فصلنامه پژوهش های چینه نگاری و رسوب شناسی
سال سی و هشتم شماره 1 (پیاپی 86، بهار 1401)

To developing and explore hydrocarbon fields, it is necessary to know geological models and reservoir quality changes. Therefore, the Ilam Formation with approximate 145m thickness was examined in the Susangerd oilfield. The sedimentary model was considered carbonate ramp type due to the lack of slump and reef structures. Two complete and one incomplete third-order sedimentary sequences were identified. Close to the sequence boundary in the middle part of the Ilam Formation, reservoir conditions improve with the development of vug and mold dissolutions, especially in green algae by corrosive fluids passing through stylolites and fractures. But near the sequence boundary in the upper part of the Ilam Formation, however intergranular porosity develops between peloids and ooids but almost destroyed with cementation. The southeastern domain of the oilfield contains shallower microfacies in the lower sedimentary sequence and most part of transgressive system tract of the middle sequence in comparison with the northwestern domain. It seems that the southeastern part of the oilfield is located on paleo high and the lower values ​​of porosity log in the northwestern part are probably due to the distance from this paleo high. Therefore, the southeast of the oilfield is suggested for the development of the Susangerd oilfield and future explorations.

The carbonate Sargelu Formation (Middle Jurassic) in the Lorestan area mainly consists of black calcareous shales and thin- to thick-bedded limestones with remarkable organic material. Integrated binary diagrams of trace elements and stable isotopes such as Na-Mn, Sr-Na, Sr/Na-Mn and petrography, SEM and CL suggest an original low Mg calcite  (LMC) composition of the carbonate Sargelu Formation. Deep-sea sedimentation of the Sargelu Formation and its outer ramp to basin depositional settings are consistent with the formation and preservation of the LMC carbonate sediments. Binary diagrams of 1000Sr/Ca-Mn and δ18O-Mn as well as the presence of the framboidal and cubic pyrites, organic material and pseudocement infilling the bivalves display a predominant anoxic to the sub-oxic condition of the sedimentary substrate of the Sargelu Basin and marine and burial diagenetic effects in a closed to the semi-closed diagenetic system. The high content of the Mn of the studied sediments supports the anoxic condition in the sedimentary substrate as this condition facilitates incorporating Mn within calcite composition. Greenhouse condition in the Middle Jurassic period, which led to the stratification of the seawater of the Sargelu Basin and as a function of low water circulation, poor ventilation and formation of the anoxic condition, caused this high-temperature degree.

The Permian–Triassic sedimentary succession in the Julfa region is lithostratigraphically composed of limestones and shales of the Julfa Formation, the Zal Member shales, and the Paratirolites Limestone of the Ali-Bashi Formation, as well as shales and carbonate beds of the Elikah Formation. The Lower Julfa Beds are rich in benthic organisms such as brachiopods and corals, while the Upper Julfa Beds and Ali-Bashi Formation contain pelagic assemblages including ammonoids, fishes, and conodonts. These rocks have already been studied from different palaeontological and geochemical perspectives, but most have focused on the strata near the Permian–Triassic boundary. In the present study, ammonoids from older intervals around the Wuchiapingin–Changhsingian boundary in the four stratigraphic sections Aras Valley, Ali-Bashi 1, Ali-Bashi 4, and Zal are outlined. Fourteen genera and 22 species of ammonoids were identified and assigned to five successive biozones. The Araxocers latissimum Zone in the Lower Julfa Beds and the Vedioceras ventrosulcatum Zone in the Upper Julfa Beds document the Wuchiapingian. Following upwards, the Iranites transcaucasius-Phisonites triangulus Zone, Dzhulfites nodosus Zone and Shevyrevites shevyrevi Zone in the Zal Member confirm an early Changhsingian age. This follows the previously presented ages based on conodonts.

An important issue in the operation of the Gotvand reservoir is the presence of salt formation in this reservoir. The present study has sought to identify whether it is possible to modify the operation curve of the Gotvand Dam to reduce the destructive effects of the Anbal salt dome and increase the operation of this reservoir. To achieve this goal, the performance of the Gotvand Dam reservoir was modeled from the time it was used (from August 2011 to March 2017), as well as the behavior of active salt karstic activity inside the reservoir with the dynamic system tools. The results showed that during the simulation period, the average amount of corrosion of the salt dome in contact with the reservoir water was 20.1 cm per day. Besides, 43.72 million tons of salt has been accumulated in the reservoir. Analysis of the results showed that 39% of the reservoir salinity is due to the dissolution of the salt formation. The rest of the salinity enters the reservoir with the upstream runoff. Based on the results of sensitivity analysis, changes in water level in the reservoir is the most important factor determining the salinity distribution parameters in the reservoir and the best remaining solution to reduce the effects of the salt dome inside the Gotvand Reservoir is to implement the curve of the optimal use of the reservoir for water released through the Gotvand Dam and the upstream basin of this dam.

Cheshmeh Khorzan conglomerate is located in the Arak Mighan Playa watershed. Determining the texture characteristics and origin of conglomerates is one of the objectives of this study. In the Pliocene loose conglomerate, the particle diameters of the conglomerates varied in a wide range of granule, pebble, cobble and boulders in 51 areas. In addition, the components of conglomerates are mostly good rounded and poorly sorted. Regression analysis and probability level tests such as Student and Fisher test at 95% confidence level showed no significant relationship between roundness and particle diameter with transporting distance. The conglomerate is re-transported in the river and alluvial fan environment. They are close to their source, and their secondary components result from erosion of compact and old conglomerates. Multivariate statistical analyzes showed that the conglomerate components were rich in sandstone (45%), andesite (37%), limestone (7%), tuff (5%), rhyolite (3%), chert and basalt (3%). They have been obtained from the erosion of older formations in Central Iran. Good roundness shows high groundwater potential and high abundance of volcanic fragments indicate the good potential of the placer in the area.

The Persian Gulf possesses the largest gas fields in the world. The reservoir part of these fields includes upper Dalan and Kangan formations dating back to the Permo–Triassic age, equivalent to Arab countries' Khuff Formation in the south of the Persian Gulf. The present study was based on the core and petro-physical information, including gamma, sonic, and neutron log, in well A in the central part of the Persian Gulf with a thickness of 174 meters. Findings obtained from examining 628 thin sections indicated 12 microfacies in five groups of microfacies (sedimentary environment). These five facies belts consisted of supratidal, intertidal, lagoon, shoal, and open marine (middle and outer ramp). The sum of these facies and their sequence indicated the formation of carbonate-evaporative successions of Kangan in a ramp environment with a gentle slope deposited in the hot and dry climate of the former Triassic. Regarding the combination of microfacies studies and petrophysical logs, two third-order depositional sequences named DS1 and DS2 were identified. Investigations indicated that the Kangan Formation could be divided into four reservoir zones. Zones 1 and 3 with evaporitic or mudstone facies and very low porosity were considered impermeable and non-production capacity parts of the reservoir. These zones were in correspondence to the supratidal microfacies and lagoon. On the other hand, zones 2 and 4 included ooid-bearing grain-supported microfacies with very high porosity. These zones were regarded as the permeable and production capacity parts of this formation and corresponded to the microfacies group of the back shoal.