نشریه رسوب شناسی کاربردی
پیاپی 22 (پاییز و زمستان 1402)

The Sarvak formation (Cenomanian-Turonian) has one of the main stratigraphical units in SW of Iran oil fields and Abadan plain due to huge amounts of trapped oil. Investigation of Sarvak Formation based on 3 wells data including core, petrographical analysis, petrophysical logs, and POROPERM data which was received from the core laboratory, led to recognized seven microfacies in this succession. The identified facies were deposited in five sub-environments; restricted lagoon, shoal rudist biostromes, and open marine. The situation of each facies and their depositional setting proposed the homoclinal carbonate ramp platform.Petrographical investigation shows the several diagenetic realms that affected this formation. the main diagenetic process identified in this research are, micritization, mechanical and chemical compaction, dissolution, cementation, stylolitization, dolomitization, silicification, and fracture. The depositional environment of Sarvak formation affected by a sea-level change in Cenomanian-Turonian time which is affected the reservoir section by several diagenetic phenomena. To investigate the reservoir rock typing two methods were analyzed. One of them is based on microfacies analysis and diagenetic process investigation and the other is based on RQI/FZI index. integration data and analysis show that dissolution has a positive effect, especially in grain-supported facies and cementation has a negative impact on reservoir quality index. the result of this study could be implemented in the planning of hydrocarbon exploration and development stage.

The studied area includes the Neogene alluvial deposits of Birjand city that is geologically considered as a part of the structural zone of eastern Iran. In petrographic studies, 200 gravel clasts and 10 sandstone samples were used, while for sediment chemistry analysis, 13 mudstone samples and 2 sandstone samples were used. The petrographic examination of the above samples shows the presence of particles with different mineralogy in the studied alluvial sediments, in which, in addition to quartz, the abundance of igneous and sedimentary rocks is significant. Drawing the results of the major oxide and minor elements analysis of the above samples in the presented graphs in this context, reflects the presence of mafic to intermediate igneous rocks and sedimentary rocks in the source area. The examination of the rock units around the studied area shows that the origin of the gravel fragments and the sand particles is probably from the Cretaceous ophiolitic units, Late Cretaceous to Eocene flysch units, Paleogene clastic-carbonate sedimentary rocks and younger volcanic and sedimentary rocks which are exposed around the target area. Study of mineralogy, textural and compositional maturity of sediments along with the evaluation of chemical weathering indicators show that the mentioned alluvial sediments were deposited near the source and suffered moderate to weak weathering.

The Upper Cretaceous (Upper Albian-Lower Torunian) Sarvak Formation (and its equivalents) is the second most important carbonate reservoir in Iran that was developed under the warm tropical climate. The Sarvak Formation with the predominant limestone lithology in the Omid, Mansuri, and Ahvaz Oil Fields was conformably overlies the shaly Kazhdumi Formation and overlain by the Ilam Formation. Detailed petrographical studies led to determine twelve carbonate microfacies that indicate deposition on a homoclinal ramp-type carbonate setting. The identified ramp-type setting was composed of four main facies belts including inner, middle and outer ramp associated with an intrashelf basin. Biostratigraphic assemblage zones, facies analysis, depositional sub-environments, and wire-line logs assisted in mapping the paleogeography of the study area into four third-order stratigraphic sequences. The upper boundary of third and forth depositional sequences (DS3-4) in all studied oil fields defined as a disconformity confirmed by the evidence of extensive dissolution and other meteoric diagentic features that reveal a platform exposure. The other sequence boundaries defined as a correlative unconformity. The modified sequences and sequence boundaries of the Sarvak succession are clearly correlatable with sequence boundaries and surfaces derived from the Arabian Plate sequenc stratigraphy in Iran and adjacent Arabian countries.

The lack of oxygen is considered as one of the main causes of mass extinction before Permian-Triassic boundary (ca. 251.9 million years ago). In this study redox conditions of Upper Permian shales of the Ali Bashi and Zal sections in SW Iran have been evaluated using redox-sensitive trace elements in order to examine the geochemical status of the seawater during Late Permian as well as before and after extinction horizon. According to V/Cr and V/(V +Ni) ratios the redox conditions throughout Upper Permian successions in both sections change from oxic/dysoxic to suboxic and in uppermost Changhsingian Aras Member which is located immediately after extinction horizon the increase of these for-mentioned rations are evident. The Mo/U authigenic ratios in both sections demonstrate their lower values than Mo/U molar ratio of seawater ( ~7.5-7.9) and MoEF and UEF covariate plots show the position of samples of two study sections in oxic/dysoxic to suboxic zones. Considering the obtained results, although there is evidence of low oxygen conditions in some parts of the Upper Permian deposits, the lack of oxygen and anoxic condition did not happen in extinction horizon and even in the Aras Member there was a suboxic status. Therefore, other mechanisms should be investigated for the low faunal diversity of the Upper portion of the Upper Permian deposits of the Ali Bashi and Zal sections.

The aim of this research is to investigate the primary mineralogy and the type of diagenesis systems in the south-west of Lorestan based on elemental geochemistry. The Talezang F ormation is formed in this section with a thickness of 84.5 meters and the predominant lithology is limestone and sandy limestone. From this section , 20 samples were selected for EDS and EPMA analysis. The change trend of Sr element to Na compared to Mn showed that the studied samples are in the range of Gordon aragonite limestones of Tasmania . Also, the trend of changes of Mn element against Sr in the studied samples showed that are in the same range as the primary mineralogy of the aragonite of the Mo zdooran Formation, which is a proof of the aragonite mineralogy of the primary limestones of the formation. The high ratio of strontium to manganese (average 63.70) strontium to calcium (average 21.01), low amounts of iron elements (average 1.75 ppm) manganese (average 24.70 ppm), the high concentration of Sr (average of 750.35 ppm) in the studied samples and the plotting of Sr / Ca values ​​against Mn , Mg, Fe indicate a closed to slightly open diagenesis system with exchange of water to rock bottom. (W/R) is for carbonates of Talezang Formation. The relatively open diagenesis system in this formation can be due to the influence of atmospheric fluids due to the presence of an erosional discontinuity that is at the top of the Talez ang Formation.

In this study, two stratigraphic sections of the Tarbur Formation have been investigated in the Structural Zagros Fold-Thrust Belt. The Tarbur Formation in the studied area conformably underlaid the Gurpi Formation but its top boundary overlaid by Bakhtiyari and Sachun formations as a disconformity. Due to the abundance of fossil content in these sediments, most of the studies had been focused on paleontology aspects. Lithological and facies studies on the sediments of this formation in the Interior Fars Province show the existence of 12 carbonate sedimentary facies in these sediments, which are scattered in four sub-environments including open marine, bioclastic shoal, lagoon and tidal flat. The abundance of benthic foraminifera in these sediments shows the change of conditions from deep to shallow environments. The results of field investigations and the trend of lateral and vertical changes of facies and sub-environments reflect the presence of a homoclinal carbonate ramp during the deposition of these sediments. The trend of changes in the depth of the facies, their distribution pattern and the characteristics of the upper boundary of the formation in the studied sections show that the Mandegan section was located in the shallow part of this ramp and there was a deepening trend in the environment towards the south-east towards the Khanehnahr section. In addition, the replacement of large benthic foraminifers by bivalves such as rudist is another confirmation of the decreasing depth trend over time. These results can be used in reconstructing the evolution of the sedimentary basin in this region.

The Fars region in Iran is one of the richest places in the world in terms of gas reserves. The area investigated in this study is located on the coastal Fars subzone. In this research, probable source rocks were evaluated by organic geochemical methods and then 1D petroleum system modeling in two wells from the gas fields of southern Iran was contracted and studied. Based on the vitrinite reflectance data and the current temperatures of the formations in the existing wells, the calibrated model and burial history diagram, kerogen to hydrocarbon transformation ratio (TR), the time and amount of light hydrocarbons (c1-c5) and Liquid hydrocarbons (C6+) expulsed from source rock were determined. The geochemical studies in the area reveals that, the Sarchahan Formation is the main candidate source rock. The results obtained from the modeling show that the Sarchahan Formation in the well of the first and second field generated hydrocarbons from 130 and 140 million years ago respectively. Currently, the thermal maturity of the Sarchahan Formation in the both wells is in the gas window. Up to now, the amount of liquid (C6+) hydrocarbons expulsed from the Sarchahan Formation in the first and the second field is about 23500 kg/m2 and 21300 kg/m2 and for gaseous (C1-C5) hydrocarbons is 23000 kg/m2 and 21200 kg/m2, respectively.

This study integrates the results of petrographic studies of core samples and thin sections with petrophysical data for the evaluation of heterogeneities in the distribution of reservoir properties in the Upper Dalan and Kangan formations. Reservoir rock types, hydraulic flow units, and reservoir zonation of these formations are determined in a sequence stratigraphic framework. Accordingly, 7 rock types and 8 hydraulic flow units are defined using the petrographic evidence and flow zone indicator values, respectively. Based on the Lorenz approach, 15 reservoir, barrier, and baffle zones have been differentiated. Among the defined rock types, PRT1 (compacted anhydrite) has lowest reservoir quality, and PRT7 (dolomitized packstone and grainstone) has highest reservoir quality. A combination of depositional facies characteristics and diagenetic alterations controlled the reservoir properties in these formations. There are one reservoir and one baffle zone in K1 unit, two reservoir zones and one speed zone in K2, one reservoir, baffle, barrier, and speed zone in K3, and three reservoirs, two baffle, and one barrier zone in K4 unit. All macroscopic and microscopic heterogeneities are determined in sequence stratigraphic framework. Results showed that non-reservoir units of Permian–Triassic formations are concentrated within the RSTs (regressive systems tracts) of UDS3 and KS1 sequences. On the other hand, reservoir zones correspond to KS2 and UDS4 sequences.

In this study, the microfacies and sedimentary environment of the Kazhdumi Formation in the Marin section (southern flank of the Khami Anticline) in the east of Izeh Zone have been investigated. Sedimentological studies turned into the identification of 6 sedimentary microfacies in this formation. These microfacies are formed in a deep environment due to the presence of skeletal grains such as radiolarians, plankton foraminifers, oligosteginids, sponge spicules, ammonite and echinoderm spines. The abundance of phosphate and the presence of glauconite are other characteristics of these microfacies. The presence of shale layers with ammonite and rich in organic matter alternating with the limestone of the Kazhdumi Formation in the Marin section confirms the depth of the sedimentary basin of the Kazhdumi Formation. The above-mentioned evidence shows the deposition of the microfacies of the Kazhdumi Formation in an intrashelf basin in this section. The microfacies identified in the Marin section are formed in the sub-environments of the basin and the outer platform, and no trace of shallower sub-environments can be seen in them.

In this study, formation of pyrite framboids was investigated in two Upper Devonian marine black shale intervals in south of the Tabas Block in southeast of central Iran. In order to identify and study the morphology of the pyrite framboids, polished section slides were prepared from pyrite-bearing black shale samples. The section slides from the shale samples were examined and photographed with a scanning electron microscope (SEM). The identified pyrite framboids are spheroid to sub-spheroid in shape and consist of several euhedral and octahedral-euhedral compact microcrystals. Generally, the diameter of the pyrite framboids range from 5 μm to 18 μm. The relative abundance and size distribution of the pyrite framboids in the studied shale intervals show that the identified pyrite framboids were mainly formed during the early diagenesis below the redox boundary, within the sediments of a nearshore marine substrate, under a slightly dysoxic–oxic water column. The absence of pyrite framboids in the other shale intervals in the studied Devonian succession may indicate fully oxic conditions in the water column.

In this study, a detailed investigation the trace fossils of Mishan Formation in the Omidieh section has been carried out. The Miocene Mishan Formation has a diverse collection of trace fossils, which includes 4 ichnogenera and 5 ichnospecies. These trace fossils show locomotion, feeding, sheltering and grazing behaviors and include Protovirgularia cf. pennatus, Planolites isp., Helminthopsis isp., Thalassinoides isp. and Thalassinoides suevicus. The trace fossils of Mishan Formation are in the alternation of massive, medium to thick beded limestone, medium to thick beded sandstones, and marl. In this study, 5 bioturbation indexes were identified according to the standard bioturbation indicators, as well as 5 ichnofabrics related to delta, tidal flat and lagoon environments. The great variety of ethological classification including locomotion, feeding, sheltering and grazing, along with the trace fossil record of sediment feeding organisms and, to a lesser extent, suspension feeding animals, confirm very shallow sedimentary environment with a high oxidation content.

The Campanian-Maastrichtian sequence in the Shiraz region of Inner Fars includes Gurpi and Tarbur formations. In order to study petrography and evaluate the evolution of the sedimentary environment and sequence stratigraphy of these sediments, two stratigraphic sections of Koh-e Qhale and Koh-e Shahrek Abarj in Marvdasht city of Fars province were selected and analyzed. Based on field studies, manual samples, and a detailed microscopic study of 250 thin sections prepared from the studied formations, it was identified that 14 facies (11 carbonate microfacies and three lithofacies) were deposited in a shelf carbonate platform. By studying about 1400 meters of Maastrichtian sediments, depending on the location of the sections under study, three third depositional sequences were identified and separated in terms of sequence stratigraphy. During the Campanian period and the formation of the LST systems tract towards the northeast of the studied area (Koh-e Qhale section), the clastic sediments of the lower part of the Tarbur formation and in the deeper parts of the sedimentary basin of the Gurpi formation were deposited equivalently. At the time of the highstand systems tract (HST facies) and high carbonate production rate in the steep part of the sedimentary environment, interlayered reef deposits (rudist packstone and rudist grainstone) were deposited between the limestones of the Tarbur Formation.

A large part of the hydrocarbons of the Abadan plain region is located in the Bangestan Group, including the Ilam Formation. Ilam Formation of Santonian age is considered as one of the important oil reservoirs of Abadan plain, whose upper boundary is conformable with Gurpi Formation and its lower boundary with Laffan Formation, and it is mainly composed of limestone with regular layering and between thin layers of shale. The purpose of this study is to determine the role of microfacies and influential diagenetic processes in the reservoir part of Ilam Formation. Petrographic studies on the number of 142 thin section samples related to the drilling debris led to the identification of 7 microfacies related to the three belts of lagoon, Shoal and open marine facies. Among the most important diagenetic processes of Ilam Formation, we can mention cementation, micriteization, compaction, dolomitization, Hematization, glauconitization, pyritization, dissolution, and various types of porosity. In general, the results relating the distribution of reservoir characteristics of Ilam Formation with the study of sedimentary microfacies and influential diagenetic processes show that the best reservoir prone microfacies are wackestone to bioclast packstone and wackestone containing planktonic foraminifera. Also, among the effective diagenetic processes in the reservoir section, we can mention the dissolution and resulting porosity (vuggy porosity), porosity resulting from fracture, recrystallization (caused by microscopic porosity) and dolomitization.