Sarvak Formation Research Articles

First report of Albian–Cenomanian ichthyological microremains from the Kazhdumi Formation Fars Province, Zagros Basin, South Iran

ABSTRACT Vertebrate remains are scarce in the Cretaceous of Iran, with only a few records of fishes and one genus of chelonioid turtle known from the Dariyan and Sarvak formations. Here, we describe for the first time the paleoichthyological fauna of the Kazhdumi Formation, based on disarticulated microremains that represent the first record of vertebrates in the unit, highlighting this novel Albian–Cenomanian fossil assemblage. The fauna is composed of lamniform (cf. Dallasiella sp., cf. Paranomotodon sp., Anacoracidae indet., cf. Haimirichia amonensis, cf. Scapanorhynchus, Lamniformes indet.) sharks and pycnodontiforms Osteichthyes.

The Middle-Late Permian Yakshawa and Late Cretaceous Daresard bauxite deposits, western Iran: Elemental deportment, parental affinities and genesis of REY minerals

The Middle-Late Permian Yakshawa bauxite deposit in the Sanandaj-Sirjan Zone and the Late Cretaceous Daresard bauxite deposit in the Zagros Fold-Thrust Belt have bauxite ores with different textures and mineralogical compositions. The presence of detrital and authigenic REY-phosphates, fluorocarbonates of the bastnasite group, and cerianite with diverse generations reflect an intricate formation process of the REY-bearing minerals in these two deposits. This clearly shows that the studied deposits have a multi-stage evolution including diagenetic and/or low-grade metamorphic, in situ fluid-assisted dissolution–reprecipitation reactions, and later bauxitization. The high abundance of monazite, textural relationships, and its LREE pattern indicate this mineral is the main LREE reservoir in the studied bauxite ores whereas xenotime, both authigenic and detrital, is the main host for HREE. The R-mode factor analyses indicates that the bauxitization process induces, at large, similar interelemental relationships in the studied deposits involving the attitude of Al oxyhydroxides, Ti oxides, and Fe oxyhydroxides to concentrate critical metals and the capability of REY-phosphate of being the main reservoir of REE + Y. As for provenance, the Eu/Eu*, Sm/Nd, and Nb/Ta proxies suggest that the bauxite ores of the Yakshawa deposit have a genetic relationship with Middle-Late Permian mafic volcanic rocks, whereas the ores of the Daresard deposit show parental affinity with the underlying fine-grained clayey limestone of the Sarvak Formation.

Controls of tectonics and paleoclimate on depositional–diagenetic evolution and pore types of Upper Cretaceous successions (Sarvak Formation) in the Abadan plain, Iran

This study examines the influence of tectonics and paleoclimate on the diagenetic evolution of Upper Cretaceous carbonate reservoirs (Sarvak Formation) in the Abadan Plain, Zagros Basin, Iran. Through petrographic analysis and scanning electron microscopy, the sedimentary facies, diagenetic processes, and pore system within the Sarvak Formation were analyzed. Facies analysis identified eight microfacies, which were deposited in shallow to deep settings of a carbonate ramp. Among these, the grain-dominated facies (packstone and grainstone) of shoal complexes and reef-talus settings were identified as the most important reservoir facies. The formation's complex diagenetic history includes marine, meteoric, and burial diagenetic processes, with meteoric diagenesis playing a significant role beneath paleoexposure surfaces. The Sarvak Formation was categorized into eight flow units, seven pore size classes, eleven electrofacies, eight velocity deviation zones, and thirteen Lorenz zones. The study's findings indicate that the most favorable reservoir units are located beneath the Cenomanian–Turonian exposure surface, characterized by significant dissolution of rudist-dominated facies due to meteoric processes and karstification. Tectonic uplift, along with weathering and erosion within a warm and humid (tropical) paleoclimate, significantly impacts the diagenetic evolution, pore types, and petrophysical properties of the Sarvak Formation. This research underscores the complex interplay between tectonics, paleoclimate, and petrophysical properties in the Abadan Plain, enhancing our understanding of carbonate systems in tectonically active and paleoclimatically dynamic regions.

Comparative evaluation of pore volume compressibility of carbonate reservoir rocks: different laboratory measurement approaches

The pore volume compressibility (C PV) of a reservoir plays an essential role in estimating the reserve oil and recovery factor. Therefore, selecting an appropriate laboratory measurement approach for determining the C PV is crucial. This study aims to compare three different routine approaches to C PV measurement. Fifty-eight carbonate core plug samples with diverse porosity from the Sarvak Formation in the Southwest of Iran were selected. The measurements were performed at different saturation, pressure, and temperature conditions in triaxial core holders. Results show that samples with the lowest porosity have the highest compressibility value, and samples with the highest porosity have the lowest value. Furthermore, the compressibility measured on brine-saturated cores at pressure and temperature conditions was higher than only brine-saturated, and consequently higher than dry core plugs. Hence, the measurement methods and approaches severely affect the resultant compressibility. The values obtained at representative reservoir conditions are advised to be utilized in simulation and modeling studies. A novel reciprocal model was also proposed to appropriately correlate the data with porosity. The results of this work give insight into the selection of the best laboratory measurement approach to estimate the actual C PV value in reservoir conditions, especially at varying pore pressure and temperature.

Estimation of pore-type distribution utilizing petrophysical data and rock physics modeling on an Iranian carbonate reservoir

Pore types in carbonate reservoirs are more complex than their sandstone counterparts due to the wide spectrum of their depositional environments and their more complicated post-depositional processes. This means that a good knowledge of pore types is vital in determining carbonate formation’s elastic and reservoir properties. This study aims to develop a multi-physics approach to determine pore-type variations in a carbonate reservoir using well log information from one of the oilfields of the Abadan Plain in southwest Iran. Firstly, we determined lithology, porosity, and fluid content by interpreting conventional well logs (gamma ray, resistivity, density, neutron, photoelectric, and P-wave sonic). Then, nuclear magnetic resonance data were used to determine different pore types within the Main Ilam and Sarvak formations. We distinguished between clay, micro, meso, and macro pore types. We confirmed our interpretation results using thin sections, scanning electron microscopy photographs, and pore-size distribution on the available core plugs. Finally, a carbonate rock physics model was employed to model sonic velocities using petrophysical interpretation along with pore-type determination results. A good match between modeled and measured sonic velocities confirmed that using nuclear magnetic resonance data for pore-type determination can reasonably estimate pore-type variations needed for rock physics modeling. The standard industry procedure for carbonate rock physics modeling uses sonic logs, core data, or thin sections to determine pore types. We offer a substitute approach with reasonable accuracy for pore-type modeling needed for carbonate rock physics modeling. We modeled pore types independently from sonic velocity and used them to predict P-wave velocity with a correlation coefficient of 92 and 64 percent accuracy in the Main Ilam and Sarvak formations.Graphical abstract

Diagenetic evolution of Upper Cretaceous carbonate sequences (Sarvak Formation) in the Abadan Plain, Iran: evidence from petrographic, elemental, stable, and strontium isotopic data

ABSTRACT This study investigates the diagenetic evolution and geochemical variations within the Sarvak Formation by integrating the core, thin-section, scanning electron microscopy, X-ray diffraction, trace elements, carbon-oxygen, and strontium isotopic data from three wells in the Abadan Plain. The formation displays diverse diagenetic processes, influenced by multiple exposure surface condition including the Cenomanian-Turonian boundary (CT-ES) and middle Turonian (mT-ES). Meteoric diagenesis, shaped by tectonic activities and eustatic sea level changes, played a pivotal role, leading to dissolution (karstification), brecciation, cementation, and palaeosol formation. Geochemical analysis revealed significant elemental variations across the studied wells. Magnesium (Mg) exhibited peaks at sequence boundaries, responding to facies changes, while iron (Fe) and manganese (Mn) concentrations increased below disconformities, reflecting changes in redox conditions and karstification. Sodium (Na) concentrations varied with facies changes, showing peaks at sequence boundaries, indicative of palaeoexposure events. Strontium (Sr) isotopes aided in delineating stratigraphic positions of palaeoexposure surfaces and absolute dating of sequences. Increases in87Sr/86Sr ratios below disconformities suggested subaerial exposure events, with shorter durations at CT-ES and longer durations at mT-ES, contributing to dissolution features development. The δ13Ccarb and δ18Ocarb values indicated severe meteoric diagenesis below sequence boundaries, characterized by dissolution and reprecipitation of calcium carbonate as low-magnesium calcite (LMC) cements. Cross plots of δ13Ccarb vs. δ18Ocarb displayed inverted-J patterns, indicative of meteoric diagenesis effects. Comparison with previous studies highlighted similar trends in geochemical variations, with notable differences in element concentrations between wells. This study enhances our understanding of diagenetic history of the Sarvak Formation in the Abadan Plain and, in a larger scale, provides insights into diagenetic evolution of palaeoexposure-related carbonate sequences.

Determination of Reservoir Rock Type in Sarvak Reservoir of an Iranian Oilfield

Integrated reservoir rock typing in carbonate reservoirs is a significant step in reservoir modelling. The key purpose of this study is the identification of integrated rock types in the Sarvak Formation of an Iranian oilfield. In this study, electrofacies (EFAC) analysis of the Sarvak reservoir was done in detail to determine the reservoir quality and rock types of the Sarvak Formation in the studied field. The core data and conventional petrophysical logs were used for rock typing. Some petrophysical logs such as porosity, sonic, neutron, density, and Photo electric factor were applied as input data for electrofacies analysis. Multi-Resolution Graph-Based Clustering was used among six approaches, resulting in four electrofacies after merging similar clusters. EFAC-1 has the lowest porosity, and EFAC-4 has the highest porosity. In addition, based on Winland and Amaefule approaches, three rock types were determined using core data (porosity and permeability). As a result, three rock types were defined; rock type-1 has the smallest pore-throat size and flow zone indicator while rock type-3 has the highest ones. The correlation coefficient between permeability and porosity in each rock type is more than 80%. Rock type 1 is mainly composed of EFAC-1 and EFAC-2, while rock types 2 and 3 share mostly the EFACs 1, 3 and 4.

The mineralogy and geochemistry of the Pirashkaft bauxite, Zagros belt, Iran: New inferences on the paleo-climate at the time of formation and on the nature of the parent rock

The Pirashkaft karst type bauxite is located in north of the Paskuhak anticline, Pirashkaft region, in the Fars province (Iran). The deposit is composed of six bauxite horizons, occurring between upper Cretaceous Sarvak and Gurpy Formations, and formed in the late Turonian. Mineralogical composition includes boehmite, hematite, diaspore, kaolinite, goethite, anatase and minor minerals like, gibbsite, spinel, rutile, and kaolinite. The major oxides in the bauxite horizons mainly comprise Al2O3 (46.93–61.34 wt%), SiO2 (5.78–14.60 wt%), Fe2O3 (11.22–28.00 wt%), and TiO2 (1.66–3.03 wt%). The bauxite has a pisolitic–oolitic texture. Pisoids are mainly composed of large Al-bearing hematite cores, surrounded by layers of gibbsite or boehmite. This structure suggests that bauxite formed under a humid climatic regime, punctuated by periodic arid periods. In the Pirashkaft deposit, LREEs vary between 136.49 ppm and 928.56 ppm, whilst HREEs range from 16.48 ppm to 59.5 ppm. Positive anomalies of Ce (Ce/Ce* between 1.08 and 2.36) indicate dominant oxidizing conditions during the bauxitization process. The Eu/Eu* anomalies of the bauxite samples vary between 0.44 and 0.92, this suggesting that parent material likely derived from two geochemically distinct source rocks. Stratigraphical, mineralogical, and geochemical evidences, such as the REE distribution between the bauxitic horizons and the Sarvak marly limestone, suggest that the underlying marly carbonates were one of the sources of the Pirashkaft bauxite.

First record of Reticulinella? kaeveri in the Laffan and Ilam formations (Zagros fold and thrust belt, SW Iran): new constraints on the chronostratigraphic calibration of the Arabian Plate sequence stratigraphy

Over the past two decades, sequence stratigraphy has proven to be a valuable tool in addressing challenges arising from the inconsistent application of lithostratigraphic nomenclature across diverse geological provinces. It has been particularly successful in achieving regional correlations within the Cretaceous deposits of the Arabian Plate. The key sequence stratigraphic surface in the currently adopted standard sequence stratigraphic scheme is the K150 SB, marking the transition from a passive to an active margin along the eastern edge of the Arabian plate. Despite its importance, the chronostratigraphic age of this surface is poorly constrained. In the Zagros FTB, the uppermost part of the Sarvak Formation, just below the K150 SB, was recently assigned to the upper part of middle Turonian to upper Turonian, based on the presence of Reticulinella? kaeveri. Our research reveals that the range of R.? kaeveri extends above the K150 SB as this species is also present in the Laffan and base of Ilam formations. The Last Occurrence of this taxon provides a new chronostratigraphic marker of the upper Turonian within the Ilam Formation. This finding proves that more Turonian deposit are present in the study area than was previously recognized. We have studied the deposits enclosing the K150 SB in outcrop sections and wells along a transect from basin to platform margin to inner platform setting. Based on planktonic foraminifera and calcareous nannofossils biostratigraphy of basinal sections and sequence stratigraphic correlation to carbonate platform sections, the age of the uppermost part of the Sarvak Formation, immediately below the K150 SB, is firmly constrained within the interval lower Turonian–lower part of middle Turonian. Sequence stratigraphic correlation from basin to platform interior reveals also that the oldest deposits above the K150 SB are not younger than late Turonian. Our new data represents a significant improvement in the chronostratigraphic calibration of sequence stratigraphy in the Cretaceous of the Zagros fold-and-thrust belt.

Maximum entropy spectral analysis of gamma ray logs for cyclostratigraphic analysis of the Late Albian- Early Turonian Sarvak Formation in the Anaran exploration block, southwestern Zagros, Iran

Maximum entropy spectral analysis of gamma ray logs for cyclostratigraphic analysis of the Late Albian- Early Turonian Sarvak Formation in the Anaran exploration block, southwestern Zagros, Iran

Pore-Type Characterization and Reservoir Zonation of the Sarvak Formation in the Abadan Plain, Zagros Basin, Iran

Carbonate reservoirs are characterized by their complex depositional–diagenetic history and, consequently, intricate pore networks. In this study, pore types and reservoir zones of the Sarvak Formation, which is the second most important oil reservoir in Iran, are investigated by integrating core, thin-section, porosity–permeability, scanning electron microscopy (SEM), and mercury injection capillary pressure (MICP) data. Depositional, diagenetic, and fracture pores are identified in macroscopic to microscopic scales. Frequency analysis of pore types revealed that diagenetic pores, including vuggy, moldic, intercrystalline, and fracture pores, formed the majority of pore spaces of the Sarvak Formation. They are dominantly recorded in the regressive systems tracts of both the Cenomanian and Turonian sequences, just below the paleoexposure surfaces. Away from the disconformities, there are two general trends regarding the pore-types distribution: (a) grain-supported facies of shoal and talus settings with dominant primary (depositional) pores including interparticle, intra-skeletal, and growth-framework pores, which are concentrated in the middle part of the formation, and (b) microporous mud-dominated facies with various types of primary and secondary pore types in the TSTs of third-order sequences, especially in the lower part of the Sarvak Formation. Stratigraphic modified Lorenz plot differentiated reservoir, speed, barrier, and baffle zones in this formation. Depositional–diagenetic characters, petrophysical properties, and dominant pore types are defined for each zone in a sequence stratigraphic framework.

Effects of diagenetic processes on the velocity evolution in carbonate reservoirs

Carbonate rocks are geologically complex due to the diagenetic processes they experience before and after lithification. Diagenetic processes alter their matrix and pore structure leading to the modification in their sonic velocities. Understanding the effect of these diagenetic features on the seismic velocities is crucial to have a reliable image of the subsurface. The dataset used in this study comprises well logs, and core data. Core data were analyzed using different methods (i.e., thin section analysis, X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM images)) to investigate the presence or absence of different diagenetic processes in each depth interval of the Sarvak formation. In order to minimize porosity effects on velocity variations, we divided all porosity data into five equal porosity classes and performed bar chart analysis in each class. The results indicated that bioturbation (through stiff pore creation and infilling with stiff minerals) and compaction (through pore space volume reduction) increase velocities, but dissolution increased velocities only for the low porosity samples (through moldic and vuggy pores creation) while reduced it in the high porosity samples (through the interconnection of the isolated pores). Furthermore, porosity enhancement (through increasing pore space volume), micritization (through porosity reduction inhibition during compaction), open fracture (through creation of soft pores and cracks), and neomorphism (through the creation of microporosity during compaction) reduce sonic velocities.

Deposition, Diagenesis, and Geochemistry of Upper Cretaceous Carbonates (Sarvak Formation) in the Zagros Basin and the Persian Gulf, Iran

Upper Cretaceous carbonate rocks of the Sarvak Formation are among the most important hydrocarbon reservoirs of the Middle East. This paper summarizes the depositional history, diagenetic alteration, and geochemistry of this formation in south and southwest Iran. The combined effects of tectonic activities and the paleoclimate had remarkable control over the depositional–diagenetic evolution of this formation. Large changes in facies types, their frequency, and thickness were recorded across the study area and attributed to the tectonic activities. The complex diagenetic history of this formation includes minor marine, intensive meteoric, and burial diagenesis. Two pronounced paleoexposure events were recorded at the Cenomanian–Turonian boundary (CT-ES) and the middle Turonian (mT-ES), which are associated with remarkable changes in facies and diagenetic characteristics and special geochemical trends. Elemental (Mg, Fe, Rb, Mn, Sr), isotopic (carbon, oxygen), and 87Sr/86Sr ratios provide useful tools for delineating the paleoexposure surfaces, their associated diagenetic alterations, and chemo-sequence stratigraphic interpretation. Variable exposure durations resulted in immature to over-matured karstified profiles below the CT-ES and mT-ES, respectively. Meteorically dissolved rudist-dominated facies provided the best reservoir zone of the Sarvak Fm. in the regressive systems tract of the Cenomanian sequence, just below the CT-ES. Microporous and dolomitized mud-dominated facies formed the second important reservoir intervals, especially within the transgressive systems tracts. The results of this study revealed that the reservoir characteristics of the Sarvak Fm. were considerably controlled by paleoexposure events and, therefore, it can be considered as a typical paleokarst hydrocarbon reservoir in Iran.

The Reticulinella?-Mangashtia foraminiferal association: Characterisation of the (upper?) middle-?lower upper Turonian interval in the Sarvak Formation of SW Iran and its bearing upon Upper Cretaceous Arabian Plate sequence stratigraphy

During the mid-Cretaceous the Arabian Plate underwent a major tectonically-driven stratigraphic reorganization, with the development of a major regional unconformity, often ascribed to the middle Turonian. However, evidence for the age calibration of this unconformity (e.g., from biostratigraphy) is limited. A previously undescribed assemblage of larger benthic foraminifera has been discovered in a section at Khormuj in the Coastal Fars region of the Iranian Zagros. This section lies in the uppermost Sarvak Formation, directly beneath the major unconformity surface. Dominant components of the assemblage are Mangashtia viennoti Henson, and Reticulinella? kaeveri Cherchi, Radoičić and Schroeder. Their presence places the uppermost Sarvak Formation at this locality in the age range (upper?) middle – ?lower upper Turonian, and in the context of evidence from other localities in the region it is most likely upper middle Turonian. This provides new constraint on the timing of the unconformity and stratigraphic organisation, and the tectonic events leading to its creation (e.g., forebulge creation related to ophiolite obduction). The presence of R.? kaeveri is reported for the first time from outside of the Mediterranean region and indicates that this taxon is useful for biostratigraphic calibration. As a result, biozonation schemes for the upper Sarvak Formation can be updated.

UPPER CRETACEOUS FORAMINIFERA MURGEINA APULA (LUPERTO SINNI, 1968): A METHUSALEM AND CENOMANIAN-TURONIAN BOUNDARY SURVIVOR TAXON

The benthic foraminifera Murgeina apula (Luperto Sinni), originally described as Nummofallotia apula from the Upper Cretaceous of southern Italy, represents a common taxon in the middle-upper Cenomanian Sarvak Formation of southwestern Iran as well as the Campanian-early Maastrichtian of Croatia. The specific identity of the Iranian and Italian as well as Croatian specimens is confirmed by equivalent biometric data and identical test structure. The agglutinated wall (inner part) of M. apula excludes its assignment to the porcelaneous Nummofallotia Barrier & Neumann. Nummofallotia is currently placed in the family Meandropsinidae Henson, while for Murgeina with its bilamellar wall inclusion in the Ventrolaminidae Weynschenk is favoured. Nummofallotia cenomana Luperto Sinnni, described from the Cenomanian of southern Italy and lacking any hard facts for species discrimination (e.g., dimensions, inner structure) is herein considered tentatively a junior synonym of Murgeina apula requiring re-examination of the type-material. M. apula represents another survivor of the Cenomanian–Turonian boundary extinction event and, with a long stratigraphic range from the middle Cenomanian to the (early) Maastrichtian, is similar to another Methusalem taxon such as Moncharmontia apenninica (De Castro) with a comparable range.

Geochemistry of Upper Cretaceous bauxite deposits, Zagros Fold Thrust Belt, SW Iran: Paleoenvironment and provenance constraints

Iranian's karst bauxites, as part of the Irano-Himalayan bauxite belt, are related to four structural zones, including the Zagros Fold-Thrust Belt (ZFTB). The ZFTB consists of 7–12 km of the Paleozoic to Pliocene carbonate sediments deposited on the Precambrian basement and Arabian continental margin. In the ZFTB occur several karst bauxite deposits between the Cenomanian-Turonian shallow-marine argillaceous of the Sarvak Formation and the Santonian-Campanian carbonates of the Ilam Formation. Compositional data of karst bauxite deposits may address paleogeographic restoration and record paleoclimate, and here we discuss a dataset including the Mombi, Bidgol, Tang-e Pirzal, and Bagoushi bauxite deposits in the ZFTB. Although the ZFTB karst bauxites show similar texture, they are different in composition, reflecting different paleoenvironmental formation conditions. In most of the studied bauxites, the composition of ooidal concretions indicates the occurrence of long periods of humid tropical climate followed by short and intermittent dry-to-wet conditions. In the Bidgol deposit, a different pattern is recognized, reflecting the predominance of dry climates and characterized by low aqueous silica activity and low water activity. The formation of iron oxyhydroxide minerals is likely due to a multistage process involving the circulation of Fe-rich solutions in micro-cracks or veins due to pyrite and chamosite dissolution and Fe2+ oxidization. In the ZFTB bauxites, two main trends of Ce/Ce* distribution with depth, a proxy of paleo-redox evolution during bauxitization, are recognized. In the Bidgol and Mombi deposits, Ce/Ce* is negative in the basal portion and increases upward. The positive Ce-anomalies in the upper parts of these deposits are due to ceranite formation under oxidizing environment near the surface. In the Bagoushi and Tang-e Pirzal deposits, Ce/Ce* is close to the unit with the notable exceptions of a few samples characterized by significant and positive Ce/Ce* anomalies. This feature suggests that cerianite, originally ubiquitous in these deposits, was affected by congruent dissolution through cerium reduction with Ce3+ transportation by acidic downward percolating solutions as fluoride complexes (REEF2+ and REEF2+), carbonate complexes (REECO3+) and carbonate–fluoride complexes (REECO3F) followed by fluorocarbonate minerals' precipitation. This process has been repeated several times with maximum distribution throughout the profile due to environmental changes, including periodic fluctuations in the groundwater level. Finally, the Sm/Nd, Nb/Ta, and Eu/Eu* provenance proxies indicate that the protolith(s) from which the ZFTB bauxite deposits are derived is UCC-like, thus excluding supply of mafic material and pointing toward a source material primarily derived from the bedrock argillaceous sediments. In addition, several samples record provenance from siliciclastic craton-derived sediment, suggesting that the ZFTB was connected with a continental margin during the Jurassic-early Cretaceous.

Geochemical characteristics and oil–oil correlation of the upper Cretaceous oils from the Iranian part of the Persian Gulf Basin

The Cenomanian Sarvak oil reservoirs are distributed over large areas of the Persian Gulf basin. The purpose of this study is analyzing the geochemical characteristics of the Sarvak oil reservoirs and their inter-relationships in the Persian Gulf, classification of the Sarvak oil samples and investigation of the possible causes of the genetic difference in oil families. In the previous studies, limited samples of Sarvak oil reservoir in scarce oilfields were studied individually and local interpretations are made accordingly. The current study employs a more complete set of geochemical from the Iranian part of Persian Gulf and regional interpretations are drawn. To achieve this goal, the geochemical data of 41 oil samples from 10 oilfields were collected and assessed based on gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), and stable carbon isotope analysis. It was demonstrated with the evaluations that the oils accumulated in the Upper Cretaceous Sarvak reservoirs in the Persian Gulf basin originate from different source rocks. The oil samples are genetically classified into four oil families based on the similarities and differences of parameters related to the depositional environment-dependent parameters using hierarchical cluster analysis (HCA), star diagram, and stable carbon isotope diagram. The source rocks were mainly deposited in anoxic marine carbonate environments. The thermal maturity of the oils was evaluated using steranes and trisnorhopanes. Oil families 2 and 4 (located at the center of the Persian Gulf) have the highest thermal maturity compared to the other samples; in contrast, oil family 1 (located at the west of the Persian Gulf) has the lowest thermal maturity. The possible source rocks of oil family 1 and family 3 (located at eastern Persian Gulf) using C28/C29 steranes are the upper Cretaceous successions; in contrast, the possible source rocks of oil families 2 and 4 are the upper Triassic and Lower Jurassic formations. Results of the study show a high similarity between the oils of families 2 and 4, which are located at the center of the Persian Gulf; these results also recognize the significant difference between these two oil families and oil families 1 and 3. The significant issue raised in this study is to find out the reason for this difference. The structural analysis of the central Persian Gulf shows that huge vertical faults created by salt domes intrusion provided the migration pathways for trapping oil in Sarvak reservoirs. The oils of the Surmeh, Fahliyan, and Dariyan reservoirs have migrated upwards through these faults to be trapped in the Sarvak Formation across the central Persian Gulf.

Depositional and diagenetic controls on reservoir properties along the shallow-marine carbonates of the Sarvak Formation, Zagros Basin: Petrographic, petrophysical, and geochemical evidence

As the second important oil reservoir of Iran, shallow-marine carbonates of the Sarvak Formation were deposited in a tectonically active setting, under a warm and humid paleoclimatic condition. This study focuses on sedimentological, geochemical, and petrophysical analysis of this formation in the Abadan Plain, Zagros Basin. Petrographic studies have resulted in the recognition of five microfacies deposited in lagoon, patch-reefs and their taluses, shoal, and open marine environments. They represent a ramp-like depositional model for the Sarvak Formation. Two third-order depositional sequences are differentiated and dated as Cenomanian–Turonian, according to the results of biostratigraphic analysis. Effects of intensive meteoric diagenesis are recorded beneath the two paleoexposure surfaces (Cenomanian–Turonian boundary, and middle Turonian). They include pervasive dissolution (karstification), collapsed brecciation, silicification, recrystallization, and development of paleosols. Additionally, decreased values of δ18Ocarb and δ13Ccarb and a sharp inverted-J pattern in the δ18Ocarb vs. δ13Ccarb cross plot indicate major meteoric phases. The most decreased values of δ18Ocarb (−7 ‰) and δ13Ccarb (−8.6 ‰) are measured from the meteoric cements and paleosols. Reservoir quality evaluation revealed that the best reservoir zones of the Sarvak Formation are developed within the rudist-dominated facies with intensive meteoric dissolution. They are concentrated within the regressive systems tract of the Cenomanian sequence. Fractured and dolomitized mud-dominated facies provided the speed/baffle zones in transgressive systems tracts, and highly compacted/cemented facies formed the barrier units. It could be concluded that the facies characteristics and meteoric diagenetic processes had major controls on reservoir quality of the Sarvak Formation, in the Abadan Plain. These controlling factors are predictable in a high-resolution sequence stratigraphic framework.

Assessment of linking diagenesis history to sequence stratigraphy evidences (systems tract), (a case study: Sarvak Formation in one of the super oil fields, SW, IRAN)

Assessment of linking diagenesis history to sequence stratigraphy evidences (systems tract), (a case study: Sarvak Formation in one of the super oil fields, SW, IRAN)

Deep dive into net pay layers: An in-depth study in Abadan Plain, South Iran

This study focuses on the intricate process of discerning productive layers within reservoir formations, taking the Sarvak Formation as a primary case. The employed methodology combines geology, comprehensive log interpretations, and petrophysical analyses to facilitate the evaluation of these productive layers. Initially, routine well logs are interpreted to determine key petrophysical parameters such as shale content, porosity, and water saturation. Subsequently, core measurement results are utilized for calibrating these log interpretations. The study further determines cut-off values through a calculated method of 5% cumulative hydrocarbon volume against porosity, shale content, and water saturation. These cut-off values are then applied to the petrophysical results to enhance their reliability. To resolve any inconsistencies or uncertainties in petrophysical evaluation, petrographical analyses, including scanning electron microscope imaging and thin section studies, are employed. The Sarvak Formation is categorized into seven distinct subzones, each thoroughly investigated to ascertain their respective productivity potential. The final results illustrate a substantial heterogeneity within the Sarvak Formation, revealing a range of diagenetic processes including compaction, dissolution, and cementation. Despite this complexity, three subzones are identified as the most productive layers with the maximum net pay, demonstrating the efficacy of the integrated approach.