: the northern Khorramabad section (92.3 m), the southeastern Khorramabad section (90 m), and the southwestern Khorramabad section (190 m). A total of 380 samples from these sections were collected for petrographic studies. In these sections, the lower boundary of the Asmari Formation with the Shahbazan Formation is a conformable contact, while the upper boundary with the Gachsaran Formation is an erosional unconformity.Thirteen microfacies were identified, including: anhydrite microfacies; fenestral dolomudstone; nodular dolomudstone; wackestone/packstone with extraclasts; mudstone to wackestone with bioturbation; wackestone/packstone with non porous foraminifera; wackestone/packstone with both non porous and porous foraminifera; wackestone/packstone with porous foraminifera; bioclastic grainstone containing foraminifera; packstone to grainstone with foraminifera, echinoderms, bryozoans, and coralline algae; rudstone/floatstone containing foraminifera and corallinacean algae; bioclastic floatstone/rudstone with red coralline algae; and mudstone to wackestone containing planktonic foraminifera. These microfacies indicate a carbonate ramp depositional environment. Several diagenetic processes were observed, including micritization, neomorphism, cementation (marine, meteoric, and burial types), dissolution and porosity (fabric selective and non fabric selective), and replacement (pyritization, silisification, and dolomitization). Based on petrographic evidence, the paragenetic sequence of the Asmari Formation deposits in these sections was interpreted in four diagenetic environments: marine, meteoric, burial, and uplift. These processes reflect early diagenesis (eogenesis), middle diagenesis (mesogenesis), and late diagenesis (telogenesis). https://jssr.ui.ac.ir/article_30392.html A complete stratigraphic succession of the Elika Formation (Early–Middle Triassic) and the Shemshak Group (Late Triassic–Middle Jurassic) is well exposed in northeastern Jajarm, in the Eastern Alborz structural zone. In this study, the upper part of the Elika Formation and the Shemirzad Formation of the Shemshak Group were investigated using stratigraphic and petrographic methods. The upper part of the Elika Formation consists of approximately 80 m of medium- to thick-bedded dolomite and dolomitic limestone, which are mostly stromatolitic boundstone and lack fossils. These features indicate deposition in a tidal-flat environment. This formation disconformabaly overlies the Mobarak Formation. The Shemirzad Formation is approximately 285 m thick and is predominantly composed of shale with interbeds of sandstone. Based on stratigraphic characteristics, it can be divided into several units. Marker beds include a middle sandstone layer rich in Calamites plant fossils and a fossiliferous limestone layer in the upper part. Petrographic analyses reveal that the predominant petrofacies of the Shahmirzad Formation are predominantly composed of siltstone, litharenite with clay matrix and cement, litharenite with carbonate cement, feldspathic litharenite with carbonate cement, feldspathic litharenite with siliceous cement, lithic arkose with carbonate cement, and lithic arkose with clay cement. litharenite represents one of the principal sandstone types associated with uplifted orogenic belts and deposition in foreland basins. Key words, Jajarm, Shemshak Group, Elika Formation, Shahmirzad Formation, Petrofacies https://jssr.ui.ac.ir/article_30406.html The aim of this research is the investigation of lithofacies and sedimentary environment of the Kashkan Formation (middle Eocene) in the Lorestan sedimentary basin. In this regard, four stratigraphic sections were selected. The Kashkan Formation in the studied area, with a variable thickness between 150 to 200 meters, consists of conglomerate, sandstone, and to a lesser extent fine-grained facies. Field studies on the four sections led to the identification of 12 lithofacies, which include five conglomeratic lithofacies (Gm, Gms, Gp, Gh, Gt), five sandstone lithofacies (Sp, St, Sm, Sh, Sr), and two mudstone lithofacies (Fm, Fl). Based on the vertical and lateral changes of facies, the depositional environment of these deposits was a braided river system with a gravelly and sandy bed in proximal areas. The clastic sequences of the Kashkan Formation overlie the marine carbonates of the Taleh Zang Formation, which indicates a sea regression and the progradation of braided rivers toward the basin. A decrease in accommodation space relative to sediment supply caused the progradation of braided river facies within the LST facies suite for the Kashkan Formation. Paleocurrent analysis in this formation shows a northeast to southwest trend, which is evidence of unidirectional (fluvial) currents in the Kashkan Formation.