Asmari Formation Research Articles

Micro-Bio Stratigraphy of Asmari Formation in Mamoolan Stratigraphy Section, Sout of Khorramabad

In this paper, Biostratigraphy of Asmari formation to San oligo-Myocene was studied in North Mamulan, South West Lorestan Province. In this region, Shahbazan-Asmari carbonate formation is located between Kashkan formation beneath and evaporative formation of Gachsaran as old as lower Miocene in the form of disconformity above. From 180 m real thickness observed in this study, 23 genera and 15 species of benthic foraminifera were identified and studied. These foraminifera were found in four assemblage zones suggesting that this formation dates back to Oligocene (Chattian-Rupelian) and early Miocene (Aquitanian-Burdigalian). Moreover, based on microscopic studies and visual observations, nine microfacies were identified that were deposited in three zones: lagoon, dam, and shallow open marine.

Seismic spectral decomposition and inversion for buried channels delineation: a case study from the Asmari Reservoir, southwestern Iran

Dense data sampling in 3D seismic surveys provides a clear image from subsurface layers, but it may fail in the cases where the thickness of a layer is below seismic resolution (e.g. thin beds and channels). Seismic spectral decomposition and inversion are among methods that can help us to delineate and study the sub-seismic resolution features. This paper shows how the buried channels of the Asmari formation in an oil field located in southwest Iran were delineated and are studied using seismic spectral decomposition and inversion. The Gabor wavelet transform was used to divide seismic frequency spectra into eight bands out of which three frequency bands with central frequencies of 24, 29, and 35 Hz have been selected for RGB blending for its perfectness in exposing the channels. Several distinct channels were detected at 15ms interval below the top of the Asmari formation. The P-impedance data coincide with the findings further giving information on lithological aspects. Detailed delineation of the channels became possible by using the Matching Pursuit Decomposition (MPD) method. In order to delineate sub-seismic resolution features, spectral decomposition is used to transform time-domain seismic data into the frequency domain and then the special elements corresponding to these subsurface features are enhanced with spectral blending.

Understanding Karst Leakage at the Kowsar Dam, Iran, by Hydrogeological Analysis

The Kowsar Dam is constructed on the Kheyrabad River at the northern limb of the Duck Anticline, close to its NW plunge, SW Iran. The dam is built on the karstic Asmari Formation, and the reservoir is in direct contact with this formation from the dam body to the upstream impermeable Gachsaran Formation. After reservoir impounding, several new springs emerged from the Asmari Formation, adjacent to the old small springs at the southern limb of the Duck Anticline. The discharge of these downstream springs was not reduced despite grout curtain treatment works. The main water leakage route is not below or through the grout curtain, as shown by considering rock permeability in pilot and check holes, cement consumption in grouting boreholes, borehole water levels, spring locations, and discharge of the dam galleries. The Duck Anticline is hydrogeologically connected to the adjacent Dill and Pahn Anticlines, comprising the Asmari Karst Aquifer, which is divided into three karst sub-aquifers. The general flow direction of karst water in each sub-aquifer was determined using hydrogeological analysis and water balance calculations. Two alternative models are proposed for water flow at the northern limb of the Duck Anticline. The main leakage route toward the downstream springs is most probably through a relict karst conduit system, developed along bedding planes of the Asmari Formation at the NW plunge of the anticline, according to the first proposed model. In this case, the water leakage can be significantly reduced by extension of the grout curtain further into the upstream impermeable Gachsaran Formation.

Three‐dimensional modelling of magnetotelluric data to image Sehqanat hydrocarbon reservoir in southwestern Iran

ABSTRACTA detailed magnetotelluric survey was conducted in 2013 in the Sehqanat oil field, southwestern Iran to map the geoelectrical structures of the sedimentary Zagros zone, particularly the boundary between the Gachsaran Formation acting as cap rock and the Asmari Formation as the reservoir. According to the electrical well logs, a large resistivity contrast exists between the two formations. The Gachsaran Formation is formed by tens to hundreds of metres of evaporites and it is highly conductive (ca. 1 Ωm–10 Ωm), and the Asmari Formation consists of dense carbonates, which are considerably more resistive (more than 100 Ωm). Broadband magnetotelluric data were collected along five southwest–northeast directed parallel lines with more than 600 stations crossing the main geological trend. Although dimensionality and strike analysis of the magnetotelluric transfer functions showed that overall they satisfied local 2D conditions, there were also strong 3D conditions found in some of the sites. Therefore, in order to obtain a more reliable image of the resistivity distribution in the Sehqanat oil field, in addition to standard 2D inversion, we investigated to what extent 3D inversion of the data was feasible and what improvements in the resistivity image could be obtained. The 2D inversion models using the determinant average of the impedance tensor depict the main resistivity structures well, whereas the estimated 3D model shows significantly more details although problems were encountered in fitting the data with the latter. Both approaches resolved the Gachsaran–Asmari transition from high conductivity to moderate conductivity. The well‐known Sehqanat anticline could also be delineated throughout the 2D and 3D resistivity models as a resistive dome‐shaped body in the middle parts of the magnetotelluric profiles.

A laboratory study on capillary sealing efficiency of Iranian shale and anhydrite caprocks

Abstract Caprock has the most important role in the long term safety of formation gas storage. The caprocks trap fluid accumulated beneath, contribute to lateral migration of this fluid and impede its upward migration. The rapid upward passage of invasive plumes due to buoyancy pressure is prevented by capillary pressure within these seal rocks. In the present study, two main seal rocks, from the Zagros basin in the southwest of Iran, a shale core sample of Asmari formation and an anhydrite core sample of Gachsaran formation, were provided. Absolute permeabilities of shale and anhydrite cores, considering the Klinkenberg effect, were measured as 6.09 × 10 −18 and 0.89 × 10 −18 m 2 , respectively. Capillary sealing efficiency of the cores was investigated using gas breakthrough experiments. To do so, two distinct techniques including step by step and residual capillary pressure approaches were performed, using carbon dioxide, nitrogen and methane gases at temperatures of 70 and 90 °C, under confining pressures in the range 24.13–37.92 MPa. In the first technique, it was found that capillary breakthrough pressure of the cores varies in the range from 2.76 to 34.34 MPa. Moreover, the measurements indicated that after capillary breakthrough, gas effective permeabilities lie in range 1.85 × 10 −21 – 1.66 × 10 −19 m 2 . In the second technique, the minimum capillary displacement pressure of shale varied from 0.66 to 1.45 MPa with the maximum effective permeability around 7.76 × 10 −21 – 6.69 × 10 −20 m 2 . The results indicate that anhydrite caprock of the Gachsaran formation provides proper capillary sealing efficacy, suitable for long term storage of the injected CO 2 plumes, due to its higher capillary breakthrough pressure and lower gas effective permeability.

Parametric analysis of caprock integrity in relation with CO2 geosequestration: capillary breakthrough pressure of caprock and gas effective permeability

AbstractCaprock integrity is a primary criterion for evaluating depleted oil and gas reservoirs for long‐term safety of carbon dioxide geosequestration. The occurrence of capillary leakage is inevitable in caprock. This phenomenon occurs whenever the buoyancy pressure due to accumulated CO2 plumes dominates the capillary pressure of caprock, and thereby the plumes intrude into the pore throats. In this study, experimental investigation of the effective parameters, including overburden pressure, ambient temperature, and CO2 impurities in the form of non‐condensable and non‐reactive CH4 and N2 gases on the capability of shale and anhydrite cores to preserve CO2 gas is conducted. In this regard, capillary breakthrough pressure and CO2 gas effective permeability analysis were performed applying two distinct techniques, step by step and residual capillary pressure. Experiments were conducted at the temperatures of 35, 70, and 90 °C and overburden pressures in the range of 3500–5800 psi. Two main seal rocks, including shale and anhydrite core samples, from middle Asmari and Gachsaran formations of the Zagros Basin located in the southwest of Iran were used. Regarding the high capillary breakthrough pressure and low gas effective permeability after outbreak of the leakage, all three parameters have noticeable effects on capillary sealing efficiency of the caprocks. The results indicate that impurities such as CH4 and N2 have a significant effect on capillary breakthrough pressure of caprock and gas effective permeability. © 2015 Society of Chemical Industry and John Wiley & Sons, Ltd

Introduction of sealing fault in Asmari reservoir by using FMI and RFT in one of the Iranian naturally fractured oil fields

Historically, in Iran, the borehole imaging tools are acquired to locate major structural features, such as faults, fractures, and sedimentary environment interpretation. The well GS-314 was drilled to evaluate the Asmari formation. It was drilled with an 8.5-in, vertical hole. To get a better understanding of reservoir characteristics of the Asmari formation on the Gachsaran field, to fullbore formation microimager (FMI), repeat formation tester (RFT) and a full set of open hole logs are acquired. The main objective of running FMI was to determine fracture characteristics and locate high porous layers/streaks so that points for RFT survey could be chosen for better results. In this study, it indicated a possible sealing/non-permeable fault in the Asmari reservoir in GS-314 of the Gachsaran field. The RFT pressure data indicated a change of 100 psi in the oil gradients across the fault, indicating possible reservoir compartmentalization in the area around well GS-314. The FMI images indicated a dense layer of 1 m thickness with a dip very different from the bedding around 2173.6 m in the Asmari reservoir. It was interpreted as a sealing fault based on the dense appearance of its plane. In this work, we will show you that how by using a combination of FMI and RFT can interpret a sealing fault in one of the Iranian naturally fractured oil fields.

Erratum to: Biostratigraphy and paleoecology of Upper Paleocene to Lower Oligocene deposits in Southeast of the Hajiabad Area (Zagros Basin, SW Iran)

This study deals with foraminiferal biostratigraphy and paleoecological interpretations for the Late Paleocene to the Early Oligocene carbonate sequence in the Zagros Basin, the Hajiabad Area, SW Iran. This sequence could be divided into three parts, basal fossil-less strata have related to Sachun Formation base on their stratigraphical position, the middle part involves dolomitic to dolomitic limestone of Jahrum Formation, and the upper part consists of the Lower Asmari Formation. In general, the bulk of the section consists of marine deposits, and it is possible to divide them in to several parts base on benthic foraminiferal biozones. The Late Paleocene to the Early Eocene (the Lower Jahrum Formation) has been distinguished by the Miscellanea, Kathina, Alveolina, Fallotella, Lokhartia, Orbitolites, and small rotalids Assemblage Zone. The Middle Eocene (the Middle Jahrum Formation) has been characterized by the Somalina Total Range Zone and the Nummulites-Alveolina Assemblage Zone. The Late Eocene (the Upper Jahrum Formation) has been distinguished by the Chapmanina, Pllatispira, Baccullogypsinoides Assemblage zone, and the outcrop of the Asmari Formation belongs to the Early Oligocene Base on the Nummulites vascus, Nummulites fichteli Assemblage Zone. Clustering content benthic foraminifera in the studied section base on their morphology, coiling, feeding habits, and habitats lead to identification of 15 distinct assemblages with similar ecological niches. Paleoecological reconstructions base on benthic foraminiferal communities indicate that five major ecological habitats include tidal flat and shore, restricted shelf, nonrestricted shelf, oligotrophic shelf, and open marine.

Crustacean microcoprolites from Lower Cretaceous and Oligo-Miocene deposits, Persian Gulf, Iran

Biostratigraphical observations in different locations and horizons of carbonate strata from four oil wells drilled in the Persian Gulf Basin document three new ichnospecies of crustacean microcoprolites. Favreina irannensis n. sp. occurs in Valanginian–Hauterivian strata of the Buwaib Formation (Gadvan Formation), Palaxius asmariensis n. sp. in the Lower Miocene strata of the Asmari Formation, and Palaxius minaensis n. sp. in the Aptian strata of the Dariyan Formation.

Sedimentary facies, depositional environment, and sequence stratigraphy of Oligocene–Miocene shallow water carbonate from the Rig Mountain, Zagros basin (SW Iran)

This study investigates the depositional environment and sequence stratigraphy of the Asmari Formation that is exposed at the Rig anticline in the Izeh zone of the Zagros foreland basin with a thickness of 374 m as a sequence of thin, medium, thick, and massive carbonate rocks. The Asmari Formation is Oligocene–Early Miocene (Burdigalian) in age at the study area. In this area 12 microfacies are distinguished on the basis of their depositional texture, petrographic analysis, and fauna. These facies deposited in four major depositional environment including tidal flat, lagoon, shoal, and basin. The Asmari Formation represents sedimentation on a homoclinal carbonate ramp system. Based on the microfacies analysis, sequence stratigraphic studies, and distribution of planktonic and benthic foraminifera, six third-order sequences, three in Oligocene (Rupelian, Early Chattian, and Late Chattian in age) and three in Miocene (Early Aquitanian, Late Aquitanian, and Early Burdigalian in age), in Rig mountain section were identified.

Magnetotelluric signature of anticlines in Iran's Sehqanat oil field

The magnetotelluric (MT) method has proved to be an effective tool in hydrocarbon exploration especially in areas with geological structures/formations where seismic reflection provides neither good quality data nor images. The Sehqanat oil field located in the sedimentary zone of Zagros in SW of Iran is a typical example. It is covered by the high velocity and heterogeneous formation of Gachsaran, which is exposed at the surface and has a thickness varying from 500m to more than 2km in the region. Gachsaran is composed mainly of salt and evaporites overlying, as a cap rock, the Asmari limestone formation which is the main reservoir in all oil fields of Iran along the Zagros range. The main geological interface which is targeted to be imaged with the MT method is the contact between the highly conductive evaporites of the Gachsaran formation and the underlying more resistive carbonates of the Asmari formation. MT data at more than 600 stations along five parallel SW–NE profiles crossing the main geological trend of the study area and transient electromagnetic data over 400 stations to be used for static shift corrections of the MT data were available. Dimensionality and strike analysis of the MT data show dominant two-dimensional (2-D) conditions in almost all sites and periods. The 2-D resistivity models resolved the boundary between Gachsaran and Asmari formations as a transition zone from highly conductive to resistive structures. The Sehqanat anticline has also been delineated throughout the 2-D resistivity sections as a resistive dome-shaped body located in the middle part of the MT profiles. There is a considerable correlation between the 2-D resistivity models and the adjacent 2-D reflection seismic sections so that a more reliable interpretation on the hydrocarbon trap of the Sehqanat anticline can be obtained.

Application of Mixture of Gaussian Clustering on Joint Facies Interpretation of Seismic and Magnetotelluric Sections

Seismic and magnetotelluric (MT) methods are the most applicable geophysical methods in exploration of hydrocarbon resources. In this paper, mixture of Gaussian clustering is used to combine seismic and MT images under the scheme of Expectation/Maximization (EM) algorithm. Pre-Stack Depth Migration (PSDM) velocity, Root Mean Square (RMS) velocity and vertical gradient of RMS velocity of seismic and resistivity model of MT along 19.3 km MUN-21 profile in Munir Block that has been located in Southwest of Iran in Dezful embayment over the Seh-Qanat anticline are applied. The anticline is the most important oil trap of this area. The Expectation/Maximization (EM) method that has been applied includes: (1) creation of data vectors from the seismic and MT images using image processing techniques, (2) normalizing and mapping using Principal Component Analysis (PCA) procedure (3) unsupervised learning of dataset matrix, (4) setting the matrix in Expectation/Maximization (EM) iteration algorithm (5) remapping to physical space. The final model consists fof six classes which could be given to eight formations that belong to Eocene to Neocomian geological age. Pre-Stack Depth Migration (PSDM) velocity model obtained from seismic study on Seh-Qanat anticline only detected 2 horizons of formations, Asmari and Sarvak Formations; however, the current methodology introduces subdivision anticline into six classes by matching it to the log information of Seh-Qanat Deep-1 (SQD-1) borehole where it was excavated over the anticline with total depth of 2876 m.

Benthic foraminifera as biostratigraphical and paleoecological indicators: An example from Oligo-Miocene deposits in the SW of Zagros basin, Iran

The Asmari Formation is a predominantly carbonate lithostratigraphic unit that outcrops in the Zagros Basin. Micropaleontological studies of the Asmari Formation in the Dehdasht area led to the identification of 51 species of foraminifera taxa. Among the foraminifera, Nummulites cf. vascus, Operculin sp., Operculina complanata, Eulepidina dilatata, Eulepidina elephantine, Ditrupa sp., Miogypsina sp., Elphidium sp. 14, and Borelis melo curdica are the most important. The Lepidocyclina-Operculina-Ditrupa assemblage zone represents the Rupelian–Chattian age. The Aquitanian age is also defined by co-occurrence of Miogypsina sp. and Elphidium sp. 14, and finally, the first occurrence of Borelis melo curdica represents the Burdigalian. Based on faunal assemblages, the following paleoenvironmental settings are determined for the deposition of the study section: (1) the deep, offshore settings in the aphotic zone dominated by pelagic and small benthic foraminifera; (2) the low energy, turbid and low light parts of the oligophotic zone characterized by large and flat lepidocyclinids (Eulepidina) and Nummulitidae; (3) the low turbidity, deeper part of the inner ramp dominated by Miogypsinoides, Neorotalia, Lepidocyclina, Operculina and Archias; (4) the shallow, marginal marine environment exposed to salinity fluctuations (short-term salinity fluctuations or fully marine conditions) dominated by small benthic Foraminifera (Ammonia and Elphidium); (5) highly translucent, shallowest part of the inner ramp dominated by representatives of Borelis, Meandropsina and Peneroplis. The biotic assemblages represent warm tropical waters with oligotrophic to slightly mesotrophic conditions.

Facies, depositional sequences, and biostratigraphy of the Oligo-Miocene Asmari Formation in Marun oilfield, North Dezful Embayment, Zagros Basin, SW Iran

The Asmari Formation in Marun oilfield (south-west Iran), is about 440m-thick marine carbonate succession with subordinate siliciclastic rocks, characterized by abundant benthic foraminifera (perforate and imperforate). Foraminiferal biostratigraphy indicates that this unit is Oligocene–Miocene in age. The distribution of benthic foraminifera and other components have led to the recognition of three siliciclastic and ten carbonate facies that were deposited in inner ramp (shoreline, tidal flat, restricted and open lagoon and shoal), middle and outer ramp sub-environments. Based on vertical facies trends, three third-order sequences in the Oligocene and three third-order sequences in the Miocene sediments have been identified. These depositional sequences are bounded by both type 1 and type 2 sequence boundaries. The transgressive systems tracts (TST) of sequences show deepening-upward facies trend with a gradual upward increase in perforate foraminifera, whereas the highstand systems tracts (HST) have a shallowing-upward facies trend and contain predominantly imperforate foraminifera. Deposition of these depositional sequences (DS) were controlled by both eustasy and tectonic subsidence.

Facies analysis and carbonate ramp evolution of Oligo-Miocene Asmari Formation in the Gachsaran and Bibi-Hakimeh oilfields and the nearby Mish Anticline, Zagros Basin, Iran

Facies analysis and carbonate ramp evolution of Oligo-Miocene Asmari Formation in the Gachsaran and Bibi-Hakimeh oilfields and the nearby Mish Anticline, Zagros Basin, Iran

A fusion approach to identify reservoir facies based on rock physics modeling

Rock physics modeling is the process of finding a relationship between seismic data and well logs. The concluded model can play a crucial role in seismic reservoir characterization studies as well as prediction of reservoir geomechanical parameters. The present study is aimed at proposing a fusion approach based on rock physics modeling to identify reservoir facies. First, we determine suitable rock physics models to predict lithology type of formation under study (the Asmari Formation) in the Mansuri oil field, south of Iran. On the basis of pore fluid type, each of the identified lithologies is divided into two classes as oil saturated facies and water saturated facies. Then we use Support Vector Machine classifier to identify desired facies. The classification is performed in two stages: single well analysis and multi-well analysis. Finally, optimistic ordered weighted averaging method is employed to fuse the results of more than one training wells.

The Effect of Gas Injections on Oil Recovery

Core flood experiments on gas injection behavior have been conducted for one of the Iranian oil fields. Different gas injection experiments in horizontal and vertical situations are designed to study the effects of type of injected fluid and gravity force on oil recovery. A sand pack from the sands of Asmari formation of one of the Iranian oil fields is prepared and is saturated with the live oil of that field. To study the effect of gravity force, methane is then injected into the prepared sand pack in the horizontal and vertical positions. The results show that a vertical injection has a stronger effect on oil recovery due to gravity force effects. Then different gases, including nitrogen, methane, and carbon dioxide, are vertically injected in the sand pack. The maximum recovery is obtained for methane injection. When CO2 is injected some signs indicate that asphaltene is precipitated.

Geological analysis and updating velocity model by restoration of a cross section in Zagros fold–thrust belt in the southwest of Iran

The Zagros region (SW Iran) contains dominant parts of the remaining Iranian natural resources. Covering Abadan Plain, Dezful Embayment, and Izeh zone, this paper studies a structural cross section constructed across the Zagros belt using 2D and 3D seismic data, well data, surface and subsurface geological maps, satellite images, and field reconnaissance. Restoration assists us for validation and modification of the constructed cross section, better understanding of geology and stratigraphy of the Zagros basin, and computing an updated velocity model in seismic time to depth conversion. In the area of interest, Hormuz basal decollement and Gachsaran detachment play the most significant role in structural style and deformation of the Zagros. More complication links with interval decollements such as Triassic evaporites, Albian shales, and Eocene marls. Estimation of shortening is complex because of a variety of lithotectonic units and detachment surfaces. Here, we are about to introduce a new approach to update seismic velocity model by means of deduced balanced depth model by restoration and seismic two-way time. This is valuable in the case of thick sedimentary cover and steep dip of faults, where acquired seismic images are ambiguous and unreliable. The seismic velocity of Asmari and Gachsaran formations mainly depends on lateral facies and thickness variations within the overburden sediments and generally increases across the Zagros belt from foreland towards hinterland.

Fracture characterization by fusion of geophysical and geomechanical data: a case study from the Asmari reservoir, the Central Zagros fold-thrust belt

Fractured reservoirs contain a large proportion of hydrocarbon reserves in the Middle East. In these types of reservoirs, a variety of fracture types and networks provide the required permeability for hydrocarbon storage and flow. Fractured reservoir characterization has been challenging to petroleum geoscientists and reservoir engineers in terms of developing new approaches in this direction. A variety of techniques have been developed in the literature to study the distribution and the impact of fracture pore types on reservoir characterization. However, such techniques are not suitable for subsurface cases where prediction of fractures become troublesome and each of the developed techniques has its own advantages and limitations. In this study, an integrated approach is proposed for fracture characterization by employing different sources of data including 3D seismic attributes, geomechanical parameters, unconventional logs (image log and nuclear magnetic response (NMR) log), velocity-deviation log (VDL), conventional well logs, and routine core analysis data. Based on the azimuths of horizontal principal stresses and natural fractures, location of the wells over the structure hanging wall is determined. Interpretation of the seismic profiles from the study area indicated a fault-related fold structure style with fault throws controlling the magnitude of curvature. Moreover, fracture distribution of the Asmari reservoir is predicted by using curvature attribute, geomechanical parameters and horizontal slices of VDL. It seems that fractures probably have a much higher distribution at zone 1 and zone 3 of the Asmari formation.

The Estimation of Stoneley Wave Velocity from Conventional Well Log Data: Using an Integration of Artificial Neural Networks

Stoneley wave velocity (Vst) is capable of providing accurate data for reservoir characterization objectives, such as permeability estimation, fracture evaluation, formation anisotropy identification, etc. At the first stage of this study, different types of artificial neural networks, including generalized regression neural network, radial basis neural network, and feed-forward backpropagation neural network were utilized to predict Vst from conventional well log data. Consequently, a generalized regression neural network was employed to combine results of mentioned artificial neural networks for overall estimation of Vst. This novel hybrid method can enhance the accuracy of final prediction through reaping the benefits of individual artificial neural networks. The proposed methodology, hybrid neural network, was applied in Asmari formation, which is the major carbonate reservoir rock of Iranian southern oil field. A group of 1,640 data points was used to establish the intelligent model, and a group of 800 data points was employed to assess the reliability of the constructed model. Results showed that integration of different artificial neural networks using generalized regression neural network can significantly improve the accuracy of final prediction.