Origin Of Dolomite Research Articles

Direct physical evidence of dolomite recrystallization

AbstractThe possibility of recrystallization is a long‐standing barrier to deciphering the genetic origin of dolomites. There is often uncertainty regarding whether or not characteristics of ancient dolomites are primary or the consequence of later recrystallization unrelated to the original dolomitization event. Results from 65 new high‐temperature dolomite synthesis experiments (1 m, 1·0 Mg/Ca ratio solutions at 218°C) demonstrate dolomite recrystallization affecting stoichiometry, cation ordering and nanometre‐scale surface texture. The data support a model of dolomitization that proceeds by a series of four unique phases of replacement and recrystallization, which occur by various dissolution–precipitation reactions. During the first phase (induction period), no dolomite forms despite favourable conditions. The second phase (replacement period) occurs when Ca‐rich dolomite products, with a low degree of cation ordering, rapidly replace calcite reactants. During the replacement period, dolomite stoichiometry and the degree of cation ordering remain constant, and all dolomite crystal surfaces are covered by nanometre‐scale growth mounds. The third phase (primary recrystallization period), which occurs in the experiments between 97% and 100% dolomite, is characterized by a reduced replacement rate but concurrent increases in dolomite stoichiometry and cation ordering. The end of the primary recrystallization period is marked by dolomite crystal growth surfaces that are covered by flat, laterally extensive layers. The fourth phase of the reaction (secondary recrystallization period) occurs when all calcite is consumed and is characterized by stoichiometric dolomite with layers as well as a continued increase in the degree of cation ordering with time. Inferences of recrystallization, in natural dolomite, based on cation order or stoichiometry of dolomite, usually depend on assumptions about the precursor dolomite subjected to recrystallization. If it is assumed that the experimental evidence presented here is applicable to natural, low‐temperature dolomites, then the presence of mounds is direct evidence of a lack of recrystallization and the presence of layers is direct evidence of recrystallization.

Magnesium-isotope and REE compositions of Lower Ordovician carbonates from eastern Laurentia: Implications for the origin of dolomites and limestones

Carbonates representing different depositional and diagenetic settings (near-surface to deep burial), including modern sabkha, were collected from the Lower Ordovician St. George Group in eastern Laurentia (western Newfoundland, Canada). Based on petrographic examination, three dolomite phases were identified: D1 dolomicrite (crystals ranging from 4 to 50μm), D2 dolomite (50 to 150μm) and D3 saddle dolomite (≥500μm). They occur as replacements and cements, and exhibit dull (for D1 and D3) to zoned (for D2) luminescence. The occurrence of near-micritic dolomites (D1) suggests that dolomitization started at low temperatures under near-surface conditions during an early stage of diagenesis, whereas microthermometric studies of the D2 and D3 dolomites confirmed their formation at higher temperatures, on average, of 112±19°C and 153±30°C, respectively, under deeper burial conditions.The D1 dolomicrite yields an average δ26Mg value of −1.92±0.30‰ (DSM3), which is slightly more negative than those of the D2 dolomite with −1.75±0.34‰ (DSM3) and D3 saddle dolomite with −1.58±0.31‰ (DSM3). The slightly more positive δ26Mg values of the higher-temperature dolomites (D2 and D3) suggest insignificant kinetic fractionation and slight overprinting by 26Mg-enriched diagenetic fluids recirculating in siliciclastic and 26Mg-rich crustal rocks under closed to semi-closed diagenetic conditions.The Sr-isotope signatures support the formation of D1 dolomicrite in an early diagenetic stage and the REE compositions of the investigated dolomites favor their formation in semi-closed to closed diagenetic systems with fluid compositions evolving by circulation through crustal rocks with progressive burial. Furthermore, the Mg isotopes suggest that the lime mudstones of the St. George Group had an algal origin, whereas the D2 dolomite was sourced and subsequently altered mostly from the D1 dolomicrites of the succession.

Origin of dolomite in the Late Jurassic platform carbonates, Bolkar Mountains, Central Taurides, Turkey: Petrographic and geochemical evidence

The Late Jurassic-early Senonian Cehennemdere Formation extending in an E-W direction in a wide area at the south of the Bolkar Mountains (Central Taurides, Turkey) is composed of platform carbonates. The formation was deposited in an environment that was being transformed from a shallow carbonate platform to an open shelf and a continental slope, and was buried until late Paleocene uplift. The formation, with a thickness of about 360m, was chiefly developed as textures consisting of mudstone and wackestone and has been commonly dolomitized. Based on petrographic and geochemical properties, four types of replacement dolomites and two types of dolomite cements were distinguished. Replacement dolomite (RD), which is cut by low-amplitude stylolites developed as (1) fine crystalline planar-s dolomite (RD1); (2) medium crystalline planar-s dolomite (RD2); (3) medium-coarse crystalline planar-e dolomite (RD3) and; (4) coarse crystalline planar-s (e) dolomite (RD4). Two types of dolomite cements (CD) observed in low abundance and overlie low-amplitude stylolites: (1) coarse crystalline dolomite cement (CD1) filling dissolution voids and fractures in RD1 dolomites, and; (2) rim dolomite cement (CD2) that commonly develops on the space-facing surfaces of RD4 dolomite. Replacement dolomites are non-stoichiometric (Ca54–59Mg41–46), have similar geochemical properties, and are generally dull red/non luminescent in appearance. Replacement dolomite is represented by δ18O values from −4.5 to −0.5‰ VPDB, δ13C values of −0.7 to 2.7‰ VPDB, and 87Sr/86Sr ratios ranging from 0.707178 to 0.707692. Petrographic and geochemical data indicate that replacement dolomite (particularly RD2, RD3, and RD4 dolomite) was formed at shallow-intermediate burial depths during the Late Jurassic-Early Cretaceous, from seawater and/or from slightly modified seawater. The replacement dolomite (RD) was then recrystallized at increased burial depths and temperatures. Dolomite cements are similar to replacement dolomites in that they are non-stoichiometric (Ca55Mg45) and have similar trace element compositions. CD1 dolomite, which cuts low-amplitude stylolites, was formed during intermediate to deep burial following stylolite development. CD2 dolomite was precipitated in intercrystal pores in association with RD4 dolomite. Remaining pore space was filled with bitumen.

Origin of dolomite in Lake Bayanchagan, Inner Mongolia and its palaeoclimatic implications

Origin of dolomite in Lake Bayanchagan, Inner Mongolia and its palaeoclimatic implications

Polysaccharide-catalyzed nucleation and growth of disordered dolomite: A potential precursor of sedimentary dolomite

The origin of dolomite is a long-standing enigma in sedimentary geology. It has been proposed that microorganisms, especially anaerobic microorganisms, can overcome kinetic barriers to facilitate dolomite precipitation, although their specific role in dolomite formation is still unclear. Our experimental results demonstrate that disordered dolomite can be synthesized at room temperature abiotically from solutions containing polysaccharides such as carboxymethyl cellulose or agar. We propose that when dissolved in solution, polysaccharides can be strongly adsorbed on Ca-Mg carbonate surfaces through hydrogen bonding. The adsorbed polysaccharides may help weaken the chemical bonding between surface Mg2+ ions and water molecules, which can lower the energy barrier to the desolvation of surface Mg2+-water complexes, enhance Mg2+ incorporation into the precipitating carbonate, and thereby promote disordered dolomite formation. In natural environments, it is possible that polysaccharides produced by microorganisms, e.g., extracellular polysaccharides, may play a key role in promoting disordered dolomite nucleation and crystallization. In marine sediments, the accumulated dissolved carbohydrates produced from organic matter degradation during early diagenesis may also serve as catalysts for disordered dolomite formation.

Hydrothermal Dolomites in the Early Albian (Cretaceous) Platform Carbonates (NW Spain): Nature and Origin of Dolomites and Dolomitising Fluids

This study documents the temporal and lateral variation in petrographic and geochemical signatures of fault-related dolomite bodies in the Ranero and El-Moro areas (Karrantza valley, Cantabrian mountains; NW Spain). These dolomite bodies are hosted in Albian carbonates, which were deposited in the Basque-Cantabrian Basin as a result of an intense rift-related subsidence with associated faulting along various orientations. Fluid circulations generated replacive and cement dolomites, paragenetically followed by various calcite cements. Petrography, mineralogical and geochemical investigations (XRD, ICP, XRF, stable and Sr isotopes) helped in distinguishing hydrothermal stages. Two major dolomite facies were observed according to their Fe-content. Early dolomites are ferroan and replace limestone more significantly than the later, non ferroan dolomites. Dolomites are generally stoichiometric (49.76 to 51.59 M% CaCO<sub>3<sub/>) and exhibit a broad range of depleted δ<sup>18<sup/>O values (–18.7 to –10.5‰ V-PDB), which may indicate multiphase dolomitisation and/or different degrees of recrystallisation. Decreasing δ<sup>18<sup/>O values correlate with decreasing Fe content in dolomites. In the Ranero area, dolomites show less slightly depleted δ<sup>13<sup/>C values (–0.15 to +2.13‰ V-PDB) relative to the host limestone δ<sup>13<sup/>C signature, while these values are substantially more depleted in El-Moro area (down to –2.18‰ V-PDB). Hydrothermal calcites predating dolomitisation show less depleted δ<sup>18<sup/>O values (–14.15 to –12.1‰ V-PDB) than postdolomitisation calcite (–18.1‰ V-PDB). Sr isotope data suggest that the fluids interacted with siliciclastic lithologies (sandstone, shale). The dolomite fabric is variably altered through dedolomitisation and cataclastic deformation. Dolomitisation occurred in at least two main episodes. A first episode of pervasive ferroan dolomitisation probably resulted from compactional dewatering of basinal fluids from the nearby Basque trough and hydrodynamic fluid flow along the faults/fractures in the Albian carbonate platform. Subsequently, a second episode of very hot and localised dolomitisation may be related to igneous activity and convective flow.

Dolomite in Permian Paleosols of the Bravo Dome CO2 Field, U.S.A.: Permian Reflux Followed by Late Recrystallization at Elevated Temperature

Abstract Lower Permian Abo–Tubb subsurface strata of northeastern New Mexico, U.S.A. consist of 30 to 120 m of fluvial and eolian deposits, containing numerous dolomitic paleosols. Dolomite in the Abo–Tubb interval occurs in discrete intervals, interpreted as paleosols on the basis of recognizable horizonation and the presence of associated features such as ped structures and rhizoliths. The dolomite commonly forms veins, rhizoliths, and discrete and coalesced nodules. Petrographic analysis indicates four dolomite types present in paleosol horizons: microsparitic, pseudospherulitic, clear cement, and cloudy (inclusion-rich) cement. Microsparitic dolomite constitutes most of (70%) the dolomite, and is stoichiometric, with relatively low Sr and Na and elevated Fe and Mn. Pseudospherulitic dolomite forms 15% of the dolomite, and consists of stoichiometric microspar and spar, with abundant fluid inclusions, low Sr, Na, and Fe, and elevated Mn. Clear dolomite cement constitutes approximately 12% of the dolomite and is finely to coarsely crystalline with a relatively calcian composition, low Sr and Na, and elevated Fe and Mn. The cloudy cement forms a minor (2%) phase of very coarsely crystalline rhombs with a calcian composition, low Sr and Na values, and relatively high Fe and Mn. All dolomite types exhibit a mottled texture of moderate to bright orange and red luminescence. Fluid-inclusion analysis was not possible on the most common (microsparitic) dolomite type, but analyses on the coarser varieties reveal homogenization temperatures typically ranging from 70°C to 110°C and final melting temperatures of ice ranging from −20°C to −34°C, suggesting fluids of high (25% by weight) salinity. Bulk-rock carbon and oxygen isotopes of carbonate nodules and rhizoconcretions from four paleosol profiles most commonly exhibit mean values typically ranging between~ −3 to −5.5‰ (VPDB) and −0.5 to 2.8‰ (VPDB), respectively. Sulfur isotopes were analyzed for the overlying Cimarron Anhydrite interval and an anhydrite nodule near the base of the study interval. Both samples yielded similar results, suggesting that brines that precipitated the Cimarron Anhydrite also influenced the study interval. The dolomite in the Abo–Tubb interval is interpreted to have precipitated in multiple stages. Solid calcite microinclusions in microsparitic and pseudospherulitic dolomite suggest that pedogenensis and groundwater penecontemporaneous with Early Permian deposition led to precipitation of calcite rather than dolomite. Oxygen and sulfur isotope data indicate dolomitization, from later reflux of brines associated with deposition of the overlying (Leonardian) Cimarron Anhydrite. Petrographic relations and elevated temperatures and salinities recorded by the fluid inclusions in the coarse dolomite are consistent with recrystallization of the earlier dolomite and precipitation of dolomite cement from warm saline fluids of a possible Tertiary hydrothermal origin, just predating migration of mantle-derived gas into the Bravo Dome reservoir. These findings offer insight into questions about the climate significance of dolomite in paleosols, origin of dolomite in hydrothermally altered hydrocarbon reservoirs, and the long-term effect of CO2 sequestration in saline aquifers. The results show that dolomitic nodules in paleosols may form as replacements of paleosol calcite long after the paleosol formed. Primary pedogenic dolomite would undoubtedly have climatic and paleoenvironmental significance, but recognizing it as forming during soil formation, as opposed to later dolomitization, is key in establishing that significance. Also, this study provides an example of high-temperature recrystallization of low-temperature dolomite, a process hotly debated in discussions of dolomitized hydrocarbon reservoirs with evidence for hydrothermal fluid flow. Finally, as a natural CO2 reservoir, the Bravo Dome system shows that the long-term result of dissolution of CO2 into brines is predominantly carbonate mineral dissolution.

Microbial dolomite precipitation using sulfate reducing and halophilic bacteria: Results from Qinghai Lake, Tibetan Plateau, NW China

Past studies have revealed the important roles of certain types of microbes in precipitating dolomite in coastal hypersaline lakes and lagoons, and marine sediments, however, it is still poorly known if other types of microbes can precipitate dolomite in low-salinity lakes without any marine influence. The objective of this study was therefore to expand the microbial dolomite precipitation model by investigating the origin of dolomite in the sediments of Qinghai Lake, an inland, slightly saline lake (salinity 12.5 g/L) on the Tibetan Plateau, NW China. This objective was accomplished via an integrated approach combining geochemical investigations of natural dolomite present in the lake sediments with laboratory precipitation experiments. A 5-m sediment core was collected and pore water chemistry was analyzed. Calculation of saturation index indicated that dolomite was oversaturated in sediment pore water. Sub-spherical grains of dolomite with abundant pores and microstructures, similar to microbially precipitated dolomite, were found in the sediments, suggesting a biogenic origin. Dolomite precipitation experiments were set up in the laboratory to simulate the natural conditions of the Qinghai Lake. Dolomite was precipitated using sulfate reducing bacterium (SRB) Desulfotomaculum ruminis and aerobic halophilic bacterium Halomonas marina present in the Qinghai Lake sediments. Aqueous geochemistry and mineral characterization data collectively suggest that dolomite precipitation was accomplished via increase of pH and alkalinity to overcome the kinetic inhibitors and exopolymeric substances (EPS) and cell walls as possible nucleation sites. These results expand the current microbial dolomite precipitation model and have implications for dolomite formation in slightly saline, inland lake environments.

The preferential association of dolomite with microbes in stalactites from Cayman Brac, British West Indies

Abstract Nani Cave, located in dolostones of the Cayman Formation (Miocene) on Cayman Brac, contains numerous stalactites that are formed largely of aragonite and calcite along with lesser amounts of calcium-rich dolomite, gypsum, and minor amounts of Mg–Si needles. Morphologically, the dolomite is divided into blocky, filamentous mat, crust, and “beehive” types whereas the gypsum is divided into the tabular and sheet types. A diverse array of filamentous microbes and spores (probably actinomycetids) and their associated exopolysaccharides (EPS) are unevenly distributed throughout the stalactites. Although microbes are commonly present on the surfaces of the calcite and aragonite crystals, none were found inside these crystals. Similarly, no microbes were found with the gypsum. The common association of the dolomite and the Mg–Si needles with the microbes and their EPS suggests that the microbes played a formative role in the precipitation of the dolomite and Mg–Si needles. The intimate association of microbes and dolomite in these stalactites has significant implications for the origin of dolomite under low-temperature and low-pressure conditions.

Origin of dolomites in the Boat Harbour Formation, St. George Group, in western Newfoundland, Canada: implications for porosity development

Abstract The lower part of the St. George Group of western Newfoundland consists of Tremadocian shallow marine platform carbonates of the Boat Harbour (about 180 m thick) and the underlying Watts Bight (about 60 m thick) formations. In the Boat Harbour Formation, dolomitization is pervasive at the top of most shallowing-upward, metre-scale, peritidal hemicycles. Petrographic examination of the Boat Harbour Formation carbonates suggests that the succession has been affected by at least three phases of dolomitization, which influenced the final rock porosity. These phases have crystal-size ranges of about 4 to 40 μm (earliest dolomite D1), 50 to 150 μm (D2), and 300 μm to 20mm (saddle dolomite D3), respectively. They occur as both replacements and cements and exhibit dull (D1 and D3) to zoned (D2) luminescence under the cold cathodoluminoscope. The occurrence of near-micritic size dolomites (about 4–40 μm) may suggest that dolomitization started at low temperatures during early stages of diagenesis. The lack of evaporite interbeds in the formation and the depleted δ18O values (−6.2±0.8‰ VPDB) as well as the low Sr contents (168±45ppm) of the earliest dolomites likely exclude a brine origin from evaporated seawater. The Sr/Ca molar ratios (0.0069 to 0.0017), calculated for the earliest dolomitizing fluid, suggest a mixture of marine and meteoric waters possibly in a mixing zone environment. The petrographic features and geochemical attributes of D2 and D3 phases, such as their depleted δ18O values (−6.9±1.5‰ and −8.3±0.9‰ VPDB, respectively) and Sr contents (177±76 and 117±33ppm, respectively), suggest that they were formed under relatively deeper burial conditions and possibly from hydrothermal fluids which is supported by homogenization temperatures (up to 135° C) and estimates of salinities (up to 23 wt. % NaCl) in the latest dolomites (D3). Based on visual estimates from thin sections, the porosity varies from <1% in most of the formation to about 10% in a dolomitized algal lime mudstone bed in the upper part of the formation, a few metres below the Boat Harbour Disconformity. Except for some vugs, the majority of pores are intercrystalline and associated with D2. The dolomitization events recorded by the Boat Harbour Formation carbonates resemble, in their succession and geochemical signatures, those previously documented in the Aguathuna Formation. However, significant differences are noted with the dolomitization events recorded in the Lower Ordovician Romaine Formation on Anticosti Island; these differences are tentatively associated with the very distinct tectonic settings of each case.

Dolomite Mountains and the origin of the dolomite rock of which they mainly consist: historical developments and new perspectives

AbstractBeginning in the late 18th Century, the Dolomite Mountains in Northern Italy have been the location for major sedimentological developments, from the discovery of the mineral dolomite to the formulation of the coral‐reef hypothesis to explain the origin of the massive dolomite structures that define the splendid scenery of the region. Further, the Dolomite Mountains have inspired voluminous research into the origin of dolomite, questioning whether dolomite is a primary precipitate or a secondary replacement product. Recently, with the recognition that microbes can mediate dolomite precipitation, a new geomicrobiological approach, combining the study of modern natural environments with bacterial culture experiments, is now being used to calibrate or interpret microbial evidence derived from the dolomite rock record. This three‐pronged methodology applied to the study of dolomite formation holds great promise for future research into the ‘Dolomite Problem’ and provides a new impetus to revisit the Dolomite Mountains in the 21st Century.

Structurally controlled hydrothermal dolomite reservoir facies: An overview: Discussion

One of the issues of the AAPG Bulletin (v. 90, no. 11) was concerned with “hydrothermal dolomite reservoirs,” specifically “hydrothermal dolomite fields” (Davies and Smith, 2006). The term “hydrothermal dolomite” is confusing and meaningless (Machel and Lonnee, 2002). To study the origin of dolomite and its hydrothermal depositional setting, I examined modern settings, including the Mediterranean Sea (Friedman, 1991) (Figure 1). The shallow-water facies in this modern setting consists of dolomite and gypsum. The rocks range from almost pure gypsum to pure tough dolostone so thoroughly cemented that sampling required hammer and chisel. X-ray diffraction analysis indicates the presence of minor anhydrite. Sporadic aragonitic gastropods occur, but fossils are otherwise absent. Under the microscope, it is a peloidal micrite (dolomicrite). The δ13CPDB of the dolostone is −39.3 to −31.2‰. These strongly negative δ13C values suggest that carbon of the dolomite came from methane. Methane forms when all sulfate has been reduced. Bacterial reaction was responsible for dolomite formation through the methane pathway. This syngenetic dolomite is like most dolomites of hydrothermal origin, but my students and I avoid this confusing bandwagon term. Figure 1 Approximate temperatures at which Holocene intertidal carbonate sediments formed, calculated from isotopic signatures. Three classes of dolostone have been recognized, which have been termed syngenetic, diagenetic, and epigenetic (Friedman and Sanders, 1967). The purpose of this classification has been for the orderly discussion of the subject and to facilitate descriptive study and genetic interpretation. The term “syngenetic” has been adapted for characterizing biological and chemical origin in its …

A Comparison of Nanometer-Scale Growth and Dissolution Features on Natural and Synthetic Dolomite Crystals: Implications for the Origin of Dolomite

Abstract This paper describes experiments in which mechanisms of crystal growth are inferred from the surface nanotopography of natural and synthetic dolomite using atomic force and scanning electron microscopy. Synthetic dolomite was formed by replacement of calcite at 218°C in Mg–Ca–Cl solutions. The first products to form during dolomite synthesis experiments, as detected with X-ray diffraction (XRD), are invariably poorly ordered (nonideal) dolomite. As the reaction proceeds, the degree of cation order increases, yet the products remain relatively disordered. Upon examination, the surfaces of nonideal dolomite are covered with mounds 10–200 nm wide and 1–20 nm high. Following depletion of the calcite reactant, dolomite becomes relatively well ordered and stoichiometric (ideal). The surfaces of ideal dolomite are covered with broad, flat layers separated by steps that measure tenths to tens of nanometers high. Comparison of natural and synthetic dolomite surfaces after etching in 0.5% H2SO4 for 10–20 seconds indicates that high-temperature synthetic and low-temperature natural nonideal dolomite surfaces are covered with mounds identical to the growth features found on unetched nonideal synthetic dolomite. In contrast, synthetic and low-temperature ideal dolomite surfaces, when etched, are characterized by flat layers with deep, euhedral etch pits. These results suggest that despite the wide range in formation conditions, natural and synthetic dolomites form by the same growth mechanisms. Furthermore, the two different surface nanotopographies described here are consistent with a model in which nonideal dolomite forms by precipitation of amorphous mounds that quickly crystallize to nonideal dolomite while ideal dolomite later replaces nonideal dolomite and grows by spiral growth.

The origin of dolomite associated with salt diapirs in central Tunisia: Preliminary investigations of field relationships and geochemistry

Black and white dolomite crystals (mm to cm width) of different isotopic composition are associated with Triassic diapirism in central Tunisia, as well as with evaporite minerals and clays. The white dolomites occur mostly in the Jabal Hadifa diapir near the contact with Cretaceous limestones, whereas the smaller black dolomites occur in the Jabal Hamra diapir. The former dolomite has a narrow range of δ 18O and δ 13C values (− 3.83‰ to − 6.60‰ VPDB for δ 18O; − 2.11‰ to − 2.83‰ VPDB for δ 13C), whereas the latter dolomite has a wider range and more depleted values (− 4.92‰ to − 9.97‰ for δ 18O; − 0.55‰ to − 6.08‰ for δ 13C). However, the 87Sr / 86Sr ratios of most of the samples are near Triassic seawater values. Dolomite formation is due to at least two different fluids. The main fluid originated from deeper hydrothermal or basinal sources related to the Triassic saliferous rocks and ascended through faults during the diapiric intrusion. The second, less important fluid source is related to meteoric water originating from Cretaceous rocks.

Isotopic composition and origin of dolomite in Upper Cretaceous Duwi Formation of Abu-Tartur, Egypt

Isotopic composition and origin of dolomite in Upper Cretaceous Duwi Formation of Abu-Tartur, Egypt

Origin of dolomite in the Late Cretaceous–Paleocene limestone turbidites, Eastern Pontides, Turkey

The Late Cretaceous–Paleocene Tonya Formation of the Hacımehmet region in Trabzon (Eastern Pontides, Turkey) is composed of limestone turbidites. These limestone turbidites were deposited at water depths of around 500 to 600 m. and remained buried until uplifted in Late Paleocene time. The lower part of this formation, which is made up of clast-supported calcirudites and pebbly calcarenites, is partially to completely dolomitized. Two morphological types of dolomite are identified: (1) fine to medium crystalline dolomite (RD1), (2) coarse to very coarse crystalline dolomite (RD2). Each one of the dolomite types is non-stoichiometric, Ca-rich (Ca 57–61Mg 43–39), and has similar geochemical features and uniform dull red/brown luminescence. These dolomites have δ 18O PDB values from − 2.6‰ to − 3.3‰ (average = − 3.0‰), δ 13C PDB values from + 2.3‰ to + 3.1‰ (average = + 2.7‰) and 87Sr / 86Sr ratios from 0.707719 to 0.707772. Fluid inclusion data of RD2 dolomites yield homogenization temperatures ( T h) of 48–58 °C and salinities of 20.4 to 21.7 wt.% NaCl equivalent. The Sr concentration in dolomites, varying from below a detection limit of 20 to 360 ppm, is not homogeneous within each dolomite crystal. The Na concentrations of dolomites are similar, varying from 600 to 760 ppm (average = 680 ppm), and are higher than those for dolomites formed in normal marine environments. The Fe and Mn concentrations range from 2500 to 7000 ppm (average = 3800 ppm) and from 50 to 850 ppm (average = 330 ppm), respectively. Considering the petrography, paragenesis, trace element and stable isotope data, the dolomites in the Tonya Formation are interpreted to have formed in the subsurface during mechanical compaction at shallow to intermediate burial depths. Dolomitization in the Tonya Formation was most probably driven by seawater that was slightly modified through rock–water interaction and, in part, at elevated temperatures.

Origin of Dolomite in the Arab-D Reservoir from the Ghawar Field, Saudi Arabia: Evidence from Petrographic and Geochemical Constraints

A significant proportion of oil production from the Kim- meridgian Arab-D strata in the Ghawar field, Saudi Arabia originates from dolomitized rocks. Stratigraphic, petrographic, and geochemical data suggest that at least four episodes of dolomitization affected these sediments. The lower portion of the Arab-D, Zone 3, is only partially dolomitized, with the dolomite frequently being associated with firm- grounds. We propose that these dolomites were formed on an outer- ramp setting with a maximum water depth of 50 m, during a period of nondeposition, with the dolomitization process being promoted by the oxidation of organic material and the diffusion of Mg 21 from the overlying seawater. The dolomites in Zone 2B are geochemically dis- tinct compared to those in Zone 3 in that they have relatively positive oxygen isotope compositions ( 2 1t o22‰ compared to 26.5 ‰). The relatively positive oxygen isotope composition and the geochemical sim- ilarity of Zone 2B to the dolomites in Zone 1, which are intimately associated with the overlying evaporites, has led us to conclude that the Zone 2 dolomites probably formed by the reflux of hypersaline fluids through the sediments. These hypersaline fluids bypassed Zone 2A by moving through the grain-dominated sediments. Early cemen- tation and dolomite formation made these units more susceptible to later fracturing that affected the entire Arab-D formation. This frac- turing allowed higher-temperature fluids to leach the dolostones, there- by removing any remaining calcite and partially resetting the oxygen isotope composition of some of the dolomites. As a result of this later dolomitization event, rocks that were only partially dolomitized were leached, creating units with extremely high permeability and porosity (super-k intervals). Dolomites in the lower Zone 3 were recrystallized during burial by the normal geothermal gradient, leading to the pre- sent negative oxygen isotope values. Zone 1 dolomites are petrograph- ically distinct from Zone 2 dolomite in that they are mimetic and fabric preserving, although they are geochemically similar. This mimetic style of dolomitization occurs immediately adjacent to the overlying anhy- drite and is interpreted to have formed very shortly after deposition from hypersaline brines. recovery, and their presence is inferred from a combination of production data and the diameter of the bore hole as measured using the caliper log. As a result of the importance of dolomite in the formation of the super-k intervals within the Arab-D, an understanding of the paragenesis of these zones is important not only in predicting super-k formation within the Arab-D but also as an analogue for understanding other dolomitized res- ervoirs with high permeability and porosity. Previous characterization work of dolostones from the Arab-D in the Ghawar field (Cantrell et al. 2004) has identified three types of dolomite with distinctive petrographic, stratigraphic, and geochemical characteristics: fabric-preserving, non-fabric-preserving, and baroque dolomite. Fabric-pre- serving dolomite is very finely crystalline in which petrographic details of the original limestone fabric are usually well preserved. Fabric-preserving dolomite contains low concentrations of Fe (average 247 ppm Fe) and relatively positive oxygen isotope values (average

Modeling seismic reflections from thin‐bed dolomite layers in diatomite formation

Abstract The origin of dolomite and its diagenesis are still not fully understood. The timing and occurrence of dolomite beds remain key problems. Many thin‐bed layers of dolomite in marine sediments were discovered at drill sites in the forearc basin west of the Japan Trench during the Ocean Drilling Program Leg 186. The thickness of the dolomite layers ranges from 0.5 to 2 m, estimated from the well log. These thin‐bed dolomite layers were able to be identified on seismic data, through quantitative analysis and comparison of the data with synthetic seismograms. Amplitude‐preserved seismic data processing with broadband high‐resolution enhancement helps to estimate the reflectivity series dominated by the large impedance contrast between the dolomite layers and the host sediments. In this report, the first geophysical evidence of large‐scale dolomitization is provided. The occurrence of thin dolomitic layers embedded in diatomaceous sediments is on a basin scale rather than being an isolated event, which constrains the type of dolomitization and the timing of layered dolomitization. It was found that thin‐bed dolomite layers occur where there is an abrupt change in sedimentation rate. Layered dolomitization starts when sedimentation rate changes (from high to low or vice versa). Two dolomite layers coincide with the Miocene–Pliocene and Pliocene–Pleistocene age boundaries. Formation of dolomite beds could be a timing signal of a dramatic change in paleoceanographic and paleoclimatic environments.

Dolomites in SE Asia — varied origins and implications for hydrocarbon exploration

Abstract Carbonates in SE Asia range in age from Palaeozoic to Recent, but are most important as reservoirs in the Neogene where they comprise a major target for hydrocarbon exploration (e.g. Batu Raja Formation, South Sumatra, Sunda and Northwest Java basins). Carbonates of pre-Tertiary, Palaeogene and Neogene age all show a strong diagenetic overprint in which dolomite occurs as both cementing and replacive phases associated with variable reservoir quality. This paper reviews published data on the occurrence and types of dolomites in SE Asian carbonates, and considers the models that have been used to explain the distribution and origin of dolomite within these rocks. Pre-Tertiary carbonates form part of the economic basement, and are little studied and poorly understood. Although some, such as in the Manusela Formation of Seram, may form possible hydrocarbon reservoirs, most are not considered to form economic prospects. They are best known from the platform carbonates of the Ratburi and Saraburi groups. in Thailand, and the oolitic grainstones of the Manusela Formation of Seram. The Ratburi Group shows extensive dolomitization with dolomite developed as an early replacive phase and as a late-stage cement. Palaeogene carbonates are widely developed in the region and are most commonly developed as extensive foraminifera-dominated carbonate shelfal systems around the margins of Sundaland (e.g. Tampur Formation, North Sumatra Basin and Tonasa Formation, Sulawesi) and the northern margins of Australia and the Birds Head microcontinent (e.g. Faumai Formation, Salawati Basin). Locally, carbonates of this age may form hydrocarbon reservoirs. Dolomite is variably recorded in these carbonates and the Tampur Formation, for example, contains extensive xenotopic dolomite. Neogene carbonates (e.g. Peutu Formation, North Sumatra) are commonly areally restricted, reef-dominated and developed in mixed carbonate-siliciclastic systems. They most typically show a strong diagenetic overprint with leaching, recrystallization, cementation and dolomitization all widespread. Hydrocarbon reservoirs are highly productive and common in carbonates of this age. Dolomite is variably distributed and its occurrence has been related to facies, karstification, proximity to carbonate margins and faults. The distribution and origin of the dolomite has been attributed to mixing-zone dolomitization (commonly in association with karstic processes), sulphate reduction via organic matter oxidation, and dewatering from the marine mudstones that commonly envelop the carbonate build-up. Dolomite has a variable association with reservoir quality in the region, and when developed as a replacive phase tends to be associated with improved porosity and permeability characteristics. This is particularly the case where it is developed as an early fabric-retentive phase. Cementing dolomite is detrimental to reservoir quality, although the extent of this degradation generally reflects the abundance and distribution of this dolomite. Dolomitization is also inferred to have influenced the distribution of non-hydrocarbon gases. This is best documented in North Sumatra where carbon dioxide occurs in quantities ranging from 0 to 85%. There are a number of possible mechanisms for generating this CO 2 (e.g. mantle degassing), although the most likely source is considered to be the widely dolomitized Eocene Tampur Formation that forms effective basement for much of the basin. High heat flows are suggested to have resulted in the thermogenic decomposition of dolomite with CO 2 produced as a by-product.

Concepts and models of dolomitization: a critical reappraisal

Concepts and models of dolomitization: a critical reappraisal