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Line 3: − + − 1University of Naples+ − 2 University of Swabi − Introduction + + − + + − + − + − + − + − − − − − − − − − − − − − − − + + − − − − − − − − − − − − − − − − − − − − Figure 3: Outcrop photographs of dolostone / dolomite-rich rocks. (A) Cambrian Jutana Dolomite, Salt Range, Pakistan. (B) Triassic Kingriali Dolomite, Kala-Chitta Range, Pakistan. (C) Thick bedded dolostone in Eocene Chorgali Formation, Khair-e-Murat Range (KMR), Pakistan. (D) Algal laminations containing dolomite and evaporites in Eocene Chorgali Formation, KMR, Pakistan. (from Awais et al., 2020b) − − Figure 4: Cores of dolostones. A-D Dolostones cores of southern Estonia and northern Latvia (Kleesment et al., 2013) − Composition, Petrography and Texture / Fabric of Dolomite / Dolostones − Compositionally, dolomite can be pure dolomite, calcian dolomite / limy dolomite, slightly calcian dolomite, argillaceous dolomite and ferroan dolomite (<10 mol percent FeCO3) (Frisia, 1994; Machel, 2004). The dolomite containing rock(s) can have different names based on quantity of dolomite such as dolostone, impure dolostone, calcitic dolomite, dolomitic limestone, impure dolomitic limestone and impure calcitic dolomite (Table 1; Fig. 5; Leighton and Pendexter, 1962). Line 68:
Line 32: + + + + + − − Figure 5: Compositional classification of dolomite (modified after Leighton and Pendexter, 1962). − − Figure 6: Photomicrographs of dolomite. (a) Cathodolumiscence (CL) image of dolomite showing zoning. (b) ARS calcitized dolostone. The calcite is stained red and dolomite is unstained. The photomicrograph is illustrating replacement of dolomite by calcite. (c) ARS of partially dolomitized dolopackstone (finely crystalline dolomite are shown by gray color). a and c from Scholle and Ulmer-Scholle (2003) and b from Janson et al. (2014). − − Different terms are used to define dolomite crystal sizes such as dolomicrostone (<4 µm), dolomicrosparstone (4–10 µm) and dolosparstone (>10 µm) (Wright, 1992). Similarly, Lucia (1995) classification of dolomite crystal size includes fine to medium crystalline dolomite (<20 µm to 100µm) and coarse crystalline (>100 µm). Other textural terms based on crystal size includes cryptomere (very fine crystalline), dolomicrite / micrite dolomite / microlite (crystal size <1 µm), dolosparite, dololutite (clay or mud size, <1/16 mm), dolosiltite (silt size, 1/16-1/256 mm) , dolarenite (sand size, 1/16-2 mm), dolorudite (coarser than sand, >2 mm) and macrocrystalline dolomites (Calver and Baillie, 1990; Bojiang et al., 2012; Barlow et al., 2016; Yang et al., 2017).
Dolomite and dolomitization: Implications for reservoir characterization (view source)
Revision as of 16:49, 21 April 2022
, 16:49, 21 April 2022no edit summary
An Entry from the AAPG [[2021 Middle East Wiki Write Off]]!
An Entry from the AAPG [[2021 Middle East Wiki Write Off]]!
Muhammad Awais1,2
by Muhammad Awais, University of Naples, University of Swabi
Dolomite is a rhombohedral carbonate mineral having chemical formula CaMg(CO3)2 (Tucker and Wright, 1990). The process of formation of dolomite is called dolomitization (Tucker and Wright, 1990). Dolomite is dominantly a marine mineral / rock but it can also be formed in continental environments e.g., karst and lakes (Baldermann et al., 2020). There can be primary and secondary dolomites but the former one is very rare and most of the dolomites are secondary in origin (Tucker and Wright, 1990). According to Flügel (2010), there are three genetic groups of dolostones i.e. syn-genetic, diagenetic and epi-genetic dolostones.
Dolomite is a rhombohedral carbonate mineral having chemical formula CaMg(CO3)2 (Tucker and Wright, 1990). The process of formation of dolomite is called dolomitization (Tucker and Wright, 1990). Dolomite is dominantly a marine mineral / rock but it can also be formed in continental environments e.g., karst and lakes (Baldermann et al., 2020). There can be primary and secondary dolomites but the former one is very rare and most of the dolomites are secondary in origin (Tucker and Wright, 1990). According to Flügel (2010), there are three genetic groups of dolostones i.e. syn-genetic, diagenetic and epi-genetic dolostones.
The origin of dolomite is still debateable and different models have been proposed by different researchers such as sabkha/evaporation, seepage-reflux, mixed marine-meteoric water, Coorong type, Kohout convection, burial compaction and falling sea-level/evaporative drawdown (Fig. 1; Tucker and Wright, 1990). Usually, dolomites not associated with evaporites are considered to be formed in mixed marine-meteoric zone while dolomites associated with evaporites are formed in seepage-reflux and evaporative drawdown model(s) (Morrow, 1982a, b; Awais et al., 2020a). Some other models of dolomitization includes organogenic/microbial, hydrothermal, geothermal, hydrofrigid, fault-controlled and fracture-related dolomitization (Machel and Lonnee, 2002; Scholle and Ulmer-Scholle, 2003; Shah et al., 2016; Rustichelli et al., 2017). There is still confusion in all dolomitization models about the source of Mg2+ and the mechanism through which dolomitizing fluids are pumped through carbonate sediments (Tucker and Wright, 1990). On a local scale, dolomitized sediments can be the source of magnesium i.e. Mg2+ originally present in or adsorbed by Mg-calcites, biogenic silica, clay minerals, chlorophyll, organic matter and or pre-existing detrital dolomites (Lyons et al., 1984; Baker and Burns, 1985).
The origin of dolomite is still debateable and different models have been proposed by different researchers such as sabkha/evaporation, seepage-reflux, mixed marine-meteoric water, Coorong type, Kohout convection, burial compaction and falling sea-level/evaporative drawdown (Fig. 1; Tucker and Wright, 1990). Usually, dolomites not associated with evaporites are considered to be formed in mixed marine-meteoric zone while dolomites associated with evaporites are formed in seepage-reflux and evaporative drawdown model(s) (Morrow, 1982a, b; Awais et al., 2020a). Some other models of dolomitization includes organogenic/microbial, hydrothermal, geothermal, hydrofrigid, fault-controlled and fracture-related dolomitization (Machel and Lonnee, 2002; Scholle and Ulmer-Scholle, 2003; Shah et al., 2016; Rustichelli et al., 2017). There is still confusion in all dolomitization models about the source of Mg2+ and the mechanism through which dolomitizing fluids are pumped through carbonate sediments (Tucker and Wright, 1990). On a local scale, dolomitized sediments can be the source of magnesium i.e. Mg2+ originally present in or adsorbed by Mg-calcites, biogenic silica, clay minerals, chlorophyll, organic matter and or pre-existing detrital dolomites (Lyons et al., 1984; Baker and Burns, 1985).
Dolomitization can takes place at different phases of diagenesis, for example, early diagenesis to deep burial (Fig. 2). Similarly, it can occur due to fluids of variety of compositions such as sea water, mixed marine-meteoric water, hypersaline and burial brines (Fig. 2; Adams and MacKenzie, 1998).
Dolomitization can takes place at different phases of diagenesis, for example, early diagenesis to deep burial (Fig. 2). Similarly, it can occur due to fluids of variety of compositions such as sea water, mixed marine-meteoric water, hypersaline and burial brines (Fig. 2; Adams and MacKenzie, 1998).
Tools and Techniques for studying dolomite and dolomitization
==Tools and Techniques for studying dolomite and dolomitization==
The fundamental material for studying dolomite is outcrop, cores, polished-slabs / peels and thin sections (Figs. 3 and 4). The tools and techniques used to study dolomite and dolomitization includes visual and hand lens inspection of outcrops, cores, polished slabs and peels, petrographic microscopy using thin section, scanning electron microscopy (SEM) using rock slab and thin section, backscattered electron imaging (BSEM), electron microprobe (EMP), Cathodoluminescence (CL), Energy Dispersive X-ray Spectroscopy (EDS), X-Ray Diffraction (XRD) analysis, X-ray Fluorescence, Transmission Electron Microscopy (TEM), QEMSCAN (quantitative evaluation of minerals by scanning electron microscopy) and geochemical analysis (alizarin red staining; major and trace elements geochemistry; isotopes of carbon, magnesium, oxygen and strontium). Dolomite’s fluid inclusion analysis is also conducted to determine microthermometry and salinity. The sub-surface dolomite or dolostone can be studied using conventional petrophysical logs. Now-a-days, research focused on dolomite precipitation experimentally in the laboratory is also going on. Furthermore, testing of dolomitization process using numerical simulations (i.e. reactive transport modelling) is also one of the latest research trend for carbonate sedimentologists.
The fundamental material for studying dolomite is outcrop, cores, polished-slabs / peels and thin sections (Figs. 3 and 4). The tools and techniques used to study dolomite and dolomitization includes visual and hand lens inspection of outcrops, cores, polished slabs and peels, petrographic microscopy using thin section, scanning electron microscopy (SEM) using rock slab and thin section, backscattered electron imaging (BSEM), electron microprobe (EMP), Cathodoluminescence (CL), Energy Dispersive X-ray Spectroscopy (EDS), X-Ray Diffraction (XRD) analysis, X-ray Fluorescence, Transmission Electron Microscopy (TEM), QEMSCAN (quantitative evaluation of minerals by scanning electron microscopy) and geochemical analysis (alizarin red staining; major and trace elements geochemistry; isotopes of carbon, magnesium, oxygen and strontium). Dolomite’s fluid inclusion analysis is also conducted to determine microthermometry and salinity. The sub-surface dolomite or dolostone can be studied using conventional petrophysical logs. Now-a-days, research focused on dolomite precipitation experimentally in the laboratory is also going on. Furthermore, testing of dolomitization process using numerical simulations (i.e. reactive transport modelling) is also one of the latest research trend for carbonate sedimentologists.
Figure 1: Dolomitization models illustrating various mechanisms for moving dolomitizing fluids through the sediments (modified after Land, 1985 and Tucker and Wright, 1990).
[[File:GeoWikiWriteOff2021-Awais-Figure1.png|thumbnail|Figure 1: Dolomitization models illustrating various mechanisms for moving dolomitizing fluids through the sediments (modified after Land, 1985 and Tucker and Wright, 1990). ]]
[[File:GeoWikiWriteOff2021-Awais-Figure2.png|thumbnail|Figure 2: Cartoon illustrating diagenetic environments in a carbonate shelf (modified after Adams and MaKenzir, 1998).]]
[[File:GeoWikiWriteOff2021-Awais-Figure3.png|thumbnail|Figure 3: Outcrop photographs of dolostone / dolomite-rich rocks. (A) Cambrian Jutana Dolomite, Salt Range, Pakistan. (B) Triassic Kingriali Dolomite, Kala-Chitta Range, Pakistan. (C) Thick bedded dolostone in Eocene Chorgali Formation, Khair-e-Murat Range (KMR), Pakistan. (D) Algal laminations containing dolomite and evaporites in Eocene Chorgali Formation, KMR, Pakistan. (from Awais et al., 2020b)]]
Figure 2: Cartoon illustrating diagenetic environments in a carbonate shelf (modified after Adams and MaKenzir, 1998).
[[File:GeoWikiWriteOff2021-Awais-Figure4.png|thumbnail|Figure 4: Cores of dolostones. A-D Dolostones cores of southern Estonia and northern Latvia (Kleesment et al., 2013)]]
==Composition, Petrography and Texture / Fabric of Dolomite / Dolostones==
Compositionally, dolomite can be pure dolomite, calcian dolomite / limy dolomite, slightly calcian dolomite, argillaceous dolomite and ferroan dolomite (<10 mol percent FeCO3) (Frisia, 1994; Machel, 2004). The dolomite containing rock(s) can have different names based on quantity of dolomite such as dolostone, impure dolostone, calcitic dolomite, dolomitic limestone, impure dolomitic limestone and impure calcitic dolomite (Table 1; Fig. 5; Leighton and Pendexter, 1962).
Table 1: Carbonate rock types having varying percentages of dolomite and calcite (after Leighton and Pendexter, 1962).
Table 1: Carbonate rock types having varying percentages of dolomite and calcite (after Leighton and Pendexter, 1962).
Carbonate rock type Dolomite percentage (%) Calcite percentage (%)
Carbonate rock type Dolomite percentage (%) Calcite percentage (%)
Petrographically, dolomite is usually identified due to its rhombohedron crystal habit / rhombic crystal shape (Fig. 6a). The dolomite is usually zoned and untwinned (Fig. 6a; Scholle and Ulmer-Scholle, 2003). In dolomitic limestones and calcitized dolostones, dolomite is identified via Alizarin Red-staining (ARS) technique i.e. dolomite remains unstained and calcite becomes red-colored (stained) (Figs. 6b and 6c).
Petrographically, dolomite is usually identified due to its rhombohedron crystal habit / rhombic crystal shape (Fig. 6a). The dolomite is usually zoned and untwinned (Fig. 6a; Scholle and Ulmer-Scholle, 2003). In dolomitic limestones and calcitized dolostones, dolomite is identified via Alizarin Red-staining (ARS) technique i.e. dolomite remains unstained and calcite becomes red-colored (stained) (Figs. 6b and 6c).
[[File:GeoWikiWriteOff2021-Awais-Figure5.png|thumbnail|Figure 5: Compositional classification of dolomite (modified after Leighton and Pendexter, 1962).]]
[[File:GeoWikiWriteOff2021-Awais-Figure6a.png|thumbnail|Figure 6a: Photomicrographs of dolomite.(a) Cathodolumiscence (CL) image of dolomite showing zoning. from Scholle and Ulmer-Scholle (2003)]]
[[File:GeoWikiWriteOff2021-Awais-Figure6b.png|thumbnail|Figure 6b: Photomicrographs of dolomite.(b) ARS calcitized dolostone. The calcite is stained red and dolomite is unstained. The photomicrograph is illustrating replacement of dolomite by calcite. from Janson et al. (2014).]]
[[File:GeoWikiWriteOff2021-Awais-Figure6c.png|thumbnail|Figure 6b: Photomicrographs of dolomite.(c) ARS of partially dolomitized dolopackstone (finely crystalline dolomite are shown by gray color).from Scholle and Ulmer-Scholle (2003)]]
Different terms are used to define dolomite crystal sizes such as dolomicrostone (<4 µm), dolomicrosparstone (4–10 µm) and dolosparstone (>10 µm) (Wright, 1992). Similarly, Lucia (1995) classification of dolomite crystal size includes fine to medium crystalline dolomite (<20 µm to 100µm) and coarse crystalline (>100 µm). Other textural terms based on crystal size includes cryptomere (very fine crystalline), dolomicrite / micrite dolomite / microlite (crystal size <1 µm), dolosparite, dololutite (clay or mud size, <1/16 mm), dolosiltite (silt size, 1/16-1/256 mm) , dolarenite (sand size, 1/16-2 mm), dolorudite (coarser than sand, >2 mm) and macrocrystalline dolomites (Calver and Baillie, 1990; Bojiang et al., 2012; Barlow et al., 2016; Yang et al., 2017).
Sibley and Gregg (1987) defined the texture of dolomite, for example, planar and non-planar crystals based on crystal shapes and hence there are different mosaics of dolomite (Fig. 7). The planar crystals can be euhedral (planar-e) and subhedral (planar-s) and results in the formation of idiotopic and hypidiotopic mosaics respectively. The non-planar dolomite crystals are anhedral and making xenotopic mosaic (Fig. 7; Sibley and Gregg, 1987). In planar dolomites, the crystal boundaries between dolomite crystals are straight and planar. On the contrary, non-planar dolomites have curved, irregular or lobate crystal boundaries between dolomite crystals (Machel, 2004).
Sibley and Gregg (1987) defined the texture of dolomite, for example, planar and non-planar crystals based on crystal shapes and hence there are different mosaics of dolomite (Fig. 7). The planar crystals can be euhedral (planar-e) and subhedral (planar-s) and results in the formation of idiotopic and hypidiotopic mosaics respectively. The non-planar dolomite crystals are anhedral and making xenotopic mosaic (Fig. 7; Sibley and Gregg, 1987). In planar dolomites, the crystal boundaries between dolomite crystals are straight and planar. On the contrary, non-planar dolomites have curved, irregular or lobate crystal boundaries between dolomite crystals (Machel, 2004).