Behrooz Esrafili-Dizaji | University of Tehran (original) (raw)

Papers by Behrooz Esrafili-Dizaji

CARBONATE RESERVOIR ROCKS AT GIANT OIL AND GAS FIELDS IN SW IRAN AND THE ADJACENT OFFSHORE: A REVIEW OF STRATIGRAPHIC OCCURRENCE AND PORO-PERM CHARACTERISTICS, 2019

SW Iran and the adjacent offshore are prolific petroleum-producing areas with very large proven o... more SW Iran and the adjacent offshore are prolific petroleum-producing areas with very large proven oil and gas reserves and the potential for significant new discoveries. Most of the oil and gas so far discovered is present in carbonate reservoir rocks in the Dehram, Khami and Bangestan Groups and the Asmari Formation, with smaller volumes in the Dashtak, Neyriz, Najmeh, Gurpi, Pabdeh, Jahrum, Shahbazan, Razak and Mishan (Guri Member) Formations. The Permo-Triassic Dehram Group carbonates produce non-associated gas and condensate in Fars Province and the nearby offshore. The Jurassic-Lower Cretaceous Khami Group carbonates are an important producing reservoir at a number of offshore fields and in the southern Dezful Embayment, and are prospective for future exploration. Much of Iran's crude oil is produced from the Oligo-Miocene Asmari Formation and the mid-Cretaceous Sarvak Formation of the Bangestan Group in the Dezful Embayment. This review paper is based on data from 115 reservoir units at 60 oil-and gasfields in SW Iran and the adjacent offshore. It demonstrates that the main carbonate reservoir units vary from one-another significantly, depending on the particular sedimentary and diagenetic history. Ooidal-grainstones and rudist-and Lithocodium-bearing carbonate facies form the most important reservoir facies, and producing units are commonly dolomitised, karstified and fractured. In general, reservoir rocks in the study area can be classified into six major types: grainstones; reefal carbonates; karstified, dolomitised and fractured carbonates; and sandstones. The stratigraphic distribution of these reservoir rocks was principally controlled by the palaeoclimatic conditions existing at the time of deposition. A comparative reservoir analysis based on core data shows that dolomitised and/or fractured, grain-dominated carbonates in the Dehram Group, Lower Khami Group and Asmari Formation typically have better reservoir qualities than the Cretaceous limestones in the Upper

The Aptian carbonate reservoirs of Persian Gulf, known as the Dariyan Formation (Shu’aiba equival... more The Aptian carbonate reservoirs of Persian Gulf, known as the Dariyan Formation (Shu’aiba equivalent), are among the most important oil reservoirs of Iran. Despite its significance, a little is known about the facies characteristics, diagenetic history, sequence stratigraphy, and reservoir quality of this formation. Using the new core data, this study presents an integrated geological reservoir characterization of this formation in four giant fields in the Persian Gulf. Using the results of the facies analysis it could be suggested that this unit formed in a shallow carbonate platform surrounding intra-shelf basins of Kazhdumi and Bab in the western and eastern Persian Gulf, respectively. A major relative sea-level fall in the Late Aptian resulted in exposure of this unit and subsequent clastic influx as well as meteoric diagenesis. This phase of meteoric diagenesis has resulted in some important diagenetic features, special geochemical trends, and considerable variations in reservoir quality of the Dariyan Formation. Sequence stratigraphic interpretation of this formation has resulted in the recognition of 2nd and 3rd order depositional sequences in the studied wells. They are closely correlated across the studied wells in the Persian Gulf, SW Iran, and other places in the Arabian Plate. Reservoir characterization has revealed that the high quality units are positioned at two stratigraphic intervals in the concerned reservoir. The first reservoir zone is located beneath the unconformity surfaces, where meteoric dissolution of grain-to mud-dominated facies has resulted in considerable amounts (10 to 40%) of vuggy, moldic, and micro-porosity in the highstand systems tracts (HSTs). The second reservoir zone of this formation has formed within the Lithocodium-algal facies, floatstone and boundstone, of inner ramp setting. The latter reservoir facies are mainly concentrated within the fields of the eastern Persian Gulf.

The Sarvak reservoirs are characterized by thick rudist-dominated intervals in the south and sout... more The Sarvak reservoirs are characterized by thick rudist-dominated intervals in the south and southwest Iran. During the middle Cretaceous, rudistid communities were widely developed on the shallow parts of the Sarvak platform, mainly in the central Zagros and eastern Persian Gulf. Regional distribution and subsequent diagenesis of the rudist-dominated facies were strongly controlled by a number of paleostructures (i.e., fault-related paleohighs and salt domes) in the area. Facies analysis in 16 Sarvak reservoirs reveals the rudistid units can be classified into three main facies groups, developing thick depositional cycles in the Sarvak Formation. Integrated petrographic and geochemical data shows the rudist-dominated facies were subjected to fresh water diagenesis caused by a considerable drop in relative sea level after the deposition. Subsequently, where they were subaerially exposed over the crests and flanks of the paleohighs, the initial porosity of the rudist facies was enhanced by extensive meteoric dissolution (types A and B). Farther away from the paleohighs, same facies have poor reservoir quality, because the porosity was reduced by meteoric and shallow to deep burial cements, and mechanical/chemical compaction (type C). Despite their deep burial depth, significant amounts of porosity (>10 %) are still preserved in the rudist-dominated facies of the Sarvak Formation, especially below disconformity surfaces. Results of this study reveal the controls of early diagenesis on later diagenetic modification and porosity evolution in the Sarvak Formation, as the second important oil-producing reservoir of Iran.

Carbonate platformsformedandevolvedindifferentclimaticconditions(i.e.aridvs.humid)show diversefea... more Carbonate platformsformedandevolvedindifferentclimaticconditions(i.e.aridvs.humid)show diversefeatureswhichisreflectedintheirinternalreservoirarchitectures.ThePermo-TriassicDalan– Kangan andUpperCretaceousSarvakcarbonateshosthugehydrocarbonaccumulationsinsouthand southwest Iran.Thesesuccessions,alongwiththeirequivalentsintheMiddleEast,areamongthebest examplesofcarbonateplatformsformedandevolvedintwodifferentclimaticconditions(i.e.aridand humid, respectively).Disparateclimaticconditionshadledtodrasticchangesintheirfaciescharacter- istics, laterdiageneticalterations,reservoircharacteristicsandarchitecture.Toinvestigatetheimpactof paleoclimatic conditionsontheirreservoircharacteristics,integratedsedimentological,geochemicaland reservoirzonationstudiesarecarriedouton(core)samplesfromselectedsuccessionsinsevenoiland gas fields acrosstheDezfulEmbayment,inSWIranandinthePersianGulf.Integrationofdetailed sedimentological studieswithpetrophysicalevaluationshasresultedintheperceptionofsome field-scale variationsinthesecarbonatesequences.

The Middle-Late Triassic Dashtak Formation provides an effective, regional seal for the Permo-Tri... more The Middle-Late Triassic Dashtak Formation provides an effective, regional seal for the Permo-Triassic Dalan-Kangan (Khuff) reservoirs and also forms the subordinate reservoir in a number of gas fields in the Zagros domain. The evaporite-dominated formation is subdivided into six members. Lithofacies and isopack maps show strong controls of deep-seated faults, particularly Kazerun and Balarud during deposition of this formation. Comparison of isopack maps of the Dalan and Dashtak formations indicates that there is a shifting of depocenter from east- to westward of Kazerun Fault from Permian to Triassic. This shifting is interpreted to be the effect of the fault reactivation caused by the Neotethys opening. The Dashtak Formation consists of four large-scale carbonate-evaporite cycles, which was deposited in a carbonate/evaporite platform. Periodic development of carbonate and anhydrite intervals was controlled by relative sea-level fluctuations under a prevailing arid paleoclimatic conditions. The Dashtak Formation consists of four large-scale carbonate-evaporite cycles, which was deposited in a carbonate/evaporite platform. Periodic development of carbonate and anhydrite intervals was controlled by relative sea-level fluctuations under a prevailing arid paleoclimatic conditions. Strontium isotope ratios (87Sr/86Sr=0.7074-07085) record varies age from Early to Middle/Late Triassic for the studied formation.

The Early Triassic Kangan Formation is one of the most prolific gas reservoirs in Iran. The forma... more The Early Triassic Kangan Formation is one of the most prolific gas reservoirs in Iran. The formation consists in two reservoir units: K2 and K1 in the area of investigation. Ten facies are distinguished in the Kangan Formation. These facies are interpreted as peritidal, lagoonal, shoal and fore shoal deposits. Two depositional sequences, Khuff sequence-2 (KS-2) and KS-1 are differentiated based on facies stacking patterns.

Four "supergiant" and numerous giant gasfields have been discovered in the Zagros area of SW Iran... more Four "supergiant" and numerous giant gasfields have been discovered in the Zagros area of SW Iran. The gasfields are concentrated in the eastern part of the foldbelt, in Fars Province and the adjacent offshore, and produce from Permo-Triassic carbonates equivalent to the Khuff Formation. The carbonates belong to the upper member of the Dalan Formation and the overlying Kangan Formation. Reservoir rock quality is strongly influenced by tectonic setting and depositional environment, and also by diagenesis. The highest quality reservoirs occur in oolitic shoal facies; fracturing (especially in onshore fields) and dolomitisation (in offshore fields) have also influenced reservoir quality. Anhydrite plugging is common in reservoirs in offshore fields, while calcite cementation is dominant in onshore reservoirs.

The South Pars Field, discovered in 1990, is part of the world's largest single gas accumulation ... more The South Pars Field, discovered in 1990, is part of the world's largest single gas accumulation located in the Gulf. The Iranian part of this immense gas accumulation accounts for 5% of the world's and 60% of Iran's total gas reserves. This field produces from Dalan/Kangan carbonates (Khuff analogues). These Permian-Triassic carbonate reservoirs in the field are highly stratified in nature and display layer-cake geometry. Combined core analysis and detailed thin section studies are used for facies analysis. Accordingly, 14 major facies were recognized in these carbonates. Facies analysis shows that their depositional setting was located along the inner part of an epiric carbonate system that extended from a peritidal setting to a shallow subtidal zone (back-shoal setting), passing over to a high-energy shoal and fore-shoal facies ).

The Dalan-Kangan Permo-Triassic aged carbonates were deposited in the South Pars gas field in the... more The Dalan-Kangan Permo-Triassic aged carbonates were deposited in the South Pars gas field in the Persian Gulf Basin, offshore Iran. Based on the thin section studies from this field, pore spaces are classified into three groups including depositional, fabric-selective and non-fabric selective. Stable isotope studies confirm the role of diagenesis in reservoir quality development. Integration of various data show that different diagenetic processes developed in two reservoir zones in the Kangan and Dalan formations. While dolomitisation enhanced reservoir properties in the upper K2 and lower K4 units, lower part of K2 and upper part of K4 have experienced more dissolution. Integration of RQI, porosity-permeability values and pore-throat sizes resulted from mercury intrusion tests shows detailed petrophysical behavior in reservoir zones. Though both upper K2 and lower K4 are dolomitised, in upper K2 unit non-fabric selective pores are dominant and fabric destructive dolomitisation is the main cause of high reservoir quality. In comparison, lower K4 has more fabric-selective pores that have been connected by fabric retentive to selective dolomitisation.Les carbonates permo-triasiques de Dalan-Kangan se sont déposés dans le champ de gaz de South Pars, dans le Golfe persique, offshore de l’Iran. A partir d’études en lame mince dans ce champ, les espaces poraux sont classés en trois groupes : dépositionnel, sélectif de texture, non sélectif de texture. Les isotopes stables confirment le rôle de la diagenèse dans le développement de la qualité du réservoir. En intégrant des données variées, on montre que des processus diagénétiques différents se développent dans deux zones réservoir, dans les formations de Kangan et Dalan. Tandis que la dolomitisation renforce les propriétés de réservoir, dans les unités supérieure K2 et inférieure K4, la partie inférieure du K2 et la partie supérieure de K4 sont l’objet d’une plus forte dissolution. L’intégration de RQI, des valeurs de porosité-perméabilité et de la taille de l’ouverture des pores résultant de tests d’intrusion de mercure, montre le comportement pétrophysique détaillé dans les zones réservoir. Bien que les unités supérieure K2 et inférieure K4 soient toutes les deux dolomitisées, dans l’unité supérieure K2, les pores non sélectifs de texture sont dominants et la dolomitisation destructrice de texture est la cause principale de la haute qualité du réservoir. En comparaison, l’unité inférieure K4 a plus de pores sélectifs de texture, qui ont été connectés par une dolomitisation rétentrice à sélective de texture.

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 significantly decreased.As emphasized in previous studies and confirmed here, dolomitization and anhydrite cementation, together with original facies type, are the major factors controlling reservoir quality in the Dalan – Kangan carbonates at South Pars. When associated with minor anhydrite cementation, replacive dolomitization has enhanced reservoir quality by increasing permeability. However, porosity in fabric-retentive dolomite was apparently inherited from the precursor rock and therefore reflects the original depositional environment.Low-temperature dolomitization is commonly fabric-selective and partially fabric-retentive. Whole rock stable isotope thermometry indicates that fabric-destructive dolomites in the reservoir rocks formed at temperatures above 22°C, whereas fabric-retentive dolomites and associated anhydrites formed in surface and near-surface conditions. Fabric-destructive dolomite or dolomite neomorphism post-date fabric-retentive dolomite and continued to form in deep burial conditions (∼1400m). These observations may explain why fabric-retentive dolomite and anhydrite fabrics are traversed by stylolites.

The largest non-associated gas reserve of the world is hosted by the Upper Dalan-Kangan (Upper Kh... more The largest non-associated gas reserve of the world is hosted by the Upper Dalan-Kangan (Upper Khuff equivalent) Permo-Triassic carbonateevaporite successions. Detailed characterization of these strata in the South Pars field has shown that the reservoir properties are a function of both sedimentary and diagenetic processes at the field scale. Facies analysis of the studied units indicates that the sediments were deposited in the inner regions of a homoclinal carbonate ramp and were subsequently subjected to shallow diagenesis and minor burial. The vertical distribution of the facies shows cyclic patterns that impact reservoir quality.

CARBONATE RESERVOIR ROCKS AT GIANT OIL AND GAS FIELDS IN SW IRAN AND THE ADJACENT OFFSHORE: A REVIEW OF STRATIGRAPHIC OCCURRENCE AND PORO-PERM CHARACTERISTICS, 2019

SW Iran and the adjacent offshore are prolific petroleum-producing areas with very large proven o... more SW Iran and the adjacent offshore are prolific petroleum-producing areas with very large proven oil and gas reserves and the potential for significant new discoveries. Most of the oil and gas so far discovered is present in carbonate reservoir rocks in the Dehram, Khami and Bangestan Groups and the Asmari Formation, with smaller volumes in the Dashtak, Neyriz, Najmeh, Gurpi, Pabdeh, Jahrum, Shahbazan, Razak and Mishan (Guri Member) Formations. The Permo-Triassic Dehram Group carbonates produce non-associated gas and condensate in Fars Province and the nearby offshore. The Jurassic-Lower Cretaceous Khami Group carbonates are an important producing reservoir at a number of offshore fields and in the southern Dezful Embayment, and are prospective for future exploration. Much of Iran's crude oil is produced from the Oligo-Miocene Asmari Formation and the mid-Cretaceous Sarvak Formation of the Bangestan Group in the Dezful Embayment. This review paper is based on data from 115 reservoir units at 60 oil-and gasfields in SW Iran and the adjacent offshore. It demonstrates that the main carbonate reservoir units vary from one-another significantly, depending on the particular sedimentary and diagenetic history. Ooidal-grainstones and rudist-and Lithocodium-bearing carbonate facies form the most important reservoir facies, and producing units are commonly dolomitised, karstified and fractured. In general, reservoir rocks in the study area can be classified into six major types: grainstones; reefal carbonates; karstified, dolomitised and fractured carbonates; and sandstones. The stratigraphic distribution of these reservoir rocks was principally controlled by the palaeoclimatic conditions existing at the time of deposition. A comparative reservoir analysis based on core data shows that dolomitised and/or fractured, grain-dominated carbonates in the Dehram Group, Lower Khami Group and Asmari Formation typically have better reservoir qualities than the Cretaceous limestones in the Upper

The Aptian carbonate reservoirs of Persian Gulf, known as the Dariyan Formation (Shu’aiba equival... more The Aptian carbonate reservoirs of Persian Gulf, known as the Dariyan Formation (Shu’aiba equivalent), are among the most important oil reservoirs of Iran. Despite its significance, a little is known about the facies characteristics, diagenetic history, sequence stratigraphy, and reservoir quality of this formation. Using the new core data, this study presents an integrated geological reservoir characterization of this formation in four giant fields in the Persian Gulf. Using the results of the facies analysis it could be suggested that this unit formed in a shallow carbonate platform surrounding intra-shelf basins of Kazhdumi and Bab in the western and eastern Persian Gulf, respectively. A major relative sea-level fall in the Late Aptian resulted in exposure of this unit and subsequent clastic influx as well as meteoric diagenesis. This phase of meteoric diagenesis has resulted in some important diagenetic features, special geochemical trends, and considerable variations in reservoir quality of the Dariyan Formation. Sequence stratigraphic interpretation of this formation has resulted in the recognition of 2nd and 3rd order depositional sequences in the studied wells. They are closely correlated across the studied wells in the Persian Gulf, SW Iran, and other places in the Arabian Plate. Reservoir characterization has revealed that the high quality units are positioned at two stratigraphic intervals in the concerned reservoir. The first reservoir zone is located beneath the unconformity surfaces, where meteoric dissolution of grain-to mud-dominated facies has resulted in considerable amounts (10 to 40%) of vuggy, moldic, and micro-porosity in the highstand systems tracts (HSTs). The second reservoir zone of this formation has formed within the Lithocodium-algal facies, floatstone and boundstone, of inner ramp setting. The latter reservoir facies are mainly concentrated within the fields of the eastern Persian Gulf.

The Sarvak reservoirs are characterized by thick rudist-dominated intervals in the south and sout... more The Sarvak reservoirs are characterized by thick rudist-dominated intervals in the south and southwest Iran. During the middle Cretaceous, rudistid communities were widely developed on the shallow parts of the Sarvak platform, mainly in the central Zagros and eastern Persian Gulf. Regional distribution and subsequent diagenesis of the rudist-dominated facies were strongly controlled by a number of paleostructures (i.e., fault-related paleohighs and salt domes) in the area. Facies analysis in 16 Sarvak reservoirs reveals the rudistid units can be classified into three main facies groups, developing thick depositional cycles in the Sarvak Formation. Integrated petrographic and geochemical data shows the rudist-dominated facies were subjected to fresh water diagenesis caused by a considerable drop in relative sea level after the deposition. Subsequently, where they were subaerially exposed over the crests and flanks of the paleohighs, the initial porosity of the rudist facies was enhanced by extensive meteoric dissolution (types A and B). Farther away from the paleohighs, same facies have poor reservoir quality, because the porosity was reduced by meteoric and shallow to deep burial cements, and mechanical/chemical compaction (type C). Despite their deep burial depth, significant amounts of porosity (>10 %) are still preserved in the rudist-dominated facies of the Sarvak Formation, especially below disconformity surfaces. Results of this study reveal the controls of early diagenesis on later diagenetic modification and porosity evolution in the Sarvak Formation, as the second important oil-producing reservoir of Iran.

Carbonate platformsformedandevolvedindifferentclimaticconditions(i.e.aridvs.humid)show diversefea... more Carbonate platformsformedandevolvedindifferentclimaticconditions(i.e.aridvs.humid)show diversefeatureswhichisreflectedintheirinternalreservoirarchitectures.ThePermo-TriassicDalan– Kangan andUpperCretaceousSarvakcarbonateshosthugehydrocarbonaccumulationsinsouthand southwest Iran.Thesesuccessions,alongwiththeirequivalentsintheMiddleEast,areamongthebest examplesofcarbonateplatformsformedandevolvedintwodifferentclimaticconditions(i.e.aridand humid, respectively).Disparateclimaticconditionshadledtodrasticchangesintheirfaciescharacter- istics, laterdiageneticalterations,reservoircharacteristicsandarchitecture.Toinvestigatetheimpactof paleoclimatic conditionsontheirreservoircharacteristics,integratedsedimentological,geochemicaland reservoirzonationstudiesarecarriedouton(core)samplesfromselectedsuccessionsinsevenoiland gas fields acrosstheDezfulEmbayment,inSWIranandinthePersianGulf.Integrationofdetailed sedimentological studieswithpetrophysicalevaluationshasresultedintheperceptionofsome field-scale variationsinthesecarbonatesequences.

The Middle-Late Triassic Dashtak Formation provides an effective, regional seal for the Permo-Tri... more The Middle-Late Triassic Dashtak Formation provides an effective, regional seal for the Permo-Triassic Dalan-Kangan (Khuff) reservoirs and also forms the subordinate reservoir in a number of gas fields in the Zagros domain. The evaporite-dominated formation is subdivided into six members. Lithofacies and isopack maps show strong controls of deep-seated faults, particularly Kazerun and Balarud during deposition of this formation. Comparison of isopack maps of the Dalan and Dashtak formations indicates that there is a shifting of depocenter from east- to westward of Kazerun Fault from Permian to Triassic. This shifting is interpreted to be the effect of the fault reactivation caused by the Neotethys opening. The Dashtak Formation consists of four large-scale carbonate-evaporite cycles, which was deposited in a carbonate/evaporite platform. Periodic development of carbonate and anhydrite intervals was controlled by relative sea-level fluctuations under a prevailing arid paleoclimatic conditions. The Dashtak Formation consists of four large-scale carbonate-evaporite cycles, which was deposited in a carbonate/evaporite platform. Periodic development of carbonate and anhydrite intervals was controlled by relative sea-level fluctuations under a prevailing arid paleoclimatic conditions. Strontium isotope ratios (87Sr/86Sr=0.7074-07085) record varies age from Early to Middle/Late Triassic for the studied formation.

The Early Triassic Kangan Formation is one of the most prolific gas reservoirs in Iran. The forma... more The Early Triassic Kangan Formation is one of the most prolific gas reservoirs in Iran. The formation consists in two reservoir units: K2 and K1 in the area of investigation. Ten facies are distinguished in the Kangan Formation. These facies are interpreted as peritidal, lagoonal, shoal and fore shoal deposits. Two depositional sequences, Khuff sequence-2 (KS-2) and KS-1 are differentiated based on facies stacking patterns.

Four "supergiant" and numerous giant gasfields have been discovered in the Zagros area of SW Iran... more Four "supergiant" and numerous giant gasfields have been discovered in the Zagros area of SW Iran. The gasfields are concentrated in the eastern part of the foldbelt, in Fars Province and the adjacent offshore, and produce from Permo-Triassic carbonates equivalent to the Khuff Formation. The carbonates belong to the upper member of the Dalan Formation and the overlying Kangan Formation. Reservoir rock quality is strongly influenced by tectonic setting and depositional environment, and also by diagenesis. The highest quality reservoirs occur in oolitic shoal facies; fracturing (especially in onshore fields) and dolomitisation (in offshore fields) have also influenced reservoir quality. Anhydrite plugging is common in reservoirs in offshore fields, while calcite cementation is dominant in onshore reservoirs.

The South Pars Field, discovered in 1990, is part of the world's largest single gas accumulation ... more The South Pars Field, discovered in 1990, is part of the world's largest single gas accumulation located in the Gulf. The Iranian part of this immense gas accumulation accounts for 5% of the world's and 60% of Iran's total gas reserves. This field produces from Dalan/Kangan carbonates (Khuff analogues). These Permian-Triassic carbonate reservoirs in the field are highly stratified in nature and display layer-cake geometry. Combined core analysis and detailed thin section studies are used for facies analysis. Accordingly, 14 major facies were recognized in these carbonates. Facies analysis shows that their depositional setting was located along the inner part of an epiric carbonate system that extended from a peritidal setting to a shallow subtidal zone (back-shoal setting), passing over to a high-energy shoal and fore-shoal facies ).

The Dalan-Kangan Permo-Triassic aged carbonates were deposited in the South Pars gas field in the... more The Dalan-Kangan Permo-Triassic aged carbonates were deposited in the South Pars gas field in the Persian Gulf Basin, offshore Iran. Based on the thin section studies from this field, pore spaces are classified into three groups including depositional, fabric-selective and non-fabric selective. Stable isotope studies confirm the role of diagenesis in reservoir quality development. Integration of various data show that different diagenetic processes developed in two reservoir zones in the Kangan and Dalan formations. While dolomitisation enhanced reservoir properties in the upper K2 and lower K4 units, lower part of K2 and upper part of K4 have experienced more dissolution. Integration of RQI, porosity-permeability values and pore-throat sizes resulted from mercury intrusion tests shows detailed petrophysical behavior in reservoir zones. Though both upper K2 and lower K4 are dolomitised, in upper K2 unit non-fabric selective pores are dominant and fabric destructive dolomitisation is the main cause of high reservoir quality. In comparison, lower K4 has more fabric-selective pores that have been connected by fabric retentive to selective dolomitisation.Les carbonates permo-triasiques de Dalan-Kangan se sont déposés dans le champ de gaz de South Pars, dans le Golfe persique, offshore de l’Iran. A partir d’études en lame mince dans ce champ, les espaces poraux sont classés en trois groupes : dépositionnel, sélectif de texture, non sélectif de texture. Les isotopes stables confirment le rôle de la diagenèse dans le développement de la qualité du réservoir. En intégrant des données variées, on montre que des processus diagénétiques différents se développent dans deux zones réservoir, dans les formations de Kangan et Dalan. Tandis que la dolomitisation renforce les propriétés de réservoir, dans les unités supérieure K2 et inférieure K4, la partie inférieure du K2 et la partie supérieure de K4 sont l’objet d’une plus forte dissolution. L’intégration de RQI, des valeurs de porosité-perméabilité et de la taille de l’ouverture des pores résultant de tests d’intrusion de mercure, montre le comportement pétrophysique détaillé dans les zones réservoir. Bien que les unités supérieure K2 et inférieure K4 soient toutes les deux dolomitisées, dans l’unité supérieure K2, les pores non sélectifs de texture sont dominants et la dolomitisation destructrice de texture est la cause principale de la haute qualité du réservoir. En comparaison, l’unité inférieure K4 a plus de pores sélectifs de texture, qui ont été connectés par une dolomitisation rétentrice à sélective de texture.

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 significantly decreased.As emphasized in previous studies and confirmed here, dolomitization and anhydrite cementation, together with original facies type, are the major factors controlling reservoir quality in the Dalan – Kangan carbonates at South Pars. When associated with minor anhydrite cementation, replacive dolomitization has enhanced reservoir quality by increasing permeability. However, porosity in fabric-retentive dolomite was apparently inherited from the precursor rock and therefore reflects the original depositional environment.Low-temperature dolomitization is commonly fabric-selective and partially fabric-retentive. Whole rock stable isotope thermometry indicates that fabric-destructive dolomites in the reservoir rocks formed at temperatures above 22°C, whereas fabric-retentive dolomites and associated anhydrites formed in surface and near-surface conditions. Fabric-destructive dolomite or dolomite neomorphism post-date fabric-retentive dolomite and continued to form in deep burial conditions (∼1400m). These observations may explain why fabric-retentive dolomite and anhydrite fabrics are traversed by stylolites.

The largest non-associated gas reserve of the world is hosted by the Upper Dalan-Kangan (Upper Kh... more The largest non-associated gas reserve of the world is hosted by the Upper Dalan-Kangan (Upper Khuff equivalent) Permo-Triassic carbonateevaporite successions. Detailed characterization of these strata in the South Pars field has shown that the reservoir properties are a function of both sedimentary and diagenetic processes at the field scale. Facies analysis of the studied units indicates that the sediments were deposited in the inner regions of a homoclinal carbonate ramp and were subsequently subjected to shallow diagenesis and minor burial. The vertical distribution of the facies shows cyclic patterns that impact reservoir quality.

Standard microfacies (SMF's) of a carbonate shelf