Ooid Grainstone Research Articles

Architecture and sedimentary evolution of the southwestern Gissar carbonate platform (Uzbekistan) during the Middle‒Late Jurassic

Abstract Sedimentological investigations have been carried out in the southwestern part of the Gissar Range (Uzbekistan) in order to document the stratigraphic evolution of a vast Middle–Upper Jurassic carbonate platform developed on the northern margin of the Amu Darya Basin. Two major sequences are distinguished, based on their contrasting facies pattern, platform configuration and carbon isotope signature: (1) a Callovian Sequence and (2) an Upper Jurassic Sequence, potentially Oxfordian to Kimmeridgian in age. A carbonate ramp with a double proximal-distal polarity developed during the Callovian Sequence. Oo-bioclastic grainstones were produced in two permanently agitated shallow water areas developing to the north and to the south. Patch and pinnacle reefs mainly built up close to the storm-wave base, passing distally to pellet- and foraminiferal-oncoid-rich mid-to outer-ramp deposits. The top of the Callovian Sequence is a regional discontinuity that could have formed by subaerial exposure of the platform. This probable hiatus could record a major climatic change associated to the Middle‒Late Jurassic transition. The Upper Jurassic Sequence consists of metre-thick alternations of pellet/algal-pack- to grainstones, microbial oncoid floatstones, ooid grainstones, and mudstones, which have likely accumulated in a vast lagoon protected by a large-scale barrier reef observed south of the study area. Gypsum and anhydrite content gradually increased towards the top of the Upper Jurassic Sequence, ending up with a thick chicken-wire gypsum accumulation recording the progradation of a large-scale sabkha over the lagoon. This phenomenon is related to a progressive aridification and restriction of the Amu Darya Basin that could have begun during the Middle/Late Oxfordian and reached its climax during the Tithonian with massive halite deposition. Comparable events occurred in Jurassic platforms located along the Tethyan margins, pointing to regional and global controls on sedimentation.

Foraminiferal biostratigraphy and facies analysis of the Permian Longge Formation in the Rongma Area, Tibet, China: implications for the palaeogeography of the South Qiangtang Block

Wang, Z-B., Gao, J-H., Wang, G-H. & Kang, Z-H., June 2018.2018. Foraminiferal biostratigraphy and facies analysis of the Permian Longge Formation in the Rongma Area, Tibet, China: implications for the palaeogeography of the South Qiangtang Block. Alcheringa XX, x–x.The Permian Longge Formation in the South Qiangtang Block, Tibet, is overwhelmingly composed of carbonate rocks with various skeletal (brachiopods, foraminifers, gastropods, echinoderms, ostracods, corals, bivalves, algae, and bryozoans) and non-skeletal (intraclasts and ooids) components. Three stratigraphic sections of the Longge Formation in the Rongma area of north-central Tibet—known as South Yibug Caka, Niushan and East Yibug Caka—were selected for this study, which examined both sedimentary facies and foraminiferal assemblages. The foraminifers in these sections consist of at least 38 species belonging to 18 genera. Based on the distribution of the foraminifers throughout the composite section, two foraminiferal assemblages were established, and the age of the Longge Formation was determined to be late Kungurian to Capitanian. During lithological studies, ten microfacies were identified using depositional textures, petrographic analysis and faunal content: mudstone, bioclast wackestone, bioclast perforated-foraminifera packstone, bioclast crinoid grainstone, intraclast wackestone, breccia, intraclast grainstone, ooid grainstone, fine crystalline dolostone and residual-grain dolostone. These microfacies are interpreted to represent four depositional environments—restricted lagoon, open marine, shoal and slope—which together suggest a shoal-rimmed carbonate platform. The non-fusuline foraminifers show transitional palaeobiogeographic affinities (Tethyan Cimmerian subregion), and the assemblage is considered to be influenced by the northward drift of the South Qiangtang Block, the climatic warming after the Late Paleozoic Ice Age and warm-water oceanic currents caused by the newly formed Neotethys Ocean. This indicates that the South Qiangtang Block was located in a relatively warm-water, low-latitude area during the middle Permian. The Permian depositional sequences in the Rongma area were also influenced by the palaeogeographic evolution of the South Qiangtang Block.Zhong-Bao Wang [zbwang@cugb.edu.cn] and Zhi-Hong Kang [kangzh98@cugb.edu.cn] School of Energy Resource, China University of Geosciences (Beijing), 29 Xueyuan Road, Beijing 100083, PR China; Jin-Han Gao* [wang_zhongbao@126.com] and Gen-Hou Wang [wgh@cugb.edu.cn] School of Earth Sciences and Resources, China University of Geosciences (Beijing), 29 Xueyuan Road, Beijing 100083, PR China.

Facies variability of a lower Aptian carbonate platform succession, Tirgan Formation, eastern Kopet Dagh Basin, northeast Iran

The Lower Cretaceous (Hauterivian-lower Aptian) Tirgan Formation is a carbonate platform succession in the Kopet Dagh Basin and is up to nearly 800 m thick. This study focuses on facies variability and depositional settings of the Tirgan in the eastern part of eastern Kopet Dagh where the formation constitutes the basal part of the lower Aptian deposits. The succession developed during the final stage of evolution of Cretaceous carbonate platform in the southern Eurasian margin and could demonstrate a synchronicity with comparable successions in other areas. In the study area, the Tirgan (48 m thick) overlies Berriasian–Barremian continental strata and underlies the lower Aptian deep marine deposits. Facies analysis of Tirgan determined several facies related to open marine, ooid grainstone shoal, protected lagoon, and lower intertidal facies belts of a carbonate system. Abundant ooids, calcareous green algae, mud size carbonate, lack of evaporite evidence, and the absence of basinal deposits record deposition in warm water shallow marine settings of a carbonate ramp under a relatively humid climate. The Tirgan encompasses a single third-order depositional sequence composed of transgressive and highstand systems tracts. The transgressive package consists of intertidal, lagoon, and open marine facies intercalated with storm and shale layers showing an overall retrogradational trend. However, the highstand package consists mainly of ooid grainstone facies and displays prograding clinoforms. The lowermost lower Aptian Tirgan sequence in the study area corresponds to the basal Aptian global third-order sea level cycle. Contact of Tirgan sequence with the overlying middle to late Aptian deep marine strata is abrupt, suggesting a drowning event, which was likely the result of early Aptian extraordinary global warming.

Effect of Thermal and Freeze-thaw Stress on the Mechanical Properties of Porous Limestone

This paper focuses on the effect of high temperature on the mechanical properties of a porous limestone that is widely used as dimension stone in Hungary. The changes in physical properties of 3 types of porous limestone were analyzed at 22 °C, 300 °C and 600 °C, respectively. The limestone specimens were also subjected to freeze-thaw cycles to assess the other extrinsic factor that influences the behavior. Parameters such as material density, bulk density, ultrasonic pulse velocity and tensile strength were measured and compared in different test conditions. The tests results indicate that fabric differences significantly influences the durability of tested limestones. Bioclastic grainstone is more frost resistant that ooidal grainstone or bioclastic packstone, but heating seems to reduce the pulse velocity and tensile strength of all tested limestone. It is suggested that cyclic freezing-thawing reduces the strength depending on the micro-fabric, while heating to 600 °C and the reduction of strength is less controlled by the fabric of the porous limestone.

Reservoir quality and diagenesis of the Permian Lucaogou Formation tight carbonates in Jimsar Sag, Junggar Basin, West China

The Lower Permian Lucaogou Formation in the Jimsar sag, Junggar Basin is a typical tight-oil reservoir in China. For effective exploration and production, the formation of a high-quality reservoir must be thoroughly studied. In this work, the tight-oil reservoir was examined using a variety of methods, including core and thin-section observations, XRD, SEM, CL and fluid inclusion and isotope testing. The tight-oil reservoirs were primarily deposited in saline lake environments, which are dominated by variable admixture of dolomite, quartz, feldspar, tuff, calcite and pyrite. Nine main lithofacies were identified: (1) siliceous mudstone, (2) dolomitic siliceous mudstone, (3) dolomitic mudstone, (4) intraclast packstone/grainstone, (5) ooid grainstone, (6) bioclast grainstone, (7) dolomitic siltstone, (8) mixed siliclastic and intraclast grainstone and (9) brecciated dolomitic mudstone. The pore types are classified into four categories: primary intergranular, moldic, intercrystalline and fracture pores. The properties of tight-oil reservoirs are quite poor, with low porosity (ave. 7.85%) and permeability (ave. 0.110 mD) and a small pore-throat radius (ave. 0.086 μm). The tight-oil reservoirs are dominated by the aggradation of a repetitive meter-scale sedimentary facies succession that records distinct lacustrine expansions and contractions. These tight carbonates have also undergone significant diagenetic alterations, such as dolomitization, dissolution, neomorphism and fracture created intercrystalline and moldic pores, vug and fractures; chemical and mechanical compaction and carbonate cementation have decreased the reservoir quality. Variations in reservoir quality in the Jimusar sag are due to a combination of lithofacies type, high-frequency cyclic depositional architecture, dissolution intensity, dolomitization and tectonic related deformation. This integrated study has helped in understanding the reservoir heterogeneity and hydrocarbon potential of the Jimusar fine-grained rocks.

Facies, Depositional Environment and Sequence Stratigraphy of Dalan Formation in Persian Gulf (Qatar-South Fars Arch) Well SP-A Subsurface Section

Objectives: Facies, depositional environment and sedimentary cycles of Dalan Formation in the South Pars gas field are the aspects that this paper is focused on. Methods/Statistical Analysis: Three hundred and seventy-eight thin-sections prepared from cutting and cores were evaluated by using petrographic microscope. Corel Draw X6, Autodesk Map, Corel Photo-Paint, Autodesk Map, DN2 Microscopy Image Processing System, Geocalc and Auto Cad 2014 employed for this research. Findings: Twenty-four major facies in five facies associations were identified comprising Tidal flat (A), Lagoon (B), Shoal (C), Open marine (D) and Mid ramp (E). The Dalan Formation was deposited in inner to mid parts of a homoclinal ramp. The sequence stratigraphy studies revealed the seven main third– order sedimentary cycles in the well SP-A. The shoal ooid grainstone facies with intergranular, moldicand fracture porosity comprise the main reservoir facies of the Dalan Formation. Dissolution, dolomitization, anhydritization, compaction and cementation are the main diagenetic processes in this formation which played an important role in the quality of the reservoir. Applications/Improvements: The results obtained from this paper, can be used to construct a major depositional framework in the Qatar-South Fars Arch for later exploration purposes.

Silicification and organic matter preservation in the Anisian Muschelkalk: Implications for the basin dynamics of the central European Muschelkalk Sea

Anisian Muschelkalk carbonates of the southern Germanic Basin containing silicified ooidal grainstone are interpreted as evidence of changing pH conditions triggered by increased bioproductivity (marine phytoplankton) and terrestrial input of plant debris during maximum flooding. Three distinct stages of calcite ooid replacement by silica were detected. Stage 1 reflects authigenic quartz development during the growth of the ooids, suggesting a change in the pH–temperature regime of the depositional environment. Stages 2 and 3 are found in silica-rich domains. The composition of silica-rich ooids shows significant Al2O3 and SrO but no FeO and MnO, indicating that late diagenetic alteration was minor. Silicified interparticle pore space is characterized by excellent preservation of marine prasinophytes; palynological slides show high abundance of terrestrial phytoclasts. The implications of our findings for basin dynamics reach from paleogeography to cyclostratigraphy and sequence stratigraphy, since change...

Lateral variability of shallow-water facies and high-frequency cycles in foreland basin carbonate platforms (Pennsylvanian, NW Spain)

The kilometer-sized and 100-meter-thick carbonate platforms of the Escalada Fm. I and II (Middle Pennsylvanian) accumulated in the foredeep of a marine foreland basin during the transgressive phases of 3rd-order sequences and were buried by prograding siliciclastic deltaic systems in the course of the subsequent highstand. The carbonate successions show a general upward trend from grain- to mud-supported carbonates, interfingering landwards with siliciclastic deposits of a mixed siliciclastic-carbonate shelf (Fito Fm.) adjacent to deltaic systems. The spatial variability of the carbonate facies and the high-frequency (4th–5th order) cycles, from the platform margin-outer platform to the deltaic systems, has been interpreted from basin reconstruction. Carbonate facies include skeletal grainstone to packstone, ooidal grainstone, burrowed skeletal wackestone, microbial and algal boundstone to wackestone forming mounds, various algal bafflestone and coral biostromes in areas with siliciclastic input. These high-frequency transgressive–regressive cycles are interpreted to record allocyclic forcing of high-amplitude glacioeustasy because they show characteristic features of icehouse cycles: thickness >5 m, absence of peritidal facies, and in some cases, subaerial exposure surfaces capping the cycles. In the mixed cycles, siliciclastics are interpreted as late highstand to lowstand regressive deposits, whereas carbonates as transgressive-early highstand deposition. The lateral and vertical variability of the facies in the glacioeustatic cycles was a response to deposition in a rapidly subsiding, active foreland basin subjected to siliciclastic input, conditions that might be detrimental to the growth of high-relief carbonate systems.

Unravelling hinterland and offshore palaeogeography from pre-to-syn-orogenic clastic sequences of the Betic Cordillera (Sierra Espuña), Spain

The Malaguide Complex (Betic Cordillera) occurs in the Sierra Espuña area (SE Spain) providing a favorable setting to study a sedimentary successions from continental and shallow-marine to deep-marine environments using structural and stratigraphic relations, and petrological and geochemical signatures. The aim of this work is to outline the sedimentary evolution of the Malaguide Complex and the El Niño Fm. This succession is arranged into pre-orogenic (Paleocene-Early Oligocene) and syn-orogenic (Late Oligocene-Early Miocene) stages. The detrital modes of sandstones of the overall succession are heterogeneous and indicate a multi-source area, marked by exhumation till an unknown Malaguide basement and probably lower units of the Internal Betic Zone (e.g. Alpujarride Complex) in the case of the El Niño Fm. Pre-orogenic deposits are mainly carbonate with abundant skeletons of intrabasinal to extra-basinal larger foraminifera, calcareous lithic fragments and quartz grains, and metamorphic lithic fragments. Minor metamorphic lithic fragments occur in the Eocene deposits (i.e. Espuña Fm). Calcareous lithic fragments and extra-basinal fossils (lithoclastic bioclasts) are mostly present in the Early Oligocene pre-orogenic formations (i.e. As Fm) and were derived from the Paleocene-Eocene sedimentary successions of the Malaguide Complex. Syn-orogenic sandstones are quartzolithic with abundant metamorphic and sedimentary lithic fragments (slate, phyllite, fine-grained schist, ooidal grainstone, mudrock and quartz-rich siltstone fragments). Sedimentary lithic fragments were derived from the Mesozoic successions of the Malaguide Complex while metamorphic detritus is related to the Malaguide basement (probably also Alpujarride from Burdigalian on) that was exhumed starting from the Late Oligocene. Mudrocks of the syn-orogenic clastics, record an increase of phyllosilicate, quartz and feldspars and an abrupt decrease in calcite and dolomite. The abundance of calcite and dolomite, and traces of hematite occur dominantly in the pre-orogenic mudrocks. The geochemical signatures suggest a provenance mostly derived from felsic source rocks with a minor supply from mafic metavolcanic rocks in some samples of the syn-orogenic stage. The syn-orogenic formations (i.e. Rio Pliego Fm and El Niño Fm) are characterized by higher Cr/V values than the pre-orogenic formations suggesting a mafic supply for the syn-orogenic samples. In particular, the contents of Cr (average=85ppm) and Ni (average=48ppm) for the samples of the Rio Pliego and El Niño Formations are higher than those samples (Cr average=20ppm; Ni average=14ppm) of the pre-orogenic formations. Palaeoweathering indices suggest low to moderate weathering conditions for the source area(s). The Cenozoic studied succession played a key role in the geodynamic evolution of the Betic Cordillera, representing the key tectonic element of the western Mesomediterranean domains.

Characterization of a reservoir ooid shoal complex and Artificial Neural Networks application in lithofacies prediction: Mississippian St. Louis formation, Lakin fields, western Kansas

Residing in the Hugoton embayment of western Kansas, the Lakin field has produced over 1.8 million bbl of oil from reservoir quality zones within the St. Louis Limestone (Mississippian, Meramecian) oolitic deposits. This study focuses on improving the understanding of the orientation, geometry, and spatial distribution of ooid shoal complexes in Kearny County, Kansas. To this end, the interpretation of petrophysical log data integrated with a facies analysis conducted on 400ft (122m) of core from three separate intervals provided the basis, in this study, for understanding the vertical stacking patterns and how facies transitions relate to reservoir quality zones within the ooid shoal complexes. A supervised Artificial Neural Network (ANN) has been trained and validated, based on input-layer of geophysical well-logs and an output-layer of core-lithofacies, for lithofacies prediction where core samples were not available. The porous ooid grainstone is the principal reservoir facies, with an average log-derived porosity measurement of eighteen percent. Ooid shoal complexes are present in St. Louis Zone B and C, with thicknesses reaching up to 10ft (3m) and 24ft (7m) respectively. These complexes are NW-SE trending and laterally extend up to 16mi (26km), with economically viable patches covering up to 8mile (13km), and record roughly 2mile (3km) in width. The apex of the shoal contains the highest porosity values, with significant porosity reduction towards the shoal margins due to the increased cementation. This study provides information on the characteristics of oolitic deposits for the purpose of understanding controls on reservoir heterogeneity to aid in finding additional hydrocarbon reserves in the St. Louis Limestone in western Kansas.

Early Cretaceous Shallow-Water Platform Carbonates of the Bolkar Mountains, Central Taurides - South Turkey: Facies Analysis and Depositional Environments

The study area comprises southern non-metamorphic part of the Bolkar Mountains which are situated in southern Turkey, eastern part of the Central Taurides. The studied five outcrops form geologically parts of the tectonostratigraphic units called as allochthonous Aladag Unit and autochthonous Geyikdagi Unit. The aim of this study is to describe microfacies and depositional environments of the Bolkar Mountains Early Cretaceous shallow- water platform carbonates. The Lower Cretaceous is represented by continuous thick- bedded to massive dolomite sequence ranging from 100 to 150 meters thick, which only contains locally laminated limestone intercalations in the Yüğlük section and thick to very thick-bedded uniform limestones ranging from approximately 50 to 120 meters, consist of mainly laminated- fenestral mudstone, peloidal-intraclastic grainstone-packstone, bioclastic packstone- wackestone, benthic foraminiferal-intraclastic grainstone-packstone, ostracod-fenestral wackestone-mudstone, dasycladacean algal packstone-wackestone and ooidal grainstone microfacies. Based on a combination sedimantological data, facies/microfacies and micropaleontological (predominantly dasycladacean algae and diverse benthic foraminifera) analysis, it is concluded that Early Cretaceous platform carbonates of the Bolkar Mountains reflect a tidally affected tidal-flat and restricted lagoon settings. During the Berriasian- Valanginian unfavourable facies for benthic foraminifera and dolomitization were predominate. In the Hauterivian-early Aptian, the effect of dolomitization largely disappeared and inner platform conditions still prevailed showing alternations of peritidal and lagoon facies, going from peritidal plains (representing various sub-environments including supratidal, intertidal area, tidal-intertidal ponds and ooid bars) dominated by ostracod and miliolids, to dasycladacean algae-rich restricted lagoons-subtidal. These environments show a transition in the vertical and lateral directions in all studied stratigraphic sections.

Skeletal fossils and microfacies analysis of the lowermost Cambrian in the southwestern margin of the North China Platform

The Xinji and Zhushadong formations are the oldest record of Cambrian deposits in the southwestern margin of the North China Platform. The two lithostratigraphic units are carbonate-dominated in lithology and are Cambrian Series 2 in age. The basal unit, the Xinji Formation yields diverse skeletal fossils, including sponge spicules, chancelloriid sclerites, hyoliths, micromolluscs, trilobites, echinoderm ossicles, cambroclaves, tianzhushanellids, protoconodonts and other small shelly fossils. The fossil assemblage, as the earliest fauna of the Cambrian in North China, provides an important window to investigate the Cambrian explosion in the region and contains fossil elements with biostratigraphical significance. Seven microfacies types of the Xinji and Zhushadong formations are recognized: (1) phosphorized and selectively dolomitized bioclastic grainstone, (2) dolomitized ooid grainstone, (3) dolomitized peloid wackstone, (4) lime mudstone preserved as dolomitic microsparite, (5) siliciclastic wackstone, (6) calcareous siltstone, and (7) breccia. The different microfacies are traced to three successive depositional environments, beginning with a bioclastic shoal, then a mixed-depositional back-shoal and finally an oolitic shoal. The system of the shoal and back-shoal is interpreted as representing the mixed siliciclastic-carbonate deposition during the initial transgression of the North China Platform as well as the early development of an epeiric environment.

Lithofacies, architecture, and reservoir heterogeneity of tidal-dominated platform marginal oolitic shoal: An analogue of oolitic reservoirs of Lower Triassic Feixianguan Formation, Sichuan Basin, SW China

The Yudongliang Outcrop, located in the northern part of the Sichuan Basin, nicely exposes oolitic shoal complexes of the Lower Triassic Feixianguan Formation. The outcrop is an analogue for the subsurface Longgang and Yuanba gas fields and other Permo-Triassic reservoirs with dolomitized and nondolomitized oolite-dominated grainstone bodies as the main producing zones.Based on five measured sedimentological sections, and petrographic and petrophysical analysis of 200 plugs, the architecture of the reservoir bodies has been reconstructed in detail and placed within a high-frequency sequence-stratigraphic framework.The studied interval belongs to the highstand system tract (HST) of a single composite sequence and comprises three shallowing-upward high-frequency sequences, which consist from base to top of oncoid/peloid/ooid grain-dominated packstone, cross-bedded ooid grainstone, dolograinstone, fenestra-laminated dolomite, and crystalline dolomite. The internal architecture of the oolitic shoal is the result of complex tidal-dominated hydrodynamics.Facies and diagenetic partitioning with each high-frequency sequence are the primary controls on pore types and reservoir properties. Original lithofacies texture and subsequent cementation resulted in interbedded thin effective reservoirs and permeability barriers that created a complex internal architecture of reservoir bodies. The shapes of reservoir bodies generally follow those of the lithofacies, but with the addition of intercalated thin flow barriers.

Facies, Sedimentary Environment and Sequence Stratigraphy of Dalan Formation in South Fars, Iran —(Qatar-South Fars Arch) Well ASL-A

Dalan Formation is one of the most important gas reservoirs of south and southwest Iran which it belongs to Dehram Group and its age is Middle to Late Permian. The Dalan formation is interpreted as reflecting a major tectono-eustatic event related to the onset of rapid thermal subsidence of the early Neo-Tethys passive margin in Arabia and Iran, and the drowning of its rift shoulders. The Dalan formation consists mainly of medium to thick-bedded oolitic to micritic shallow-marine carbonate, with intercalations of evaporates. This formation overlies the Faraghan formation and extends up into the Lower Triassic kangan formation. The current paper is focused on the facies, sedimentary environment and sequence Stratigraphy study of the Middle to Upper Permian Dalan formation in the West Assaluyeh gas field the subsurface section of well ASL-A. Based on microfacies analysis and significant founa and flora, nineteen major facies in five facies associations including Tidal flat (A), Lagoon (B), Shoal (C), Open marine (D) and Mid ramp (E) were recognized in the Dalan formation. Facies analysis and those comparisons with modern and ancient environments indicated that the Dalan formation was deposited inner to mid parts of a homoclinal ramp. The sequence stratigraphy studies on the Dalan formation in this gas field led to assessment of seven main sedimentary sequences of the third-order in the Assaluyeh field, well ASL-A. The boundary between the third-order sequences with the Faraghan formation and between the Nar and Upper Dalan members are Subaerial Unconformity (SU) and the boundaries between the third-order sequences with each other and also with the Kangan formation are Correlative Conformity (CC). The main diagenetic processes in this formation are dolomitization, dissolution, anhydritization, cementation and compaction which played a significant role in improving reservoir quality. The shoal ooid grainstone facies with intergranular and oomoldic porosity comprise the main reservoir facies of the Dalan formation.

Facies, Depositional Environment and Diagenesis of the Qom Formation in Rameh Section (Northeastern Garmsar)

Deposits of the Qom Formation (Oligocene-Miocene) in Rameh section are located in northeastern Garmsar and contain dominantly 420 m Limestone, pebble-rich to sandy Limestone and marl Limestone. This formation is unconformably overlaid and underlaid with siliciclastic deposits of Upper Red Formation and Lower Red Formation. Field observations, along with laboratory investigations, have resulted in identifying tidal flat, lagoon, shoal and open marine environments in the studied formation. Open marine facies association consists of bioclast mudstone, bioclast wackestone, bioclast packstone and bioclast roadstone; shoal facies association consists of ooid grainstone, bioclast grainstone and coral boundstone; lagoon facies association is composed of dolomitic mudstone, intraclast bioclast wackestone and bioclast packstone; and tidal flat facies association is sandy dolomudstone and stromatolite boundstone. The qom formation rocks in Rameh section are deposited in a rimmed shelf carbonate ramp. This formation undergoes various diagenetic processes including dissolution, porosity, cementation, micritization, compaction and dolomitization.

Analysis of Sedimentary Environment and Sequence Stratigraphy of Middle-Late Permian Sediments in Coastal Fars, Iran (Zireh Gas Field, Well ZH-A)

The current paper is focused on the facies, sedimentary environment and depositional cycles study of the Middle-Late Permian sediments in the South Fars Zireh Gas Field, the subsurface section of well ZH-A. Four hundred thin-sections obtained from cores and cuttings were examined under standard petrographic microscope. For this study research, Corel Draw X6, Corel Photo-Paint, DN2 Microscopy Image Processing System, Scope Photo, Autodesk Map, Geocalc and Auto Cad 2014 were utilized. Based on microfacies analysis and significant founa and flora, fourteen major facies in four facies associations comprising tidal flat (A), lagoon (B), shoal (C) and open marine (D) identified in the well ZH-A. This formation was deposited in inner part of a homoclinal ramp. Based on depositional cycles, seven main 3rd order sequences were revealed in the Well ZH-A. The ooid grainstone facies with interparticle and oomoldic porosity has high reservoir potential. The diagenetic processes like dolomitization and dissolution have significant effect in the reservoir quality. Based on research results, a major framework can be weaved and used to correlate reservoir layering.

Sedimentary Environment and Sequence Stratigraphy of the Kangan Formation in Kish Gas Field (Kish Well A1 Subsurface Section)

Kangan formation is one of the most important gas reservoirs of south and southwest Iran which it belongs to Dehram group and its age is Lower Triassic (Scythian). It was formed in an extensional basin in the expanded Neotethys Ocean. The basin was formed as a result of the Late Permian continental drift between Cimeria and Gondwana. The Kangan Formation is mainly composed of dolomite, dolomitic limestone with shale and anhydrite. It is unconformably underlain by the upper Permian Dalan formation and is conformably overlain by the upper Triassic Dashtak formation. In order to study facies, sedimentary environment and sequence stratigraphy in the Kish gas field, subsurface section of well A1 have been studied. Based on microfacies analysis, three facies belts include Tidal flat (A), Lagoon (B) and Shoal (C) were recognized in the Kangan formation. Facies analysis and those comparisons with modern and ancient environments indicate that the Kangan formation was deposited in an inner part of a homoclinal ramp platform. The sequence stratigraphy studies on the Kangan formation in this gas field led to assessment of three main sedimentary sequences of the Third-order in the Kish Well A1. The boundary between the Third-order sequences with each other and also the boundary between Third-order sequences with Dashtak formation were unconformity boundary of type 2 (SB2) and the boundary between Third-order sequences with Dalan formation which is the Permian-Triassic boundary is an unconformity boundary of type 1 (SB1). The main diagenetic processes in this formation are dissolution, cementation, micritization, anhydritization, dolomitization, recrystallization, physical and chemical compaction which played a significant role in improving reservoir quality. The dominantly ooid grainstone facieses with intercrystalline, moldic and Fracture porosity comprise the main reservoir facies of the Kangan Formation. Keywords: Dehram Group, Homoclinal Ramp, Kangan Formation, Scythian, Sequence Stratigraphy, Unconformity Boundary

Hydraulic flow units, depositional facies and pore type of Kangan and Dalan Formations, South Pars Gas Field, Iran

It is highly essential to characterize the reservoir as rigorously as possible to implement enhanced oil recovery operations and field development scenarios. In this scope, use of novel concepts and techniques for classification of reservoir rocks and definition of producing and non-producing zones will be greatly helpful. In the current research, hierarchical clustering is employed to characterize hydraulic flow units of four wells of South Pars Gas Field using core data. Studying the clustering tree, also known as dendrogram, it is proved that six is the optimal number of flow units for reservoir zonation. Cross plot of porosity versus permeability for each flow unit shows high cross correlation value, which confirmed the accuracy of employed technique for grouping the FZI data. The available core data were studied to identify the possible microfacies. Ten micofacies are distinguished including massive to laminated anhydrite, evaporite bearing fenestral dolomudstone, pellet grainstone, pellet wackestone to packstone, bioclastic wackestone to packstone/mudstone, bioclastic wackestone to mudstone, bioclastic ooid grainstone, ooid grainstone, intraclastic grainstone, and thrombolite boundstone. The frequency of each facies for each hydraulic flow units was investigated. The results indicate that presence of anhydrate severely reduced reservoir quality. According to thin section and core slab analyses results, high quality flow units mainly consist of bioclastic ooid grainstone, ooid grainstone, and intraclastic grainstone and thrombolite boundstone, which are associated with shoal facies. Finally, thin section and scanning electron microscope (SEM) analysis was done to recognize pore types of each flow unit.

Biostratigraphy, facies analysis and paleoecology of the Asmari Formation in the northwest of Behbahan, south-western Iran

In this research, the biostratigraphy analysis, paleoecological interpretation and paleoenvironmental reconstruction of carbonate sediments of the Asmari Formation are based on the identified foraminiferal assemblages. The Asmari Formation was measured in the southern flank of Bangestan Anticline in north-western Behbahan (south-western Iran). Based on the foraminiferal assemblages, the Asmari Formation is late Oligocene–early Miocene (Aquitanian–Burdigalian) in age. Nine microfacies were identified and interpreted. These facies include: (1) anhydrite, (2) bioclastic mudstone, (3) peloidal wackestone–packstone, (4) mollusk packstone–grainstone, (5) bioclast imperforate foraminifera packstone–grainstone, (6) bioclast, bryozoa wackestone–packstone, (7) coral and corallinacea boundstone, (8) bioclast, foraminifera wackestone–packstone, and (9) ooid grainstone. The carbonate facies represent dominant marine conditions in a euphotic zone. The presence of non-skeletal grains such as ooids and peloids as well as the foraminifers and corals (Porites) indicates that the carbonate sediments of the Asmari Formation were deposited in a tropical to subtropical zone. Two third-order depositional sequences were recognized. These depositional sequences were separated by a type-2 sequence boundary. The faunal associations and the analyzed facies represent oligotrophic to mesotrophic conditions.

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

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.