Late Dolomite Research Articles

Crystallisation of trapped carbonate–silicate melts terminating at the carbonated solidus ledge: a record of carbon immobilisation mechanism in the lithospheric mantle

Orogenic peridotites in the crystalline basement of the northwestern Bohemian Massif contain multiphase solid inclusions (MSI), which are interpreted to be crystallisation products of trapped former carbonate–silicate melts metasomatizing their host rocks. We applied conventional thermobarometry and forward thermodynamic modelling to constrain the P–T evolution ranging from the peak metamorphic conditions of the investigated harzburgite and lherzolite, through entrapment of the melts in the outer parts of garnets, to the (re)-equilibration of the MSI assemblages. The peak conditions of c. 1100 °C/4.5–5.5 GPa are recorded by garnet cores and large pyroxene porphyroclasts. The melt entrapment, during which garnet outer parts grew, was associated with influx of the metasomatizing liquids and probably took place during the early stage of the exhumation. Thermodynamic model of amphibole-free MSI assemblage comprising kinoshitalite/Ba-rich phlogopite (approximated by phlogopite in the model), dolomite, magnesite, clinopyroxene, orthopyroxene, garnet and chromite provided robust estimate of P and T of its (re)-equilibration, c. 900–1000 °C, 1.8–2.2 GPa. Furthermore, the lack of olivine reflects co-existence of COH fluid with high X(CO2) = CO2/(CO2 + H2O) ≥ 0.6. Models employing identical P–T–X(CO2) parameters successfully reproduced the other two amphibole-bearing assemblages observed. The modelled stability fields show perfect alignment with a characteristic isobaric segment of the solidus curve of carbonated peridotite. This co-incidence implies that the (re)-equilibration corresponds to the termination of the melt crystallisation once the near-isothermal exhumation path intersected the solidus. Decreased solubility of silicates at the carbonated peridotite “solidus ledge”, inferred from the published experimental data, as well as concentric textures of some MSI indicates sequential crystallisation from the early silicates to late dolomite. The carbonated “solidus ledge” is a relatively narrow boundary in the lithospheric mantle capable of an abrupt immobilisation of fluxing or transported carbonated melts. The investigated rocks are estimated to store approximately 0.02 kg C/m3 (or 6 ppm C) occurring as carbonates in the MSI.

Diagenetic control on porosity types of carbonate rocks of Late Triassic Baluti Formation in Northern Thrust Zone, Iraqi Kurdistan region, Northern Iraq

The carbonate rocks of Late Triassic Baluti Formation in Sararu outcrop, Northern Thrust Zone, Northern Iraqi Kurdistan region consist of brecciated marly limestone and sandy dolomitic limestone interbedded with shale that characterized by complex heterogeneity. The microscopic description of the porosity types and diagenetic processes controlled it were studied in current work. The petrographic study of 12 thin section of Baluti carbonate displayed that the main skeletal grains are shallow marine derivative fossils and non-skeletal grains are only monocrystalline quartz. The main matrix is micrite which sometimes changed to microspar and sparry calcite. Seven pore types were distinguished in the studied rocks of the formation from both primary and secondary stages. The primary porosity are interparticle and intraparticle pores. While, the secondary types were moldic, intercrystalline, fracture, vuggy and stylolitic porosities. The formation in studied outcrop was undergone in several diagenetic processes, that enhanced or reduced the porosity of the Baluti carbonates. Early micritization protect the primary pores from destroying by compaction. While, solution was assists in creating different secondary pores in different diagenetic stage of the formation particularly the dominant moldic porosity. Calcite cements aid in reducing all pore types in the studied formation. Late dolomitization contribute to formation intercrystalline and dolomoldic porosities. stylolites with dissolution helps to creating stylolitic porosity. Neomorphism, silicification and pyritization generally reduced porosity in the formation.

Güneydoğu Anadolu Bölgesi Mardin Grubu dolomitlerinin dolomitleşme yaşlarının ve derinliklerinin kümelenmiş izotoplar ve 1B basen modeli kullanılarak belirlenmesi

The widespread Cretaceous aged Mardin Group is shallow marine carbonates and is the most proliferous section in SE Anatolia (SE Turkey). Clumped isotope temperatures were measured for Mardin dolomites between 92oC and 125oC. Using 1D basin model, clumped isotopes temperatures were converted to a depth and time for dolomitization from 2200 m (22 Ma) to 3420 m (14 Ma). Both the range of paleotemperature and burial depth reflects different degree of recrystalization event as a result of burial event. The oxygen isotopic composition of dolomitizing fluid varied between 3.32‰ and 6.31‰ which is significantly heavier than Cretaceous marine waters. The results indicates that late dolomitization are present in the study area and that a deeply buried dolomitizing pore fluid with high T has a negative influence on porosity system of the Mardin Group Reservoir.

New progress and prospect of Middle Permian natural gas exploration in the Sichuan Basin

In recent years, natural gas exploration in the deep marine carbonates of the Sichuan Basin has been strengthened continuously. And based on the research results in the regional geological setting, a great number of new geological understandings are obtained in terms of lithofacies palaogeography, reservoir genesis and hydrocarbon accumulation evolution. What's more, new progress of natural gas exploration is achieved in many strata and domains in the Sichuan Basin, e.g. Qixia Fm of Middle Permian, Maokou Fm and Emeishan basalt, and the prospect of natural gas exploration is promising. In this paper, the recent exploration achievements of Middle Permian natural gas in the Sichuan Basin were summarized. First, under the control of palaeogeomorphology of late Caledonian, marginal platform shoal and girdle intra-platform shoal that are distributed in a large scale were developed at the edge of western Sichuan sea basin and in the periphery of central Sichuan paleo-uplift during the sedimentation of Qixia Fm, and they are large in scale and extensive in distribution range. Dolomite reservoirs are developed with good gas bearing property. Second, two kinds of reservoir bodies are mainly developed in the Maokou Fm of the Sichuan Basin. One is the shoal body which is developed along the marginal platform and the intra-platform high belt under the control of facies belt superimposed with late dolomitization, and the other is the effective karst reservoir which is distributed at the karst slope belt under the action of large-scale erosion in the late Maokou period. Third, volcanic gas reservoirs are discovered in the areas of Chengdu and Jianyang, and their quality reservoirs are of volcanic eruption facies and characterized by great thickness, good physical properties and extensive distribution. The discovery of this new type of gas reservoir expands the exploration domain of Middle Permian natural gas in the Sichuan Basin. In conclusion, the improvement of the large-scale natural gas exploration potential in many strata and domains of Middle Permian in the Sichuan Basin promotes it to be one important replacement domain of the further increase of natural gas production and reserves in the Sichuan Basin.

Mineralogy and mineral chemistry of carbonates from the Zhulingou Ge-Zn deposit in Guizhou Province and its geological significance

以碳酸盐岩为容矿岩石的后生热液铅锌矿床（Mississippi Valley-type，MVT矿床）是世界上一种重要的铅锌矿床类型，也是锗的重要工业来源之一。热液碳酸盐矿物是MVT矿床中最主要的脉石矿物，其形成贯穿整个MVT矿床成矿过程。因此，碳酸盐矿物携带丰富的成矿信息，是认识MVT矿床成因的重要补充。位于贵州省贵定县境内的竹林沟锗锌矿床，是近年来新发现的富锗锌矿床（平均品位97.9×10^-6 Ge，6.54% Zn），赋存于泥盆系碳酸盐岩中。本次工作发现该矿床不同期次热液白云石的矿物学和微量元素地球化学特征存在明显差异：成矿期前白云石（Dol1）主要呈细脉状穿插围岩，被成矿期白云石和硫化物脉穿插，部分呈细粒状被后期白云石包裹；成矿早期白云石（Dol2）主要呈粗粒状，与闪锌矿共生；主成矿期白云石（Dol3）主要呈脉状、团块状，与闪锌矿和黄铁矿共生；成矿晚期白云石（Dol4）呈团块状充填于闪锌矿矿石或者围岩中；成矿期后白云石（Dol5）呈脉状穿插或包裹其它期次白云石/闪锌矿-黄铁矿条带。C-O同位素研究表明，成矿期白云石主要来源于碳酸盐围岩的溶解，成矿流体中的C来源于围岩，而较低的δ¹⁸O值可能是亏损¹⁸O的成矿流体和围岩间水/岩反应过程中O同位素发生交换的结果。激光剥蚀电感耦合等离子体质谱（LA-ICPMS）白云石原位微量元素分析结果表明，Dol2-Dol4的Y/Ho比值相对稳定（30.5~47.9），结合区域成矿特征，认为形成成矿期白云石的流体主要为盆地卤水，与该矿床属于MVT矿床事实吻合。Dol1和Dol5具有相对较低的稀土总量（∑REE=3.97×10^-6~29.7×10^-6），很可能与白云岩围岩（∑REE=25.2×10^-6~61.3×10^-6）溶解作用有关，直接继承围岩的稀土元素组成特征；成矿期Dol2-Dol4的∑REE较高（∑REE=39.2×10^-6~117×10^-6），暗示成矿流体本身也携带了部分稀土元素。成矿各期次白云石均具有明显的Eu负异常（δEu=0.38~0.72）特征，指示成矿温度可能较高（>200℃），Ce正异常相对稳定（δCe=1.10~1.28），暗示其成矿流体pH值保持弱酸性。综上，本文认为竹林沟锗锌矿床的形成经历如下过程：在伸展背景下，深循环盆地卤水萃取下伏地层和基底岩石中的Zn、Ge和REE等元素，形成富金属流体，受构造作用驱动沿区域性断裂不断向上运移，在赋矿层位与富硫流体发生混合作用，导致闪锌矿等硫化物沉淀；在整个成矿过程，成矿环境经历了还原→氧化的转变，成矿元素发生了共生分异，在竹林沟形成富锗锌矿体，在牛角塘形成富镉锌矿体，而pH始终保持弱酸性，直至被围岩碳酸盐岩中和。;Carbonate-hosted epigenetic hydrothermal Pb-Zn deposits, i.e. Mississippi Valley-type (MVT) deposits, are important lead-zinc resources in the world and a major industrial source of germanium (Ge). Wherein hydrothermal carbonates are the most important gangue minerals and their formation runs through the whole metallogenic process of MVT deposits. Therefore, hydrothermal carbonates contain rich mineralization information and are an important supplement for understanding the genesis of MVT deposits. The Devonian carbonate-hosted Zhulingou Ge-Zn deposit (97.9×10^-6 Ge, 6.54% Zn) located in Guiding County, Guizhou Province, is newly discovered in recent years. The hydrothermal dolomite of the Zhulingou deposit have various mineralogical features:the pre-ore dolomite (Dol1) occurs mainly as veinlets or interspersed in host rocks and cross-cut by ore-stage dolomite and sulfides, or occurs as fine crystals enclosed by the late dolomite; the early ore-stage dolomite (Dol2) is mainly coarse-grained and coexists with sphalerite; the main ore-stage dolomite (Dol3) is mainly vein-like or clumpy, coexisting with sphalerite and pyrite; the late ore-stage dolomite (Dol4) fills into sphalerite or host rocks in the form of clumps; the post-ore dolomite (Dol5) crosscuts/encloses early stages of dolomite/sphalerite-pyrite bands. The C and O isotopic data show that the formation of Dol2-Dol4 is likely related to the dissolution of the carbonate host rocks, and the C in the ore-forming fluid derives from the country rocks, the lower δ¹⁸O may be contributed to the isotope exchange between the host rock and the ore-forming fluid with ¹⁸O depletion. The laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) in situ analytical results show that the Y/Ho ratios of Dol2-Dol4 are relatively constant (30.5~47.9). This indicates that the fluids formed metallogenic dolomite are basin brine, which is supported by the MVT genesis of the Zhulingou deposit. Dol1 and Dol5 with lower ∑REE values (3.97×10^-6~29.7×10^-6) may be related to the dissolution of the host rock dolostone (∑REE=25.2×10^-6~61.3×10^-6), and directly inherit the REE characteristics of the latter. The relatively high ∑REE values (39.2×10^-6~117×10^-6) of Dol2-Dol4 suggest an REE source from the ore-forming fluids. Hydrothermal dolomite at Zhulingou is characterized by significantly negative Eu anomalies (δEu=0.38~0.72), implying a relatively high ore-forming temperatures (>200℃). The positive Ce anomalies (δCe=1.10~1.28) suggest weakly acidic ore-forming fluid. In conclusion, this study proposed that the formation of the Zhulingou Ge-Zn deposit experienced the following process:under the regional extensional stress, the deep circulated brine extracted the ore-forming elements (e.g. Zn, Ge and REE) from underlying strata and basement rocks, and formed the metal-rich fluids, which continued to transport upward along the regional faults driven by tectonic action. The metal-rich fluids were mixed with the sulfur-rich fluids in the ore-bearing strata to form the sulfide such as sphalerite. During the ore-forming process, the metallogenic environment went through the transition from reduction to oxidation accompanied by symbiosis and differentiation of ore-forming elements, forming Ge-rich zinc ore body at Zhulingou, and Cd-rich zinc ore body at Niujiaotang, while the fluids remain weakly acidic until neutralized by the host rock carbonates.

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.

Diagenesis and its controls on reservoir quality of the Tambar oil field, Norwegian North Sea

A study of the diagenetic evolution of the Late Jurassic sandstones of the shallow marine facies in the Tambar oil field, Norwegian North Sea was carried out to understand its controls on reservoir quality. Core samples and a set of wireline logs obtained from three wells were used to carry out petrographic studies including light optics and scanning electron microscopy, well log analysis, fluid inclusions, X-ray diffraction (XRD) for whole rock samples, clay mineral extracts and stable isotopes. The Tambar reservoir sandstones range mainly from siltstone to fine-grained sandstones, exclusively arkoses cemented mainly by microcrystalline quartz, euhedral quartz overgrowth, dolomite cement, illite and chlorite. Early diagenetic/eodigenetic minerals include pyrite, calcite, and microcrystalline quartz and late diagenetic/mesodiagenetic minerals include quartz overgrowth, dolomite, illite and chlorite. The δ18O values of dolomite cement in the Tambar sandstone reservoir range from −11.77‰ to −3.57‰ PDB, while the δ13C values of the dolomite cement are from −5.07‰ to −1.12‰ PDB. Homogenization temperature for fluid inclusions trapped in authigenic quartz fall between 130 °C and 169 °C. The reservoir quality is controlled mainly by early formed grain-coating microcrystalline quartz that precipitated from the dissolution of sponge spicules which significantly inhibits quartz overgrowth precipitation. Oil emplacement do not show significant control on reservoir quality because considerable quartz cementation has taken place before the onset of major oil charge. Only the coarse-grained sands show a correlation between quartz cement volume and water saturation. However, the interpreted temperature for dolomite cementation is significantly higher for water leg-suggested effect of oil emplacement on the late dolomite. Understanding these controls on the reservoir quality will enhance the exploration strategy for the Tambar oil field.

Reservoir characteristics, pore structure, and main controlling factors of oolitic shoal reservoir in Feixianguan Formation: a case study from eastern Kaijiang-Liangping Trough

The understanding of pore structures and controlling factors of different oolitic shoal reservoir types in Lower Triassic Feixianguan Formation around Kaijiang-Liangping Trough has been a key issue restricting efficient exploration and development of such reservoirs. Based on thin section identification, scanning electron microscopy, mercury injection, three-dimensional CT scanning, cathodoluminescence analysis, stable isotope, and rare earth element analysis, the Feixianguan Formation oolitic shoal reservoirs are classified into three types: mold-oolitic dolostone type, residual-oolitic dolostone type, and sparry-oolitic limestone type. The residual-oolitic dolostone possesses the best pore structure and reservoir connectivity. By comparison, the pore structure and connectivity of mold-oolitic dolostone and sparry-oolitic limestone are relatively poor. Based on deposition of oolitic shoals, the main controlling factors for pore structure difference are indicated to be meteoric freshwater leaching and dolomitization. The pore structure of mold-oolitic dolostone reservoir is transformed near the exposed surfaces in the upper-middle portion of shallowing-upward meter-scale cyclic sequences. The mold pores are formed by meteoric freshwater leaching, and then transformed by the late dolomitization. The development of residual-oolitic dolostone is not strongly affected by meteoric freshwater leaching effects, while transformed by seepage-reflux dolomitization. Hydrothermal fluids are involved in the dolomitization process of the oolitic dolostones mentioned above. The emplacement of hydrocarbon preserves primary intergranular pore in sparry-oolitic limestone. Other diagenetic effects are less important for the pore structure of oolitic shoal reservoirs in the study area.

Geochemical constrains on dolomitization pathways of the Upper Jurassic carbonate rocks in the Geneva Basin (Switzerland and France)

The Kimmeridgian-Tithonian carbonate rocks of the Geneva Basin represent potential reservoirs for geothermal energy exploitation. Based on petrographic data, a previous study (Makhloufi et al. 2018) reported three different stages of dolomitization affecting these carbonate rocks, followed by dedolomitization. The present study focuses on the geochemical characterization of these stages based on O, C and Sr isotopes of dolomites and dedolomites. The oxygen isotopic values of early dolomite are depleted compared to the values of Late Jurassic marine cements. This is interpreted to reflect dolomite precipitation from an oxygen-enriched fluid, likely evaporitic. Carbon isotopic values are close to the composition of well-preserved Late Jurassic cements suggestive of abiotic precipitation. These findings are consistent with a scenario of reflux type dolomitization induced by high-frequency sea-level changes producing pulses of dolomitizing brines. Late dolomitization represents an advanced level of replacement. Isotopic data exhibits depleted oxygen composition pointing towards burial diagenesis and is interpreted as the results of shallow burial over-dolomitization. Ages provided by radiogenic strontium isotopes data are consistent with an early first stage of dolomitization followed by late burial dolomite. Dedolomitization is observed at different orders of magnitude and might results from the interaction with meteoric water initiating the dissolution of both early and late dolomites. This dedolomitization would have taken place during long-term emersion events or after the exhumation. The results presented in this work provide further understanding of the processes involved in dolomitization under the influence of high-frequency sea-level fluctuations and the evolution of dolomitic fabrics during burial.

Carbonate diagenesis in the Barremian-Aptian Tirgan Formation (Kopet-Dagh Basin, NE Iran): Petrographic, geochemical and reservoir quality constraints

This study aims to establish the diagenetic evolution of carbonate rocks of the Barremian-Aptian Tirgan Formation (Kopet-Dagh Basin, NE Iran), with special emphasis in their impact on reservoir quality to be used as analog for their equivalent carbonate reservoirs. To achieve this target, basic petrography complemented by elemental and stable isotopic (δ18O and δ13C) analyses of the studied carbonate rocks was used. In addition, the visible porosity was detected. The recognized carbonate diagenetic processes include micritization, cementation, dolomitization, compaction, dissolution, stylolitization, fracturing, silicification and neomorphism. These diagenetic processes took place in four diagenetic environments: marine-phreatic, meteoric-phreatic, meteoric-vadose and burial. Early dolomitization of lime muds, micritization of skeletal allochems and cementation by isopachous equant calcite represent marine-phreatic diagenesis. The meteoric-phreatic diagenesis was accompanied with the development of granular non-ferroan calcite cement, neomorphosed carbonate matrix and recrystallized bioclasts. The meteoric-vadose diagenesis led to prevalence of dissolution process and sparry calcite infilling the fractures. Late dolomitization, ferroan blocky and poikilotopic calcite cement, stylolitization and silicification processes were developed during burial diagenesis. An integration of the petrographic study with the geochemical analysis confirms that the studied carbonate rocks were originally composed of aragonite, which was eventually recrystallized to calcite and replaced by dolomite during diagenesis. In terms of reservoir quality, the recognized diagenetic alterations and visible porosities show that the diagenetic processes have a direct effect on modification of both inter- and intra-granular/crystalline porosity. In this manner, it is found that the calcite cementation, micritization and compaction decline the pore spaces, whereas the dolomitization, dissolution, formation of moldic pores and fracturing increase the pore volumes. Consequently, prevalence of dolomitization, dissolution, occurrence of moldic pores and fracturing in the Tirgan carbonate rocks gives the Tirgan Formation and its equivalent rocks a good opportunity to be an excellent reservoirs.

Trace element and stable (C, O) and radiogenic (Sr) isotope geochemistry of stromatolitic carbonate rocks of the Mesoarchaean Pongola Supergroup: Implications for seawater composition

The Mesoarchaean Pongola Supergroup hosts one of the oldest well-preserved stromatolitic carbonate successions. Major- and trace-element geochemistry along with C, O, and Sr isotopic analyses of three stratigraphic sections of stromatolitic carbonate rocks were undertaken in order to infer the depositional environment, and to unravel the chemical characteristics of contemporaneous seawater. The carbonate rocks consist mainly of dolomite and, to a lesser extent, ankerite. Their variable Sr concentrations of ~95 to 450mg/kg are interpreted as resulting from early to late dolomitization of metastable carbonates (aragonite and/or high-Mg calcite), facies control and calcitization during meteoric diagenesis. δ18O values range from −19.6 to −12.0‰ VPDB and reflect recrystallization of carbonates caused by high water-rock interactions during diagenesis and metamorphism. A δ13C value of 2.5‰ VPDB was measured in carbonate samples least affected by alteration, and we suggest that it approximates the composition of dissolved inorganic carbon in coeval seawater. The samples display variable 87Sr/86Sr isotopic ratios, from as low as 0.708 to 0.777. These ratios are considerably higher than those expected in Archaean open-ocean water masses buffered by hydrothermal interaction with the mantle. We interpret the highly radiogenic Sr isotopic ratios in some carbonate samples to reflect isotopic exchange with radiogenic Sr from siliciclastic/volcaniclastic components in the samples and fluids from clay-rich layers intercalated with the carbonates. The least radiogenic Sr isotopic ratio may approximate the mixed isotopic composition of seawater in the epicontinental basin and of riverine freshwater draining the Kaapvaal Craton. The dolomitic carbonate samples show shale-normalized rare earth element and yttrium distribution patterns that resemble those of modern ocean water with positive LaSN, GdSN and YSN anomalies, superchondritic Y/Ho ratios, a small negative CeSN anomaly and depleted light rare earth elements relative to the heavy rare earth elements. These anomalies are not as pronounced as expected for an open marine setting, which together with the Sr isotopic results support a more restricted setting with riverine input and fluctuations in marine ingressions. A convex-upward shale-normalized rare earth element and yttrium pattern is probably a result of several factors, including organic matter complexation, and element fractionation during estuarine mixing. The small negative CeSN anomaly may suggest small amounts of free oxygen in shallow waters due to local oxygenic microbial activity.

Relationship between karstification and burial dolomitization in Permian platform carbonates (Lower Khuff — Oman)

Large breccia fabrics associated with karst constitute an important structure in massive limestone successions. The dimensions and shapes of breccia structures are controlled by the initial fracture pattern of the limestone and preferential pathways of the karstifying fluids, but subsequently breccia fabrics can also govern the migration of later fluids. Therefore, breccias are highly relevant features to capture for reservoir characterisation. Outcrop analogues for Lower Khuff units in the Middle East present in the Central Oman Mountains reveal brecciated fabrics up to 10s of metres in diameter. These brecciated units are closely associated with dolomite bodies of late diagenetic origin.Based on an integrated set of data, the breccias are interpreted as collapsed karst cavities either formed by meteoric or hypogenic fluids. The exact origin of the fluids could not be constrained due to an overprint by later dolomitizing fluids. Based on the composition of the clasts and matrix in the breccias, two dolomitization events are interpreted to have affected the succession, one prior to (early diagenetic [ED] dolomite) and one after brecciation (late diagenetic [DT2] dolomite). Dolomite of shallow burial origin (ED dolomite) was only observed as clasts within breccia and is much more frequent than late diagenetic (medium to deep burial) dolomite clasts. Thus, the timing of the brecciation and collapse is assumed to postdate shallow burial early diagenetic dolomitization. Late diagenetic replacive dolomite (DT2 dolomite) forms 90% of the matrix in the breccia fabrics with the exception of a small area that was not affected by dolomitization, but is rarely present as clasts. Stable isotope measurements [δ18O: −2.5‰ to −6‰ VPDB and δ13C: 2.9‰ to 4.8‰ VPDB] suggest a burial origin for the late diagenetic dolomite potentially with the participation of hydrothermal fluids. The dolomitized matrix indicates a migration of late dolomitizing fluids subsequent to or postdating the collapse of the karstic cavities. Thus, early karstification processes seem to have played a big role in controlling subsequent loci of late dolomitization in the Oman Mountains, and potentially in other similar settings elsewhere.

Do CO2-charged fluids contribute to secondary porosity creation in deeply buried carbonates?

Carbonate dissolution by acidic CO2-charged fluids is one possible mechanism leading to the creation of burial porosity. The Late Permian carbonates of the Lower Saxony Basin, NW Germany, are a suitable target to study the influence of such CO2-charged fluids on porosity evolution. The so-called Zechstein 2 carbonates (Ca2) consist of alternating dolomitic and calcitic intervals and contain locally very high amounts of CO2 (>80%) and variable amounts of H2S in reservoir gas and vein cement hosted fluid inclusions. Composition and isotope data from fluid inclusions suggest that the CO2 is sourced from the thermal decomposition of underlying Devonian carbonates. Far-reaching fault systems connect the Devonian CO2 source with the Permian Ca2 reservoir, while thick rock salt deposits above the Zechstein 2 carbonates prohibit an influence of surface-derived meteoric fluids. The aim of the study is to evaluate the contribution of CO2 on carbonate dissolution. Seven wells were preferentially sampled along mineral veins, which are interpreted as the preferred migration pathways for the present gas. Classic petrography methods, such as transmitted light microscopy and cathodoluminescence, were combined with fluid inclusion analysis, and porosity data. Petrographic analysis reveals no great impact of the massive CO2 presence on porosity creation, as fluids get quickly saturated with respect to Ca2+. Minor vuggy porosity is associated with fractures in only one of the studied wells, but the dominant porosity types are early diagenetic fabric-selective oomoldic and intercrystal porosity. To evaluate whether the porosity values are higher than usual, a comparison with a global porosity dataset is included, which, however, shows no increase in average porosities compared to global values. The results of this case study suggest that the contribution of CO2 alone on carbonate dissolution and therefore reservoir quality is negligible, as long as other processes, such as late dolomitization or pyrite formation by thermochemical sulphate reduction-derived H2S are absent.

Beyreli - Çamiçi (Hadim-Konya) Dolayındaki Jura Yaşlı Dedebeleni Formasyonu Dolomitlerinin Sedimantolojik ve Jeokimyasal İncelenmesi

The objective of this study is to determine the sedimentary properties of dolomites in terms of petrography and geochemistry, which belong to the Dedebeleni (Jurassic) formation existing at the surroundings of Beyreli Village of Hadim District lying at the southeast of Konya City. Jurassic-aged Dedebeleni formation, locating Central Taurus composed of limestone, mudstone, dolomitic limestone and dolomite. Dedebeleni formation microfacies: wackestone, packstone, crystalline limestone and dolosparite.The Sr content in the Dedebeleni formation (73 to 88 ppm in the early dolomites, and 58 to 77 ppm in the late dolomites, respectively) are compatible with the Sr concentration shallow-deep burial dolomites. The Na content in the Dedebeleni formation (148 ppm in the early dolomites, and 74 to 148 ppm in the late dolomites, respectively) are compatible with the Na concentration shallow-deep burial dolomites. The investigated dolomites suggest to occur from normal sea water or weakly evaporative sea water δO18 compositions (‰ -2,37 to -3,35) in the early diagenetic dolomites of Dedebeleni formation. The δO18 compositions (‰ -4,39 to -5,9) of late diagenetic dolomites in the Dedebeleni formation propose to occur from basinal brines at elevated temperatures. The δC13 values (early dolomites: ‰ +1,29 ile +2,65; late dolomites: ‰ +0,79 ile +3,14) in the dolomites of Dedebeleni formation stress to occur biogenic and non-biogenic. As a result, the Dedebeleni formation dolomites have been formed as early diagenetic at the shallow-burial environment and as the late diagenetic at the medium burial environment

Primary dolomite in the Late Triassic Travenanzes Formation, Dolomites, Northern Italy: Facies control and possible bacterial influence

AbstractIn the late Carnian (Late Triassic), a carbonate‐clastic depositional system including a distal alluvial plain, flood basin and sabkha, tidal flat and shallow carbonate lagoon was established in the Dolomites (Northern Italy). The flood basin was a muddy supratidal environment where marine carbonates and continental siliciclastics interfingered. A dolomite phase made of sub‐micrometre euhedral crystals with a mosaic microstructure of nanometre‐scale domains was identified in stromatolitic laminae of the flood basin embedded in clay. This dolomite is interpreted here as primary and has a nearly stoichiometric composition, as opposed to younger early diagenetic (not primary) dolomite phases, which are commonly calcian. This primary dolomite was shielded from later diagenetic transformation by the clay. The stable isotopic composition of dolomite was analyzed along a depositional transect. The δ13C values range between ca −6‰ and +4‰, with the most 13C‐depleted values in dolomites of the distal alluvial plain and flood basin, and the most 13C‐enriched in dolomites of the tidal flat and lagoon. Uniform δ18O values ranging between 0‰ and +3‰ were found in all sedimentary facies. It is hypothesized that the primary dolomite with mosaic microstructure nucleated on extracellular polymeric substances secreted by sulphate reducing bacteria. A multi‐step process involving sabkha and reflux dolomitization led to partial replacement and overgrowth of the primary dolomite, but replacement and overgrowth were facies‐dependent. Dolomites of the landward, clay‐rich portion of the sedimentary system were only moderately overgrown during late dolomitization steps, and partly retain an isotopic signature consistent with bacterial sulphate reduction with δ13C as low as −6‰. In contrast, dolomites of the marine, clay‐free part of the system were probably transformed through sabkha and reflux diagenetic processes into calcian varieties, and exhibit δ13C values of ca +3‰. Major shifts of δ13C values strictly follow the lateral migration of facies and thus mark transgressions and regressions.

A fluid inclusion study of diagenetic fluids in Proterozoic and Paleozoic carbonate rocks, Victoria Island, NWT

AbstractDespite the presence of known economic resources in Canada's Arctic archipelago, Victoria Island remains understudied. This study addresses the fluid history and economic potential of two major carbonate units on Victoria Island by integrating fluid inclusion microthermometry with SEM‐EDS analysis of evaporate mounds. Three cements containing fluid inclusion assemblages (FIA) occur in the Neoproterozoic Wynniatt Formation: saddle dolomite, brown dolomite and calcite, in paragenetic order. The two dolomite‐hosted cements have average homogenisation temperatures (Th) for FIAs (n = 3) of 108°C (saddle) and 101 and 116°C, but metastability precluded determining salinities; most calcite‐hosted fluid inclusions are too small and/or necked to obtain Th values, but rare larger inclusions have salinities from 1.7 to 0.4 wt. % NaCl equiv. SEM‐EDS analysis of evaporate mounds indicates the fluid changed from an early K‐rich (saddle dolomite), to a later K + Na (brown dolomite), and finally Na‐rich (calcite), which suggests mixing of two end‐member fluids (i.e. Na‐rich and K‐rich). Dolostone of the lower Paleozoic ‘Victoria Island formation’ contains two cements: early quartz and late dolomite. Quartz‐hosted FIAs (n = 2) have an average Th value of 126°C, and salinity of 23.2 wt. % NaCl equiv., whereas FIAs (n = 3) in dolomite have average Th values of 109, 116 and 124°C; metastability precluded determining salinity. Evaporate mound analysis for the cements indicates evolution from a Na‐rich to a Na + K fluid through interaction with reservoir rocks. A reduced, metal‐rich fluid was present during quartz precipitation, as implied by the presence of pyrite framboids along growth zones and nanoparticles of barite and sulphide minerals (Zn, Cu and Pb) in evacuated inclusions, which suggests the area may have potential to host base‐metal mineralisation. Importantly, distinguishing different fluid compositions in both of the case studies would not have been possible without evaporate mound analysis and therefore the results emphasise integrating this technique into diagenetic studies.

Onshore to offshore trends in carbonate sequence development, diagenesis and reservoir quality across a land-attached shelf in SE Asia

Although isolated Miocene buildups in SE Asia commonly form prolific hydrocarbon reservoirs, their equivalents on clastic-dominated land-attached shelves remain poorly known and underexplored. Here, onshore to offshore trends in carbonate development and reservoir quality are assessed across the NW Borneo shelf through study of surface outcrops and subsurface wells. A multidisciplinary programme of fieldwork, petrography and geochemical analyses allowed evaluation of spatio-temporal variations in deposition, diagenesis and pore system development, together with an assessment of controlling influences. In addition to field logging and sample collection >200 samples were studied via transmitted light, cathodoluminescent and scanning electron microscopy together with stable isotopic characterisation (O, C and Sr).Carbonates developed as localised low-, and higher-relief buildups, as well as more continuous sheet-like deposits in near-coast to shelf margin positions. Molluscs, corals, larger benthic foraminifera and coralline algae are common constituents. Most samples show evidence for marine micritisation, and just in shelf margin positions isopachous cements. However, burial diagenesis predominates in the form of compaction, neomorphism, fracturing, late leaching and dolomitisation. Near-coastal carbonates commonly contain siliciclastics, as do some shelf margin deposits that interdigitate with, or are covered in siliciclastics. Some early, probable meteoric leaching affected inner shelf deposits prior to pervasive neomorphic to blocky/poikilotopic calcite cement formation. On the basis of δ18O V-PDB values of −4.5 to −7.9‰ equivalent to δ18O V-SMOW values of 0 to −4‰ at 25–40 °C and δ13C V-PDB values of −0.6 to +1.6‰ cementation probably reflects alteration from terrestrial groundwaters in meteoric aquifers derived from the humid landmass of Borneo. Despite this cementation, moderate energy inner- to mid-shelf grainstones from the core of mounded carbonates still retain, or have enhanced porosity (<8%) over their lower energy counterparts (<4% porosity). Retention of primary porosity and/or late burial dissolution (often associated with saddle dolomite formation) enhancing predominantly primary and minor secondary porosity is key to reservoir quality development in outer-shelf deposits. Best porosity (<20–35%) is in high energy grainstones and rudstones from outer-shelf to shelf-margin positions that experienced minimal clastic influx, most commonly from backstepping to aggradational carbonate sequences. Although stable isotopes for shelf margin calcite cements are consistent with precipitation from marine-derived fluids (δ18O V-PDB values of −3.6 to −5.4‰), those for the late dolomites are suggestive of fluids of meteoric origin (δ18O V-PDB values of −5.2 to −7.4‰ equivalent to values of −0.3 to −6.3‰ V-SMOW at 40–60 °C). Critical factors for reservoir quality development in carbonates from siliciclastic-dominated shelves in the equatorial tropics are: (1) development and preservation of primary porosity, (2) cementation associated with meteoric aquifers draining large humid equatorial landmasses, and (3) burial leaching and fluid pathways.

Fluid channeling along thrust zones: the Lagonegro case history, southern Apennines, Italy

AbstractA regional study on late burial, syntectonic dolomites and syntectonic calcite and quartz veins occurring in the hemipelagic Triassic–Jurassic succession of the Lagonegro Basin (southern Apennines) was carried out by integrating fieldwork with structural, fluid inclusion, and geochemical investigations on both surface and subsurface samples. The main goal was to characterize the different parent fluids, channeled along a major, out of sequence thrust zone. We also investigated calcite veins from the deeper tectonic mélange zone. The results point out that the fluids at peak burial conditions, during the first deformation stage, were characterized by temperatures of 120–140°C and salinities averaging 2 wt% NaCl eq. Fluids channeled along the late, out of sequence thrust zone and responsible for partial dolomitization of the succession had lower temperatures (Th = 95 ± 10°C) and salinities in the range of slightly modified to normal marine seawater (mean at 3.7 wt% NaCl eq). Similar low salinities are encountered in late dolomites from other fold‐and‐thrust belts of Italy. The most probable source for the fluids channeled along the investigated out of sequence thrust zone is represented by Miocene marine pore waters expelled from the tectonic mélange zone. A comparison of the Lagonegro dolomites with similar late stages carbonates formed in other fold‐and‐thrust belts suggests that fluid salinities appear to be controlled by the different lithologies of the fold‐and‐thrust belts décollement surfaces.

Late Dolomitization in Basinal Limestones of the Southern Apennines Fold and Thrust Belt (Italy)

The Triassic pelagic carbonates of the Lagonegro Units from the southern Apennines fold and thrust belt host discordant bodies of dolostone. These rocks show textural features typical of zebra or saddle dolomites. In this contribution, the results of field observations, petrography and geochemical analyzes performed on samples from three different outcrops are presented.Field data indicate that the peculiar zebra-type rock and brecciated fabrics were controlled by regular bedding of the micritic, pelagic limestones. The dolomitization resulted in the replacement of the host limestones and subsequent void-filling precipitation of dolomite in a dilatational stress field.Homogenization temperatures (Th) are in the range 80-120°C, with a clear mode of 95 ÷ 10°C. When corrected for maximum pressure, they indicate an upper limit for dolomite formation around 110-115°C. Melting temperatures of ice (Tmi ) point to salinities in the range 2-6 wt% NaCl eq, with a mean of 4.2%. δ 13 C values overlap those of the Upper Triassic seawater, whereas the δ 18 O values are significantly depleted compared to the coeval seawater. 87 Sr/86 Sr values are higher with respect to Upper Triassic seawater, and partially overlap Middle-Upper Miocene values.The performed analyzes indicate a dolomitization process driven by warm fluids (110-115°C) with a salinity close to that of seawater and O-isotope ratios comprised between seawater and formation waters. It is suggested that the dolomitization was accomplished by formation waters squeezed out or from surrounding lithologies or from the underlying melange units. Integration with available thermal data on the collisional history of the belt suggests that the fluid-flow took place after maximum burial, in the early stages of exhumation, in the assumption of a fluid in thermal equilibrium with the host rocks.According to literature data, the dolomitization event was quite widespread. It can be argued that it was part of a major fluid-flux associated with fold and thrust belt development, which possibly affected the Apulian Platform carbonates located in the foreland. The latter, now buried below the nappe stack, hosts some of the major oil fields of continental Europe.

Petrography and geochemistry of the magnesites and dolostones of the Ediacaran Ibor Group (635 to 542 Ma), Western Spain: Evidences of their hydrothermal origin

The Ediacaran deposits (between 635 and 542 Ma) of the Central Zone of the Iberian Massif consist of alternating siliciclastic and carbonate beds. These carbonates are dolostones and magnesites which are interpreted to have been formed by the replacement of primary peritidal limestones. Through petrographic and geochemical analyses, we recognize different types of dolomites (D1 to D4) and magnesites (M1 and M2). Despite distinct petrographic features of the four types of dolomite, their oxygen and carbon isotopes overlap with δ 18O values ranging from + 15.45 to + 17.51‰ (SMOW) and δ 13C from − 0.13 to 3.21‰ PDB. Sr isotope values for D1 and D2 range from 0.7028 to 0.7091. Magnesites (M1 and M2) show oxygen values higher than + 17.87.0‰, and δ 13C values show the same variability as for the dolomites. D3 and D4 oxygen isotope values are between + 18.91 and 19.61, and the carbon isotope values range are similar to the other diagenetic phases. Sr isotope values for the magnesites and late dolomites (D3 and D4) are 0.7095 to 0.7104, being higher than those of the D1 and D2 dolomites. D1 is a relatively early dolomite phase formed by the replacement of fine grained peritidal limestones. The coarser crystal size of D2, which shows similar geochemical features as D1, suggests formation by dolomitization of coarser grained limestones. The replacement of D1 and D2 by M1 and M2 advanced along stylolites, fractures and bedding planes. This replacement is interpreted to have occurred by hydrothermal fluids, which is suggested by the presence of talc and forsterite. D3, a coarse dolomite, completely destroyed any previous texture and D4 (dolomite cement) post-dates magnesite formation. Interactions of hydrothermal fluids with the prior carbonates reset the oxygen isotopes of the earlier dolomite. The study of these magnesites and related dolostones may offer new insights into the model of formation of sparry magnesites hosted by mixed siliciclastic–carbonate platform deposits. The establishment of the factors and mechanism that control the diagenetic evolution of these carbonates has a great importance in order to understand and predicts porosity and permeability variations of rocks formed under similar geological conditions.