JOURNAL OF SCIENCE (UNIVERSITY OF TEHRAN) (JSUT)
Year:2009 | Volume:34 | Issue:4 (SECTION: GEOLOGY)|Papers Count:12

Themost important reservoir unit in NW Iran, known as Zivah Formation, is well described and its characteristics/are accomplished in the sequence stratigraphic framework. Using both field studies and laboratory measurements the constituent facies/facies associations are di6stinguished. The facies are grouped in to parasequences, system tracts and sequences based on their sedimentological characteristics, stratigraphic position, fining-up or coarsening-up trend, and the nature of their stratal surfaces. Discontinuous and thin coal seams, development of carbonate and Fe-oxide cements, distribution of turbidites, and establishment of incised valleys were used for detection of major strata surfaces. Sequence boundaries and maximum flooding surfaces have received greater attention due to their higher utility in sequence stratigraphic studies. The distinguished sequences are classified in to 2nd to 6th order sequences based on their sedimentological characteristics, thickness, stratigraphic position, and the age of whole formation.Correlating with available sea-level curves from other parts of the world, the 2ndorder cycles are related to worldwide sea-level changes (Eustacy). The 3rd to 4th order cycles are related to structural deformation of the source region and surrounding mountains, affected from regional tectonic. Local environmental conditions (bounding faults activities, waves, and currents, erosion rate) are found responsible for development of 5th to 6th order sequences. The facies/facies associations of LST and FRST are found more susceptible for hydrocarbon reservoir.

The various morphology of Anhydrite in Gachsaran anhydrite facies at Ab-Teymure Oil Field indicate This facies underwent different degrees of diagenetic alterations during surface, shallow and deep burial diagenetic environments. Diagenetic processes affected this anhydrite include nodule growth, pyritization, anhydritization, dissolution, silicification, compaction, recrystalization, pore filling, calcitization and anhydrite and gypsum veins. The succession of diagenetic processes and their paragenetic relationships within the Gachsaran anhydrite facies display a complex diagenetic evolution during syndepositional, early and late diagenetic stages. Many controlling factors such as, intensity of evaporation, sulfate reduction bacteria, depth of burial, tectonic activity, temperature variation, chemistry of brine and ground water, anhydrite preservation because of high stability in highly saline and 02-poor water, have been important factors influencing the anhydrite facies in Gachsaran Formation. The results of this can be used for evaluation of-.diagenetic processes in similar sulphate facies in evaporative basins, specially as a cap rocks of hydrocarbon reservoirs.

The Permian succession in Labnesar section (Eastern Alborz) is composed of Middle Permian Ruteh and Upper Permian Nesen formations. These sediments, which show characteristics of marine to continental environments, were deposited in landward margins of the Paleotethys Ocean. Based on sedimentological characteristics, four depositional sequences were identified in the studied area. The first and second depositional sequences, which belong to the Ruteh Formation, show mixed siliciclastic-carbonate nature, whereas the third and fourth depositional sequences (comprising Nesen Formation) have terrigenous nature. The supersequence containing the Permian deposits is correlated to the Middle Absaroka (Transpecos) supersequence worldwide. Deposition of the supersequence was begun by Gondwanaland's deglaciation and global sea level rise in the Middle Permian and lasted to the Upper Permian. Global sea level fall at the end of Permian resulted in termination of deposition and initiation of a widespread unconformity at the Permian- Triassic boundary.

Well logs are one of the important tools in reservoir characterization. Parameters such as porosity, volume of shale, water saturation, permeability, litho logy and production zones could be computed from processing and interpreting of well logs. For many reasons, such as incomplete logging in old wells, destruction of logs due to inappropriate data storage and measurement errors due to problems with logging apparatus or hole conditions, log suites are either incomplete or unreliable. Considering coverage and continuity of seismic data, log estimation in each location of reservoir by using seismic data, not only before the drilling but also after it will be applied and invaluable. So that using the predicted wells log from seismic data, reservoir evaluation and assessment over the seismic data coverage is possible. In this research, a formulation is established between well log data including sonic, density; porosity and their corresponding seismic attributes at well location using neural networks.For this purpose, optimal input seismic attributes were obtained using a forward stepwise regression.Predicted well logs using seismic data could be used for reservoir parameters evaluation and reducing risks of future exploration and fields' development.

Abderaz Formation at its type section with an age of Turonian-early Campanian and a thickness of 300 m contains light grey shale and marl. The study of the planktonic foraminifera in isolated form led to differentiate three morphotype groups. The first group is characterized by trochospiral tests usually indicate shallow waters, the second group contains forms with strong ornamentations and the primary keels representing mid waters and finally compact trochospiral tests with keels known as deep water indices are included in the third group. Studies on the morphotypes showed a regressive cycle for Abderaz Formation. Also the planktonic to benthic ratio was calculated using Depth = e (3.58718 + (0.03534x%*p) equilibrium which explains that at the deposits 400 meter at total part of in this Formation. This study was intended to explore the marine sedimentation of Abderaz Formation in (outer neritic-upper bathyal) restrict and the provided curves from morphotypes changes are in full agreement with the curves of the sea level changes and-planktonic. foraminifer to epifauna benthic ratio.

Shear wave velocity in well logging is measured by DSI (Dipole Shear Sonic imager) tool. Due to high measurement costs it runs in a limited number of wells in a hydrocarbon field. From these wells, only some intervals may have Vs data. Unlike Shear wave velocity data, Compressional wave velocity data and other conventional log data such as Neutron porosity, Density and resistivity data are obtained and measured easily by related tools in well logging. These logs have mathematical and physical relations with Shear wave velocity data, and as in reservoir intervals, Shear wave velocity data are important in geophysical studies such as AVO and VSP, lithology and fluid type identification, interpretation of elastic parameters and rock mechanical properties calculation, by using conventional log data and finding logical relations between these data and Shear wave velocity data, we can estimate Shear wave velocity in equivalent and partially similar intervals or wells with no Shear wave velocity data. So, because of the importance and usefulness of shear wave velocity, in wells with no shear wave velocity log, we should use a method to predict shear wave velocity. Genetic algorithms technique as a subset of evolutionary computing is an important part of intelligent systems for solving optimization problems. In this study shear wave velocity was modeled by genetic algorithms technique from petrophysical data in Ghar member of Asmari Formation, Hendijan field. For measuring there liability of the method, predicted values were compared with the real shear wave velocity data in Ghar member of Asmari Formation, Abozar field. Comparing the estimated results from Ghar Formation in Hendijan field with real data in Ghar formation in Abozar field, we can conclude that changes in, lithology,. fluid type and other petrophysical properties can have a strong effect on Shear wave velocity variation and reliability of predicted models, and this factor shows the importance of modeling and estimation methods like GA, in verifying the effects of changes in reservoir properties in large distances and their effect on reliability of intelligent systems Multiple Regression Analysis was used as another technique to evaluate the accuracy of the optimized model with GA. The results of this study show that GA could be considered as an efficient, fast and cost effective method for predicting VS- and reducing the exploration risks in reservoir characterization studies.

In order to revise the biostratigraphical zonation of the Surgah Formation, a total of 70 samples from this formation at the type section were: studied. Five genera and 23 species of planktonic foraminifera were recorded, which permit for the first time, the recognition of three biostratigraphic zones. Zone 1 (Marginotruncana schneegansi zone) is Middle- Late Touronian in age. Zone 2 (Dicarinella concavata zone) suggests Late Touronian-Early Santonian. Zone 3 (Dicarinella asymetrica zone) shows Santonian age. These biozones are corralatable with global standard biozones. Study of morphotype groups of planktonic foraminifera indicates that deposition of the Surgah Formation took place in a relatively deep marine environment.

Characteristics of fertile organs of a fern, Marattiopsis asiatica Kawasaki, and morphologic features of their in situ Punctatosporites scabratus spores, from the Shemshak Formation, northeast of Shahrud, are investigated and compared with similar taxa reported from Iran and elsewhere. Marattiopsis asiatica compared with other species of this taxon is distinguished by its possession of comparatively more crowded veins and longer synangia. The linear synangia are composed of two valves attached on secondary veins of the pinna margin. Each synangium is made up of two laterally joined long strips. Each sporangium contains a large number of relatively small, ±elliptical, bilateral, granulate, monolete spores which can be positively ascribed to the dispersed Punctatosporites scabratus.

The reservoir part of Kangan Formation with thickness of 115 m is a carbonate-evaporate sequence that is considered as a part of the largest carbonate reservoir in the South Pars Field in the Persian Gulf. Based on the facies studies the Kangan Formation consists of 12 microfacies and 4 facies belts, such as tidal flat, lagoon, shoal and open marine. The elemental studies and oxygen carbon isotopic trends in limestone samples indicate that these sediments were deposited in a tropical shallow environment and aragonite was original carbonate mineralogy. Kangan Formation has undergone a meteoric diagenesis in a closed diagenetic system. Variation of minor elements and their ratios along the stratigraphic column, and particularly between Dalan-Kangan Formations, clearly determined the boundary between these two formations. This variation is related to diagenetic and facies variations.

Structural analysis of folds across the western part of the Lorestan zone in Zagros fold-thrust belt showed that geometry of the folds are varies in different structural levels and leads to change in the folds style to depth. Competency contrast of rock units is the main controlling parameter of such a diver's behavior. Rock units with lower competency, as detachment surfaces, play a significant role in deformation style and its diversity down to depth. The Pabdeh, Gurpi and Illam Formation which are overlaying on incompetent Garu Formation compose outcrops of the study area.It is interpreted that the Garu and Pabdeh Formations are the major detachment zones control fold style across the study area. Geometric test of the folds constrain the effect of the detachment zones on variation of deformation style between their upper and lower rock units. In such a manner that folds in lower units to these detachments have larger wavelength comparing to ones developed within the overlying units. Similarly, strain partitioning within the whole sequence between competent and incompetent rock units demonstrate that change in geometry and folding style from surface to depth, in Lorestan zone, is mainly affected by diversity in mechanical behavior of rock units or in other words the effect of mechanical stratigraphy.