Non-skeletal Grains Research Articles

Diagenetic evolution and cementation mechanism in deep Carbonate reservoirs: A case study of Dengying Fm. 2 in Penglai, Sichuan Basin, China

The Dengying Formation in the Penglai region of the Sichuan Basin is renowned for hosting high-quality dolomite reservoirs. However, the understanding of their formation mechanisms and spatial distribution patterns remains enigmatic. This study undertakes a comprehensive petrographic analysis of the second member of the Dengying Formation, primarily focusing on its burial history, reservoir characteristics, and an examination of the types, stages, and developmental patterns of cementation within the dolomite reservoir. This is achieved through the application of isotopic geochemistry, inclusion temperature analysis, and laser U-Pb dating. The findings reveal that cements within the second member of the Dengying Formation predominantly occur in pores and fractures, often exhibiting euhedral and subhedral morphologies. These cements also exhibit evidence of replacing original non-skeletal grains and micrites in semi-euhedral and other forms. Broadly, the cements can be classified into four types: calcium cement (the most abundant), dolomite cement, quartz, and pyrite. The formation of calcareous and siliceous cements closely aligns with the sedimentation and burial processes of the Dengying Formation as a whole. The early to late stages of early-middle diagenesis play a crucial role in the formation of dolomite within the second member. Notably, the degree of euhedral dolomite formation directly correlates with the specific diagenetic stage. Our research indicates that the development of high-quality reservoirs in deep-to-ultra-deep microbial dolomites is a result of complex multi-factor interactions and multi-stage discontinuous modifications, leading to strong heterogeneity within the reservoirs. Microbial dolomite carbonates inherently exhibit high initial porosity, with the favorable sedimentary facies serving as the fundamental basis for reservoir development. Dissolution processes during the syngenetic and quasi-syngenetic periods, in conjunction with epigenetic karstification, further augment porosity and expand the reservoir's capacity. During burial and deep burial stages, various acidic fluids play a pivotal role in preserving and modifying the early-formed pores. In the deep environment characterized by high temperature and pressure, microbial dolomite reservoirs undergo intricate diagenetic evolution and possess complex pore preservation mechanisms. Remarkably, even under exceptional ultra-deep conditions, high-quality microbial dolomite carbonate reservoirs can still develop and be preserved, presenting significant potential for oil and gas exploration.

Microfacies Analysis and Evolution of Porosity of the Albian-Early Cenomanian Mauddud Formation from Selected Wells in Ratawi Oilfield, Southern Iraq

Mauddud Formation (Albian stage-the Early Cretaceous) is an important oil reservoir in Ratawi field of southern Iraq. Four wells, R T-2, R T-3, R T-6, and R T-7, located 70 km northwest of Basra, were selected to study microfacies properties and petrophysical associations with the probability of oil production. Seventy-seven core samples are collected, and thin sections for petrographic analysis. The self-potential, Gamma-ray, resistivity, and porosity logs are used to determine the top and bottom of the Mauddud Formation. Water saturation of the invaded and uninvaded zones, shale volume, and porosity were calculated. The study area results showed that the quantity of shale is less than 15% for most of the wells, and the dominant porosity is the secondary porosity. In contrast, the primary porosity is low in all study wells, and the formation contains varying proportions of producing hydrocarbons. The results suggest the formation comprises light-colored dolomitized lime and pseudo-oolitic white limestone with green to blue-gray shale. The petrographic analyses of 65 thin sections of the Mauddud Formation reveal that most skeletal grains are shallow marine-derived faunas, while non-skeletal grains include peloids and ooids. The facies analysis showed that the Mauddud Formation was deposited within different sedimentary environments ranging from deep to very shallow environments, which was represented by a large variation in Five microfacies and ten submicrofacies, the microfacies "Mudstone, wackestone, packstone, grainstone, and Rudstone "submicrofacies" Peloidal wackestone to packstone, Bioclastic wackestone to packstone, Miliolids wackestone to packstone,Orbitolina wackestone, Peloidal packstone to grainstone, Orbitolina packstone, Mudstone to wackestone,Bioclastic wackestone, Miliolids and Bioclastic wackestone to packstone, Bioclastic packstone, and Peloidai grainstone". This variance suggested that the formation might be deposited within the carbonate platform's variety of environment, such as "deep sea, restricted, shallow open marine, mid-ramp, rudist biostrome, and shoal." Seven different kinds of pores have been found in the carbonate sideman of the Formation: Interparticle, Intraparticle, moldic, Vuggy and fracture.

Microfacies and Sedimentary Environments of Nfayil Formation (Middle Miocene) in Karbala Using Stable Isotope

A surface section of the Nfayil Formation (Middle Miocene) was studied approximately 80 km from the holy city of Karbala. The study area includes the Malutiyat area's Nyfail Formation (Middle Miocene age). The Nfayil Formation contains marls interbedded with limestones, as well as marly limestone. Fieldwork and petrographic analysis are being conducted on Miocene sediments from the Karbala region. Details obtained from textural and faunal properties aided in the designation of microfacies zones, which led to environmental subdivisions as a ramp. The carbonate lithic arenite sandstone has a significant amount of carbonate fragments. Microfacies characteristics of the whole successions showed skeletal and non-skeletal grains. The oysters, an index fossil of the Nfayil Formation are found in the lower part of the Nfayil Formation. The stable isotopes 18O/16O and 13C/12C in carbonates are analyzed to define the depositional characters and the basin paleo depth. The studied samples have negative δ18O isotope values (average -11.79 %). The carbon isotope composition is of positive values (average 2.16‰), The Z values, higher than 120, reflect the marine sediments of the formation. Four facies were identified, (Lime mudstone Microfacies, Sandy Lime mudstone Microfacies, Peloidal Bioclastic Packstone-Wackestone Microfacies, Bioclastic Wackestone Microfacies) and based on them, the formation environment was determined (restricted marine, and shallow open marine environments).

Diagenetic control on porosity types of carbonate rocks of Late Triassic Baluti Formation in Northern Thrust Zone, Iraqi Kurdistan region, Northern Iraq

The carbonate rocks of Late Triassic Baluti Formation in Sararu outcrop, Northern Thrust Zone, Northern Iraqi Kurdistan region consist of brecciated marly limestone and sandy dolomitic limestone interbedded with shale that characterized by complex heterogeneity. The microscopic description of the porosity types and diagenetic processes controlled it were studied in current work. The petrographic study of 12 thin section of Baluti carbonate displayed that the main skeletal grains are shallow marine derivative fossils and non-skeletal grains are only monocrystalline quartz. The main matrix is micrite which sometimes changed to microspar and sparry calcite. Seven pore types were distinguished in the studied rocks of the formation from both primary and secondary stages. The primary porosity are interparticle and intraparticle pores. While, the secondary types were moldic, intercrystalline, fracture, vuggy and stylolitic porosities. The formation in studied outcrop was undergone in several diagenetic processes, that enhanced or reduced the porosity of the Baluti carbonates. Early micritization protect the primary pores from destroying by compaction. While, solution was assists in creating different secondary pores in different diagenetic stage of the formation particularly the dominant moldic porosity. Calcite cements aid in reducing all pore types in the studied formation. Late dolomitization contribute to formation intercrystalline and dolomoldic porosities. stylolites with dissolution helps to creating stylolitic porosity. Neomorphism, silicification and pyritization generally reduced porosity in the formation.

Facies Association and Depositional Environment of the Sarmord Formation (Valanginian-Aptian), Kurdistan Region, Northeastern Iraq

The facies analysis, mineralogy and depositional environment of the Cretaceous Sarmord Formation are studied in two outcrops in the Imbrication and High Folded Zones of Northeastern Kurdistan region of Iraq. The formation comprises of rhythmic alternation of medium to thick-bedded marly limestone with marls. Petrographic investigation of marly limestones is on 60 thin sections and showed a variety of pelagic (open sea) faunas such as; ammonites, planktonic forams (Globigerina), radiolarians, calcispheres, ostracods, bioclasts, echinoderm plates and rare calpionellids. Peloids are the only non-skeletal grains components. Depending on detailed microfacies analysis of limestones, three main microfacies types and nine sub-microfacies are identified in the studied sections of the Sarmord Formation, these facies were incorporated into one facies association type relating to their environmental interpretation, that is basinal facies association. X-ray diffraction analysis of selected marl samples, displayed that the main clay minerals in the Sarmord Formation are illite, kaolinite, chlorite and mixed-layers of illite – chlorite. The high abundance of illite indicates that hot-arid climatic conditions prevailed during the deposition of the Sarmord Formation in the study area. From all of the petrographic, facies, textural and mineralogical analyses, it is concluded that the Sarmord Formation in the northeastern Iraq, was deposited in deep water, basinal (outer ramp) environment.

Subenvironments of Jeribe Formation (M. Miocene) deduced from microfacies study, Zurbatiya Area, South-East of Iraq

The current study of Jeribe Formation (Middle Miocene) at Zurbatiya area, South-East of Iraq. It is, structurally, located in the Low Folded Zone. The topic depends mainly on the study of microfacies under a microscope. One surface section in the Zurbatiya area is represented in the Wadi Al-Numur section. It is composed basically of dolomitic limestone. Petrography provides a diversity of fauna, such as benthic foraminifera Miliolid, Borelis melo curdica, and Rotalia in addition to Ostracoda, Algae, Coral, and Mollusks. Borelis melo curdica is considered an index fossil for the Jeribe Formation. The non-skeletal grains like peloids, ooids intraclasts, and extraclasts. The Jeribe Formation was affected by many diagenetic processes like neomorphisim, dissolution, cementation, dolomitization, compaction, and silicification. Sedimentary microfacies are classified into three main microfacies, which are dolomudstone, dolowackestone, and dolopackstone. These microfacies were divided into many submicrofacies, which are peloidal dolomudstone, ostrocods dolomudstone, rotaliids dolowackstone, miliolid dolowackstone, bioclasts dolowackstone, green and red algae dolowackstone, red algae dolopackestone, and coral dolopackestone. The evidence from petrography and microfacies analysis supports that the Jeribe Formation was deposited in the back reef, reef, and lagoon environments.

Petrography and diagenesis of the Middle to Upper Jurassic succession from Sargelu section, northeastern Iraq

Petrographic and diagenetic analysis of the Middle-Upper Jurassic successions (Sargelu, Naokelekan, and Barsarin) formations and boundaries between them in the Sargelu area, Kurdistan region, N.E. Iraq was conducted based on the lithologic description, thin section analysis, and scanning electron microscopy. The study aims to define the petrographic components and diagenetic processes that affect the carbonate rocks of Jurassic succession in the studied section. Thirty-eight thin sections have been prepared, with five samples selected using the S.E.M. technique to reveal the petrographic components and diagenetic processes. The Jurassic succession is composed mainly of carbonates (limestone and dolostone) interbedded with shale units. Petrographically, the Sargelu, Naokelekan, and Barsarin formations are composed of skeletal grains (pelagic pelecypods, radiolaria, calcispheres, planktonic and benthonic foraminifera such as miliolid, ostracods, bioclasts, and stromatolites) which are the most common, in addition, non-skeletal grains such as poloids, micritic groundmass, and recrystallized micro spars, Many diagenetic processes affected the studied carbonate rocks such as micritization, dolomitization compaction and stylolite formation, authigenic minerals (pyrite), cementation, neomorphism, dissolution and porosity formation as represented by moldic, vuggy, channel and fracture porosity.

Depositional environment of the Sarmord Formation (Valanginian-Aptian) in selected areas, northeastern Iraq

The current study deals with microfacies and the depositional environment of the Lower Cretaceous Sarmord Formation at selected sections in Sulaimani and Erbil Governorates, northern Iraq. The Sarmord Formation alternates rhythmically between yellowish grey marly limestones and grey black marls. These lithologies are observed in all studied sections of Sarmord Formation in northern Iraq. Petrographic investigation of this study based on 240 thin sections demonstrated that the carbonate constituents are mainly composed of skeletal and non-skeletal grains. The skeletal grains include a variety of foraminiferas (planktonic and benthonic), bioclasts, calcispheres, ostracods, radiolaria, echinoderms, sponge spicules, ammonoids and belemnites. The non-skeletal grains consist of rare intraclasts. Based on field observations, two major lithofacies are recognized in the studied sections, namely marly limestone and marl lithofacies. Depending on the detailed microfacies analysis, two main microfacies, which in turn are divided into four sub-microfacies, were identified. These facies were subdivided upon their environmental interpretation into two types of facies associations: deep shelf and basinal. From the results of petrographic, facies, and textural analyses, it is concluded that Sarmord Formation in the studied sections was deposited at a rimmed carbonate platform. With a gradual shallowing of the basin from basinal sediments at the lower part overlying by deep shelf sediments in the uppermost part of the formation in the studied sections.

Microfacies Analysis and Depositional Environment of the Middle Eocene Avanah Formation in Haibat Sultan Area, Iraqi Kurdistan Region

The Avanah Formation (Middle Eocene) was studied in the Haibat sultan section at Bina Bawi Anticline, Northeast of Erbil city within the Iraqi Kurdistan region. The field observation indicated that the formation occurs as Carbonate interbeds within the lower and middle parts of the Gercus Formation, which consists mainly of light yellowish to creamy color, well-bedded, marly, and dolomitic limestone with some intercalations of sandstones and marls reaching about 32 m thick characterized by lateral variations in facies and thicknesses. A petrographic study of 16 thin slices of carbonate rocks indicated that most of the matrix is composed of micrite, with a few microspars. Foraminifers, dasycladacean green alge, and Ostracods make up the skeleton grains. Non-skeletal grains mainly include peloids, intraclasts, and extraclasts. Depending on detailed microfacies analysis in the current study, five main microfacies and fourteen submicrofacies were discovered. The total of all petrographic, facies and textural investigations indicates that the Avanah Formation in the Haibat Sultan area was deposited in the shallow marine inner platform within subtidal, intertidal, and supratidal environments

Microfacies and depositional environment of Jeribe Formation (Middle Miocene) in the selected sections in Bekhair anticline, northwestern Kurdistan region of Iraq

Microfacies analysis and depositional environment of Jeribe Formation (early Middle Miocene) were studied for the first time in two outcrops (Karin and Qadia villages) within the Bekhair anticline in the High Folded Zone, Zakho area, northwestern part of Kurdistan region-northern Iraq. In the Bekhair anticline, the Jeribe Formation overlies the Dhiban Formation and underlies the Fatha Formation with gradational and conformable boundaries. Based on field observations two informal lithostratigraphic units were identified: a thick massive dolomitic limestone unit, and a fractured limestone unit. The petrographic diagnosis of 27 thin sections show a variety of shallow marine derivative fossils with non-skeletal grains like peloids, ooids, intraclasts, and quartz grains. Relying on the detailed microfacies analysis, five major microfacies types including lime mudstone, lime wackestone, lime packstone, grainstone, and boundstone were identified and then divided into eleven sub-microfacies types according to dominant grain types. They were integrated according to their environmental differentiation into two facies associations which are: intertidal and lagoon. From the sum of field observation and facies analysis, the depositional environment of the Jeribe Formation was a shallow marine environment, deposited in the intertidal setting in the lower part and changed to a semi-restricted lagoonal environment toward the upper part.

Microfacies study of the Neogene sequences and determination of sedimentary environments in the Jambour oil field / Kirkuk

The current research deals with microfacies study of the four wells drilled in the area south-east of Kirkuk within Jambour oil field and these are wells Jambour Ja-42 ,Ja-55,Ja-62 and Ja-66. The study included the bed of a Basal anhydrite cycle (Aquitanian) and configurations Serikagni, Euphrates and Dhiban Formation from early Miocene second secondary cycle (Burdigalian) and Jeribe Formation from secondary middle Miocene cycle (Langhian).
 The petrographic study showed that the most important components of limestone is skeletal grains, non-skeletal grains , cement and matrix. The current study described sedimentary facies, which are divided into: - mudstone facies , wackestone facies , packstone facies and grainstone facies and these microfacies have been divided into submicrofacies. The facies changes within the studied formations have been identified on the depositional environment and put their environmental models. Overall, the Burdigalian cycle characterized by swallowing upward , which began transgression making several and different sedimentary environment , which lasted from pelagic environment formation(Serikagni Formation) and to shallower environment(Euphrates Formation), which ended with marine regression (evaporites of Dhiban Formation) in the basin was originally inherited from the basin of Euphrates and Serikagni Formations. The lower part of Langhian cycle represented by the Jeribe Formation also affected by another transgression.

Porosity Evolution and Microfacies Analysis of the Khurmala Formation (Paleocene-Early Eocene) from Selected Sections in Kurdistan Region, Northern Iraq Revealed by Cathodoluminescence Spectroscopy

The microfacies analysis and porosity evolution of the Khurmala Formation (Paleocene-Early Eocene) were studied in the Gomaspan and Sheraswar sections in the High Folded Zone, northeastern Erbil City, Kurdistan Region (N-Iraq). Lithologically, the Khurmala Formation in the Gomaspan section is about 23.5 m comprised of thick to massive bedded brecciated grey dolomitic limestone in lower and middle parts and interbedding of medium to thick beds of grey dolomitic limestone and yellow calcareous shale in the upper part. The Sheraswar section comprises 18.5 m yellow marl, thick yellowish-grey dolomitic marly limestone in the lower part, and bedded grey limestone interbedded with thin blue marl, shale, and mudstone in the upper part. The petrographic and cathodoluminescence (CL) studies of 20 thin sections of the Khurmala carbonates show that most skeletal grains are shallow marine derivative faunas and non-skeletal grains represent intraclasts, peloids, ooids, and extraclasts (monocrystalline quartz). Five main microfacies were identified in the studied rocks, and they integrated into two facies associations relating to their environmental interpretation: back reef/lagoon and patch reef. A new eight pore types were distinguished in the carbonate rocks of Khurmala Formation: interparticle, intraparticle, growth framework, intercrystalline, moldic, vuggy, fracture, and stylolite porosities. Most of the primary porosity was reduced by compaction and cementation due to effect of intensive diagenesis. Secondary porosity is predominant in both studied sections, the most common being the moldic porosity associated with different microfacies fabrics. CL investigations revealed that dolomitized lime wackestone microfacies has two types of different cementations in the late stages of blocky calcite cement which are discontinuous light to dull orange luminescence without zoning, and pure calcite cement of coarse to very coarse blocky calcite cement with characteristic bright orange luminescence and tight zoning.

Microfacies and depositional model of the Lower Mahil Formation (outcrop Khuff-equivalent (KS-1)) in Jabal Akhdar, Northern Oman: Implications for reservoir characterization

This study presents a detailed microfacies analysis of the Lower Triassic Mahil Formation (Khuff outcrop equivalent (KS-1)) in Jabal Akhdar, at the Saiq Plateau and Wadi Sahtan areas of Northern Oman.Two sections were measured and sedimentologically logged along a depositional dip profile emphasizing color, lithology, grain components, bed thickness, the nature of bed contact, lateral continuity of beds, and fossil content.One hundred and eighty fresh rock samples were collected from various lithofacies.The collected samples were thin-sectioned after blue epoxy was impregnated, and then they were half stained with Alizarin Red S and Potassium Ferricyanide for mineralogical identification.The point-counted compositions were determined by counting 300 points for each thin section to reconstruct the original components, obtain precise quantification, develop representative microfacies, and construct the depositional model for the Lower Mahil Formation.Accordingly, eighteen microfacies types representing different carbonate ramp environments were identified; two are only recognized in the Wadi Sahtan logged section, ten are restricted to the Saiq Plateau logged section, and six are common for both sections.The Lower Mahil Formation consists of backshoal (inner ramp) to foreshoal (mid-ramp) carbonate facies, with shoal (ramp crest) facies restricted to the Saiq Plateau logged section.The backshoal facies was characterized by abundant non-skeletal grains with breccia.The shoals were predominantly oolitic and intraclastic packstone-grainstone.Foreshoal carbonate facies, on the other hand, has mixed skeletal and non-skeletal grains with weakly developed hummocky cross-stratified beds.The shallow-water foraminiferal and oolitic packstone/grainstone facies are the potential reservoir intervals, whereas the wackestone facies is the non-reservoir intervals within the Lower Mahil Formation.The detailed sedimentological and microfacies analysis helped characterize the Lower Mahil Formation for better recognizing the potential reservoir vs.no-reservoir.This study can be a key reference to correlate the Lower Mahil Formation locally and regionally.

Stratigraphy and Mineralogy of Balambo Formation (Aptian - Cenomanian) near Chomabrok village, Imbrication Zone, Iraqi Kurdistan Region

The stratigraphy and mineralogy of Balambo Formation (Aptian - Cenomanian) in exposed surface section near Chomabrok village in the Imbrication zone, northeastern Iraq is studied. The Formation is 450 meters thick and consists of thin to medium to thick bedded yellowish-brown, and black to dark gray limestones, marly limestones, with thin- medium bedded, grayish - black marl and shale, with thin bands of black cherts and abundant nodules and lenses. From the field observations and petrographic studies, the formation mainly made from two main lithofacies types; marly limestone-marl lithofacies and cherty limestone lithofacies. The petrographic study of carbonate rocks deepened on 79 thin sections and demonstrated that the carbonates of Balambo Formation composed of micrite ground mass with abundant skeletal grains; planktonic and some benthic foraminifera, radiolaria, calcisphere, ostracods, pelecypods, larva-ammonoids, small-brachiopods, sponge spicules, echinoid spines, and bioclasts. Non-skeletal grains include peloids only. Using extraction method and according to an assessment of foraminiferal assemblages and based on the defined planktonic foraminifera the assigned age of Balambo Formation in the studied section is Aptian to Cenomanian as upper Balambo age. The results of X-ray diffraction (XRD) analysis for bulk mineralogy of selected shale and marl samples suggested the presence of phyllosilicate with abundant calcite, common quartz, and moderate Fluorapatite with minor dolomite content. Illite is the dominant clay mineral in the Balambo Formation and indicates a hot, arid climate during most of the Cretaceous period in the Imbrication zone, Northeastern Iraq.

Reconstruction of a Ross lost CambrianSeries 2mixed siliciclastic–carbonate platform from carbonate clasts of the Shackleton Range, Antarctica

ABSTRACTThe presence of archaeocyath-bearing clasts from Cenozoic tills and Cambrian Mount Wegener Formation reveal erosion of a hidden Cambrian carbonate platform in Shackleton Range, Antarctica. We provide microfacies, paleontological, diagenetic and tectonically induced fabric data from carbonate clasts which, in addition to available geochemical and geochronological data from Shackleton Range, allow the paleoenvironmental reconstruction of a lost CambrianSeries 2mixed siliciclastic–carbonate platform that was developed and eroded during the Ross orogeny. Carbonate production was dominated by non-skeletal grains in possibly restricted platform-interior and oolitic shoal complex settings, while open subtidal sub-environments (calcimicrobe carpets, calcimicrobe–archaeocyath patch reefs, muddy bottoms) were dominated by a diverse calcimicrobe assemblage and/or by secondary to accessory heterozoan assemblage (archaeocyaths and other sponges, chancelloriids, hyoliths, coralomorphs, trilobites, echinoderms). We describe a Botoman assemblage with 34 archaeocyathan species among 12 existing archaeocyathan genera. A new archaeocyath family Shackletoncyathidae is proposed. New species (Rotundocyathus glaciussp. nov.,Buggischicyathus microporusgen. et sp. nov.,Paragnaltacyathus hoeflei,Shackletoncyathus buggischigen. et. sp. nov.,Santelmocyathus santelmoigen. et sp. nov.,Wegenercyathus sexangulaegen. et sp. nov.) andTabulaconus kordaecoralomorph are reported from Antarctica for the first time. Archaeocyathan fauna share few species with contemporary fauna of South Australia (9) and even fewer with the Antarctic platforms of the Shackleton Limestone (2) or the Schneider Hills limestone (1). Similarity is greater with Antarctic allochthonous assemblages of Permo-Carboniferous tillites from Ellsworth Mountains (2), Cenozoic deposits from King George Island (4) or Weddell Sea (1). The Shackleton Range lost/hidden platform shows a distinct entity related with its tectonosedimentary evolution, in a possible back-arc basin on the Mozambique seaway during the E and W Gondwana amalgamation, which distinguishes it from those developed on the palaeo-Pacific margin of the E Antarctic craton.

Microfacies analysis and depositional environment of the Upper Devonian Dankovo-Lebedyansky sediments, Tatarstan, Volga-Ural Basin, Russia

The Upper Devonian, Middle Famennian Dankovo-Lebedyansky sediments in Southeast Tatarstan territory are a carbonate sequence composed of limestone and dolomite, which is important hydrocarbon reservoir units, therefore, it is necessary to conduct in-depth study on its microfacies and depositional environment. In this study, a multidisciplinary approach that combines core observation with thin section examination is used. The limestone contains abundant skeletal grains (echinoderms, foraminifera, algae, gastropods, and calcispheres), as well as non-skeletal grains (intraclasts and peloids). On the basis of detailed petrographic investigations, six sedimentary microfacies can be identified, including (i) peloidal grainstone (MF 1), (ii) cemented bioclastic peloidal grainstone (MF 2), (iii) echinoderm-concentrated packstone (MF 3), (iv) algae packstone (MF 4), (v) bioclastic wackestone (MF 5), (vi) whole-fossil wackestone (MF 6), as well as dolomite or dolostone as diagenetic facies (MF 7). Based on microfacies analysis, the Dankovo-Lebedyansky sediments were deposited in three distinct sedimentary facies belts (shoal, lagoon and open marine environment). In order to reflect dispositional energy condition, the microfacies were grouped into facies associations: (i) low-energy microfacies associations including MF 5 and MF 6, (ii) moderate energy microfacies associations including MF 1, MF 2, MF 3, and MF 4. The dolomite, or diagenetic facies (MF 7), is the result of slightly to extensively dolomitization of limestone in the Dankovo-Lebedyansky sediments. Most of frequent dolostone types are euhedral planar-e and subhedral planar-s. According to petrographic characteristic of the dolostone, a seepage reflux model can be used to explain the dolomitization process of Dankovo-Lebedyansky sediments.

Depositional Setting of Khurmala Formation (Paleocene-Early Eocene) in Nerwa section, Berat Anticline, Kurdistan Region, Northern Iraq

The depositional environment of the Khurmala Formation (Paleocene-Early Eocene) has been investigated in the Nerwa section, the southern limb of the Berat anticline in the High Folded Zone of Iraq. The thickness of the formation in the studied section was about 113m and can be divided into three units; fossiliferous bedded limestone unit (Unit A), massive dolomitic limestone unit (Unit B) and bedded marly dolomitic limestone unit (Unit C). The petrographic study of the 81 thin sections of the Khurmala carbonates revealed that the majority of skeletal grains are shallow marine derivative faunas and non-skeletal grains include peloids, cortoids and intraclasts. Depending on detailed field observations and petrographic study of the carbonate rocks, five main microfacies were recorded, these are; lime mudstone, lime wackestone, lime packstone, grainstone and boundstone in addition to one lithofacies which is an intraformational conglomerate. Based on the integration of the recognized microfacies and the field criteria of the Khurmala Formation, two facies associations were identified which elucidate deposition in a lagoonal environment including reef patches. Accompanying facies indicate that the formation's sedimentary model is a ramp carbonate platform.

Lithostratigraphy and microfacies analysis of Avanah Formation (Middle Eocene) in Gomaspan section northeast Erbil City, Kurdistan region, Iraq

Lithostratigraphy and microfacies analysis of the Avanah Formation (Middle Eocene) were studied in the Gomaspan section in the Bina Bawi anticline, northeast of Erbil city, Kurdistan Region, Iraq. The field observations refer that the formation attains 56 m of medium to thick bedded yellow limestone, grey dolomitic limestone and blue marly dolomitic limestone interbedded with thin beds of blue marl and dark grey shale with an interval of sandy limestone in the middle part and thin to medium bedded limestone interbedded with red mudstone. The petrographic study of 29 thin sections of Avanah carbonates revealed that the majority of the matrix is carbonate mud (micrite) with few microspar. The skeletal grains include benthic foraminifera, dasycladacean green algae, ostracods, calcispheres, pelecypods, rare planktonic foraminifera and bryozoa in addition to bioclasts. Non-skeletal grains encompass peloids, oncoids, intraclasts and extraclasts with common monocrystalline quartz. Based on the field observation and petrographic analysis, three different lithostratigraphic units were identified. They are in ascending order: A-Thick bedded dolomitic marly limestone interbedded with shale. B- Bedded interbedded with red mudstone. Depending on detailed microfacies analysis of carbonate rocks, three main microfacies and 12 submicrofacies are recognized. From the sum of all petrographic, facies, textural analyses, it is concluded that Avanah Formation in Gomaspan section, was deposited in shallow marine environment, semi restricted lagoon, in lower and upper parts and open lagoon environment in the middle part interval.

Lithostratigraphy and microfacies analysis of Avanah Formation (Middle Eocene) in Gomaspan section northeast Erbil City, Kurdistan region, Iraq

Lithostratigraphy and microfacies analysis of the Avanah Formation (Middle Eocene) were studied in the Gomaspan section in the Bina Bawi anticline, northeast of Erbil city, Kurdistan Region, Iraq. The field observations refer that the formation attains 56 m of medium to thick bedded yellow limestone, grey dolomitic limestone and blue marly dolomitic limestone interbedded with thin beds of blue marl and dark grey shale with an interval of sandy limestone in the middle part and thin to medium bedded limestone interbedded with red mudstone. The petrographic study of 29 thin sections of Avanah carbonates revealed that the majority of the matrix is carbonate mud (micrite) with few microspar. The skeletal grains include benthic foraminifera, dasycladacean green algae, ostracods, calcispheres, pelecypods, rare planktonic foraminifera and bryozoa in addition to bioclasts. Non-skeletal grains encompass peloids, oncoids, intraclasts and extraclasts with common monocrystalline quartz. Based on the field observation and petrographic analysis, three different lithostratigraphic units were identified. They are in ascending order: A-Thick bedded dolomitic marly limestone interbedded with shale. B- Bedded dolomitic limestone interbedded with shale and marl. C- Thin to medium bedded limestone interbedded with red mudstone. Depending on detailed microfacies analysis of carbonate rocks, three main microfacies and 12 submicrofacies are recognized. From the sum of all petrographic, facies, textural analyses, it is concluded that Avanah Formation in Gomaspan section, was deposited in shallow marine environment, semi restricted lagoon, in lower and upper parts and open lagoon environment in the middle part interval.

Quantitative compaction trends of Miocene to Holocene carbonates off the west coast of Australia

In this paper, we describe porosity variations in Miocene to Holocene carbonates off the west coast of Australia and assess their compaction trends. The porosity values were measured using discrete samples of Sites U1459–U1464 obtained by the International Ocean Discovery Program Expedition 356. The carbonate deposits have been influenced by a range of textures and diagenetic conditions throughout a nearly continuous sequence of geological ages from the Miocene to Holocene and at core depths from 0 to 1100 m below the seafloor. The collected samples were mostly grainstone, packstone, wackestone and mudstone textures. Dolostones and dolomitic carbonates were described at the Miocene intervals. Compaction trends were estimated exponentially and linearly based on cored sites, carbonate textures and dominant mineralogies (dolomite, calcite/aragonite). At all six sites, porosity distribution and reduction were generally depth-dependent. The porosity converged to about 30% between 750 and 1100 m, which suggests that the carbonates were close to the densest packing by mechanical compaction at a burial depth of ∼750 m. The porosity deviations are associated with textures and dominant mineralogies. Increasing mud content from grainstone to mudstone is a controlling factor for initial porosity and porosity reduction rate. Dolomitisation, dolomitic cementation, aragonite needle-rich mud and non-skeletal grains cause deviations from the depth-dependent compaction trends. Reflux-related cementation generally decreases porosity in Miocene dolomitic intervals. Higher porosity values of the Quaternary wackestone and mudstone at Site U1461 resulted from the presence of aragonite needle-rich mud hosting abundant micropores and from a high sedimentation rate. The occurrence of non-skeletal grains, such as ooids and peloids, as well as occasional meteoric exposure led to porosity inversion, occluding interparticle permeability and the creation of moldic pores. KEY POINTS Porosity variations of Miocene–Holocene carbonates off the west Australian coast are primarily depth-dependent. Mechanical compaction affected porosity reduction down to a value of ∼30% at a burial depth of ∼750 m. Differences in texture-based compaction trends were affected by increasing mud content. Major porosity deviations are associated with dolomitisation, dolomitic cementation, aragonite needle-rich mud and non-skeletal grains.