Calcite Fracture Research Articles

Surface Characterization of Oleic Acid Coated Marble Dust

Calcite, being the most abundant mineral on earth crust, have wide application areas especially in polymer industry as a micronized functional filler material. It is hydrophilic in natural form, and made hydrophobic after surface modification to meet the requirements of polymer industry: the incompatibility between high energetic hydrophilic surface of calcite and the low-energy surface of hydrophobic polymers is a major problem. Treatment of micronized calcite with fatty acids is one of the most common method to obtain modified mineral surface. In present study, oleic acid (OA), fatty acid type surface modifying agent was used for the surface characterization of OA coated marble dust. Fine tailings of slab cutting unit of a marble processing plant was supplied. The sample was subjected to wet classification process to obtain micronized calcite fraction for experimental works. Surface modification of finely sized fraction was performed in a laboratory type flotation unit. Thermogravimetric analysis (TGA) and Transmission Electron Microscopy (TEM) were used as characterization techniques.

Polarized lidar and ocean particles: insights from a mesoscale coccolithophore bloom.

Oceanographic lidar can provide remote estimates of the vertical distribution of suspended particles in natural waters, potentially revolutionizing our ability to characterize marine ecosystems and properly represent them in models of upper ocean biogeochemistry. However, lidar signals exhibit complex dependencies on water column inherent optical properties (IOPs) and instrument characteristics, which complicate efforts to derive meaningful biogeochemical properties from lidar return signals. In this study, we used a ship-based system to measure the lidar attenuation coefficient (α) and linear depolarization ratio (δ) across a variety of optically and biogeochemically distinct water masses, including turbid coastal waters, clear oligotrophic waters, and calcite rich waters associated with a mesoscale coccolithophore bloom. Sea surface IOPs were measured continuously while underway to characterize the response of α and δ to changes in particle abundance and composition. The magnitude of α was consistent with the diffuse attenuation coefficient (Kd), though the α versus Kd relationship was nonlinear. δ was positively related to the scattering optical depth and the calcite fraction of backscattering. A statistical fit to these data suggests that the polarized scattering properties of calcified particles are distinct and contribute to measurable differences in the lidar depolarization ratio. A better understanding of the polarized scattering properties of coccolithophores and other marine particles will further our ability to interpret polarized oceanographic lidar measurements and may lead to new techniques for measuring the material properties of marine particles remotely.

Fused Filament Fabrication 3‐D Printing of Reactive Porous Media

AbstractUnderstanding the impacts of porous media properties on geochemical reactions is challenging due to the highly heterogeneous nature of natural samples. This work explores the use of 3‐D printing to create synthetic rock samples with reactive properties mimicking those of natural samples. Here, X‐ray Computed Tomography and 3‐D printing were used to recreate the pore network structure and reactive properties of a Paluxy sandstone. Novel, reactive filaments for 3‐D printing synthetic rock samples were constructed from mixtures of high impact polystyrene and calcite. The distribution and accessible calcite surface area of printed samples were evaluated and compared to values for the real rock sample. Only a small fraction of the total calcite was on the surface of the printed sample, but the accessible calcite surface area was comparable to real samples such that 3‐D printing may be a feasible means of fabricating reactive porous media samples.

Prescreening Hydraulic Lime-Binders for Disordered Calcite in Caesarea Maritima: Characterizing the Chemical Environment Using FTIR

ABSTRACTHydraulic lime binders are considered a technological marvel which revolutionized construction techniques in antiquity. The core material is made of a binder that is a mixture of calcite and hydraulic phases, which are amorphous silicate compounds that nanostructurally polymerize into insoluble phases that harden even underwater, formed during the reaction between lime and reactive silicates such as volcanic ash. This is also what makes hydraulic lime so hard to radiocarbon (14C) date. These insoluble phases contain carbonates that may set centuries following their application, resulting in younger ages, which may contaminate the calcite fraction that is favorable for 14C dating. This calcite fraction forms upon the incorporation of atmospheric carbon dioxide during the setting of the hydrated lime. Therefore, different characterization methods are being constantly developed for identifying and characterizing the components of hydraulic lime-binders. In this work, we present a rapid characterization technique based on Fourier transform infrared spectroscopy (FTIR) that characterizes the atomic disorder and chemical environment of the carbonates and silicates fractions in the binder. The atomic disorder of the calcite crystallites was determined by the ν2 and ν4 vibrational modes, and the silicates were characterized by the main peak asymmetry and full width at half maximum (FWHM). Different hydraulic binders from Caesarea Maritima were examined, including Herodian mortars from the underwater breakwater and on-land plasters and mortars from the portʼs warehouse and vaults. Hydraulic binders, in which the calcite fraction in the binder shows atomic disorder that is comparable to modern plaster binders, was associated with silicates that have asymmetry and FWHM of clays and quartz. These materials are considered to be in good preservation state for 14C dating since their carbonates crystallites are disordered and did not interact with the environment chemically to form stable and ordered crystals. Interestingly, the atomic disorder of binders that underwent chemical alterations and recrystallization processes, are associated with reactive silicates aggregates such as volcanic ash (pozzolana). These results suggest a new way to prescreen materials for radiocarbon dating based on the composition of lime-binders and preservation state of the carbonate fraction and hydraulic products.

Aragonite Fraction Dating of Vermetids in the Context of Paleo Sea-Level Curves Reconstruction

ABSTRACTIdentifying and tackling recrystallization is a critical factor in the reliable radiocarbon (14C) dating of carbonates, since exogenous carbon can be incorporated and thus mask the real age of the samples. Vermetids are among the most important bioindicators used for paleo sea-level reconstruction, and the accuracy of their chronology can significantly impact sea-level curves. Age differences larger than 1 14C kyr before and after acid etching, combined with X-ray diffraction (XRD) analysis that indicates a significant amount of calcite still remains in the shell, led us to apply the previously developed carbonate density separation protocol (CarDS). Using a solution of sodium polytungstate, with density of 2.80 g/cm3, we successfully separated different carbonate fractions for a set of 10 vermetid samples from the coast of Rio de Janeiro, southeast of Brazil. Each separation was verified by XRD analysis and the 14C concentrations of different fractions were compared. The results show that the calcite fraction in the studied vermetid samples varied from 12 to 63% and aragonite fraction ages are up to 2 14C kyr older than the raw samples, thus confirming the efficacy of CarDS in removing young carbonates and the importance of density separation to vermetids prior to accelerator mass spectrometry (AMS) dating.

Discrete Element Modelling for biocemented sand: effect of calcite distribution at the microscopic scale

The mechanical efficiency of the biocementation process is directly related to the microstructural properties of the biocemented soil, such as, the volume fraction of calcite, its distribution within the pore space (whether localized at the contact between grains or over the grain surfaces) and the contact properties: coordination number, contact surface area, contact orientation, type of contacts (frictional even after treatment, purely cohesive via a calcite bridge or combining friction between particles and cohesion of the localized calcite). Dadda et al, (2018) have used microscopic properties computed from 3D images obtained by X-ray tomography of biocemented sand samples with different levels of biocementation as an input in current analytical models to estimate the elastic properties (Young’s and shear modulus) and the strength properties (Coulomb cohesion). They pointed out the important role of some microstructural parameters, notably those related to the contact, on such effective parameters. However, the precise evaluation of the effect of microstructural parameters such as the contact surface distribution on the global mechanical behaviour of the soil requires the use of more advanced modelling methods. The paper presents the results of Discrete Element Modelling of triaxial tests with the open source code Yade in which the real microstructural properties of biocemented soil computed on 3D X-ray microtomography images are used as input parameters. A particular attention has been paid to take into account the actual distribution of contact surface in the model and not only the average value. It appears that the model is then able to reproduce the evolution of the macroscopic properties (in particular that of the cohesion) with the calcite content.

Flotation of Calcium Minerals with Combination of Reagents of Different Molecular Structure

The influence of non-ionic compounds in combination with sodium oleate on the contrast behavior of flotation response of calcium minerals is investigated theoretically and experimentally. The flotation and adsorption analyses are carried out with monomineral fractions of calcite and scheelite, while the flotation process analysis is conducted with an ore material screened into size grades of -44 and -15 urn. The process conditions of the highest difference in adsorption of oleate and contrast in extraction of scheelite and calcite are found. The conditions of the maximum depression of calcite in the rougher flotation circuit are determined.

An Increase in Process Characteristics of Flotation of Low-Grade Fine-Disseminated Scheelite Ores

The results of investigations into increasing the contrast of process properties of calcite and scheelite due to the combined application of water glass with salts such as sulfates of aluminum, zinc, iron, magnesium; mixtures of water glass and calcium chloride; sodium carboxymethyl cellulose (CMC); combinations of sodium oleate with low-polar compounds (neonol and fatty iso-alcohols); and data on the ultrasonic treatment of the liquid phase and oleate are described. The lowest recovery of calcite during the flotation of a monomineral calcite fraction in the Hallimond tube is attained when jointly using salts of iron(II) and water glass (3(4) : 1). The joint application of water glass and CaCl2 during the flotation of the low-grade scheelite ore with a high carbonate modulus using tap water lowers the calcite flotability. When using recycled water, adding calcium chloride to water glass leads to a certain increase in the yield of the rough concentrate (from 13.8 to 14.1%) with a significant decrease in the recovery of WO3 into the final selection concentrate of scheelite (from 72.7 to 53.3%) and worsens the concentrate quality. The replacement of water glass by CMC showed no satisfactory results. The ultrasonic treatment of the pulp, liquid phase, and collector leads to a certain increase in calcite flotation activity, possibly due to an increase in the liquid phase temperature and in the fraction of the ionic form of oleate. The use of neonols in the reagent flotation mode of the scheelite-containing ore with a high carbonate modulus does not confirm a decrease in the flotation activity of calcite formed when studying monomineral calcite fractions, in contrast with fatty iso-alcohols, which make it possible to fabricate the higher quality concentrates in the selection cycle when compared with one oleate.

Improvement of the technological effects of flotation of lean fine disseminated scheelite ores

The paper describes the results of studying ways to improve the contrast of calcite and scheelite technological properties using water glass combined with aluminum, zinc, iron, magnesium sulphate salts, a mixture of water glass and calcium chloride, sodium carboxymethyl cellulose (CMC), combinations of sodium oleate with low-polar compounds (neonol, fatty isoalcohols), liquid phase and oleate ultrasound treatment. The monomineralic fraction of calcite floated by mechanical cell demonstrated that the minimum recovery of calcite is achieved by combining the Fe(II) salt and water glass (3(4) : 1). When f loating lean sheelite ore with a high carbonate modulus on domestic water, the combined use of water glass and CaCl2 reduces the floatability of calcium. Calcium chloride added to water glass on recycling water leads to a certain increase in the rough concentrate yield (13.8 to 14.1 %) with a significant decrease of WO3 recovery to the finished selection concentrate (72.7 to 53.3 %) and a deterioration in the concentrate quality. Replacement of water glass with CMC did not show satisfactory results. Ultrasonic treatment of pulp, liquid phase, collector leads to a certain increase in the calcite floatability, possibly due to the higher liquid phase temperature and increased proportion of the oleate ionic form. The use of neonols in the reagent scheme of flotation of scheelite-containing ore with a high carbonate modulus found no evidence of a decrease in the flotatability of calcite obtained when studying monomineralic calcite fractions unlike fatty isoalcohols that provided better concentrates in the selection cycle in comparison with a single oleate.

Fractionation of Rare Earth Elements in Greisen and Hydrothermal Veins Related to A-Type Magmatism

This study focuses on concentrations and fractionation of rare earth elements (REE) in a variety of minerals and bulk materials of hydrothermal greisen and vein mineralization in Paleoproterozoic monzodiorite to granodiorite related to the intrusion of Mesoproterozoic alkali- and fluorine-rich granite. The greisen consists of coarse-grained quartz, muscovite, and fluorite, whereas the veins mainly contain quartz, calcite, epidote, chlorite, and fluorite in order of abundance. A temporal and thus genetic link between the granite and the greisen/veins is established via high spatial resolution in situ Rb-Sr dating, supported by several other isotopic signatures (δ34S, 87Sr/86Sr, δ18O, and δ13C). Fluid-inclusion microthermometry reveals that multiple pulses of moderately to highly saline aqueous to carbonic solutions caused greisenization and vein formation at temperatures above 200–250°C and up to 430°C at the early hydrothermal stage in the veins. Low calculated ∑REE concentration for bulk vein (15 ppm) compared to greisen (75 ppm), country rocks (173–224 ppm), and the intruding granite (320 ppm) points to overall low REE levels in the hydrothermal fluids emanating from the granite. This is explained by efficient REE retention in the granite via incorporation in accessory phosphates, zircon, and fluorite and unfavorable conditions for REE partitioning in fluids at the magmatic and early hydrothermal stages. A noteworthy feature is substantial heavy REE (HREE) enrichment of calcite in the vein system, in contrast to the relatively flat patterns of greisen calcite. The REE fractionation of the vein calcite is explained mainly by fractional crystallization, where the initially precipitated epidote in the veins preferentially incorporates most of the light REE (LREE) pool, leaving a residual fluid enriched in the HREE from which calcite precipitated. Fluorite occurs throughout the system and displays decreasing REE concentrations from granite towards greisen and veins and different fractionation patterns among all these three materials. Taken together, these features confirm efficient REE retention in the early stages of the system and minor control of the REE uptake by mineral-specific partitioning. REE-fractionation patterns and fluid-inclusion data suggest that chloride complexation dominated REE transport during greisenization, whereas carbonate complexation contributed to the HREE enrichment in vein calcite.

Origin of strontium and calcium in pedogenic needle fibre calcite (NFC)

Strontium (Sr) and calcium (Ca) isotopic compositions and Sr/Ca ratios have been analysed in different fractions of pedogenic needle fibre calcite (NFC) from the Swiss Jura Mountains (Villiers, Neuchâtel, Switzerland) in order to determine their Ca sources and to investigate the potential role of fungi in the origin of NFC. 87Sr/86Sr ratios have been measured to trace the Ca trapped within different pedogenic carbonate. This ratio was compared for previously identified microscopic morphological groups of NFC, the bulk soil fine earth (<2 mm), the carbonate host rock (CHR), a late calcitic cement (LCC, another pedogenic carbonate precipitated physicochemically at the same depth as NFC in soil pores), the soil solution, and the throughfall. The 87Sr/86Sr indicated a mixing of weathered allochthonous minerals (an atmospheric component) and the dissolved CHR (the local geogenic component). Moreover, the comparison of 87Sr/86Sr ratios of the different types of NFC with LCC suggests that the contribution of Ca from the main Ca sources slightly differs between NFC morphotypes and LCC. In addition, the three NFC morphotypes displayed Sr isotopic compositions different from each other, emphasizing a direct relationship between the microscopic shapes of NFC and the processes involved in their respective formation, including the origin of the Ca. Strontium concentrations and δ44/40Ca values of NFC and LCC crystals were used to determine possible differences in their growth rate and/or micro-environmental conditions during their formation. Crystals described micromorphologically as simple needles (SN) were enriched in 44Ca and depleted in Sr compared to LCC, suggesting that the elongated shape of the SN crystal cannot be related to a rapid precipitation rate, but rather to a slow precipitation. Calculations applied to the soil solution evolution during calcite precipitation demonstrates the utilisation of large portions of the Ca pool for NFC formation within a closed system, e.g. inside an organic mould, such as fungal hyphae. The differences between the three microscopic groups can be explained by a mixing of the SN with a LCC-like compound originating from the soil solution.

Sensitivity of clumped isotope temperatures in fossil benthic and planktic foraminifera to diagenetic alteration

Applying the clumped isotope (Δ47) thermometer to foraminifer microfossils offers the potential to significantly improve paleoclimate reconstructions, owing to its insensitivity to the isotopic composition of seawater (unlike traditional oxygen isotope (δ18O) analyses). However, the extent to which primary Δ47 signatures of foraminiferal calcites can be altered during diagenesis is not well known. Here, we present Δ47 data as well as high-resolution (∼10 kyr) δ18O and δ13C middle Eocene time series, measured on benthic and planktic foraminifera from ODP/IODP Sites 1408, 1409, 1410, 1050, 1260 and 1263 in the Atlantic Ocean. The sites examined span various oceanographic regimes, including the western tropical to mid-latitude North Atlantic, and the eastern mid-latitude South Atlantic. Comparing data from contemporaneous foraminifera with different preservation states, we test the effects of diagenetic alteration on paleotemperature reconstructions for the deep and surface ocean. We find that overall, primary Δ47 signatures appear similarly sensitive to diagenetic overprinting as δ18O, with differences in sensitivity depending on pore fluid chemistry and the amount of secondary calcite. Where planktic foraminifera are significantly altered, sea surface temperatures derived from Δ47 and δ18O values are biased towards cool temperatures. In comparison, Δ47 and δ18O values of benthic and well preserved planktic foraminifera are less affected by diagenesis and thus likely to yield robust foraminiferal calcification temperatures. With independent estimates of diagenetic calcite fractions, secondary overprints could be corrected for, using end-member modeling and Δ47-based temperatures from benthic foraminifera.

Mineral assemblages and temperature associated with Cu enrichment in the Seival area (Neoproterozoic Camaquã Basin of Southern Brazil)

The Neoproterozoic sequence of volcanic rocks in the Camaquã Basin included in the Lavras do Sul Shoshonitic Association hosts disseminated Cu deposits. The volcanic sequence in the Seival Mine area includes andesitic lava flows, lapilli tuff, volcanic agglomerate and andesitic dikes with pervasive alteration. The hydrothermal alteration is interpreted to be a product of late–magmatic fluids or the mixing of magmatic with meteoric fluids or basinal brines. The late–magmatic hydrothermal system started after volatile oversaturation and degassing in a magmatic system, which was partially segregated into vesicles. The similar REE contents in the andesine–labradorite and albite indicate that the fluid temperatures ranged from 600 to 250 °C. The crystallization of titanite and Fe–clinochlore to Mg–chamosite followed the albitization process. The temperature range associated with the chloritization process was estimated using a chlorite geothermometer and varied from 251 ± 56 to 183 ± 39 °C. The precipitation of Mg–saponite and smectite–rich chlorite/smectite mixed–layers in some andesitic lava flows and the precipitation of quartz, calcite, barite and hematite in fractures suggest fluid circulation with temperatures lower than the chloritization process. The alteration minerals are represented by chlorite and albite associated with pyrite–chalcopyrite, while chlorite/smectite mixed–layers and barite or hematite are associated with bornite–chalcocite–covellite. Thermodynamic calculations confirm the potential coprecipitation of pyrite–chalcopyrite with chlorite and albite. The occurrence of bornite–chalcocite–covellite and barite seems to be favored by low–temperature chlorite/smectite with neutral to mildly acidic water influx. Consequently, albitization and the process of chloritization at high temperatures (>ca. 251 ± 56 °C) can be used as an exploration guide for primary pyrite–chalcopyrite enrichment, and chlorite/smectite at low temperatures (ca. 250 to 50 °C) can be related to bornite–chalcocite–covellite or the process of Cu enrichment.

Fracture Fillings and Implication of Fluid Activities in Volcanic Rocks: Dixi Area in Kelameili Gas Field, Junggar Basin, Northwestern China

The Carboniferous Batamayineishan Formation of the Kelameili Gas Field is a specific weathered crust-related volcanic reservoir that has a significant production rate in the Junggar Basin, Northwestern China, attributed to debatable processes of fluid evolution. The results suggest that various types of fluids occurring in volcanic rocks lead to the filling of quartz and calcite in fractures and their associated alteration haloes. The silica that formed quartz veins was mainly derived from deep hydrothermal fluids, while the carbon dioxide that formed calcite veins originated from sources characterized by mixing and alteration of deep hydrothermal and hydrocarbon fluids. Siliceous hydrothermal fluids rich in sulphur dioxide and other volatile components were driven by a pressure gradient and buoyancy, and circulated both laterally and vertically along the fractures, forming quartz veins and tension fractures under different temperature conditions. Moreover, changes in salinity, pressure, and carbon dioxide of deep fluids, varying from acidic to weakly alkaline, resulted in earlier calcite precipitation in contraction fractures and weathered fractures. Tectonic uplift resulted in the long-term exposure of volcanic rocks, where fresh water mixed with the partially alkaline fluid escaping the basin to form calcite cements, thus retaining the characteristics of a seepage environment in the weathered fractures. Structural fractures occurred due to tectonic movements during the burial period. Filling and leakage of hydrocarbons caused pore fluids to convert from acidic to alkaline, precipitating late sparry calcite in dissolution fractures. Late hydrothermal fluid metasomatism, brought about by infiltration into the permeable zone, caused partial dissolution of local calcite along cleavage cracks.

Pulsed Terahertz Radiation for Sensitive Quantification of Carbonate Minerals.

Quantification of natural carbonate minerals, namely, aragonite, high- and low-Mg calcite, and dolomite provides essential information about biomineralization, carbon cycling on Earth, and the evolution of ocean chemistry, and is also useful in many other scientific, pharmaceutical, and industrial fields. However, X-ray diffractometer has previously been the only practical tool to identify and quantify carbonate minerals, including calcium carbonate (CaCO3) polymorphs. We propose new fingerprint terahertz (THz) absorption and reflective index spectra in the 1–6 THz range that probe the lattice phonon modes and can be used for sensitive quantification of these four carbonate minerals, including polymorphs. In THz time-domain spectroscopy with our unique attenuated total reflection system, high- and low-Mg calcite and aragonite show different absorbance and reflective index amplitudes at 3.32 THz, which corresponds to the transverse optic mode. Dolomite shows a distinct absorbance peak and reflective index at 4.82 THz because its space group (R3̅) is different from that of calcite (R3̅c). THz absorbance and reflective index curves of the mixed carbonate materials, which typically occur in natural environments, correspond well to the curves calculated from the results of single-mineral samples (R2 > 0.98). Remarkably, the absorbance and reflective index can quantify small fractions (<1%) of low-Mg calcite in an aragonite matrix with high linearity (R2 = 0.99). Our findings provide a new method for screening low-Mg calcite diagenetic overprints on primary aragonitic skeletons such as corals, which is crucial for climate reconstructions using the isotopic analyses because a 1% overprint can cause estimated temperature deviations of ∼1 °C. THz spectra of carbonate minerals offer not only a new high-sensitivity quantification tool for interdisciplinary fields, but also safer light-source handling than X-ray diffractometer.

Evaluation study for phosphorus mobilisation-release behaviour on different marine sediments: focus on phosphate sorption characteristics

Environmental contextPhosphorus is a key nutrient element associated with eutrophication in aquatic ecosystems. We studied phosphate sorption processes, which play a key role in phosphorus cycling, in sediments collected from 23 sites including estuary, coastal sea and aquaculture areas in China. The results show the influence of sediment type on phosphorus buffering capacity and allow better interpretation of phosphorus migration in aquatic ecosystems. AbstractPhosphorus is well known as an important nutrient element associated with eutrophication in the marine ecosystem, and its sorption on sediments plays a key role in its immobilisation in the bio-geochemical cycle. In this paper, the sorption behaviour of phosphorus onto sediments collected from 23 different sites in estuary, coastal sea and aquaculture areas of China was studied. The main aim is to determine the phosphorus sorption characteristics of these sediments thereby assessing their phosphorus buffering capacities. Both kinetic and equilibrium isotherms of the sorption and desorption of phosphorus were assessed. The resultant sorption and desorption kinetic curves fit well to a two-compartment first-order equation. The equilibration time was considered as 48h. The isotherms agreed well with the Freundlich and Langmuir equations. The hysteresis coefficient values showed an obvious sorption-desorption hysteresis. Decreasing salinity was favourable for the sorption ability in the range from 30 to 3. The mean values of the phosphorus sorption-desorption equilibrium concentration and the maximum phosphorus sorption capacity were 0.098mgL−1 and 0.086mgg−1 for sediments in the Changjiang Estuary and East China Sea shelf, and 0.138mgL−1 and 0.067mgg−1 for the sediments in the aquaculture area. The fractions of clay, calcite and organic matter influenced the sorption and retention abilities and the effects were different for sediments obtained from different origins.

Influence of the microstructural properties of biocemented sand on its mechanical behavior

SummaryThe mechanical efficiency of the biocementation process is directly related to the microstructural properties of the biocemented sand, such as the volume fraction of calcite, its distribution within the pore space, coordination number, contact surface area, and types of contact. In the present work, some of these microscopic properties are computed, from 3D images obtained by X‐ray tomography of biocemented sand. These properties are then used as an input in current analytical models to estimate the elastic properties (Young and shear moduli) and the strength properties (Coulomb cohesion). For the elastic properties, the analytical estimates (contact cement theory model) are compared with classical bounds, self‐consistent estimate and numerical results obtained by direct computation (FEM computation) on the same 3D images. Concerning the cohesion, an analytical model initially developed to estimate the cohesion due to suction in unsaturated soils is modified to evaluate the macroscopic cohesion of biocemented sands. Such analytical model is calibrated on experimental data obtained from triaxial tests performed on the same biocemented sand. In overall, the presented results point out the important role of some microstructural parameters, notably those related to the contact, on such effective parameters.

Characterization of contact properties in biocemented sand using 3D X-ray micro-tomography

The mechanical efficiency of the biocementation process is directly related to the microstructural properties of the biocemented sand, such as the volume fraction of calcite, its distribution within the pore space (localized at the contact between grains, over the grain surfaces) and the contact properties: coordination number, contact surface area, contacts orientation and types of contact. In the present work, all these micromechanical properties are computed, for the first time, from 3D images obtained by X-ray tomography of intact biocemented sand samples. The evolution of all these properties with respect to the volume fraction of calcite is analyzed and compared between each other (from untreated sand to highly cemented sand). Whatever the volume fraction of calcite, it is shown that the precipitation of the calcite is localized at the contacts between grains. These results are confirmed by comparing our numerical results with analytical estimates assuming that the granular medium is made of periodic simple cubic arrangements of grains and by considering two extreme cases of precipitation: (1) The calcite is localized at the contact, and (2) the grains are covered by a uniform layer of calcite. In overall, the obtained results show that a small percentage of calcite is sufficient to get a large amount of cohesive contacts.

Lithofacies and reservoir characteristics of the Lower Cretaceous continental Shahezi Shale in the Changling Fault Depression of Songliao Basin, NE China

The increasing enthusiasm for continental shale gas exploration in China has made the Shahezi shale in the Changling Fault Depression an important research target due to its impressive demonstrated gas capacity. In this work, geochemical and petrologic analyses, field emission-scanning electron microscopy (FE-SEM) observation, low-pressure adsorption isotherms and mercury intrusion porosimetry (MIP) were conducted on 19 core samples to comprehensively analyze the reservoir characteristics of different lithofacies. The results show that the Shahezi shale has the characteristics of low content of organic matter (OM), high Ro, high content of clay minerals and small fractions of calcite. The kerogen is type III, dominated by vitrinite and inertinite. Clay minerals are dominated by the mixed illite-smectite, followed by illite and a small amount of chlorite. The Shahezi shale develops three kind of stratification structures according to the thickness of laminae: laminated structures, bedded structures, and massive structures. On the basis of TOC, mineral composition and petrologic texture, eight types of lithofacies were recognized. The organic pores show strong heterogeneity and are poorly developed, while clay-related pores are ubiquitous. The Shahezi shale has a high pore volume (PV) (0.005–0.031 ml/g, averaging 0.0173 ml/g) and specific surface area (SSA) (2.57–27.48 m2/g, averaging 16.61 m2/g), indicating an excellent storage capacity. Low-pressure CO2 and N2 isotherms and MIP were utilized to construct the whole-range pore size distribution (PSD). Based on the whole-range PSDs, mesopores were observed to contribute most to the PV, followed by macropores. Micropores and mesopores account for more than 99% of SSA. The reservoir capacity of different lithofacies is following the order in terms of PV and SSA, from high to low: organic-medium massive mixed shale (OMMMS), organic-medium massive argillaceous shale (OMMAS), organic-rich laminated argillaceous shale (ORLAS), organic-medium laminated argillaceous shale (OMLAS), organic-rich bedded mixed shale (ORBMS), organic-rich bedded siliceous shale (ORBSS), organic-medium bedded argillaceous shale (OMBAS), organic-poor shale (OPS). The main controlling factors of pore structure for the Shahezi shale are clay minerals rather than OM, which is similar to the continental Chang 7th shale but contrary to the Longmaxi shale. However, the Shahezi shale is mainly of low TOC and strong heterogeneity pores development result from OM macerals, while the Chang 7th shale is mainly of low maturity. Ro play an important role in promoting pore development. Only high content of calcite can greatly improve the pore space due to its solubility. This work contributes to the theory of continental shales and how to identify continental high-quality shale reservoirs.

An Evaluation of the Loss-on-Ignition Method for Determining the Soil Organic Matter Content of Calcareous Soils

ABSTRACTThe Loss-on-Ignition (LOI) method is widely employed for measuring the organic matter (OM) content of soil samples. There is a risk of carbonate losses when calcareous soil samples are analyzed through LOI, but this has never been investigated in detail. Moreover, a worldwide standard protocol for determining the carbonate content of soils is not available. The aims of this study were (i) to evaluate two commonly employed carbonate analysis procedures using calcareous and non-calcareous soil samples: the gravimetric method with (GMF) and without (GM) the addition of the antioxidant iron(II) chloride (FeCl2) and the acetic acid dissolution procedure (AAD); (ii) to evaluate the effect of ignition temperature on losses of pure calcite, calcite-quartz and calcareous soil samples. We found that the average apparent carbonate content of the non-calcareous soils was greatest for the GMF method followed by the AAD procedure. The GM method showed the smallest apparent carbonate contents. For the calcite-quartz sand mixture, ignition losses started at 600°C and increased with temperature in a sigmoidal way. LOI values stabilized at 750°C when 80% of the carbon dioxide was released. We recommend the GM procedure for carbonate analysis because the apparent carbonate contents of the non-calcareous soil samples were smallest. Furthermore, we recommend an LOI temperature of 550°C because at this ignition temperature 99.8% of the total calcite fraction remains in the soil samples.