Blocky Calcite Cement Research Articles

THE APPLICATION OF SEM-EDS FOR THE SURFACE TEXTURE OF FORAMINIFERA: IDENTIFICATION OF EARLY DIAGENESIS OF REWORKED L. DIMIDIATUS WITHIN MODERN COASTAL DEPOSITS

Sediment recycling has been known to occur within Quaternary coastal barriers of the greater Coorong Coastal Plain, southern Australia. A high degree of reworking of skeletal carbonates from Late Pleistocene deposits (Robe Range) is evident based on the novel application of amino acid dating of the single-foraminifera species Lamellodiscorbis dimidiatus. More importantly, some apparently transparent, well-preserved tests indicate anomalously high extents of amino acid racemization, implying that reworked fossils could not be easily distinguished based on taphonomic signatures such as corrosion. Here, we examine the surface microtexture of this species, constrained with well-preserved specimens, on a modern beach of Canunda, southern Australia, using scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS). The aim is to identify surface features of foraminifera in more detail, capturing signs of early diagenesis associated with weakly consolidated Late Pleistocene coastal barriers. The results reveal that some well-preserved tests show localized blocky calcite cementation, most notably within intraseptal spaces or impact sites. The EDS spectra of cement indicate lower Mg content than unaltered foraminifera surface. This suggests low-Mg calcite precipitation due to meteoric diagenesis experienced by the onshore Late Pleistocene coastal barrier. It implies that these foraminifera shells are reworked fossils originated from older successions and were subsequently redeposited within the present-day beach. SEM-EDS used in this study demonstrates its capability in examining small-scale carbonate diagenesis products beyond the traditional binocular microscope. Thus, this tool is recommended to aid amino acid dating in detecting reworked fossils.

PRE‐, SYN‐ AND POST‐TECTONIC DIAGENETIC EVOLUTION OF A CARBONATE RESERVOIR: A CASE STUDY OF THE LOWER CRETACEOUS FAHLIYAN FORMATION IN THE DEZFUL EMBAYMENT, ZAGROS FOLDBELT, SW IRAN

Lower Cretaceous carbonates of the Fahliyan Formation form prolific reservoir rocks at oilfields in the Dezful Embayment, central Zagros fold‐thrust belt, SW Iran. The carbonates have undergone significant diagenetic alteration in phases which can in general be linked to the pre‐, syn‐ and post‐tectonic evolution of the fold‐thrust belt. This paper investigates the impact of diagenetic processes on the reservoir quality of the carbonates using integrated petrographic, geochemical and sedimentological analyses of subsurface and outcrop samples of the formation. Diagenetic alterations include:(i) pre‐tectonic eogenesis in the marine and shallow‐burial realm, which resulted in micritization of allochems and cementation by equant and isopachous calcite rims and framboidal pyrite together with limited dolomitization and dissolution of metastable bioclasts. The isotopic compositions of micrite and early calcite cement depart from postulated values of Lower Cretaceous marine carbonates, signifying early stabilization of precursor metastable carbonate minerals and the possible effects of the incursion of meteoric waters and/or increasing burial temperatures;(ii) mesogenesis during the subsequent syn‐tectonic phase, which included Late Cretaceous ophiolite obduction at the northern margin of the Arabian Plate and the later Zagros orogeny in the Miocene‐Pliocene. Diagenetic modifications included the emplacement of hydrocarbons, the development of stylolites and fractures, and the precipitation of saddle dolomite, replacive rhombic dolomite, discrete pyrite, microcrystalline quartz, kaolin and anhydrite. The average stable isotope compositions of saddle dolomite (δ18O: ‐6.9 ‰ ± .9 and δ13C 0.5 ‰ ± 1.6, respectively) also reflects the influence of high temprature basinal fluids;and (iii) “late” (telogenetic, post‐tectonic) uplift‐related modification starting in the Pliocene, when the incursion of meteoric waters resulted in the formation of vugs, the calcitization of dolomite, and cementation by fracture‐filling blocky calcite. The negative δ18O and δ13C stable isotope values (average: ‐5.5 ‰ ± 1.5; and ‐3.6 ‰ ± 5.9, respectively) of late blocky calcite cement suggest the incursion of meteoric water into the system.This study demonstrates that diagenetic processes in carbonates in the Fahliyan Formation, which exerted a significant control on the distribution of secondary porosity, can be related to the tectonic evolution of the central Zagros fold‐thrust belt. Thus, constraining the diagenetic history of carbonate successions within the context of their wider tectonic evolution is important for the prediction of the spatial and temporal distribution of reservoir quality.

Palaeo-environmental significance of fibrous carbonate cement in Marinoan cap carbonates

Marinoan cap carbonates are the first sedimentary rocks deposited after the Marinoan Snowball Earth glaciation and likely record the marine geochemical signals across the Cryogenian-Ediacaran boundary. The nature of the parent fluids, the possible precursor mineralogies and the dolomitisation models of these carbonates are debated to the present day. Cap carbonates and their fibrous dolomite cement samples from the Jiulongwan and Huajipo sections in South China were analysed for their petrography and geochemistry. Data obtained shed light on the deposition and alteration history of cap carbonates. Two types of fibrous dolomite cement precipitated penecontemporaneously with platform carbonate sediment deposition: (i) isopachous fascicular-optic dolomite fabrics with a characteristic botryoidal habit and mottled cathodoluminescence colour; they likely formed via an aragonitic precursor; and (ii) radial-slow dolomite cement, representing a direct marine precipitate. Growth zones are recognised under the SEM and visible in alternating dull- and bright-red zones under the cathodoluminescence microscope. Basal cap carbonates (Ce anomalies: 0.85 ± 0.11; Mn: 3324.3 ± 558.2 ppm) and the base (Ce anomalies: 0.83 ± 0.22; Mn: 2349.2 ± 1276.4 ppm) of fascicular-optic dolomite cement from platform facies indicate anoxic marine seawater with high Mn concentrations. Based on the geochemical data of fibrous dolomite and blocky calcite cement, the marine porewater evolved from the Mn reduction zone (Ce anomalies: 1.04 ± 0.26) to the sulphate reduction zone (Ce anomalies: 1.00 ± 0.48) and the Fe reduction zone (Ce anomaly: 1.40). In situ trace and rare earth elemental contents and the transition from a dull to bright fluorescence colour along the fibrous dolomite transects point to the former activity of microbial sulphate reduction and the degradation of organic matter. These two factors likely facilitated the formation of cap dolostone and multiphase dolomite cement crusts in a marine seawater (porewater) environment. Data shown here are significant for those concerned with the palaeoenvironmental archive and the dolomitisation process of Marinoan cap dolostones.

Constraining the diagenesis of the Puga cap carbonate from U–Pb in‐situ dating of seafloor crystal fans, southern Amazonian craton, Brazil

AbstractCap carbonates overlying Marinoan (635 Ma) glacial deposits worldwide record extreme paleoceanographic changes that may have triggered the rise of macroscopic life. However, the lack of geochronological constraints on these successions hinders reconciling climate and environmental change. Their upper part is marked by anomalous aragonite precipitation associated with large transgression and oxygenation of the post‐glacial ocean. Neomorphism of these deposits to low‐Mg calcite often includes centimetre‐scale seafloor crystal fans. We used in‐situ U–Pb dating and C and Sr isotopic analyses in the Puga cap carbonate to estimate the nature and timing of the neomorphism process. An age of 623 ± 3 [9] Ma (δ13C ~ −6‰ and 87Sr/87Sr ~ 0.7071–0.7075) for a blocky calcite cement is consistent with solution‐reprecipitation within the fans during final filling and diagenetic stabilization of the cap carbonate fabric. This result improves previous cap carbonate direct dating, providing a rare Lower Ediacaran chronostratigraphic reference.

Diagenesis and the effects of cataclastic deformation on the Permo-Triassic New Red Sandstone, Isle of Arran, Scotland

Diagenesis in the Permo-Triassic New Red Sandstone, Isle of Arran is characterized by early cementation of hematite, clay, and calcite minerals, followed by burial compaction, quartz, feldspar, and pyrite cementation. Cataclasis post-dated the quartz and feldspar cementation and reduced the grain and pore aperture size in deformed samples. Samples with cataclastic bands typically have 18% porosity and 8.81 mD permeability on average. Whereas, undeformed samples have an average porosity of 22% and an average permeability of 381 mD. Cataclasis was not as important as diagenesis in controlling sandstone porosity and permeability. However, cataclasis resulted in lower porosity and very poor to medium permeability in deformed samples. Cataclastic bands compartmentalize reservoir sands and cause a high heterogeneity in undeformed porous sandstones. Poikilotopic and blocky calcite cement postdates early clay and hematite cement. In addition, burial quartz and feldspar overgrowths also postdate the early clay and hematite. However, the poikilotopic calcite fills in framework grains that have larger void volumes than the grain/grain contacts where quartz overgrowths are present. Cataclasis resulted in fracturing of quartz and feldspar overgrowths. Therefore, the cataclasis occurred after the development of quartz and feldspar cementation. Dissolution postdated the formation of authigenic feldspar and pyrite formation resulted from hematite reduction. The distributions of grain and pore sizes against cumulative mercury volumes in studied samples shows a high level of reduction of grain and pore aperture sizes for deformed samples from single-cataclastic and multi-cataclastic bands. The distribution of apex volumes illustrates that the effective mercury porosity of the multi-cataclastic band sample may be reduced up to > 2 times in comparison to undeformed samples. However, the sample of a thin single cataclastic band has only a slightly lower apex volume in comparison to the host sample.

Porosity Evolution and Microfacies Analysis of the Khurmala Formation (Paleocene-Early Eocene) from Selected Sections in Kurdistan Region, Northern Iraq Revealed by Cathodoluminescence Spectroscopy

The microfacies analysis and porosity evolution of the Khurmala Formation (Paleocene-Early Eocene) were studied in the Gomaspan and Sheraswar sections in the High Folded Zone, northeastern Erbil City, Kurdistan Region (N-Iraq). Lithologically, the Khurmala Formation in the Gomaspan section is about 23.5 m comprised of thick to massive bedded brecciated grey dolomitic limestone in lower and middle parts and interbedding of medium to thick beds of grey dolomitic limestone and yellow calcareous shale in the upper part. The Sheraswar section comprises 18.5 m yellow marl, thick yellowish-grey dolomitic marly limestone in the lower part, and bedded grey limestone interbedded with thin blue marl, shale, and mudstone in the upper part. The petrographic and cathodoluminescence (CL) studies of 20 thin sections of the Khurmala carbonates show that most skeletal grains are shallow marine derivative faunas and non-skeletal grains represent intraclasts, peloids, ooids, and extraclasts (monocrystalline quartz). Five main microfacies were identified in the studied rocks, and they integrated into two facies associations relating to their environmental interpretation: back reef/lagoon and patch reef. A new eight pore types were distinguished in the carbonate rocks of Khurmala Formation: interparticle, intraparticle, growth framework, intercrystalline, moldic, vuggy, fracture, and stylolite porosities. Most of the primary porosity was reduced by compaction and cementation due to effect of intensive diagenesis. Secondary porosity is predominant in both studied sections, the most common being the moldic porosity associated with different microfacies fabrics. CL investigations revealed that dolomitized lime wackestone microfacies has two types of different cementations in the late stages of blocky calcite cement which are discontinuous light to dull orange luminescence without zoning, and pure calcite cement of coarse to very coarse blocky calcite cement with characteristic bright orange luminescence and tight zoning.

Diagenetic characterization and the influence on the porosity properties of a mixed bioclastic-siliciclastic rift lacustrine deposit of the Sergipe-Alagoas Basin, northeastern Brazil

Bioclastic and siliciclastic deposits of the Morro do Chaves Formation, Early Cretaceous of the Sergipe-Alagoas Basin, northeastern Brazil, show depositional and diagenetic analogies with the Brazilian Pre-salt rift reservoirs. A detailed petrographic study carried out on 221 thin sections from four well-cores were used to define the stratigraphic and sedimentological controls of the diagenetic events on coquinas supported by cathodoluminescence and EDS-MEV analysis. The study of cement distribution allowed identification in high- and low-frequency cycles of the subaerial exposure zones and intervals with porosity and permeability enhancement. The eodiagenetic processes and products show a good correlation with the maximum regressive events marked in the lower carbonate-dominated sequence, by the occurrence of nodules, micritization, pseudomicrokarst, mechanical compaction, three types of calcite fringes, coarse calcite cement, and three dissolution events. This sequence shows less than 25% of terrigenous constituents, thus favoring a pervasive early calcite cement precipitation and consequently an occlusion of the porosity. The upper sequence displays a predominance of poikilotopic and blocky calcite cement related to the mesogenetic stage. This sequence shows a dominance of mixed carbonate and siliciclastic constituents, which reflects directly the reduced early cementation, and the higher porosity and permeability relatively to the lower sequence. This study on cement occurrence and distribution on mixed deposits and their relationships with sedimentological and stratigraphic controls adds new and useful data for helping in the understanding of the complex coquina successions of the analogous lacustrine bioclastic reservoirs of the Pre-salt interval. The diagenetic model can support further studies regarding to predictive models of porosity and permeability distribution in other bioclastic lacustrine reservoirs.

Multistage fracturing history in the Paleocene lacustrine shale oil reservoirs of the Subei Basin, Eastern China

This study examines sealed fractures in core samples from the Paleocene organic-rich lacustrine shales of the Gaoyou Depression, Subei Basin, China, using a multi-pronged approach to elucidate the interplay between fracture propagation and fluid circulation in shaly successions. Based on petrography, geochemistry, and geochronology of fracture-filling minerals, four generations of vein emplacement (denoted Types A-D) were identified. These were attributed to successive episodes of burial diagenesis, tectonism, and uplift occurring at different phases of basin evolution. These vein generations were then inserted into the temporal-spatial framework derived from basin modeling, leading to a reconstruction of the fracture diagenesis history in the Funing shales. Two of the vein generations contain evidence for two phases of oil migration marked by two types of oil inclusions with different fluorescence.It has been shown that the studied lacustrine shales were deposited at ca. 60.2–58.0 Ma. The earliest vein generation, represented by antitaxial beef calcite veins sub-perpendicular to shale bedding, propagated in incompletely consolidated shales within a regional thrust regime related to the Wubao Movement. The vein nucleation during a short-lived burial following initial deposition was also evidenced by the presence of monoliquid inclusions in the veins and calcite LA-ICP-MS U–Pb age (57.7 ± 3.7 Ma). Tectonic compressive stress together with overpressures arising from horizontal and vertical disequilibrium compaction were interpreted as being responsible for this fracturing episode. The second vein generation is bedding-parallel, oil inclusion-rich stretching calcite veins with a crack-seal texture, formed concurrently with kerogen maturation and early oil generation at moderate burial depths. The vein crack-seal fills containing yellow fluorescing oil inclusions provide evidence for episodic oil expulsion in response to cyclic rises and drops in pore pressure during the oil window. Bedding-parallel beef calcite veins characterized by antitaxial growth represent the next vein generation, emplaced during the maximum burial period (ca. 41.9 ± 2.7 Ma). The vein fibers have tracked the horizontal displacement of the wall rocks, likely related to lateral shortening before or at the initiation of Late Eocene basin inversion. The first three vein generations were generated in a relative closed fluid system, with vein fill sourced locally. Host-rock carbonate components and/or earlier diagenetic carbonates were dissolved and then mobilized into these veins. By contrast, the last vein generation with blocky calcite and barite cementation is associated with input of externally derived hydrothermal fluids. The evidence comes from abnormally high fluid-inclusion homogenization temperature (Th) and salinity data, as well as the fact that Ba and sulfate were sourced from the underlying evaporite layers rather than local shales. We thus infer that cross-stratal faulting during the uplift related to the Sanduo Movement resulted in the opening of initial hydraulically closed fluid system. This episode of tectonically-driven hydrothermal circulation from deeper locations in the basin into the Funing shales led to the mixing of oil-bearing pore fluids with exotic basinal hot brines. This also accounts for the presence of abundant blue-green fluorescing oil inclusions and significantly elevated fluid salinity. Overall, this study emphasizes that during basin evolution, differences in tectonic settings and the diagenetic boundary conditions under which veins formed results in different fracturing mechanisms and associated fluid-flow regimes in shale sequences, with consequent effects on vein attributes and mineralization.

LA-ICP-MS U-Pb dating and geochemical characterization of oil inclusion-bearing calcite cements: Constraints on primary oil migration in lacustrine mudstone source rocks

Abstract To date, few isotope age constraints on primary oil migration have been reported. Here we present U-Pb dating and characterization of two fracture-filling, oil inclusion-bearing calcite veins hosted in the Paleocene siliciclastic mudstone source rocks in Subei Basin, China. Deposition age of the mudstone formation was estimated to be ca. 60.2–58.0 Ma. The first vein consists of two major phases: a microcrystalline-granular (MG) calcite phase, and a blocky calcite phase, each showing distinctive petrographic features, rare earth element patterns, and carbon and oxygen isotope compositions. The early MG phase resulted from local mobilization of host carbonates, likely associated with disequilibrium compaction over-pressuring or tectonic extension, whereas the late-filling blocky calcite phase was derived from overpressured oil-bearing fluids with enhanced fluid-rock interactions. Vein texture and fluorescence characteristics reveal at least two oil expulsion events, the former represented by multiple bitumen veinlets postdating the MG calcite generation, and the latter marked by blue-fluorescing primary oil inclusions synchronous with the blocky calcite cementation. The MG calcite yields a laser ablation–inductively coupled plasma–mass spectrometry U-Pb age of 55.6 ± 1.4 Ma, constraining the earliest timing of the early oil migration event. The blocky calcite gives a younger U-Pb age of 47.8 ± 2.3 Ma, analytically indistinguishable from the U-Pb age of 46.5 ± 1.7 Ma yielded by the second calcite vein. These two ages define the time of the late oil migration event, agreeing well with the age estimate of 49.7–45.2 Ma inferred from fluid-inclusion homogenization temperature and published burial models. Thermodynamic modeling shows that the oil inclusions were trapped at ~27.0–40.9 MPa, exceeding corresponding hydrostatic pressures (23.1–26.7 MPa), confirming mild-moderate overpressure created by oil generation-expulsion. This integrated study combining carbonate U-Pb dating and fluid-inclusion characterization provides a new approach for reconstructing pressure-temperature-composition-time points in petroleum systems.

Diagenetic controls on the elastic velocity of the early Triassic Upper Khartam Member (Khuff Formation, central Saudi Arabia)

The Permo-Triassic intervals of the Arabian Plate host the most important gas reservoirs in the world, the Khuff Formation and its equivalents. Understanding the main controls on the elastic properties of the Khuff carbonates is crucial for hydrocarbon exploration and quantitative interpretation of sonic and seismic data. In total, 88 samples were collected from the Upper Khartam Member of the Khuff Formation (central Saudi Arabia) to investigate the controls and variations in petrophysical properties and the associated acoustic velocities. Samples were analyzed using optical microscopy, XRD, and SEM to identify their texture, mineralogy and pore types. Porosity, permeability, and compressional and shear wave velocities were measured for all samples. A significant scatter is observed in the velocity-porosity relationship of the studied samples. Depositional environment, lithofacies and minerology show no clear impact on such observed scatter. Our results show that variations in pore types, predominantly of diagenetic origin, are the main factor controlling variations of velocity at any given porosity. Samples characterized by vuggy and non-cemented moldic porosity have a higher velocity when compared to samples characterized by infilled molds, interparticle and microporosity. The impact of pore types on velocity is investigated by means of velocity-porosity trajectories obtained by the differential effective medium (DEM) modeling, which incorporates variable equivalent pore aspect ratio (EPAR) values. Three main diagenetic processes, all being non-EPAR-preserving, contribute to the evolution of velocity and porosity: 1) dolomitization creating connected pore system between the euhedral dolomite rhombs which increases permeability (best reservoir unit) but decreases pore aspect ratio and thus velocity; 2) dissolution of microporous ooids and skeletal grains creating high EPAR molds and vugs at the expense of low EPAR micropores, 3) blocky calcite cementation that does not preserve the rounded shape of open oomolds reducing EPAR from more than 0.4 to less than 0.1 for extensively infilled molds. The established impedance-porosity trajectories in this study suggest that elastic properties can provide important insights about diagenetic overprints, pore types and reservoir quality in Khuff carbonates.

Microfacies and diagenetic overprints in the limestones of Middle Jurassic Fort Member (Jaisalmer Formation), Western Rajasthan, India: Implications for the depositional environment, cyclicity, and reservoir quality

The present study investigates the microfacies and the effects of diagenetic processes on the reservoir quality of limestones of the Fort Member, Jaisalmer Formation, Western Rajasthan. The study interpolates an integrated approach including field studies, petrography, staining, and X‐ray diffraction techniques. The Fort Member is composed of medium‐ to thick‐bedded, nodular highly fossiliferous limestones, which are fine‐ to medium‐grained, poorly‐ to very well‐sorted, and comprise angular‐ to well‐rounded grains of low to high sphericity. These limestones contain abundant skeletal (bivalves, brachiopods, echinoderms, foraminifera, algae, gastropods, sponge, coral, and calcispheres) and non‐skeletal grains (intraclasts, ooids, and peloids). On the basis of detailed petrographic investigations, eight microfacies have been recognized including: bioclastic wackestone, whole‐fossil wackestone, bioclastic lithoclastic packstone, aggregate grainstone, peloidal grainstone, ooidal grainstone, bioclastic grainstone, and coated bioclastic grainstone. Based on the microfacies analysis, the limestones of the Fort Member are inferred to have been deposited in shoal, lagoon, and open marine environments. The diagenetic overprints of the Fort Member limestones is characterized by the several diagenetic features such as micritization, cementation, dolomitization, compaction, dissolution, silicification, neomorphism (aragonite to calcite transformation and development of microspar), and secondary porosity. Such main diagenetic features affecting the carbonates are developed in marine phreatic, meteoric phreatic, mixed meteoric phreatic and burial diagenetic environments. Micritization of allochems, dolomitization of lime muds, and cementation by isopachous calcite, granular calcite cement, neomorphism, and recrystallization of bioclasts are developed in meteoric‐phreatic diagenesis. The meteoric‐vadose diagenesis led to prevalence of dissolution process and sparry calcite infilling the fractures. Dolomitization, blocky calcite cement, and silicification processes were developed during the burial diagenesis. These diagenetic processes have direct effects on the modification of both intergranular and intragranular/crystalline porosity. In this manner, it is found that the calcite cementation, micritization, and compaction were responsible for the reduction of the pore spaces, whereas dolomitization, dissolution, formation of moldic pores, and fracturing increased the pore volumes. Therefore, the enhanced pore volumes improved the carbonate reservoir characteristics, suggesting the limestones of the Fort Member to be a better carbonate reservoir for hydrocarbon accumulation.

Insight into Heterogeneous Calcite Cementation of Turbidite Channel-Fills from UAV Photogrammetry

Diagenesis is a key controlling factor on sandstone porosity and permeability. Understanding type, paragenetic sequence and spatial patterns of cements is thus important for assessing sandstone hydrocarbon reservoir properties. In this study Unmanned Aerial Vehicle (UAV) photogrammetry is used to evaluate the shape and spatial distribution of calcite concretions developed within the sand-prone fill of a turbidite channel. The studied channel-fill is entrenched into hemipelagic marlstones and include a lower conglomeratic sandstone loaded with marlstone rip-ups and an upper fill featuring a range of turbidite bed types, which, up-section and off the channel axis, are progressively finer grained and less amalgamated. Concretion shape analysis highlighted a continuum of equant to oblate shapes with flat-lying major axes and a cumulative volume fraction of ca. 22%. Equant to sub-equant concretions are ubiquitous and occur at different heights within beds, often developing around marlstone rip-ups. Conversely, elongated concretions are either strata-bound concretions or completely cemented beds which become volumetrically dominant up section and off the channel axis. The interparticle pore-space of concretions represents on average ca. 22% and is tightly filled by poikilotopic and blocky calcite cement precipitated near to maximum burial depth, whereas host sandstones lack calcite cements and show smectite clay cement and an average preserved porosity of ca. 15%. The oxygen and carbon isotopes of calcite cements point to the marlstone as the main source of carbonate ions, suggesting concretions developed during burial by either diffusion from rip-ups and mud caps or recrystallization of, matrix micrite. Results suggest that the process by which the carbonate-rich component was eroded from the substrate and trapped within the channel-fill is a key control on spatial distribution of calcite concretions, likely to reflect on spatial variability of reservoir properties.

Fluid compartmentalization of Devonian and Mississippian dolostones, Western Canada Sedimentary Basin: petrologic and geochemical evidence from fracture mineralization

Integrated petrographic, geochemical, and fluid inclusion study of fracture mineralization and associated host rock in selected Mississippian and Devonian carbonates extending from southeastern Alberta to northwestern British Columbia, Canada, aims to quantify the type and nature of fluid precipitated saddle dolomite and late calcite cement and their origin. Petrographic and isotopic evidence from both the Devonian and Mississippian fracture-filling carbonates indicate the presence of a hydrothermal fluid source. The δ18O isotopic values for the Devonian saddle dolomite (−14.62‰ to −3.75‰ VPDB, average −11.12‰) combined with enriched 87Sr/86Sr isotopic ratios (0.70827–0.71599, average 0.71006) and higher homogenization temperatures (Th = 74–194.6 °C, average 126.8 °C) and salinity values (7.7–26.6 wt.% NaCl, average 16.2 wt.% NaCl) show significant differences from the Mississippian saddle dolomite, which is characterized by less negative δ18O isotopic values (−12.53‰ to −7.82‰ VPDB, average −9.14‰), less radiogenic 87Sr/86Sr isotopic ratios (0.70859–0.70943, average 0.70887), and lower homogenization temperatures (Th) and salinity values of fluid inclusions (87.6–214.2 °C, average 136.3 °C; 2.0–13.2 wt.% NaCl, average 9.6 wt.% NaCl). Later fracture- and vug-rimming blocky calcite cement records comparable or slightly lower values of δ18O (−16.31‰ to −4.08‰ VPDB, average −9.76‰) and 87Sr/86Sr (0.70784–0.709743, average 0.70868) and much lower salinity values (0–22.5 wt.% NaCl, average 2.86 wt.% NaCl) for samples mostly from the Mississippian age group. These results possibly suggest two different hydrothermal episodes related to early (Antler) and late (Laramide) tectonic events that affected the Western Canada Sedimentary Basin with possible compartmentalization of hydrothermal systems and their associated brines in the basin.

Dolomitization of the Upper Jurassic carbonate rocks in the Geneva Basin, Switzerland and France

The Upper Jurassic carbonates represent important potential targeted reservoirs for geothermal energy in the Geneva Basin (Switzerland and France). Horizons affected by dolomitization, the focus of the present study, are of particular interest because they proved to be productive in time-equivalent deposits currently exploited in Southern Germany. The study is based on sub-surface samples and outcrops in the Geneva Basin. Petrographic analyses allowed to constrain the paragenesis of the Upper Jurassic units prior to discussing the cause(s) and effect(s) of dolomitization. Data reveal that the facies are affected by early and late diagenesis. All samples show at least two stages of burial blocky calcite cementation with the exception of those from the sub-surface, which display an incomplete burial blocky cementation preserving primary intercrystalline porosity. Dolomitization affected all units. The results point to an early dolomitization event, under the form of replacement dolomite. Dedolomitization, through calcitization and/or dissolution, is an important process, creating secondary pore space. Results of the present study favor a reflux model for dolomitization rather than the mixing-zone model suggested in earlier work. However, considering the geodynamic context, other dolomitization models cannot be excluded for the subsurface. The presence of secondary pore space might contribute to the connectivity of the porous network providing enhanced reservoir properties. These results are a first step towards a better understanding of the diagenetic history of the Upper Jurassic in the Geneva Basin. Moreover, it provides a reasonable framework for further geochemical analyses to constrain the nature and timing of fluid migration. The paragenesis and the dolomitization model hold the potential to help in ongoing exploration for geothermal energy beyond the Geneva Basin.

Diagenetic history and reservoir properties of the Cenomanian-Turonian carbonates in southwestern Iran and the Persian Gulf

Shallow-marine carbonates of the mid-Cretaceous Sarvak Formation are important reservoir rocks in southern Iran and the Persian Gulf region. These carbonates were deposited on the margin of the Arabian Plate and rest on the Kazhdumi Formation, which is one of the major hydrocarbon source rocks in the region. The top of the Sarvak Formation coincides with the regional Turonian unconformity. Most of the observed diagenetic features are genetically related to meteoric waters entering the Sarvak Formation during Cenomanian-Turonian and mid-Turonian uplift and the subsequent paleoexposure.Integration of field and petrographic studies and isotope geochemistry reveals the history of a variety of diagenetic processes, which include dissolution and development of secondary porosity which enhance reservoir properties of the Upper Sarvak carbonates. Various types of calcite cements were identified as the main cause for porosity loss in these carbonates. Their diagenetic environment is discussed using the geochemical data acquired as part of the present study.The δ18O and δ13C values (−12.3 to −0.6‰ and −5.8–3.6‰ VPDB, respectively) of the cements indicate precipitation from marine, meteoric and/or mixed meteoric-marine fluids. Some drusy calcite cements exhibit δ18O and δ13C values (−5.1 and 0.8‰ VPDB, respectively) and higher 87Sr/86Sr ratios, comparing to mid-Cretaceous carbonates, (i.e., 0.70747), which may indicate precipitation from meteoric waters. Lower δ18O and δ13C values (e.g. −5.1, 0.8‰ VPDB) combined with higher 87Sr/86Sr ratios (e.g., 0.70747) of some drusy calcite could confirm their precipitation from meteoric waters.Lower δ18O values of some blocky calcite cements (e.g., −12.3‰ VPDB) and matrix-replacive compression-related dolomites (i.e., −7.3 to −3.4‰ VPDB) suggest their precipitation at rising temperatures during burial.The range of δ13C values (−5.8–3.6‰ VPDB) suggests that the main source of carbon in the calcite cements was primarily marine mixed with isotopically more negative carbon from atmospheric/soil-derived CO2.

Activation of stylolites as conduits for overpressured fluid flow in dolomitized platform carbonates

Abstract This study investigates the Late Aptian–earliest Albian platform carbonates of the Benicàssim area (Maestrat Basin, Spain) in order to assess the relationship between bed-parallel stylolites and the flow of diagenetic fluids during dolomitization and subsequent hydrothermal alteration. Dolostones and burial dolomite and calcite cements were studied by a combination of field geology and standard petrographic and isotope analysis. Field data indicate that dolostones are closely associated with seismic-scale synsedimentary faults, preferentially replace grain-dominated facies and typically show wavy dolomitizing fronts that mostly correspond to bed-parallel stylolites. The dolostones are corroded and contain bed-parallel pores that are filled with hydrothermal saddle dolomite and blocky calcite cements. This late calcite cement frequently engulfs clasts of the host dolostones, suggesting that hydraulic brecciation likely associated with overpressured fluid occurred. Results indicate that stylolites play a key role in the distribution of dolostones and subsequent hydrothermal mineralization. During the replacement stage, stylolites acted as baffles for the dolomitzing fluids controlling lateral fluid flow and resulting in the stratabound dolostone distribution. During the post-dolomitization stage, stylolites became preferred pathways for overpressured hydrothermal corrosive and mineralizing fluids that likely came from the underlying basement, and increased bed-parallel stylolitic porosity and probably also permeability.

High resolution ion microprobe investigation of the δ18O of carbonate cements (Jurassic, Paris Basin, France): New insights and pending questions

The scope of this work is to investigate, at a high resolution, the oxygen isotope composition (δ18Ocarb) of diagenetic products (synsedimentary and burial calcite cements) in shallow-marine carbonates. SIMS (Secondary Ion Mass Spectrometry) microprobe analyses were performed on thin sections from Oxfordian and Kimmeridgian Formations of the eastern Paris Basin and compared to data obtained on the same diagenetic products by conventional mass spectrometry (acid digestion). Hereby obtained, δ18O are similar, but the SIMS dataset displays a larger range of values.The isotopic zonation obtained by SIMS transects through sequences of cements filling pores, reveals an (expected) isotopic depletion from older stage synsedimentary calcites to younger stage blocky calcites and that follows the CL (cathodoluminescence) zonation. SIMS analyses however show that synsedimentary cements precipitated in intra-skeletal pores, have heavier δ18O than their inter-particle counterparts, with an offset of +4‰V-PDB, despite similar petrographical characteristics. This difference is maintained in the δ18O of the first stages of blocky calcite cements, intra-skeletal blocky calcites showing heavier δ18O than the time equivalent and petrographically identical inter-particle calcites, with an offset of +5‰V-PDB. These offsets are tentatively explained by the precipitation of cements under non-equilibrium conditions in intra-skeletal pores, where organic matter decay may have played a key role, acting notably on the pH.The occurrence of isolated micro-diagenetic environments, co-existing at the thin section scale, is tentatively proposed as an explanation to these small scale and high amplitude δ18O heterogeneities. These results may question the sampling strategy for future works. Microdrilling may miss the observed range of variation, but averaging the values may not necessarily lead to real misinterpretations if a critical selection of samples is performed, targeting potentially similar micro-diagenetic environments and avoiding potentially specific ones, i.e. closed intra-skeletal pores.These results also definitely underscore the need for additional experiments to improve the reliability of SIMS, in order to develop the use of this very high resolution technique for carbonate diagenesis studies.

Estimating the impact of early diagenesis on isotope records in shallow-marine carbonates: A case study from the Urgonian Platform in western Swiss Jura

The poor preservation of sedimentological and geochemical evidence often hampers the identification of subaerial exposure surfaces in carbonate platform successions. Such discontinuities may represent significant time gaps, justifying the quest for diagnostic indications. We investigated a lower Hauterivian to upper Barremian sedimentary succession from Switzerland to highlight the stratigraphic impact of meteoric diagenesis on platform carbonates. Petrographic observations revealed the presence of five generations of calcitic cements. Spot measurement of carbon and oxygen stable isotope compositions (δ13C and δ18O values, respectively) measurements relate them to specific diagenetic environments: selected blocky calcite cements exhibit an enrichment in light isotopes (12C, 16O), indicative of meteoric eogenesis, whereas very negative δ18O values link the last phase of cementation to mesogenesis. Meteoric calcitic cements formed during karstification of the top of the upper Barremian succession; burial diagenetic phases overlap these eogenetic phases, which thus cannot be related to recent telogenesis. We estimated the ratio of early meteoric versus burial cements within the studied succession: its stratigraphic repartition reveals that lower Aptian eogenesis influenced the geochemistry of platform carbonates as deep as 45m below the exposure surface. In this interval, negative whole-rock δ13C values do not reflect contemporaneous variations of the δ13C curves documented in sections devoid of strong diagenetic impact. Such an impact is a function of the amount of meteoric cement filling pore space as well as of the primary carbon isotope composition of the carbonate sediments and of the meteoric cement. The vertical influence of a karst event is not restricted to the first metres directly below the exposure surface. Although meteoric diagenesis may not significantly alter microfacies, it may strongly perturb isotope systems. Building onto this case study, the application of δ13C chemostratigraphy to platform carbonates can only be performed with great caution and after a careful examination of diagenetic features.

Saddle dolomite and calcite cements as records of fluid flow during basin evolution: Paleogene carbonates, United Arab Emirates

Field observations indicate that tectonic compression, anticline formation and concomitant uplift events of marine Paleogene carbonates in eastern United Arab Emirates, which are related to the Zagros Orogeny, have induced brecciation, karstification, and carbonate cementation in vugs and along faults and fractures. Structural analysis, stable isotopes and fluid inclusion microthermometry are used to constrain the origin and geochemical evolution of the fluids. Fluid flow was related to two tectonic deformation phases. Initially, the flux of moderately 87Sr-rich basinal NaCl–MgCl2–H2O brines along reactivated deep-seated strike-slip faults have resulted in the precipitation of saddle dolomite in fractures and vugs and in dolomitization of host Eocene limestones (δ18OV-PDB −15.8‰ to −6.2‰; homogenization temperatures of 80–115 °C and salinity of 18–25 wt.% eq. NaCl). Subsequently, compression and uplift of the anticline was associated with incursion of meteoric waters and mixing with the basinal brines, which resulted in the precipitation of blocky calcite cement (δ18OV-PDB −22‰ to −12‰; homogenization temperatures of 60–90 °C and salinity of 4.5–9 wt.% eq. NaCl). Saddle dolomite and surrounding blocky calcite have precipitated along the pre- and syn-folding E–W fracture system and its conjugate fracture sets. The stable isotopes coupled with fluid-inclusion micro-thermometry (homogenization temperatures of ≤50 °C and salinity of <1.5 wt.% eq. NaCl) of later prismatic/dogtooth and fibrous calcites, which occurred primarily along the post-folding NNE–SSW fracture system and its conjugate fracture sets, suggest cementation by descending moderately 87Sr-rich, cool meteoric waters. This carbonate cementation history explains the presence of two correlation trends between the δ18OV-PDB and δ13CV-PDB values: (i) a negative temperature-dependent oxygen isotope fractionation trend related to burial diagenesis and to the flux of basinal brines, and (ii) positive brine-meteoric mixing trend. This integrated study approach allows better understanding of changes in fluid composition and circulation pattern during evolution of foreland basins.

Diagenetic evolution of the upper Cretaceous limestone and sandstone exhumed reservoirs in the western Basque–Cantabrian Basin, North Spain

The Basque–Cantabrian Basin (NE Spain) has been considered one of the most interesting areas for hydrocarbon exploration in the Iberian Peninsula since the 60th to 70th of last century. This basin is characterized by the presence of numerous outcrops of tar sands closely associated with fractures and Triassic diapirs. The aims of this work is to establish the diagenetic evolution of the Upper Cretaceous reservoir rocks with special emphasis in the emplacement of oil and their impact on reservoir quality. The studied rocks are constituted of carbonates and sandstones that contain massive quantities of bitumen filling vugs and fractures.Petrographic results indicate that the carbonate rocks from Maestu outcrops are bioclastic grainstones and wackestones, whereas the tar sandstones from Atauri and Loza outcrops are dominated by quartzarenites and subordinated subarkoses. The paragenetic sequence of the main diagenetic phases and processes include, pyrite, bladed and drusy calcite cement, calcite overgrowths, silicification of bioclasts and microcrystalline rhombic dolomite cement, and first stage of oil emplacement, blocky calcite cement, coarse crystalline calcite cement, calcitized dolomite, calcite veins, saddle dolomite and stylolites filled by the second phase of oil entrance. Together with the above mentioned diagenetic alterations, the arenites are affected by early kaolinitization of feldspars and the scarce formation of clay rim and epimatrix of illite. All sandstones and dolomitized carbonate rocks show high intercrystalline and intergranular porosity which is full by biodegraded hydrocarbons (solid bitumen). The biodegradation affects alkanes, isoprenoids and partially hopanes and steranes saturated hydrocarbons. Aromatics hydrocarbons, like naphthalenes, phenanthrenes, dibenzothiophenes and triaromatics are also affected by biodegradation. Results indicate that the first HC emplacement corresponds to early stage of calcite and dolomite cementation, and the second and more important emplacement is related to fracturation processes resulting in the formation of excellent reservoirs.