نشریه زمین شناسی نفت ایران
سال سیزدهم شماره 2 (پیاپی 26، پاییز و زمستان 1402)

Electrofacies determination of the reservoir plays an important role in the petrophysical evaluation of reservoir zones to optimize production and development of oil fields. The process is based on data clustering that all unique petrophysical set are put in one group to separate from other groups. The present study was done in Asmari Formation, Ghaleh Nar oil field. The primary electrofacies model determined using different clustering methods such as SOM, MRGC, and DYNCLUST in several drilled wells. In the next step, they correlated with fluid units of porosity and permeability of core plot. Of these methods, SOM indicates more correlation and so it was selected to data clustering. According to Gamma and porosity plots, electrofacies were generated and developed to the whole of the field. This is resulted to a model with the potential of separation parts of the reservoir. The model showed that some parts of the reservoir especial zone 1 and zone 3 can be considered as more suitable reservoir quality than other parts. Zone 4 shows normal reservoir quality but two other zones are not in suitable reservoir condition.

Today, reservoir rock evaluation is very important in the oil industry. The ability of petrophysical evaluation is very important in determining reservoir zones and investigating them in petroleum geology. The studied gas field is one of the gas fields in the south of Iran in the Zagros sedimentary basin. In this research, using petrophysical data and porosity-permeability information of the cores and with the help of the possible evaluation method, the petrophysical parameters of the upper Dalan Formation were evaluated in two wells 4 and 6. The purpose of the research is to check the quality of the studied gas field and evaluate the reservoir properties using petrophysical studies. According to the results of this petrophysical evaluation, in the upper Dalan Formation in wells 4 and 6, the average shale volume is 1.7 and 2.4, respectively, the average water saturation is 30, 22.4, respectively, and the average effective porosity is 3.7 and 4.4, respectively. 6 and the ratio of useful thickness to total thickness is 0.68 and 0.92 respectively.The closer the value of this parameter is to 1, the studied formation has more reservoir quality. For a more detailed investigation, the upper Dalan formation in the studied field was divided into 3 zones (UD-3-UD-1) and UD-3 zone was divided into two sublayers (UD-3b-UD-3a). UD-2 zone with the dominant lithology of dolomite and some calcite, due to the volume of gas and greater porosity than other zones, was recognized as the best reservoir horizon. Such studies are used to increase production and EOR.

The Ilam Formation is considered one of the most important reservoir formations in Iran due to its significant extent in the Upper Cretaceous of the Zagros Zone. The changes in facies, lithology, and depositional environment from shallow to deep marine settings have played a considerable role in enhancing the reservoir characteristics of this formation. This study focuses on the Ilam Formation in the well No.1 of Babaghir oilfield and two surface sections of Tange-Hamam and Azgeleh in the Lorestan subzone for reconstruction of the depositional environment and sea-level changes during the Upper Cretaceous. In the study area, the Ilam Formation primarily composed of limestone and shaly limestone. The microfacies identified include mudstone to mudstone with planktonic foraminifera, wackestone with planktonic foraminifera, wackestone-packstone with planktonic foraminifera and oligosteginids, and wackestone-packstone with planktonic foraminifera and skeletal fragments. Based on textural characteristics and evidence of gravity currents, it is believed that the Ilam Formation in the study area was formed in a carbonate shelf. Sequence stratigraphy analysis in the study area has revealed a third-order depositional sequence with transgressive-regressive system tracts. The evolution of carbonate platform of the Ilam Formation has been significantly influenced by global sea level fluctuations, dominant warm climate, and palaeogeographic conditions during the Upper Cretaceous.

Folding patterns in the Zagros thrust belt are highly influenced by both the mechanical behavior of its rock units and the activity of the Bedrock faults with each Zagrosian and Arabian orientation, and this factor significantly influences the upper deposit sedimentation patterns throughout many areas, and on other occasions it also affects facies distribution and alternate thickness changes. The main goal of this article is to assess the effect of Hendijan-Izeh Fault on deposition of Tarbur Formation (Maastrichtian) in Keynow anticline and its vicinity during field study, satellite imagery geological mapping; and using results from thin sections (33 samples), Four lithofacies were identified, which points toward a carbonate ramp depositional environment, whereas the sediments biostratigraphically display Maastrichtian age. The findings of the article demonstrate that with respect to the Hendijan-Izeh Fault, Tarbur Formation transitions into Gurpi Formation with distance, while Gurpi Formation is deposited where Tarbur Formation would be from the southern limb of Keynow anticline. Therefore, it is concluded that the Hendijan uplift was consequential during the time of Tarbur Formation deposition.

An integrated analysis of subsurface data, encompassing seismic and well log information, serves as an effective approach for refining sequence stratigraphic interpretations and enhancing the understanding of oil fields. This method elucidates the connections between sedimentary sequences, facies, and reservoir characteristics, facilitating more precise predictions of facies distribution and the identification of hydrocarbon reservoirs. In this study, the deposits of the Kazhdumi Formation in a field located in the northwestern Persian Gulf were analyzed across three wells. To achieve a more detailed evaluation of the Kazhdumi Formation, seismic data were calibrated with well data using check shots. Following this calibration, seismic data interpretation was conducted according to sequence stratigraphy principles, focusing on seismic reflector terminations on seismic sections to delineate the sequence boundaries and elements within the Kazhdumi sequence. Seismic analysis revealed a single seismic sequence within the Kazhdumi Formation in the studied field. This sequence comprises several components: the base of the Dariyan Formation (lower sequence boundary), the base of the Maddud Formation (upper sequence boundary), the Borgun B Formation (lowstand systems tract), the Borgun A Formation (transgressive systems tract), the Dair limestone (overlying the maximum flooding surface), and the upper Kazhdumi along with a portion of the Madud member of the Sarvak Formation (highstand systems tract). The Kazhdumi sequence was mapped and interpreted on 2D seismic sections within the study field and adjacent areas. Subsequently, subsurface isochron maps were created by interpolating points between 2D seismic lines.

 Petrography and clay mineralogical study on DII, DI memebers of shurjeh gas reservoir  ( sand stones) , and upper  member of DII shows , presence of different type of  minerals,  as well as clay minerals , such as calcite, gluconate, Pyrite, Iron oxide,  interbedd with silty & clay layers. Petrography studies along with XRD, XRF,  EDX, SEM on the present samples, indicates presence of, kaolinite , illite  chlorite and expandable  clay  minerals. Expandable clay comprise smectite and mixed layer or interstratified Illite / Smectite ( I/S) .Clay minerals have a tendency to change permeability of  the rock, and hinder the passage of fluids.  Kolinite  may also cause varying degrees of impaired permability on contct with fresh water.The presence of interstitial clay drastically increase microprosity whilst at the  same time  reducing effective prosity. Petrography study shows , the mineralogical composition of reservoir rock comprised, Sublitharenites , using nomenclature of Folk, 1980., in  general , 75% Quartz, 6% Feldspar and 15% Rock Fragments. Presence of different clay minerals in pore networks may have a significant impact on petrophysical properties of reservoir and hence affect reservoir productivity.