Microcrystalline Calcite Research Articles

Diagenetic features and porosity dense evolution of Chang 8 tight sandstone reservoir in Hujianshan area, Ordos Basin

Diagenesis is the main reason for tight reservoir lithology, complex pore-throat structure, strong heterogeneity, and variable occurrence and distribution characteristics of fluid in the pore-throat, which directly restricts the Chang 8 oil layer group in the Ordos Basin. Exploration and development. Based on the analysis of a large number of cores and thin slices, through the application of scanning electron microscopy, high-pressure mercury intrusion and other experimental testing methods, the diagenesis characteristics of the Chang 8 tight reservoir in the Hujianshan area of the Ordos Basin and the mechanism and process of tight porosity evolution are analyzed in depth. The relationship between different diagenetic facies belts, pore structures and their impact on reservoir quality is further explored. The results show that the Chang 8 reservoir in the Hujianshan area is in the mid-diagenetic stage A, which mainly experienced compaction-pressure dissolution, cementation, dissolution and other diagenesis. The compaction caused the loss of a large number of primary pores in the sandstone, which is the most important factor for the densification of the Chang 8 reservoir in the study area, followed by cementation of carbonate and clay minerals. The chlorite clay film formed in the early diagenesis retained the primary particles. The inter-pores maintain good reservoir physical properties, and are affected by the acidic diagenetic environment. The unstable feldspar minerals dissolve. The dissolution pores produced effectively improve the connectivity and permeability of the pores. The compaction reduces the porosity on average by 19.37% and the loss rate reaches 51.86%. The cementation reduces the porosity on average by 11.97% and the loss rate reaches 32.66%. The dissolution increases the porosity by 3.26% on average. In the process of pore evolution, the compaction of the early diagenetic stage A and the authigenic clay minerals and carbonate cementation in the middle diagenetic stage are the main reasons for the substantial decrease in porosity. The early diagenetic stage B is due to its short duration. Less porosity is lost. The secondary pores generated in the early diagenetic stage A are basically filled with microcrystalline calcite, while the organic acid dissolution in the middle diagenetic stage is limited. According to different diagenetic minerals and pore evolution characteristics, combined with logging response characteristics, the whole area is divided into four diagenetic facies. Among them, the residual chlorite-membrane intergranular-pore facies and the feldspar dissolution facies of chlorite film are the most favorable diagenetic facies. The findings of this study can help for better understanding of the diagenetic characteristics and quantitative evolution of pores, reveal the diagenetic-pore evolution rules of Chang 8 sandstone reservoirs in the study area, and provide a basis for reservoir evaluation and prediction in the study area.

Effects of Diagenesis on the Reservoir Quality in the Upper Sands of Lower Goru Formation, Badin Block, Lower Indus Basin, Pakistan

The core samples of B member of upper sand reservoir rocks of the lower Goru Formation from three wellsof the Badin block were studied using thin section, XRD, and SEM techniques to investigate the diagenetic trends andtheir effect on reservoir quality. Microscopic study indicates that the B sand unit is mature with quartz as thepredominant mineral constituent with variable amount of feldspar and lithic minerals. The QFL plot indicates that mostof the samples are plotted in the field of quartz arenite, sub-litharenite and sub-arkose respectively. A few samplesbelong to the category of litharenite and feldspathic litharenite. The feldspars were partially to completely altered tokaolinite and other clay minerals. Coarse-crystalline or micro-crystalline calcite is the predominant cementing material.Bulk rock XRD analysis also confirms that the main mineral constituents of there samples are quartz and calcite invariable proportions. The undulose extinction and fracturing of quartz grains indicate that the area remained understress. Moreover, such fracturing is post-depositional and therefore is the product of late diagenesis. Scanning ElectronMicroscopic (SEM) images at 50 micrometer (μm) size show irregular type of fracturing within the quartz grains. Thislate stage fracturing of quartz has also generated various types of channels which may serve as secondary porosity. Thequartz overgrowth was observed in some samples due to late stage diagenesis. The micro-crystalline cement in the formof calcite is mostly present within the pores in fractured quartz. The results indicate that the diagenesis played asignificant role in improving the reservoir characteristics of B sand by increasing the porosity due to quartz fracturing,feldspar dissolution, alteration and quartz corrosion. Hence, better understanding of reservoir heterogeneities inporosity through diagenetic studies can be helpful in evaluating potential reservoir horizons for hydrocarbonaccumulation on spatial and temporal scales.

The origins of carbonate minerals of a source-controlled lacustrine carbonate succession in the Shulu sag, Bohai Bay Basin: Implications for porosity development and paleoenvironment

Lacustrine carbonates are widely distributed in the third member of the Eocene Shahejie Formation (Es3L) of the Shulu sag and other rift basins in the Bohai Bay Basin, which can act as reservoirs for hydrocarbon and provide insights into paleoenvironment. However, the origins of lacustrine carbonate deposition and their implications for porosity development and paleoenvironment are not well understood. Based on micropetrological characteristics, X-ray diffraction (XRD), stable carbon and oxygen isotope analyses, and organic geochemistry data, four types of calcite (calcite debris; calcareous biological detritus; micrite in fine-grained marlstone; fibrous sparry calcite) and two types of dolomite (dolomite debris; rhombic euhedral dolomite) with distinctly different characteristics and origins are identified. The δ13C and δ18O values of calcite and dolomite debris are consistent with that of the Cambrian-Ordovician carbonate rock in the provenance area, suggesting that the carbonate debris was sourced from the surrounding Cambrian-Ordovician carbonate bedrock. Microcrystalline calcites are commonly developed in organic-rich marlstones with a large number of algae. The δ13C values of the microcrystalline calcites are higher than that of carbonate debris and the positive excursion of the δ13C value is along with the increase of TOC content. These indicate that origin of the microcrystalline calcite was authigenic calcite and was formed by biochemical process. Bedding-parallel fibrous sparry calcite occurs in intervals with high TOC contents and within the thermally mature zone of hydrocarbon generation, indicating that oil generation should be responsible for the sparry calcite formation. Rhombic euhedral dolomite commonly occurs as isolated micro-crystals with euhedral to subhedral rhombs shape. Micro-sized spherule-like dolomite in the early stage of crystal growth were also present. Rhombic euhedral dolomite are commonly associated with organic matter and developed in organic-matter rich intervals with plenty of fossilized bacterial filaments and framboidal pyrite. These suggest that rhombic euhedral dolomite were probably microbially mediated dolomite. Mineral components can affect reservoirs quality. The porosities of samples show a positive correlation with dolomite content and the porosities of fibrous sparry calcite-bearing intervals are higher than non-parry calcite-bearing intervals. The increase in authigenic microcrystalline calcite content reflects a rise in lake level and an increase in water depth and primitive productivity.

Origin of a global carbonate layer deposited in the aftermath of the Cretaceous-Paleogene boundary impact

Microcrystalline calcite (micrite) dominates the sedimentary record of the aftermath of the Cretaceous–Paleogene (K–Pg) impact at 31 sites globally, with records ranging from the deep ocean to the Chicxulub impact crater, over intervals ranging from a few centimeters to more than seventeen meters. This micrite-rich layer provides important information about the chemistry and biology of the oceans after the impact. Detailed high-resolution scanning electron microscopy demonstrates that the layer contains abundant calcite crystals in the micron size range with a variety of forms. Crystals are often constructed of delicate, oriented agglomerates of sub-micrometer mesocrystals indicative of rapid precipitation. We compare the form of crystals with natural and experimental calcite to shed light on their origin. Close to the crater, a significant part of the micrite may derive from the initial backreaction of CaO vaporized during impact. In more distal sites, simple interlocking rhombohedral crystals resemble calcite precipitated from solution. Globally, we found unique calcite crystals associated with fossilized extracellular materials that strikingly resemble calcite precipitated by various types of bacteria in natural and laboratory settings. The micrite-rich layer contains abundant bacterial and eukaryotic algal biomarkers and most likely represents global microbial blooms initiated within millennia of the K–Pg mass extinction. Cyanobacteria and non-haptophyte microalgae likely proliferated as dominant primary producers in cold immediate post-impact environments. As surface-water saturation state rose over the following millennia due to the loss of eukaryotic carbonate producers and continuing river input of alkalinity, “whitings” induced by cyanobacteria replaced calcareous nannoplankton as major carbonate producers. We postulate that the blooms grew in supersaturated surface waters as evidenced by crystals that resemble calcite precipitates from solution. The microbial biomass may have served as a food source enabling survival of a portion of the marine biota, ultimately including life on the deep seafloor. Although the dominance of cyanobacterial and algal photosynthesis would have weakened the biological pump, it still would have removed sufficient nutrients from surface waters thus conditioning the ocean for the recovery of biota at higher trophic levels.

Slope destabilization provoked by dissociation of gas hydrates in the Outer Carpathian basin during the Oligocene: Sedimentological, petrographic, isotopic and biostratigraphic record

Sedimentological, biostratigraphic, petrographic, and stable C and O isotope study was carried out on chaotic complexes hosting carbonate concretions in the uppermost Oligocene successions of the Outer Carpathians. These chaotic complexes reveal a range of sedimentary features indicative of intrabasinal, submarine mass-wasting deposition, including soft-sediment deformation, brecciation, homogenization and fluidization of the parent material. Apart from carbonate concretions, they enclose blocks of rocks having lithologies typical for the underlying Oligocene strata. The concretions exhibit extremely depleted δ13C (by down to −51.3‰) indicating anaerobic oxidation of methane, whereas their δ18O values are strongly enriched relative to coeval brackish basin water indicating dissociation of gas hydrates as the main fluid source. These data and the geotectonic context imply that the chaotic complexes formed as submarine slumps on a N-inclined paleoslope of the basin in consequence to tectonically-induced dissociation of gas hydrates. Hydrates were destabilized within the sediments due to synorogenic uplift, related to northward progradation of the accretionary prism. These slumps and hosted concretions were dated by the combination of calcareous nannoplankton and organic-walled dinoflagellate cysts. The age of concretions indicated by dinoflagellate cyst assemblages always overlaps with that of the surrounding slump matrix determined by both micropaleontological groups. Together with sedimentological and petrographic evidence, this confirms that the concretions formed in situ within the sediment shortly after deposition and prior to slumping. Hydrate dissociation, seepage of methane-charged fluids and the resultant slope destabilization was not an isolated event, but a continuous process occurring diachronously in front of N-prograding accretionary prism during Oligocene, at least from early NP23 to late NP25 biozones.The stable C and O isotope analysis of carbonate concretions was crucial for this research, because the low δ13C and high δ18O values were the only preserved indicators of anaerobic oxidation of methane and dissociation of gas hydrates in the basin, respectively. Because, the concretions are composed of a mixture of dolomite and calcite, a semiautomatic method for measuring C and O isotope composition selectively for calcite and dolomite (Baudrand et al., 2012) was applied, which provided conclusive results. These measurements were combined with detailed petrographic investigations, which enabled assessment of isotopic composition for various carbonate constituents separately, e.g. sedimentary calcite (biogenic and detrital), early and late calcite cements, dolomite cement. They showed that the concretionary microcrystalline calcite was the main cement that precipitated due to hydrate dissociation and subsequent methane oxidation. This approach can provide valuable information when applied to other authigenic carbonates composed of a mixture of dolomite and calcite.

Experimental stabilization of carbonate sediments to calcite: Insights into the depositional and diagenetic controls on calcite microcrystal texture

Aragonite and high-magnesium calcite are abundant constituents of modern shallow marine carbonate sediments, but are rare in ancient carbonate rocks. Because these minerals are metastable, they tend to stabilize to microcrystalline calcite which is ubiquitous in most Phanerozoic marine and lacustrine limestones. Calcite microcrystals exhibit a range of textures that have been classified in terms of crystal size, morphology, and contact geometry. These textures have the potential to reveal information about the precursor sediments as well as the diagenetic conditions of stabilization, and thus may serve as a proxy for paleoenvironmental conditions. However, despite their ubiquity and importance, the origin of these textures is controversial. Hypotheses regarding the origin of calcite microcrystal textures typically involve inferences regarding the precursor depositional sediments, diagenetic conditions of stabilization, or diagenetic processes. Here, hundreds of laboratory experiments were conducted in which metastable carbonate sediments were stabilized to calcite. The goal of these experiments was to investigate the depositional and diagenetic controls on calcite microcrystal textures. The depositional variables investigated were precursor mineralogy, type, and size, and the diagenetic variables were temperature, fluid chemical composition, and fluid:solid ratio. Our results show that experimental stabilization of carbonate sediments produces calcite with a wide variety of textures depending on the various depositional and diagenetic parameters. These textures are remarkably similar to those commonly observed in the rock record. Calcite microcrystal morphology is controlled by fluid chemistry, fluid:solid ratio, and reactant type. Calcite microcrystal size is controlled by temperature, fluid chemistry, fluid:solid ratio, and reactant size. The degree to which calcite microcrystals are fitted together is controlled by reactant size and reactant microstructure. Our findings challenge most of the existing hypotheses regarding the origin of calcite microcrystal textures. We specifically show that textures that have been interpreted to indicate late diagenetic processes such as dissolution and cementation, may be the result of the early diagenetic processes of stabilization.

Effects of micrite microtextures on the elastic and petrophysical properties of carbonate reservoirs

Apparent differences in sedimentation and diagenesis exist between carbonate reservoirs in different areas and affect their petrophysical and elastic properties. To elucidate the relevant mechanism, we study and analyze the characteristics of rock microstructure and elastic properties of carbonates and their variation regularity using 89 carbonate samples from the different areas The results show that the overall variation regularities of the physical and elastic properties of the carbonate rocks are controlled by the microtextures of the microcrystalline calcite, whereas the traditional classification of rock- and pore- structures is no longer applicable. The micrite microtextures can be divided, with respect to their morphological features, into porous micrite, compact micrite, and tight micrite. As the micrites evolves from the first to the last type, crystal boundaries are observed with increasingly close coalescence, the micritic intercrystalline porosity and pore-throat radius gradually decrease; meanwhile, the rigidity of the calcite microcrystalline particle boundary and elastic homogeneity are enhanced. As a result, the seismic elastic characteristics, such as permeability and velocity of samples, show a general trend of decreasing with the increase of porosity. For low-porosity rock samples (φ 5%) dominated by tight micrite, the micritic pores have limited contributions to porosity and permeability and the micrite elastic properties are similar to those of the rock matrix. In such cases, the macroscopic physical and elastic properties are more susceptible to the formation of cracks and dissolution pores, but these features are controlled by the pore structure. The pore aspect ratio can be used as a good indication of pore types. The bulk modulus aspect ratio for dissolution pores is greater than 0.2, whereas that of the intergranular pores ranges from 0.1 to 0.2. The porous and compact micrites are observed to have a bulk modulus aspect ratio less than 0.1, whereas the ratio of the tight micrite approaches 0.2.

Upper Devonian (Frasnian) stromatactis-bearing mud mounds, western Alberta, Canada: reef framework dominated by peloidal microcrystalline calcite

ABSTRACT Two well-preserved mud mounds, approximately 50 m thick, in the Mount Hawk Formation (Upper Devonian, Frasnian) in western Alberta provide an unparalleled opportunity to study the microstructure of this reef type in detail for this time interval. This reveals that they are composed dominantly of peloidal sediments—more than a quarter of mud mound volume—along with dense micrite, particulate micrite, bioclasts, and stromatactis cavities. Five types of peloids are differentiated: bacterial, cyanobacterial, bioclastic, intraclastic, and pseudo-peloids. Bacterial peloids were generated by bacterial metabolic activities with possibly some contribution from organomineralization in areas within spicular networks. Three subtypes of cyanobacterial peloids are distinguished based on whether they are physically reworked calcified filaments, aggregates of calcified coccoids, or precipitated within stromatolite-forming cyanobacterial mats or biofilms. Bioclastic peloids are fully micritized fragments of skeletons and shells. Intraclastic peloids are eroded fragments of early-lithified matrix. Pseudo-peloids represent artifacts of poorly preserved sponge spicular networks reflecting the interplay between dissolution of spicules and organomineralization. The distribution of the various peloid types shows specificity in different microfacies. They prove to be valuable paleoecological indicators, and taken together suggest that the mud mounds accreted within the photic zone above storm wave base but below the fair-weather wave base, during deposition of a transgressive and perhaps highstand systems tracts. They are the products reflecting a dynamic balance between constructional versus destructive processes. Bacterial peloids combined with dense micrite constitute the framework, which in turn makes up close to 40% of the mud mounds. This indicates that microbial activities are responsible for the early-indurated framework. The onset of bacterial peloid formation in the bedded sediment immediately underlying the mud mounds further demonstrates that it may have been a necessary precursor to mud-mound initiation. Bacterial peloids appear to characterize most Paleozoic and Mesozoic reefal mud mounds as an essential framework element.

Composition and biostratinomy of sponge-rich biogenic crusts in submarine caves (Aegean Sea, Eastern Mediterranean)

Submarine caves constitute “natural laboratories” for studying biodiversity, community structure and inter-specific relationship in cryptic habitats. These unique systems are of strategic importance to reconstruct the complex biotic processes developed in cavities of skeletal frameworks through geologic time. In this study, the biogenic crusts in two submarine caves of the Aegean Sea were characterized with regards to the organisms involved in their formation. The crusts were examined with the aim to investigate the possible influence of environmental parameters and/or biotic associations on the framework. The walls and ceilings of the studied Fara and Agios Vasilios Caves (Lesvos Island, Greece) are covered mainly with sponges, coralline algae, scleractinian corals, serpulid polychaetes, and bryozoans. Skeletons of these organisms are often cemented together and form extensive crusts of variable thickness. Optical and electron microscopy were utilized to detect quantitative relationships among skeletal components inside the crusts. Coralline algae and corals dominate close to the opening of the caves, whereas serpulids, bryozoans and sponges are the main crust builders in the innermost cave sectors. Among skeletal components, the micrite (microcrystalline calcite) is a minor component of the crusts and consists mainly of an autochthonous fraction, mineralized directly inside the crust through organomineralization processes. Crusts show abundance of endolithic and insinuating sponges, whose organic tissue decay leaves borings filled with spicules. The abundance of sponges competing with carbonatogenetic bacteria for the same living cryptic spaces prevented the development of microbialites. This competition explains the morphological differences between the studied biogenic crusts and the large biostalactites, which are common in other Mediterranean caves (i.e. Sicily, Apulia and Cyprus) and are characterized by only a few sponges but abundant microbialites.

Micromorphological peculiarities of the Pleistocene soils in the Middle Pobuzhzhya (Ukraine) and their significance for paleogeographic reconstructions

The basic micromorphological features of fossil Pleistocene soils of the Middle Pobuzhzhya region are revealed. The early Neopleistocene (Shyrokyne, Martonosha, Lubny) and Middle Pleistocene Zavadivka soils are characterized by bright brownish, reddish and brownish colors of plasma, compact composing structural separations in the form of cleave blocks with densely packed nodular formations of ferruginous matter, cracksman ship of the mass, a significantamount of ferruginous, manganese and carbonate new formations. In the soils of the early optimum of the Kaydaky stage, signs of the eluvial-illuvial processes were observed (impoverished on the thin clays and humus fields with the «washed» grains of the mineral skeleton, the destroyed microaggregates in the eluvial and the impregnation of the plasma by calomorphic clays in the form of streaks, films, streams in the illuvial horizons). In the Pryluky soils there is a well-expressed microagregation of the mass, a branched net of twisted pores, humus coagulation in the humus and humus transition horizons, various forms of carbonate new formations as the impregnation and plasma cementation by microcrystalline calcite, presence of isolated crystals of crypto-, micro- and small crystalline calcite. The specific individual features of the Vytachiv soils are cleave block microstructure, the presence of the nodular concentric organo-iron-clay formations, and microorsteins. Dofinivka soils are characterized by a loose microstructure, fuzzy rounded microaggregates, a developed system of twisted pores, and enrichment of mass on carbonates. In order to clarify the genetic types of fossil Pleistocene soils of the Middle Pobuzhzhya, identification of the signs of elementary soil formation processes was carried out on the basis of micromorphological analysis data. The significance of the results of micromorphological researches for paleogeographical reconstruction is outlined. It has been established that certain groups of soil formation processes are characteristic for fossil soils of separate paleogeographical stages. In the soils formed up to the Dnieper glaciation (Shyrokyne, Martonosha, Lubny and Zavadivka), signs of processes of claying, rubbification, ferralization, cleaving were displayed, but weakly expressed humus formation, thoughcarbonization were diagnosed. In the soils formed after the maximum glaciation (Kaydaky, Pryluky, Vytachiv, Dofinivka), signs of the such processes as humus formation, podzolization, lessive, leaching, damp-meadow soil formation on floodplain, migration of carbonates, etc., are established. It is processes which are predominating in the modern soils of the territory of Ukraine. Assertaintment of elementary soil-forming processes, diagnosed in multi-annual fossil soils, have made it possible to identify their genetic types and as a result, to reconstruct the soil cover in separate stages of the Neopleistocene, to establish regional patterns of evolutional stadial changes and the natural environment in the Pleistocene on the territory of the Middle Pobuzhzhya.

The fabric and its formation mechanism of porous-compact laminae in the stalagmite from Shihua Cave, Beijing

The stalagmites in Shihua Cave, Beijing have been intensely studied as archives of regional palaeoclimate. They yield high resolution time series of multi-proxy records which have greatly improved the understanding of the relationship between speleothem formation and climate changes. The stalagmites studied in this cave were composed of fibrous calcite with clear visible and fluorescent laminae. Recently, porous-compact layers at the top 4 mm were found on an actively growing stalagmite (XMG). It consists of visible transparent-opaque layers. A noteworthy observation is that the laminae of calcite fabrics occurred on the top of XMG which formed after the cave open for tourism. The results from scanning electron microscopy analysis on this stalagmite indicates that the opaque microcrystalline calcite (MCC) porous sub-layers are composed of granular mosaic structures, comparing to the compact transparent fibrous columnar calcite (FCC) sub-layers with structures almost oriented perpendicular to the growth surface. Further analysis on the modern carbonate of this site was carried to investigate its formation mechanisms. Based on the size and habit of crystal, these modern carbonate crystallite formations can be categorized into three types. Type I contains larger rhombohedral grains, most of which are formed in the dry season of January–March. Most crystals with c -axis (along the growth direction) represent scales larger than 10 μm and structures of FCC. Type II includes the mixture of large rhombohedrons and small microcrystalline with a scale of less than 4 µm during the transition period of dry to rainy season, with deposition in April−May and October−December. During this period, the scale of large crystals with a diameter of more than 10 µm gradually decreases and the microcrystalline crystals begin growing. Type III consists of calcite precipitates with a spherical habit in the rainy season of June–September. Crystallites with diameters of less than 1 µm can be observed in this period, which is highly related to the new water recharge during this season. To investigate their elemental compositions, both the rhombohedral crystallites and microcrystalline were analyzed through an elemental spectrum analyzer. The results show that the elemental consistency is highly associated with the scales of crystallites. Trace elements, including silicon, aluminum, magnesium, etc. could be obtained on the microcrystalline specimen. On the contrary, only the basic formation elements of calcium carbonate as carbon, oxygen and calcium are detected on rhombohedral structures. Based on these observations, there are two hypotheses on this formation mechanism. Hypothesis I is that, FCC corresponded to the relatively decreased drip rates. During the dry season, the recharge contains high calcite saturation from a stable soil-water-rock interaction over the cave. On the other hand, new water inputs during the rain seasons of increased drip rates result in increased variables including lower index of calcite saturation, complex soil-water-rock interaction and variable amounts of prior calcite precipitation. Hereby, it presents a result of increased MCC with more impurities. Hypothesis II is that, the changed structure might be related to the cave tourism conditions. As the new structure is observed after the cave open to the public, it is expected to suggest a correlation to the tourist patterns. Moreover, both the increased CO2 level and aerosol levels inside cave systems are highly associated with increased visitors, which result in more venting as well as external air inputs. It is likely that the impurities from the external aerosol are the key factors of MCC Formation during the transition period. These anthropogenic factors might play a vital role in the thicker microcrystalline calcite layers formation.

Macroscopic, optical and diffraction assessment of encrustations and SEM analyses of phototrophic microbial mats from wellheads and select zones of emergence of mineral water in Serbia

Investigations focusing on wellhead encrustations within select zones of emergence of mineral water, in different hydrogeological provinces, were conducted from 2014 to 2017 in Serbia. They included: well BB-1 in Bogatic (Inner Dinaric Alps of western Serbia), wells LB-4 and LB-5 in Lukovska Banja (Sumadija-Kopaonik-Kosovo Province), and wells VG-2 and VG-3 in Vranjska Banja (Serbian Crystalline Core). The studied occurrences belong to the group of hot mineral waters, with temperatures ranging from 62°C to 93.9°C, and total dissolved solids (TDS) from 600 to 1267 mg/L. They are mildly acidic to mildly alkaline (pH 6.6 – 8.0) and their genetic types are HCO3--Na++K+ (Bogatic and Lukovska Banja) and SO42-, HCO3--Na++K+ (Vranjska Banja). Macroscopic, optical and diffraction assessment revealed that the encrustations comprised calcite and aragonite minerals, along with some local quartz, muscovite, albite and clinochlore. The conclusion was that there was a predominant presence of calcium carbonate in all the occurrences, represented by crystalline calcite grains, characteristically birefractive, and dark, semitranslucent grains in the form of microcrystalline calcite highly stained by iron hydroxides (Bogatic), as well as crystalline calcite with radially developed carbonate - aragonite (Lukovska Banja) and rod-like and filamentous aggregates of aragonite and calcite, highly translucent in places (Vranjska Banja).The texture of the samples exhibits alternating horizontal or wavy carbonate laminae. In Vranjska Banja, they include thin micritic to medium crystalline laminae and laminae composed of aggregates of aragonite needles in the form of bunch. In Bogatic and Lukovska Banja, there are thick micritic and highly porous laminae, 0.025 to 1 mm thick, or interlayers of a coarser crystalline structure. Apart from encrustations, all the occurrences exhibit phototrophic microbial mats, which were analyzed by scanning electron microscopy. The onset and progress of encrustation are primarily influenced by the physical and chemical composition of the mineral water (T, pH, TDS, HCO3-, Ca2+, CO2, etc.), as corroborated by hydrogeochemical modeling software PHREEQC 3.4.0. In addition, phototrophic biofilms might be contributing to the creation and accumulation of minerals in the encrustations.

Katian oncoids of the lower Ordovician Lianglitage Formation in the Tarim Basin, Northwest China

This study examines oncoids and their diagenetic structures in the lower Ordovician (Katian Stage) Lianglitage Formation in the Tarim Basin, Northwest China. Oncoid morphologies range from spherical and sub-spherical to irregular. Individual oncoids generally range from 4 to 10 mm in diameter, with an average of 8 mm and a maximum of 18 mm. Microbial fossils, including gastropods, sponges, trilobite skeletons, and bryozoans, are abundant in oncoid nuclei. Densely packed, calcified microbial clumps are also observed in some nuclei. Microbial remnants in oncoid cortices appear mainly as filaments. Alternating light and dark laminae are obvious in the oncoid cortices, and both sets of laminae contain extracellular polymeric substances, nanoparticles, and microcrystalline calcite. This suggests that oncoid formation was the result of interaction between microbial chemical processes and the external environment. Microbes and heterotrophic organisms provide the building material for oncoid nuclei formation, and frequent fluctuations in sea level provide favorable hydrodynamic conditions.

Structural diagenesis of carbonate fault rocks exhumed from shallow crustal depths: An example from the central-southern Apennines, Italy

This contribution focuses on field and laboratory analyses of carbonate fault cores pertaining to high-angle extensional fault zones currently exposed in the central and southern Apennines, Italy. The fault zones studied crosscut Mesozoic platform-related carbonate rocks, strike ca. NW-SE, and dip steeply SW. They formed during the Plio-Quaternary downfaulting of the Apennine fold-and-thrust belt and were exhumed from shallow crustal levels (<1.5 km). The carbonate fault cores include grain-supported, matrix-supported, and cement-supported fault rocks, fluidized layers of ultracataclasites with injection veins, and main slip surfaces. Our results of microstructural, petrographic, and cathodoluminescence analyses highlight the contrasting diagenetic evolution of calcite- and dolomite-rich fault rocks. Physical compaction was common within the dolomite-rich fault rocks, whereas chemical compaction coupled with dissolution characterized the calcite-rich fault rocks. Furthermore, multiple generations of calcite cements are documented in the fault zones. The first generation consists of a microcrystalline calcite cement, which developed around survivor grains and lined intergranular pores. The second generation is made up of light-luminescent, fibrous calcite crystals, which precipitated within open fractures and around survivor grains. The third generation consists of an euhedral calcite cement that surrounded survivor grains and infilled both open fractures and intergranular pores.

Diagenetic Environment of Sandstones from the Ermaying Formation (Middle Triassic), North-eastern Margin of the Ordos Basin, China: Implications for the Erosion Vulnerability of the Pi Sandstone

A hard layer of calcite cemented sandstones overlaid weakly consolidated mudstones occur in the Middle Triassic, Ermaying Formation in the eastern margin of the Ordos Basin, China, were examined to reveal the diagenetic environment of Pi sandstone. The Pi sandstone refers to a type of terrigenous clastic rock assemblage composed mainly of sandstone, siltstone and mudstone with red and white color, which is characterized by exposing or being covered with sand or loess, vulnerable to weathering and erosion, distributed in the contiguous area of Shaanxi province, Inner Mongolia autonomous region and Shanxi province in China, and formed during Late Paleozoic–Mesozoic. The “Pi” comes from the folk name (pishuang) of arsenic in Chinese, the appellation of the “Pi sandstone” means that it is harmful to soil and water conservation. The Pi sandstone samples were collected from a sheet-like sandstones deposited in ephemeral river. Petrographic observation show that the samples studied carry a semi-arid climate signal, are cemented by mictrie (microcrystalline calcite), sparry (coarse-crystalline) calcite as well as quartz overgrowths. The δ13C and δ18O values of calcite in the studied are –6.2 to – 3.8 ‰ PDB and from–10.2 to –8.9‰ PDB, respectively. The evidence from petrography and stable isotopes suggests that the cementation of the sandstone mainly occurred in the vadose - phreatic environment.

An XRPD and EPR spectroscopy study of microcrystalline calcite bioprecipitated by Bacillus subtilis

We report in this study the first XRPD and EPR spectroscopy characterisation of a biogenic calcite, obtained from the activity of the bacterium Bacillus subtilis. Microcrystalline calcite powders obtained from bacterial culture in a suitable precipitation liquid medium were analysed without further manipulation. Both techniques reveal unusual parameters, closely related to the biological source of the mineral, i.e., to the bioprecipitation process and in particular to the organic matrix observed inside calcite. In detail, XRPD analysis revealed that bacterial calcite has slightly higher c/a lattice parameters ratio than abiotic calcite. This correlation was already noticed in microcrystalline calcite samples grown by bio-mineralisation processes, but it had never been previously verified for bacterial biocalcites. EPR spectroscopy evidenced an anomalously large value of W6, a parameter that can be linked to occupation by different chemical species in the next nearest neighbouring sites. This parameter allows to clearly distinguish bacterial and abiotic calcite. This latter achievement was obtained after having reduced the parameters space into an unbiased Euclidean one, through an isometric log-ratio transformation. We conclude that this approach enables the coupled use of XRPD and EPR for identifying the traces of bacterial activity in fossil carbonate deposits.

Ecophysiology of the hydrothermal vent snail Ifremeria nautilei and barnacle Eochionelasmus ohtai manusensis, Manus Basin, Papua New Guinea: Insights from shell mineralogy and stable isotope geochemistry

Deep-sea vent communities live on a limited area characterized by sharp physico-chemical (temperature, salinity, pH) gradients. Around the vent, the fauna is distributed accordingly, showing characteristic niche partitioning for different groups of animals. In this study we investigate shell microstructure, minor elements and stable isotope compositions of two groups of organisms such as a snail, Ifremeria nautilei, and a crustacean, Eochionelasmus ohtai manusensis. Both organisms occupy distinct niches within the same hydrothermal vent field of the Manus Basin, Western Pacific. Powder XRD and electron microbeam analysis of a polished cross-section indicate that the shells are composed of microcrystalline calcite, with distinct Na, Mg, Sr, and S element contents. For both specimens 20–30 µm large weddellite crystals were found. The δ18O profiles were obtained perpendicular to the growth increments of I. nautilei and E. o. manusensis calcitic shells. Those profiles reveal isotopic variations of 0.5 and 0.6‰, respectively for both intra- and inter-shell measurements. For E. o. manusensis, the Mg content suggests continuous shell growth during the year, both δ18O and Mg data supporting cyclical variation of temperature at vent site. The calculated temperatures at sites with I. nautilei and E. o. manusensis range from 17° to 21.5°C and from 2.1° to 7.2°C, respectively, showing a similar variability of 5–6 °C. The δ13C values of the Ifremeria calcitic shell range from 3‰ to 4.6‰ (V-PDB), the isotopic composition being 13C-enriched relative to the surrounding inorganic pool. The δ13C values of the chitine layer covering the shell range from − 33 to − 31.1‰. The δ13C values of Eochionelasmus vary between 0‰ and 1‰, reflecting the surrounding inorganic DIC pool.

Oxygen Isotope Microanalysis By Secondary Ion Mass Spectrometry Suggests Continuous 300-million-year History of Calcite Cementation and Dolomitization in the Devonian Bakken Formation

Calcite cementation and dolomitization are key diagenetic processes in many sedimentary systems. Unravelling detailed histories and timescales of cementation and replacement is, however, often compromised by the limited spatial resolution of many analytical techniques; in some cases multiple grains are co-analyzed so that diagenetic histories are blurred and reaction periods are difficult to establish. In this study we have used 10-micrometer-resolution, in situ secondary ion mass spectrometry to determine the oxygen isotope composition of 197 individual, 10-50-micrometer-size crystals of dolomite and calcite from six samples in a single core of Upper Devonian Middle Bakken Member siltstones and sandstones, the major tight oil formation of the Williston Basin, USA. This amount of data places important constraints on the range of temperatures and times that carbonate cementation and replacement occurred. Petrographic data show that microcrystalline calcite cement is an early phase, and combined with mineralogical data suggest that much of the dolomite replaces calcite. Over spatial scales of less than a centimeter, analyses of individual calcite crystals have a range of 5‰ for 18O in the group of crystals, and for the group of individual dolomite crystals, 10‰. These sub-centimeter ranges are as great as those observed in previous studies of carbonate cements sampled over many meters and remind us that previous low-resolution studies may have inadvertently analyzed mixed phases. There is no relationship between dolomite texture and isotopic composition at this spatial scale; microscale backscattered electron imagery and scanning electron microscopy cathodoluminescence zoning is seen, but cannot be resolved with a 10 micrometer spot size. Assuming, since it is an early cement, that calcite precipitated from seawater (δ18O = -1.5‰), it formed at ca. 15-40°C, mainly at the lower temperatures. Present-day formation waters in Devonian rocks in this region have oxygen isotope compositions of 7-8‰ VSMOW. Using these values as a likely dolomitizing fluid, we suggest that dolomitization occurred continuously between 40 and 140°C over 150 – 200 million years, most likely in a fluid with a high Mg/Ca ratio resulting from gypsum formation in local evaporites. We suggest that this exceptionally low rate of dolomitization was controlled by the low rate of supply of Mg in a very sluggish flow regime; dolomitization is incomplete because of a limited supply of Mg.

Characterization of Mortar from Church Ruins in Barangay Budiao, Daraga, Albay

The lack of information about the correct ways of restoring historical masonry materials from cultural heritage structures tends to create more damage to the said structures. According to Cesare Brandi’s theory of restoration, existing historical materials must be replaced with their equivalent. This paper presents the chemical composition of historical mortars acquired from church ruins of barangay Budiao, Daraga, Albay, Philippines. The historical mortar sample was characterized using petrographic analysis, sieve analysis, atomic absorption spectroscopy (AAS), thermogravimetric analysis (TGA), and infrared spectroscopy (IR). Petrographic analysis shows that the mortar sample is bounded by 90% organic material particularly coral and is held together by microcrystalline calcite (micrite) matrix that comprises of 10% of the total sample mortar composition. Sieve analysis shows a well-graded particle distribution. Atomic absorption spectroscopy (AAS) was also utilized to determine the %Ca for each sieve fraction using HCl for acid digestion. TGA thermograph shows the non-linear drop in weight between 740°C and 850°C, which corresponds to the degradation temperature of calcium carbonate or limestone. IR analysis shows essential characteristic peaks of CaCO3 at 712 cm-1, 874 cm-1, and 1437 cm-1.

Cryptic serpulid-microbialite bioconstructions in the Kakoskali submarine cave (Cyprus, Eastern Mediterranean)

The biostalactites from the Kakoskali cave in Cyprus represent a new example of the complex biotic relationships between skeletal organisms and microbial communities in building bioconstructions of cryptic marine environments. Biostalactites are mainly constituted of polychaetes of the family Serpulidae and, to a lesser degree, foraminifers and bryozoans. Within the skeletal framework of these organisms, two types of microcrystalline calcite (micrite) have been recognized: autochthonous and detrital micrite. The autochthonous fraction is syndepositionally lithified and occurs as clotted peloidal and, subordinately, aphanitic (structureless) textures, suggesting the presence of heterotrophic microbial activities thriving on decaying metazoan organic matter. This fraction is limited to the protected portions of the bioconstructions, especially in the inner and lower parts. The presence of iron and manganesiferous oxidizing bacteria is suggested by the deposition of ferromanganesiferous crusts and Frutexites-like structures. These microbial-induced biomineralizations are the main evidence of carbonatogenetic and Fe–Mn, autotrophic and chemoheterotrophic, bacterial activities. The Kakoskali cave is frequently visited by divers who, during their immersions, resuspend the fine bottom sediment, which later covers the surface of the bioconstructions, disturbing the delicate equilibrium of the biotic association. This perturbation, which is also caused by strong waves and currents, during winter months, reflects on the bioconstruction morphologies, community composition, and colonization pattern. Bioconstructions exhibit an upper smooth surface, produced by few taxa (e.g., polychaetes, foraminifers), hosting a low number of living individuals, and a lower comparably rough surface, colonized by a more abundant community showing a higher species richness. The ratio surface roughness/smoothness is related to micrite sediment type: the upper part is mainly characterized by loose detrital micrite while the internal and lower parts by syndepositional cemented autochthonous micrite.