The Origin of Dolomite in the Asmari Formation (Oligocene‐Lower Miocene), Dezful Embayment, SW Iran (original) (raw)
Related papers
Journal of Petroleum Geology, 2009
Iran indicate that the carbonates have been subjected to extensive diagenesis including calcite cementation and dolomitization. Diagenetic modification occurred in different diagenetic realms ranging from marine, meteoric and finally burial. Asmari carbonates were in general deposited in a ramp setting and are represented by intertidal to subtidal deposits together with lagoonal, shoal and low-energy deposits formed below normal wave base. Lithofacies include bioclastic grainstones, ooidal and bioclastic, foraminiferal and intraclastic packstones, and mudstones. Multiple episodes of calcite cementation, dolomitization and fracturing have affected these rocks to varying degrees and control porosity. Four types of dolomites have been identified: microcrystalline matrix replacement dolomite (D1); fine to medium crystalline matrix replacement dolomite (D2); coarse crystalline saddle-like dolomite cement (D3); and coarse crystalline zoned dolomite cement (D4). Microcrystalline dolomites (D1) (6-12 μm) replacing micrite, allochems and calcite cements in the mud-supported facies prior to early compaction show δ 18 O and δ 13 C values of -4.01 to +1.02 ‰ VPDB and -0.30 to +4.08 ‰ VPDB, respectively. These values are slightly depleted with respect to postulated Oligocene-Miocene marine carbonate values, suggesting their precipitation from seawater, partly altered by later fluids. The association of this type of dolomite with primary anhydrite in intertidal facies supports dolomitization by evaporative brines. Fine to medium crystalline matrix dolomites (D2) (20-60μm) occur mostly in grainstone facies and have relatively high porosities. These dolomites formed during early burial and could be considered as recrystallized forms of D1 dolomite. Their isotopic values overlap those of D1 dolomites, implying precipitation from similar early fluids, possibly altered by meteoric fluids. Coarse crystalline saddle-like dolomites (D3) (200-300 μm) partially or completely occlude fractures and vugs. The vugs developed through the dissolution of carbonate components and rarely matrix carbonates, while fractures developed during Zagros folding in late Oligocene to early Miocene times. A final diagenetic episode is represented by the precipitation of coarse crystalline planar e-s zoned dolomite (D4) (80-250 μm) that occurs in fractures and vugs and also replaces earlier dolomite and post-dates stylolitization. Fluids responsible for the formation of D3 and D4 dolomites are affected by brine enrichment and increasing temperatures due to increasing burial. Dolomitization in the Oligocene-Miocene Asmari Formation, Gachsaran area, SW Iran
Shallow burial dolomitization of an Eocene carbonate platform, southeast Zagros Basin, Iran
GeoArabia
Here, a case example of a dolomitized Eocene ramp setting from the southeastern Zagros Basin is documented and discussed in the context of published work. This is of significance as well-documented case examples of Eocene dolomitized inner platforms are comparably rare. The same is true for detailed diagenetic studies from the Zagros Basin in general. Three measured field sections were combined with detailed petrographic and geochemical analyses and four main dolomite types were defined. The most significant dolomite type is present in the form of a volumetrically significant occurrence of meter-thick beds of strata-bound dolostones. These dolomites are characterized by near-stoichiometric composition, fabric-retentive and fabric-destructive textures, subhedral to anhedral in shape and most being in the tens-of-microns range. Dolomite δ18O (averaging -2.6‰) values are depleted relative to that expected for precipitation from Eocene seawater (averaging 0‰), while δ13C (averaging -0.1...
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...
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.
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.
Journal of Petroleum Geology, 2000
Dolomitization in the early Eocene Jirani Formation in the Gabes-Tripoli Basin (offshore western Libya) occurred in two stages. Stage I dolomites are composed of two types, one associated with anhydrite (Type I) the other anhydrite free (Type II,). The stratigraphic and sedimentological settings together with petrographic and geochemical criteria suggest that dolomitization was effected by refluxed evaporative seawater. Stable isotope and trace element analyses suggest dolomitization of both Types from a fluid of near-surface seawater composition under oxidising conditions modified by evaporation. Non-luminescence and lack ofzonation of all the dolomite indicate that the dolomitizing fluids maintained a relatively constant composition. The geologic setting during the early Eocene, interpreted as hypersaline lagoon, supports an evaporative reflux origin for the anhydritic dolomite Type I. Type II developed under less saline conditions in the transition zone between lagoon and open marine shelf.Stage II dolomitization is recorded by negative isotope values in both Types I and II indicating their dissolution and recrystallization (neomorphism) by dilute solutions. A period of exposure of the overlying Jdeir Formation following a relative sea-level fall allowed ingress of meteoric waters into both the Jdeir and the underlying Jirani Formations. Flushing by meteoric waters also resulted in development of excellent secondaly porosity and caused major dissolution of anhydrite to form the anhydritic-free dolomite facies typical of Type II. Following, and possibly during, both Stages I and II, low temperature dolomites (Type IIIa) precipitated in pore spaces from residual jluids at shallow burial depths, partially occluding porosity. In the late stage of basin evolution, medium clystalline, pore-filling saddle dolomite precipitated, causing some filling of mouldic and vuggy porosity (Type IIIb). Very light oxygen isotopic signatures confirm that it developed from high temperature fluids during deep burial diagenesis. Calculation of temperatures and timings of the dolomitization and cement phases show that the main dolomitization phases and Type IIIa cements occurred in the early Eocene, and that the saddle dolomite precipitated in the Miocene; these results are consistent with age relationships established from stratigraphic, petrographic and geochemical signatures. The most common porosity includes intercrystal, vuggy and mouldic types. Porosity is both pre-dolomitization and syn-dolomitization in origin, but the latter is the most dominant. Hence, reservoir quality is largely controlled by fluid dynamics.
Carbonates and Evaporites, 2007
This study investigates the depositional environment and sequence stratigraphy of the Asmari Formation (Oligoeene-Miocene) in Gaehsaran Area. The formation is carbonate sequence, which is laid down in the southern side edge Neotethys Ocean (Zagros area). The Asmari Formation represents sedimentation on a carbonate ramp. Seven major microfacies and three subfacies are recognized which include: MF-I) planktonic foraminifer wackestone-packstone, MF-2) nummulitic-bioclastic-corallinacean wackestonepackstone, MF-3) bioclastic grainstone. MF-4) ooid-grainstone, MF-5) bioclastic-miliolid-borelisid, MF-6) miliolid-intraclast-bioclast and MF-7) carbonate mudstones (laminated mudstone, fossiliferous mudstone and algal mudstones).
Dolomitization of middle miocene buildups, Um Gheig area, Red Sea coast, Egypt
Carbonates and Evaporites, 1997
Middle Miocene reefal carbonates of the Urn Mahara Formation at Urn Gheig area near the Red Sea coast of Egypt had a complex history ofdolomitization. Various dolomite petrotypes are recorded including; I) dolomite replacing allochems with complete fabric preservation, 2) mimic dolomite replacement for originally Mg-calcite radial fibrous cements, 3) fibrous, spar, and micrite dolomite cements that fill some secondary pores, and 4) dolomite matrix which possesses non-planar crystal boundaries and poly modal size distribution. The investigated dolomites exhibit a narrow 0 18 0 range (-3.6 to -6.3 %0 PDB) relative to their OUC values (1.2 to -11.2%0PDB). The petrographic and isotopic characteristics are in accord with multiple phases ofdolomitization, related to sealevel variations. The first dolomitization phase was an early and pervasive event that took place during a stillstand period within the zone of circulating marine pore-fluids ahead of a mixing zone. This resulted in dolomitization ofthe bioclasts and early Mg-calcite marine cements. Excellent fabric preservation ofthese components is attributed to pre-dolomitization diagenesis. Silica interruption for this early dolomitization phase is indicated by the sharp contacts observed between the silica minerals and dolomite and the presence of dolomite inclusions within quartz.
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).
Himalayan Geology, 2019
The Lower Cretaceous carbonates succession (Shurijeh and Tirgan formations) in west Kopet-Dagh basin (NE-Iran) was examined to study the diagenetic modifications. Various diagenetic processes were controlled by the original facies characteristics, carbonate mineralogy, climatic condition, sea-level fluctuations and burial history.Four types of dolomite are identified including 1: microcrystalline matrix replacement dolomite (xenotopic, D1), 2: fine to medium euhedral-to subhedral crystalline matrix replacement dolomite (D2e), 3: fine to medium euhedral-to subhedral porphyrotopic crystalline matrix replacement dolomite (D2p), and 4: fracture filling, euhedralto subhedral dolomite (D3). All dolomite samples were analyzed for carbon and oxygen stable isotopes. The microcrystalline dolomites (xenotopic, D1) define a relatively wide range of δ O values from 0.3 to-4.52‰V-PDB (pee dee belemnite), and narrower range of δ C l8 l3 values from 1.96 to 3.20‰V-PDB, which are slightly depleted compared with the original isotopic signatures for the Lower Cretaceous marine dolomites. The planar-e dolomites (fine crystals) in the matrix show δ O and δ C values ranging from-l8 l3 2.17 to-5.33‰ and 3.20 to 3.50‰ V-PDB respectively. Dolomitized orbitolinid and euhedral mosaic dolomite crystals with planar boundaries (D2e, medium crystals) show depleted δ O and δ C values ranging from-8.12 to-4.11‰ and 0.3 to l8 l3 3.34‰ V-PDB respectively. Fine crystal dolomites (D2e) formed during early burial and could be considered as recrystallized forms of D1 dolomite. Fluids responsible for the formation of medium crystals dolomites (D2e, medium) and orbitolinid filled dolomite (D2e), suffered higher temperature due to increasing the burial depth. Consequently, heavier δ O l8 values of finer dolomite crystals and elevated temperatures to lighter δ O values indicate in higher burial depths that led to l8 coarser euhedral crystals during dolomitization.