JOURNAL OF SCIENCE (UNIVERSITY OF TEHRAN) (JSUT)
Year:2009 | Volume:35 | Issue:3 (SECTION: GEOLOGY)|Papers Count:12

The studied intrusion is located in the Central Alborz tectonic zone of Iran, in the south of Taleghan valley near the Nesa-e-pain village. The Nesa-e-pain post Eocene intrusion is composed mainly of monzonite along with some gabbro and syenitic dykes. This intrusion is emplaced in the acidic tuff of Karaj Formation with a chilled margin in the contact. The intrusive rocks with a granular texture are mainly composed of plagioclase, alkali feldspar and clinopyroxene as rock forming minerals. These rocks are metaluminous and belong to SH (shoshonitic) type granites. High Zr/Nb, Nb/Ta and Rb/Zr ratios with accompanying the sample position on Srn/Yb vs Sm and La/Sm diagrams display that the intr .isive rocks have probably originated from an enriched mantle (Sp- gran lherzolite). According to the discrimination and spider diagrams, it seems that the magma of these rocks was generated in a subduction zone because of their enrichment in LILE and cepletion in HFSE along with low Ti02 oxide and high Ba/Ta and BalNb ratios.

Having knowledge about elastic prosperities of different layers of Earth has many useful applications in geophysical and geotechnical engineering projects. One of the important parameters in determination of static constants and the key parameter controlling the amplification of seismic motion during earthquake is shear-wave velocity. Because of the dispersive characteristic of Rayleigh waves when traveling through a layered medium and the important role of Swave velocity in determination of Rayleigh wave velocity, dispersion of phase velocity of surface waves can be used for near-surface S-wave delineation. In this study Common Mid Point Cross-Correlation analysis of multi-channel surface-wave data (CMPCC MASW method) is used for dispersion curve determination of 2-D seismic reflection survey data. CMPCC MASW analysis is a further extension of Multi-channel Analysis of Surface Waves (MASW) that improves the accuracy and resolution of MASW by overcoming the problem of improved lateral resolution trading off against accuracy of phase velocity at low frequencies. It also enables Spectral Analysis of Surface Waves (SASW) method to perform a pseudo multi-channel analysis in order to distinguish fundamental mode from higher modes visually and therefore solving the spatial aliasing problem of SASW method.

Several studies have been carried out on engineering geology and geotechnical properties of Tehran alluvium with emphasis on cementation, cohesion, strength, etc. However, there are few studies with focus on excavation stability and pattern. This paper focuses on excavation pattern of fine grained alluvium in southern Tehran. Thus, physical and mechanical properties of soil were determined from field investigation and laboratory tests. Afterward, stability analyses were performed using GeoStudio software with Morgenstern-price method based on limit equilibrium. According to the results, the studied region was divided into three zones: D 1, D2 and D3 based on excavation stability pattern. Because of low presence of sand in D 1 zone, safe excavation depth is more than the others. The excavation depth of vertical wall in D2 and D3 zones is the same, but in deeper excavations in both zones, D3 zone showed more instability.

Distributions of larger benthic foraminifera in east Dow-Gonbadan section are used to determine the age of the Asmari Formation. Two assemblage zones have been recognized by distribution of the larger benthic foraminifera in the study area. The distribution of the Oligocene larger benthic foraminifera indicate that shallow marine carbonate sediments of the Asmari Formation at the study areas have been deposited in the photic zone of tropical to subtropical oceans. Based on analysis of larger benthic foraminiferal assemblages and microfacies features three major depositional environments are identified. These include inner shelf, middle shelf and outer shelf. The inner shelf facies characterized by wackestone-packstone, dominated by various taxa of imperforate foraminifera. The middle shelf is represented by packstone with a diverse assemblage of larger foraminifera with perforate wall. Basinwards is dominated by argillaceous wackestone characterized by planktonic foraminifera and large and flat nummulitidae and lepidocyclinidae. Planktonic foraminifera wackestone is the dominant facies in the outer shelf.

C-axis quartz measurement and petrography studies of Lakhshak granodiorite mylonite showed that most of the quartz grainshad recrystallized with grain boundary migration mechanism. This recrystall-ization tookk place over 630 ± 30°C temperatures. Most of the quartz crystals were deformed by prism [C] slip and maximum C-axis of the grains is closed X-axis on the stereogram. The prism [C] slip occurs over 550-600°C and presences water. Quartz was deformed during flatting ellipsoid stress and this situation caused that C-axes occur with girdles shape around stereogram Z axis. Opening angle of the C-axes girdles range from 79- 114 that measured in The XZ plane through Z and using of Opening angle thermometer show that Lakhshak granodiorite mylonites were deformed at 585-730±50oC. Based on the following evidences these rocks were deformed in presence of a little fluid: (1) replacement of plagioclase by intergrowth quartz and feldspar with granophyric texture, (2) the filled cracks in the primary feldspar by fine grains of biotite, K-feldspar and quartz aggregates (3) the presence of the undeformed granophiric texture in the margin of deformed orthoclase crystals, (4) recrystallization of biotite laths and myrmekite formation around K-feldspar porphyroclass core. The above evidences illustrate that Lakhshak granodiorite mylonites were deformed at 585-730oC at present of some fluid.

Cementation factor is the most important parameter in the fluids saturation models and small error in the evaluation of it has large effect on estimation of water saturation and therefore in-situ hydrocarbon reserve. Based on type of heterogeneity in heterogeneous carbonates the m value could be differed from 1 in fractured carbonates to 5 in separated vugs and molds porosity carbonate rocks. The increases of separated porosity in the rock will cause increase of m. Most of the time an experimental model for determination of m can be presented by accurate measurement of porosity and electrical resistivity of cores from drilling wells of a hydrocarbon field. Consider to different secondary digenesis conditions in hydrocarbon fields, the use of an experimental model from a field for another field will increase the risk of in-situ hydrocarbon reserve determination. In the present study to evaluate m for one of heterogeneous carbonate gas reservoir from south of 'Iran first porosity, permeability and pore size distribution of core samples from 3 drilled wells of this field were determined and by using this information and geological study of thin sections the reservoir rocks were grouped. Then to perform electrical resistivity measurements, the core plug samples were selected to cover the range of porosity in each rock type. Due to compressibility of reservoir rocks, which is occurred from increase in net overburden pressure after production and to investigate the effect of compressibility on cementation factor, the electrical resistivity tests were performed in 4 different net confining pressure additions to ambient condition. The results showed that the m value is equal to 2.086 in ambient condition for dolostone with intercrystalline and interparticle porosity and will be increased to 2.148 in 4000 psi hydrostatic net confining stress. For other rock types, which were limestones with separated vug and mold porosity, an experimental model were presented for m that shows increase in m value with increase in porosity. In this type of limestones the effect of net confining stress on cementation factor is more dominant than dolostones.

To understand interrelationship between geological and petrophysical parameters, the petrophysical and petrographical data from Ilam reservoir in one of the oil fields in North Dezful (SW of Iran) were integrated. 10 different microfacies were recognized via detailed petrographical studies. In upper Ilam the Ooidal-peloidal grainy fabrics are abundant indicating a high energy shoal environment whereas, in middle and lower parts the muddy textures are common, wackestone, rarely packstone in middle part and lime mudstone to wackestone in lower part, suggest low energy and fairly deep depositional environment. The most important diagenetic processes are early stage physical compaction and calcite cementation in upper part and dolomitization and pyritization in middle/lower parts. The grain- supported fabrics in upper part are intensely cemented, occluding the intergranular pore spaces in most cases. Therefore, the diagenesis has a serious destructive effect on the reservoir quality comparing to the depositional environment. The most common visible porosity types are mold ie, vuggy and intergranular in upper part, moldic, enlarged moldic, intrafossil and occasional fractures in middle part, and moldic and vuggy in lower part. The most suitable zone for hydrocarbon production is located in upper part coinciding with grain supported textures having intergranular porosity. Despite of the high total porosity occurring in middle part, the production potential is very low due to a pore network with narrow pore throats and microporosity which greatly reduces the permeability. Water saturation abruptly increases in the boundary of middle -lower parts probably due to variation in microfacies, higher clay conent and low porosity which altogether form a porosity system with tight pore throats and conduits, weakening the oil accumulation assumption in the ilam reservoir through vertical migration.

Source rock is one of main elements in petroelum system. In order to investigate a petroleum system in an oil region, it's necessary to recognize the characteristics of source rocks. In this study the source rocks of the Darkhovain oil field in Abadan plan are investigated. In this way, gamma rays and sonic log are used and source rock zones separated in Pabdeh, Garu, Gotnia, Najmeh, Sargelu and Nyriz formations. These zones sampled and total organic carbon of samples determined with Rock Eval analysis. Acording to Rock Eval results, Sargelu and Garu zones have good to excellent total organic carbon and good to moderate genetic potential. These zones have more thickness then the other zones. Kazdumi source rock samples show range of good to very good in total organic carbon and a range of moderate to good in genetic potential. Pabdeh, Sarvak and Surgah samples are poor in organic matter and have low genetic potential. Acording to this study, Sargrlu and Garu zones are main source rocks and Kazdumi and Nyriz zones are minor source rocks in Darkhovain field.

The middle Jurassic Shir-Kuh granitic batholith in Central Iran intruded to the sandstones and shales of Nayband-Shemshak Formation. The batholith consists of three main granodioritic, monzogranitic and leucogranitic units. The granodiorites composed of plagioclase, quartz and biotite and is limited to the northern margin of the batholith. The monzogranites which made the dominant unit range from relatively mafic (cordierite-bearing) to felsic (muscovite-rich) rocks. The leucogranites character- ized by white color and absence of opaque minerals consist mainly of quartz, K-feldspar, and sodic plagioclase and located along the northwestern margin. Separation of restite from initial melt composition appears to have been an effective differentiation process in the formation of more mafic rocks of the batholith. The relatively felsic was formed due to fractional crystallization. The boundary between the two types is at about 70% Sio2. Relatively mafic granites are rich in normative corundum and the more felsic one is close to the minimum melt. The Shir-Kuh granitic rocks are enriched in large ion lithophile elements LILEs (K, Rb, Th and U) and depletion in high field strength elements HFSEs (Nb and Ti) which are indicative of subduction-related magma. The batholith is peraluminous, calc-alkaline, and S-type granite. The widespread dykes which are exist in the batholith and country rocks support the settlement of mafic magma below the lower continental crust.