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The Fahliyan Formation, a significant carbonate reservoir in southwestern Iran encompassing the Izeh and Dezful Zones, underwent detailed petrographic investigations. These analyses revealed eight distinct microfacies associated with four different depositional settings within a homoclinal ramp model. From a diagenesis perspective, the formation has undergone various processes, including micritization, dissolution, compaction, cementation, dolomitization, stylolitization, and fracturing. These diagenetic features affected the Fahliyan Formation from early marine–meteoric to late burial diagenetic realms. Notably, dissolution developed as the most effective and widespread diagenetic feature, improving reservoir quality. Likewise, fracture and dolomitization positively impact reservoir quality, while compaction and cementation have destructive effects. Micritization and early isopachous calcite cement have a retentive role in reservoir characteristics. In addition, the Flow Zone Indicator (FZI) approach introduced three Hydraulic Flow Units (HFUs). The correlation between microfacies types and their petrophysical features indicates that the bioclastic peloid packstones and grainstones have better reservoir quality, which resulted from dissolution, and initial isopachous calcite cements. Also, Planktonic foraminifer’s bioclastic mud/ wackestone and Quartz-bearing mudstone, equivalent to HFU1, indicate lower reservoir quality due to the compaction (stylolitization) and cementation.

Tirgan formation of the Kopet-Dagh Basin (northeast Iran) represents one of the Urgonian carbonate platforms that were deposited during the Early Cretaceous time in the northern Alpine Tethys and deformed during the Alpine orogeny. In this study, six stratigraphic sections of the shallow-water platform sediments (Tirgan formation) were measured based on microfacies and fauna abundance. Detail study of petrography, fossil content, and sedimentary structures led to the identification of fifteen microfacies belonging to four facies belts including open marine, shoal, protected lagoon, and tidal flat. The sediments of the Tirgan formation exhibit calcareous green algae, abundant ooids, oysters, bryozoans, and crinoids in inner and middle platform ramp facies and planktonic bivalves and sponge spicules in outer-platform facies. Furthermore, the absence of basinal deposits and lack of evaporate evidence point to deposition under warm-temperate and humid climate conditions. Sequence stratigraphy analysis of Tirgan formation led to distinguish a single depositional sequence in all of the sections which are composed of transgressive and highstand systems tracts with sequence boundaries of type II (SB2). The lowermost lower Aptian Tirgan sequence in the study area relatively shows a similar trend in comparison with the global curve. This basin was deepened over time so that shaly and marly sediments of Sarcheshmeh formation were placed over Tirgan conformably and may suggest a drowning event that was likely related to unusual global warming. Last, this study contributes to the better understanding of the high distribution of facies assemblages in the Urgonian carbonate platforms.

The purpose of this study is to identify the pore types and distinguish the pore facies based on the diagenetic performance. The petrographic examination of 230 thin sections lead to the identification of a variety of primary and secondary porosities and show that the vuggy, moldic and intercrystalline pores have more frequency than the intraparticle, interparticle and fenestral pores. The plot of data obtained from thin sections on the ternary porosity graph used for determining the pore facies and their diagenetic trends and led to the identification of6 pore facies including depositional (PF1), moldic (PF4), and vuggy (PF6) pore facies, and also mixture pore facies PF2 (mixture of PF1 and PF4), PF3 (mixture of PF1 and PF6) and PF5 (mixture of PF4 and PF6). The comparison of petrography results with the well logs, core permeability and the calculated RQI and FZI values show that the pore facies which have touching pores, especially PF6 and PF1, have played the most important role in improving the quality of the reservoir, especially in the upper part of the studied zone. Also the pore facies which have separate pores, especially PF4, have the most influence in the reduction of reservoir quality relative to other pore facies, especially in the lower part of the studied zone. Most of the pores identified in the studied zone are the secondary porosity and diagenetic in origin. This demonstrates that the reservoir characteristics in the studied zone have been strongly modified by diagenetic overprinting. The most important diagenetic processes affecting the pore systems of the Asmari reservoir are cementation, selective and massive dissolution and dolomitization. Massive dissolution plays the most active role in the formation of touching vugs as well as enhancement of reservoir quality. Cementation and fabric selective dolomitization play the most active role in decrease of the reservoir quality.

In this study, using a multi-layer perceptron neural network (MLPNN) model, total organic carbon (TOC) and hydrogen index (HI) values for Pabdeh and Gurpi Formations in the oil fields of Naft Sefid (NS-13), Kupal (KL-36, KL-38, and KL-48) and Palangan (PL-2) were calculated in the North Dezful Embayment located in the southwest of Iran. To build the MLPNN model, the geochemical data calculated by the Rock–Eval pyrolysis method (TOC and HI) and the conventional petrophysical well log data, including sonic transit time log (DT), formation density log (RHOB), total resistivity log (RT), spectral gamma-ray log, computed gamma-ray log and neutron porosity log from the NS-13 well were used. The log data were the input layer, and the geochemical data were the output layer of the model. Twenty-four datasets were used for MLPNN training, and seven datasets were used for MLPNN testing. Two hidden layers were considered in this technique. Each hidden layer has an activation function (tanh) and a solver parameter (lbfgs). The accuracy of measurement of TOC and HI indices of Pabdeh and Gurpi Formations in terms of R2 was 0.93 and 0.90, respectively. This model has higher accuracy than the ΔlogR technique (R2: 0.28). Considering the relationships between the input data and other wireline logs is an advantage of this technique. These two formations have five source rock zones. Pabdeh Formation has three zones. The middle zone of the Pabdeh Formation (Pz. II) has the highest TOC (2.6 wt%) and source rock potential. Pabdeh Formation has kerogen type II. Gurpi Formation has a weaker source rock potential than Pabdeh Formation due to its low TOC content (< 1%). Both source rock zones of this formation have low TOC, but in some layers of the lower zone of the Gurpi Formation (Gz. II), high values for TOC were predicted. Gurpi Formation has Kerogen types II and III.

The purpose of this study is to use the geochemical analysis of recent sediments from the Rayen River (length of about 25 km), located in the Central Iranian zone and Urumieh-Dokhtar Magmatic Belt, to interpret their compositional maturity, chemical weathering, source rocks, and tectonic setting. Geochemical analysis (major and trace elements) of ten sediment samples suggests that mafic and intermediate igneous rocks are the sources of these sediments. Based on discriminant function, binary and ternary diagrams, it is concluded that the tectonic setting of the study area is similar to a continental island arc. The Chemical Index of Alteration (CIA) ranges between 50 and 57, suggesting low-to-moderate chemical weathering that reflects tectonic activity, erosion, and rapid deposition under arid climatic conditions, which gradually increased through time in the source areas. The ICV (Index of Compositional Variability) values show that these sediments have low compositional and mineralogical maturity. It is also shown that the bed load in the Rayen River has a mixed source from undifferentiated volcanic rocks. The data obtained from both petrography and geochemical analysis, which reflect the tectonic and climatic conditions in the study area, can be used as a guide for the interpretation of similar ancient sedimentary records.

Routine core analysis data (porosity and permeability)—used in various methods for hydraulic flow unit (HFU) determination of reservoir rocks—are unavailable in all drilled wells. On the other hand, raw petrophysical wireline logs—applied to determine reservoir electrofacies (EF)—are usually available in all wells. Since cores provide accurate data on reservoir characteristics, the lack of cores has always interested petroleum geologists and engineers. Therefore, introducing a new method to give almost accurate data about reservoir rocks in uncored wells has always interested petroleum geologists and engineers. As the type of input data that was used to determine HFUs and reservoir EFs are fundamentally different from each other, providing an approach that can create a better match between the results of these two rock typing methods is always one of significant interest for researchers. In this research, capillary pressure (Pc) test results are vital in obtaining reservoir EFs compatible with HFUs for the Oligo-Miocene Asmari Formation in Qale Nar Oilfield. So that only EFs that are compatible with Pc test results are approved. Flow zone indicator (FZI) method was applied to determine five HFU including A (Log FZI > − 0.05, average of core porosity and permeability are 5.8% and 0.37 mD) to E (Log FZI < − 0.65, average of core porosity and permeability are 0.07% and 0.03 mD). Furthermore, based on raw petrophysical wireline logs and MRGC algorithm in Geolog software, five electrofacies (EF) were indicated containing EF 1 (average of core porosity and permeability are 5.91% and 0.38 mD) to EF 5 (average of core porosity and permeability are 0.08% and 0.02 mD). The correlation between HFUs and EFs shows that HFU A to HFU E is compatible with EF 1 to EF 5. Also, examining the obtained electrofacies in the modified Lorenz plot indicates that EF 1 and 2 perfectly match intervals with a high fluid flow regime. By this method, it is possible to provide an almost accurate estimation of hydraulic flow unit distribution for wells and intervals without cores.

Petrographic studies were carried out to explain the main variations in sedimentary facies and environment characteristics. Based on these studies, 16 microfacies were recognized for the Fahliyan strata, which were deposited in various segments of the ramp platform, including tidal-flat, lagoon, shoal, and open marine settings. Moreover, interpreted depositional environment and fauna components and sedimentary textures of microfacies were applied to identify three-order depositional sequences (sequences A to C). Sequence A, the lowest part of the Fahliyan strata, comprises sedimentary facies related to shoal and open marine settings. Sequence B constitutes the middle part of the formation, and the open marine segment was considered the primary depositional environment for most of the recognized microfacies in this sequence. Microfacies in the upper part of the Fahliyan Formation were mainly deposited in shallow (lagoon) to deeper water (open marine), and they form the last depositional sequence of the formation (sequence C). The results of this study reveal that the lower successions mainly were deposited coincided with a sea-level fall in the basin, while the sea level shows a slight rise during deposition of the middle units of the formation. However, the prevailing microfacies related to the shallow water environment, such as lagoon setting through the uppermost parts of the Fahliyan Formation, represent a fall in sea level at that time. This study shows that the Mongasht section (center of the study area) with microfacies associated with deep-water environments was the deepest part of the basin (depocenter). Hopefully, it is attempted to make a sequence stratigraphy framework to analyze the variations of microfacies and sedimentary environment and the reasons behind these changes for this formation.

Qadir Member of Nayband Formation, located in East of Central Iran, has developed to a great extent. Investigation of the lithofacies and sedimentary environment, resulted in identification of the deltaic and marine deposits. Based on field evidence and facies features, Qadir Member consists of two lithofacies, including carbonate and siliciclastic facies. The siliciclastic facies were identified as having four sandstone facies including Sr, Sh, Sp, St, three fine-grained lithofacies, including FI, Fm, Fl (Sr) / Sr (FI) and one coal facies. Also, regarding the field, laboratory studies, and identification of lithofacies, the coastal plain, deltaic (including deltaic plain, proximal delta front, distal delta front, and prodelta) and open marine environments were identified for Qadir Member which is is under the impact of tidal currents. The chemical weathering index (71%) indicated semi-arid to semi-humid conditions and plotting the geochemical data showed the provenance of re-cycling and active continental margin and because of Chemical Index of Alteration, the weathering rate was found to be rather medium to high. The geochemical diagrams also showed a probable source of the intermediate igneous and sedimentary rocks. The active continental margin conditions for this deposit could suggest the Neotethys subduction under Iran’s plate and volcanic activity at the end of Triassic, which coincided with the early Cimmerian orogeny in Alborz and Central East Iranian Microcontinent.

Despite the previous interpretations of the evolutionary scenarios of the Qatar Arch, the present study aimed to present a new complementary scenario to lay out the probable effects of another basement fault, corresponding to the physical shape of the Arch. A remote sensing study was carried out based on geological and geophysical data interpretations, obtained from some national and international databases, including GIS-based global geology data of Harvard University, the free-air gravity anomaly data from the World Gravity Map (WGM), and the airborne geophysical project of the National Geosciences Database of Iran (NGDIR). Results revealed that offshore extrapolation of the Trans Arabian-Bostaneh Fault (TABF) lineament through the morphological boundary of Hauterivian–Barremian (HB) formations and the offshore extrapolation of the Kazerun fault could produce a new complementary model for the Qatar Arch evolution. Based on the particular shape of the Arch (i.e., narrowing and uplifting to the south influenced by the Kazerun basement fault and the rotation of TABF), we can propose that these faults crossed each other after the Barremian age, created a particular shape of the Qatar Arch, and influenced beneath the Fars hinterland. The obtained geophysical gravity data also showed a good match between structural units and faults with maximum values corresponding to the Qatar Arch and minimum values relating to the thick sediment field in the Fars region. In conclusion, the present study contributes to a new proposition about exploring the possible faults and a new gas field situated in the interior territory of Iran. All of these interpretations will provide a new insight to improve the extant knowledge in the geology and petroleum evolution of the study area, such as the Plate tectonic events and the creation of possible reservoirs in future.

Qadir Member of Nayband Formation, located in East of Central Iran, has developed to a great extent. Investigation of the lithofacies and sedimentary environment, resulted in identification of the deltaic and marine deposits. Based on field evidence and facies features, Qadir Member consists of two lithofacies, including carbonate and siliciclastic facies. The siliciclastic facies were identified as having four sandstone facies including Sr, Sh, Sp, St, three finegrained lithofacies, including FI, Fm, Fl (Sr) / Sr (FI) and one coal facies. Also, regarding the field, laboratory studies, and identification of lithofacies, the coastal plain, deltaic (including deltaic plain, proximal delta front, distal delta front, and prodelta) and open marine environments were identified for Qadir Member which is is under the impact of tidal currents. The chemical weathering index (71%) indicated semi-arid to semi-humid conditions and plotting the geochemical data showed the provenance of re-cycling and active continental margin and because of Chemical Index of Alteration, the weathering rate was found to be rather medium to high. The geochemical diagrams also showed a probable source of the intermediate igneous and sedimentary rocks. The active continental margin conditions for this deposit could suggest the Neotethys subduction under Iran’s plate and volcanic activity at the end of Triassic, which coincided with the early Cimmerian orogeny in Alborz and Central East Iranian Microcontinent. El miembro Qadir de la Formación Nayband, ubicada en el este del centro de Irán, se ha desarrollado en gran medida. La investigación de la litofacies y el medio sedimentario resultó en la identificación de los depósitos deltaicos y marinos. Basado en evidencia de campo y características de facies, Qadir Member consta de dos litofacies, incluidas facies de carbonato y siliciclástico. Se identificó que las facies siliciclásticas tenían cuatro facies de arenisca, incluidas Sr, Sh, Sp, St, tres litofacies de grano fino, incluidas FI, Fm, Fl (Sr) / Sr (FI) y una facies de carbón. Además, con respecto al campo, los estudios de laboratorio y la identificación de litofacies, se identificaron la llanura costera, deltaica (incluida la llanura deltaica, frente deltaico proximal, frente delta distal y prodelta) y ambientes marinos abiertos para el Miembro Qadir que se encuentra bajo el impacto. de las corrientes de marea. El índice de meteorización química (71%) indicó condiciones semiáridas a semihúmedas y el trazado de los datos geoquímicos mostró la procedencia del reciclaje y el margen continental activo y, debido al índice químico de alteración, la tasa de meteorización resultó ser bastante media. en lo alto. Los diagramas geoquímicos también mostraron una fuente probable de rocas ígneas y sedimentarias intermedias. Las condiciones del margen continental activo para este depósito podrían sugerir la subducción de Neotethys bajo la placa de Irán y la actividad volcánica al final del Triásico, que coincidió con la orogenia cimeria temprana en Alborz y el microcontinente iraní centro-este.