Controls on depositional facies in Upper Cretaceous carbonate reservoirs in the Zagros area and the Persian Gulf, Iran (original) (raw)

2015, Facies

https://doi.org/10.1007/S10347-015-0450-8

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Abstract

Deep-marine clastics Shallow-marine clastics Deep-marine carbonates Continental deposits Open-marine carbonate Shallow-marine carbonate Evaporites Coastal-deltaic deposits Upper Cretaceous (Turonian-Maastrichtian) Obducted ophiolites Neo-Tethys Neo-Tethys

FACIES ANALYSIS AND DEPOSITIONAL SEQUENCES OF THE UPPER JURASSIC MOZDURAN FORMATION, A CARBONATE RESERVOIR IN THE KOPET DAGH BASIN, NE IRAN

Journal of Petroleum Geology, 2009

Upper Jurassic carbonates of the Mozduran Formation constitute the principal reservoir intervals at the giant Khangiran and Gonbadli gasfields in the Kopet Dagh Basin, NE Iran. These carbonates were investigated using detailed field studies and petrographic and wireline log analyses in order to clarify their depositional facies and sequence stratigraphy. Facies were interpreted to reflect deep basin, fore-shoal, shelf margin, lagoon, tidal flat and coastal plain depositional systems.The Mozduran Formation is composed of six depositional sequences. Thickness variations were controlled by differential subsidence. Aggradation on the platform margin and reduced carbonate production in the deep basin together with differential subsidence resulted in the creation of a narrow seaway during the late Oxfordian. Petrographic studies suggest that Mozduran Formation carbonates had a low-Mg calcite mineralogy during the Oxfordian, and an aragonite to high-Mg calcite mineralogy during the Kimmeridgian. Reservoir pay zones are located in highstand systems tracts within the lower and middle Kimmeridgian depositional sequences. The rapid lateral thickness variations of these sequences were controlled by tectonic factors, leading to compartmentalization of the Mozduran Formation reservoir with the possible creation of stratigraphic traps, especially at the Khangiran field.

Tectono-sedimentary controls on Lower Cretaceous carbonate platforms of the central Zagros, Iran: An example of rift-basin carbonate systems

Marine and Petroleum Geology, 2019

This study focuses on the variations in facies characteristics, depositional environments, and sedimentary sequences of the Fahliyan Formation carbonate succession (Neocomian-Barremian) as one of the petroliferous units in the Middle East. Accordingly, four surface and subsurface sections were selected from the Dezful Embayment and Izeh Zone of the central Zagros, SW Iran. A shoal-algal facies dominated homoclinal ramp is proposed as the general depositional setting of this formation. It shows remarkable variations in facies belts, their abundance and thickness in the study area. Four third-order depositional sequences are differentiated in studied outcrops of the Izeh Zone. Interaction of tectonics, eustasy, and sedimentation (carbonate production) rates are discussed for depositional sequences and three tectono-sedimentary models are presented for the Fahliyan Formation that correspond to depositional sequences I to III of this formation. Variations in thickness, facies, diagenesis, and allochemical grains are discussed for each tectono-sedimentary model. Comparison of the acquired results with the proposed models of active tectonic basins suggests a fault-block platform in an extensional rift basin for the Fahliyan Formation in the study area.

Depositional History and Sequence Stratigraphy of Outcropping Tertiary Carbonates in the Jahrum and Asmari Formations, Shiraz Area (SW Iran)

Journal of Petroleum Geology, 2004

The Oligo-Miocene Asmari Formation is one of the most important petroleum reservoir units in the Zagros Basin of south and SW Iran. It mainly consists of limestones and dolomitic limestones with interbedded shales, together with a few intervals of sandstone and gypsum assigned to the Ahwaz and Kalhur Members, respectively. The Asmari Formation rests on the thin-bedded limestones of the Jahrum Formation (Paleocene-Eocene). In this paper, we report on the lithofacies characteristics of these two formations using data from three measured outcrop sections near Shiraz in SW Iran. From field and petrographic data, we have identified four major lithofacies and twelve subfacies which are interpreted to have been deposited in open-marine, shoal, lagoon and tidal flat settings. We show that the Asmari and Jahrum Formations constitute two separate depositional sequences which are separated by a thin palaeosol, representing a type-one sequence boundary which can be correlated with global curves of relative sea-level. Each depositional sequence is composed of many metre-scale shallowing-upward parasequences. This is the first time that the Asmari and Jahrum Formations have been differentiated in the study area. We hope that this study will lead to a better understanding of the Asmari Formation in the subsurface in other parts of the Zagros Basin.

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

Marine and Petroleum Geology, 2015

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 2 nd and 3 rd 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

Paleoclimate and tectonic controls on the depositional and diagenetic history of the Cenomanian–early Turonian carbonate reservoirs, Dezful Embayment, SW Iran

Facies, 2013

Integrated facies and diagenetic analyses within a sequence stratigraphic framework were carried out on mid-Cretaceous Sarvak carbonate reservoirs in five giant and supergiant oilfields in the central and southern parts of the Dezful Embayment, SW Iran. Results of facies analysis indicate a homoclinal ramp-type carbonate platform for this formation with the frequencies of different facies associations in six wells reflecting their approximate position in the sedimentary model. Diagenetic studies indicate periods of subaerial exposure with different intensities and durations in the upper Sarvak carbonates producing karstified profiles, dissolution-collapse breccias, and thick bauxitic-lateritic horizons. Sequence stratigraphic interpretations show that the tectonic evolution of the NE margin of the Arabian Plate (Zagros Basin) during Cenomanian-Turonian times shaped the facies characteristics, diagenetic features, and strongly influenced reservoir formation. Reactivation of basement-block faults and halokinetic movements (related to the Hormoz salt series) in the middle Cretaceous, resulted in the development of several paleohighs and troughs in the Dezful Embayment hydrocarbon province. Movements on these structures generated two and locally three disconformities in the upper parts of Sarvak Formation in this region. The paleohighs played an important role in reservoir evolution within the Sarvak Formation in three giant-supergiant oilfields (including Gachsaran, Rage -Safid, and Abteymour oilfields) but where these structures are absent reservoir quality is low.

Inorganic control on original carbonate mineralogy and creation of gas reservoir of the Upper Jurassic carbonates in the Kopeh-Dagh Basin, NE Iran

The Upper Jurassic Mozduran Formation (Oxfordian-Tithonian) is the main petroleum reservoir in the Kopeh-Dagh Basin, northeast Iran which consists predominantly of carbonate rocks with subordinate evaporites and siliciclastics that were deposited in platform to deep marine settings of a subtropical sea. Detailed field surveys, petrographic investigations, facies and wire line logs analyses were carried out at eight surface sections and four wells in the Kopeh-Dagh Basin. Integration of petrographic and isotopic data suggests primary low-Mg calcite (LMC) mineralogy of Oxfordian and Tithonian ooids. On the other hand, in the wells, Kimmeridgian ooids and cements are dominantly aragonitic and high-Mg calcite mineralogy (HMC). Marine cements with isopachous, fibrous and isopachous bladed fabrics indicate original aragonite and HMC mineralogy, respectively. The domination of aragonite mineralogy could be related to increased hypersalinity, evaporite precipitation and consequently an increase in Mg/Ca ratio, which resulted in formation of aragonite in preference to calcite. Preserved ooids with radial and concentric cortices in shallow-water settings that are nearby siliciclastic source, together with aragonitic and HMC ooids accompanied by evaporites in the drilled fields, suggest original mineralogy was probably controlled by inorganically following local salinity variations. This study suggests that Kimmeridgian pay zones are mainly controlled by depositional facies, aragonitic and HMC mineralogy, and diagenetic processes such dolomitization and dissolution.

Facies analysis and depositional sequences of the middle Cretaceous Sarvak Formation in the northwest of Behbahan, Zagros basin, Iran

Episodes, 2017

Sarvak Formation, a thick carbonate succession with the age of Middle Cretaceous, is one of the most significant hydrocarbon reservoirs in the Zagros basin. The Tang-e Solak section of the Sarvak Formation in the Bangestan anticline with a total thickness of about 850 m has been studied, in order to determinate its microfacies types, depositional setting model and depositional sequences. Detailed textural and petrographic analysis led to the identification of 11 microfacies that arranged in three facies associations: inner ramp, mid-ramp, outer ramp and basin. Examination of vertical and lateral facies changes indicated these sediments were deposited in a carbonate ramp depositional system. Frequency analysis of facies indicated outer ramp and basin facies associations are the most frequent facies. Sea level fluctuations during in deposition of Sarvak successions led to formation of two depositional sequences. These depositional sequences are composed of transgressive systems tract and highstand systems tract. The highstand systems tract can be separated into Early HST and Late HST. Stacking pattern in Early HST part is aggradational and in Late HST is progradational.

Arabian carbonate reservoirs: A depositional model of the Arab-D reservoir in Khurais field, Saudi Arabia

AAPG Bulletin, 2013

The Upper Jurassic Arab Formation in the Arabian Peninsula, the most prolific oil-bearing interval of the world, is a succession of interbedded thick carbonates and evaporites that are defined stratigraphically upsection as the Arab-D, Arab-C, Arab-B, and Arab-A. The Arab-D reservoir is the main reservoir in Khurais field, one of the largest onshore oil fields of the Kingdom of Saudi Arabia. In Khurais field, the Arab-D reservoir is composed of the overlying evaporitic Arab-D Member of the Arab Formation and the underlying upper part of the Jubaila Formation. It contains 11 lithofacies, listed from deepest to shallowest: (1) hardground-capped skeletal wackestone and lime mudstone; (2) intraclast floatstone and rudstone; (3) pelletal wackestone and packstone; (4) stromatoporoid wackestone, packstone, and floatstone; (5) Cladocoropsis wackestone, packstone, and floatstone; (6) Clypeina and Thaumatoporella wackestone and packstone; (7) peloidal packstone and grainstone; (8) ooid grainstone; (9) crypt-microbial laminites; (10) evaporites; and (11) stratigraphically reoccurring dolomite. The Arab-D reservoir lithofacies succession represents shallowing-upward deposition, which, from deepest to shallowest, reflects the following depositional environments: offshore submarine turbidity fans (lithofacies 1 and 2); lower shoreface settings (lithofacies 3); stromatoporoid reef (lithofacies 4); lagoon (lithofacies 5 and 6); shallow subtidal settings (lithofacies 7 and 8); peritidal settings (lithofacies 9); and sabkhas and salinas (lithofacies 10). The depositional succession of the reservoir represents a prograding, shallow-marine,

Facies analysis and depositional environments of the Upper Cretaceous Sadr unit in the Nakhlak area, Central Iran

Revista Mexicana De Ciencias Geologicas, 2012

The up to 258 m thick, carbonate-siliciclastic Upper Cenomanian to Campanian rocks (Sadr unit), which crop out widely in the Nakhlak area of central Iran, consist of conglomerates, sandy limestones and dolostones, calcareous sandstones, sandy-argillaceous limestones and reefal limestones. The lower boundary of the studied section is an angular unconformity and its upper boundary is faulted. Sedimentological and palaeontological data indicate that Upper Cretaceous Sadr unit of Nakhlak area is equivalent to shallow carbonate platform successions of Upper Cretaceous rocks in central Iran, which belong to the central Iranian Plate and were deposited in marginal marine, shallow shelf and moderately deep marine environments. This geological unit can be divided into carbonates, siliciclastics, and mixed carbonate-siliciclastics groups. The siliciclastic facies group was deposited as shorelines, tidal flats, lagoons, and barrier bars indicating shallow shelf environments. The mixed carbonate-siliciclastics facies group was formed in a coastal-delta complex and the carbonate facies group took initially place on a homoclinal ramp which later developed into a rimmed platform due to the expansion of the rudist barrier facies.

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Inversion of well logs into rock types, lithofacies and environmental facies, using pattern recognition, a case study of carbonate Sarvak Formation

Carbonates and Evaporites, 2017

The ''facies'' is a frequently used term for describing sedimentary units. In the literature, this term has been used for different purposes, as depositional environment, rock type, lithofacies, etc. In subsurface geology, the core samples are essential for facies studies. While lacking cored intervals, the well logs provide precious subsurface information, but the complexity of well log responses leads most of the time to the complexity of interpretations. In this paper, a method is proposed to study the facies types through well logs. The case study is a carbonate platform system, deposited in the upper Cretaceous, named Sarvak Formation, in one of Iranian onshore oilfields, located in the Abadan Plain. For this purpose, parametric and nonparametric (k-nearest neighbor) classifiers were applied to the dataset. Detailed petrography, assisted by core descriptions, led to 37 microfacies, grouped into three main lithofacies, four carbonate rock types and six environmental facies. Classifiers could not identify the microfacies due to a limited number of observations and high variations. However, the environmental facies were truly classified. In addition, lithofacies classification and Dunham carbonate rock typing were carried out correctly. It is discussed that k-nearest neighbor is clearly the outperformed classifier, and the classical parametric models were inadequate due to the nature of the input well logs (dependency of well logs and may be their distribution), and the overlapping of the input feature space.

Late Pennsylvanian carbonate platform facies and coral reef: new insights from southern China (Guizhou Province)

Facies, 2020

The Pennsylvanian is characterized by intense paleoenvironmental changes related to glacio-eustatic sea-level fluctuations and major tectonic events, which affected the evolution of biocommunities. Most known Pennsylvanian tropical reefs and mounds are predominantly composed of calcareous algae (e.g. phylloid algae,Archaeolithophyllum), calcareous sponges, fenestrate bryozoans,Tubiphytes, and microbialites. However, in Houchang (southern China), the Late Pennsylvanian carbonate platform records a large coral reef lacking any analogs in age (Gzhelian), size (80–100 m thick) and composition (high biodiversity). The large coral reef developed at the border of the Luodian intraplatform basin. The intraplatform basin is characterized by the deposition of green algal grainstone, coated grain grainstone and bioclastic packstone, grainstone, floatstone and rudstone in shallow-waters. In the deep-water shelf, lithofacies are composed of burrowed bioclastic wackestone, microbioclastic peloida...

Depositional and diagenetic controls on reservoir properties along the shallow-marine carbonates of the Sarvak Formation, Zagros Basin: Petrographic, petrophysical, and geochemical evidence

Sedimentary Geology

The seventh oil layer of the Upper Triassic Yanchang Formation (Chang-7) tight oil sandstone reservoirs is a major exploration target. A significant amount of hydrocarbons has been discovered in these reservoirs in the southwestern Ordos basin in China. The Chang-7 tight sandstones are characterised as tight with low porosity, low permeability, and strong heterogeneity. This study investigates the sedimentary facies, diagenesis, and their impact on the reservoir quality of the Chang-7 tight oil sandstones. The sandstones were deposited in a deltaic-lacustrine depositional system. Three major depositional facies are identified consisting of delta front fed by braided rivers and meandering rivers, and slump turbidite fans. The depositional environment exerts a key control on reservoir quality. The distinct low-energy sedimentary environment produced fine to very fine-grained sandstones with high matrix and mica contents, characterised by low initial porosity and permeability. Diagenesis mainly comprised mechanical compaction and cementation by quartz, carbonate minerals and various clay minerals. The reservoir properties of the Chang-7 sandstones are generally poor, with porosity of 1.4-20.7% (average porosity 8.6%) and permeability of 0.001-116.7 mD (average 0.2 mD), which are attributed to significant compaction and cementation. Mechanical compaction was more important than cementation for reducing porosity, whereas secondary dissolution porosity was significant for the Chang-7 tight oil sandstones due to closer proximity to the underlying Chang-7 3 source rocks.

Middle Cretaceous Carbonate Reservoirs, Fahud Field and Northwestern Oman: DISCUSSION

AAPG Bulletin, 1986

Reservoir facies in Fahud field and tliroughout northwestern Oman are in shallow-shelf carbonates of the middle Cretaceous Mishrif and Mauddud Formations. Interparticle porosity formed in the Mishrif as sand aprons of lithoclast and skeletal grainstones surrounding fault-block islands, and less commonly in the Mauddud as biostromes of rudist packstones. Moldic porosity after fine rudist debris is more common than interparticle porosity and occurs in thicker stratigraphic units, interpreted to have formed locally in meteoric-water lenses associated with islands, and regionally during subaerial exposure associated with sea level lows. LOCATION The carbonate reservoir rocks discussed in this paper are located rn northwestern Oman (Figure 1). The area is situated near the eastern margin of the Arabian subplate and lies southwest of the Oman Mountains. Known reservoirs and exploration targets in the area occur primarily in shallow-shelf carbonates of Lower Cretaceous (Berriasian through Aptian) and middle Cretaceous (Albian-Cenomanian) ages. This paper describes lithologies and porosity types of middle Cretaceous reservoir rocks that we have examined in the subsurface and in outcrop in northwestern Oman, and that are similar to those of large fields in the immediate area. The largest of these fields, Fahud field, has been described by Tschopp (1967) and is summarized here. CRETACEOUS GEOLOGIC HISTORY The middle Cretaceous carbonates of northwestern Oman represent the culmination of a period of dominantly carbonate deposition that lasted about 150 m.y. from the Late Permian to the earliest Tlironian of the Late Cretaceous (Saint-Marc, 1978; Murris, 1980). In general, carbonate deposition was dominant because the area was located a considerable distance away from terrigenous detritus shed from the Saudi Arabian shield (Figure 2). Unconformities in the sedimentary sequence are inter

Palaeo-exposure surfaces in Cenomanian – Santonian carbonate reservoirs in the Dezful Embayment, SW Iran

Journal of Petroleum Geology, 2013

Cretaceous carbonates host major hydrocarbon reserves in SW Iran and elsewhere in the Arabian Plate. Tectonic activity combined with eustatic sea-level changes resulted in periodic exposure of these carbonates which were subsequently modified by meteoric diagenesis under a warm and humid climate. Long-term exposure led to the formation of several disconformity surfaces within the middle Cretaceous succession which had important effects on the interval's reservoir characteristics. These disconformity surfaces in the Dezful Embayment were investigated using microfacies, diagenetic and geochemical studies at five subsurface sections. Facies differences across these boundaries, together with features such as karstification, palaeosol development and collapse-dissolution breccias, were used to identify emergent surfaces. Stable oxygen and carbon isotope ratios and trace element profiles indicate intense meteoric diagenesis. Disconformities were dated using biostratigraphic studies. The results indicate the presence of two major erosional disconformities: one is located at the Cenomanian-Turonian boundary separating the middle Sarvak Formation from its upper part; and the other is in the mid-Turonian at the boundary between the Sarvak and Ilam Formations. The latter disconformity is correlatable throughout the Arabian Plate.

Depositional sequences, diagenesis and structural control of the Albian to Turonian carbonate platform systems in coastal Fars (SW Iran)

Marine and Petroleum Geology, 2015

This study covers a 300 km long, NE/SW oriented transect including five outcrop sections, and provides new biostratigraphic data and a sequence stratigraphic interpretation of the Albian, Cenomanian and Turonian sediments (Kazhdumi and Sarvak Formations) present in Coastal Fars (SW Iran). Two different carbonate depositional systems are distinguished, one for the Albian and one for the Cenomanian/ Turonian. During the Albian eustatic control was the dominant factor influencing sedimentation, while during the Cenomanian and Turonian large and small scale tectonics were dominant. This resulted in significant thickness variations and local diagenetic overprint of the sequence boundaries.

Original mineralogy of the Upper Jurassic carbonates in the Kopet-Dagh Basin , NE Iran

2011

The Kopet Dagh Basin in northeast Iran contains giant Khangiran and Gonbadli gas fields. This study deals with the main hydrocarbon reservoir of Upper Jurassic (Oxfordian-Tithonian) Mozduran Formation, which is composed mainly of limestone and dolomite, with minor amounts of marl/shale, siliciclastics and evaporites. The objective of this study is carbonate mineralogy of the Mozduran Formation. Thin sections were stained by alizarin-red S to detect dolomitization of grains and cements. Regarding diagenetic products and their diagenetic environments, selected samples were observed with a cathodoluminescent microscope (Nikon CL, CCL 8200) at the Research Institute of Petroleum Industry (R.I.P.I). Detailed field studies, petrographic investigations and facies analyses of eight surface sections and four wells, led to the recognition of several facies that define deep basin, fore-shoal, shelf margin, lagoonal, tidal flat and coastal plain facies belts, which deposited on a rimmed-shelf a...

Diagenetic History of the Mid-Cretaceous Carbonates in Southwestern Iran and the Persian Gulf

The Mid-Cretaceous Sarvak Formation in southern Iran and the Persian Gulf represents a carbonate ramp that developed at the north eastern edge of the Arabian Plate. The carbonate platform which records the Anoxic Oceanic Event (OAE) was periodically subaerially exposed during Cenomanian-Turonian. Regional Turonian unconformity that marked the top of these carbonates greatly influenced the diagenesis of the underlying carbonates. In this study a detailed investigation of the Sarvak Formation diagenesis were carried out in surface and subsurface sections where the effects of the unconformities were documented. A combination of petrographic and geochemical analysis is utilized to unravel the diagenetic history of the Cenomanian-Turonian carbonates in the study area. Over 300 thin sections were examined using transmitted light and over 100 representative samples studied using a Technosyn 8200 MKII model cold cathodoluminescence (CL) microscopy. Rudist's shells, calcite matrix and different types of cements were micro-sampled. Powdered samples were analyzed for stable oxygen and carbon isotopes analysis using a Finnigan Mat Delta Plus mass spectrometer. All the results for oxygen and carbon isotope analysis are reported in per mil (‰) notation relative to the Vienna Pee Dee Belemnite (VPDB) standard. Precision for both isotopes was better than 0.05‰. Trace elements data were obtained using an ICP-MS at Great lake Environmental Research institute, University of Windsor. Based on field and petrographic investigations, the most important diagenetic processes which have influenced these strata could be summarized as; dissolution, compaction, dolomitization, pyrite formation and calcite cementation. The most abundant calcite cements observed include: Drusy mosaic, blocky, equant and syntaxial from which the drusy and blocky calcite cements were sampled for geochemical analysis. The δ 13 C and δ 18 O values of the calcite matrix range from-6.4‰ to 4.1‰ and-9.4 to-0.9‰, and drusy mosaic calcite cements display values ranging from-5.8 ‰ to 3.6‰ and-9.3‰ to-0.6‰ respectively. In blocky calcite cements the δ 13 C shows values between-2.4‰ to +3.6‰ and δ 18 O from-12.3‰ to-2.8‰ VPDB. Considering the petrographic and chemical analysis results, the Mid-Cretaceous carbonates in this area went through diagenesis in variety of environments ranging from marine to meteoric and burial. Although the results of the δ 13 C analysis of most of the drusy mosaic and blocky calcite cements indicate the marine origin and even the OAE traces in these cements, depleted δ 18 O values confirms their precipitation in mixed marine-meteoric environment. A likely mechanism that could cause δ 13 C depletion in some of drusy mosaic cements (i.e.,-5.8 ‰) is meteoric diagenesis associated with oscillations in sea level (mainly the Cenomanian-Turonian and mid Turonian sea-level fall) that episodically exposed these shallow-water carbonates. Low concentrations of Sr (= 59 ppm) in these cements could also confirm the influence of meteoric waters on them. Higher depletion of δ 18 O values (i.e.-12.3 ‰ VPDB) and two-phase fluid inclusions in some blocky calcite cements suggest their precipitation at higher temperatures in burial environment.

Stratigraphic and Lithological Characteristics of Upper Cretaceous Carbonates in Central Iraq

Journal of Petroleum Geology, 1996

Shallow-marine carbonates were deposited in Central Iraq during the Lute Campanian-Maastrichtian. These carbonates consist of foraminiferal-shoal and bryozoan-algalrudist bank deposits, separated by argillaceous, oligosteginal wackestones and mudstones. The build-ups are believed to have developed on block-faulted topographic highs, while marly limestones were deposited in the intervening basinal lows. Based on facies analysis and log characteristics, these sediments are here divided into eight lithostratigraphic units.

Diagenetic history and reservoir properties of the Cenomanian-Turonian carbonates in southwestern Iran and the Persian Gulf

Marine and Petroleum Geology

Shallow-marine carbonates of the mid-Cretaceous Sarvak Formation are important reservoir rocks in southern Iran and the Persian Gulf region. These carbonates were deposited on the margin of the Arabian Plate and rest on the Kazhdumi Formation, which is one of the major hydrocarbon source rocks in the region. The top of the Sarvak Formation coincides with the regional Turonian unconformity. Most of the observed diagenetic features are genetically related to meteoric waters entering the Sarvak Formation during Cenomanian-Turonian and mid-Turonian uplift and the subsequent paleoexposure. Integration of field and petrographic studies and isotope geochemistry reveals the history of a variety of diagenetic processes, which include dissolution and development of secondary porosity which enhance reservoir properties of the Upper Sarvak carbonates. Various types of calcite cements were identified as the main cause for porosity loss in these carbonates. Their diagenetic environment is discussed using the geochemical data acquired as part of the present study. The δ 18 O and δ 13 C values (-12.3 to-0.6 ‰ and-5.8 to 3.6‰ VPDB, respectively) of the cements indicate precipitation from marine, meteoric and/or mixed meteoricmarine fluids. Some drusy calcite cements exhibit δ 18 O and δ 13 C values (-5.

Depositional environment, and sequence stratigraphy of the Lower Cretaceous Fahliyan Formation (Zagros Basin, SW Iran)

Neues Jahrbuch für Geologie und Paläontologie, 2022

This study is focused on the sedimentary environments, facies distribution and sequence stratigraphy of the Santonian intervals (Ilam Formation of Bangestan Group) that host enormous hydrocarbon reserves in five giant and supergiant oilfields in the central and southern parts of the Dezful Embayment (SW Iran). This reservoir formation is investigated using detailed petrographic analysis assisted by microscopic image analyses to explain its depositional facies and sedimentary environment in the subsurface sections of this embayment. Petrographic studies led to the recognition of 18 microfacies that formed in four facies belts: inner ramp (including shoal facies and open to restricted lagoons), mid-ramp (including channels and patch reef talus facies), outer ramp and basin. To locate the approximate position of the studied wells in the conceptual depositional model, frequency analyses for facies associations are carried out. The studied intervals consist of two, thick shallowing-upward 3rd-order sequences. Facies variations of the Ilam Formation investigated throughout the studied oilfields using correlation in a sequence stratigraphic framework.

Inorganic control on original carbonate mineralogy and creation of gas reservoir of the Upper Jurassic carbonates in the Kopet-Dagh Basin, NE, Iran

Carbonates and Evaporites, 2014

The Upper Jurassic Mozduran Formation (Oxfordian-Tithonian) is the main petroleum reservoir in the Kopet-Dagh Basin, northeast Iran, which consists predominantly of carbonate rocks with subordinate evaporites and siliciclastics deposited in platform to deep marine settings of a subtropical sea. Detailed field surveys, petrographic investigations, facies and wire line log analyses were carried out at eight surface sections and four wells in the Kopet-Dagh Basin. Integration of petrographic and isotopic data suggests primary low-Mg calcite mineralogy of Oxfordian and Tithonian ooids. On the other hand, in the wells, Kimmeridgian ooids and cements are dominantly aragonitic and high-Mg calcite mineralogy (HMC). Marine cements with isopachous, fibrous and isopachous bladed fabrics indicate original aragonite and HMC mineralogy, respectively. The domination of aragonite mineralogy could be related to increased hypersalinity, evaporite precipitation and consequently an increase in Mg/Ca ratio, which resulted in formation of aragonite in preference to calcite. Preserved ooids with radial and concentric cortices in shallow-water settings that are nearby siliciclastic source, together with aragonitic and HMC ooids accompanied by evaporites in the drilled fields, suggest original mineralogy was probably controlled inorganically following local salinity variations. This study suggests that Kimmeridgian pay zones are mainly controlled by depositional facies, aragonitic and HMC mineralogy, and diagenetic processes such as dolomitization and dissolution.

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