Geological reservoir characterization of the Lower Cretaceous Dariyan Formation (Shu'aiba equivalent

Geological reservoir characterization of the Lower Cretaceous Dariyan Formation (Shu'aiba equivalent) in the Persian Gulf, southern Iran

Hamzeh Mehrabi, Hossain Rhimpour, Elham Hajikazemi, Behrooz Esrafili-Dizaji

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 (10–40%) 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.

Impact of contrasting paleoclimate on carbonate reservoir architecture

Impact of contrasting paleoclimate on carbonate reservoir architecture: Cases from arid Permo-Triassic and humid Cretaceous platforms in the south and southwestern Iran

Hamzeh Mehrabi, Hossain Rahimpour-Bonab, Amir Hossain Enayati-Bidgoli, Behrooz Esrafili-Dizaji

2015, Journal of Petroleum Science and Engineering

Carbonate platforms formed and evolved in different climatic conditions (i.e. arid vs. humid) show diverse features which is reflected in their internal reservoir architectures. The Permo-Triassic Dalan–Kangan and Upper Cretaceous Sarvak carbonates host huge hydrocarbon accumulations in south and southwest Iran. These successions, along with their equivalents in the Middle East, are among the best examples of carbonate platforms formed and evolved in two different climatic conditions (i.e. arid and humid, respectively). Disparate climatic conditions had led to drastic changes in their facies characteristics, later diagenetic alterations, reservoir characteristics and architecture. To investigate the impact of paleoclimatic conditions on their reservoir characteristics, integrated sedimentological, geochemical and reservoir zonation studies are carried out on (core) samples from selected successions in seven oil and gas fields across the Dezful Embayment, in SW Iran and in the Persian Gulf. Integration of detailed sedimentological studies with petrophysical evaluations has resulted in the perception of some field-scale variations in these carbonate sequences.

Characterization of rudist-dominated units as potential reservoirs in the middle Cretaceous Sarvak F

Characterization of rudist-dominated units as potential reservoirs in the middle Cretaceous Sarvak Formation, SW Iran

Behrooz Esrafili-Dizaji, Hossain Rahimpour-Bonab, Hamzeh Mehrabi, Samineh Afshin, Farkhondeh Kiani Harchegani, Nayyer Shahverdi

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.

Controls on Reservoir Quality in the Early Triassic Kangan Formation, Iran

Controls on Reservoir Quality in the Early Triassic Kangan Formation, Iran

B. Esrafili-Dizaji, F. Kiani Harchegani, H. Rahimpour-Bonab and M.R. Kamali

 

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. Detailed diagenetic studies suggest that the Kangan sequence was exposed to early diagenesis after deposition. Two styles of early diagenetic processes are interpreted: dissolution with cementation (type A) and dolomitisation with anhydrite plugging (type B). Differences in early diagenesis are interpreted as a function of palaeosalinity on the platform. This in turn is controlled by relative sea-level fluctuations and palaeoclimatic conditions. The Kangan Formation is now buried at over 2.7 km and exposed to temperatures of about 100°C. This study indicates that the spatial and temporal development of fracturing and stylolites (types, diversity and frequency) are related to early diagenetic modifications. Our investigations conclude that the pattern and intensity of burial diagenetic and reservoir quality are mainly controlled by primary textural variation in the Kangan Formation.

Generation and evolution of oolitic shoal reservoirs in the Permo-Triassic carbonates

Generation and evolution of oolitic shoal reservoirs in the Permo-Triassic carbonates, the South Pars Field, Iran

Behrooz Esrafili-Dizaji, Hossain Rahimpour-Bonab

Ooid grainstone is the main reservoir rock in the Permo-Triassic Dalan and Kangan formations (Khuff equivalents) in many gas fields of the Persian Gulf and neighbouring areas. Ooids with a dominant aragonite mineralogy accumulated in a series of linear shoals and sand banks parallel to the shoreline, on the shallow parts of a vast epeiric carbonate platform. On the basis of the sequence stratigraphic analysis, these reservoir facies mainly developed during relative sea-level rises and increases in accommodation space. Integrated petrographic and geochemical studies reveal that ooid grainstone was altered through a complex diagenetic history, largely controlled by water chemistry, as a result of relative sea-level fluctuations. Two main types of ooid grainstone are the result: dolomitised grainstone or type H and oomouldic grainstone or type M. Dolomitised grainstone is commonly associated with the transgressive systems tract (TST), and developed under hypersaline conditions. In comparison, oomouldic grainstone is predominant in the early highstand systems tract (HST), which was affected by intensive meteoric diagenesis. Petrophysical study indicates that the reservoir properties of these rocks are largely a function of diagenesis. On the basis of their dominant pore types and diagenetic modifications, the ooid grainstone facies of the studied formations are grouped into five reservoir rock types. The spatial and temporal distribution of these rock types and their diagenetic evolution can be predicted within the sequence stratigraphic framework. Among these rock types, porosity has been greatly enhanced by dolomitisation and dissolution in the dolomitised grainstone (DG) and oomouldic grainstone (MG). On the other hand, it has been reduced by cementation and compaction in tightly cemented and compacted grainstones (CEG and COG rock types). The primary porosity has been relatively well preserved in grainstone with interparticle porosity (IPG rock type). The later porosity reduction during burial was also controlled by rock type. This study shows that despite the great burial depth, a significant amount of porosity is still preserved in oomouldic grainstone.

A review of Permo‐Triassic reservoir rocks in the Zagros area, SW Iran

A review of Permo‐Triassic reservoir rocks in the Zagros area, SW Iran: influence of the Qatar‐Fars Arch

 B Esrafili‐Dizaji, H Rahimpour‐Bonab

2013; Journal of Petroleum Geology

Abstract: 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. Facies variations in the Dalan-Kangan Formations appear to correspond to syndepositional palaeohighs and depocentres. In the Eastern Zagros (Fars area), thickening of the Dalan Formation corresponds to a Mid-Late Permian depocentre referred to here as the Permian Fars Basin. As a result of sea level fall, this depocentre evolved into a hypersaline lagoon with evaporite deposition (Nar Member). In the Triassic, the depocentre evolved into a palaeohigh as indicated by thinning and facies changes in the Kangan Formation. The results of this study draw attention to variations in the reservoir quality of the Dalan-Kangan Formations. Much of this variation was due to the influence of the Qatar-Fars Arch.

DOLOMITIZATION AND ANHYDRITE PRECIPITATION IN PERMO-TRIASSIC

DOLOMITIZATION AND ANHYDRITE PRECIPITATION IN PERMO-TRIASSIC CARBONATES AT THE SOUTH PARS GASFIELD, OFFSHORE IRAN: CONTROLS ON RESERVOIR QUALITY

H Rahimpour‐Bonab, B Esrafili‐Dizaji, V Tavakoli

Journal of Petroleum Geology 33 (1), 43-66

 

Abstract : 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.

Effects of depositional and diagenetic characteristics on carbonate reservoir quality

Effects of depositional and diagenetic characteristics on carbonate reservoir quality: a case study from the South Pars gas field in the Persian Gulf

B. Esrafili-Dizaji and H. Rahimpour-Bonab

Petroleum Geoscience, 15, 325-344, 26 October 2009, https://doi.org/10.1144/1354-079309-817

 

ABSTRACT: The largest non-associated gas reserve of the world is hosted by the Upper Dalan–Kangan (Upper Khuff equivalent) Permo-Triassic carbonate–evaporite 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. The rock type classes have been grouped on the basis of the dominant pore spaces, and have enabled distinct fields to be identified. This approach has demonstrated a relationship between poroperm values and rock type groups. Diagenetic overprinting has significantly affected the reservoir properties. Although the original poroperm heterogeneities in the studied reservoir are inherited from the Upper Dalan–Kangan palaeoplatform, they have been modified strongly by diagenetic overprinting. Consequently, tentative correlation may be possible between facies types and reservoir properties based on diagenetic effects. Therefore, for precise characterization of the Upper Dalan–Kangan reservoir properties it has been necessary to integrate both the depositional history and diagenetic features.