The mid-Cenomanian Mishrif Formation (Fm.) is considered as one of the most important rudist-bear... more The mid-Cenomanian Mishrif Formation (Fm.) is considered as one of the most important rudist-bearing reservoir horizons in the Sirri Oil Fields of the Persian Gulf. Due to the general heterogeneity of carbonate reservoirs, the use of an integrated approach is helpful for investigating porosity and permeability distribution along with recognizing controlling pore system factors in the reservoir. Thus, for the reservoir characterization of the Mishrif Fm., an integrated approach including facies analysis, diagenetic history and sequence stratigraphic analysis is considered. Detailed petrographic studies showed a total of eight microfacies and seven facies belts, related to inner ramp to the basin of a homoclinal carbonate ramp. Humid climatic condition and tectonic activity, associated with eustatic sea-level fluctuations during the mid-Cretaceous, led to meteoric diagenesis of the Mishrif carbonates during subaerial exposures (mid-Cenomanian and Cenomanian-Turonian disconformities). ...
Journal of Petroleum Exploration and Production Technology, 2020
The Upper Arab reservoir in the central Persian Gulf was examined for depositional, diagenetic, s... more The Upper Arab reservoir in the central Persian Gulf was examined for depositional, diagenetic, sequence stratigraphic and petrophysical features. This succession is composed of eight sedimentary facies that deposited on a carbonate ramp. Three-third-order sequences prograded across the Late Jurassic Arab Platform. This interval was complicated by multiple diagenetic phases including eogenesis and mesogenesis that strongly influenced reservoir properties. Dolomitization, dissolution, cementation and compaction are major diagenetic processes which played an essential role in increasing or decreasing reservoir quality. Four hydraulic flow units (HFU) were determined by flow zone indicator approach for evaluation of the reservoir quality. In addition to depositional features, diagenetic alterations have changed general HFUs characterizations such as porosity, permeability and pore-throat size. Likewise, via Lucia classification, HFUs of the Arab reservoir were grouped based on the inte...
Inland basins such as lakes and playas are promising sedimentological archives for recording pale... more Inland basins such as lakes and playas are promising sedimentological archives for recording paleoenvironmental conditions. In this study, a 5-m long sediment core from Jazmurian Playa in southeast Iran was investigated to understand the transport mechanism and palaeohydrological conditions in this region from the late Pleistocene to Holocene. Grain size analysis, total organic materials, calcimetry, elemental analysis, 14C-age dating, and scanning electron microscopy were used for this purpose. The core was divided into 8 zones (Jaz-A to H) based on macroscopic, sedimentological, and geochemical characteristics. Si and Al represented as fine-grained particles were considered as allochthonous inputs into the playa. Comparing carbonate content and size analysis of the grains, it was demonstrated that carbonate particles are terrigenous and have been transported to the basin. Total organic materials of the samples were used to infer the transporting and environmental energies as well as oxidation state of the depositional setting. The presence of Ti and Zr indicated dry conditions, whereas Rb indicated wet conditions. The alternation of wet and dry conditions in the region was evident from the different forms of sediment transport mechanisms and deposition. Windy, arid, and calm conditions were dominant from the base of the core to the 11.45 ky BP. This condition was inferred based on the low Rb and high Ti and Zr, grain surface characteristics (based on scanning electron microscopy imaging), and presence of evaporative minerals. The climate was relatively wet ca. 11.45 to 10.6 ky BP which was deduced from high Ca, Si, and total organic materials. From 10.6 to 3.5 ky BP, a transition zone from wet to dry climate was dominant in the basin. The sediments in the playa were transported mainly by the wind under dry conditions at the topmost interval of the studied core.
This study focuses on fining-upward sequences of carbonate-evaporite series of the Kangan Formati... more This study focuses on fining-upward sequences of carbonate-evaporite series of the Kangan Formation in the central Persian Gulf area. The studied sequences shows a typical succession of tempestite deposits. Comparison of these units with sedimentary deposits of other graded bedding forming mechanisms, showed that these strata have been formed during the occurrence of storms at the time of Kangan deposition in this region. Formation of these units in shallow depths of gently sloped ramp setting, well sorting especially at the middle of the unit, absence of slumping related structures in these sediments, centimeter to decimeter scale, lack of terrigenous sediments and their considerable repetition in middle to upper parts of the Kangan Formation confirms the role of storms in their formation during lower to middle Triassic in the Persian Gulf Basin. Study of these strata could be a great help for understanding the climate change at the Triassic time in Persian Gulf Basin.
Abstract Understanding the pattern of spatial permeability distribution in carbonate reservoirs s... more Abstract Understanding the pattern of spatial permeability distribution in carbonate reservoirs strongly depends on its relationships to geological factors. Despite this importance, the relationship between dolomitization and permeability changes is poorly understood. This study represents the effects of dolomitization on permeability variations in carbonate Permian–Triassic strata of the central Persian Gulf. Porosity and permeability measured on 1389 plug samples using Boyle and Darcy lows, respectively. Petrographical studies under a polarizing microscope showed that various types of facies are recognizable including boundstone, mudstone, wackestone, packstone and grainstone. Dolomitization and dissolution are the main diagenetic processes. Permeability increases along with dolomite content in the samples with higher than 1 mD permeability. Dolomitization of the allochems increased the interparticle spaces and so improved the pore connectivity. High permeability is also due to the intercrystalline pores between dolomite crystals and vuggy porosities which have been created as the result of high degree of dolomitization. Classifying the dolomitic rocks based on dolomite crystal shapes indicates that nonplanar dolomites are more frequent in the samples with less than 0.001 mD permeability while samples with more than 1 mD permeability have more euhedral dolomites. The subhedral dolomites are more frequent in the sample with 0.1–1 mD permeabilities. Nonplanar dolomites are more abundant in K1 and K3 units which are non-reservoir parts. Also, pore spaces in coarse crystal dolomites are connected to each other more effectively, which mainly occurs in K2 and K4 reservoir units. Pore typing in the studied carbonates demonstrates that moderate amounts of dolomites (about 50%) connect the other pore types while they produce a contiguous pore network in higher quantities. In the samples with more than about 60% dolomites, the intercrystalline pores are combined together and touching vugs are created, which strongly increase the permeability of the samples.
The mid-Cenomanian Mishrif Formation (Fm.) is considered as one of the most important rudist-bear... more The mid-Cenomanian Mishrif Formation (Fm.) is considered as one of the most important rudist-bearing reservoir horizons in the Sirri Oil Fields of the Persian Gulf. Due to the general heterogeneity of carbonate reservoirs, the use of an integrated approach is helpful for investigating porosity and permeability distribution along with recognizing controlling pore system factors in the reservoir. Thus, for the reservoir characterization of the Mishrif Fm., an integrated approach including facies analysis, diagenetic history and sequence stratigraphic analysis is considered. Detailed petrographic studies showed a total of eight microfacies and seven facies belts, related to inner ramp to the basin of a homoclinal carbonate ramp. Humid climatic condition and tectonic activity, associated with eustatic sea-level fluctuations during the mid-Cretaceous, led to meteoric diagenesis of the Mishrif carbonates during subaerial exposures (mid-Cenomanian and Cenomanian-Turonian disconformities). ...
Abstract The Permian–Triassic boundary corresponds with the largest mass extinction in the Earth&... more Abstract The Permian–Triassic boundary corresponds with the largest mass extinction in the Earth's history, during which most living taxa were diminished. The delayed recovery time after this boundary was also one of the longest among all mass extinction events, and resulted from enduring unfavorable environmental conditions for living organisms. Such long-lasting harsh conditions during the Early Triassic resulted not only in the delayed recovery of organisms but also considerable changes in depositional processes, which formed ‘anachronistic facies’ all around the world. In this study, anachronistic facies and features are reported, for the first time, from the Early Triassic intervals of the Persian Gulf. Also, it aims to interpret the factors governing formation of these facies using sedimentological evidence. Accordingly, three types of microbial facies (including stromatolitic boundstones, oncoidal facies and thrombolytic facies), highly cemented facies/layers, large-ooid facies and coarse intraclastic facies are introduced as the anachronistic facies of the Early Triassic sequences in four giant and supergiant gas fields in the Persian Gulf region. Seemingly, the oscillations in CaCO 3 saturation can be considered as the main cause for the formation of anachronistic facies in these intervals. The evidence shows that an increase in dissolved CO 2 caused acidification of the sea water, CaCO 3 dissolution and extinction. Carbonate dissolution causes reduction in dissolved CO 2 and increase in Ca 2 + and HCO 3 − (super-saturation situation). Therefore, sea-water under-saturation and subsequent super-saturation in an organism-free substrate were the main causes for development of such anachronistic facies (in the super-saturation situation phase).
SpringerBriefs in Petroleum Geoscience & Engineering
The ultimate goal of reservoir studies is predicting the properties and their controlling factors... more The ultimate goal of reservoir studies is predicting the properties and their controlling factors in a reservoir body based on limited data mainly from boreholes. Reservoir heterogeneity means variations of reservoir properties in space and time, and so this concept is the most important factor in reservoir studies. Despite such importance, relatively few works have been focused or documented different aspects of this subject. Evaluating heterogeneity is more complicated in carbonates which have diverse facies and are more prone to diagenetic processes. Textures and allochems vary considerably at small scales of carbonate reservoirs. Different facies are deposited in various depositional environments. They also change in response to sea-level changes and climate conditions. These building blocks integrate to create various facies belts and geometries of the depositional settings. These geometries are used for propagation of reservoir properties in field scale. Diagenetic processes modify these properties. They follow the primary textural characteristics in many cases, especially in early diagenesis. Anyway, many late diagenetic processes, such as fractures, crosscut the primary facies as well as other diagenetic features. While facies variations create heterogeneity at larger scales, diagenesis is responsible for changes at smaller scales.
Dolomitization and related anhydrite cementation can complicate the characterization of carbonate... more Dolomitization and related anhydrite cementation can complicate the characterization of carbonate reservoirs. Both processes have affected the Permo-Triassic Upper Dalan – Kangan carbonates, the main reservoir at the South Pars gasfield, offshore Iran. The carbonates were deposited in a shallow-marine ramp or epeiric platform and, according to previous studies, underwent intense near-surface diagenesis and minor burial modification. Detailed petrographical and geochemical analyses indicate that dolomitization and anhydrite precipitation can be explained in terms of the sabkha/seepage-reflux models. The early dolomites then re-equilibrated or re-crystallized in a shallow burial setting. Evaluation of poroperm values in different reservoir intervals indicates that replacive dolomitization in the absence of anhydrite precipitation or with only patchy anhydrite has enhanced the reservoir quality. Where anhydrite cement is pervasive and has plugged the rock fabric, poroperm values are si...
The mid-Cenomanian Mishrif Formation (Fm.) is considered as one of the most important rudist-bear... more The mid-Cenomanian Mishrif Formation (Fm.) is considered as one of the most important rudist-bearing reservoir horizons in the Sirri Oil Fields of the Persian Gulf. Due to the general heterogeneity of carbonate reservoirs, the use of an integrated approach is helpful for investigating porosity and permeability distribution along with recognizing controlling pore system factors in the reservoir. Thus, for the reservoir characterization of the Mishrif Fm., an integrated approach including facies analysis, diagenetic history and sequence stratigraphic analysis is considered. Detailed petrographic studies showed a total of eight microfacies and seven facies belts, related to inner ramp to the basin of a homoclinal carbonate ramp. Humid climatic condition and tectonic activity, associated with eustatic sea-level fluctuations during the mid-Cretaceous, led to meteoric diagenesis of the Mishrif carbonates during subaerial exposures (mid-Cenomanian and Cenomanian-Turonian disconformities). ...
Journal of Petroleum Exploration and Production Technology, 2020
The Upper Arab reservoir in the central Persian Gulf was examined for depositional, diagenetic, s... more The Upper Arab reservoir in the central Persian Gulf was examined for depositional, diagenetic, sequence stratigraphic and petrophysical features. This succession is composed of eight sedimentary facies that deposited on a carbonate ramp. Three-third-order sequences prograded across the Late Jurassic Arab Platform. This interval was complicated by multiple diagenetic phases including eogenesis and mesogenesis that strongly influenced reservoir properties. Dolomitization, dissolution, cementation and compaction are major diagenetic processes which played an essential role in increasing or decreasing reservoir quality. Four hydraulic flow units (HFU) were determined by flow zone indicator approach for evaluation of the reservoir quality. In addition to depositional features, diagenetic alterations have changed general HFUs characterizations such as porosity, permeability and pore-throat size. Likewise, via Lucia classification, HFUs of the Arab reservoir were grouped based on the inte...
Inland basins such as lakes and playas are promising sedimentological archives for recording pale... more Inland basins such as lakes and playas are promising sedimentological archives for recording paleoenvironmental conditions. In this study, a 5-m long sediment core from Jazmurian Playa in southeast Iran was investigated to understand the transport mechanism and palaeohydrological conditions in this region from the late Pleistocene to Holocene. Grain size analysis, total organic materials, calcimetry, elemental analysis, 14C-age dating, and scanning electron microscopy were used for this purpose. The core was divided into 8 zones (Jaz-A to H) based on macroscopic, sedimentological, and geochemical characteristics. Si and Al represented as fine-grained particles were considered as allochthonous inputs into the playa. Comparing carbonate content and size analysis of the grains, it was demonstrated that carbonate particles are terrigenous and have been transported to the basin. Total organic materials of the samples were used to infer the transporting and environmental energies as well as oxidation state of the depositional setting. The presence of Ti and Zr indicated dry conditions, whereas Rb indicated wet conditions. The alternation of wet and dry conditions in the region was evident from the different forms of sediment transport mechanisms and deposition. Windy, arid, and calm conditions were dominant from the base of the core to the 11.45 ky BP. This condition was inferred based on the low Rb and high Ti and Zr, grain surface characteristics (based on scanning electron microscopy imaging), and presence of evaporative minerals. The climate was relatively wet ca. 11.45 to 10.6 ky BP which was deduced from high Ca, Si, and total organic materials. From 10.6 to 3.5 ky BP, a transition zone from wet to dry climate was dominant in the basin. The sediments in the playa were transported mainly by the wind under dry conditions at the topmost interval of the studied core.
This study focuses on fining-upward sequences of carbonate-evaporite series of the Kangan Formati... more This study focuses on fining-upward sequences of carbonate-evaporite series of the Kangan Formation in the central Persian Gulf area. The studied sequences shows a typical succession of tempestite deposits. Comparison of these units with sedimentary deposits of other graded bedding forming mechanisms, showed that these strata have been formed during the occurrence of storms at the time of Kangan deposition in this region. Formation of these units in shallow depths of gently sloped ramp setting, well sorting especially at the middle of the unit, absence of slumping related structures in these sediments, centimeter to decimeter scale, lack of terrigenous sediments and their considerable repetition in middle to upper parts of the Kangan Formation confirms the role of storms in their formation during lower to middle Triassic in the Persian Gulf Basin. Study of these strata could be a great help for understanding the climate change at the Triassic time in Persian Gulf Basin.
Abstract Understanding the pattern of spatial permeability distribution in carbonate reservoirs s... more Abstract Understanding the pattern of spatial permeability distribution in carbonate reservoirs strongly depends on its relationships to geological factors. Despite this importance, the relationship between dolomitization and permeability changes is poorly understood. This study represents the effects of dolomitization on permeability variations in carbonate Permian–Triassic strata of the central Persian Gulf. Porosity and permeability measured on 1389 plug samples using Boyle and Darcy lows, respectively. Petrographical studies under a polarizing microscope showed that various types of facies are recognizable including boundstone, mudstone, wackestone, packstone and grainstone. Dolomitization and dissolution are the main diagenetic processes. Permeability increases along with dolomite content in the samples with higher than 1 mD permeability. Dolomitization of the allochems increased the interparticle spaces and so improved the pore connectivity. High permeability is also due to the intercrystalline pores between dolomite crystals and vuggy porosities which have been created as the result of high degree of dolomitization. Classifying the dolomitic rocks based on dolomite crystal shapes indicates that nonplanar dolomites are more frequent in the samples with less than 0.001 mD permeability while samples with more than 1 mD permeability have more euhedral dolomites. The subhedral dolomites are more frequent in the sample with 0.1–1 mD permeabilities. Nonplanar dolomites are more abundant in K1 and K3 units which are non-reservoir parts. Also, pore spaces in coarse crystal dolomites are connected to each other more effectively, which mainly occurs in K2 and K4 reservoir units. Pore typing in the studied carbonates demonstrates that moderate amounts of dolomites (about 50%) connect the other pore types while they produce a contiguous pore network in higher quantities. In the samples with more than about 60% dolomites, the intercrystalline pores are combined together and touching vugs are created, which strongly increase the permeability of the samples.
The mid-Cenomanian Mishrif Formation (Fm.) is considered as one of the most important rudist-bear... more The mid-Cenomanian Mishrif Formation (Fm.) is considered as one of the most important rudist-bearing reservoir horizons in the Sirri Oil Fields of the Persian Gulf. Due to the general heterogeneity of carbonate reservoirs, the use of an integrated approach is helpful for investigating porosity and permeability distribution along with recognizing controlling pore system factors in the reservoir. Thus, for the reservoir characterization of the Mishrif Fm., an integrated approach including facies analysis, diagenetic history and sequence stratigraphic analysis is considered. Detailed petrographic studies showed a total of eight microfacies and seven facies belts, related to inner ramp to the basin of a homoclinal carbonate ramp. Humid climatic condition and tectonic activity, associated with eustatic sea-level fluctuations during the mid-Cretaceous, led to meteoric diagenesis of the Mishrif carbonates during subaerial exposures (mid-Cenomanian and Cenomanian-Turonian disconformities). ...
Abstract The Permian–Triassic boundary corresponds with the largest mass extinction in the Earth&... more Abstract The Permian–Triassic boundary corresponds with the largest mass extinction in the Earth's history, during which most living taxa were diminished. The delayed recovery time after this boundary was also one of the longest among all mass extinction events, and resulted from enduring unfavorable environmental conditions for living organisms. Such long-lasting harsh conditions during the Early Triassic resulted not only in the delayed recovery of organisms but also considerable changes in depositional processes, which formed ‘anachronistic facies’ all around the world. In this study, anachronistic facies and features are reported, for the first time, from the Early Triassic intervals of the Persian Gulf. Also, it aims to interpret the factors governing formation of these facies using sedimentological evidence. Accordingly, three types of microbial facies (including stromatolitic boundstones, oncoidal facies and thrombolytic facies), highly cemented facies/layers, large-ooid facies and coarse intraclastic facies are introduced as the anachronistic facies of the Early Triassic sequences in four giant and supergiant gas fields in the Persian Gulf region. Seemingly, the oscillations in CaCO 3 saturation can be considered as the main cause for the formation of anachronistic facies in these intervals. The evidence shows that an increase in dissolved CO 2 caused acidification of the sea water, CaCO 3 dissolution and extinction. Carbonate dissolution causes reduction in dissolved CO 2 and increase in Ca 2 + and HCO 3 − (super-saturation situation). Therefore, sea-water under-saturation and subsequent super-saturation in an organism-free substrate were the main causes for development of such anachronistic facies (in the super-saturation situation phase).
SpringerBriefs in Petroleum Geoscience & Engineering
The ultimate goal of reservoir studies is predicting the properties and their controlling factors... more The ultimate goal of reservoir studies is predicting the properties and their controlling factors in a reservoir body based on limited data mainly from boreholes. Reservoir heterogeneity means variations of reservoir properties in space and time, and so this concept is the most important factor in reservoir studies. Despite such importance, relatively few works have been focused or documented different aspects of this subject. Evaluating heterogeneity is more complicated in carbonates which have diverse facies and are more prone to diagenetic processes. Textures and allochems vary considerably at small scales of carbonate reservoirs. Different facies are deposited in various depositional environments. They also change in response to sea-level changes and climate conditions. These building blocks integrate to create various facies belts and geometries of the depositional settings. These geometries are used for propagation of reservoir properties in field scale. Diagenetic processes modify these properties. They follow the primary textural characteristics in many cases, especially in early diagenesis. Anyway, many late diagenetic processes, such as fractures, crosscut the primary facies as well as other diagenetic features. While facies variations create heterogeneity at larger scales, diagenesis is responsible for changes at smaller scales.
Dolomitization and related anhydrite cementation can complicate the characterization of carbonate... more Dolomitization and related anhydrite cementation can complicate the characterization of carbonate reservoirs. Both processes have affected the Permo-Triassic Upper Dalan – Kangan carbonates, the main reservoir at the South Pars gasfield, offshore Iran. The carbonates were deposited in a shallow-marine ramp or epeiric platform and, according to previous studies, underwent intense near-surface diagenesis and minor burial modification. Detailed petrographical and geochemical analyses indicate that dolomitization and anhydrite precipitation can be explained in terms of the sabkha/seepage-reflux models. The early dolomites then re-equilibrated or re-crystallized in a shallow burial setting. Evaluation of poroperm values in different reservoir intervals indicates that replacive dolomitization in the absence of anhydrite precipitation or with only patchy anhydrite has enhanced the reservoir quality. Where anhydrite cement is pervasive and has plugged the rock fabric, poroperm values are si...
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