نشریه زمین شناسی نفت ایران
سال یازدهم شماره 1 (پیاپی 21، بهار و تابستان 1400)

Rock typing is the process of assigning reservoir properties to geological facies, and an identified rock type has similar geological and reservoir properties. Due to the importance of identifying and separating rock types in hydrocarbon reservoirs, various methods have been proposed and developed today for the determination of rock types. One of the simplest methods is the porosity chart against permeability and cut-off determination, and one of the most important and practical of these methods is to determine rock types by flow zone indicator. In this study, we examine one of the most important hydrocarbon fields in the south of Iran where core, well and seismic data are available for the field studied so that by designing a new workflow and with use of the most important and efficient methods of rock typing, including FZI, porosity, Winland-Pitman index, FZI*, and logarithms of FZI and FZI* we have identified rock types in three dimensions and through the whole field. After the final validation, the correlation coefficient values were more than 83%, 57%, 70%, 70%, 73% and 78% for the methods used, indicating greater accuracy and efficiency of the FZI method for the rock types separation in this field. The partial comparison of the validation results after each method employment also confirms the highest accuracy belongs to the FZI method. As a result, by integrating this method with seismic attributes, the rock types have been separated in four different groups throughout the field in three dimensions.

The Ilam formation is one of the important carbonate oil reservoirs of the Zagros and Dezful embayment basins. In order to identify the microfacies and depositional conditions and diagenesis processes, 100 microscopic thin sections from one well of this reservoir in one of the Dezful embayment oil fields were evaluated. Porosity and permeability data were used to check reservoir quality. The petrographic study led to the identification of 9 microfacies belonging to the facies belts of lagoon, barier, middle ramp and outer ramp, which were deposited in the homoclinal ramp environment. The most important diagenesis processes identified in Ilam reservoir include cementation, dissolution, fracture, micriteization, stylolitization, and dolomitization. Based on the qualitative classification of the reservoir and Lucia's petrophysical diagrams, the lagoon facies and the middle ramp have an average reservoir status, and the carbonate barier facies and the outer ramp have a weak reservoir status. The reservoir quality of the lagoon and middle ramp facies is related to the existence of interconnected and channel porosities. Due to strong cementation and the presence of unrelated porosity such as mold porosity, the carbonate barier facies has low permeability and has a poor reservoir status. In general, the Ilam Formation in the studied field is in a weak state in terms of reservoir, which can be important in addition to the facies controllers in relation to the lack of expansion of fracture and dolomitization and the excessive expansion of cementation in these facies. Therefore, the Ilam Formation in the studied field has a weak reservoir performance due to diagenetic processes, despite having more shallow sequences than deep ones.

Sorkh Shale Formation in Rabat-e-Shur stratigraphic section, west of the Tabas Depression, has been biostratigraphically investigated in this research. The Sorkh Shale Formation, with 48 meters thickness in this section, consists of shale, marl, and marly limestones, which are underlain by the Permian Jamal Formation and are overlain by the Triassic Shotori Formation dolomites. Nine conodont species belonging to four genera, Ellisonia, Hadrodontina, Parachirognathus, and Pachycladina were identified for the first time from this formation. The mentioned conodont assemblage is divided into three biozones: Hadrodontina aequabilis, Hadrodontina anceps, and Pachycladina obliqua. These taxa are in good agreement with the previously reported euryhaline assemblages from the near-shore and shallow water Western Tethyan realms in the south and east of Europe in the Early Triassic (Late Griesbachian-Middle Smithian). These conodont biozones are used for a better understanding of the distribution and affinities of the Early Triassic conodonts in West Asia, their paleobiogeographical rank, their significant role in the Early Triassic chronostratigraphy and their importance in bio-correlation of different section worldwide. Examination of the conodont alteration index of the obtained taxa from the Sorkh Shale Formation demonstrates CAI=5 during the late Griesbachian, CAI=4 in the Smithian, and CAI=6-8 in the Dienerian substages which is placed in the barren Zone for hydrocarbon production.

The Middle Permian Jamal Formation have been investigated for sedimentological and geochemical aspects in the Chah-Riseh section, northeast Isfahan. According to the field studies the Jamal Formation with 251 m thickness divided into eight lithostratigraphic unit. Lower boundary of this formation with an unconformity is underlained by the Sardar Formation which belongs to the Carboniferous period and upper boundary with an unconformity reaches to the Lower Triassic Sorkh-Shale Formation. Facies and microfacies studies of the Jamal Formation led to the identification of two petrofacies and 14 carbonate microfacies. According to the recognized carbonate allochems, petrofacies and microfacies of the Jamal Formation and some evidence such as transitional microfacies changes, we can consider a depositional environment of a shallow mixed siliciclastic-carbonate ramp platform. Petrographically, four types of dolomites are recognized in the Jamal Formation. The dolomitization model for the type I dolomite is considered forming in tidal flat and burial dolomitization for types II, III and IV. Geochemical studies including major and trace elements analysis comprised of elements such as Ca, Mg, Sr, Mn and Fe. Using ratios of the elements and also by plotting some of these elements cross carbon and oxygen isotopes in various diagrams have been used in determining the original mineralogy of carbonate deposits and efficient diagenetic system on the Jamal Formation. The results indicate that the dominant diagenetic environment effected on the carbonate deposits of Jamal Formation was occurred in a semi-closed system and the original mineralogy was aragonite. Evaluation of major and trace elements contents of the four types dolomites, confirmed different characteristics of theses dolomite resembling crystal sizes in petrographic studies. Carbon and oxygen isotopes data of dolomites also defined their diagenetic situations.

Tirgan Formation (Lower Cretaceous) in the Kopeh-Dagh sedimentary basin consists mainly of Oolitic, Orbitolina, dolomitic limestones that conformably overlies the Shurijeh and Sarcheshmeh formations, respectively. Upper contact of Tirgan Formation at Jozak section with Abderaz Formation is faulty and were placed under white chalky limestone of the Abderaz Formation. Two stratigraphic sections at Estarkhi and Jozak were measured and sampled. Biostratigraphicic studies lead to two benthic foraminifera and calcareous green algae fossil assemblages have been identified. Twenty one genera and 24 species of benthic foraminifera and 16 genera and 26 species of calcareous green alge at Estarkhi section, 21 genera and 26 species of benthic foraminifera and 10 genera and 10 species of calcareous green alge at Jozak section in west of Kopeh-Dagh sedimentary basin have been identified. This study has been suggested the Barremian- Early Aptian( Bedulian) stage for the Tirgan Formation at these localities that is corresponded with age of central and east basin. Also based on paleobio-events study were determined base of the Aptian stage in east of the kopeh-Dagh sedimentary basin base of FA(First Appearance) Dictyoconus pachymarginalis and LA(Last Appearance) Montseciella arabica and in center and west of the basin base of the FAs Palorbitolina lenticularis, Salpingoporella muehlbergii.

Ilam Formation is deposited at the late Cretaceous (Santonian) in a neritic/pelagic environment. Following the Ilam deposition, a general deepening has occurred and the Gurpi formation (shaly layer) was deposited. Ilam has thickness variation between 75-110m and structure has created as a result of upward movement of deep seated salt. Ilam reservoir in this field is regarded as a secondary hydrocarbon potential, and since now, no oil has been produced from this horizon. petrophysical and Seismic interpretation has been done for the Ilam reservoir, the geological models (structural and property models) are generated, with both deterministic and stochastic approaches. The seismic attributes as secondary variables, improved the kriging and Sequential Gaussian Simulation (SGS) algorithm results for modeling of Ilam. This study reveals that Water Saturation is generally high, indicating that Ilam reservoir has low hydrocarbon potential, within the five reservoir potential zones, the zone 5 indicated a good original oil in place potential, especially in the western and central parts of the field.