Pabdeh Formation Research Articles

PETROLEUM GEOLOGY OF THE CENOZOIC SUCCESSION IN THE ZAGROS OF SW IRAN: A SEQUENCE STRATIGRAPHIC APPROACH

The Cenozoic stratigraphy of the Zagros records the ongoing collision between the Arabian and Eurasian Plates and the closure of NeoTethys. A Paleogene NW‐SE trending foreland basin was inherited from a Late Cretaceous precursor. Widespread progradation into the foredeep was a feature of both margins which, allied to ongoing tectonism, had by the late Eocene led to the narrowing and subsequent division of the foredeep into the Lurestan – Khuzestan and Lengeh Troughs, separated by the northward continuation of the rejuvenated Qatar‐Fars Arch. This sub‐division strongly influenced subsequent deposition and the petroleum geology of the area. In addition, the diachronous nature of the Arabian – Eurasian collision led to strong diachroneity in lithostratigraphic units along the length of the Zagros. Hence its petroleum geology is best understood within a regional sequence stratigraphic framework. This study identifies three tectono‐megasequences (TMS 10, TMS 11a, TMS 11b) and multiple depositional sequences.The Cenozoic contains a world class hydrocarbon province with prolific oil reservoirs in the Oligo‐Miocene Asmari Formation sealed by the evaporite‐dominated Gachsaran Formation, mostly contained within giant NW‐SE trending “whaleback” anticlines concentrated in the Dezful Embayment. Reservoirs in the SW are dominantly siliciclastic or comprise mixed siliciclastics and carbonates, whereas those to the east and NE are dominated by fractured carbonates. There remains untested potential in stratigraphic traps, especially in deeperwater sandstone reservoirs deposited along the SW margin of the foredeep.Late Miocene to Pliocene charge to the Asmari reservoirs was mostly from Aptian – Albian Kazhdumi Formation source rocks. In some fields, an additional component was from organic‐rich late Eocene to earliest Oligocene Pabdeh Formation source rocks confined to the narrowing Lurestan – Khuzestan Trough. Where mature, the latter source rock is also a potential unconventional reservoir target, although the prospective area is limited due to recent uplift and erosion. Deeper Jurassic source rocks contributed to the Cheshmeh Khush field in Dezful North. Silurian source rocks charged gas‐bearing structures in the Bandar Abbas region.

Biostratigraphy of the Upper Cretaceous Gurpi Formation: Enhancing correlation potential by identification of abundance events of planktonic foraminifera as auxiliary indicators

The Upper Cretaceous Gurpi Formation represents a pelagic interval composed of shale and marly limestone rich in planktonic foraminifera. This study focuses on the analysis of planktonic foraminifera from the Gurpi Formation in the Zard-Shehneh Anticline section in southwestern Iran. The formation is of significant importance in parts of the Zagros foreland basin, as it acts as both a cap rock and a source rock. The presence of the Cretaceous/Paleogene (K/Pg) boundary at the upper boundary of this formation further emphasizes the significance of studying it. Therefore, the study of its biozonation and bioevents is crucial. Through the recovery of planktonic foraminifera from 125 samples in thin sections and washed residues, 30 genera, 88 species, and 10 biozones were identified for the Gurpi Formation, as well as 7 genera, 12 species, and one biozone for the base of the overlying Pabdeh Formation. In addition, high accuracy was achieved in the placement of bioevents, including: 1) the occurrence of Globotruncana linneiana at the base of the Gurpi Formation, indicating the Santonian age; 2) the highest occurrences of Dicarinella asymetrica and most species of marginotruncanids, marking the Santonian/Campanian boundary; 3) the earlier occurrence of Globotruncana aegyptiaca; 4) the occurrence of the Campanian/Maastrichtian boundary in the lower part of Gansserina gansseri Zone; 5) the occurrences of five abundance events including: V. eggeri in the upper half of the Dicarinella asymetrica Zone, benthic foraminifera in the middle to upper half of the Radotruncana calcarata Zone, G. linneiana, G. lapparenti, and G. esnehensis in the lower part of Globotruncana falsostuarti Zone, G. esnehensis in the upper part of Globotruncana falsostuarti Zone, and G. gansseri at the end of the Pseudoguembelina hariaensis Zone; and 6) the disappearance of most keeled planktonic foraminifera species during the late Maastrichtian, starting from the middle parts of the Abathomphalus mayaroensis Zone. In this study, the K/Pg boundary, which is considered the upper boundary of the Gurpi Formation, exhibits unconformity and condensation. The high-resolution biozonation and bioevents framework not only allows for accurate dating of the Gurpi Formation in the Zard-Shehneh section, but also enhances the ability to precisely correlate sedimentary sequences and paleoenvironmental intervals at regional or global scales in future studies.

Effect of thermal maturation and organic matter content on oil shale fracturing

The Pabdeh Formation represents organic matter enrichment in some oil fields, which can be considered a source rock. This study is based on the Rock–Eval, Iatroscan, and electron microscopy imaging results before and after heating the samples. We discovered this immature shale that undergoes burial and diagenesis, in which organic matter is converted into hydrocarbons. Primary migration is the process that transports hydrocarbons in the source rock. We investigated this phenomenon by developing a model that simulates hydrocarbon generation and fluid pressure during kerogen-to-hydrocarbon conversion. Microfractures initially formed at the tip/edge of kerogen and were filled with hydrocarbons, but as catagenesis progressed, the pressure caused by the volume increase of kerogen decreased due to hydrocarbon release. The transformation of solid kerogen into low-density bitumen/oil increased the pressure, leading to the development of damage zones in the source rock. The Pabdeh Formation’s small porethroats hindered effective expulsion, causing an increase in pore fluid pressure inside the initial microfractures. The stress accumulated due to hydrocarbon production, reaching the rock’s fracture strength, further contributed to damage zone development. During the expansion process, microfractures preferentially grew in low-strength pathways such as lithology changes, laminae boundaries, and pre-existing microfractures. When the porous pressure created by each kerogen overlapped, individual microfractures interconnected, forming a network of microfractures within the source rock. This research sheds light on the complex interplay between temperature, hydrocarbon generation, and the development of expulsion fractures in the Pabdeh Formation, providing valuable insights for understanding and optimizing hydrocarbon extraction in similar geological settings.

Calcareous nannofossil distribution in the upper Maastrichtian–lower Thanetian interval in the Izeh zone (Southwest Iran): biostratigraphic framework and stage boundary identification in the Eastern Tethys

The distribution of calcareous nannofossil species in the upper part ofGurpi Formation and lower part of Pabdeh Formation has been investigated in the Karta section (northwest of Izeh in Zagros Basin, Iran, Eastern Tethys) to provide a biostratigraphic framework from Maastrichtian to Thanetian interval. In this study, 31 genera and 52 species of calcareous nannofossils have been identified and based on the recognized bioevents and the resulting biozones, the studied interval spans from the top of the nannofossil zone UC19TP to the base of the NP7 zone and covers ~10.8Myr. In this study, the Gurpi Formation spans from the upper UC19TP to NP4 zones, while the lowest part of Pabdeh Formation covers NP5, NP6 and NP7 zones. The studied sequence is continuous and encompasses Cretaceous-Paleogene (K/Pg), Danian/Selandian and Selandian/Thanetian boundaries. The K/Pg boundary is characterized by nannofossil assemblage recognized worldwide, such as the occurrence of Biantholithus, Cruciplacolithus, Braarudosphaera, Praeprinsius, and calcareous blooms of Thoracosphaera spp. Adecrease in the abundance of nannofossils is observed in the upper part of the Maastrichtian to the lower part of the Danian; poor to moderate preservation of nannofossils and low diversified nannofossil assemblages cover most of the studied samples. Here, as a result of species poor to moderate preservation, we observe patterns of stepwise extinctions upward in the top of CC25 zone, that culminate with the last occurrence of eight species, just prior to the base of the UC20bTP. We interpret these patterns to be mainly the result of diagenetic alteration. Our dataset provides a record over a long-lasting interval of stepwise extinctions toward the K/Pg boundary that was likely caused by the diagenetically-enhanced Signor-Lipps effect.

Metallogeny and temporal-spatial distribution of sedimentary phosphorite mineralization in Iran, relation to tethyans oceans evaluation and implications for future exploration

There are several deposits and mineral occurrences of sedimentary phosphorite (SP) mineralization in Iran. The main tectonic-structural zones of Iran that host SP deposits are: (1) Alborz Magmatic Belt (AMB), (2) the Central Iran and (3) Zagros Folded Thrust Belt (ZFTB). These deposits were formed during separate time: (1) Neoproterozoic-Early Cambrian (Soltanieh Formation), (2) Ordovician-(Silurian) (Shirgesht, Mila and Seyahou Formations), (3) Upper Devonian (Jeirud Formation), (4) Permian to Jurassic (Abkhory, Taleqan and Hosseinabad area) and (5) Cretaceous to Paleogene (Gurpi and Pabdeh Formations). The stratigraphic sequences of these deposits are commonly formed from sedimentary rocks, which are similar to other SP districts in the Middle East and North Africa. The main period of SP mineralization in Iran was from the Neoproterozoic-Early Cambrian, Upper Devonian and Late Cretaceous to Paleogene. The Jeirud Formation of the Upper Devonian and the Pabdeh Formation of the Paleocene-Eocene contained the largest sedimentary phosphorites in Iran. The formation and distribution of sedimentary phosphorite deposits in the different structural zones can be explained by divergent and convergent events related to the Proto, Paleo, and Neo-Tethyan oceans. The SP deposits hosted by the Neoproterozoic-Early Cambrian Soltanieh Formation are related to the Proto-Tethys Ocean. Mineralization in the Ordovician Mila and Shirgesht formations and Upper Devonian Jeirud Formation are age related to the Paleo-Tethys Ocean, and finally, the SP deposits formed in the Cretaceous- Paleogene (Gurpi and Pabdeh formations) sequence were formed the evolution of the Neo-Tethys Ocean cycle. Passive margins are the principal plate tectonic settings for Iranian SP deposits. Iranian SP deposits are stratigraphically controlled in which phosphorite mineralization occurs as laminae, disseminated grains, and replacement. SP mineralization is commonly closely related to framboidal pyrite and biologic events. The AMB and ZFTB are the main tectonic zones for Iranian sedimentary phosphorite deposits. These zones are the most favorable metallogenic provinces for the exploration of sedimentary phosphorite deposits, representing in excess of 80% of the as of now known SP deposits and occurrences in Iran. High sea level, low oceanic anoxic events (OAE's), warm climates, latitude and longitude close to the equator and shallow slope environments are among the most important factors controlling Iranian SP deposits.

Geochemical evaluation and source rock zonation by multi-layer perceptron neural network technique: a case study for Pabdeh and Gurpi Formations-North Dezful Embayment (SW Iran)

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.

Mineralogy, geochemistry, and depositional environment of phosphates in the Pabdeh Formation, Khormuj anticline, SW of Iran

Mineralogy, geochemistry, and depositional environment of phosphates in the Pabdeh Formation, Khormuj anticline, SW of Iran

Lower Eocene carbonate ramp clinoforms of the southern Tethys; Zagros Foreland Basin, SW Iran: Sequence stratigraphy architecture, basin physiography and carbonate factory controlling parameters

AbstractExcellent cliff exposures in the Khush‐Ab and Chenareh anticlines (Zagros Foreland Basin, SW Iran) offer a good opportunity to document stratal geometries, paleofacies heterogeneity, depositional architecture and depositional cycles of the Lower Eocene sedimentary basin system. These unique outcrops containing six logged sections have been studied across a large‐scale transect, covering ca. 10 km of continental (Kashkan Formation), carbonate‐dominated platforms (Taleh‐Zang Formation) and submarine fan to basin‐floor settings (Amiran and Pabdeh formations). Field observations of the bedding geometries revealed a set of NE–SW oriented carbonate ramp clinoforms (clinoformal units 1–4) with sigmoidal cross‐sectional shapes and an internal fore‐stepping architecture. Based on detailed facies analysis, six facies associations (FA) were identified (FA.I to FA.VI), which are interpreted to have been deposited laterally in the continental, proximal to distal and deep‐water settings of a distally steepened carbonate ramp. According to the stratal stacking pattern, bounding surface, facies architecture and internal makeup of carbonate clinoforms, four H‐F‐cycles (cycle I–IV) corresponding to a lower hierarchical rank (fourth‐order cycle) were recognized and nested within the regressive stacking pattern (HST) of a higher hierarchical rank (third‐order sequence). These H‐F‐cycles are arranged in three segments (bottomset, foreset and topset) of each clinoformal unit. Higher rank transgressive blocks (TST) discriminated each clinoformal unit by up‐deepening sets of the H‐F‐cycle V. In total, five third‐order depositional sequences were identified. The sequence, stratigraphic framework and internal makeup of this carbonate platform indicate that these carbonate sloping successions are the type of accretionary carbonate ramp clinoforms that display an ascending ramp‐slope break trajectory. Evolutionary episodes of biogenic communities, climatic change, local tectonic movements, physical processes (e.g. waves and storms) and water depth gradient are major forcing parameters that controlled the carbonate factory and depositional geometry of this Lower Eocene succession; however, carbonate‐producing organisms and eustatic sea‐level fluctuations played the first role, and local tectonic movements in response to tectonic activities of the Zagros Foreland Basin played the second role. As a result, Taleh‐Zang carbonate platforms are rich in diverse assemblages of LBFs without fragments of coral and red algae, which appear to be a consequence of a hothouse state that diminishes the global thermal gradient, weakens pycnoclines and thereby limits the turbulence.

Biostratigraphy and Lithostratigraphy of Pabdeh Formation is based on planktonic foraminifera in the section of Jahangirabad (south Ilam_ Zagros basin)

Biostratigraphy and Lithostratigraphy of Pabdeh Formation is based on planktonic foraminifera in the section of Jahangirabad (south Ilam_ Zagros basin)

Geochemical and Tectonic Characterization of Oil–Source Rocks in the Shadegan Oilfield, Dezful Embayment, SW Iran

Geochemical methods allow us to develop a systematic understanding of petroleum system elements. Geochemical analyses were done out on the shale samples of the Pabdeh and Asmari Formations and on the Asmari reservoir oil samples in the Shadegan oilfield, southwest Iran. Rock–Eval pyrolysis, gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) data were utilized. Oil–oil and oil–source correlations using GC and GC–MS analyses indicated that the Pabdeh Formation and the Asmari Formation basal shale layer are two possible sources of Asmari reservoir oil. The results of Rock–Eval pyrolysis revealed the values of the organic matter in the samples of the Asmari Formation basal shale layer are consider poor to fair in total organic carbon content. Therefore, this layer cannot be known as the source rock for the oil in the Asmari reservoir. Based on the thermal maturity parameters including the sterane isomerization ratios and homohopanes isomerization ratio together with Rock–Eval data and 1D burial history model, both Pabdeh and Kazhdumi Formations are in the maturity levels for oil generation. Different biomarker ratios in two oil samples (for well numbers 4 and 17) compared to samples from the Shadegan oilfield, revealed that the oil of these two wells originated from carbonate source rock(s) unlike oils of other wells that originated from a shaly source rock. Oil migration from adjacent oilfields with carbonate source rock (e.g., the Maroon, Ahvaz and Mansouri oilfields) into these wells through natural fractures in paleohighs is considered as one of the controlling factors of observed difference in biomarker ratios.

Biostratigraphy of larger foraminifera from the Middle Eocene Jahrum-Pabdeh formations (Zagros region, SW Iran) and their correlation with the planktonic foraminiferal zones

Eocene larger benthic foraminifera including orthophragmines, nummulitids and alveolinids from the Jahrum and Pabdeh formations in the Zagros region are presented. The studied 20-m-thick stratigraphic interval contains, Asterocyclina Gumbel 1870, Nemkovella Less 1987, Orbitoclypeus Silvestri 1907, Discocyclina Gumbel 1870, Nummulites Lamarck 1801, Assilina d'Orbigny 1826 and Alveolina d'Orbigny 1826. This larger benthic foraminiferal assemblage testifies for the shallow benthic zones (SBZs) 15 to 17, which ranges from the middle Lutetian up to the early Bartonian. Further micropaleontological analysis made on intercalated planktonic-rich levels helps calibrate the larger foraminifera biostratigraphy, demonstrating a correlation with the E11 biozone. This confirms that the chronostratigraphic calibration of the SBZs made by previous authors for the peri-Mediterranean area can, through the middle Lutetian to the early Bartonian, be applicable in the Middle East as well.

Biostratigraphy and paleoenvironmental significance of Paleogene foraminiferal assemblages from Dashte Zari area in High Zagros, West Iran

The Paleogene carbonate deposits of Pabdeh and Jahrum formations are widespread from the northwest (Dashte Zari area) to the southwest of the Shahrekord region in the High Zagros Mountains of Iran and record the lateral and upward transition from open marine into the shallow water environment. The Pabdeh Formation shows a succession of open marine pelagic and hemipelagic limestone, argillaceous limestone, and argillaceous chert. It consists of planktonic wackestone, pellet-planktonic wackestone, mudstone with planktonic foraminifera, and radiolarian siliceous wackestone, which accumulated within the Zagros Foreland Basin. The planktonic foraminifers are assigned to the Late Paleocene–Late Eocene and correspond to subtropical and tropical Zones P4b–E15. The Jahrum Formation is represented by bioclast-bearing limestone and calcarenite. It consists of benthic foraminiferal wackestone, benthic foraminiferal-red algal packstone, and bioclast-intraclast packstone deposited in a shallow platform environment. The Jahrum Formation is inter-fingered in the upper part of the Pabdeh Formation and finally overlies it conformably during the Bartonian–Priabonian. Shallowing and off-lap relationships record basin shrinking, while repeated inter-fingering signals moderate tectonic subsidence. Both formations are disconformably covered by the Late Oligocene–Miocene Asmari Formation. Keywords: biostratigraphy, Pabdeh Formation, Zagros, Paleogene, Iran.

Interaction of inherited structures and contractional deformation in the South Dezful Embayment: Insights from the Gachsaran oilfield, SW Iran

Identifying the timing, geometry, mechanism and multiphase reactivation of the inherited structures on the structural style of the fold-and-thrust belt is important for hydrocarbon exploration and production.In this work, we present a multidisciplinary approach including field data, seismic profiles, well data analyses, isopach maps, 2D balanced cross-sections construction, and 2D sequential restoration. Such an approach is applied to decipher the multiphase deformation of the NNE-SSW trending deep-seated Kharg-Mish Fault (KMF) that controlled the tectono-sedimentary evolution and structural style of the Gachsaran Anticline (South Dezful Embayment - SDE). Kinematic analysis of the KMF reveals two different mechanisms from normal to reverse faulting (positive inversion) since the Cenomanian. From the Late Cretaceous-Late Miocene, the sedimentary cover thickness ∼2500 m decreases along-strike of the Gachsaran Anticline towards the east, over the KMF, due to regional erosion and pinch-out.The eastern sector of the Gachsaran Anticline shows an angular and smaller wavelength fold in the levels of Asmari and Sarvak formations due to the efficient local middle décollements of the Kazhdumi and Pabdeh formations. Different amounts of shortening, 19.4% for the western and 16.7% for the eastern sector, and fore-thrust propagation developed a tear fault that accommodated differential deformation. Long-term structural evolution and strain localization in the eastern sector of the Gachsaran Anticline over the KMF produced dense fracturing. Our study suggests that mechanism and timing of deformation along the pre-existing basement structures are important in structural evolution of the sedimentary cover, oil migration and accumulation in such a hydrocarbon province.

Enhanced storm-induced turbiditic events during early Paleogene hyperthermals (Arabian continental margin, SW Iran)

Early Paleogene hyperthermal episodes including the Paleocene-Eocene Thermal Maximum (PETM) have long been viewed as analogues of the Anthropocene global warming. Few studies, however, have analyzed the environmental consequences of such climatic anomalies in deep-water turbidite-rich successions. This integrated sedimentological, biostratigraphic, and stable-isotope study of the Paleogene Pabdeh Formation, deposited along the Arabian continental margin of southwestern Iran, allowed us to document the geological response of hyperthermal events in deep Neo-Tethyan Ocean. The late Thanetian event (Pre-Onset Excursion or long-term late Paleocene climatic perturbation), the Early Eocene Climate Optimum, and the Middle Eocene Thermal Maximum were successfully identified within the Pabdeh Formation. The PETM event could not be documented because the Paleocene/Eocene boundary corresponds to a prolonged non-depositional hiatus marked by a glauco-phosphorite interval. Based on high-resolution microfacies analysis, three different processes in a carbonate slope to basin-margin environment were distinguished including pelagic settling, upwelling-condensation-reworking, and storm-induced turbiditic deposition. Detailed sedimentological analysis revealed an anomalous abundance of storm-induced proximal to distal turbidites represented by packstones with deep-water and reworked shallow-water bioclasts occurring during the hyperthermal intervals. A close causal link between climate warming and tropical storms during the early Paleogene hyperthermal events is thus envisaged. As a principal mechanism, we propose that rapid warming in response to massive carbon release triggered pronounced sedimentological changes along low-latitude tropical margins, leading to generation of storm-induced calciturbidite and re-deposition in the deep sea during hothouse stages.

Uranium and rare earth elements in the phosphate deposits of the Pabdeh Formation, Zagros Zone, Iran: concentrations and geochemical patterns comparison

Uranium and rare earth elements in the phosphate deposits of the Pabdeh Formation, Zagros Zone, Iran: concentrations and geochemical patterns comparison

Organic geochemistry of Paleogene Pabdeh Formation in northern Dezful embayment, Zagros, SW Iran

In this paper, the organic geochemistry of Upper Eocene-Oligocene Pabdeh Formation, in northern Dezful Embayment, SW Iran, was studied using combination of geochemical measurements including Rock-Eval pyrolysis, Gas Chromatography and Gas Chromatography-Mass Spectrometry. A total of 14 shale samples of Pabdeh Formation were obtained from 14 wells in 11 oil fields in Dezful Embayment. The results show that most of the analyzed samples of the Pabdeh Formation have total organic carbon (TOC) content more than one percent. High TOC values and low volatile hydrocarbons (S1) content in the analyzed samples indicate that the hydrocarbons are autochthonous. On the basis of the plot of S1+S2 versus TOC, the Pabdeh Formation can be classified as fair to excellent source rock. Based on the binary plot of HI against OI, kerogen in the Pabdeh Formation belongs to type II. In terms of maturity, the pyrolysis results also show that, the studied Pabdeh samples are in the immature and early mature stages.A comparison of the obtained data with the previous data on Pabdeh Formation in the southern parts of Dezful Embayment shows the difference in the characteristics of this formation so that in the southern parts the average TOC is less and kerogen type II and III have been reported. GC and GC-MS data confirm the presence of predominantly marine organic matter and less continental input. These results also show that the maturity of samples of Pabdeh Formation is more than what was determined by pyrolysis results.

Chemometric investigation of oil families and geochemical characterization of crude oils in the Northern Dezful Embayment Zone, SW Iran

Chemometric analysis was performed on 19 crude oil samples from 14 different oil fields to reveal oil families and their distributions in the Northern Dezful Embayment Zone. The geochemical characteristics and candidate source rock(s) of the defined oil families were also investigated in this study. The chemometric classification revealed three different oil families in the studied area. Group I includes Naft Sefid, Ramin, Kupal, Marun, and Aghajari oil fields. Group II comprises Ziloie, Masjid-e Soleyman, Karanj, Haftkel, Balarud, Qale Nar, and Karoun oil fields, and group III contains Azar and Dehluran oil fields. All studied samples were expelled from open marine settings with anoxic conditions, type II kerogen, and marine organic matter inputs. The group I oil fields have C29/C30 > 1, TAR<1, moderate C22/C21 and C24/C23 tricyclic terpanes, an insignificant amount of Oleanane, mid-oil window maturity, and an average carbon isotope of −26.98‰, which were charged from the Kazhdumi source rock. The group II oil family has C29/C30H < 1, moderate C22/C21 tricyclic terpanes, TAR<1, moderate C24/C23 tricyclic terpanes, and a high Oleanane Index (average of 22%), mid-oil window maturity, and average carbon isotope of −25.35‰. These oils were originated from the Pabdeh Formation. The group III samples have C29/C30H > 1, high C22/C21 tricyclic terpanes, TAR<1, low C24/C23 tricyclic terpanes, mid-oil window maturity, and an average carbon isotope of −27.67‰, which were originated from the Garau or Sargelu settings.

Calcareous nannofossil biostratigraphy and paleoenvironment of the Eocene–Oligocene interval in the Pabdeh Formation in southwestern Iran

Calcareous nannofossil biostratigraphy and paleoenvironment of the Eocene–Oligocene interval in the Pabdeh Formation in southwestern Iran

Structural geometry and evolution of the Dehdasht Basin in the Central Zagros fold-and-thrust belt: Implications for the timing of oil generation and trap formation

Structural analysis and temporal evolution of the Dehdasht Structural Basin in the Central Zagros through the constructed cross-sections and 2D sequential restoration have defined the timing of trap formation in the basin. At the level of the competent layer (Early Cretaceous to Oligo-Miocene), the basin is deeper toward the center where it is covered by the thicker Miocene evaporites and syn-tectonic clastic deposits. The evolutionary history of the basin based on the sequential restoration proposes that the anticlinal traps are formed since the Pabdeh deposition at Paleocene-Eocene on the boundaries and during the middle Miocene Mishan deposition within the basin. The proposed structural evolution and the suggested oil window using the burial depth and the present thermal gradient for the Dehdasht Basin give insight into its hydrocarbon potential. Accordingly, the Cretaceous Kazhdumi source rock has started oil generation since Paleocene-Eocene (deposition of the Pabdeh Formation) in the deeper central part until the present in the subsurface relatively large-amplitude anticlines close to the boundaries. We also suggest that the Paleocene-Eocene Pabdeh Formation contained oil mainly in the subsurface low-amplitude anticlines in the central part of the basin during deposition of the Gachsaran and Mishan formations at early-middle Miocene. The older oil generation in the basin is attributed to its thicker deposits in the Paleocene-Eocene and early Miocene resulting in deeper burial depth for the source rocks. The obtained results propose that the subsurface relatively large-amplitude anticlines near the basin margins with the possible hydrocarbon (mainly generated from the Cretaceous Kazhdumi shale) are the most likely targets for future oil exploration. Additionally, we obtained that the boundary fore-thrusts and the caprock erosion due to the lateral anticlinal growth control the oil and gas seepages on the Dehdasht Basin boundaries.

Hydrocarbon source rocks in Kazhdumi and Pabdeh formations—a quick outlook in Gachsaran oilfield, SW Iran

Geochemical study of Kazhdumi and Pabdeh Formations as potential source rocks in Gachsaran Oilfield demonstrates that the Kazhdumi Formation has a fair to good capability of hydrocarbon generation and predominately contains type II-III kerogen. On the other hand, the Pabdeh Formation has a poor to good petroleum potential and contains different kerogen types, including type II, type II-III, type III and even for one sample, type IV, indicating different depositional conditions for this formation. The geochemical log of the Kazhdumi Formation shows that there is a close correlation between different geological parameters as noticed prominently in well number 55, which suggests the more extensive the anoxic condition, the higher the petroleum potential is for Kazhdumi Formation. By contrast, a poor correlation between TOC and other Rock–Eval-derived parameters for the Pabdeh Formation at a depth of more than 2100 m may demonstrate the inert organic matter and mineral matrix effects at this depth interval. However, biomarkers show differences in lithology and depositional environment for the Kazhdumi Formation in well numbers 55 and 83. On the other hand, the Pabdeh Formation has a mixed lithology (carbonate-shale) deposited in a marine setting under suboxic–anoxic condition. Moreover, thermal maturity indicators suggest that Pabdeh and Kazhdumi Formations are immature and early mature, respectively.