فصلنامه زمین شناسی ایران
پیاپی 51 (پاییز 1398)

The Qom Formation is the only hydrocarbon reservoir in the central Iran sedimentary basin. In most parts of the central Iran, the Qom Formation conformably overlies the Lower Red Formation with an erosional discontinuity and is in turn conformably overlain by the Upper Red Formation. In the Madabad celestite deposit (south of Zanjan), the Qom Formation is composed of 190 m of medium to thick-bedded and massive limestone, marly limestone and marl. Five main microfacies are identified in the limestone units of the Qom Formation in the Madabad area. These microfacies probably were deposited on a shelf carbonate platform. Petrographic studies suggest original calcite mineralogy for limestone units of the Qom Formation in the Madabad area. Geochemical studies (Ca, Mg, Na, Sr, Mn and Fe) also represent the original calcite mineralogy in a closed diagenetic system with low dissolution rates. These evidence show significant role of fractures rather than diagenetic processes such as dissolution for increasing the reservoir quality of the Qom Formation for the oil and gas fields (such as Serajeh and Alborz) of central Iran.

Ahwaz Bangestan reservoir (Ilam and Sarvak formations with approximately 1000 meter thickness) were deposited in the Upper Cretaceous period, which is deposited in different sedimentological and tectonic situations. Ahwaz Bangestan reservoir is divided into four reservoir zones (C-E-G-I) which are separated by zones of dense limestone (A-B-D-F-H). This reservoir is formed due to repeating shallowing upward of shoal-bar to lagoonal facies with distribution of rudist debris, with the exception of younger sequence which belongs to deeper environment. Back shoal facies show frequent depositional changes in vertical and horizontal directions in zones C3-E-F-G-I, the changes which can be seen in reservoir. Furthermore, the effect of basement faults, with trend nearly N-S in Ahwaz Bangestan reservoir, resulted in changes of ancient paleogeography. These changes had an important effect on diagentic changes, depositional setting, and finally productivity in conventional and carbonate fractured reservoirs. Matrix porosity is fabric selective in zones C3-E-G-and I. So, grainstone, and packstone facies have better reservoir quality than wackestone and mudstone facies. Diagensis and meteoric waters affected the top of every exposure cycles, and caused the best reservoir quality in grainstone, and packstone facies and productivity of wells. But, zone C1 of Ahwaz Bangestan reservoir (chalky limestone) experienced dominantly solution and leaching, which caused an increase in matrix porosity, but little changes in permeability. Furthermore, the open fractures developed in zones E, F and G which are located in the central and southern parts of the field. Existence of open fractures are confirmed by mud losses, analysis of sedimentary structure and dynamic reservoir data.  Increase of mud losses in some area of limestone zones (D, F, and H) could be an indication of vertical relationship of two reservoirs and absence of sedimentological barrier in these areas.

Sahib Az Zaman Mountain is located 8 km North East of the Kerman city. Study area is part of Central Iran structural zone and Orumiyeh Dokhtar volcanic arc. In this area several young intrusions with intermediate composition intruded into Cretaceous limestone. According to stratigraphic evidence, these granitoids have Cenozoic age and produced thin skarnic halo. Based on mineralogical properties and whole rock geochemical analyses, Sahib Az Zaman skarn is a calc -silicate skarn. Based on this study, migration of silicon, magnesium, iron and trace elements from igneous intrusions towards calcareous host rock and the reverse migration of calcium and magnesium occurred from carbonate host rocks. Major minerals of igneous rocks include plagioclase, alkali feldspar, clinopyroxene, and amphibole. In the boundary of igneous rock body (endoskarn), monticellite is observed. In the skarns, garnet, clinopyroxene, idocrase, epidote and calcite are major minerals. Based on geochemical studies, garnets include grossular-andradite, clinopyroxenes are diopside and wollastonite is also present. Comparison of geochemical composition between skarns and igneous rocks, illustrated that emplacement of igneous body in the beginning, metasomatism affected the host rock fluids and after replacement and crystallization of the main intrusive body had the major role in providing fluids derived from the intrusive body.

The study area is located in the NW of Tasuj city, East Azerbaijan province. The mineralization in the Tupchi area is of type copper.  The ore minerals include   pyrite, chalcocite, bornite, chalcopyrite and other minerals include malachite and azurite that occur within the Miocene sandstone sedimentary rocks. Based on the Index Combined Variety, deposition of mineralized sandstone in Tupchi area, accurred in the first sedimentary cycle. This was confirmed by the presence of unstable minerals such as lithic grains, feldspar and mafic minerals. The study of the source rock and the type of mineralization that occurred in these sandstones, indicate their formation from the erosion of mafic to intermediate igneous rocks in semi-arid climatic conditions. Two and multi variable statistical studies indicate that there is a good correlation between copper –silver and zinc, which is due to the similar behavior of these elements. The Red Bed type mineralization took place  in this area according to type of mineralized host rock, ore mineralogy, ore structure, type of alteration, reduce factor and mineralization control (plant remains), and finally, the comparison with known deposits in Iran.

The Torud-Chah Shirin volcanic-sedimentary arc, in the south of Kavir-e-Chah Jam depression (SE of Damghan), hosted many Pb, Zn, Cu, Ag and Au occurrences and deposits. Abgareh copper deposit is located in the northeastern part. Field and petrographic studies indicate that deposit area consist of andesite, basaltic andesite and basalt rocks and to a lesser extent crystal tuffs with a middle–upper Eocene age. The rocks are of high-K, calc-alkaline to shoshonitic in nature, and are formed in a magmatic arc setting in a subduction zone. According to the field observations and mineralogical studies, the mineralization in the region occurred in two stages: hypogene and supergene and weathering. Hypogen zone minerals are generally pyrite, chalcopyrite and bornite, while chalcocite, covellite, malachite and chrysocolla are considered as the main minerals in the supergene zone. Fractures resulting from faults in the rocks of the region created a favorable location for the influence of hydrothermal solution and it is considered as the main controller of mineralization. Most of the textures observed in the mineralization include vein-veinlets, open space filling, radial, replacement and disseminated forms. Geochemical studies indicate that copper has the most relative correlation with silver. Since silver has not been found as an independent crystalline phase, therefore copper was replaced by silver in chalcopyrite and chalcocite. Compared with chondrite and primitive mantle normalizing diagrams, the studied rocks show significant enrichment with respect to LREE and LILE and depletion in HREE and HFSE and negative anomalies in Ti and Nb elements. Based on the relevant diagrams, differential crystallization of mantle rocks had the essential role in the evolution of the studied rocks which were probably derived from enriched mantle. Based on petrography, structural control of mineralization, alteration type and its extention and simple mineralogy, it can be concluded that mineralization at Abgareh district has characteristics of an individual mineralization system. This system is related to evolution of hydrothermal fluid mineralization resulted in vein-type Cu mineralization.

Spid iron ore deposit is situated in the Urumieh-Dokhtar Magmatic Arc and is located 60 km West of Qom. The host rock varies in composition from diorite, quartz diorite to monzo diorite, and has been altered by the influence of hydrothermal fluids. The most important ore in the deposit is primary hematite (oligist) which is associated with pyrite, limonite, goethite, malachite and azurite. The most dominant alteration types in the Spid iron deposit are propylitic and argillic, identified by the presence of chlorite, epidote, carbonate, titanite, clay minerals and sericite. Grant Isocon method has been employed to study the behavior of elements and to calculate mass changes during hydrothermal alteration processes. The mass change calculations show that Fe2O3, MgO, K2O and LOI have been enriched, whereas P2O5, TiO2, Na2O and SiO2 have been depleted during propylitic alteration. The depletion of elements in this alteration points to the decomposition of primary minerals of the host rock, the presence CO2 rich solutions and the increase of fluid/rocks ratio during the development of the alteration system. During argillic alteration, SiO2, K2O, Na2O and P2O5 increased and TiO2, Fe2O3, MgO, CaO and LOI decreased. The increase in Na in this zone can be ascribed to the acidification of the plagioclase and its albitization, and the reduction of Fe and Mg results from the alteration of ferro-magnesin minerals such as amphibole, and the release of Fe in alterative solutions and outflow from the environment. Chemical index of alteration calculated for propylitic and argillic alteration zones are equal to 58.76 and 55.94 percent respectively, suggesting that propylitic alteration was more intense in the Spid ore deposit.

Geomechanical studies have important applications in various topics such as wellbore stability, well completion, well orientation, hydraulic fracturing plans and operations, sand production and hydrocarbon fields subsidence. It is necessary to prepare earth mechanical model of the well in the field. In addition, one of the necessary subjects to prepare mechanical earth model (MEM) is providing continuous rock mechanical parameters in the well. Rock mechanical parameters change by any variation in lithology. In this study, rock mechanical parameters are provided in continuous form, for Faraghan, Zakeen and Sarchahan formations for a field in the Persian Gulf and these parameters are clustered. Clustering resulted in recognizing six clusters with various rock mechanical characteristics. Petrographic study (i.e. determining facies, cementation and diagenesis) recognized five facies with different petrographic and cementation characteristics. These facies include quartz arenite and arkosic sandstones, shales, red mudstone and carbonates. A correlation between sedimentary and geomechanical facies was found. According to petrographic and geomechanical studies, the studied interval was divided into 7 sections. Accordingly, variation of rock mechanical parameters with regard to change in lithology was investigated. In addition, the impact of rock composition, cementation and compaction changes on rock mechanical parameters were evaluated as well.

Basiran area in the south of Birjand city is well-known for its important mineral potential and is located in the volcanic region of eastern Iran. . In 2005, the geological survey of Iran (GSI) conducted a high resolution airborne magnetic survey over the area about 850 km2 to create a suitable geospatial database for further explorations and investigations. Aeromagnetic survey is an effective tool for magnetic basement mapping and detecting concealed magnetic lineaments in a volcanic area. This work focuses on two main subjects using aeromagnetic data: (1) depth estimation of sediment sequences (i.e. depth to the magnetic basement), and (2) extracting magnetic lineaments. Necessary corrections such as diurnal, compensation, lag and IGRF applied over the raw aeromagnetic data to generate residual magnetic intensity data. Then, required filters were sequentially applied. They were reduced-to-pole (RTP), upward continuation (UP), Euler and Werner deconvolutions and Tilt derivation (TDR). To estimate the thickness of sedimentary sequences, a combination of depth estimators that were the Euler and Werner were utilized. Consequently, the outputs showed thick volcanic (magnetic basement) outcrops in some portions, and the thick sedimentary sequences that extended from a depth of a few meters to 1200 meters. Magnetic lineament in the form of magnetic fabric and fault were detected by tilt derivation method, in which positive and negative peaks in TDR map were in association with a magnetic fabric in the area. Meanwhile, magnetic fabrics showed small linear magnetic traces which were used for recognition of linear structural pattern. In addition, magnetic faults were detected through implementation of a collection of filters namely TDR of RTP_UP of 50, 200, 500 and 1000 meters. Note that, minor magnetic faults (order 1) were only detectable in the low level TDR_UP, but major ones were detectable in the high level of TDR_UP. This work emphasizes the important role of the magnetic data processing to better understand the relation between magnetic anomalies and the subsurface geology. Totally, more than 20 first and second order faults were identified and mapped.