Fracture characterization of Asmari Formation carbonate reservoirs in G Oilfield, Zagros Basin, Middle East

The Asmari Formation in the G oilfield on the Iran-Iraq border is a fractured-porous multi-lithology mixed reservoir, for which fracture is an important factor affecting oil productivity and water cut. The characterization and modeling of fractures in the carbonate reservoir of G oilfield are challenging due to weak conventional well log responses of fractures and a lack of specific logs, such as image logs. This study proposes an integrated approach for characterizing and modeling fractures in the carbonate reservoir. The features, formation mechanism, influencing factors, and prediction methods of fractures in the Asmari Formation carbonate reservoirs of G oilfield were studied using core observation, thin section, image log, cross-dipole acoustic log (CDAL), geomechanics numerical simulation (GNS), and production data. According to CDAL-based fracture density interpretation, GNS-based fracture intensity prediction between wells, and DFN-based rock fracture properties modeling, the quantitative fracture characterization for G oilfield was realized. This research shows that the fractures in the Asamri Formation are mainly medium-to high-angle shear fractures. The substantial compression stress during the Miocene played a major role in the formation of the prominent fractures and determined their trend in the region, with primary trends of NNW-SSE and NNE-SSW. The fracture distribution has regularity, and the fractures in zone A dolomites are more highly developed than that in zone B limestones vertically. Horizontally, fractures intensity is mainly controlled by faults and structural location. The results of this study may benefit the optimization of well design during field development. From 2019 to 2021, three horizontal wells pilot tests were deployed in the fractures belt in zone A, and these fractures prominently increased the permeability of tight dolomite reservoirs. The initial production of the wells is four to five times the average production of other wells in the area, showing a good development effect. Meanwhile, the updated numerical simulation validates that the history match accuracy of water cut based on the dual-porosity model is significantly improved, proving the fracture evaluation and prediction results to be relatively reliable and applicable.

Biostratigraphy,Microfacies and Depositional Environment of the Sarvak Formation at the Pyun Anticline(Zagros Basin,Southwest of Iran)

The Sarvak Formation is a carbonate sequence of Late Albian–Early Turonian age in Zagros Basin in Iran.In this investigation,the Sarvak Formation at the Pyun Anticline section(Izeh Zone)has been studied.It consists of 797 m thick limestone which overlies the Kazhdumi Formation conformably and underlies the Gurpi Formation unconformably at the Pyun Anticline.A very rich fossil association(85 genera and 132 species)characterizes the Pyun section.Based on the stratigraphic distribution of diagnosed foraminifera,five biozones are established:1,Muricohedbergella-Globigerinelloides sp.assemblage zone(Late Albian);2,Praealveolina iberica-Chrysalidina gradata interval zone(Early Cenomanian);3,Chrysalidina gradata-Cisalveolina fraasi(fallax)and C.lehneri interval zone(Middle Cenomanian);4,C.fraasi(fallax)and C.lehneri-Praetaberina bingistani assemblage zone(Late Cenomanian);5,Nezzazatinella picardi-MangashtiaDicyclina assemblage zone(Early Turonian).Based on the petrographic and sedimentological analyses,some 13 carbonate microfacies were identified.The investigated microfacies confirm a ramp-type paleoenvironment.These microfacies,from distal to proximal environments,consist of:MF1:planktonic foraminifera wackestone-packstone;MF2:Oligostegina planktonic foraminifera wackestone to packstone;MF3:planktonic-benthic foraminifers sponge spicules wackestonepackstone;MF4:rudist floatstone;MF5:rudist rudstone;MF6:bioclast intraclast grainstone;MF7:peloid intraclast grainstone;MF8:benthic foraminifers rudist grainstone-packstone;MF9:peloid bioclast grainstone-packstone;MF10:bioclast(benthic foraminifers)wackestone-packstone;MF11:peloid bioclast packstone;MF12:dacycladacea benthic foraminifera packstone-wackestone;MF13:miliolids wackestone-packstone.

Larger Foraminiferal Biostratigraphy and Facies Analysis of the Oligocene–Miocene Asmari Formation in the Western Fars Sub-basin, Zagros Mountains, Iran

The Oligocene–Miocene carbonate record of the Zagros Mountains, known as the Asmari Formation, constitutes an important hydrocarbon reservoir in southern Iran. This marine carbonate succession, which developed under tropical conditions, is explored in terms of larger foraminiferal biostratigraphy, facies analysis and sequence stratigraphy in a new section at Papoon cropping out in the western Fars sub-basin, in the south-east of the Zagros belt. Facies analysis shows evidence of re-working and transport of skeletal components throughout the depositional system, interpreted here as a carbonate ramp. The foraminifera-based biozones identified include the Globigerina–Turborotalia cerroazulensis–Hantkenina Zone and Nummulites vascus–Nummulites fichteli Zone, both of Rupelian age, the Archaias asmaricus–Archaias hensoni–Miogypsinoides complanatus Zone of Chattian age and the ‘Indeterminate’ Zone of Aquitanian age. The vertical sedimentary evolution of the formation exhibits a progressive shallowing of the facies belts and thus the succession is interpreted as a high-rank low-order regressive systems tract. This long-lasting Rupelian–Aquitanian regressive event is in accordance with accepted global long-term eustatic curves. Accordingly, long-term eustatic trends would have been a factor controlling accommodation during the deposition of the Asmari Formation studied in the western Fars sub-basin.

Sedimentary Environment and Sequence Stratigraphy of the Asmari Formation at Khaviz Anticline, Zagros Mountains, Southwest Iran

The Oligocene-Miocene Asmari Formation is a thick sequence of shallow water carbonates of the Zagros Basin. Khaviz Anticline outcrop [near Behbahan city/Iran] was studied in this research in order to interpret the facies, depositional environment and sequence stratigraphy of the Asmari Formation succession. In this study, twelve different microfacies types have been recognized, which can be grouped into five (micro) facies associations: peritidal, lagoon, shoal, semi restricted marine and open marine. The Asmari Formation represents sedimentation on a carbonate ramp. According to the fauna data, the Asmari Formation is Oligocene (Rupelian/Chattian) to Early Miocene (Burdigalian) in age at the study area. Eight third-order depositional sequences are identified on the basis of deepening and shallowing patterns in the microfacies. The depositional sequences 0 and 1 (Rupelian-Chattian), 2, 3 and 4 (Chattian) were referred to the lower while sequences 5 and 6 (Aquitanian) were referred to the middle and sequence 7 (Burdigalian) was referred to the upper Asmari Formation. The relative sea-level curve of the Asmari basin and its matching with the global sea-level curves documented that Global eustatic phenomena affected this basin.

Impacts of Depositional Facies and Diagenesis on Reservoir Quality:A Case Study from the Rudist-bearing Sarvak Formation,Abadan Plain,SW Iran

An integrated geological-petrophysical analysis of the rudist-bearing sequence of the Cretaceous Sarvak Formation is given one giant oilfield,and provides an improved understanding of this main reservoir in the Abadan Plain,in the Zagros Basin,SW Iran.The main objective of this study is to evaluate reservoir potential of the Sarvak Formation,and then to utilize the calibrated well log signature to correlate reservoir potential in un-cored wells.Eight main facies are recognized and categorized in five facies groups:lagoon,shoal,rudist-biostrome,slope,and shallow open marine,deposited on a shelf carbonate platform.Given the distribution of diagenetic products and their effects on pore systems,three diagenetic facies namely,(DF-1)low dissolution and cementation;(DF-2);high dissolution;and(DF-3)high cementation are differentiated.The initial sedimentary characteristics in combination with distribution of diagenetic products play an important role in reservoir quality heterogeneity.The effect of diagenetic processes related to disconformities mainly depends on the facies nature below these surfaces.Grain-dominated facies of shoal and rudist debris,observed below the Cenomanian–Turonian disconformity,are mostly characterized by high dissolution and interconnected pore systems.Finally,depositional and diagenetic facies in the studied wells are correlated by petrophysical well log data,leading to distribution of the reservoir zones.Data obtained can be utilized for efficient reservoir characterization of the Sarvak Formation and its equivalent units in the Arabian Plate.

Microbiostratigraphy of Kazhdumi Formation in the Northwestern Shiraz (Southwest Iran) on the Basis of Foraminifera and Calcareous Algae

The middle Cretaceous Kazhdumi Formation,with a thickness of 222 m,belongs to the Bangestan Group and occurs in the Zagros folded zone in southwest Iran.The lower boundary with the Dariyan Formation is disconformable,which is recognized by iron oxides and glauconite.The recognized microfossils are Valvulammina sp.,Scandonea sp.,Daxia cenomana,Choffatela sp., Pseudolituonella reicheli and calcareous algae-Lithocodium aggregation（which belongs to the Sarvak Formation）,representing the beginning of Cenomanian.Other microfossils are：Ammobaculites goodlandensis,Marsonella trochus,Hemicyclammina sigali,Praechrysalidina infracretacea,Orbitolina gr.concava,Orbitolina（conicorbitolina）conica,Orbitolina subconcava,Salpingoporella sp.,Trinocladus tripolitanus,Trinocladus sp.,Permocalculus irenae and Dissocladella deserta.These microfossils can be classified into five assemblage zones.

An intelligent prediction method of fractures in tight carbonate reservoirs

An intelligent prediction method for fractures in tight carbonate reservoir has been established by upgrading single-well fracture identification and interwell fracture trend prediction with artificial intelligence,modifying construction of interwell fracture density model,and modeling fracture network and making fracture property equivalence.This method deeply mines fracture information in multi-source isomerous data of different scales to reduce uncertainties of fracture prediction.Based on conventional fracture indicating parameter method,a prediction method of single-well fractures has been worked out by using 3 kinds of artificial intelligence methods to improve fracture identification accuracy from 3 aspects,small sample classification,multi-scale nonlinear feature extraction,and decreasing variance of the prediction model.Fracture prediction by artificial intelligence using seismic attributes provides many details of inter-well fractures.It is combined with fault-related fracture information predicted by numerical simulation of reservoir geomechanics to improve inter-well fracture trend prediction.An interwell fracture density model for fracture network modeling is built by coupling single-well fracture identification and interwell fracture trend through co-sequential simulation.By taking the tight carbonate reservoir of Oligocene-Miocene AS Formation of A Oilfield in Zagros Basin of the Middle East as an example,the proposed prediction method was applied and verified.The single-well fracture identification improves over 15%compared with the conventional fracture indication parameter method in accuracy rate,and the inter-well fracture prediction improves over 25%compared with the composite seismic attribute prediction.The established fracture network model is well consistent with the fluid production index.

Determination of hydrocarbon generation potential of a non-isothermal pyrolysis of Faraghun and Sarchahan Formations in Coastal Fars and the Persian Gulf,Iran

Source rock assessment is a key step in any petroleum exploration activity.The results of Rock-Eval analysis showed that Sarchahan Formation was in the late oil window,while the Faraghun and Zakeen Formations were just in the early stages of the oil window.Furthermore,Sarchahan,Zakeen and Faraghun Formations exhibited different kerogen types(types-Ⅱ,types-Ⅲand type-Ⅲ,respectively).Refining the kinetic parameters using the OPTKIN software,the error function returned error values below 0.1,indicating accurate optimization of the kinetic parameters.Based on the obtained values of activation energy,it was clear that Sarchahan Formation contained type-Ⅱkerogen with an activation energy of 48-52 kcal/mol,while Zakeen and Faraghun Formations contained type-III kerogen with activation energies of 70-80 kcal/mol and 44-56 kcal/mol,respectively.The geographical distribution of the samples studied in this work,it was found that the organic matter(OM)quantity and quality increased as one moved toward the Coastal Fars in Sarchahan Formation.The same trend was observed as one moved from the southern coasts of Iran toward the shaly and coaly portions of Faraghun Formation in the center of the Persian Gulf.

Investigation of Main Planktonic Foraminiferal Bio-Events in Surgah Formation at Pol-e-Dokhtar Area, South Western of Iran

Surgah Formation at Male-kuh well section with 82 meters thickness has been studied at Pol-e-Dokhtar area. Surgah Formation at mentioned section was composed of limestone and shaly limestone. The lower and upper contacts of studied formation with Sarvak and Ilam formations are conformed. Based planktonic foraminifera study 37 species belonging to 10 genera in frame of two planktonic foraminiferal biozones were identified respectively as below: 1) Dicarinella concavata Interval Zone;2) Dicarinella asymetrica Total Range Zone. Finally, based on recent investigation have been detected middle Turonian-Late Santonian age for studied section. According to (Robaszynski and Caron 1979), the first occurrence of Dicarinella primitiva has been shown base of the Coniacian stage. The first occurrence of Di. primitiva was placed under the Turonian-Coniacian boundary. Robaszynski and Caron 1995 recorded the first occurrence of Dicarinella primitiva and Dicarinella concavata were simultaneous and assigned to late Turonian which correlated with ammonite’s species Subprionocyclus neptuni. In the correlation between ammonite and Dicarinella primitiva at Tethyan realm first occurrence of Di. primitiva was close to the Middle-Late Turonian sub stage. The base of Coniacian stage was identified based on the FODs of Archaeoglobigerina cretacea and Dicarinella concavata from planktonic foraminifera with FOD of Cremnoceramus walterdorfensis walterdorfensis from inoceramid group, while the base of Santonian stage recorded using LOD of Dicarinella primitiva, Whiteinellids group with FODs of Dicarinella asymetrica and Inoceramus aff. vistulensis (inoceramid species).

Indicating the Role of Geological Conditions in Shaping the Hareer Anticline, Iraqi Kurdistan Region

The Hareer anticline is within the High Folded Zone, facing southwards towards the Low Folded Zone. Both zones are part of the Outer Platform of the Arabian Plate. Moreover, both zones are part of the Zagros Fold-Thrust Belt located within the Zagros Foreland Basin. Although the Hareer anticline is a double plunging anticline oriented in a NW-SE trend, both plunges are abnormal. The northwestern plunge is gradually passing to the southwestern limb of the Pirat anticline;although there is a very shallow syncline in between them, giving a right-hand en-echelon form to the plunge. Whereas, the southeastern plunge disappears between the Kamosk anticline located southeast wards and the Shakrook anticline is located southwards. Both the Hareer and the Kamosk anticlines are thrusted over the Shakrook anticline by means of two long thrust faults that run almost parallel to the Hareer anticline. It is clearly observed that the anticline is growing northwest wards;according to the recognized geomorphological and structural features. High-Quality satellite images were interpreted to elucidate the abnormal form of the Hareer anticline. The interpreted data and different types of geomorphological features including the estimation of the rate of stream incision were checked in the field.

Formation evaluation and rock type classification of Asmari Formation based on petrophysical-petrographic data:A case study in one of super fields in Iran southwest

The Oligo-Miocene Asmari Formation is one of the most important hydrocarbon reservoirs in the Middle East.The oilfield under study is one of the largest oilfields in the Zagros basin with the Asmari Formation being the major reservoir rock.In this study,petrographic analyses,petrophysical data and neural network clustering techniques were used for identifying rock types in the Asmari reservoir.Facies analysis of the Asmari Formation in the study area has resulted in the definition of 1 clastic lithofacies and 14 carbonate microfacies types.Using petrophysical logs from 43 wells and their correlation with capillary pressure(Pc)curves,led to the recognition of 7 electrofacies(EF1-EF7).Microscopic evidence of Electrofacies group C1 and S1 show that the sedimentary facies of these electrofacies are most commonly found in restricted and shoal facies belts zone.Also,petrographic studies show that the sedimentary facies of C2,C3,C4,S2 and S3 were formed in the open marine,Lagoon,and Tidal flat facies belt zone of homoclinal ramp sedimentary environment during the Oligo-Miocene based on relative sea level changes respectively.The link between electrofacies and geological data indicated that both sedimentary and diagenetic processes controlled the reservoir quality of the Asmari Formation.Porosity,permeability and water saturation were used to estimate the reservoir quality of each electrofacies.EFs 1 and 2 with high porosity and permeability,low water saturation is considered as the best reservoir with regard to sedimentary textures(dolowackestone and dolograinstone)and the effect of diagenetic processes such as dolomitization processes.Vuggy,growth framework and interparticle porosities are major in EF-2,while the intercrystalline porosity is the major type in EF-1.EFs 3 and 4 show low values of porosity,permeability and high percentage of water saturations,which characterizes them as poor reservoir rocks.Finally,EF-5 is the only electrofacies in the siliciclastic parts of the Asmari reservoir,which is composed of rounded and well-sorted quartz grains that are slightly cemented.In sandstone electrofacies,electrofacies EF-5(S1),is the best type of sandstone reservoir rock and to move towards electrofacies EF-7(S3),will reduce reservoir quality.In carbonate electrofacies,also,electrofacies no 1,the best type of carbonate reservoir rock can be observed and move towards electrofacie number 4,lower quality of reservoir rocks is seen.

Lithostratigraphy and Biostratigraphy Based on Calcareous Nannofossils at the Late Campanian to Thanetian Transition in the Izeh Zone,Southwestern Iran(Eastern Neo-Tethys)

This study investigates the Gurpi Formation in the northeast of Izeh,southwestern Iran.In this study,59 species and 34 genera were determined.Biostratigraphy allows the identification of zones CC21 to CC26,which is equivalent to the UC15c^(TP)-UC20d^(TP) that spans the Late Campanian to Late Maastrichtian.Subsequently,the presence of NP1 to NP6,equivalent to CNP1 to CNP8,indicates that the sequence extends to Selandian.Then the Pabdeh Formation,which dates back to Thanetian,covers the sequence.Several major changes were recorded here in order of importance.The first change is the Campanian-Maastrichtian transition,which was identified based on the last occurrence of the Aspidolithus parcus constrictus.Subsequently,the Early/Late Maastrichtian boundary was determined based on the last occurrence of Reinhardtites levis.In addition,another change in the Late Maastrichtian–Early Danian is associated with an abrupt decrease in the richness of nannofossils,although a significant increase in the abundance of Early Paleocene new species has been observed along with the abundance of Thoracosphaera spp.blooms.Eventually,the final change coincides with an increase in Iridium throughout NP1.These changes may indicate changes in environmental conditions in this part of Iran during the Late Cretaceous–Paleogene transition in the eastern Neo-Tethys domain.