Ilam Formation Research Articles

DASYCLADALES (GREEN ALGAE) AND SOME BENTHIC FORAMINIFERA FROM THE UPPER CRETACEOUS ILAM FORMATION (LATE CONIACIAN–SATONIAN), SW IRAN (ONSHORE AND OFFSHORE)

New data (onshore, offshore) on the Upper Cretaceous Ilam Formation of southwestern Iran are provided concerning the common presence of dasycladalean green algae (families Triploporellaceae, Polyphysaceae) that have so far been treated mostly in open nomenclature. The assemblage consists of the five taxa Salpingoporella ubaiydhi Radoičić, 1979, Dissocladella ondulata (Raineri, 1922), Trinocladus tripolitanus Raineri, 1922, Clypeina cf. dusanbrstinai Radoičić, 1997, and gen. et sp. indet. The taxa typically prevail in wackestones-packstones but exhibit different distributional patterns. While S. ubaiydhi and T. tripolitanus, rarely associated with D. ondulata, form a characteristic assemblage in a distal infralittoral facies (more diverse assemblage; zone no. 31 of Wynd, 1965), C. cf. dusanbrstinai occurs in a proximal infralittoral facies associated with miliolids among also complex forms (rhapydioninids) (assemblage zone no. 30 of Wynd, 1965). It is concluded that the two assemblage zones refer to different palaeoenvironments, instead of different ages (‘lower’ and ‘upper’). Based on benthic foraminifera, the main part of the Ilam Formation can be ascribed to the Santonian. For the lower part, a late Coniacian age is suggested implied indirectly by the comparably long vertical range of the rotaliid Orbitokathina vonderschmitti Hottinger, 1966 of the studied core. This would be in conformity with the K 150 Arabian Plate K 150 sequence boundary referring to a late Turonian?–early Coniacian hiatus. From a palaeobiogeographic perspective, T. tripolitanus is a cosmopolitan species, S. ubaiydhi is restricted to the area of the former Arabian Plate (subsurface Iraq, Saudi Arabia and Egypt), while C. dusanbrstinai has been recorded only from Serbia (type-locality). In addition to the algae, the stratigraphic discussion includes the occurrence of the benthic foraminifera Reticulinella? kaeveri Cherchi, Radoičić & Schroeder, 1989 and Orbitoides mid-orientalis (Eames & Smout, 1956) in the Ilam Formation.

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

In-situ stress regime analysis and mechanical earth modeling of the southwestern sector of the Zagros folded belt, SW Iran: applications for acid fracturing in Ilam and Sarvak carbonate formations

In-situ stress regime analysis and mechanical earth modeling of the southwestern sector of the Zagros folded belt, SW Iran: applications for acid fracturing in Ilam and Sarvak carbonate formations

Discerning species among the foraminiferal “Rotalia skourensis” morphotypes of the Middle East

In the Upper Cretaceous of the Middle East, rotaliid Foraminifera have conventionally been attributed to Rotalia skourensis Pfender (= Rotorbinella skourensis), a key element of biozones #30 and #31 as commonly recognized in the Iranian Zagros. However, R. skourensis is a Paleocene species and is morphologically distinct from the rotaliid specimens present in the Upper Cretaceous of theMiddle East. Through detailed sampling of the Ilam Formation (Turonian?–Campanian) in the Zagros basin, two new species previously classified as R. skourensis have been described. Rotorbinella pachypila n. sp. andOrbitokathina parva n. sp. contribute to our understanding of the diversity of rotaliids during the Late Cretaceous of the Arabian Plate and highlight biogeographic connections with the western Tethys carbonate platform domains.

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.

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.

Determining the drilling mud window by integration of geostatistics, intelligent, and conditional programming models in an oilfield of SW Iran

Accurate knowledge of pore and fracture pressures is essential for drilling wells safely with the desired mud weight (MW). Overpressure occurs when the pore pressure is higher than the normal hydrostatic pressure. There is a challenge regarding the pressure studies domain in an oilfield in SW Iran, where lack of geo-mechanical data limits exact mud window calculation. Also, the reservoir generally consists of carbonate rocks and contains no shale interbeds, so mechanical stratigraphy based on Gamma ray could not be applied. This study is to provide safe drilling considering MW to prevent the flow or loss in the vicinity of the new wells in the studied field. In this research, the formation pressures and mud window models are determined by combining geostatistical, intelligent, and conditional programming models and compared with real data. The conditional programming was also used to correct small out-of-range data. The highest correlation between the final effective pressure and velocity cube was observed in lower Fahliyan Formation with 0.86 and Ilam with 0.71.The modeled MW difference ranged between 2.5 and 30 PCF. Also, the maximum modeled MW is 150 PCF in the upper Fahliyan Formation. Heavy mud of more than 130 PCF is suggested for drilling the Khalij member and continues to the end of stratigraphy column. Best observed correlation comparing the drilled and modeled MW, especially achieved in the Fahliyan reservoir Formation with more than 100 PCF and the Ilam Formation with 80–100 PCF. Finally, 3D formation pressures are presented and recommended for further safe drillings.

Role of facies cycles on the mid-upper Cretaceous Bangestan Reservoir rock typing in the Zagros Basin

The principal objectives of this research were to examine the role of sedimentary cycles and tectonics in the Ahwaz Bangestan Reservoir quality, view the vertical and lateral changes of these sedimentary cycles, and identify rock typing units. There is an urgent need to understand how interpretive data are perceived among diagenetic events, sedimentary cycles, exposure surfaces, disconformities, carbonate fabric units, porosity-permeability relationship, wettability index, and relative permeability and capillary pressure vs. saturation curves. The Ahwaz Bangestan Sequence consists of Ilam and Sarvak Formations from the late Albian to Santonian Stages, in which disconformity surfaces separated the third-order sequences. Early diagenetic processes of the Bangestan sequence include meteoric dissolution and the deposit of marine cement. The meteoric diagenesis plays an essential role in the reservoir quality of these sequences and sedimentary cycles, and the burial cement process reduces the Bangestan Reservoir quality. The Bangestan sediments experience significant disconformity surfaces in the upper Cenomanian and Post-Turonian Stages. Meteoric waters affect these exposure surfaces to 50 m, increase porosity by about 15%, and have a permeability of 5 mD. With this, the results showed that there is a positive relationship between permeability and porosity data of the Bangestan sedimentary facies. In this regard, grainstone and packstone facies in the Ilam Formation demonstrated a higher reservoir quality than other facies, except for secondary chalky facies in Zone C1, where severe meteoric diagenesis occurred before lithification. Besides, the sedimentary model of the Bangestan Reservoir revealed six facies association, including tidal flat, lagoon, back shoal, rudist biostorm, shoal, and deep-marine with terrigenous layers. Given rock typing classification, four geological static rock typing (GSRT) were recognized with good correlation among lagoon, back shoal, rudist biostorm, and shoal facies association. The results demonstrated a positive relationship between permeability and porosity characteristics, leading to higher porosity and permeability in facies association of shoal and rudist biostorm with better GSRT. Besides, facies association of back shoal and lagoon with lower porosity and permeability represented worse GSRT. However, due to different diagenesis events, open-marine facies did not follow an adequate GSRT. Furthermore, three oil-wet geological dynamic rock typing (GDRT) and two water-wet GDRTs were identified, indicating the dynamic behavior of facies association. As a result, facies association with better reservoir quality (i.e., higher permeability and porosity) present higher GDRT, while facies association with worse reservoir quality (i.e., lower permeability and porosity) show lower GDRT.

Discrete fracture modeling by integrating image logs, seismic attributes, and production data: a case study from Ilam and Sarvak Formations, Danan Oilfield, southwest of Iran

Understanding the fracture patterns of hydrocarbon reservoirs is vital in the Zagros area of southwest of Iran as they are strongly affected by the collision of the Arabian and Iranian plates. It is essential to evaluate both primary and secondary (fracture) porosity and permeability to understand the fluid dynamics of the reservoirs. In this study, we adopted an integrated workflow to assess the influence of various fracture sets on the heterogeneous carbonate reservoir rocks of the Cenomanian–Santonian Bangestan group, including Ilam and upper Sarvak Formations. For this purpose, a combination of field data was used including seismic data, core data, open-hole well-logs, petrophysical interpretations, and reservoir dynamic data. FMI interpretation revealed that a substantial amount of secondary porosity exists in the Ilam and Sarvak Formations. The upper interval of Sarvak 1-2 (3491 m to 3510 m), Sarvak 1-3 (3530 m to 3550 m), and the base of Sarvak 2-1 are the most fractured intervals in the formation. The dominant stress regime in the study area is a combination of compressional and strike-slip system featuring reverse faults with a NW–SE orientation. From the depositional setting point of view, mid-ramp and inner-ramp show a higher concentration of fractures compared to open marine environment. Fracture permeability was modeled iteratively to establish a realistic match with production log data. The results indicate that secondary permeability has a significant influence on the productivity of wells in the study area.

Investigating for hydrocarbon potential in the Sarvak and Ilam Formations using Fuzzy logic in the Fars Region, Iran

Investigating for hydrocarbon potential in the Sarvak and Ilam Formations using Fuzzy logic in the Fars Region, Iran

Geomechanical analysis of an oil field: Numerical study of wellbore stability and reservoir subsidence

Geomechanics as the knowledge of rock deformation and stability is an indispensable part of all field development plans. Conducting geomechanical analyses leads to a safer and more efficient operation otherwise different kinds of instability and distortion might occur. In this study, the geomechanical behavior of Ilam and Sarvak formations of an oil field in southwest of Iran was investigated. The research objectives can be summarized as wellbore stability evaluation and predicting the value of reservoir subsidence due to pressure drop as a result of reservoir fluid production. To fulfill these, a set of petrophysical logs run in the exploration well of this green field were collected. Next, using empirical correlations and statistical methods, required data for evaluating wellbore stability during drilling, specifying safe mud window to discover reservoir breakdown pressure, predicting the possibility of wellbore collapse in field lifetime, and assessing reservoir subsidence were determined. The results revealed that the average subsidence value as the consequence of production within 21 years is 0.275 ft Which is not significant. In terms of wellbore stability, it was concluded that all horizontal and vertical wells remain stable during this time period. Briefly to conclude, field development is not associated with alarming incidents from geomechanical aspect.

Variation of geochemical data and sedimentary characteristics in the Upper Cretaceous Ilam Formation, a case study from southwest Iran

Variation of geochemical data and sedimentary characteristics in the Upper Cretaceous Ilam Formation, a case study from southwest Iran

Age, duration, and geochemical signatures of paleo-exposure events in Cenomanian–Santonian sequences (Sarvak and Ilam formations) in SW Iran: Insights from carbon and strontium isotopes chemostratigraphy

This study addresses the timing implications of chemical stratigraphy and chronostratigraphic correlation of unconformity-related shallow water (neritic) carbonate rock units. Cenomanian–Santonian sequences (i.e., Sarvak and Ilam formations) host considerable volumes of hydrocarbon in the Zagros region of SW Iran, which represent good examples of unconformity-related reservoirs worldwide. Geochemical traits including carbon, oxygen, and strontium isotopes of these sequences are measured in three wells from the Abadan Plain and one well from the Dezful Embayment. The altered intervals below the exposed erosional surfaces are characterized by remarkable variations in both elemental and isotopic compositions. These variations are marked by increasing in Mg, Al, Fe, Mn, and Rb elemental contents as well as decreasing in Sr and Ca elemental contents. The altered intervals show a positive shift in 87Sr/86Sr isotopic values and a negative trend in δ13Ccarb and δ18Ocarb isotopic values. The paleontological age that is available from previous studies provide a valuable basis for numerical dating of the studied intervals using the rubidium-corrected 87Sr/86Sr isotopic values. The duration of the stratigraphic hiatuses at the Cenomanian–Turonian and mid-Turonian paleo-exposures are calculated as 0.53 Myr and 2.7 Myr, respectively. The calculated durations corroborate the results of previous studies of these paleo-exposures within the Zagros region of western Iran. The concluded (carbon and Sr-isotope) chemostratigraphic correlation of the Cenomanian–Santonian sequences in the Abadan Plain and other parts of the Zagros region can provide a more reliable chronostratigraphic framework of larger scale. The latter can be used in regional correlation and interpretation of the geological-structural evolution of the Zagros region during Cenomanian–Santonian, particularly in western Iran.

Evaluation and prediction of the rock static and dynamic parameters

Determination of rock properties as materials, foundations and sites of civil projects, is one of the priorities. This study aimed to assess the relationship between static elastic modulus (Es) and dynamic elastic modulus (Ed) and to estimate static properties and shear wave velocity (Vs) using simple regression (SR), support vector regression (SVR), multivariate linear regression (MVLR) and artificial neural network (ANN) methods based on compressional wave velocity and physical properties. For this purpose, first geomechanical characteristics of 80 specimens of the limestone rocks from the Asmari and Ilam formations in Karun 4 (K4) and Karun 2 (K2) dam sites, in southwestern Iran were measured. Then, data related to the various studies from different parts of the world were collected and a global relationship was presented. The average Ed obtained from the various relationships of different researchers was equal to 19.90 GPa, which is less than the average Ed of the present study (31.20 GPa). According to the most accurate fit, the presented relationship between Es and Ed was power. The analysis of all model hypotheses by MVLR showed that it is possible to estimate the static and dynamic properties. Predicting dynamic and static parameters of the limestone rocks using Sigmoid transfer functions and Hyperbolic tangents and various training rules showed that the Sigmoid transfer function and Levenberg-Marquardt training law have the best performance in predictions. Comparison of the methods performance in estimating Vs and static properties showed that the SVR has higher accuracy than other methods.

Depositional environment, seismic stratigraphy, and Sr-isotope geochronology, Bangestan reservoir, Ahwaz oilfield, SW Iran

The Bangestan reservoir, which occurs in the Ahwaz oilfield, consists of the middle Cretaceous limestone Ilam and Sarvak Formations that were deposited in the Zagros Basin. The reservoir is divided into ten Zones (A to J) formed in the upper Albian-Santonian and contains considerable hydrocarbon accumulations. The limestones were deposited on an extensive shallow carbonate platform on a passive margin and are dominated by rudist biostrome and grainstone facies. Paleogeographical changes mean that identification of the facies is complex. Seismic stratigraphy and isotopic data are used to better understand the structural and geological setting and develop an understanding of the sedimentary environment. The results show that the rudist biostrome facies extends to a shallow interior shelf as a patch reef, indicating a regressive trend. Petrographic studies on 3174 thin sections identified twelve microfacies in the Bangestan reservoir. In the Sarvak Formation, the dominant microfacies consist of rudist biostrome, shoal bar, and lagoon facies, with a high sedimentation rate in the basin, whereas the Ilam Formation involves open and deep-marine facies. Seismic tectono-stratigraphy shows that there are seven sedimentary sequences with mostly regressive cycles and six maximum flooding surfaces from the upper Albian to the Santonian. In addition, the Ahwaz oilfield had experienced pre-tectonic extension before the Early Cretaceous and post-tectonic extension after the Late Cretaceous. The isotopic data from the Bangestan reservoir demonstrate two important disconformities (Cenomanian-Turonian and post-Turonian) that had an important influence on the evolution of the area evolution. Sr geochronology confirms long-term exposure to two main disconformities within the Ahwaz Bangestan reservoir. • C/O geochemical data show that Ilam Formation experienced fewer diagenesis event. • Ilam Formation represents open- and deep-marine facies. • Sarvak consists of rudist biostrome, shoal, back shoal, lagoon and tidal flat facies. • 7 sedimentary sequences with regressive cycles and 6 MFS from Albian to Santonin. • Bangestan reservoir involves two important disconformities of Cenomanian-Turonian.

Planktonic foraminiferal biostratigraphy and determination of the Santonian-Campanian boundary in the Zagros sedimentary basin, SW Iran

The carbonate-dominated Upper Cretaceous Ilam Formation is representative of an important reservoir in the Zagros Basin. In this study, planktonic foraminiferal biostratigraphy of the Ilam Formation was investigated in the Kuh-e-Surgah section in western Iran. In the studied section, the 170 m thick Ilam Formation is mainly composed of imestones with shale intercalations. The formation conformably overlies and underlies the Surgah and Gurpi formations, respectively. Twelve genera and 24 species of planktonic foraminifera were identified. The following planktonic foraminiferal biozones were identified: the Dicarinella asymetrica Total Range Zone (latest Coniacian- late Santonian), the Globotruncanita elevata Partial Range Zone (Santonian to early Campanian), the Globotruncana ventricosa Interval Zone (middle to late Campanian.), the Radotruncana calcarata Total Range Zone (late Campanian.), the Globotruncanella havanensis Partial Range Zone (late Campanian) and the Globotruncana aegyptiaca Interval Zone (late to latest Campanian). In general, the biozones show that the Ilam Formation in the studied section is latest Coniacian-late Campanian in age. The Santonian-Campanian boundary in the Kuh-e-Surgah section correlates with the Globotruncanita elevata Partial Range Zone. The recognized biozones in the Ilam Formation are comparable with those recorded in other parts of the Tethyan Realm. The crinoid species Marsupites testudinarius is absent in the studied section. Generally, the last occurrence of this cosmopolitan species indicates the base of the Campanian stage. The Santonian/Campanian boundary was placed in the lower part of the Globotruncanita elevata Partial Range Zone at the base of the Ilam Formation in the studied section.

Determination of the mud weight window, optimum drilling trajectory, and wellbore stability using geomechanical parameters in one of the Iranian hydrocarbon reservoirs

The challenges behind this research were encountered while drilling into the Ilam, Mauddud, Gurpi, and Mishrif Formations, where severe drilling instability-related issues were observed across the weaker formations above the reservoir intervals. In this paper, geomechanical parameters were carried out to determine optimum mud weight windows and safe drilling deviation trajectories using the geomechanical parameters. We propose a workflow to determine the equivalent mud window (EMW) that resulted in 11.18–12.61 ppg which is suitable for Gurpi formation and 9.36–13.13 ppg for Ilam and Mishrif Formations, respectively. To estimate safe drilling trajectories, the Poisson’s ratio, Young’s modulus, and unconfined compressive strength (UCS) parameters were determined. These parameters illustrate an optimum drilling trajectory angle of 45° (Azimuth 277°) for the Ilam to Mauddud Formations and less than 35° for the Gurpi Formation. Our analysis reveals that maximum horizontal stress and Poisson’s ratio have the most impact on determining the optimum drilling mud weight windows and safe drilling deviation trajectories. On the contrary, vertical stress and Young’s modulus have minimum impact on drilling mud weight windows and safe drilling deviation trajectories. This study can be used as a reference for the optimal mud weight window to overcome drilling instability issues in future wellbore planning in the study.

Planktonic foraminiferal distribution of the Late Cretaceous to late Paleocene deposits of the Bandar Abbas area (Zagros Belt, SW Iran): An update of the bio- and lithostratigraphic framework

In this paper, we present a biostratigraphic study of the planktonic foraminiferal associations of the Gurpi Formation outcropping in the Bandar Abbas area of the Zagros Belt (SW Iran). Two stratigraphic sections, Kuh-e Khush and Kuh-e Khamir, were logged and sampled, and the identification of the planktonic foraminiferal assemblages has allowed the recognition of eight Cretaceous biozones, namely Dicarinella asymetrica Zone, Globotruncanita elevata-Globotruncana ventricosa Zone, Radotruncana calcarata Zone, Globotruncanella havanensis Zone, Radotruncana calcarata–Globotruncana aegyptiaca Zone, Globotruncana aegyptiaca-Gansserina gansseri Zone, Gansserina gansseri Zone and Contusotruncana contusa-Abathomphalus mayaroensis Zone. This planktonic foraminiferal zonal framework reveals a latest Coniacian to late Maastrichtian age for the deposits of the Gurpi Formation in the two studied sections. The Cenozoic Interval of the studied sections, represented by the Pabdeh Formation, is attributed to the P3–P6a integrated Zone in the Khush section and the P4c Subzone (Acarinina soldadoensis) in the Khamir section. The data show that the Gurpi Formation at the Khamir locality rests disconformably upon the shallow neritic facies of the Ilam Formation, while in the Khush section this formation disconformably overlies the shallow carbonates of the Sarvak Formation. In addition to the in situ biota, reworked microfossils, such as calpionellids, calcareous algae and benthic foraminifera from older Jurassic (Surmeh Formation) and Cretaceous formations (Surgah equivalent, Sarvak, Kazhdumi and Dariyan formations) are recorded within several stratigraphic levels of the Gurpi Formation.

Controls of depositional facies and diagenetic processes on reservoir quality of the Santonian carbonate sequences (Ilam Formation) in the Abadan Plain, Iran

Ilam Formation (Santonian) is a member of the Bangestan Group, as an important oil reservoir of SW Iran. Various geological and reservoir quality aspects of this formation were discussed in the Zagros area. However, depositional–diagenetic controls on pore types of this formation are not defined, especially in the Abadan Plain. In this study, an integrated sedimentological and petrophysical evaluation has been carried out in six subsurface sections of this formation throughout the Abadan Plain. Facies analysis indicates deposition in shallow- to deep marine settings of a ramp-like carbonate platform. It includes lagoon, shoal, middle ramp, outer ramp and basinal settings. Various diagenetic alterations have been recorded including carbonate cementation, dissolution, mechanical to chemical compaction and fracturing. They indicate marine to meteoric and shallow- to deep burial diagenetic realms for the Ilam Fm. Controls of depositional and diagenetic processes on reservoir quality and pore types have been evaluated by integrating well logs and porosity–permeability data with petrographic evidence in sequence stratigraphic framework. Accordingly, a higher-order and three lower-order sequences have been defined and correlated across the study area. Various pore types of the reservoir are differentiated and classified into five classes: interparticle, intra-particle/skeletal, vuggy/moldic, fractures and micropores. Accordingly, two reservoir intervals are defined in the Ilam formation that include the following: (1) microporous mud-dominated reservoir in TST of higher-order sequence, in the lower Ilam, and (2) macroporous grain-supported facies in RST of higher-order sequence, in upper part of the Ilam formation.

Investigating the souring mechanism in two giant carbonate oil reservoirs, southwestern Iran

In general, a hydrocarbon reservoir with a produced gas stream having more than three parts per million by volume (ppmv) of hydrogen sulfide is considered a sour reservoir. This study aims to present novel ideas on the main souring mechanisms in Bangestan Group, a well-known Mid-Cretaceous hydrocarbon hosting carbonate in Iran, including Ilam, Sarvak, and Kazhdumi formations. Especially, several scenarios are discussed on potential source of H2S in Sarvak Formation and controversial sweetness of the Upper Ilam Reservoir. At first, both of the bacterial or microbial sulfate Reduction (BSR or MSR) and thermochemical sulfate reduction (TSR) mechanisms were rejected based on the reported bottom-hole temperature and lack of anhydrite in rock matrix, respectively. Moreover, the possibility of souring due to migration was disregarded mainly due to the reported constant concentration of H2S over time and among the target oilfield. At the end, thermal decomposition of organic sulfur compounds within the kerogen or crude oil was introduced as the only potential souring mechanism that was further proven through sulfur isotopic analysis. Furthermore, the possibility of H2S absorption in lower horizons of Ilam Formation, and so sweetness of this formation, due to the presence of ferrous minerals was evidenced through detailed petrographic analysis.