Fabric-destructive Dolomite Research Articles

Resistance of eogenetic dolomites to geochemical resetting during diagenetic alteration: A case study of the lower Qiulitage Formation of the Late Cambrian, Tarim Basin

Rare earth elements (REEs), carbon, oxygen and Mg isotopes have been widely used to explain the origin of dolomite, but dolomite may undergo complex diagenetic alteration during long geological processes, and whether the geochemical signals of precursor dolomite can be preserved is still unknown. In order to further explore the formation mechanism of dolomite and the influence of diagenetic alteration processes on geochemical parameters, this paper takes the thick-layer dolomite of the Lower Qiulitage Formation of the Late Cambrian in the Tazhong uplift area as an example and divides the dolomite into two categories through petrologic observation. One category is fabric-retentive dolomite (FRD), including algal clotted dolomite, algal psammitic or psammitic grain dolomite, and aphanocrystalline dolomite; the other category is fabric-destructive dolomite (FDD), including crystalline dolomite (CD) and siliceous dolomite (SD). Cation ordering, unit cell parameters, major/trace elements, REEs as well as C–O–Mg isotopes of the pure matrix powders of different types of dolomites are further analyzed. The results show that the dolomites are near-source/far-source seepage reflux type and sabkha type eogenetic origin, and the magnesium ions mainly come from Late Cambrian seawater; dolomite crystals of different sizes are the result of the precursor dolomite suffering varying degrees of recrystallization during the early diagenesis period, inheriting the original geochemical characteristics; hydrothermal alteration occurs in the late stage of diagenesis, with limited magnesium ions in the hydrothermal fluid. Its impact on the dolomite is mainly reflected in local dolomite distortion and a large amount of aphanocrystalline silica and coarse-grained quartz cement filling, which fails to obviously reset the geochemical information of the matrix dolomite. The above results demonstrate that, under favorable conditions, early genetic marine dolomites may resist geochemical resetting over time intervals of more than 400 million years.

Dolomitization of early-post rift Lower Jurassic carbonate platforms along the Moroccan Atlantic Margin: Origin and significance

Dolomitization is the most significant diagenetic process to affect Jurassic carbonate reservoirs along the Central Atlantic Margin (CAM). Despite several studies on dolomitization from different parts of CAM, the origin of these dolomites and their influence on the subsequent diagenetic evolution of Jurassic carbonate systems remains enigmatic. In addition, while dolomitization is evident at the surface and in the subsurface of the Moroccan Atlantic Margin, virtually no detailed studies have been conducted to determine the origin, mechanism, and significance of dolomitization in this basin. Therefore, the principal objective of this study is to assess the origin and occurrence of dolomite in the of Upper Sinemurian-Lower Pliensbachian carbonates of the Arich Ouzla Formation in the Essaouira-Agadir Basin by using petrography and geochemistry.The shallow marine carbonates of the Arich Ouzla Formation have been partially dolomitized and are exposed on the salt-cored Amsittene Anticline. The dolomite is stratabound, and predominantly fabric-retentive, although in some parts it is partially replaced by non-stratabound, fabric-destructive dolomites. From petrographic observations and geochemical proxies, the fabric-preserving dolomites show dolomitization by reflux of mesohaline seawater (δ18Odolomite average = −3.5 ‰ VPDB, and δ13Cdolomite average = 2.0 ‰ VPDB). In contrast, petrographic and geochemical characteristics of the fabric destructive dolomites suggest precipitation from modified seawater/formational brines convected along faults and fractures evidenced by depleted δ18O isotopic values (average = −4.1 ‰ VPDB) with high fluid temperatures (average = 78 °C; range = 66–90 °C) where fluids interacted with the basal Triassic evaporites and siliciclastic sediments.Fabric preserving dolomite has higher porosity (average = 6.0 %) than the precursor limestones (average = 0.4 %), whereas permeability in both rock types (average = 0.48 mD, and average = 0.02 mD, respectively) is low. Fabric destructive dolomite has low porosity in proximity to fracture corridors (average = 1.9 %) due to dolomite recrystallization (overdolomitization), whereas porosity increases to an average of 7.4%, away from fracture corridors. The dolomites are post-dated by calcite cement which occludes vugs, intercrystalline pores and fractures. The calcite is interpreted to be meteoric in origin, because of its non-cathodoluminescence and depleted δ18O (average = −4.7 ‰ VPDB) and δ13C (average = −9.3 ‰ VPDB) isotopic values with respect to Jurassic marine carbonates. The meteoric calcites co-exist with bitumen suggesting that hydrocarbon migration in the basin likely occurred at the same time, most likely during basin inversion and exposure. This work considers dolomitization to be a localised process due to salt diapirism and demonstrates that the coincidence of hydrocarbon emplacement with basin inversion results in degradation and probably leakage of hydrocarbons. This emphasises the importance of local and regional tectonics, including salt diapirism, on patterns of diagenetic overprint in sedimentary basins.

Multistage dolomitization of deeply buried dolomite in the Lower Cambrian Canglangpu Formation, central and northern Sichuan Basin

The dolostone units in the Cambrian Changlangpu Formation of the Sichuan Basin have demonstrated substantial potential for natural gas exploration. A systematic understanding of the various dolomite (and non-carbonate) phases that build these rocks and the diagenetic pathways of the Canglangpu Formation carbonates still needs to be developed. This study combines the petrographic properties of dolomite (and non-carbonate) phases with their geochemical evidence (carbon, oxygen and 87Sr/86Sr isotope data). Moreover, fluid inclusion evidence and in situ rare-earth element (REE) data are presented. The samples studied were taken from hand specimens collected in the field and plugs from cores to explore the properties and evolutions of dolomite in the Cambrian Canglangpu Formation in the central-northern Sichuan Basin. Six dolomite phases are identified: silty crystalline dolomite (D1), fabric-retentive dolomite (D2), fabric-destructive dolomite (D3), fine crystalline dolomite (D4), medium to coarse crystalline dolomite cement (C1a and C1b) and saddle dolomite cement (C2). Silty crystalline dolomite represents the earliest dolomitization and shows δ18O values comparable to those of Cambrian seawater. Fabric-retentive dolomite, fabric-destructive dolomite and fine crystalline dolomite represent reflux dolomitization during shallow burial. Specifically, Fine crystalline dolomite with a turbid-core and clean-rim may have recrystallized from secondary dolomitization of marine-derived porewater. Fabric-destructive dolomite shows the depleted δ13C values, resulting from the secondary dolomitization involving organics degradation in medium to deep burial environment. The fluid inclusion data demonstrates that the medium to coarse crystalline dolomite cement (C1a) formed in relatively elevated temperature and salinity brines. Intrusion of sulfur- and metal-rich (Zn and Fe) medium-temperature hydrothermal fluids gave rise to the formation of medium to coarse crystalline dolomite cement (C1b) and sphalerite mineralization. Cementation of saddle dolomite was promoted by sulfate-rich reducing hydrothermal fluids. δ13C depletion, extremely low REE content and positive δEu in the saddle dolomite cement suggest the presence of thermal sulfate reduction.

Petrographic and geochemical characteristics of the Middle−Upper Cambrian Xixiangchi dolomites in the central Sichuan Basin, southwest China: Implications for dolomite origins and dolomitization mechanisms

The Middle−Upper Xixiangchi Formation of the Sichuan Basin consists of mixed carbonates and siliciclastics. The carbonates are dominated by dolomitized lime-mudstones and dolo–grainstones of shallow warm-water environments. Petrographic examination of carbonates reveals two types of dolomite: (i) fabric retentive dolomicrite (D1, micritic to near-micritic, 4–50 μm) and (ii) fabric-destructive dolomite (eu-to subhedral crystals with cloudy cores and clear rims, 50–300 μm). The mean ∑REE values of D1 (15.3 ± 6.3 ppm) and D2 (14.6 ± 7.1 ppm) and the nearly identical mean PAAS-normalized patterns that coincide at some points, suggest an origin from similar parent fluids that were not dramatically changed by water-rock interaction through basinal sedimentary rocks. The estimated Sr/Ca molar ratios of D1 (0.0043–0.0055) parent fluids and their estimated δ18O values imply an origin from a mixture of meteoric and seawaters, which is also consistent with the petrographic evidence. The partially coinciding normalized REE pattern of D2 with that of the earlier D1 suggests that the latter was formed at a mid-burial setting rather than deep burial, which is consistent with higher homogenization temperatures of primary two-phased fluid inclusions retained in D2 (Th = 118.1 ± 5.3 °C). Therefore, D1 is suggested to have formed (1) during a penecontemporaneous period that resulted from a slight evaporative pumping effect within a near-surface diagenetic environment, and/or (2) during post-penecontemporaneous or penecontemporaneous seepage-reflux dolomitization, induced by sea-level fluctuations, during early stages of diagenesis at near-surface conditions. D2 formed likely through the recrystallization of D1 or a continuation of reflux dolomitization of the same precursor at mid-burial rather than deep burial settings.

Geochemical Constraints on the Hydrothermal Dolomitization of the Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin, China

The Middle-Upper Cambrian Xixiangchi carbonates in the Sichuan Basin have been pervasively dolomitized. In the presented work, petrographic investigation revealed three generations of the Xixiangchi dolomites, consisting of dolomicrite (D1, 5–20 μm) with a planar-s to non-planar texture, fabric destructive dolomite (D2, 50–150 μM) with a planar-s to planar-e texture, and saddle dolomite (D3, 300 μm to 4 mm) with a planar-s to planar-e texture. D1 and D2 dolomites are presented as matrix dolomites, whereas D3 dolomites are observed as fracture-filling dolomites. Compared with the matrix D1 and D2 dolomites, which are interpreted as products of dolomitization under near-surface or at shallow burial conditions, the depleted δ13C and δ18O values of D3 than D1 and D2 dolomites are probably caused by the temperature-controlled isotopic fractionation within an increasing fluid–rock interaction at burial. The enriched Mn, Sr, and Ba concentrations of D3 than D1 and D2 dolomites suggest a newly introduced type of diagenetic fluids, which is probably related to the upwelling of magmatic activities (Emeishan large igneous province). By contrast, the abnormally depleted Fe concentration in D3 dolomites is attributed to its preferential incorporation into other solid phases rather than its true concentration. The similar rare earth element (REE) partition patterns of D1 and D2 dolomites demonstrate similar dolomitization fluids related to seawater or marine-origin fluids. The hydrothermal-derived D3 dolomites exhibit a different REE partition pattern by contrast. The negative Eu anomalies of D3 dolomites may represent hydrothermal fluid cooling or an association with intermediate-felsic igneous rocks. The findings of the presented work would enhance our understanding on the hydrothermal dolomitization of the Middle-Upper Cambrian Xixiangchi Formation.

Coevolution of diagenetic fronts and fluid-fracture pathways

Diagenetic boundaries are paleo-reaction fronts, which have the potential to archive the termination of metasomatic processes in sedimentary rocks. They have not been extensively studied, perhaps because they appear simple in outcrop. Recent work has demonstrated the significance of paleo-reaction fronts to decipher multiphase recrystallization processes and provide high porosity zones. This paper provides a detailed documentation of reaction front evolution in a tectonically active salt basin and reveals a high level of complexity, associated with multiple fluid flow and tectonic events. Here, consistent patterns of increasing dolomite stoichiometry and ordering, along with a change from seawater-derived, fabric-retentive dolomite to fracture-controlled, fabric-destructive hydrothermal dolomite are observed vertically across the stratabound dolomite bodies. These patterns, coupled with a decrease in porosity, increase in ∆47 temperature and δ18Owater values indicate multiphase recrystallization and stabilization by warm, Mg-rich fluids. The stratabound dolomite bodies apparently terminated at a fracture-bound contact, but the presence of dolomite fragments within the fracture corridor suggests that fracturing post-dated the first dolomitization event. The termination of dolomite formation is therefore interpreted to be associated with a decrease in the capacity of the magnesium-rich fluids to dolomitize the rock, as indicated by the presence of non-stoichiometric and poorly ordered dolomite at the reaction fronts. The fracture corridors are interpreted to exploit dolostone-limestone boundaries, forming prior to a later, higher temperature, hydrothermal dolomitization event, which coincided with the formation and growth of the anticline. Karstification subsequently exploited these fracture corridors, widening fractures and leading to localized collapse and brecciation. The results demonstrate that an apparently simple reaction front can have a complex history, governed by the inheritance of prior diagenetic events. These events modified rock properties in such a way that fluid flow was repeatedly focused along the original dolomite-limestone boundary, overprinting much of its original signature. These findings have implications to the prediction of structurally controlled diagenetic processes and the exploration of naturally fractured carbonate reservoirs for energy exploration globally.

Effects of dolomitization on porosity – Permeability distribution in depositional sequences and its effects on reservoir quality, a case from Asmari Formation, SW Iran

The Oligocene-Miocene Asmari reservoir is dominated by heterogeneity in various aspects, especially porosity and permeability caused mainly by dolomitization. The consequences of petrographic studies of core samples from five cored wells lead to four depositional environments to be identified: inner ramp, mid ramp, outer ramp and basin. This succession is arranged in six third-order depositional sequences. The formation has undergone complex diagenetic alteration represented mainly by multiple generations of dolomites. Dolomite types comprise (i) very finely-crystalline, fabric-retentive dolomite (D1), (ii) fine to medium-crystalline, fabric-retentive dolomite (D2); (iii) medium to coarse-crystalline, fabric-destructive dolomite (D3); and (iv) very coarsely-crystalline, non-planar saddle dolomite (D4). D1 substituted by micritic matrix in mud-supported facies before early compaction. D2 and D3 were formed by the reflux of warmer and more saline fluids during intermediate burial, and also by the recrystallization of D1. D4 was precipitated from hot, saline fluids during deep burial. Fine-to medium-crystalline, fabric-retentive dolomites occur in the upper part of the formation, whereas coarse-crystalline fabric-destructive dolomites mostly occur in the middle and lower parts. This causes distribution of porosity and permeability in the depositional sequences, thereby affecting the reservoir quality. Accordingly, dolomitization resulted in enhanced porosity and permeability and improved reservoir quality.

Reflux dolomitization and subsequent hydrothermal dolomitization induced by the Alkaline Igneous Province in the Middle Devonian Nowshera Formation (Peshawar Basin, NW Pakistan)

Dolomitized strata are potential exploration targets because they host economic mineral and hydrocarbon deposits around the globe. Establishing a petrogenetic history for dolomite is thus very vital. In this study, dolomitic bodies present in the Devonian carbonates of Nowshera Formation in Peshawar Basin, North-West Lesser Himalayas in north Pakistan are investigated through field observations, petrographic studies, and geochemical analysis. The carbonates of Nowshera Formation show evidence for multistage dolomitization and a complex diagenetic history. In a first stage the succession was completely dolomitized probably by the reflux of penecontemporaneous mesohaline seawater. This process resulted in both fabric-retentive and fabric-destructive dolomite types under near surface to shallow burial realms. In the subsequent stage saddle dolomites (matrix & cement) were formed under intermediate and/or deep burial realm. This later saddle dolomite phase likely resulted from the circulation of exotic fluids at comparatively high temperature. This local hydrothermal event, evidenced from the fluid inclusion homogenization temperatures, and stable isotopic signatures, also resulted in vuggy, fracture, and dissolution enhanced porosity. Microthermometric analysis of primary fluid inclusions in saddle dolomite confirm the presence of hot (125–178 °C) and highly saline brines (17–25 mass% NaCl equivalent). The δ18Owater (+2 to +9.2‰V-SMOW) calculated from fluid-inclusion homogenization temperatures in conjunction with the comparatively high salinity values is compatible with a magmatic origin of the hydrothermal fluid. It is suggested that the investigated hydrothermal dolomites in the Nowshera Formation formed in Carboniferous-Permian time. During this time, thermal convection heated by the Peshawar Plain Alkaline Igneous Province (PPAIP) may have provided the high temperature and sufficient magnesium (Mg) flux for several millions of years (~40 M.y.), ultimately leading to the formation of hydrothermal dolomite bodies. The pore spaces resulted from the leaching of these hydrothermal fluids were subsequently occluded by meteoric calcite related to the Eocene to Middle Miocene Himalayan exhumation of the studied area. This study provides important implications for potential source of Mg in understanding the genesis and timing of dolomitization.

Porosity enhancement potential through dolomitization of carbonate reservoirs, a case of study from the Euphrates Graben fields, East Syria

The dolostones and dolomitic limestones of the lower Cretaceous Judea formation are a key target of hydrocarbons in most of the Euphrates Graben fields. Core materials investigation, thin sections petrographically examinations, and petrophysical evaluations were obtained to determine enhancement of the porosity through dolomitization. Results showed that the lagoon-shallow marine carbonates of the Judea formation are subdivided into two main zones; the upper “limestone zone” is micritic limestones dominated with no reservoir potential, and the lower “dolomite zone” is dolomitic limestones and dolostones dominated with good to very good reservoir potential. Dolomitization of the mudstones and wackestones of the micritic limestones resulted in formation of microcrystalline dolomicrite and early fabric destructive dolomites. Conversely, dolomitization of the packstones and grainstones resulted in formation of the fabric destructive and saddle dolomites. Based on petrography data, dolomitization of the “limestone zone” is interpreted by the seawater dolomitization mechanism at low-temperatures, while dolomitization of the “dolomite zone” is interpreted by the burial dolomitization mechanism under high temperature and pressure conditions. The “limestone zone” is characterized by the poorest reservoir quality, while the “dolomite zone” is characterized by the best reservoir quality. The seawater dolomitization did not significantly enhance the porosity, while the burial dolomitization contributes to enhancing the preserved secondary porosity. Stylolites microfractures and dissolution seams associated with dolomitization played as major factors in porosity enhancement of the dolostones and dolomitic limestones and serving as pathways for hydrocarbon migration. Dissolution processes increased the porosity and more permeability unless they are filled with the precipitated dolomite and/or calcite. Calcification had significant effects on the porosity by blocking the cavities and channels and decreased the effective pore volume.

Origin of Middle-Upper Cambrian dolomites in eastern Laurentia: A case study from Belle Isle strait, western Newfoundland

The Middle and Upper Cambrian Port au Port Group of western Newfoundland consists mainly of mixed carbonates and siliciclastics and is composed of the March Point, Petit Jardin, and Berry Head formations. Samples were collected from the core NF-02 that spans the uppermost Hawke Bay (40 m), the March Point (~83 m thick) and the lowermost Petit Jardin (~47 m). Petrographic examination of carbonates of the March Point and Petit Jardin carbonates reveals three generations of dolomite: (i) micritic to near-micritic (D1, 4 μm −35 μm) fabric-retentive dolomite, (ii) coarse crystalline sucrosic (D2, 60 μm−150 μm) fabric-destructive dolomite (sub-to euhedral zoned rhombs with clear rim and cloudy core), and (iii) coarse anhedral (D3, >0.5 mm) fracture-filling saddle dolomite. The microthermometric measurements of primary two-phased fluid inclusions retained in D2, and D3 samples yielded homogenization temperatures (Th) showing a general increase with burial. The mean values of Th and salinity are 89.1 ± 6.4 °C vs 21.2 ± 2.2 eq. wt% NaCl, and 110 ± 8.2 °C vs 21.1 ± 4.7 eq. wt% NaCl, respectively. The investigated dolomites show a decrease of Sr from D1 (102 ± 49 ppm) to D3 (47 ± 15 ppm) which reflects the influence of progressive burial. The mean δ18O values decrease from D1 (−6.3 ± −1.1‰ VPDB), to D3 (−9.7 ± 1.7‰ VPDB) where D2 (−7.6 ± 1.2‰ VPDB) is in the middle. The estimated δ18O values of the D1 parent fluid (−5 to −10‰ VSMOW) are bracketed by those suggested for the Middle Cambrian meteoric and seawaters. The fluid-inclusion gas analyses provided CO2/CH4 and N2/Ar ratios coherent with the conclusions from the petrographic examination and geochemical compositions.

Effect of dolomitization on isotopic records from Neoproterozoic carbonates in southwestern Mongolia

Carbon isotope values from shallow-marine carbonate rocks, including those from many dolomitized successions, are the primary lens through which we interpret the ancient carbon cycle. Carbon isotopes are typically regarded as being robust to alteration during dolomitization due to the high carbon content of the rock compared to the fluid. However, chemostratigraphic studies of the Neoproterozoic Tsagaan-Olom Group in southwestern Mongolia exhibit 3–12‰ differences in carbon isotopes between stratigraphically equivalent limestone and dolomitized successions. To understand the origins of this geochemical variation, we conducted detailed geological mapping, petrographic, isotopic (δ13C, δ18O, δ44/40Ca, and δ26Mg), and fluid inclusion analyses of carbonates in the Taishir, Ol, and Shuurgat formations of the Tsagaan-Olom Group. Stratigraphic and textural constraints distinguish fabric retentive early dolomitization (Dolomitization Event 1) that occurred in the Taishir and Ol formations during and soon after deposition of the ca. 635 Ma Marinoan cap dolomite of the Ol Formation and fabric destructive dolomitization that occurred after the deposition of the younger Shuurgat Formation (Dolomitization Event 2), either prior to or during early Cambrian foreland basin formation. The dolomitizing fluids moved through porous stratigraphic units bound by impermeable shale of the Khongor and Zuun-Arts formations. Dolomitization homogenized the isotopic records of all Neoproterozoic Tsagaan-Olom Group dolomites; undolomitized successions show more extreme negative excursions from more positive background values. The salinity and isotopic values of seawater, the dolomitizing fluids, and the original platform carbonate were estimated from fluid inclusion data and a numerical model of diagenetic dolomitization. These results demonstrate that both early and late dolomitization can have a profound effect on carbon isotopic records of carbonates; however, with a multi-isotope approach the original carbon isotopic composition of platform-top carbonate can be distinguished from that of seawater and other dolomitizing fluids.

Petrological and geochemical constraints on the origin of dolomites: a case study from the early Cambrian Qingxudong Formation, Sichuan Basin, South China

Several types of dolomite had occurred pervasively in the lower Cambrian Qingxudong Formation in the Sichuan Basin. However, the origin of these dolomites remains particularly controversial until now. Here, we describe the petrographic and geochemical characteristics of dolomites from the lower Cambrian Qingxudong Formation in the southeastern Sichuan Basin, which can potentially provide useful information to constrain the origin and evolution of dolomitization fluid. Three fabric types and five dolomite generations were identified in this study: fabric-retentive dolomites (dolomicrite, Dol-1, and dolomitic microbialite, Dol-2), fabric-destructive dolomites (sucrosic dolomite, Dol-3) and pore-filling dolomites (medium- to coarse-crystalline dolomite, Dol-4, and saddle dolomite, Dol-5). The δ13C and δ18O values, 87Sr/86Sr ratios and rare earth element (REE) pattern of Dol-1, Dol-2 (except the δ13C values) and Dol-3 are approximately consistent with original seawater values, suggesting that the dolomitization fluids were mediated by seawater. The calculated dolomitization temperature (25.66 °C) and Z values (125.2) of Dol-1 with evaporite minerals indicate that the dolomitization fluid was high-salinity concentrated seawater derived from evaporation. The calculated dolomitization temperature (24.20 °C) and Z value (121.8) of Dol-2, and no traces of evaporite minerals in Dol-2 imply that the dolomitization fluid was slightly hypersaline seawater. The more negative δ13C and abundant microbial features of Dol-2 indicate that microbial mats were involved in dolomite formation. The dolomitization temperature (30.55 °C) and Z value (125) of Dol-3 also indicate that the dolomitization fluid was high-salinity concentrated seawater. However, Dol-3 is commonly distributed in reef and shoal facies (high porosity and permeability) throughout the Qingxudong Formation. Moreover, the abundant organic matters in the dolomitization fluid likely overcame the kinetic and thermodynamic barriers in the process of dolomitization. The geochemical data (except the δ18O values) of Dol-4 are approximately consistent with fabric-retentive dolomites, suggesting that the Dol-4 had been inherited from precursor carbonates. The negative δ18O values of Dol-4 revealed a further burial environment. High LREE/HREE ratios, positive europium anomalies and negative δ18O values of Dol-5 suggest that these dolomites were affected by hydrothermal fluids under deep burial conditions. Thus, Dol-1, Dol-2 and Dol-3 are likely attributed to sabkha, organogenic (microbial) and seepage-reflux dolomitization models during early-stage diagenesis, respectively. Dol-4 and Dol-5 are commonly attributed to burial and hydrothermal dolomitization models during deep burial diagenesis. This study provides new insights into the origin of dolomites and is especially useful for the further understanding of dolomite reservoir formation in the Qingxudong Formation.

A Palaeoproterozoic dolomite (Vempalle Formation, Cuddapah Basin, India) showing Phanerozoic-type dolomitisation

The Palaeoproterozoic Vempalle Formation of the Cuddapah Basin, India, significantly adds to our understanding of the evolution of Precambrian marine carbonate systems and the redox state of the Earth’s early oceans. A facies-microfacies-diagenetic-geochemical examination of samples from a ∼1000-m long exposure in a freshly-cut canal section shows that 10–15% of precursor limestone is still preserved in the Vempalle Formation in the form of remnant patches of calcimicrite and ooids with calcite spar cement. The ooids, preserving primary radial and concentric fabrics and radial fractures, are considered to have been originally precipitated as calcite, which may have been low-Mg. In places the preserved calcite spar, that is partially replaced by fabric-destructive dolomite, shows Type I calcite twin lamellae. Petrographic observations demonstrate that Vempalle Formation dolomite formed through very early precipitation, which in stromatolites preserved microbial filaments, as well as through fabric-destructive dolomitization during shallow to moderate burial. Vempalle Formation dolomite is characterized by micritic dolomite crystals which suggest rapid early dolomitization of lime mud and micritic calcite from a supersaturated Mg-Ca-rich solution, probably near-surface or during shallow burial. Depletion of Na and Sr contents of Vempalle Formation dolomite along with negative δ18O values indicate dolomite recrystallisation during burial and further replacement. Dolomite δ13C values of −0.5 to 2‰ are likely inherited original marine values. Geochemical proxies (trace elements and rare earths) imply that Cuddapah Basin seawater and dolomitizing fluids were anoxic and ferruginous but not euxinic. Geochemical analyses also indicate that the burial diagenetic fluids evolved from Eu-enriched seawater that probably resulted from continental rifting around 1.9–2.0 Ga. This probable ocean chemistry is in contrast with the anoxic, ferruginous and extremely high Mg/Ca “dolomite oceans” that prevailed during Proterozoic time. The Vempalle dolomite shows more similarities with dolomitised Phanerozoic platform carbonates than typical Precambrian dolomite with its well-preserved textures and burial dolospar cements.

Geochemical characteristics of Upper Jurassic – Lower Cretaceous platform carbonates in Hazine Mağara, Gümüşhane (northeast Turkey): implications for dolomitization and recrystallization

The Upper Jurassic – Lower Cretaceous Berdiga Formation of the Eastern Pontides, Turkey, represents a carbonate platform succession composed of pervasively dolomitized intra-shelf to deep-shelf facies. In this area, polymetallic deposits occur as veins and lenses within the Berdiga Formation in close proximity to its upper contact with the overlying formation. Three different types of replacive dolomites occur in the formation: (i) microcrystalline dolomite, (ii) fabric-preserving dolomite, and (iii) fabric-destructive dolomite. Replacive dolomites are Ca rich and nonstoichiometric (Ca56–58Mg42–44) and are characterized by a pronounced negative shift in oxygen (–11.38‰ to –4.05‰ Vienna Pee Dee Belemnite (VPDB)), δ13C values of 0.69‰ to 3.13‰ VPDB, radiogenic 87Sr/86Sr ratios (0.70753 to 0.70884), and extremely high Fe (2727–21 053 ppm) and Mn (1548–27 726 ppm) contents. All dolomite samples have low Y/Ho ratios (23–40), and they also contain highly variable contents of rare earth elements (REE) (7–41). REE patterns of dolomites normalized to Post-Archean Australian shale show a distinct positive Eu anomaly (1.3–2.1) and slightly flattened Ce anomalies (0.8–1.1). Integration of petrographic and geochemical studies reveals the history of a variety of diagenetic processes highly affected by hydrothermal alteration, which include dolomitization, recrystallization, dissolution, silicification, and pyrite mineralization associated with the emplacement of the polymetallic mineralization.

Pervasive early diagenetic dolomitization, subsequent hydrothermal alteration, and late stage hydrocarbon accumulation in a Middle Triassic carbonate sequence (Szeged Basin, SE Hungary)

Abstract The Middle Triassic shallow marine carbonates of the SE Pannonian Basin (Szeged Dolomite Formation) show evidence for multistage dolomitization and a complex diagenetic history. In first stage the whole sequence was completely dolomitized by reflux of slightly evaporated seawater. This process took place from the near surface till shallow burial realms and resulted in the formation of both fabric-preserving and fabric-destructive dolomite types. In the following stage nonplanar matrix dolomite and saddle dolomite cement were formed in the intermediate and/or deep burial realm. These later dolomite phases are likely generated by invasion of exotic fluids at relatively high temperature evidenced from the fluid inclusion homogenization temperatures, and stable isotope compositions. Vuggy, fracture, and solution enhanced porosity are also related to this local hydrothermal event. Microthermometry performed on saddle dolomite-hosted primary fluid inclusions confirm the presence of hot (138–235 °C) and moderately saline brines (4.1–8.7 mass% NaCl equivalent). The calculated δ18Owater and the measured δDwater values of the fluid inclusions from the saddle dolomite cement together with the relatively low salinity values indicate a metamorphogenic (and/or magmatic) origin of the hydrothermal fluid that probably was channeled along the Upper Cretaceous subhorizontal overthrust zones. The pores formed by the leaching effect of these hydrothermal fluids were subsequently partly occluded by meteoric calcite during the Paleogene–Middle Miocene subaerial exposure but a remarkable part was preserved, and currently serves as reservoir space. Such an integrated study of the different dolomite and porosity types, the understanding of their genesis, and timing relative to hydrocarbon maturation and migration could aid in exploration and development.

Origins and Geochemistry of Dolomites and Their Dissolution in the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

Triassic dolomites occur pervasively in the Western Sichuan Basin. Although these strata have been deeply buried and affected by multiple phases of dolomitization and dissolution, some intervals in the upper part of the Leikoupo Formation have developed high porosity. Based on their petrographic and geochemical features, three major types of dolomite fabrics are recognized, namely, dolomicrite, fabric-retentive dolomite, and fabric-destructive dolomite. Geochemical evidence indicates that the dolomicrite formed following the Sabkha model in a low-temperature hypersaline environment, as these rocks exhibit abnormally high Sr and Na contents, lower Fe and Mn contents, δ18O values generally ranging from −1.70‰ to −1.67‰ (with an average value of −1.69‰), and higher Mg/Ca ratios. The fabric-retentive dolomite formed following the seepage-reflux model in a shallow burial environment, and these rocks exhibit the highest 87Sr/86Sr ratios, δ18O values generally ranging from −6.10‰ to −2.50‰ (with an average value of −3.98‰), and a wide range of Fe and Mn contents, indicating that they may have been altered by meteoric water. The fabric-destructive dolomite formed following the burial model at elevated temperatures; these rocks exhibit the lowest Sr and Na contents, δ18O values generally ranging from −7.01‰ to −6.62‰ (with an average value of −6.79‰), relatively higher Mg/Ca values, and lower 87Sr/86Sr ratios. The early Sabkha, seepage-reflux dolomitization and penecontemporaneous periodic meteoric freshwater selective dissolution processes formed multi-period, overlapping moldic pores, algal framework pores, and intragranular dissolution pores. The superposition of organic acid dissolution during the burial period is the main controlling factor of the formation of deeply buried, high-quality dolomite reservoirs in the Leikoupo Formation.

Diagenesis of microbialites in the lower Cambrian Qingxudong Formation, South China: Implications for the origin of porosity in deep microbial carbonates

Understanding microbial carbonate is one of the most challenging issues in the field of carbonate sedimentology and reservoir. The biotic and abiotic processes that control microbial carbonate precipitation are becoming more established, but the influence of subsequent diagenesis on microbial carbonate reservoirs has not been adequately studied. Here, we describe microbial carbonate from the lower Cambrian Qingxudong Formation in southeastern Sichuan Basin to assess their formation, textures and subsequent diagenesis. Six stages of calcite cementation (Calcite-1 to Calcite-6) and four stages of dissolution (Dis-1 to Dis-4) were identified in microbialites of the Qingxudong Formation. Stromatolites and thrombolites have a different porosity evolution. Sedimentary processes are the fundamental controlling factors of creating pre-existing pores in stromatolites. For thrombolites, the formation of pre-existing pores is attributed to island dissolution and dolomitization in near-surface processes. Microbial metabolic activities contribute more to the dissolution and dolomitization than mixing water. Meteoric water dissolution (Dis-1) in synsedimentary processes and microbial dissolution (Dis-2) in near-surface processes are the fundamental control factors of porosity creation in microbial carbonate of Qingxudong Formation. Thermochemical sulfate reduction (TSR) is a double-edged sword for the porosity development. The destructive effect of TSR on microbial carbonate reservoirs is greater than the constructive effect. The microbial carbonate reservoirs, especially the fabric-destructive dolomite (Dol-2) evolved by thrombolites, have great potential for gas exploration in the Qingxudong Formation. This study is especially useful for further understanding deeply buried microbialite reservoir formation and development, and deep hydrocarbon exploration in this basin and elsewhere worldwide.

Dolomite and dolomitization of the Permian Khuff-C reservoir in Ghawar field, Saudi ArabiaSea

ABSTRACT The Permian Khuff-C reservoir in Saudi Arabia is known for its lateral and vertical heterogeneity caused mainly by dolomitization. Detailed petrographic analysis of 600 thin sections, from six cored wells, revealed three main replacive dolomite fabrics: (1) fabric-preserving mimetic (FPM), (2) fabric-preserving nonmimetic (FPNM), and (3) fabric-destructive (FD) dolomites. Crystal sizes are mostly less than or equal to 20 μm for FPM dolomite, less than or equal to 50 μm for FPNM dolomite, and less than or equal to 100 μm for FD dolomite. The FPM dolomite decreases in abundance, and FPNM dolomite increases in abundance, with increasing grain content of the facies. The δ 18 O values of dolostones (although considered an obsolete term, dolostone is used here to mean rock containing ≥80% dolomite) indicate early dolomitization at low temperatures in Permian seawater or evaporated seawater, with landward facies (mudstone and wackestone) generally dolomitized by more evaporated waters and seaward grainy facies generally dolomitized by less evaporated, more normal marine seawaters. Stratigraphic variations in the dolostones’ δ 18 O values track with facies variations through fourth-order depositional sequences and indicate that different stratigraphic bodies of dolomite formed from seawaters with different degrees of evaporation. The δ 13 C values of the dolostones exhibit temporal trends inherited from the precursor limestones. Variations in the lateral and vertical abundance of dolomite and dolomite fabrics, in the propensity for each facies to be dolomitized, and in the dolomites’ oxygen isotopic values all suggest that multiple dolomitization events occurred in the Khuff-C reservoir as depositional cycles accumulated, with some dolostones overprinted by younger events. Average porosities of grain-rich dolostones are greater than those of mud-rich dolostones, indicating that depositional facies preordained porosity distribution within the dolostones. However, the more evaporated the dolomitizing fluid, the more likely dolomitization resulted in lower porosity regardless of facies.

Dolomitization of Triassic microbial mat deposits (Hungary): Origin of microcrystalline dolomite

Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, diagnosing primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward, even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in the Transdanubian Range. In both sections, two fabric types occur in the upper part of the metre-scale cycles. One of that is microbial boundstone (fabric type 1)—characterised by clusters of dolomite microcrystals which display diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules. The other one is different in the two sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric type 3) that is rich in reworked dasycladalean alga fragments and consists of dolomite crystals of wide size-range from fine to coarse. The precipitation of the microcrystalline dolomite phase is interpreted as being facilitated by mats and biofilms favouring/tolerating an increasing frequency of subaerial conditions in the upper intertidal setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite dissolution and it postdated the organogenic precipitation. It took place only in the peritidal caps of the shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining that microcrystalline dolomite did not form via mimetic replacement. Accordingly the microcrystalline dolomite, which shows microbial microfabrics in the studied samples, is interpreted as an organogenic primary precipitate. Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the peritidal environment were replaced and cemented by medium and coarsely crystalline dolomite during further burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83°C) and negative stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite (fabric type 4).

EVIDENCE FOR LADINIAN (MIDDLE TRIASSIC) PLATFORM PROGRADATION IN THE GYULAKESZI AREA, TAPOLCA BASIN, WESTERN HUNGARY: MICROFACIES ANALYSIS AND BIOSTRATIGRAPHY

A shallowing-upward carbonate sequence was studied from the outcrop at Gyulakeszi, Tapolca Basin (western Hungary), and it is interpreted as a Middle Triassic ( Curionii or younger) platform progradation. Two lithostratigraphic units are distinguished. Microfacies analysis and micropaleontological investigation conducted on the red nodular, cherty limestone (Vaszoly and Buchenstein formations) suggest that the lower unit was deposited during the Reitzi and the Secedensis ammonoid zones. The overlying white platform limestone (upper unit) is typical of a prograding platform and includes gravity-driven deposits at the base followed by periplatform facies deposited in shallow marine warm waters around the fair-weather wave base. The section at Gyulakeszi was unaffected by fabric-destructive dolomitization, which is uncharacteristic of similar platform facies in the Balaton Highland. Isopachous and radiaxial fibrous calcite cement found in the grainstone and boundstone facies are indicative of early lithification and diagenesis in the marine phreatic zone. “Evinospongiae”-type cement is described for the first time from the Balaton Highland and it is similar to the outer platform cements published previously from the Alps (Italy and Austria). The progradation could have advanced over the pelagic limestones as early as the Curionii zone, which is an undocumented event in the Veszprem Plateau. Similar event, however, is well known from the Western Dolomites, where aggradation was followed by intense progradation during the Gredleri and Archelaus ammonoid zones. The length of this progradation event at Gyulakeszi, however, is ambiguous since proven Ladinian (Longobardian) rocks are not exposed in the study area and were not penetrated by boreholes in the Tapolca Basin.