نشریه رسوب شناسی کاربردی
پیاپی 15 (بهار و تابستان 1399)

In order to identify and understand the post-depositional history of carbonate rocks in Bagh-e Vang Formation, tow stratigraphy sections of Bagh-e Vang and Shesh Angosht  have been selected in north Tabas (East Central Iran). Bagh–e Vang Formation in the Bagh-e Vang section is 58/5 m thick, its lower boundry with the Sardar Formation is disconformity and the upper boundry with the Jamal Formation is gradual. This formation in the Shesh Angosht section is 62 m thick, it overlies on Sardar Formation with disconformity and its boundry with Jamal Formation is gradual. Diagenetic investigation leads to the identification of several diagenesis processes such as micritization, neomorphism (aggrading and degrading), cementation (syntaxial, epitaxial, drusy, poikilotopic, meniscus, pendant, equant and blocky), compaction ( mechanical and chemical), dissolution (fabric selective and non fabric selective), replacment (pyritization, silicification, and dolomitization). Based on the evidence of petrography, the paragenetic sequence of the Lower Permian deposits in these two sections are interpreted in four environments, marine, metoric, burial, and uplift, and into three stages including: early diagenetic (eogenentic) and middle diagenetic (mesogenetic) and late diagenetic (telogenetic).

The Pliocene deposits in northern Iran including Cheleken and Akchagyl stratigraphic units are considered as the most important elements of petroleum system in the Caspian Basin. The Cheleken stratigraphic unit is mainly composed of the coarse-grained red-coloured continental facies with the age of Early to Middle Pliocene, and the Akchagyl unit mainly consists of white-coloured carbonate-clastic facies with the Late Pliocene age. In the present research, three outcrops (Aghband, Somli Darreh and Yelli Badragh) in the northeastern part of the Gonbad-e Kavous area were measured as the thickest and oldest Tertiary sequences. The main purpose of the study is to determine original mineralogy and depositional conditions of the Akchagyl Formation using elemental geochemistry analysis in the measured surface sections.Variations of the major and minor elements in the Akchagyl carbonate samples indicate the aragonite and high-Mg calcite mineralogical composition. Sr/Ca versus Mn variations represents the presence of semi-closed diagenetic system during formation of the Akchagyl unit in the basin. The high Mn-values in mud-dominated samples indicates the sub-anoxic condition whereas the low values of Mn and high Sr contents in grain-dominated samples represent the effect of meteoric diagenesis (dissolution) in the depositional basin. Comparison of the major/minor elements values with the standard mean values in other sedimentary basins indicates a semi-closed basin during deposition of the Akchagyl in the Late Pliocene, and the upward salinity decreasing of the basin is due to the fresh water input and its connection to the global oceans in some time intervals of the Pliocene.

The Mishrif Formation is one of the most important oil reservoirs in the Persian Gulf Basin. In this research, the porosity and permeability data as well as the petro-physical logs are used respectively to determine the flow units and electrofacies of the Mishrif Formation in two wells of the Esfand oil field. Using petrophysical data, three electrofacies (EF) were detected by the MRGG method. The EF1 facies, which is consisted of predominantly carbonate shoal deposits, show the best reservoir characteristics, whereas the EF3 facies  with the weakest reservoir parameters, is composed of lagoonal and marine deposits. Analysis of the porosity and permeability data also lead to identification of four hydraulic units by using FZI index. The HFU1 unit with the least reservoir quality is characterized by the carbonate shoal, lagoon and tidal flat deposits but the HFU4 unit with the best reservoir quality is consisted of the lagoon, carbonate shoal and open marine deposits. In addition, the EF1 is equivalent to the HFU2 and HFU3 as the EF3 to the HFU1. The flow unit of HFU4 that is equivalent to the EF1 and EF2 electrofacies. The study of reservoir potential of the Mishrif Formation, based on RPI index, suggest its complicated reservoir quality. According to this index, the Mishrif reservoir is classified as the fair and poor classes in the studied wells. The sedimentary and diagenetic processes were associated with complicated effects on the reservoir sequence and its parameters. Thus, assessment of the electrofacies along with the flow units increases possibility of the reservoir horizons separation.

In this reaserch for the first time, the sandstones of the middle parts of the Upper Red Formation in the Hamzelou region, 70 Km NW of Zanjan, has been studied based on sedimentology and elemental geochemistary. The studied section is 945 m thick and is divided into four parts. These parts from button to top of the formation consist of  alteration of evaporite beds; green-marl with interbedded of the gypsum (300 m); alternation of red-marl with the layers of red and grey-sandstone and micro-conglomerate (with less frequency) (355 m); the alternation of red and green-marl with interbedded of the grey and red-sandstone (150 m); the alternation of green-grey marls with interbedded of the layers of gypsum (140 m). Based on petrographic studies, these sandstones manily contain different types of the rock fragmensts such as sedimentary, metamorphic and minor amount of volcanic, monocrystalline quartz with straight extinction and finally feldspar manily orthoclase types with a less frequency. Carbonate cements and matrix are also visible between the grains. These sandstones represent manily a feldspathic-litharenite and litharenite (mainly chertarenite) composition with submature to mature in terms of textural maturity. The micro-conglomerate layers are similar in composition to sandstones and classified as an extra-formational conglomerate, ortho-conglomerate (less than 15% matrix) and polymictic conglomerate. Based on the main components of the studied sandstone and results of chemical analysis of the trace elements, the tectonic setting of these sandstones is related to collision and recycled orogenic. Also, according to the obtained data in this research, the source rocks of these sandstones are intermediate to felsic igneous rocks, which were influenced by semi-arid climates in the source area.

The study of facies stacking and depositional environment of the Jurassic successions in the Fars region indicates important changes in the evolution history of the Zagros Basin, including subaerial exposure and drowning of the platform during that period. To investigate the different aspects of these geological events, two sections of the Jurassic sedimentary rocks in Gadvan anticline and Kuh-e Siah oilfield have been studied by sedimentology, stratigraphy and well logging. The results indicate that the Lower Jurassic sedimentary sequence is composed mainly of carbonate rocks and shales deposited in the inner part of a homoclinal ramp. This sequence terminated by a calcareous soil horizon (calcrete) which formed simultaneously with the Aalenian unconformity and related platform emergence. Subsequent marine transgression and re-establishment of the platform during early Middle Jurassic resulted in the discontinuity surface covered by peritidal carbonates, deep-marine shales and pelagic limestones. Immediate overlap of the platform deposits with the Bajocian deep-marine facies indicates a sudden increase in accommodation space and drowning of the Fars carbonate platform. The drowning was the result of rapid deepening of the basin caused by tectonic subsidence and a coeval global sea-level rise. This time slice represents a significant environmental change across the Zagros Basin during which the Early Jurassic undifferentiated platform (epeiric shelf) evolved into a new sedimentary system. This system containing deep-marine basins separated by platform areas reflects a change to more open marine conditions. Similar sedimentary environment of the Bajocian deposits in Fars and Gotnia Basin (Lurestan and northern Iraq) indicates the extension of deep-marine environment in most areas of the Zagros. Therefore, drowning of the Jurassic carbonate platform is a pervasive event and turning point in sedimentation history and structural evolution of the Zagros Basin that the sedimentary regime has since been quite different.

Biab travertine and the Pole-Arvan travertine deposits are located in anticline Aran, north of Avaj city and in the south of Qazvin Province. One section of the Pole-Arvan travertine the forming quarry body and two sections of the Biab travertine were examined. The deposits morphologically are cascading in both regions, which is affected by faults in the region.  According to the physical characteristics, four lithotypes have been identified, including laminar, coated bubbles, vegetative and dendritic. Micritic fabric with Peloidal allochems has been the most important fabric, and also dissolution and cementation are among the most important diagenetic processes in the region's travertine. Based on the results of XRD analysis, calcite in Biab region, and calcite and aragonite in the Pole-Arvan are the main minerals. It also contains small amounts of illite, chlorite, and hematite minerals, which are originated of layers of upper Miocene and Pliocene under travertine. According to the results of ICP-OES analysis, there is the highest concentration of calcium and lower amounts of magnesium, sodium, iron and strontium in travertine samples. The amount of these elements in the Pole-Arvan travertine is higher than in the Biab region and is related to the presence of cyanobacteria, the activation of the travertine springs and the deposition of aragonite. The hydrogeochemical results of this water indicate the passage of spring water through the salt layers of the region and the deposition of travertine from saline water. The water type is both Chlorocalcite, which is the result of a mixture of bicarbonate and salt water, reverse cation exchange between saline water and carbonate surrounding rocks.

The Dezful Embayment is a depressed area within the Zagros Folded Belt. This embayment represents a foreland basin where subsidence at the foot of the uplifting mountain front fault has resulted in the deposition of thick post-Oligocene sediments. It has been one of the most productive areas where several potential source rock units with different geological ages were deposited in this tectonically developed depression making this area as the most prolific region in the Middle East. It is divided into northern and southern parts. The South Dezful Embayment at the foreland part of the Zagros Fold-Thrust Belt is a post-Oligocene subsiding basin. To the southeast, the South Dezful Embayment is bounded to the Kazerun Fault Zone and to the west, it bounded by the hendijan Fault zone, which is an N-S trending paleo-structure. Due to its vast oil resources, it has attracted the attention of researchers during time. The Dariyan Formation-Aptian age- is known as a carbonate reservoir in the Zagros Basin. It overlies the Gadvan Formation conformably and is overlain unconformably by Kazhdumi Formation. In the studied area, the Dariyan Formation has a thickness ranging from 195 to 45 m and can be divided into three members that are as follows: Lower Dariyan, Middle Dariyan, and Upper Dariyan. The Lower Dariyan consists of thick to medium-bedded limestone. The middle part of the Dariyan Formation is the so-called Kazhdumi Tongue. This part consists of thick black to greenish-grey shale and marl layers associated with a high abundance of planktonic foraminifers. In the East-West trend thickness of this part changes from 100 To 0 m where middle Dariyan and also Upper Dariyan change to Kazhdumi facies toward Kazhdumi intra-shelf basin in central part of Dezful embayment. The upper Dariyan Formation is composed of buff to grey medium, thick-bedded limestone. Many studies have been focused on Reservoir properties of Dariyan Carbonates in Zagros basin and Arabian plate but this is the first time that hydrocarbon source rock of the Middle part of Dariyan potential is going to be evaluated. The result may help to define new Petroleum systems that middle Dariyan will important role as a source rock for Upper Dariyan and above reservoirs.

In this research based on the petrographic analysis 12 microfacies belonging to outer ramp, middle ramp, shoal and inner ramp were introduced, which led to recognition of some paleoeological parameters. In the studied well, the Asmari Formation sediments have been deposited from normal to hypersaline, oligophotic to euphotic and oligotrophic to eutrophic conditions. Deep parts of the basin with low hydrodynamic energy, soft and stable substrate contain foraminifera with thin and elongate tests; the shallow parts of the middle ramp (proximal) and shoal with high hydrodynamic energy, hard and unstable substrate including thick-wall,  lenticular hyaline benthic foraminifera and porcelaneous forams with thick walls are distributed in the lagoon facies belt. The high abundance of the large benthic foraminifera, coralinacea red algae and coral fragments indicate warm and tropical to subtropical environments during the deposition of the Asmari Formation. High proportion of the epifaunal to infaunal benthic foraminifera in the studied well suggest an oxygenated marine environment. In adition in this research 4 carbonate associations including Nanofer, Rhodalgal, Foralgal and Foramo were recognized. Finally, 4 positive- and 5 negative- boundary surfaces (PB and NB, respectively) were introduced; some of which are well correlated with stage boundaries.

The Marun oilfield is located in the middle part of the folded Zagros structural zone, in the Dezful embayment. In this study, using subsurface data and Geolag software, the separation zones of the reservoir anticline were determined lithologically. Factors such as porosity and shale volume were considered in determining the reservoir zones. Petrophysical studies using neutron-density, neutron-sonic, density- sonic and M-N cross plots show that the predominant lithology in this reservoir is carbonate. Based on the analysis of well log data and using neutron-density cross-plot, the Asmari Formation has a good porosity especially in the upper parts of the formation. For this reason, these parts have better reservoir quality than the lower parts. Also, the accuracy of the evaluation performed in the studied borehole had an acceptable correlation with the calculated porosity of the core. In the present study, the average shale volume based on the CGR log is 25.33%, of which the highest shale amount is concentrated in the lower part of the formation. Since each sedimentary sequence can be composed of one or more reservoir zones with their own lithological and petrophysical characteristics, so 5 reservoir zones (A, B, C, D, E) were identified in the studied borehole. In this study, zone A belongs to the upper part and zone E belongs to the lower part of the Asmari Formation sequence. Analysis of available data, appropriate porosity in the whole formation and low shale volume, especially in zones A, B, C and D indicate that almost the entire formation in the Marun oilfield has a suitable reservoir potential.

The Dariyan formation (Shuaiba equivalent) with aptian-albian in age is one of significant reservoirs in the Zagros and the Persian Gulf. This formation in the study field (western Persian Gulf) include sequence of 185 orbitolinaid shallow and deep carbonate facies. The Dariyan formation in this area is divisible into four zones: Lower Dariyan with gray Limestone and dissolution cavities, Kazhdumi Toungue with dark gray Marl and without structure, Upper Dariyan dark to light clean Limestone and Upper clastics with Sandy Siltstone and Shale. Microfacies study of the Dariyan led to recognition of 8 microfacies and 3 petrofacies in 5 facies belt inclusive Delta, Inner ramp, Mid-ramp, Outer ramp and Basin. Microfacies interpretation which is done based on the fauna mainly indicates shallow marine environment. Also uniform variation of microfacies and absence of rim and buildup faunas, and replacement of corals and rudists by the Lithocodium-Bacinella is coeval with the Oceanic Anoxic Event 1a, that Lithocodium-Bacinella had fewer abilities to build huge reefs so this depositional environment predicted as carbonate ramp with patch reefs and mounds. The three sequences were recognized in the study field. These sequences belong to Early, Middle and Late Aptian, respectively. In the first and third sequences, the sequence boundaries were identified by the unconformities and their related features, while in the second sequence, the retrogradation and alteration of the intrashelf basin facies (Kazhdumi) to shallower fscies were recognized. The maximum flooding surface was also identified by the maximum transgrassion and deep outer ramp and basin facies in addition to rather increase in oxygen and carbon isotope values. Finally, a siliciclastic sequence, due to the entrance of siliciclastic sediments to the basin, was recognized.

Shurab travertine spring is located 140 km the east of Tehran and at the beginning of the Gadook Gorge on the border of Tehran and Mazandaran provinces. This spring originates from the fractures and drainage of the Barut Formation in the train tunnel. The carbonate deposits are divided into two main categories: the first group of deposits that are forming at present time (active): and mostly of different colors such as red, yellow and orange, the second group of old (fossilized) deposits mainly adjacent to the railway line and are mostly cream-colored. The age of these deposits is less than 100 years old. The morphology of the carbonate deposits are layeres and Cascade form. Types of travertines and tufas based on morphology and environment in the stack include 1- waterfall travertine 2- travertine with layered morphology and 3- calcareous tufa or framestone phytoherm. Also, according to the petrographic results of four types of lithofacies, according to the characteristic morphologies on the ground scale and hand specimens in the travertine deposits of the saltwater travertine were identified which include: 1- lithofacies raft boundstone; 2- lithofacies algae; 3- lithofacies crystalline crusts and 4- lithofacies Bandstone shrubs. Dissolution and cementation are the most important diagenetic processes of tufa and travertine in the carbonate fascia of the region.  Hydrochemical data the water in Shurab spring is near neutral to slightly alkaline and of sodium chloride type.

The purpose of this study is to investigate the diagenetic processes and their influenceimpact on the reservoir quality of Early Cretaceous Fahliyan Formation in subsurface section of wells 1 and 2 of Arvand oil field located in Abadan plain. This study based on petrographic and petrophysical studies including 229 thin sections and well logging. Diagenesis events showed that dissolution and cementation, dolomitization, and micritization, bioaccumulation, compressive pressure dissolution and compaction are the most common diagenetic processes affecting the studied formation. Among these processes, cementation and physical compaction cause porosity to dissolve, and desroy but dissolution and dolomitization have increased porosity. Styloliticity stylolitization in some cases due to dolomitization and dissolution along it increases Reservoir quality and in some cases due to the concentration of nondisolved residues, permeability barriers are created Thin sections studied show that the porosity are intergranular, voggy and fractures types. Petrophysical studies of the studied reservoir indicate that calcareous facies of the Fahliyan Formation, is associated with inter layers of shale. Rock porosity is between 3 to 8 percent and shale volume is less than 5percent, water saturation is between 10 to 40 percent, and oil and water content is high with high water saturation. According to the microfacies study, the proposed sedimentary model is carbonate ramp and due to the limited expansion of effective diagenetic processes in reservoir quality and value of lime mud, the reservoir quality of Fahliyan Formation in Arvand field is moderate.