Changes

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).

 

Figure 7: Dolomite textures. (A) Xenotopic mosaic consisting of non-planar anhedral crystals; (B) Idiotopic mosaic consisting of planar-e (euhedral) crystals; (C) Hypidiotopic mosaic consisting of planar-s (subhedral) crystals (after Sibley and Gregg, 1987). The photomicrographs are adopted from Adams and Mackenzie (1998).  

[[File:GeoWikiWriteOff2021-Awais-Figure7.png|thumbnail|Figure 7: Dolomite textures. (A) Xenotopic mosaic consisting of non-planar anhedral crystals; (B) Idiotopic mosaic consisting of planar-e (euhedral) crystals; (C) Hypidiotopic mosaic consisting of planar-s (subhedral) crystals (after Sibley and Gregg, 1987). The photomicrographs are adopted from Adams and Mackenzie (1998). ]]

Planar dolomites have straight extinction while non-planar dolomites have segment / sweeping extinction (Machel, 2004). Sometimes, the planar fabric containing euhedral dolomite crystals is easily seen in rocks having some intercrystalline pore spaces (see idiotopic mosaic in Fig. 7; Adams and MacKenzie, 1998). Sometimes, planar dolomites are usually equicrystalline imparting sugary texture to the rock and called as Sucrosic dolomites (Machel, 2004). The sugary texture is identifiable in hand specimen and using hand-lens as light reflects-off dolomite crystals faces (Machel, 2004).

Planar dolomites have straight extinction while non-planar dolomites have segment / sweeping extinction (Machel, 2004). Sometimes, the planar fabric containing euhedral dolomite crystals is easily seen in rocks having some intercrystalline pore spaces (see idiotopic mosaic in Fig. 7; Adams and MacKenzie, 1998). Sometimes, planar dolomites are usually equicrystalline imparting sugary texture to the rock and called as Sucrosic dolomites (Machel, 2004). The sugary texture is identifiable in hand specimen and using hand-lens as light reflects-off dolomite crystals faces (Machel, 2004).

There is another variety of high temperature (60 - 150°C) Ca- and Fe-rich burial dolomite called as Baroque or saddle dolomite (Fig. 8). Such dolomites are petrographically recognized by curved crystal faces, invariably sparry, usually milky white, curved cleavages, having undulose extinction, pearly lustre and cloudy appearance due to abundant fluid inclusions (Fig. 8; Adams and Mackenzie, 1998; Scholle and Ulmer-Scholle, 2003). These dolomites are mostly occuring as cements and fractures-fill (Adams and Mackenzie, 1998).  

There is another variety of high temperature (60 - 150°C) Ca- and Fe-rich burial dolomite called as Baroque or saddle dolomite (Fig. 8). Such dolomites are petrographically recognized by curved crystal faces, invariably sparry, usually milky white, curved cleavages, having undulose extinction, pearly lustre and cloudy appearance due to abundant fluid inclusions (Fig. 8; Adams and Mackenzie, 1998; Scholle and Ulmer-Scholle, 2003). These dolomites are mostly occuring as cements and fractures-fill (Adams and Mackenzie, 1998).  

 

Figure 8: Photomicrographs of baroque or saddle dolomites. (a) Curved crystal faces of baroque dolomites (blue color shows porosity), Plane polarized light (PPL). (b) Undulose extinctions in baroque dolomites, Cross polarized light (CPL). The image is ~ 1.8 cm wide (after Warren, 2019). (c) Partially-calcitized baroque dolomites (Scholle and Ulmer-Scholle, 2003).

[[File:GeoWikiWriteOff2021-Awais-Figure8a.png|thumbnail|Figure 8a: Photomicrographs of baroque or saddle dolomites. (a) Curved crystal faces of baroque dolomites (blue color shows porosity), Plane polarized light (PPL).]]

[[File:GeoWikiWriteOff2021-Awais-Figure8b.png|thumbnail|Figure 8b: Photomicrographs of baroque or saddle dolomites. (b) Undulose extinctions in baroque dolomites, Cross polarized light (CPL). The image is ~ 1.8 cm wide (after Warren, 2019).]]

[[File:GeoWikiWriteOff2021-Awais-Figure8c.png|thumbnail|Figure 8c: Photomicrographs of baroque or saddle dolomites. (c) Partially-calcitized baroque dolomites (Scholle and Ulmer-Scholle, 2003).]] 

The texture of dolomite under the petrographic microscope gives clue(s) about origin of dolomites but the interpretation of main dolomitization model(s) is based on detailed geological and geochemical data (Flügel, 2010). The replacement fabrics of dolomite are categorized as unimodal and polymodal based on crystal size distribution (Sibley and Gregg, 1987). Unimodal and polymodal dolomites are also called as equicrystalline and inequicrystalline dolomites respectively (Adams and Mackenzie, 1998). Moreover, dolomite or dolomitization is classified as mimic or mimetic (fabric-preserving) and non-mimic or non-mimetic (fabric destroying) dolomite (Fig. 9; Sibley and Gregg, 1987; Adams and Mackenzie, 1998). Most non-mimetic dolomites contains ‘ghosts’ which are usually non-carbonate sediments and shows outline or internal pattern of precursor allochems or cements (Machel, 2004). The partial or selective dolomitization can be fabric-selective (replacing particular constituents of limestone) and non-fabric selective (replacing different or all components of the limestone in a chaotic manner) (Fig. 9; Adams and Mackenzie, 1998). Allochems like echinoderms and corraline algae are commonly mimetically replaced while brachiopods cannot be mimetically replaced (Sibley, 1982). Most replacement dolomites have cloudy cores and clear peripheries (Fig. 9I; Keyser et al., 2002).

The texture of dolomite under the petrographic microscope gives clue(s) about origin of dolomites but the interpretation of main dolomitization model(s) is based on detailed geological and geochemical data (Flügel, 2010). The replacement fabrics of dolomite are categorized as unimodal and polymodal based on crystal size distribution (Sibley and Gregg, 1987). Unimodal and polymodal dolomites are also called as equicrystalline and inequicrystalline dolomites respectively (Adams and Mackenzie, 1998). Moreover, dolomite or dolomitization is classified as mimic or mimetic (fabric-preserving) and non-mimic or non-mimetic (fabric destroying) dolomite (Fig. 9; Sibley and Gregg, 1987; Adams and Mackenzie, 1998). Most non-mimetic dolomites contains ‘ghosts’ which are usually non-carbonate sediments and shows outline or internal pattern of precursor allochems or cements (Machel, 2004). The partial or selective dolomitization can be fabric-selective (replacing particular constituents of limestone) and non-fabric selective (replacing different or all components of the limestone in a chaotic manner) (Fig. 9; Adams and Mackenzie, 1998). Allochems like echinoderms and corraline algae are commonly mimetically replaced while brachiopods cannot be mimetically replaced (Sibley, 1982). Most replacement dolomites have cloudy cores and clear peripheries (Fig. 9I; Keyser et al., 2002).

Dolomite / dolostone occurrence in nature

 

==Dolomite / dolostone occurrence in nature==

Dolomite / dolostone is found in nature in the form of thin-, medium-, thick-, very thick-bedded and massive units (Figs. 3 and 4). Laminated dolostone also occurs in nature called as dololaminite and as surficial layer called as dolomite crust. The variations in crystal size of dolomite can generate varied colored layering in carbonates, for example, brownish colored layering (fine-medium crystalline dolomites) and light colored layering (coarse crystalline dolomite), as reported by Vinci et al. (2017) in Cretaceous carbonates of Southern Apennines (southern Italy) (Fig. 10a). Nodular dolomite patches are also noticed in rock record, for example, in nodular dolomustone in Miocene Asmari Formation of Qaleh Nar Oil Field, Iran (Fig. 10b; Esrafili-Dizaji and Rahimpour-Bonab, 2019). Spheroidal and complex polyhedral of dolomites are also reported but the the former one existed in association with submarine hydrothermal vents, hydrocarbon seeps and possibly bacteria (Naiman et al., 1983; Gunatilaka, 1989; Gregg et al., 1992; Nielsen et al., 1997).  

Dolomite / dolostone is found in nature in the form of thin-, medium-, thick-, very thick-bedded and massive units (Figs. 3 and 4). Laminated dolostone also occurs in nature called as dololaminite and as surficial layer called as dolomite crust. The variations in crystal size of dolomite can generate varied colored layering in carbonates, for example, brownish colored layering (fine-medium crystalline dolomites) and light colored layering (coarse crystalline dolomite), as reported by Vinci et al. (2017) in Cretaceous carbonates of Southern Apennines (southern Italy) (Fig. 10a). Nodular dolomite patches are also noticed in rock record, for example, in nodular dolomustone in Miocene Asmari Formation of Qaleh Nar Oil Field, Iran (Fig. 10b; Esrafili-Dizaji and Rahimpour-Bonab, 2019). Spheroidal and complex polyhedral of dolomites are also reported but the the former one existed in association with submarine hydrothermal vents, hydrocarbon seeps and possibly bacteria (Naiman et al., 1983; Gunatilaka, 1989; Gregg et al., 1992; Nielsen et al., 1997).  

[[File:GeoWikiWriteOff2021-Awais-Figure10a.png|thumbnail|Figure 10a: Sedimentary features related to dolomites. (a) Layering in Cretaceous carbonates due to variations in crystal size of dolomites. Cdol for coarse crystalline dolomite and Fmdol for fine-medium crystalline dolomites (Vinci et al., 2017).]]   

Figure 10: Sedimentary features related to dolomites. (a) Layering in Cretaceous carbonates due to variations in crystal size of dolomites. Cdol for coarse crystalline dolomite and Fmdol for fine-medium crystalline dolomites (Vinci et al., 2017). (b) Nodular dolomudstone in Miocene Asmari Formation, Iran (Esrafili-Dizaji and Rahimpour-Bonab, 2019).

[[File:GeoWikiWriteOff2021-Awais-Figure10a.png|thumbnail|Figure 10b: Sedimentary features related to dolomites. (b) Nodular dolomudstone in Miocene Asmari Formation, Iran (Esrafili-Dizaji and Rahimpour-Bonab, 2019).]]   

In the rock record, most of the dolomites have replacement (secondary) origin especially in carbonates. They are formed by replacement of pre-existing aragonite, high-Mg calcite and low-Mg calcite (but very less) (Scholle and Ulmer-Scholle, 2003). There are many replacement fabrics of dolomite, for example, micro-crystalline replacements to limpid, zoned, selective / partial and pervasive / complete replacements and cements (Fig. 9). The zoning in dolomite can be compositional, due to presence of inclusions and precipitation of metastable minerals etc (Figs. 6a and 9I; Adams and Mackenzie, 1998; Scholle and Ulmer-Scholle, 2003). In most cases, the inclusions can be calcite, fluid inclusions, micropores and or inclusions of non-carbonate sediments like clay (Adams and Mackenzie, 1998; Kordi et al., 2017).  

In the rock record, most of the dolomites have replacement (secondary) origin especially in carbonates. They are formed by replacement of pre-existing aragonite, high-Mg calcite and low-Mg calcite (but very less) (Scholle and Ulmer-Scholle, 2003). There are many replacement fabrics of dolomite, for example, micro-crystalline replacements to limpid, zoned, selective / partial and pervasive / complete replacements and cements (Fig. 9). The zoning in dolomite can be compositional, due to presence of inclusions and precipitation of metastable minerals etc (Figs. 6a and 9I; Adams and Mackenzie, 1998; Scholle and Ulmer-Scholle, 2003). In most cases, the inclusions can be calcite, fluid inclusions, micropores and or inclusions of non-carbonate sediments like clay (Adams and Mackenzie, 1998; Kordi et al., 2017).  

The dolomite cements, generally found in carbonates and clastics, are precipitated directly from pore fluids during diagenesis (Tucker and Wright, 1990; Chen et al., 2019). The dolomite cement can also be primary dolomite cement or the replacement of calcite cement (Adams and Mackenzie, 1998). Dolomite cement rims on detrital grains are noticed in Permian Park City Formation of Utah, USA (Fig. 11A; Scholle and Ulmer-Scholle, 2003). Dolomite overgrowths are noticed in Lower Cretaceous Shuaiba Formation in Offshore Qatar (Fig. 11B; Scholle and Ulmer-Scholle, 2003). Dolomite fibrous cements are reported in Ediacaran Dengying Formation of China (Fig. 11C; Hu et al., 2020). The dolomite can also occur as aphanocrystalline crystals, for example, as noticed in Upper Permian Tansill Formation of New Mexico (Fig. 11D; Scholle and Ulmer-Scholle, 2003). Dolomite can occur as syntaxial overgrowth (i.e. dolomite overgrowth) around dolomite crystals (Figs. 11E and 11F; Scholle and Ulmer-Scholle, 2003). It can also occur as discrete crystals filling intergranular pores and fractures and as lining in molds (Fig. 11G; Scholle and Ulmer-Scholle, 2003; Kordi et al., 2017; Awais et al., 2019). Dolomite-cementation can takes place in burrows, for example, Middle to Upper Ordovician Red River Formation, North Dakota, USA (Fig. 11H; Scholle and Ulmer-Scholle, 2003). There can also be dolomitization front in carbonates when one part is completely dolomitized and the adjacent part contains scarce dolomites (Fig. 11I; Scholle and Ulmer-Scholle, 2003).  

The dolomite cements, generally found in carbonates and clastics, are precipitated directly from pore fluids during diagenesis (Tucker and Wright, 1990; Chen et al., 2019). The dolomite cement can also be primary dolomite cement or the replacement of calcite cement (Adams and Mackenzie, 1998). Dolomite cement rims on detrital grains are noticed in Permian Park City Formation of Utah, USA (Fig. 11A; Scholle and Ulmer-Scholle, 2003). Dolomite overgrowths are noticed in Lower Cretaceous Shuaiba Formation in Offshore Qatar (Fig. 11B; Scholle and Ulmer-Scholle, 2003). Dolomite fibrous cements are reported in Ediacaran Dengying Formation of China (Fig. 11C; Hu et al., 2020). The dolomite can also occur as aphanocrystalline crystals, for example, as noticed in Upper Permian Tansill Formation of New Mexico (Fig. 11D; Scholle and Ulmer-Scholle, 2003). Dolomite can occur as syntaxial overgrowth (i.e. dolomite overgrowth) around dolomite crystals (Figs. 11E and 11F; Scholle and Ulmer-Scholle, 2003). It can also occur as discrete crystals filling intergranular pores and fractures and as lining in molds (Fig. 11G; Scholle and Ulmer-Scholle, 2003; Kordi et al., 2017; Awais et al., 2019). Dolomite-cementation can takes place in burrows, for example, Middle to Upper Ordovician Red River Formation, North Dakota, USA (Fig. 11H; Scholle and Ulmer-Scholle, 2003). There can also be dolomitization front in carbonates when one part is completely dolomitized and the adjacent part contains scarce dolomites (Fig. 11I; Scholle and Ulmer-Scholle, 2003).  

Dolomitization and Sequence Stratigraphy

 

==Dolomitization and Sequence Stratigraphy==

Dolomitization can be controlled by sequence stratigraphic parameters i.e. rate of productivity and accumulation of carbonates; hydrologic factors and circulation pattern(s), which takes place in response to transgression and regression; pore-waters chemical composition and dynamics of marine, meteoric and mixed marine-meteoric pore-water areas and residence-time of carbonates in specific geochemical conditions which is influenced by duration of cycles of relative sea-level changes (Moss and Tucker, 1996; Morad et al., 2012). Kordi et al. (2017) related dolomitization with transgression (sequence stratigraphy) in Carboniferous Um Bogma Formation (Sinai, Egypt) (Fig. 12). These Carboniferous dolostones are formed by two marine transgression episodes (i.e. transgressive systems tract-TST, and initial time of highstand systems tract-HST) (Fig. 12; Kordi et al., 2017). In addition, similar interpretation is also made by Peyravi et al. (2016) i.e. the TST and lower part of HST are mostly containing dolomite in Lower Triassic Kangan Formation of Persian Gulf. Manche and Kaczmarek (2019) interpreted, in Cretaceous carbonates of Upper Glen Rose Formation of Central Texas (USA), that dolomite characteristics (i.e. quantity, crystal size, stoichiometry and Oxygen-18 isotope, δ18O) varies with transgression and regression. The regressive facies contains increasing dolomite quantity, dolomite stoichiomtery and δ18O (VPDB), and reducing crystal size of dolomites. On the contrary, the transgressive facies contains reducing dolomite quantity, dolomite stoichiometry and δ18O (VPDB), and increasing crystal size of dolomite (Manche and Kaczmarek, 2019).  

Dolomitization can be controlled by sequence stratigraphic parameters i.e. rate of productivity and accumulation of carbonates; hydrologic factors and circulation pattern(s), which takes place in response to transgression and regression; pore-waters chemical composition and dynamics of marine, meteoric and mixed marine-meteoric pore-water areas and residence-time of carbonates in specific geochemical conditions which is influenced by duration of cycles of relative sea-level changes (Moss and Tucker, 1996; Morad et al., 2012). Kordi et al. (2017) related dolomitization with transgression (sequence stratigraphy) in Carboniferous Um Bogma Formation (Sinai, Egypt) (Fig. 12). These Carboniferous dolostones are formed by two marine transgression episodes (i.e. transgressive systems tract-TST, and initial time of highstand systems tract-HST) (Fig. 12; Kordi et al., 2017). In addition, similar interpretation is also made by Peyravi et al. (2016) i.e. the TST and lower part of HST are mostly containing dolomite in Lower Triassic Kangan Formation of Persian Gulf. Manche and Kaczmarek (2019) interpreted, in Cretaceous carbonates of Upper Glen Rose Formation of Central Texas (USA), that dolomite characteristics (i.e. quantity, crystal size, stoichiometry and Oxygen-18 isotope, δ18O) varies with transgression and regression. The regressive facies contains increasing dolomite quantity, dolomite stoichiomtery and δ18O (VPDB), and reducing crystal size of dolomites. On the contrary, the transgressive facies contains reducing dolomite quantity, dolomite stoichiometry and δ18O (VPDB), and increasing crystal size of dolomite (Manche and Kaczmarek, 2019).  

[[File:GeoWikiWriteOff2021-Awais-Figure12.png|thumbnail|Figure 12: Stratigraphic column demonstrating sequence stratigraphy of the Um Bogma Formation, Egypt (modified from Kordi et al., 2017).]]   

Figure 12: Stratigraphic column demonstrating sequence stratigraphy of the Um Bogma Formation, Egypt (modified from Kordi et al., 2017).

Dolomitization and Porosity Evolution

==Dolomitization and Porosity Evolution==

During dolomitization, porosity may be produced, maintained and vanished (Fig. 13; Machel, 2004). The replacement of aragonite and or high-Mg calcite by dolomite leads to development of dolomoldic porosity (Fig. 13A; Scholle and Ulmer-Scholle, 2003). The replacement of limestone by dolostone increases the porosity by 13% (Fig. 13B; Machel, 2004). Dolomitization generally increases porosity, for example, in Upper Jurassic to Lower Cretaceous carbonates of southern Italy, it has increased matrix porosity up to 7 % (Rustichelli et al., 2017). Sometimes, selective dissolution of dolomites takes place during telogenetic diagenesis and hence porosity is developed in carbonates. Furthermore, seldom selective zones are also dissolved in dolostones and sometimes whole zones are completely dissolved and leached-out, for example, as noticed in Mississipian Terrero Formation, New Mexico (Scholle and Ulmer-Scholle, 2003).  

During dolomitization, porosity may be produced, maintained and vanished (Fig. 13; Machel, 2004). The replacement of aragonite and or high-Mg calcite by dolomite leads to development of dolomoldic porosity (Fig. 13A; Scholle and Ulmer-Scholle, 2003). The replacement of limestone by dolostone increases the porosity by 13% (Fig. 13B; Machel, 2004). Dolomitization generally increases porosity, for example, in Upper Jurassic to Lower Cretaceous carbonates of southern Italy, it has increased matrix porosity up to 7 % (Rustichelli et al., 2017). Sometimes, selective dissolution of dolomites takes place during telogenetic diagenesis and hence porosity is developed in carbonates. Furthermore, seldom selective zones are also dissolved in dolostones and sometimes whole zones are completely dissolved and leached-out, for example, as noticed in Mississipian Terrero Formation, New Mexico (Scholle and Ulmer-Scholle, 2003).  

The dolostone texture and reservoir properties are greatly variable i.e. in matrix-selective dolomitization, the porosity is dominantly remnant primary porosity having poor interconnection. In non-matrix-selective dolomitization or rocks with coarse crystalline matrix (sucrosic dolomites) has intercrystalline porosity and excellent permeability (Machel, 2004).

The dolostone texture and reservoir properties are greatly variable i.e. in matrix-selective dolomitization, the porosity is dominantly remnant primary porosity having poor interconnection. In non-matrix-selective dolomitization or rocks with coarse crystalline matrix (sucrosic dolomites) has intercrystalline porosity and excellent permeability (Machel, 2004).

[[File:GeoWikiWriteOff2021-Awais-Figure13.png|thumbnail|Figure 13: Photomicrographs of different porosities associated with dolomites. (A) Dolomoldic porosity. The dolomite crystals have been partially-dissolved away. (B) Intercrystalline porosity in dolostone having euhedral dolomites. (C) Dolomite cements in bivalve shell and ooids occupying biomoldic and oomoldic porosity. (D) Dolomite cement (coarse crystalline) in a cavity adjacent to fine crystalline dolomites. In all photomicrographs, the blue color indicates porosity. A, B and D from Adams and MacKenzie (1998) and C from Scholle and Ulmer-Scholle (2003).]] 

Figure 13: Photomicrographs of different porosities associated with dolomites. (A) Dolomoldic porosity. The dolomite crystals have been partially-dissolved away. (B) Intercrystalline porosity in dolostone having euhedral dolomites. (C) Dolomite cements in bivalve shell and ooids occupying biomoldic and oomoldic porosity. (D) Dolomite cement (coarse crystalline) in a cavity adjacent to fine crystalline dolomites. In all photomicrographs, the blue color indicates porosity. A, B and D from Adams and MacKenzie (1998) and C from Scholle and Ulmer-Scholle (2003).

Murray (1968) reported a relationship between dolomite quantity and porosity. He noticed porosity reduced with raising dolomite quantity when the total dolomite percentage is < 50 % in a rock while porosity raises with increasing dolomite content when dolomite percentage in a rock is > 50% (Fig. 14a; Murray, 1968). In dolomite reservoirs of Nanpu Sag (China), dolomites have reduced the porosity when they are scattered in the matrix because due to compaction they occupied pore spaces. But dolomites have also preserved and increased the porosity when coarse crystalline dolomites are formed and which resisted the influence of compaction (Bojiang et al., 2012). Therefore, Bojiang et al. (2012) interpretation also validates Murray (1968) observation. Bojiang et al. (2012) envisaged that fluid movement is efficient in coarse crystalline dolomites and less efficient in fine-crystalline and micrite dolomites. Therefore, rocks having coarse crystalline dolomites are good reservoirs as compared to fine-crystalline and micrite-dolomites (Bojiang et al., 2012).  

Murray (1968) reported a relationship between dolomite quantity and porosity. He noticed porosity reduced with raising dolomite quantity when the total dolomite percentage is < 50 % in a rock while porosity raises with increasing dolomite content when dolomite percentage in a rock is > 50% (Fig. 14a; Murray, 1968). In dolomite reservoirs of Nanpu Sag (China), dolomites have reduced the porosity when they are scattered in the matrix because due to compaction they occupied pore spaces. But dolomites have also preserved and increased the porosity when coarse crystalline dolomites are formed and which resisted the influence of compaction (Bojiang et al., 2012). Therefore, Bojiang et al. (2012) interpretation also validates Murray (1968) observation. Bojiang et al. (2012) envisaged that fluid movement is efficient in coarse crystalline dolomites and less efficient in fine-crystalline and micrite dolomites. Therefore, rocks having coarse crystalline dolomites are good reservoirs as compared to fine-crystalline and micrite-dolomites (Bojiang et al., 2012).  

Dolomite cementation (overdolomitization) in the form of syntaxial overgrowth, discrete and clustered crystals occupying intergranular pores and fractures reduces porosity and permeability (Fig. 14b; Lucia, 2004; Martín-Martín et al., 2015; Kordi et al., 2017). In Middle Eocene Avon Park Formation (South Florida, USA), dolomitization (cementation) reduced the total porosity to 9 % (Scholle and Ulmer-Scholle, 2003). In Carboniferous Um Bogma Formation (Egypt), dolomite occurence as cement in intercrystalline / intergranular pores has reduced reservoir quality (Kordi et al., 2017). However, it is not necessary that overdolomitization reduces porosity all the time, for example, in Permian–Triassic Kangan and Dalan formations of Persian Gulf, overdolomitizaton has no significant influence on the reservoir quality (Tavakoli and Jamalian, 2019). Hence, dolomite presence can enhance, impede and having no effect on geofluids (especially hydrocarbons) storage and transmitting capacity in carbonates.  

Dolomite cementation (overdolomitization) in the form of syntaxial overgrowth, discrete and clustered crystals occupying intergranular pores and fractures reduces porosity and permeability (Fig. 14b; Lucia, 2004; Martín-Martín et al., 2015; Kordi et al., 2017). In Middle Eocene Avon Park Formation (South Florida, USA), dolomitization (cementation) reduced the total porosity to 9 % (Scholle and Ulmer-Scholle, 2003). In Carboniferous Um Bogma Formation (Egypt), dolomite occurence as cement in intercrystalline / intergranular pores has reduced reservoir quality (Kordi et al., 2017). However, it is not necessary that overdolomitization reduces porosity all the time, for example, in Permian–Triassic Kangan and Dalan formations of Persian Gulf, overdolomitizaton has no significant influence on the reservoir quality (Tavakoli and Jamalian, 2019). Hence, dolomite presence can enhance, impede and having no effect on geofluids (especially hydrocarbons) storage and transmitting capacity in carbonates.  

Figure 14: Relationship between dolomite and porosity. (a) Relationship of porosity to dolomite percentage in carbonate rocks (after Murray, 1960). (b) Dolomite pores evolution and overdolomitization. Porosity is reduced going from polyhedral pores via tetrahedral to interboundary boundary pores (modified after Wardlaw, 1976).

The dolomite can also be identified indirectly in the subsurface using petrophysical logs (gamma ray, density, sonic etc; Fig. 15). The Gamma ray (GR) log response of dolostone / dolomite-rich carbonates can be lower than the shale baseline (Schlumberger, 1989; Rider, 1996). However, it can be high rarely might be due to radioactive minerals (uranium) in veins. The Photoelectric factor (PEF) of dolomite is 3.14 barns / electron (Schlumberger, 1989). The bulk density of dolomite is 2.87 g/cm3 (Schlumberger, 1989). The sonic matrix time for dolomite is 44 us/ft (Rider, 1996).  

Usually GR log is used to demarcate shale and non-shale (limestone, dolostone) lithology. However, within a carbonate succession, limestone and dolostone are differentiated using a combo of neutron and density logs. For limestone, both logs will overlap while for dolostones they will be at a distance (Fig. 15; Lucia, 2007). The interpretation of Lucia (2007) is noticed in Paleocene-Eocene Khurmala Formation and Jurassic Butmah Formation of Iraq respectively (Rashid et al., 2020; Zangana et al., 2020). Similarly, neutron-density cross-plot is also used for lithology identification (Fig. 16; Rider, 1996; Zangana et al., 2020). In addition, M-N cross-plot is also usually used to determine lithology of geological formations having heterogenous lithologies (Rider, 1996; Zangana et al., 2020).  

Usually GR log is used to demarcate shale and non-shale (limestone, dolostone) lithology. However, within a carbonate succession, limestone and dolostone are differentiated using a combo of neutron and density logs. For limestone, both logs will overlap while for dolostones they will be at a distance (Fig. 15; Lucia, 2007). The interpretation of Lucia (2007) is noticed in Paleocene-Eocene Khurmala Formation and Jurassic Butmah Formation of Iraq respectively (Rashid et al., 2020; Zangana et al., 2020). Similarly, neutron-density cross-plot is also used for lithology identification (Fig. 16; Rider, 1996; Zangana et al., 2020). In addition, M-N cross-plot is also usually used to determine lithology of geological formations having heterogenous lithologies (Rider, 1996; Zangana et al., 2020).  

[[File:GeoWikiWriteOff2021-Awais-Figure15.png|thumbnail|Figure 15: Log signatures of carbonates and commonly associated rocks. The image has been modified from an image available on AAPG Wiki website (https://wiki.aapg.org/Quick-look_lithology_from_logs). ]]   

Figure 15: Log signatures of carbonates and commonly associated rocks. The image has been modified from an image available on AAPG Wiki website (https://wiki.aapg.org/Quick-look_lithology_from_logs).  

[[File:GeoWikiWriteOff2021-Awais-Figure16a.png|thumbnail|Figure 16a: Cross-plots for lithology determination in dolostone dominated unit. (A) Neutron-density cross-plot. (after Zangana et al., 2020). ]]   

[[File:GeoWikiWriteOff2021-Awais-Figure16b.png|thumbnail|Figure 16b: Cross-plots for lithology determination in dolostone dominated unit. (B) M-N cross-plot (after Zangana et al., 2020). ]]   

Figure 16: Cross-plots for lithology determination in dolostone dominated unit. (A) Neutron-density cross-plot. (B) M-N cross-plot (after Zangana et al., 2020).  

 

Examples of hydrocarbon producing Dolostone reservoirs

==Examples of hydrocarbon producing Dolostone reservoirs==

Cambrian Jutana Formation (dominated by dolostone) is a proven producing hydrocarbon reservoir rock in Potwar Plateau, Pakistan (Khan et al., 1986). Middle - Upper Ordovician Red River Formation (North Dakota, USA) is producing from intercrystalline porosity (Scholle and Ulmer-Scholle, 2003). The Lower Triassic Kangan Formation is the major carbonate (dolomite-rich) reservoir of natural gas in southwest Iran and Persian Gulf (Peyravi et al., 2016). Upper Jurassic Arab D Carbonate (Dukhan Field, Qatar) is producing from biomoldic and oomoldic porosities (Scholle and Ulmer-Scholle, 2003). Lower Cretaceous Shuaiba Formation (Offshore Qatar) is producing from patchy and intercrystalline porosities (Scholle and Ulmer-Scholle, 2003). Oligocene to Lower Miocene Asmari Formation (containing dolomite) is a giant hydrocarbon producing reservoir in Iran (Lackpour et al., 2008).   

Cambrian Jutana Formation (dominated by dolostone) is a proven producing hydrocarbon reservoir rock in Potwar Plateau, Pakistan (Khan et al., 1986). Middle - Upper Ordovician Red River Formation (North Dakota, USA) is producing from intercrystalline porosity (Scholle and Ulmer-Scholle, 2003). The Lower Triassic Kangan Formation is the major carbonate (dolomite-rich) reservoir of natural gas in southwest Iran and Persian Gulf (Peyravi et al., 2016). Upper Jurassic Arab D Carbonate (Dukhan Field, Qatar) is producing from biomoldic and oomoldic porosities (Scholle and Ulmer-Scholle, 2003). Lower Cretaceous Shuaiba Formation (Offshore Qatar) is producing from patchy and intercrystalline porosities (Scholle and Ulmer-Scholle, 2003). Oligocene to Lower Miocene Asmari Formation (containing dolomite) is a giant hydrocarbon producing reservoir in Iran (Lackpour et al., 2008).   

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