SW Iran Research Articles

Electrofacies analysis of the Asmari reservoir, Marun oil field, SW Iran

This article integrates core and well log data to determine reservoir electrofacies of the Oligo-Miocene Asmari Formation in the western Dezfol Embayment, SW Iran. At the start, an unsupervised neural network was employed based on the selforganizing map (SOM) technique to identify and extract electrofacies groups of Asmari Formation in the Marun oilfield, which is a mixed siliciclastic and carbonate reservoir. Using inexpensive and accessible petrophysical wireline logs, such as gamma ray, sonic, density and neutron, along with calculated reservoir data (water saturation and effective porosity) from 12 reference wells and their correlation with reservoir core data, led to recognition of seven electrofacies including first four class as limestone rocks (EF1–EF4) and others three classes as sandstone rocks (EF5–EF7). Based on the porosity and permeability maps and water saturation at different depth levels, EF4 and EF7 with low porosity and permeability, high water saturation are considered as non-reservoir relating mostly to sedimentary textures of mudstone and argillaceous sandstones, respectively. By contrast, EF1 and EF5 with high values of porosity, permeability and low percent of water saturations characterize the best reservoir quality rocks, and EF2, EF3 and EF6 have medium reservoir quality. EF1 consists of dolomitic skeletal grainstone, in which biomoldic, vuggy and intercrystalline porosity is the dominant pore type, while the intergranular porosity is the major pore type in EF4 as an unconsolidated sand electrofacies. In general, the results obtained in this study indicate a satisfactory agreement between core data and log facies. This correlation allows rockfacies to be classified in the cored wells and predict those facies in the uncored wells. Using the methodology outlined here, it is possible to track reservoir electrofacies from conventional well log data.

Persiconus sarvaki gen. et sp. nov., a new complex orbitolinid (Foraminifera) from the Cenomanian of the Sarvak Formation (SW Iran, Zagros Zone)

A new orbitolinid is described as Persiconus sarvaki gen. et sp. nov. (subfamily Orbitolininae), from the Cenomanian of the Sarvak Formation, Zagros Zone, SW Iran. It is characterized by a high-conical test, a complex embryo situated in an apical position, a complex marginal zone (several orders of beams and rafters), and a central zone with a reticulate network of anastomosing main partitions. Comparisons can be made to the Aptian taxa Rectodictyoconus Schroeder, Dictyoconus? pachymarginalis Schroeder, and the Thanetian genus Karsella Sirel. The new species occurs in a foraminiferal wacke-/packstone associated with nezzazatids, alveolinids (Praealveolina, Cisalveolina), chrysalidinids, indicating a middle to middle/upper Cenomanian age. It is considered an endemic taxon restricted to the Arabian Plate. Persiconus sarvaki represents the latest orbitolinid recorded so far before the large-scale extinction event around the Cenomanian-Turonian boundary.

Heterogeneous sub-simple deformation in the Gol-e-Gohar shear zone (Zagros, SW Iran): insights from microstructural and crystal fabric analyses

Petrofabrics and microstructural characteristics of the quartz-rich mylonites were investigated in the NW–SE-trending Gol-e-Gohar shear zone with length of ca. 25 km within the Zagros Transpression Zone, SW Iran. The semi-quantitative vorticity analysis using quartz c-axis-fabric (Rxz/β method) and rigid porphyroclast geometry (Rigid Grain Net method) reveals that mean kinematic vorticity number (Wm) increases towards the Baghat Thrust Fault displaying the occurrence of spatial and temporal variations of flow during progressive deformation. The Wm values show pure shear-dominated flow progressively was changed to a simple shear-dominated flow toward to the late stages of ductile deformation, implying that the accelerating flow path was accompanied with progressive cooling and strain localization at the lower parts of the zone. The measured vertical thinning/dip-parallel elongation suggests that thrust emplacement and metamorphic rock extrusion likely occurred by a combination of simple shear with a pure shear component of deformation (i.e., general shear). The estimated deformation temperatures based on the quartz c-axis fabric patterns and their opening angles highlight greenschist-to-amphibolite-facies conditions.

Factors Controlling the Unusual Oil Composition Distribution in the Bangestan Reservoir, Dezful Embayment, SW Iran

Abstract —Petroleum geochemical characteristics are a major parameter of hydrocarbon field development. The present study is an attempt to decipher the unusual Bangestan (Cretaceous age) oil distribution throughout the Ahvaz oil field, SW Iran, based on the SARA test and GC–MS analysis of selected oil samples. The results indicated that all analyzed oils belong to the paraffinic group. Hydrocarbon indicators, such as tricyclic triterpane C22/C21 (high), C24/C23 and C26/C25 (low), and hopane C31R/C30 (high) ratios and C25/C26 ratio vs. C25/C26 tet ratio, show that these oils are sourced from carbonate–marl rocks. The high saturation/aromatic hydrocarbon ratio in these oil samples may be related to long migration or high maturity. The biomarker variation exhibits a marine environment for the source rocks deposition. The observed oil maturity trend is showing a good correlation with the prevailing geothermal gradient and possible basement faults and fractured system. The increasing oil maturity in the eastern part of the field may be related to a heat flow anomaly. It seems that the area around well C (the area of wells C, D, or E) can be considered a petrochemical separator of fluids for two sides of the field. Therefore, it can be concluded that the observed petrochemical pattern is a complicated response of several factors: the presence of a palaeohigh, basement-controlled faults, petroleum source rocks, fractured system, and geothermal gradient variation in this deep reservoir.

Geochemistry, facies characteristics and palaeoenvironmental conditions of the storm-dominated phosphate-bearing deposits of eastern Tethyan Ocean; A case study from Zagros region, SW Iran

Phosphate deposits in south-western Iran are part of the South Tethyan Phosphogenic Province, a huge carbonate-dominated strata that extends to the Middle East. The Tethyan phosphorites of Iran are dated Eocene-Oligocene (Pabdeh Formation) and categorized as low-grade ore deposits on a global scale. Depositional conditions of the facies indicate that the Pabdeh Formation was deposited on a carbonate ramp setting as a distally steepened ramp. Under such an environment, turbidity currents transported phosphate particles from the back-shoal setting to the deeper middle and outer ramp of the ocean where they were suspended and deposited as shell-lag and phosphate lamination. Microfacies studies demonstrate that all the phosphatic ooids and phosphatized foraminifera, fish scales, bones and phosphatic intraclasts reworked from shallow parts of the Tethyan Ocean to deeper parts with the help of turbidity currents. Analysis and interpretation of the data reveal positive correlation between. REE+Y and P2O5 in all studied sections which attests to their strong coherence as a geochemical group. The shale normalized REE patterns of Mondun phosphorites are characterized by negative Ce anomalies. This anomaly indicate that the depositional environment was oxic and highly reworked, bioturbated with higher energy realm during phosphate deposition, conversely Nill section with Ce enrichment reflect conditions of relatively deeper water sedimentation. These geochemical findings are in accordance with microfacies studies which indicate shallow and high energy condition for Mondun section with negative cerium anomalies and a deep ramp setting for Nill and Siah sections which denote a positive cerium anomalies in REE patterns.

Reservoir heterogeneity and fracture parameter determination using electrical image logs and petrophysical data (a case study, carbonate Asmari Formation, Zagros Basin, SW Iran)

Assessment of reservoir and fracture parameters is necessary to optimize oil production, especially in heterogeneous reservoirs. Core and image logs are regarded as two of the best methods for this aim. However, due to core limitations, using image log is considered as the best method. This study aims to use electrical image logs in the carbonate Asmari Formation reservoir in Zagros Basin, SW Iran, in order to evaluate natural fractures, porosity system, permeability profile and heterogeneity index and accordingly compare the results with core and well data. The results indicated that the electrical image logs are reliable for evaluating fracture and reservoir parameters, when there is no core available for a well. Based on the results from formation micro-imager (FMI) and electrical micro-imager (EMI), Asmari was recognized as a completely fractured reservoir in studied field and the reservoir parameters are mainly controlled by fractures. Furthermore, core and image logs indicated that the secondary porosity varies from 0% to 10%. The permeability indicator indicates that zones 3 and 5 have higher permeability index. Image log permeability index shows a very reasonable permeability profile after scaling against core and modular dynamics tester mobility, mud loss and production index which vary between 1 and 1000 md. In addition, no relationship was observed between core porosity and permeability, while the permeability relied heavily on fracture aperture. Therefore, fracture aperture was considered as the most important parameter for the determination of permeability. Sudden changes were also observed at zones 1-1 and 5 in the permeability trend, due to the high fracture aperture. It can be concluded that the electrical image logs (FMI and EMI) are usable for evaluating both reservoir and fracture parameters in wells with no core data in the Zagros Basin, SW Iran.

Integration of sonic and resistivity conventional logs for identification of fracture parameters in the carbonate reservoirs (A case study, Carbonate Asmari Formation, Zagros Basin, SW Iran)

Between all available methods for fractures evaluation, core and image logs are prime methods. However, both of these data are very expensive and practically available in less than 10% of drilled wells. Therefore, a less expensive and quick method would be very helpful for fracture analysis. The main aim of this study is to identify the fracture parameters in the carbonate reservoirs, i.e. fracture density and fracture aperture, using a combination of sonic, resistivity and other conventional logs which commonly available in all wells. For this aim, fractures and their parameters were precisely determined on the image logs in 6 wells from carbonate Asmari Formation as a typical fractured reservoir in the world. Then, the effect of fracture parameters on the sonic and resistivity logs were used to identify fracture parameters in the wells with no core and image logs. Results indicate that the zones with high fracture density and aperture are easily detectable by conventional logs. On the other hand, the behavior of these logs is a function of fracture parameters in the fractured intervals, especially in the zones with high fracture aperture. Separation between shallow (RLA2) and deep (RLA5) resistivity logs along with their cross plot are very reliable indicators for fractured intervals. Sonic transit time rapidly increases where fractures density or aperture increased as well. In this study, a combination of sonic and neutron porosity logs have also been used to determine the effect of fracture aperture, as the main effective factor on the reservoir properties. Furthermore, full waveforms in some studied wells combined with sonic transit time to improve the results of fractures evaluation. Using sonic and resistivity along with neutron porosity and gamma ray logs provides a reliable method for describing the fracture parameters which shows high correlation with image logs results. Due to the high impact of fracture aperture on both reservoir quality and conventional log responses, determination of this parameter is more crucial than other fracture parameters. This method is also applicable for OBM (Oil Base Mud) image tools which conventionally cannot detect fracture aperture.

Reticulotaberina n. gen. jahrumiana n. sp., a new soritoidean (Praerhapydioninidae) from the Eocene of Iran

A new porcelaneous larger benthic foraminifer, Reticulotaberina jahrumiana new genus and new species, from the latest middle Eocene shallow-water carbonate platform deposits of SW Iran (Jahrum Formation, Zagros Mountains) is described. Its structure is characterized by the development of a thin alveolar layer (exoskeleton) in the marginal area of the chamber lumen and by septula and interseptal pillars (endoskeleton) in the central area. The septula build a network, whereas the interseptal pillars rise from its intersections. The intercameral foramina are concentrically arranged and are intercalated with septula. R. jahrumiana n. sp. displays a strong dimorphism: A-form has a planispiral-uncoiling subcylindrical morphology, whereas the B-form presents a flared shape with oval or compressed outline. The new taxon is included in the Praerhapydioninidae family and is so far known from the south-west of Iran. The age is late Bartonian.

Geometric interpretation of Faults and surface fractures of Siyah Makan oil field(Dezful embayment, SW Iran) by Thermal Remote Sensing Techniques

امروزه، علم سنجش از دور به‌طور گسترده‌ای در علوم مرتبط با زمین به‌کار می‌رود. یکی از این شاخه‌ها شناسایی، تحقیق و تفسیر ساختارهای سطح زمین است. مطالعه سیستم‌های شکستگی‌‌، شناخت هندسه و نحوه گسترش مکانی آن‌ها، می‌تواند به توسعه میادین نفتی، کمک قابل توجهی کند. در این تحقیق، با استفاده از داده‌های ماهواره‌ای Landsat 8، الگوریتم STA و تکنیک‌های پردازش سنجش از دور، تجزیه و تحلیل هندسی ساختارهای سطحی میدان نفتی سیاه‌مکان مورد بررسی قرار گرفته‌است. سپس، دمای سطح زمین(LST)، با روش دفتر علوم لندست برای محدوده میدان نفتی محاسبه شده، در نهایت به‌بررسی ارتباط توزیع شکستگی‌های سطحی با الگوی توزیع دمای سطحی محدوده مطالعاتی پرداخته‌ شده است. نتایج نشان می‌دهد که شکستگی‌های موازی سطح محوری (SA) و برشی (SO1) که به‌ترتیب، دارای روند N-S و NW-SE هستند، بیشترین فراوانی را دارند. براساس نقشه چگالی‌، تراکم خطواره‌ها در نواحی میانی، متمایل به جنوب شرقی میدان، دارای بیشترین‌ مقدار است. از آنجایی‌که منابع زیرزمینی در امتداد خطواره‌های تکتونیکی نزدیک سطح قرار دارند، این ساختار‌ها در تصاویر ماهواره‌ای خنک‌تر و تاریک‌تر از مناطق اطراف دیده می‌شوند. خطواره‌های ساختاری با نقشه دمای سطحی، انطباق نشان می‌دهد؛ به‌طوری‌که خطواره‌ها، اغلب در مناطق با دمای سطحی متوسط و متوسط به پایین قرار گرفته‌اند. از این رو، دماهای کمتر با محدوده‌های دارای تراکم شکستگی بالا مطابقت نشان می‌دهند.

Study of hydrocarbon generation and 1D-2D modeling of hydrocarbon migration at the Karanj and Parsi oilfields, Southern Dezful Embayment, SW Iran

Abstract This study is a 2D basin modeling to determine the petroleum system (i.e. Cretaceous-Neogene play) of the Karanj and Parsi oilfields in the Southern Dezful Embayment, SW Iran. This system is characterized by occurrence of source rocks of Middle Cretaceous, Late Cretaceous and Paleocene to Eocene (Kazhdumi, Pabdeh and Gurpi Formation), reservoirs of Upper Cretaceous and Late Oligocene–Early Miocene (Ilam-Sarvak and Asmari formations), and seals of Paleocene and Miocene (Pabdeh and Gachsaran Formation). Considering temperature, transformation ratio and vitrinite reflectance, the maturity of source rocks (Kazhdumi, Pabdeh and Gurpi Formation) was investigated. A maturity history analysis showed that the maturity of Kazhdumi, Pabdeh and Gurpi Formation started during 70–60 and 4 Ma, respectively. Generation and expulsion of hydrocarbons started ~16 Ma in the Kazhdumi Formation and ~3 Ma in the Pabdeh Formation that continued up to the present. The analysis of the transformation ratio indicates transformation of significant amounts of Kazhdumi source rocks for generating hydrocarbons. Migration of hydrocarbons from the Kazhdumi Formation is mainly vertically upward. From Pabdeh Formation migration is both vertically upward (to the Asmari reservoir) and downwards (to the Ilam-Sarvak reservoir). However, some local traps have formed due to the hydrocarbon accumulation above the faults. Our results reveal that petroleum generation, migration and entrapment occurred at proper timing which facilitated the migration of hydrocarbons and their accumulation in the reservoirs.

Effect of salinity of irrigation water on soil properties (abadan plain, SW Iran)

To evaluate the effects of irrigation with saline water of Bahmanshir River in southwestern Iran, 11 water samples from the river in two flow and ebb periods and 22 soil samples from two depths of 0–20 and 20–40 cm were taken. The irrigation suitability indexes and Wilcox, USSL, and Doneen charts were used to determine the quality of agricultural water. The soil texture, SAR, EC, and pH parameters were used to determine soil quality. Important factors on the water quality are flow and ebb, wastewater discharge, hydrogeochemical and climatic processes. Based on the most studied indexes, water quality is classified as permissible to unsuitable. The Wilcox and USSL charts indicate that water is unsuitable for agricultural use. The Doneen chart showed that water had no effect on soil permeability. The region’s soils are mainly classified as saline-sodic and saline soils. The soil salinity depends irrigating with saline water, inappropriate irrigation approaches, and geographical and climatic conditions.

Priorities for conservation of endemic trees and shrubs of Iran: Important Plant Areas (IPAs) and Alliance for Zero Extinction (AZE) in SW Asia

Owing to narrow geographical distribution, endemic species indicates higher vulnerability than other taxa, so they have been used as surrogate elements to identify the priorities. Little attention has been paid to distribution patterns, hot spots, and conservative zones of endemic trees and shrubs in scale of Iran. The main purpose of this study was to identify and introduce the Important Plant Areas (IPAs) as well as Alliance for Zero Extinction (AZEs) of endemic trees and shrubs in Iran. The total number of endemic trees and shrubs in the studied area included 75 species belonging to 32 genera of 22 plant families. Chorological elements mainly (65:86.6%) belong to the Irano-Turanian region. The geomorphological barriers have confined plant immigrations in the north-south as well as east-west slopes, appearing as a center of plant speciation in Iran. Our results indicate a significant percentage (36.2%) of these endemics classified in the threatened categories. Central as well as southern Zagros, Central Alborz and with more limited range N. Zagros, Kopet Dagh as well SW Iran are classified as AZE under both Criteria of AZE.

Variation of subsidence in the Dezful Embayment, SW Iran: influence of reactivated basement structures

Subsidence analyses over the past 140 Ma in 15 wells in the Dezful Embayment, which is the main oil-producing area in the Zagros fold and thrust belt, SW Iran, have significantly improved our understanding of the post-Jurassic evolution of the basin. A number of regional-scale tectonic events since the Early Cretaceous were separated by tectonically quiescence periods, which controlled the basin architecture of Central Zagros basin evolution. Subsidence patterns in the Dezful Embayment during tectonically active periods are evidence for the influence of the Precambrian basement tectonics on sedimentation and deformation. This study shows that basement paleo-highs in the Central Zagros are regional-scale features, which may be related to regional upwarps, and are not restricted to the reactivation of deep-seated faults. However, when deep-seated faults developed over regional basement highs, their impact on sedimentation could intensify and be easily recognized in seismic by truncation/onlap features. The impacts of subsidence variations in the Dezful Embayment to the deposition and maturation of the Albian source rock, and on the development of migration path ways for hydrocarbons, are discussed.

Organic geochemical characteristics of Middle to Late Eocene Shahbazan Formation in Dezful Embayment, SW Iran: A case study from Qaleh-Nar oilfield

In a first attempt, Middle to Late Eocene Shahbazan Formation as a possible source rock in Dezful Embayment was geochemically investigated. Maturity indicators derived from Rock-Eval pyrolysis (Tmax and PI) and gas chromatography (CPI) show that the organic matter, which dominated by a mixed type II/III kerogen, is thermally mature and has already entered the oil window. A fair to good petroleum-generative potential is suggested by moderate to relatively high values of total organic carbon (TOC) and potential yield (S1+S2). Deposition of Shahbazan Formation under low-oxygen condition, which is represented by low pristane/phytane ratio (<1), conduced to preservation of organic matter. This is in accordance with considerable TOC contents, ranging from 1.01 to 1.72 wt%. The relation between pristane/nC17 and phytane/nC18 as well as terrigenous/aquatic ratio (∼1) represent the mixed marine and terrestrially sourced organic matter. Based on the results obtained from this study, Shahbazan Formation could have been acted as a prolific oil and gas source rock.

Evaluation of effect factors on water quality of Karun River in downstream and lake of the Gotvand-e-Olya Dam (SW Iran)

Outcrop of Gachsaran evaporative formation in the lake of Gotvand-e-Olya Dam in SW IRAN has posed a major challenge in terms of water quality. In the present study, multivariate statistical analysis, ionic ratios, and Piper diagram were utilized to investigate the effect of formation dissolution on water quality. Sampling was performed two times with a time interval of 6 months. The result showed that the types of downstream samples are Cl–Na and Cl–Ca, which are affected by the dissolution of Gachsaran Formation and reverse ion exchange. Due to the transmission of the saline water to the depth and layering of reservoir, the water types in the upper levels of dam’s lake are (SO4–Ca and HCO3–Ca) and (Cl–Ca and SO4–Ca) upon the first and second sampling, respectively. In both times, the clustering of the EC, TDS, Na, and Cl parameters demonstrates the effect of halite dissolution on water quality in downstream and lake of the dam. At the first sampling, the SO4 and HCO3 parameters are in one cluster that shows increasing calcareous formation dissolution. At the second time, the grouping of the Ca and SO4 parameters shows the effect of gypsum and anhydrite dissolution on water quality. In both sampling times, the stations are grouped based on distance to the dam such that upstream stations are in one cluster. Results of principal component analysis show that data of the first-time sampling are summarized into two factors that show the effect of the formation dissolution and rainfall effect on water quality, respectively. At the second-time sampling, studied parameters are summarized in one factor. Local conditions of the studied area indicate the dominant effect of formation dissolution on water quality. The results of ionic ratios confirm the effect of geological formation on water quality in the lake and downstream of the dam. Due to the water layering, in terms of salinity in the dam’s lake, the rate and discharge of water outflow of the dam also affect the water quality in downstream.

CARBONATE RESERVOIR ROCKS AT GIANT OIL AND GAS FIELDS IN SW IRAN AND THE ADJACENT OFFSHORE: A REVIEW OF STRATIGRAPHIC OCCURRENCE AND PORO‐PERM CHARACTERISTICS

SW Iran and the adjacent offshore are prolific petroleum‐producing areas with very large proven oil and gas reserves and the potential for significant new discoveries. Most of the oil and gas so far discovered is present in carbonate reservoir rocks in the Dehram, Khami and Bangestan Groups and the Asmari Formation, with smaller volumes in the Dashtak, Neyriz, Najmeh, Gurpi, Pabdeh, Jahrum, Shahbazan, Razak and Mishan (Guri Member) Formations. The Permo‐Triassic Dehram Group carbonates produce non‐associated gas and condensate in Fars Province and the nearby offshore. The Jurassic – Lower Cretaceous Khami Group carbonates are an important producing reservoir at a number of offshore fields and in the southern Dezful Embayment, and are prospective for future exploration. Much of Iran's crude oil is produced from the Oligo‐Miocene Asmari Formation and the mid‐Cretaceous Sarvak Formation of the Bangestan Group in the Dezful Embayment.This review paper is based on data from 115 reservoir units at 60 oil‐ and gasfields in SW Iran and the adjacent offshore. It demonstrates that the main carbonate reservoir units vary from one‐another significantly, depending on the particular sedimentary and diagenetic history. Ooidal‐grainstones and rudist‐ and Lithocodium‐bearing carbonate facies form the most important reservoir facies, and producing units are commonly dolomitised, karstified and fractured. In general, reservoir rocks in the study area can be classified into six major types: grainstones; reefal carbonates; karstified, dolomitised and fractured carbonates; and sandstones. The stratigraphic distribution of these reservoir rocks was principally controlled by the palaeoclimatic conditions existing at the time of deposition. A comparative reservoir analysis based on core data shows that dolomitised and/or fractured, grain‐dominated carbonates in the Dehram Group, Lower Khami Group and Asmari Formation typically have better reservoir qualities than the Cretaceous limestones in the Upper Khami and Bangestan Groups.

A new Idalina (Milioloidea, benthic Foraminifera) and some associated fauna from the upper Maastrichtian Tarbur Formation of SW Iran

ABSTRACT The upper Maastrichtian Tarbur Formation of the Zagros Zone, SW Iran, contains a rich assemblage of larger benthic Foraminifera including agglutinating, porcelaneous, and lamellar-perforate taxa. The occurrence of such a diversified foraminiferal fauna represents a prominent high diversity that took place in the Arabian platform during the latest Cretaceous. Several new species have been reported in the past decades from these carbonates, whereas many others have been described in recent times. However, the list of unknown taxa is still far from being complete. In the present contribution, we describe a new taxon referred to the miliolid genus Idalina. Idalina praecaenozoica n. sp. displays the typical basal layer that characterises the genus, whereas it differs from the other well-known Cretaceous and Paleogene allies by some morphometrical parameters. Compared to the dispersal of other Maastrichtian larger Foraminifera, the paleogeographic distribution of Cretaceous Idalina appears less extensive. Pseudonummoloculina kalantarii, another characteristic miliolid from the Tarbur Fm. is here figured with poorly known axial sections. The latest representative of the Siderolitidae, the late Maastrichtian marker taxon Canalispina iapygia is here reported as well. All such taxa give further evidence to the enhanced biogeographic radiation especially in this area before the K-Pg boundary extinction event.

Biostratigraphy and lithostratigraphy of Late Cretaceous–Early Palaeogene deposits in Tang-e-Abolhayat, Zagros basin (SW Iran)

In this study, to consider microbiostratigraphy of Cretaceous and Palaeocene planktic foraminifera, a stratigraphic section of the Gurpi and Pabdeh formations named Tang-e-Abolhayat was selected. On the basis of the identified planktic foraminiferal assemblages, seven biozones and two sub-biozones have been recorded (four biozones from the uppermost part of the Cretaceous, (Gurpi Formation), and three biozones from the lower part of the Lower Palaeocene (Pabdeh Formation). The biostratigraphic correlations based on planktic foraminiferal zonations showed a comparison between the biostratigraphic zones in this study and other equivalents of the commonly used planktic zonal scheme around the Cretaceous–Palaeogene in and outside Iran. Biostratigraphic analysis confirms that the studied section is incomplete across the Cretaceous–Tertiary boundary and a hiatus is suggested by the absence of the Plummerita hantkeninoides, Guembelitria cretacea, Parvularugoglobigerina eugubina Zones and Parasubbotina pseudobulloides Subzone (CFl to Pla zones). These absences could reflect the restriction of the above-mentioned zones to shallower settings, slow sedimentation rates or an interval of erosion or no deposition. There is no obvious evidence of erosion. On the other hand, the Pabdeh Formation overlies Gurpi Formation paraconformably.

Evidence of high-energy storm and shallow water facies in Pabdeh sedimentary phosphate deposit, Kuhe-Lar-anticline, SW Iran

Kuhe-Lar sedimentary phosphate deposit is a major phosphate prospect located in Zagros folded basin in the flank of the Kuhe-Lar-anticline in the southeast of Iran. Phosphate deposits in the Middle East extend in Neo-Tethys basin rim through several countries from North Africa, Syria, Iraq, Turkey, Iran, and Oman. The sedimentary phosphate mineralization occurred in the Pabdeh Formation, which is comprised of a marl, limestone, and shale sequence. Based on comprehensive field and laboratory studies, facies zones, standards microfacies, and standard ramp microfacies were recognized in the Pabdeh Formation. Sedimentary features such as tempestite, hummocky cross stratification, ripple marks, couplets of fine and coarse laminae, erosional surface, graded bedding, and shell lag features strongly supported a high-energy storm and shallow water depositional conditions. The main evidence of storm origin for the Pabdeh Formation is the occurrence of sandstones interbedded with bioturbated mudstones commonly as upward fine grade forms. Negative carbon isotopic composition (δ13C, − 1.85 to − 4.89) of bulk phosphatic rock shows that it may be formed within the suboxic-to-anoxic zone, almost 100-cm depths (under seafloor). These values are within the range of recent and ancient phosphorites, suggesting their good preservation.

Tectono-sedimentary controls on Lower Cretaceous carbonate platforms of the central Zagros, Iran: An example of rift-basin carbonate systems

Abstract This study focuses on the variations in facies characteristics, depositional environments, and sedimentary sequences of the Fahliyan Formation carbonate succession (Neocomian–Barremian) as one of the petroliferous units in the Middle East. Four surface and subsurface sections were selected from the Dezful Embayment and Izeh Zone of the central Zagros, SW Iran. A shoal-algal facies dominated homoclinal ramp is proposed as the general depositional setting of this formation. It shows remarkable variations in facies belts, their abundance and thickness in the study area. Four third-order depositional sequences are differentiated in studied outcrops of the Izeh Zone. Interaction of tectonics, eustasy, and sedimentation (carbonate production) rates are discussed for depositional sequences and three tectono-sedimentary models are presented for the Fahliyan Formation that correspond to depositional sequences I to III of this formation. Variations in thickness, facies, diagenesis, and allochemical grains are discussed for each tectono-sedimentary model. Comparison of the acquired results with the proposed models of active tectonic basins suggests a fault-block platform in an extensional rift basin for the Fahliyan Formation in the study area.