فصلنامه زمین شناسی ایران
پیاپی 50 (تابستان 1398)

Calcareous nannofossils are useful in determining the relative age of the Cretaceous strata. In this study, the Gurpi Formation with a thickness of 445 m at the Kuh-e Sabz syncline located in southwest of Borojen city is investigated based on the calcareous nannofossils. As a result of this study, 20 genera and 37 species of calcareous nannofossil were identified. Based on distribution of these index calcareous nannofossils, seven bio-zones were recognized including: Calculites obscurus Zone (CC17/ late Santonian- early Campanian), Aspidolithus parcus Zone (CC18/ early Campanian), Calculites ovalis Zone (CC19/ late Early Campanian), Ceratolithoides aculeus Zone (CC20/ late Early Campanian),
Quadrum sissinghii Zone (CC21/ early Late Campanian), Quadrum trifidum Zone (CC22/ late Late Campanian) and Tranolithus phacelosus Zone (CC23/ latest Campanian–early Maastrichtian), that corresponding to upper part of NC17 - NC20 and UC13-UC17. Based on the identified zones, the age of the studied intervals is Late Santonian/Early Campanian to latest Campanian/early Maastrichtian. The study of the Gurpi Formation in the Kuh-e Sabz syncline, shows that the sedimentation began at the Late Santonian/Early Campanian time and continued up to the latest Campanian/early Maastrichtian. Then, with a sudden drop in the depth of sedimentary basin, the Tarbur Formation gradually deposited on the top of the Gurpi Formation.

Clastic deposits of the Faraghan Formation (Late Permian, Sakmarian) are present in different outcrops in the Zagros area and some wells in the Persian Gulf Basin. The presence of these clastics represents the existence of an extended clastic system on the north-east of the Arabian plate. The Faraghan Formation unconformably overlies Zakeen sandstone (Devonian) and grades upward into Dalan carbonate (Upper Permian). To study the Faraghan Formation some thin sections were prepared from cuttings of some wells in the central part of the Persian Gulf and then they were studied. Results show of this study that the Faraghan Formation consists of quartz arenite, sandy siltstone and sandy claystone petrofacies and carbonate mudstone, bioclast wackestone and bioclast packstone microfacies. Based on sedimentary characteristics and spatial relationships between the petrofacies, this formation was deposited on a linear shoreface environment (floodplain, shoreface and offshore sub-environments). Main diagenetic processes consist of compaction, dissolution, cementation, neomorphism, sericitization and dolomitization. Heterogeneity of the Faraghan Formation lithology and variety of environmental conditions and diagenetic features make this formation a good candidate for hydrocarbon reservoir studies.

Wellbore instability is one of the main problems during drilling of oil and gas wells. Analyzing and wellbore instability prediction is important. In-situ stress measurement and stress regime are important for stability studies. This study was carried out in one of the oilfields in the south Caspian basin in Iran. The compressional stress regime in the studied field was determined based on regional stress indicators and larg scale tectonic observations. . This study presented the effect of borehole inclination and azimuth on borehole stability under reverse fault regimes. The magnitude of stresses was determined from measured sonic velocities, density log and predicted pore pressure utilizing the Eaton method. In this study, the modified Lade model have been utilized to perform stability calculation for different inclinations and azimuths. In this study, to perform stability calculation with the use of STABview software is presented. It is shown that drilling wells parallel to maximum in-situ horizontal stress (NE-SW) causes less stability problems. The results of this study could help in the mitigation and/ or prevention of wellbore stability issues in this oilfield.

Estaj iron deposit (EID) is located 40 km south of Mashhad. Rock units in the area include sandstone and quartzite (Cambrian), limestone (Devonian), micaschist (Silurian) and gabbro diorite. Hematite iron mineralization occured in the schist and limestone host rocks and sporadically in quartzite. Mineralization accomplished in two stages, of hypogene (including primary minerals of hematite, pyrite, and magnetite) and oxidized (including minerals, of goethite, hematite and limonite). Ground magnetic survey over main outcrop of hematite iron mineralization was carried out to investigate magnetite mineralization at depth. Magnetic maps show anomalies A and B over the main mineralization and C, D and E over the intrusive body. Small amount of mineralization is roughly cropped out along a fault with an east-west trend, coinciding with the anomalies C, D and E. Magnetic susceptibility of surface samples of intrusive body and host quartzite (  and  is low. Consequently the main source of anomalies at the location of main mineralization (with suseptibility of ) is probably magnetite. In the location of intrusive body the source is either magnetite or the intrusive, body which has higher magnetite suseptibility at depth. The latter point should be verified by drilling.

The Gol-e-Zard Zn-Pb deposit is located in northeastern of the Aligudarz region (Lorestan province). This deposit is exposed in phyllite and meta-sandstones of upper Triassic-Jurassic of the Sanandaj-Sirjan Zone. The mineralization includes sphalerite, galena and chalcopyrite, which are mainly along quartz veins. Two mineralized horizons are phyllite with high mineralization and meta-sandstone with low mineralization. The mineralogical studies show that galena, sphalerite and chalcopyrite are metallic ores and quartz is also the most abundant gang mineral in the studied deposit. The evidence indicate that the mineralization of the Gol-e-Zard deposit is syngeneic and epigenetic. The most significant structural pattern is the stratabound mineralization in the region. The aim of this study is to determine the type and characteristics the mineralizing fluid in the discriminating of mineralized horizons, besides the determining of source of the fluid in the Gol-e-Zard deposit. LREE enrichments (La/Lu average 4.8) and positive Eu anomalies (average 1.2) indicate the anoxic condition and hydrothermal fluids. Fluid inclusion data shows homogenization temperatures of 139-199.5°C, salinity 5.21-30.38 wt%equ.NaCl and density 0.9-1.1 gr/cm3 in this deposit. Investigation of evolution path of the fluids shows isothermal mixing of fluids during mineralization that comprises the mixing of magmatic water with sea water and also meteoric water. Thus, the mineralization can be summarized as: expulsion of the hydrothermal fluids from the depth, entering into sea water and reducing the temperature of hydrothermal fluids due to mixing with sea water, ascending to the sea level, moving through sediments and circulating in void spaces of and then the leaching of metals from sediments and deposited along the void space and quartz veins.

The Dash Aghol iron deposit is located in the northeast of Bukan city and is hosted by Jurassic volcano-sedimentary sequence. The main host rock of the Dash Aghol deposit includes sandstone, limy tuff and limestone. Based on geometry of ore body, two different types of mineralization were distinguished in Dash Aghol deposit: lenticular primary mineralization which occurred as parallel and concordant with layering of the host rocks. Ore textures of this type include massive, laminated and disseminated forms existing in sandstone, limy tuff and limestone. Second type, includes supergene vein-veinlet and open space filling mineralization which is associated with faults and related brecciated zones and were occurred in hanging wall rocks of the primary ore lense. According to the stratigraphy, ore body form, texture, paragenetic sequences, ore bearing chert-carbonate- tuff facies and mineralogy, the iron mieralization in Dash Aghol deposit is stratiform tuff carbonate-sandstone host rocks deposits, in rolcano-sedimentary sequence of Jurassic, which were precipitated in shallow marine environment.

In this paper, the Shazand catchment area have been simulated  using the geographical information system  (GIS) with an accuracy of 10 × 10 meters and various parameters required in the form of digital layers in the ArcGIS environment. Then, using the EPM model, the erosion zoning map of this basin located around the Saveh dam was prepared and its annual precipitation rate was estimated. In the following, the output of the model was compared with the hydrometric method. According to the results of EPM model, sediment yield from stations of Poledoab, Bazaneh, Toureh and Shazand using EPM model are 288103.81, 18963.83, 43079.2 and 20583.76 respectively, whereas hydrometric method shows values 335285.42, 23937.42, 36459.09 and 42781.69 ton per year, respectively. Also comparison of EPM model with hydrometric method shows a good correlation. Results show accuracy of 86%, 82%, 48% and 79% for Poledoab, Toureh, Shazand and Bazaneh stations, respectively. Assessments show that EPM model with acceptable accuracy provides good results in order to predict the mximum annual sediment yield.

In order to study the Lower Cretaceous deposits, Qumenjan section has been selected and measured. Field and lab studies of the Lower Cretaceous deposits in this section reveal a gradual change of the depositional setting, from a fluvial depositional system (continental environment) to a mixed siliciclastic- carbonate ramp (marine environment). The continental deposits of the studied section include conglomerate and sandstone lithofacies that are deposited in a proximal gravel- bed braided river. In the studied sedimentary sequence, the marine siliciclastic- carbonate facies associations are formed in three tidal flat, lagoon and shoal facies belt in the inner and mid ramp setting.