Secondary Clay Minerals Research Articles

Experimental Investigation on Enhanced-Oil-Recovery Mechanisms of Using Supercritical Carbon Dioxide as Prefracturing Energized Fluid in Tight Oil Reservoir

SummaryFracturing is the necessary means of tight oil development, and the most common fracturing fluid is slickwater. However, the Loess Plateau of the Ordos Basin in China is seriously short of water resources. Therefore, the tight oil development in this area by hydraulic fracturing is extremely costly and environmentally unfriendly. In this paper, a new method using supercritical carbon dioxide (CO2) (ScCO2) as the prefracturing energized fluid is applied in hydraulic fracturing. This method can give full play to the dual advantages of ScCO2 characteristics and mixed-water fracturing technology while saving water resources at the same time. On the other hand, this method can reduce reservoir damage, change rock microstructure, and significantly increase oil production, which is a development method with broad application potential.In this work, the main mechanism, the system-energy enhancement, and flowback efficiency of ScCO2 as the prefracturing energized fluid were investigated. First, the microscopic mechanism of ScCO2 was studied, and the effects of ScCO2 on pores and rock minerals were analyzed by nuclear-magnetic-resonance (NMR) test, X-ray-diffraction (XRD) analysis, and scanning-electron-microscope (SEM) experiments. Second, the high-pressure chamber-reaction experiment was conducted to study the interaction mechanism between ScCO2 and live oil under formation conditions, and quantitively describe the change of high-pressure physical properties of live oil after ScCO2 injection. Then, the numerical-simulation method was applied to analyze the distribution and existence state of ScCO2, as well as the changes of live-oil density, viscosity, and composition in different stages during the full-cycle fracturing process. Finally, four injection modes of ScCO2-injection core-laboratory experiments were designed to compare the performance of ScCO2 and slickwater in terms of energy enhancement and flowback efficiency, then optimize the optimal CO2-injection mode and the optimal injection amount of CO2 slug.The results show that ScCO2 can dissolve calcite and clay minerals (illite and chlorite) to generate pores with sizes in the range of 0.1 to 10 µm, which is the main reason for the porosity and permeability increases. Besides, the generated secondary clay minerals and dispersion of previously cemented rock particles will block the pores. ScCO2 injection increases the saturation pressure, expansion coefficient, volume coefficient, density, and compressibility of crude oil, which are the main mechanisms of energy increase and oil-production enhancement. After analyzing the four different injection-mode tests, the optimal one is to first inject CO2 and then inject slickwater. The CO2 slug has the optimal value, which is 0.5 pore volume (PV) in this paper.In this paper, the main mechanisms of using ScCO2 as the prefracturing energized fluid are illuminated. Experimental studies have proved the pressure increase, production enhancement, and flowback potential of CO2 prefracturing. The application of this method is of great significance to the protection of water resources and the improvement of the fracturing effect.

Analysis of soil clay mineral in terrestrial ecosystem using X-ray diffraction spectroscopy

Conventional X-ray diffraction spectrum and synchrotron radiation X-ray diffraction spectrum were used to study the characteristics of soil clay mineral in the terrestrial ecosystem of northern China. The former showed that mineral composition was mainly 2:1 type secondary clay mineral. The existence of clay mineral in the forest soils was identified in the synchrotron radiation X-ray diffraction spectrum, whereas there was no evidence for their presence in the other spectrum. Compared with the conventional method, the synchrotron radiation X-ray diffraction had better resolution, compactedness and scanning range. Therefore, it seemed more suitable for the soil spectroscopy study of terrestrial ecosystem.

Water-free oil reservoirs: origin and morphology

The features of the morphology and composition of secondary minerals in the reservoir zones of the White Tiger (Bach Ho) deposit (shelf of South Vietnam) and deposits in clay rocks of the Bazhenov formation of the Salym area of Western Siberia are considered. It is shown that the common for these fields is the formation of a reservoir as a result of the leaching effect of high-temperature fluids. The genesis of the reservoir is determined by a complex of secondary minerals, obviously of hydrothermal origin, partially filling caverns and cracks in the rock: native silver – zincous copper – barite – anhydrite – kaolinite – laumontite (White Tiger oilfield) and sulphates of iron, aluminum, sodium and calcium – iron-aluminum alum, alunite, jarosite, mirabilite, melanterite, gypsum (deposit in the rocks of the Bazhenov formation of the Salym area). Diagnostics of secondary minerals was established based on the results of microprobe studies of undisturbed samples and analysis of the composition of water extracts from crushed core samples.The species composition of secondary minerals indicates a high temperature of the affected solutions, which varied from 600 to 200°C in the case of the White Tiger oilfield, and in the range of 350–450°C, in the case of rocks of the Bazhenov formation. Hydrothermal alteration of crystalline rocks with the formation of secondary clay minerals (kaolinite, montmorillonite, hydromica), zeolites and minerals-sulfates, which are crystalline hydrates, occurs with the absorption of huge amounts of water by the rock (up to 4∙108 tons of water per 1 km3 of altered rock). This may be related to the waterlessness (lack of bottom water) of oilfields, the reservoir of which has a hydrothermal origin. It is possible that oil deposits in crystalline rocks with high oil-saturated zones have associated waters of hydrothermal rather than sedimentogenic origin, but they occur at considerable depths and are not penetrated by drilling. This possibility is evidenced by the discovery of hydrothermal water in the basement of the White Tiger reservoir at a depth of 4493 m, which, in terms of chemical composition and mineralization, is close to the waters of sodium chloride hydrotherms of Kamchatka.

Controls on δ26Mg variability in three Central European headwater catchments characterized by contrasting bedrock chemistry and contrasting inputs of atmospheric pollutants.

Magnesium isotope ratios (26Mg/24Mg) can provide insights into the origin of Mg pools and fluxes in catchments where Mg sources have distinct isotope compositions, and the direction and magnitude of Mg isotope fractionations are known. Variability in Mg isotope compositions was investigated in three small, spruce-forested catchments in the Czech Republic (Central Europe) situated along an industrial pollution gradient. The following combinations of catchment characteristics were selected for the study: low-Mg bedrock + low Mg deposition (site LYS, underlain by leucogranite); high-Mg bedrock + low Mg deposition (site PLB, underlain by serpentinite), and low-Mg bedrock + high Mg deposition (site UDL, underlain by orthogneiss). UDL, affected by spruce die-back due to acid rain, was the only investigated site where dolomite was applied to mitigate forest decline. The δ26Mg values of 10 catchment compartments were determined on pooled subsamples. At LYS, a wide range of δ26Mg values was observed across the compartments, from -3.38 ‰ (bedrock) to -2.88 ‰ (soil), -1.48% (open-area precipitation), -1.34 ‰ (throughfall), -1.19 ‰ (soil water), -0.99 ‰ (xylem), -0.95 ‰ (needles), -0.82 ‰ (bark), -0.76 ‰ (fine roots), and -0.76 ‰ (runoff). The δ26Mg values at UDL spanned 1.32 ‰ and were thus less variable, compared to LYS. Magnesium at PLB was isotopically relatively homogeneous. The δ26Mg systematics was consistent with geogenic control of runoff Mg at PLB. Mainly atmospheric/biological control of runoff Mg was indicated at UDL, and possibly also at LYS. Our sites did not exhibit the combination of low-δ26Mg runoff and high-δ26Mg weathering products (secondary clay minerals) reported from several previously studied sites. Six years after the end of liming at UDL, Mg derived from dolomite was isotopically undetectable in runoff.

A global survey of radiogenic strontium isotopes in river sediments

Radiogenic strontium isotopes are routinely used in provenance studies, but their application to sediments is often complicated by various grain size and weathering effects, which can influence measured 87Sr/86Sr ratios. Here, we report Sr isotopic data for a large number of sediment samples (n = 61) from the world's largest rivers and other river catchments draining particular geological and climatic settings; using both clay-rich (<4 μm) and silt-size (4-63 μm) detrital fractions to re-examine the factors controlling their distribution in sediments. In agreement with previous studies, the detrital material transported by world rivers defines general inverse Nd-Sr isotope relationships, which provide further empirical evidence for the utility of radiogenic Sr isotopes in sediment provenance studies. In a novel departure, however, we show that the 87Sr/86Sr difference between paired clay- and silt-size fractions (Δ87Sr/86Sr Clay-Silt) relates to the degree of chemical alteration of river sediments, as inferred from various relationships with weathering indices, such as the CIA, WIP and Na2O/Al2O3. The weathering dependence of Δ87Sr/86Sr Clay-Silt appears to be mainly controlled by temperature. In sub-Arctic and temperate regions, river sediments systematically display positive Δ87Sr/86Sr Clay-Silt indicative of preferential alteration of biotite in soils. In contrast, in sub-tropical watersheds characterized by mean annual temperatures >20 °C, intense feldspar weathering leads to the preferential incorporation of unradiogenic Sr into secondary clay minerals; a process which results in negative Δ87Sr/86Sr Clay-Silt values. In addition to climate forcing, the degree of size-dependent Sr isotope decoupling is also shown to be dependent on the type of weathering regime in watersheds, being more pronounced in low-elevation environments (< 2000 m), where transport-limited conditions and the presence of thick soil sequences can be associated with intense silicate weathering, than in high mountain regions (> 4000 m) dominated by kinetically-limited weathering regimes.While further studies will be required to test the validity of these conclusions at the local scale of weathering profiles, these findings suggest that combined Sr isotopic analyses of separate size fractions could be used as a new weathering proxy in sediment records, ideally complementing the conventional use of radiogenic Sr isotopes as provenance tracers. Finally, our results are also used to re-assess the mean Sr flux and 87Sr/86Sr composition of the suspended sediment exported to the ocean yearly, yielding a global flux-weighted average of 0.7160, identical to that proposed earlier in the seminal study of Goldstein and Jacobsen (1988).

Weathering of tephra and the formation of pedogenic minerals in young Andosols, South East Iceland

Frequent tephra deposition and a steady influx of aeolian material of multiple origins, dominate soil formation in Iceland. Little is known about the weathering behaviour, mineral formation and alteration of tephra and Icelandic Andosols after tephra deposition.Two sampling sites in the volcanically active area south of Vatnajökull in South East Iceland were selected according to the presence of two distinctive tephra layers; a light coloured, rhyolitic, tephra from the Öræfajökull eruption in 1362 CE and a black, basaltic, Veiðivötn tephra from 1477 CE. Through a combination of physical, chemical and mineralogical analyses, the present research improves the understanding of changes in properties, weathering processes and mineralogy in Andosols after the deposition of tephra and aeolian material. It adds significant knowledge about the impact of explosive volcanic eruptions and consequences of heavy tephra fall and erosion on soil development in Iceland in particular, but may have wide-ranging influence on the management of volcanic soils worldwide.Both pedons can be classified as Andosols, showing silandic and vitric soil properties. The soils were acidic and dominated by sand. Feo/Fed ratios above 0.75 in all soils indicated a low degree of soil development. The major portion of the clay size particles mainly derived from poorly crystalline and amorphous constituents (e.g. allophane and ferrihydrite). In spite of the low soil age (less than 650 years) and the prevailing cool climatic conditions, we observed signs of pedogenesis and the presence of secondary clay minerals in both, soils and tephra. It was mainly secondary chlorite, which could be verified. Usually it takes more time under given environmental conditions for clay minerals to form and alter. The phyllosilicates we found are supposedly attributed to aeolian influx of material from older, more weathered, more developed eroded surfaces of unknown origin and chemical composition, maybe from sources outside Iceland. This and the particular local site conditions (e.g. moisture, soil temperature, SOM), rather than the primary composition of the parent material, are the driving factors in the development of the investigated Andosols and the alteration of minerals. Additionally, the high volcanic activity in Iceland constantly provides new tephra to the soils.

The distribution of (234U/238U) activity ratios in river sediments

Uranium (U) isotopes can be used to estimate the comminution age of sediments, i.e. the time elapsed from sediment production on continents, via weathering and physical erosion, to deposition in the sedimentary record. The calculation of this comminution age is based on measured (234U/238U) activity ratios in river sediments, and inferred time-dependent recoil effect, which leads to the preferential release of 234U from mineral lattices during erosion processes. In this study, we report on a large-scale (234U/238U) investigation of modern river sediments worldwide, with the aim to determine the extent to which parameters such as grain size, lithology, weathering, climate and geomorphology may influence the distribution of U isotopes in fine-grained sediments. Our extensive dataset (N = 64) includes U isotopic measurements for many of the world’s largest rivers, but also rivers draining particular climatic and geological settings. Our results indicate that sediments collected from river basins draining mostly igneous, metamorphic or volcanic rocks often display (234U/238U) ratios > 1, with clay-size fractions (<4 µm) being less depleted in 234U (higher 234U/238U) than corresponding silt-size fractions (4–63 µm). In contrast, sediments derived from multi-lithological basins or draining sedimentary rocks are typically characterized by (234U/238U) ratios <1, with clays generally exhibiting more depleted 234U signatures than silts. Taken together, these observations suggest that the formation of secondary clay minerals in soils from basins draining mostly igneous, metamorphic is accompanied by partial incorporation by recoil injection of 234U initially released during weathering processes, possibly from U-rich minerals, such as sphene or apatite. Instead, in multi-lithological catchments draining sedimentary rocks, we propose that the erosion of recycled sediments having experienced several cycles of weathering, possibly over glacial-interglacial timescales, could explain the much lower (234U/238U) ratios observed in clay-size fractions. While no direct relationships can be identified between sediment (234U/238U) ratios and lithology, weathering intensity, climatic or geomorphic parameters in corresponding river basins, we show that the catchment size probably plays an important role in controlling the distribution of (234U/238U) in river sediments, through its direct influence on the sediment residence time. Finally, a multiple regression analysis of our data, combining various environmental parameters for the lithology, climate and geomorphology of studied river basins, indicates predicted (234U/238U) values that are very similar to measured values (with R2 ∼ 0.8). This finding provides further support for the usefulness of (234U/238U) ratios in the sedimentary record for reconstructing past landscape changes and their effect on sediment transport and residence time in river basins.

Estimation the reactive surface area of minerals during fluid-rock interaction in hydrothermal Jiangxi Province (SE China)

The reactive surface areas of the most abundant reacting minerals have been estimated in the granite hydrothermal field of Jiangxi Province in South-East China. The chemical composition of the sampled fluids was used as input data and the continuous equilibrium approach was assumed for the kinetic calculations. Dissolution rates and the reactive surface areas of different minerals were obtained by reconstructing the chemical evolution of the interacting fluids in a reaction process schema defined by a fractional degree of advance of the irreversible mass-transfer process. We found that the reactive surface area ratio of albite/K-feldspar varies by 2–3 orders of magnitude while that of biotite/K-feldspar ratio by 4 orders of magnitude. The obtained results indicate that mineral reactive surface areas are not constant during the fluid-granite interaction: higher reactive surface areas were obtained at higher pCO2 partial pressure (lower pH) compared to water with lower pCO2 partial pressure (higher pH). The eventual formation of secondary clay-minerals overlaying the primary ones may be responsible for decreasing the contact area of the rock-forming mineral in thermal waters having higher pH. The results of our investigation indicate that the reactive surface area of biotite play an important role in the CO2-neutralizing process of this area that in turn, controls the natural releasing of CO2 to the atmosphere in this area.

REE redistributions during granite weathering: Implications for Ce anomaly as a proxy for paleoredox states

Abstract Different responses of Ce to the redox state from those of the other light rare earth elements (LREEs) can be used to understand paleoredox states. To establish the possibility of using the Ce anomaly as a proxy for paleo-environments, we examined the mineralogical and chemical characteristics of bulk samples and REE-bearing minerals of a modern weathering profile developed on granite, by X-ray fluorescence analysis, laser-ablation inductively coupled plasma mass spectrometry, field emission electron microprobe analysis, field emission scanning electron microscopy, and X-ray diffractometry. Bulk samples showed no significant Ce-anomalies except for the topmost layer that had a positive Ce-anomaly reflecting significant loss of LREEs except for Ce. Allanite-(Ce), primary REE-bearing mineral, contributed to ~100% of La, Ce, Pr, and Nd in the parent rock, and gradually decreased in amount toward the topmost layer. Secondary cerianite-(Ce) [Ce(IV)O2] was observed in the weathering profile, especially at shallower depths. Secondary rhabdophane-(La), -(Ce), -(Nd), and -(Y) were also observed in the weathering profile but in less amounts in the topmost layer. The occurrences of rhabdophane-(La) and -(Nd) in contact with halloysite, a secondary clay mineral, suggest probable adsorption of REEs onto halloysite prior to their formation. Similar formation mechanisms are likely for rhabdophane-(Ce) that commonly occurred in grain boundaries and was usually formed in contact with halloysite. Rhabdophane-(Y) occurred in association with fluorapatite. The ratios of La, Pr, and Nd of rhabdophane-(La), -(Ce), and -(Nd) were similar to that of allanite-(Ce), suggesting that these LREEs are inherited from allanite-(Ce) and behave similarly before the formation of rhabdophane. Different negative Ce-anomaly values of rhabdophane [i.e., ~0.03–0.34 for rhabdophane-(La), -(Nd), and -(Y), and ~0.6 for rhabdophane-(Ce)] can result from a difference in intensity of the formation of cerianite-(Ce) prior to the precipitation of rhabdophane. We have classified LREE redistributions in both secondary minerals and bulk weathered samples during oxic weathering and suggested that Ce anomaly can provide useful information on anoxic weathering and thus atmospheric oxygen evolution in the Precambrian if Ce anomalies of both bulk samples and secondary REE-bearing minerals are determined.

Content of radionuclides (226Ra, 232Th, 40K, 137Cs) in soils of the North-West region of Russia formed on three types of soil forming rocks

The results of a study of the content of natural radionuclides (NR) (226Ra, 232Th, 40K) and technogenic 137Cs in soil samples formed on the most common types of soil-forming rocks of the North-West: lakeglacial (non-boulder and tape clay), glacial and water-glacial deposits are presented. Soil sections laid in the Pskov, Novgorod and Leningrad regions. The granulometric composition in the upper part of the soil profile is lighter than in the parent rock. This is reflected in the NR content in the profile of these soils: with a decrease in the fractions of physical clay and silt, the content of 226Ra, 232Th and 40K decreases. This is due to the fixation of natural radionuclides by secondary clay minerals. According to the increase in the average specific activity of all NRs, the soils form the following sequence: soils on water-glacial deposits soils on moraines - soils on lake-glacial deposits. A comparison of the average specific activity of natural radionuclides and their ranges in the upper part of the soil (0-5, 5-10 and 10-20 cm) and in the parent rock (90-100 cm) revealed a higher content of radionuclides in parent rocks (lake-glacial and moraine deposits). In the soils on water-glacial deposits, no significant difference was found.

Magnesium isotope fractionation during hydrothermal seawater-basalt interaction

Fluid-rock interactions in hydrothermal systems at or near mid-oceanic ridges (MOR) play a major role in determining the composition of the oceanic crust and seawater. To quantify the processes that govern cation exchange in these environments we have experimentally studied the isotopic evolution of δ26/24Mg in the fluid phase during seawater-basalt interaction at 250 and 290 °C. Mass balance constraints indicate that isotopically heavy Mg was preferentially incorporated into non-exchangeable (octahedral) sites in secondary clay minerals such as saponite (Mg-rich smectite), leaving residual fluids enriched in light Mg isotopes. The magnitude of fractionation observed during smectite precipitation in our experiments (εSmectite-Liquid26/24) ranged from 0.35‰ to 0.42‰. This observation, which contrasts with the preferential uptake of light Mg isotopes into biogenic and inorganic marine carbonates, highlights the potential utility of Mg isotopes as tracers of the precipitation dynamics of authigenic Mg-silicate and Mg-carbonate phases. Furthermore, although Mg isotopic fractionation is often masked by the almost complete removal of Mg in high temperature marine hydrothermal systems, our experiments demonstrate that it does become significant at lower temperatures where Mg removal by clay formation is incomplete. Under such conditions, this fractionation will create isotopically light fluids due to smectite precipitation, thus potentially represents an important component of the marine Mg isotope inventory.

Experimental constraints on Mg isotope fractionation during clay formation: Implications for the global biogeochemical cycle of Mg

The direction and magnitude of magnesium (Mg) isotope fractionation attendant to the formation of clay minerals is fundamental to the use of Mg isotopes to decipher the biogeochemical cycling of Mg in the critical zone and for the oceanic Mg budget. This study provides experimental data on the Mg fractionation factor for two smectite-group minerals (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. A series of experiments were performed to asses the impact of temperature and pH on isotope fractionation. Bulk solid samples were treated with ammonium chloride to remove exchangeable Mg in order to distinguish the Mg isotopic fractionation between these sites and octahedral sites.All bulk and residual solids were enriched in 24Mg compared to the initial solution and δ26Mg values of the exchangeable pool were lower than, or within error of, the initial solution. Final solutions were either within error of, or enriched in, 26Mg compared to the initial solution, depending on the fraction of Mg removed from solution (fMg). For experiments with small or negligible fMg, increasing the pH resulted in a higher reaction rate and reduced fractionation from the initial solution. This could point to a kinetic effect, but the composition of the residual solid (Mg/(Li+Mg) ratio) was also dependent on pH. The change in the composition was reflected in the wavenumber of the Mg3-OH stretch in FT-IR data, which is a proxy for bond strength, and suggests an equilibrium control. An equilibrium control is further supported by the observation of reduced fractionation compared to the initial solution with increasing temperature. Rayleigh and batch fractionation models were fitted to the data giving fractionation factors of 0.9991 and 0.9990 respectively.We compare our results with existing field and experimental data and suggest that the apparent contradictions surrounding the direction of Mg isotope fractionation into phyllosilicate minerals could be due to the similarity of Mg bond lengths between clay octahedral sites and dissolved Mg. Thus small changes in mineral structure or initial solution conditions may result in a change in bond length sufficient to alter the direction of fractionation, implying that the magnitude and direction of Mg isotope fractionation into clay minerals could be dependent on local field conditions. Alternatively, if the precipitation of secondary clay minerals in the field preferentially incorporates light Mg, as observed in this experimental study, this implies the contribution of carbonate weathering to dissolved Mg fluxes has been underestimated, with major implications for the global biogeochemical cycle of Mg.

Mineral replacement in long-term flooded porous carbonate rocks

This study reports mineralogical and physical property changes linked to geo-chemical alterations processes during three ultra-long-term tri-axial tests on outcrop-chalk from Liège (Belgium). The test core plugs were flooded with MgCl2-brines for approximately one and a half, two and three years, mimicking effective reservoir stresses (9.5–12.5 MPa) and temperature (130 °C) of important hydrocarbon deposits at the Norwegian Continental Shelf. The flooded cores were studied using electron microscopy, whole-rock and stable isotope geochemical analyses, and ion chromatography of the effluent water.All tests show altered textures and mineralogy at the flow-inlet side of the approximately 7 cm long cores. With longer duration of flooding, these alterations moved further into the cores, and for the three-year-test, the entire core was altered. When studied at nano-scale, the newly formed crystals were found to be magnesite containing minor calcium impurities, together with clay-minerals. On the outlet side of the alteration-fronts in the two shorter tests, the mineralogy still mainly consists of calcite and primary clay-minerals, together with newly formed magnesite and secondary clay-minerals. Dolomite or low- and high-Mg-calcite are not observed. The textures of larger micro-fossils are often preserved, but the mineralogy of their shells is altered.A sharp, only 4 mm narrow transition zone at the border of the alteration front towards the less altered area for the two shorter tests, shows the highest porosity in the cores. This pattern resembles what is observed in single-crystal experiments, where the alterations are driven by phase dissolution and subsequent precipitation, the progression of high porosity zones and the state of equilibrium at the boundary between the primary and new mineral phase. This is also in line with observations in nature and models for transport driven mineral replacement in porous media, where differences in dissolution and precipitation rates may cause high porosity transitions zones.During the experiments, all cores underwent severe overall compaction between 10.1% and 18.2%. However, in the two- and three-year long test-cores, the permeability, and calculated porosity, started to increase after a primary phase of reduction. As magnesite precipitates at the expense of calcite, the density increase, but the solid volume decrease. As the bulk volume is constant, porosity and permeability are increased. The changes in ion-concentration of effluents, monitored throughout the experiments, balance the changes in mineralogy, compaction and permeability within the cores. Compositional variations of the injection fluid effectively control the amount of chemical reaction in chalk. This allows for control and predicting changes in geo-mechanical parameters induced by mineralogical replacement, which has significant impact on reservoir conditions.

Chemical and minero-petrographical changes on granulite rocks affected by weathering processes

The purpose of this work is to study the weathering processes of the granulite rocks of the Serre Massif (southern Calabria, Italy) using a multidisciplinary approach based on field studies, geochemical modeling, and minero-petrographical analyses. The granulite rocks are plagioclase-rich with minor amphibole, clinopyroxene, orthopyroxene, biotite, and garnet and their texture are coarse-grained. The reaction path modeling was performed to simulate the evolution of groundwaters upon interaction with local granulite by means of the software package EQ3/6, version 8.0a. Simulations were performed in kinetic (time) mode under a closed system at a constant temperature of 11.5°C, (which reproduces the average temperature of local area) and fixing the fugacity of CO2 at 10–2.34 bar (mean value). During the most advanced stage of weathering the main mineralogical changes are: partial destruction and transformation of biotite and plagioclase associated with neoformation of ferruginous products and secondary clay minerals producing a change in the origin rock fabric. The secondary solid phases observed during the geochemical modeling (kaolinite, vermiculite and ferrihydrite) are similar to those found in this natural system. Thus, the soil-like material mainly characterized by mostly sand to gravel grain-size fractions is the final result of the weathering processes.

Microtopography-mediated hydrologic environment controls elemental migration and mineral weathering in subalpine surface soils of subtropical monsoonal China

Local topography and elevation gradients can exert important influences on soil formation processes such as elemental migration, mineral weathering, and soil organic matter (SOM) accumulation, yet these influences remain insufficiently investigated to date, particularly in surface soils of subtropical monsoonal regions. Here, we report on an investigation of a series of surface soils collected from four different topographic locations across the subalpine Dajiuhu Critical Zone Observatory (CZO), representing hillslope (planar), swale and river channel (convergent), and bulge (divergent) microtopographic sites. Evidence provided by rare-earth element (REE) patterns, immobile element ratios, clay-mineral compositions, and particle-size distributions suggests that these soils have rather uniform parent materials. X-ray diffraction (XRD) analysis revealed that secondary clay minerals in these soils are complex, being dominated by various interstratified clays. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy shows similar patterns among the swale, bulge, and river-channel soils that differ from those of the hillslope soils. Most soils at convergent sites with poor drainage contain more smectitic clays (interstratified illite/smectite and chlorite/smectite) and less vermiculitic clays (hydroxyl-interlayered vermiculite and interstratified illite/hydroxyl-interlayered vermiculite), and exhibit weaker chemical weathering and fewer elemental losses than those from non-convergent sites. The diversity of clay types can be ascribed to the complexity and heterogeneity, in particular of pH and hydrology, in these soil environments. Across the range of microtopographic sites investigated here, elemental migration and chemical weathering are generally coupled, with greater elemental losses associated with more intense chemical weathering. Soil organic carbon (SOC) content generally increases at higher elevations, which is attributable to lower temperatures and a consequent reduction of microbial remineralization, and under more reducing soil water conditions. Though influenced by eolian dust, variations in Fe/Mn, Ce anomaly, and Corg/P can reflect redox conditions of different soils. SOC retention is also closely associated with soil redox status, with more reducing conditions being more conducive to SOC preservation.

Modeling the formation of soil microaggregates

The functions of soils are intimately linked to its aggregated structure. Microaggregates formed during pedogenesis from a vast variety of mineral, organic, and biotic materials are the smallest conceivable compounds at the basis of soil structural hierarchy. Qualitative hypotheses and concepts on how aggregates form are quite elaborate, but the consistent quantitative application within a mechanistic and physically rigorous framework is still missing. This is partly caused by the fact that such an endeavor requires the combination of synergistic concepts on the transport of particles and their interactions affected by shape, size, and surface properties in aqueous suspension. Here we present a novel quantitative approach for the formation of soil microaggregates in silico that integrates diffusion-limited cluster-cluster aggregation with a probabilistic attachment following the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of particle interactions. To represent universal mineral shapes found in soils, we implemented spherical, plate-like as well as rod-like particle morphologies inspired by weathered silicates, secondary clay minerals and pedogenic iron oxides. In this way, we developed a model that produces an explicit three-dimensional structure of aggregates emerging from hetero- as well as homo-aggregation in response to the chemical milieu. We then exploited the 3D morphology to assess functional properties of aggregates exemplified with the water retention curve and pore size distribution.

Rock avalanche-debris geometry and implications for rock-avalanche genesis

Rock avalanches have an important impact on the geomorphological evolution of mountain landscapes. Rock avalanche-debris geometry holds important clues related to the processes of the disaster predisposition, occurrence and rapid runout of rock avalanche-debris flows. The morphological features of the debris of the Touzhai rock avalanche deposit in Southwest China are examined as a specific case with the help of three-dimensional laser scanning, polarization microscopy (for polished sections) and field-emission scanning electron microscopy (FESEM). The unexpectedly high roundness is the most impressive morphological characteristic of the Touzhai rock avalanche-debris, particularly that of sand-plus silt-sized particles. The shapes of 12 scanned metric-sized boulders are closer to cubes than to spheres. 90% of the 114 scanned subangular blocks with a mean particle diameter of 12.04 cm and an average specific surface area of 0.225 cm2/g fall into the spherical domain of Zingg's classification diagram according to their flatness ratio and elongation ratio, and the average sphericity of 0.798 further indicates that they are more akin to cubes compared with spheres. The average form factor of the 1201 sand-sized grains is 0.784 with a standard deviation of 0.044, indicating that they are nearly spherical, and the solidity ranging from 0.951 to 0.974 also strongly shows that these particles are close to smooth spheres. Most of the grains in the size range of <0.075 mm are sub-angular except slaty secondary clay minerals of <0.005 mm based on the visual estimation under FESEM. The detailed qualitative and quantitative contrasts between the morphometric characteristics of the Touzhai rock-avalanche debris and those of the fresh basalt fragments demonstrate that the geometry of the former is dominated by the pre-failure predesign in which the basalt, primary and secondary structures, unloading and weathering acted together. Weathering was the direct contributor to its high roundness. The dynamic fragmentation triggering newly generated ruptures during the post-failure motion was less important, which should be the case in some rock avalanches in other regions. The slope pattern of “large creeping main slide mass + lower intact locked segment” generalized according to this study should be applicable to some other rock avalanches worldwide, and can help the pre-event identification of the potential rock-avalanche sites. Our results support the hypothesis of “aerification of debris” to explain the high mobility of rock avalanches. Highly fragmented source rockmass with enough fine grains should be necessary for the occurrence of rock avalanches and long, rapid runout of the rock avalanche-debris flows.

Influence of long-term operation of supercritical carbon dioxide based enhanced geothermal system on mineralogical and microstructurally-induced mechanical alteration of surrounding rock mass

Chemico-microstructural and mineralogically-induced time-dependent mechanical alterations of the reservoir rock found in the peripheral zone of a CO2-based EGS is experimentally investigated. A series of uniaxial compressive and triaxial (under 30 MPa constant confining pressure with different temperatures; 25-300 °C) strength tests on granite specimens reacted in ScCO2+water medium for different time periods was performed by coupling acoustic emission (AE) technique. The results were compared with those for intact and water-reacted rock specimens in order to better understand this weakening phenomenon. Scanning electron microscopy (SEM) was also incorporated to evaluate the microstructural and mineralogical alterations.The results indicated that the mineralogical structure of Harcourt granite rock specimens was mainly altered by the dissolution of silicate minerals and plagioclase phase feldspar minerals. In addition, enhanced precipitation of secondary clay minerals including kaolinite, smectite, and illite was observed in the ScCO2+water-reacted rock specimens. The average compressive strength of ScCO2/water-reacted granite specimens found to be reduced with the increase of reaction period. Furthermore, 3.81%, 5.08%, 5.66% and 0.52% reductions in failure strength resulted under 30 MPa confining pressure in 6 months ScCO2/water-reacted granite specimens at 25 °C, 100 °C, 200 °C and 300 °C temperatures, respectively, compared to intact rock values at these temperatures. The post-failure analysis also revealed that the ScCO2/water reaction significantly alters the post-failure behaviour due to micro-mineralogically-induced weakening inside the rock pore structure. It was concluded that enhanced mineral precipitation and dissolution influence in time-dependent degradation of the surrounding rock mass which influence in long-term operation of the geothermal system.

Si cycling and isotope fractionation: Implications on weathering and soil formation processes in a typical subtropical area

The translocation of silicon (Si) is closely related to soil evolution. However, how to trace the transformation and migration paths of Si and further understand its effects on soil formation and evolution remains as a challenge in geochemistry and soil science. Here we studied the Si isotope (δ30Si) values and physical, chemical and mineralogical properties of rock, soil, plant and water in representative small watersheds in the south of Anhui province, subtropical China. The aims are to illustrate weathering and soil formation processes by tracing the fractionation of Si isotope among those ecosystem components as well as soil components and plant organs. Results show that the δ30Si values of bulk soil and clay are significantly related to many soil development indicators, such as total, free and amorphous Al oxides, as well as active Fe oxides, clay, silt and sand contents, Al/Si molar ratio and chemical index of alteration (CIA). These indicators evidence the relationships between Si isotope changes and mineral weathering and soil development degree. Biological resilication (accumulation of Si) by plant does not lower soil δ30Si because further fractionation occurs among plant organs, and soil phytoliths have significantly higher δ30Si than soil clay and silt particles. However, soil desilication (loss of Si) lowers soil δ30Si because much 30Si is transported to streams in runoff. The consistently positive δ30Si values of water and their relationships with Si concentrations and discharge suggest that dissolved Si (DSi) in stream water mainly comes from weathering of primary minerals; the contribution of dissolution of secondary minerals, quartz and phytolith is minor. Differential δ30Si values in clay, silt, sand, rock and water are the result of soil formation processes which directly evidence a dominate neoformation pathway of secondary clay minerals in this area. This study can help to further understand mechanisms and processes of Si translocation during soil evolution and, extend the application of Si translocation in soil genesis studies.

Trace element behavior in moderately weathered boles from the Deccan volcanic province: Implications for paleoenvironment

The present study examines the behavior of trace elements in moderately weathered boles of three different colors (i.e., red, brown and green) from the Deccan Traps in order to understand paleoenvironmental conditions during periods of quiescence between successive eruptions of Deccan flood volcanism (DFV). The minimal (<40%) losses/gains of trace elements, including rare earth elements (REE), in the brown and red boles indicate their low mobility either due to limited release from primary minerals and/or due to sorption on secondary clay minerals and Fe-Mn oxides under near neutral pH (~5.5 to 8) conditions. The green bole shows a greater loss of trace elements, on average ~40–70%, including the REE and the typically immobile elements V and Zr. The low concentrations of V and Zr in the green bole are in accordance with low Ti and high SiO2, K2O and Rb concentrations that suggest addition of petrogenetically evolved volcanic ash (e.g., andesitic composition) compared to the lower flow basalt.A negative Ce anomaly and enrichment of the light REE (LREE) in the green bole indicate precipitation of soluble trivalent LREE desorbed from Fe-Mn oxide surfaces preferentially to insoluble oxidized tetravalent Ce. This is interpreted to indicate more reducing conditions at depth, with Fe-Mn oxides in a more oxidized upper saprolite/soil horizon were eroded prior to trapping of the green bole by the upper lava flow. The light red bole, weathered under a more pervasively oxidative environment, shows loss of Mn oxides that might have initially scavenged cerianite leading to a negative Ce anomaly preserved in the residual bole. Positive Ce anomalies in the dark red and brown boles, are consistent with their gain and negligible loss of Mn, respectively, indicating Ce precipitation on the Mn oxide surfaces relative to the other trivalent LREE in an oxidizing environment. Overall, the boles show weathering signatures suggestive of near neutral pH conditions and locally changing redox, some of which may have largely escaped post-eruptive acidic precipitation possibly due to the transport of acidic gases into the upper atmosphere (stratosphere) under arid conditions.