Petrography and Diagenetic Characteristics of the Upper Oligocene  Lower Miocene Ghar Formation in Se Iraq (original) (raw)
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Genesis and characterization of dolomite, Arab-D Reservoir, Ghawar field, Saudi Arabia
GeoArabia, 2004
This study reports the results of an investigation into the nature, origin and significance of linear dolomite trends across the Arab-D reservoir in Ghawar field. In the course of this study, three distinct types of dolomite were identified based on petrographic and geochemical criteria: fabric-preserving (FP), non-fabric-preserving (NFP) and baroque dolomite. Fabric-preserving (FP) dolomite is very finely crystalline dolomite in which details of the original limestone fabric are usually well preserved. Beds of FP dolomite typically occur as thin, sheet-like or stratigraphic layers that are always intimately associated with the overlying anhydrite. This dolomite is interpreted to have formed very early in the diagenetic history of the sediment, by dense, highly evaporated magnesium-rich brines associated with the overlying anhydrite. In contrast, NFP dolomite is a medium crystalline, non-baroque dolomite in which all traces of the original limestone fabric have been obliterated. Thi...
Baluti Formation of the Rhaetian (Late Triassic) age is composed mainly of dolomite, the unit formed with dolomitic limestone, dolomitic breccias and limestone begins with gray or dark gray colored and sugar textured dolomitic limestones including micrite with shale horizons. Baluti Formation was deposited in carbonate platform, and slumped to deeper margins forming carbonate debrites and breccias of various types. Petrographic examination of the dolomites reveals various crystal habits and textures of the dolomites. Planktonic bivalve, calcisphere and echinoid spicules were found in the Baluti Formation settled in deep-margin carbonate environment. Nine dolomite-rock textures were identified and classified according to the crystal-size distribution and crystal-boundary shape. These are made of unimodal, 1) very fine to fine-crystalline planar-s (subhedral) mosaic dolomite; 2) unimodal, medium to coarse-crystalline planar-s (subhedral) mosaic dolomite; 3) coarse to very coarse crystalline planar-s (subhedral) dolomite; 4) medium to coarse-crystalline planar-e (euhedral) mosaic dolomite; 5) medium to coarse-crystalline planar-e (euhedral) dolomite; 6) coarse to very coarse-crystalline non-planar-a (anhedral) dolomite; 7) coarse to very coarse-crystalline non-planar-c (cement) dolomite; 8) polymodal, planar-s (subhedral) to planar-e (euhedral) mosaic dolomite. Dolomitization is closely associated with the development of secondary porosity; dolomitization pre and post diagenetic dissolution and corrosion and no secondary porosity generation is present in the associated limestones. The most common porosity types are non-fabric selective moldic and vugy porosity and intercrystalline porosity. These porous zones are characterized by late-diagenetic coarse-crystalline dolomite, whereas the non-porous intervals are composed of dense mosaics of early-diagenetic dolomites. The distribution of dolomite rock textures indicates that porous zones were preserved as limestone until late in the diagenetic history, and were then subjected to late-stage dolomitization in a medium burial environment, resulting in coarse-crystalline porous dolomites. Baluti dolomites have been formed as early diagenetic at the tidal-subtidal environment and as a late diagenetic at the shallow-deep burial depths.
Sedimentologic and facies evidences reveal a marine environment for the Gercus Formation. Facies analysis and associated sedimentary structures including graded beddings decide turbidity origin of the rocks. Marine environment is supported by the identifying glauconite and fossils types reported for the first time. The formation composed of seven lithotypes; shale/claystone, mudstone, sandstone, carbonate, conglomerate, breccias and debris flow, which are arranged in repeated cycles of mixed siliciclastic-carbonate turbidites in a range of gravity-flow regime. The Gercus successions are grouped into four facies associations confirming marine depositional systems, these are (from bottom to top); slump siliciclastic-calciturbidites (dolomite/shale dominated), proximal siliciclastic-calciturbidites (dolomite/sand dominated), distal siliciclastic-calciturbidites (sand/mud dominated) and slope siliciclastic turbidites (sand/clay dominated) respectively. Petrographic analysis of sandstone units show predominant of lithic fragments, most of it are carbonate with subordinate tuffaceous fragments, chert, chalcedony, volcanic ash, metamorphic and detrital iron oxides grains, with noticeable grains of glauconite. Varieties of marine fossils are identified includes planktonic bivalves and benthic forams of cool water, which support the deeper marine environment. Petrographic examination of carbonate units reveal skeletal grains of benthic and planktonic forams, stromatolite, planktonic bivalves, corals and algae, with non-skeletal grains of chert, chalcedony, tuffaceous fragments, volcanic ash, and volcanic bubbles. Petrography, lithofacies and lithostratigraphic analysis of the Gercus Formation suggest deposition in developed marine environment, mainly effected by gravity-flow turbidity currents, and displays successive submarine fans of high density turbulent currents in deeper margins. Mixed siliciclastic-carbonate cycles were deposited in intervals of weaning of turbulent currents. Based on clast type and size, it seems likely that a weakly turbulent to laminar gravity-flow phase was present when the flow event entered the basin at the end part of the fan. A change in flow behavior may have led to deposit sand-rich unit with 'turbidite' characteristics, which was subsequently grades upwards to clay-dominated unit. This paper presents new details of lithostratigraphic subdivisions and associations of the Gercus Formation in Koi Dokan area, and new suggested marine environment of deposition. The previous workers suggest continental and probably mixed with deltaic environments in the upper part.
Iraqi Bulletin of Geology and Mining, 2019
The Baluti Formation in the Galley Derash locality is comprised of 73 m brecciated dolomitic limestone in the lower part, alternating with thin bedded dolomitic limestone and dark gray shale in the middle part, then thin to medium bedded brecciated sandy limestone interbedded with dark grey shale in its upper part. The sum of 21 thin sections of the Baluti Formation carbonates are petrographically studied using polarizing microscope. The results show various types of skeletal grains including calcareous algae, ostracods, brachiopods, bivalves, benthonic foraminifera, gastropods, and echinoderms, while the non-skeletal grains include peloids, intraclasts, extraclasts and rare ooids. The Baluti Formation was subjected to differed diagenetic processes such as micritization, mechanical compaction, cementation, early dolomitization, neomorphism, stylolitization, late dolomitization, dedolomitization, silisification, soulution, fracturing and pyritization. These diagenetic modifications occurred during marine phreatic shallow burial stage and were activated during intermediate to deep burial and uplift in the late stages. Five kinds of dolomite textures are recognized: Unimodal very fine to fine-crystalline planar-s (subhedral) mosaic, unimodal very fine to fine-crystalline planar-e (euhedral) mosaic, unimodal fine to medium crystalline planar-s (subhedral) mosaic, medium to coarsecrystalline planar to non-planar-c (cement) mosaic and polymodal planar-s (subhedral) to planar-e (euhedral) mosaic. The petrographic study revealed that the secondary porosity is dominant. The paragenetic history of the Baluti Formation passed through four diagenetic environments which are: marine, meteoric, burial and uplift.
The Impact of Paleoclimate on Dolomite Reservoir Development in the Zagros and Persian Gulf Regions
University Of Tehran, 2024
Dolomite reservoirs flourished during arid climatic periods in the Middle East, primarily in the Permo-Triassic, Upper Jurassic, and Oligo-Miocene formations. These dolomitized reservoirs are frequently linked to evaporites, exhibit isotopically enriched signatures, and tend to occur predominantly in the more restricted regions of carbonate platforms. These observations strongly support their origin through sabkha and evaporative reflux processes. Consequently, dolomite formation and distribution are primarily influenced by early diagenetic processes and climatic conditions during arid periods. Dolomitization has exerted a significant influence on reservoir properties within the studied carbonate platforms. Porosity distribution and variation are jointly controlled by several factors, including dolomite content, texture, crystal size, anhydrite abundance, dolomite cementation, and the extent of burial compaction. While dolomite textures can vary from fabric-preserving to fabric-destroying, the overall reservoir properties exhibit an ascending pattern from intertidal to shoal facies. This trend is primarily determined by the proximity to the source of dolomitizing brines. The downward percolation of brines, coupled with decreasing dolomitizing potential, leads to an increase in dolomite crystal size within depositional cycles as one moves further away from the anhydrite facies. Proximal areas, characterized by fine-grained intertidal and lagoonal facies, are more susceptible to anhydrite cementation and overdolomitization, resulting in significant porosity reduction. Conversely, in more distal regions, reservoir quality substantially improves, particularly in areas dominated by sucrosic dolomite or grain-rich facies. While this trend may be altered by compaction during burial, it underscores the crucial role of dolomitization in preserving porosity, especially in deeply buried carbonate reservoirs.
AAPG Bulletin, 2013
Understanding the distribution and geometry of reservoir geobodies is crucial for netto-gross estimates and to model subsurface flow. This paper focuses on the process of dolomitization and resulting geometry of diagenetic geobodies in an outcrop of Jurassic host rocks from northern Oman. Field and petrographic data show that a first phase of stratabound dolomite is crosscut by a second phase of fault-related dolomite. The stratabound dolomite geobodies are laterally continuous for at least several hundreds of meters (~1000 ft) and probably regionally and are half a meter (1.6 ft) thick. Based on petrography and geochemistry, a process of seepage reflux of mesosaline or hypersaline fluids during the early stages of burial diagenesis is proposed for the formation of the stratabound dolomite. In contrast, the fault-related dolomite geobodies are trending along a fault that can be followed for at least 100 meters (328 ft), and vary in width from a few tens of centimeters up to 10 meters (~1 to 33 ft). Petrography, geochemistry and high homogenization temperature of fluid inclusions all point to formation of the dolomite along a normal fault under deep burial conditions during mid to Late Cretaceous times. The high 87 Sr/ 86 Sr ratio in the dolomite and the high salinity measured in fluid inclusions indicate that the dolomitizing fluids are deep basinal brines that interacted with crystalline basement. The dolomitization styles have an impact on dimension, texture and geochemistry of the different dolomite geobodies, and a modified classification scheme (compared to the one from Jung and Aigner, 2012) is proposed to incorporate diagenetic geobodies in future reservoir modeling.
AAPG Bulletin, 2010
Diagenetic illite is one of two major porosity occluding cements in the Unayzah sandstone reservoirs (Permian-Carboniferous) of Saudi Arabia. The other is diagenetic quartz. This article focuses on the origin of illite and its timing. Illite has been formed by a reaction of detrital K-feldspar and early diagenetic kaolinite as temperatures increased due to burial. When either of the two reactants is exhausted, illite ceases to precipitate. There is no evidence that hydrocarbon emplacement, deep brine migration, or unique thermal events are factors in illite precipitation. Modeling of illite precipitation as a kinetically controlled reaction using burial histories of the samples studied generally yields a reasonable match between measured and modeled ages and amounts of illite. This lends further support to the gradual formation of illite over a time-temperature interval. Although quartz overgrowths and diagenetic illite may occur in the same thin section, they appear to be mutually exclusive locally. Quartz overgrowths do not occur on detrital quartz grains that are coated with diagenetic illite, and illite is rarely observed on quartz overgrowths. Therefore, it appears that not only does diagenetic illite inhibit nucleation of quartz overgrowths but quartz overgrowths may also inhibit precipitation of diagenetic illite. The two cements appear to compete for surface area on uncoated detrital quartz grains.
Geology and Production Significance of Dolomite, Arab-D Reservoir, Ghawar Field, Saudi Arabia
GeoArabia, 2001
At least five distinct types of dolomite occur in the Arab-D Reservoir in Ghawar field, Saudi Arabia – one of which appears to be responsible for high flow or ‘super-k’. These dolomite types are distinct petrographically, geochemically and stratigraphically: a finely-crystalline non-fabric-preserving (NFP) variety of dolomite in the lower Arab-D (Zone 3) with low oxygen isotope values and generally poor reservoir quality;a medium-crystalline NFP dolomite with high oxygen isotope values and very poor reservoir quality in the upper Arab-D (Zone 2);a medium to coarsely-crystalline NFP dolomite with low oxygen isotopic values and very good reservoir quality (‘super-k’) occurring in Zone 2; anda finely-crystalline fabric-preserving (FP) dolomite in the uppermost Arab-D (Zone 1) that contains high oxygen isotope values and has generally fair to poor reservoir quality. Previous studies have documented a rare fifth type of dolomite, baroque or ‘saddle’ dolomite, that occurs locally in the r...
2020
The Faiha and Sindibad oilfields are located in the northeastern part of the Basrah Governorate, south Iraq. More than 50 thin sections of limestone samples, collected from wells penetrating the Mishrif and Yamama formations in these oilfields, are examined in this work. Several microfacies are identified in the formations studied; four main facies and seven subfacies are found in the Yamama Formation and three main Formation. The main microfacies include lime mudstone, wackestone, packstone and grainstone. They were deposited in various marine sedimentary systems of open marine, foreslope, reef and lagoonal environments. Benthonic foraminifera, Mollusk shells, echinoids, and algae, are the fossils identified in the Mishrif and Yamama formations. Dolomite and calcite are the main minerals in the studied samples. Petrographic examinations revealed that diagenesis varied in intensity from microfacies to another. Dissolution and neomorphism (recrystallization) have created and controlled the development of porosity. The dominant pore types are vuggy, interparticles, intraparticles, channel and moldic. Whereas, cementation, micritization, and compaction processes have had destructive effects, by reducing porosity and permeability that led to reducing reservoir quality. Other processes, such as dolomitization and authigenic minerals (pyrite) do not have strong effects on the reservoir quality. The depositional basin model shows that the Yamama Formation was deposited in the ramp carbonate environment, whereas, the Mishrif Formation was deposited in the rimmed carbonate shelf environment.
Dolomite textures in the Upper Cretaceous carbonate-hosted Pb–Zn deposits, Zakho, Northern Iraq
In the northeast of Zakho City, Northern Iraq, the host rocks of Pb-Zn deposits are composed predominantly of dolomites with subordinate dolomitic limestone intervals. This study is focused on the dolomites of the Bekhme Formation (Upper Campanian) carbonate-hosted Pb-Zn deposits. The amount of dolomites, however, increases toward the mineralized zone. Dolomites are dominated by replacement dolomite with minor dolomite cements. Petrography study allowed identification of six different dolomite textures. These are (1) fine crystalline, planar-s (subhedral) dolomite, RD1; (2) medium to coarse crystalline, planar-e (euhedral) to planar-s (subhedral) dolomites, RD2; (3) medium crystalline, planar-s (subhedral) to nonplanar-a (anhedral) dolomites, RD3; (4) coarse crystalline, planar-s (subhedral) to nonplanar-a (anhedral) dolomites, RD4; (5) planar (subhedral) void-filling dolomite cements, CD1; and (6) nonplanar (saddle) voidfilling dolomite, CD2. The RD1, RD2, RD3, and RD4 dolomite textures are replacive in origin and are volumetrically the most important types, whereas CD1 and CD2 dolomites with sparry calcite are commonly cements that fill the open spaces. Although the dolomites of the Bekhme Formation are not macroscopically observed in the field, their different types are easily distinguished by petrographic examination and scanning electron microscopy. It was observed that the dolomites of the Bekhme Formation are formed in two different diagenetic stages: the early diagenetic from mixing zone fluids at the tidal-subtidal (reef) environments and the late diagenetic from basinal brines which partially mixed with hydrothermal fluids at the shallow-deep burial depths. The latter occurs often with sphalerite, galena, and pyrite within mineralized zone. These dolomite types are associated base-metal mineralization (Mississippi Valley type).