Feldspar Content Research Articles

Geochemistry of the Middle Eocene Gercus Sandstone, Shaqlawa Area, Northern Iraq: Implications for Provenance, Tectonic Setting and Paleoweathering

The study of the geochemical characterization of sandstone rocks contributes to understand the nature of the source rocks, their tectonic setting and the effects of paleoclimate and paleo-weathering. Accordingly, twenty sandstone samples from the red sandstone units of the Gercus Formation were collected from the northeastern limb of Safin Anticline at Shaqlawa Area – Northern Iraq. The studied sandstone samples show depletion in major, trace elements contents, except for CaO, MgO and Ni, Co and Cr respectively, and general depletion in the rare earth elements (REE) contents. Major element geochemistry indicates that the red bed sandstones of the Gercus Formation are of an oceanic island arc (OIA) tectonic setting. On the other hand, the results revealed that the paleo-weathering of the source area of the Gercus Formation experienced intensive chemical weathering under humid and/or warm climate conditions. The maturity and paleoclimate conclusions of the studied samples suggest a low feldspar content and low values of K2O / Al2O3 ratio, which in turn resulted from the dominance of low grade metamorphic and sedimentary rock types in the source area.

Coseismic fluid–rock interactions in the Yingxiu–Beichuan surface rupture zone of the Mw 7.9 Wenchuan earthquake and their implications for the structural diagenesis of fault rocks

The mechanisms by which fluids modify the mineralogy and geochemistry of fault zones and the role of rock–fluid interactions in fault weakening are subjects of debate. We assessed the mineralogical and geochemical variations in fault rocks in the Shaba area of the Yingxiu–Beichuan surface rupture zone of the Mw 7.9 Wenchuan earthquake using analyses of their mineralogical compositions, major elements, and the microstructural characteristics of wall rocks, damage zones, and oriented fault gouge samples from the principal slip zones. The elemental and mineralogical compositions show a certain regularity across the fault zone from the wall rocks to the fault gouge so that (1) Al2O3, Fe2O3T, and K2O contents increase and exhibit significant enrichments in the fault gouge; (2) Na2O and P2O5 contents decrease gradually and exhibit significant depletions in the fault gouge; (3) quartz, feldspar, and carbonate mineral contents decline; and (4) clay mineral contents increase dramatically. Isocon analysis indicates that mass losses in the Shaba fault zone occur with losses in the damage zone < the fault gouge. The mechanisms of material loss and transformation in the fault zone are complex, and mechanical fracturing, dehydration reactions, and fluid–rock interactions are important factors in changing and controlling the material compositions and fault zone evolution, with material losses and transformation especially important within the fault core. Moreover, due to the damage zone having a higher permeability than the fault core, it was conducive to hydrothermal upwelling, fluid–rock interactions, and fracture healing.

Late Holocene peat paleodust deposition in south-western Sweden - exploring geochemical properties, local mineral sources and regional aeolian activity

Atmospheric mineral dust not only interacts with the climate system by scattering incoming solar radiation and affecting atmospheric photochemistry, but also contributes critical nutrients to marine and terrestrial ecosystems. In a high-resolution analysis of paleodust deposition, peat development and soil dust sources, we assess the interplay between dust deposition and bog development of the Davidsmosse bog in south-western Sweden. Analyses of the 5400-year record (458 cm) included radiocarbon dating, bulk density, ash content, chemical and mineralogical composition and carbon stable isotopes, subsequently explored using principal component analysis. Fourteen dust events (DEs) were recorded (cal BP) in the peat sequence: 3580–3490; 3280; 3140; 3010–2840; 2740; 2610; 2480; 2340; 2240–2130; 1690; 1240; 960, 890–760, and 620–360. The majority of the DEs were coupled to increases in peat accumulation rates and increased nutrient content (N, P and K) suggesting that the DEs contributed with nutrients to the bog ecosystem, promoting increased accumulation. We also analyzed the chemical and mineral composition of potential mineral source deposits (separated into 6 grain-size fractions) from sites within a 4 km radius as well as aeolian dunes closer to the coast (25 km). The composition deposited on the present-day bog surface indicates that the bulk of the contemporary minerals have a local origin (<1.5 km), but the DEs may be of a more distant origin. The results also indicate that quartz and plagioclase feldspar content consistently increase with increasing grain-size, both in the source samples as well as in the peat sequence, and that the Si/Al ratio can be used to infer grain size changes in the peat. Two longer phases saw numerous DEs, between 2800 and 2130 cal BP and a stepwise increase from 960 towards 360 cal BP. The episodic character of the events, together with the inferred coarse grain size, suggest that the particles were deposited by (winter) storms. Future studies should include grain size analysis as well as a more in-depth comparison with regional paleo dust and storm records to increase knowledge on both transport processes (creep, saltation, suspension) and the climate processes driving late Holocene dust and storm events in Scandinavia.

Quantification of the Fracture Complexity of Shale Cores After Triaxial Fracturing

Diagnosing fractures under compression is of great importance in optimizing hydraulic fracturing stimulation strategies for unconventional reservoirs. However, a lot of information, such as fracture morphology and fracture complexity, is far from being fully excavated in the laboratory limited by the immature fracture identification techniques. In the current study, we propose a set of methods to analyze the fracture complexity of cylindrical cores after triaxial fracturing. Rock failure under conventional compression tests is real-time controlled by monitoring the stress–strain evolutions to ensure that the cores remain cylindrical after failure. The lateral surface of the core cylinders is scanned with a 2D optical scanner to extract the fracture parameters, surface fracture rate, and inclination dispersion, which are normalized and averaged to derive the fracture complexity. After analyzing the data for 24 shale gas reservoir cores from the Sichuan Basin, the fractal dimension of fracture images shows a good linear correlation with the surface fracture rate but has no correlation with the dip dispersion. The calculated fracture complexity has nearly no relationship with the E-v–based brittleness index but demonstrates a positive correlation with the mineral content–based brittleness index. Moreover, the fracture complexity is associated with the core mineralogical compositions. The fracture complexity is positively correlated with the content of quartz, calcite, and dolomite and negatively correlated with the content of clay minerals and has no obvious relationship with the content of feldspar. The proposed method provides an experimental basis for the evaluation of fracturability of unconventional oil and gas reservoirs.

Mineral composition and seasonal dynamics of the content of heavy metals in bed loads of ravines and small rivers in the city of Kamyshin (Volgograd oblast, Russia)

The article presents the study of the mineral composition and content of heavy metals (Ni, Cu, Cd, Cr, Pb, Zn) in bed loads (0–1 cm layer) of ravines and small rivers in the city of Kamyshin (Volgograd oblast, Russia). We established that quartz is the most widespread rock-forming mineral found all-over the place. In areas, where proportion of quartz decreases, the content of hydromica and feldspars increases. We discovered that the content of heavy metals in the bed loads increases in 61.1% of cases in spring compared to summer due to the inflow of pollutants from the urban watersheds with snowmelt runoff. Bed loads with a high content of heavy metals should be considered as an object of accumulated environmental damage. Reclamation of the beds of small rivers and ravines is required in order to create a comfortable urban environment and reduce the negative impact on the Volgograd Reservoir.

Unconventional CO2 Storage: CO2 Mineral Trapping Predicted in Characterized Shales, Sandstones, and Coal Seam Interburden

Summary Carbon dioxide (CO2) capture from industrial sources including coal combustion, gas processing, cement or steel production, blue hydrogen, or direct air capture, and subsequent geological storage is part of the transition to reduce greenhouse gas emissions. Unconventional and conventional reservoirs provide opportunities for beneficial use such as enhanced recovery, supercritical CO2 (ScCO2) fracturing, and storage of gases such as CO2 and ancillary gases, or potentially hydrogen. The purpose of this study is to use Australian unconventional rock packages to understand the controls on CO2 reactivity and mineral trapping (the most secure form of storage) and compare the potential for CO2 storage. Characterization of core from the Surat, Eromanga, and Cooper basins, Australia, is used to populate CO2 and production water-rock reactivity models. Sensitivity to production water composition and temperature was also tested. Coal seam gas (CSG) reservoir interburden ranged from clay-rich mudstones to interlaminated sandstone and mudstone, and calcite cemented sandstones. The coal seam interburden samples contained high plagioclase and chlorite content. They were predicted to alter to carbonates calcite, ankerite, siderite, and dawsonite mineral trapping CO2. After 30 years, net mineral trapping varied from −0.1 to +0.3 kg CO2/m3, and pH was 4.6–4.9. Net mineral trapping after 1,000 years varied from 5.7 to 16.3 kg CO2/m3 and was 17.1 kg CO2/m3 with higher salinity water. The mineral content had the main control with different lithologies decreasing mineral trapping by 41 or 35% compared with a base case. Overlying plagioclase-rich sandstone trapped 17.1 kg CO2/m3 as calcite, ankerite, dawsonite, and siderite after 1,000 years with the pH increasing to 6. For the quartz-rich oil reservoir sandstone, however, only 0.3 kg CO2/m3 was trapped after 1,000 years. Gas shale and marine black oil shales contained high mica, chlorite, and feldspar content that could be converted to carbonate minerals, mineral trapping CO2. A marine black oil shale mineral trapped 8.3 and 13.9 kg CO2/m3 after 30 and 1,000 years, respectively, as siderite and ankerite. Unconventional reservoirs have a strong potential for mineral trapping during CO2 storage.

Shale gas potential of the lower Silurian hot shales in southern Iran and the Arabian Plate: Characterization of organic geochemistry

A significant phase of global warming appeared during the Llandovery and productive Silurian hot shale was preserved all over the world. The lower Silurian shale is the main effective source rock for most of the Paleozoic hydrocarbon in Iran and the Arabian platform. Silurian hot shales have become prospective resources for new energy such as shale gas. The regional distribution and shale gas potential of the lower Silurian hot shale in southern Iran and the Arabian plate are determined using outcrops and exploration well samples data from previous studies. The studied area has a high organic content (on average more than 2%), maximum burial depth is 5300 m, shale thickness of 30–200 m, organic matter maturities (most comparable), clay minerals content ranging from 20% to 57%, quartz content ranges from 20% to 49%, feldspar content ranges from 10% to 15% and calcite content ranges from 1.48% to 5% which all favor shale gas generation and accumulation. We concluded that southern Iran and east-central Saudi Arabia are two of the most sustainable and favorable locations for shale gas exploration and production for lower Silurian hot shale after assessing all of the key characteristics.

Chemical-mechanical coupling effects on the permeability of shale subjected to supercritical CO2-water exposure

The permeability of shale reservoir rock and caprock is the key parameters influencing the shale gas production and the storage security of CO2. During the CO2 enhanced shale gas recovery, the issue of how the coupled chemical-mechanical process control the evolution of porosity and permeability in shale remains undetermined. In this study, multiple tests were conducted to obtained the shale properties alteration induced by ScCO2-water exposure, including the mineral compositions measured by XRD, XRF and ICP-OES, the mechanical properties measured by uniaxial compression test, the pore structure and permeability of shale measured using nuclear magnetic resonance (NMR) at different confining stresses over a range of injection pressures. After ScCO2-water exposure, the contents of carbonate and clay minerals decreased, while the contents of quartz and feldspar increased. The geochemical reaction altered the pore structure and mechanical properties of shale, resulting in the enlargement of pore, the decrease in uniaxial compressive strength and elastic modulus of shale, which in turn impact the porosity and permeability evolution in shale. At the unstressed state, the pore in shale was enlarged by the pure chemical reaction, leading to the increase in porosity and the initial permeability of shale. At the stressed condition, the porosity and permeability of shale is controlled by the chemical-mechanical coupling effects, the permeability of CO2-water treated shale sample is lower than that of the untreated shale sample, which can be explained by the increase in stress sensitivity of shale permeability induced by the mechanical weakening, as the compressibility Cf and permeability change rate Δkc were increased after ScCO2-water exposure. In addition, the stress sensitivity of permeability in shale is stress-dependent, for both untreated and ScCO2-water treated shale samples, the Cf and Δkc of shale shown a negative relation with effective stress. At a higher effective stress condition, the change in the permeability of ScCO2-water treated shale is more significantly enhanced. The results demonstrated that the ScCO2-water-shale interaction induced chemical-mechanical effect may decrease the permeability of shale at in-situ stress condition, and hence adversely affecting the efficiency of gas recovery and CO2 sequestration in shale formation.

A multivariate statistical approach in correlating the acoustic properties with petrophysics and mineralogy on sandstones

SUMMARYWe examined the effects of the petrophysical and mineralogical variables on the acoustic properties of Upper Assam sandstones. All parameters were evaluated in the laboratory using recognized standard laboratory methods. Compressional wave velocity was measured in the laboratory using piezoelectric transducers of 54 kHz by using the ultrasonic-through transmission technique. Single parameter correlations among bulk density, porosity, permeability and mineralogy with compressional wave velocity showed that the compressional wave velocity correlated inversely with porosity, permeability and feldspar content and directly with bulk density and quartz content. For instance, a plug with a higher amount of feldspar content showed a corresponding decrease in compressional wave velocity. Similarly, higher quartz content showed a higher compressional wave velocity. Nevertheless, to depict a clear correlation among different parameters, a multiparameter analysis was performed. It was observed that the coefficient of determination improved from 0.596 to 0.899 when compressional wave velocity was modelled in terms of bulk density, porosity, quartz and feldspar content collectively, rather than when compressional wave velocity was modelled as a function of porosity alone. Hence, our study suggests that multiple parameters exhibit a coherent influence on compressional wave velocity. Therefore, a multivariate statistical approach involving petrophysical and mineralogical influences would be more realistic. Furthermore, major mineral fractions of sandstone rock type, that is quartz and feldspar fractions were varied for a range of porosity to determine the variations in compressional wave velocity for different saturation conditions. Capturing the fluctuations in compressional wave velocity within the geological constraints will aid in possible reservoir characterization away from the well-control or beyond the available data range. The integrated approach can result in more accurate and precise rock physics models that may help to infer key reservoir parameters. The developed models can be used in quantitative seismic interpretation for reservoir characterization and to identify prospective reservoirs.

Characteristics and Rates of Microbial Processes in Clays of Different Mineral and Elemental Composition in Relation to Safety Prediction for ESB Clay Materials

Microbial activity in clay barrier materials during radioactive waste disposal can lead to a violation of important physicochemical properties of the barrier system as a whole, thereby reducing the safety of the storage facility. This work evaluates the activity of the microbial complex of five bentonite and two kaolin clays. All clay materials were found to contain microorganisms, mostly with organotrophic aerobic and anaerobic metabolism capable of forming hydrogen sulfide and transforming basic and impurity clay minerals. The activity of microorganisms can increase with the ingress of degradation products of aluminophosphate matrices and cement barriers, as well as radiolysis products. For all clay the rates of microbial processes were shown to exhibit a direct correlation with the content of organic matter, kaolinite, and potassium feldspar, and an inverse correlation with montmorillonite content. A systematic methodological approach is proposed for clay materials and their susceptibility to microbial processes. The approach makes it possible, based on the content of organic matter, biophilic elements in the samples, parameters of the respiratory activity of the microbial complex, the formation of hydrogen sulfide, and other parameters, to assess in advance the possibility of using them in barriers.

Petrogenetic Studies of the Basement Rocks in Jimgbe and Environs, Northcentral Nigeria

Geochemical and petrographic studies of Basement rock outcropped at southeastern Jimgbe have been carried out with the aim of characterizing and delineating the petrogenesis of the basement rocks. Field results with Facies analyses of the basement rocks in the study area revealed four (4) dominant rock units: Porphyritic granite, Migmatitic gneiss, Granite gneiss and Biotite gneiss. The minor rocks include quartzo-feldspartic intrusion and quartz vein. The area has different structures such as fold, fracture and foliation which trends NW-SE and NE-SW direction, this is allotted to the Pan-African Orogeny imprints. Fourteen (14) analyzed for petrographic method and twenty-four (24) samples were analyzed for major elements using XRF geochemical method. The petrographic analysis result showed that the four rock types are granitic in origin due to their high quartz and feldspar contents. The major mineral compositions are quartz, plagioclase, microcline and biotite. Results of geochemical studies shows that the average oxide percentages are; SiO2 (69.25%), Al2O3 (12.41%), CaO (5.72%), Na2O (3.25%), K2O (3.76%), MgO (2.76%), Fe2O3 (1.79%), TiO2 (0.28%), MnO (0.33%) and P2O5 (0.47%). These results showed that the rocks are calcic, and are also of high K-Calc alkaline in nature, the rocks majorly plot in the eugeosynclinal field except for sample T4 which plots in the sedimentary field. They are mostly S-type peraluminous rocks except for T11 which is an I-type peraluminous rock and all plot in the Granodioritic field.

Characteristics and Mechanical Mechanism of In Situ Unloading Damage and Core Discing in Deep Rock Mass of Metal Mine

Based on the phenomena of core damage and core discing in deep strata of metal mines in Laizhou area, the physical and mechanical mechanism behind these phenomena and characteristics is discussed. The analysis shows that the damage effect of deep high stress rock is obvious after in-situ unloading, the core discing occurs when the damage reaches a certain degree, and the time effect of damage increases with the increase of buried depth. The characteristics of core discing are closely related to the stress environment and the mesostructure and mineral composition of the rock. Monzogranite with a large particle size and high content of potassium feldspar is easier to occur core discing than granodiorite in this area. The mechanism of core discing in Sanshandao gold mine is as follows: tensile failure occurs in the middle of the core, and composite failure of shear and tension occurs at the edge. The crack extends from the middle of the core to the outer edge, and finally, the tensile crack and shear crack intersect and connect to form a cake crack. Considering the stress condition and strength conditions of core discing, the energy release rate of rock mass in the core discing area is large, which has the conditions for strong rock burst. In addition, the ratio of maximum stress to tensile strength at the starting position of core discing is about 6.5 in this area, which is consistent with the previous analysis, so as to provide a strong basis for calculating in-situ stress based on the core discing phenomenon.

Study on Acoustic Emission Characteristics of Different Felsic Metagabbros Under Hydraulic Coupling

The mechanical properties of the deep rock are impacted not only by the complicated stress environment but also by the mineral composition. To reveal the relationship between mineral development characteristics and acoustic emission signals in the mechanical evolution of deep rocks, metagabbros with different feldspar mineral development were subjected to acoustic emission tests under hydraulic coupling. The characteristic parameter of acoustic emission signals and their correlation characteristics were investigated. The results showed that the mechanical property of the metagabbro with feldspar mineral development was better than that without feldspar mineral. The metagabbro with feldspar mineral development had much larger AE ringing counts and absolute energies, as well as stronger rock brittleness features than that without feldspar mineral development. With increasing feldspar mineral content in metagabbro, the AE signals showed a longer duration and a higher amplitude. During the unstable crack propagation stage, metagabbro without feldspar mineral development exhibited four peak frequency ranges, while that with feldspar mineral development displayed five frequency ranges. The metagabbro with nonuniform feldspar mineral development had more pores and microcracks than that with uniform mineral development. The permeability of the rocks was highly correlated with the uniformity of mineral development. This study provides a theoretical foundation for monitoring and early warning of water inrush risk in deep rock engineering.

Evaluating microdistribution of adsorbed and free oil in a lacustrine shale using nuclear magnetic resonance: A theoretical and experimental study

The occurrence states (adsorbed or free) directly control the transport and production of shale oil, and free oil is the most common potential product available using existing technologies. A variety of theories and experimental methods have been developed to estimate the adsorbed and free oil amounts in shale pore-fracture systems, however, it is difficult to clarify the microdistribution characteristics of shale oil in in-situ pore systems using these methods. In the study described here, by integrating experimental and theoretical models, an effective method was developed to reveal the microdistribution of oil confined in nanoporous shales from the Dongying Sag, in the Bohai Bay basin, China. Centrifugation–nuclear magnetic resonance (NMR) experiments were combined with the modified Langmuir equation, and then applied to estimating amounts of adsorbed and free oil. And the microdistribution characteristics of shale oil were then analyzed by the adsorption ratio theoretical model. Furthermore, the mechanisms of controlling the adsorbed, free amounts and microdistribution of shale oil were also discussed. The primary results demonstrated that the modified Langmuir equation (Langmuir-like equation) was an effective and accurate technique for evaluating the adsorbed and free oil amounts in shales. We estimated that adsorbed and free oil amounts ranged from 5.8023 to 29.7995 mg/g, and from 3.9510 to 25.6410 mg/g, respectively, at a temperature of 20 °C, while the adsorbed oil weight ratios were 0.2778–0.7023 (mean 0.5438). The average value of the adsorbed oil density was approximately 2.0231 g/cm³, which was 2.6994 times that of the free oil. The average adsorbed layer thickness was 0.7433 nm, which equaled nearly two molecular diameters. The adsorbed and free oil microdistribution was clearly described by the adsorption ratio theoretical model and the T₂ spectrum. The adsorbed oil was mainly confined to micropores, while free oil was stored in meso- and macropores—and that pore microstructures affected shale oil microdistribution characteristics. The amount of adsorbed oil was mainly controlled by micropore volume and specific surface area, while the amount of free oil was closely associated with total volume of the larger pores. We also noted that the more complex the pore structure, the lower the free oil content and the higher the adsorption ratio. The amount of adsorbed oil was closely related to the shale composition, and increased with TOC content. The amount (ratio) of adsorbed oil also increased with increasing clay mineral content, and was negatively proportional to the quartz + feldspar content. The authors are confident that the study described in this paper has delivered new insights into the microdistribution of adsorbed and free oil in shale pore-fracture systems.

Physical and mechanical properties of sandstones from Southern Zanjan, north-western Iran

In this paper, three groups of quartz-rich sandstone were selected. These three groups include sandstones with low, medium and high quartz content. Three categories of tests including petrography tests, physical and engineering index property tests were performed on 26 samples of sandstones. Physical and engineering index property tests included porosity, density, P-wave velocity, and compressive and tensile strength tests. The purpose of the experiments/article was to find a relationship between different characteristics of the studied sandstone samples. Emphasis was placed on the effect of feldspar content and quartz content on the tensile and compressive strength of specimens. According to the results, significant relationships exist between the engineering index characteristics and the compressive and tensile strength of the samples as well as their elastic properties. The main finding of this research is that due to the presence of micro-mineral cracks in feldspar, increasing feldspar content reduces the compressive and tensile strength of the samples. Packing density, packing proximity, sub-angular and angular grains and quartz content are positively correlated with dry density and P-wave velocity and negatively correlated with porosity. Compressive and tensile strength and Young’s modulus increase with increasing dry density, P-wave velocity, packing density, packing proximity, percent sub-angular and angular grains, and quartz content. Feldspar content has a positive correlation with porosity and a negative correlation with dry density and P-wave velocity.

Tight sandstone petrophysics and flow rate/water/alkali sensitivities variations: A cross section of permian in the northeastern of Ordos Basin, China

Tight gas is an important unconventional resource with continuous consumption demand, and co-production from multi-layers are becoming an inevitable choice for areas with low reserve abundance. The Ordos basin contain tight gas production from generally the whole Permian, recording sandstones deposited from transitional to terrestrial environments. In order to reveal the reservoir sensitivities of different layers, petrophysical and sensitivity experiments were conducted with samples from members of different sedimentary environment, i.e., Tai 2 Member (P1t2, coastal environment), Shan 1 Member (P1s1, deltaic environment), He 8 Member (P2x8, deltaic-fluvial environment), He 4 Member (P2s4, fluvial environment), and Qian 5 Member (P3s5, braided fluvial environment) of Permian. The results showed: (1) The sandstones of P1t2, P1s1, P2x8, P2s4, and P3s5 are dominated by feldspathic litharenite, litharenite, lithic arkose, litharenite, and lithic arkose, respectively; (2) From P1t2 to P3s5, the content of illite decreases, those of chlorite and illite/smectite (I/S) increase, and kaolinite firstly increases and then decreases (the trend changed in P2s4); (3) The permeability of all the analyzed sandstones ranges between 0.2 and 0.6 mD, the average porosity between 5.7% and 8.2%, with mercury withdrawal efficiency between 32.8% and 42.1% and average median radius between 0.10 and 0.20 μm; (4) All the sandstones have medium–weak flow rate sensitivities, with medium–weak (P1t2), medium (P1s1), medium (P2x8), medium (P2s4), and medium–weak (P3s5) water sensitivities and weak (P1t2), medium–weak (P1s1), medium–weak (P2x8), medium–weak (P2s4), and medium (P3s5) alkali sensitivities. The flow rate sensitivities are mainly influenced by pore structure and the quartz content, while water sensitivities are affected by pore structure, I/S and illite contents, and the alkali sensitivities are correlated with pore structure and kaolinite, I/S, and feldspar contents. The results can be beneficial for tight gas production engineering choices and strategies for improving gas production.

Characteristics and formation of corundum within syenite in the Yushishan rare metal deposits in the northeastern Tibetan Plateau

Abstract Corundum is rarely found in situ within alkali syenites. A corundum-bearing syenite was found in the Yushishan rare metal deposits of the eastern section of the Altyn Tagh fault in the northeastern Tibetan Plateau, but the characteristics and formation of corundum remain unknown. We describe a corundum-bearing syenite dike emplaced in biotite plagioclase gneiss that suffered overprinted deformation with characteristics of mylonitization. The corundum crystals have variable grain sizes, and the largest ones are megacrystic with growth zoning. The corundum crystals contain a variety of mineral inclusions that are divided into primary and secondary. The primary mineral inclusions within the corundum include variable contents of Fe-Ti oxide needles, ilmenite, zircon, monazite-(Ce), potassium feldspar, pyrochlore, columbite-(Fe), magnetite, samarskite-(Y), and pyrite that indicate corundum crystallized in peraluminous Zr-rich and Si-poor alkali rock with variable TiO2 contents. Secondary mineral inclusions include Zn-rich hercynite, ilmenite, magnetite, annite, fluorapatite, and intergrowths of ilmenite with columbite-(Fe) and goethite that reveal late-stage influx of Zn-, Ti-, Fe-, and F-bearing fluids into corundum that caused metasomatism and element migration and mineral precipitation. The trace element analysis of corundum shows high-Fe and -Ga contents and low-Mg and -Cr contents that are consistent with the characteristics of corundum of magmatic origin. The trace element characteristics and the oxygen isotopes (6.2–8.2‰) results indicate that corundum crystallized in melts with the involvement of Al-rich and Si-poor crustal material.

Spearman rank correlations analysis of the elemental, mineral concentrations, and mechanical parameters of the Lower Cambrian Niutitang shale: A case study in the Fenggang block, Northeast Guizhou Province, South China

The increasing complexity of reservoirs, particularly shale reservoirs, demands accurate information on the formation composition, mineralogy, and mechanical parameters for effective exploitation. The development of geochemical and geophysical logging technology allows the investigation of the correlations between the elemental, mineral contents, and the mechanical parameters of shale. Taking the Lower Cambrian Niutitang Formation shale in the Fenggang block in northeastern Guizhou as an example, 889 data sets of the main elemental (Si, Ca, Fe, S, Ti, Gd, K, Mg, and S) concentrations and minerals (carbonate, pyrite, clay, quartz, feldspar, and mica) from Elemental Capture Spectroscopy logging and rock mechanics parameters (dynamic Young's modulus, dynamic Poisson's ratio, shear modulus, bulk modulus, and brittleness index) determined via Sonic Scanner logging data in the target zone of the study area are collected. We investigated the relationships between the mechanical parameters and the elemental, and mineral concentrations by Spearman rank correlation analysis. The influence of different Si concentrations on the mechanical parameters of the rock (positive/negative, and significance) is discussed based on the mineral contents determined via X-ray diffraction of 24 experimental samples. When 22.1% < DWSi (dry weight fraction of the silicon content as a percent) < 28.40% and 28.40% < DWSi <34.20%, the static Young's modulus, shear modulus, bulk modulus, and brittleness index exhibit a highly positive/negative correlation that is significant with the increase/decrease in DWSi, WCLA (dry weight fraction of clay), and WQFM (dry weight fraction of quartz, feldspar, and mica) contents, respectively. DWSi = 28.4% is suggested as the turning point for the correlation change (positive/negative, significance). Thus, there exists uncertainty in the determination of the mechanical parameters of rock within certain elemental content ranges. More mathematical models and factors need to be considered to adequately characterize the mechanical parameters of rock.

The occurrence of tight oil in the Chang 8 lacustrine sandstone of the Huaqing area, Ordos Basin, China: Insights into the content of adsorbed oil and its controlling factors

Tight oil is a great potential resource for unconventional oil exploration and development. Determining the content of adsorbed tight oil in sandstone reservoirs and the factors controlling its content is crucial for the evaluation and development of tight oil resources. Using fluorescence petrography, environmental scanning electron microscopy, porosity as well as permeability analyses, high-pressure mercury intrusion, and nuclear magnetic resonance imaging, the present study analyzed the occurrence form, location, and relative content of free and adsorbed tight oils and effect of controlling factors on the content of adsorbed tight oil in sandstone reservoirs of the Chang 8 member in the Huaqing area of the Ordos Basin. The free tight oil mainly exists in residual intergranular, intergranular dissolved, intragranular dissolved, and dissolution-enlarged pores as well as some throats in the form of flakes with various areas and slender strips. The adsorbed tight oil mainly occurs on the surfaces of the mineral particles and mica fragments as well as in fractures, some throats or the intragranular as well as intergranular dissolved pores in the form of films with variable thicknesses and long strips. The lower limit of the pore radius at which free tight oil occurs in sandstone is approximately 0.1 μm and the adsorbed tight oil occurs on the surface of mineral particles and in spaces with pore radii of 0.02–0.3 μm. The contribution of free tight oil to total tight oil in the present study has highest and lowest content of 60.95% and 32.48%, respectively, with an average content of 50.14%. The contribution of adsorbed tight oil to total tight oil has highest and lowest content of 67.52% and 39.05%, respectively, with an average content of 49.86%. The content of adsorbed tight oil is generally proportional to the median pressure and inversely proportional to the porosity as well as permeability of the reservoir; particularly, porosity is relevant. The content of adsorbed tight oil is inversely proportional to the quartz content and directly proportional to the feldspar, carbonate cement, and clay mineral content of the sandstone.

Characteristics of rock types and exploration significance of the shale strata in the Chang 73 sub-member of Yanchang Formation, Ordos Basin, China

For a long time, the Chang 73 sub-member of the Ordos Basin was mostly investigated as a single source rock, with hardly any investigation into the characteristics of various rock types or the integration of source and reservoir. Based on correlation analysis of lithology and physical properties combined with a fine interpretation of logging, the characteristics of the Chang 73 sub-member are discussed in detail, and the reservoir characteristics and exploration significance of different types of rocks are analyzed. The results show that: (1) There are five types of lithology in the Chang 73 sub-member: black shale, dark mudstone, fine sandstone, siltstone, and tuff. (2)The storage space of sandy rocks is primarily made up of feldspar dissolution pores and residual intergranular pores, which are held together by rigid grains. The sandstone with high feldspar and quartz mineral content, low carbonate cementation, low clay mineral content, and a large accumulated thickness is the optimal shale oil reservoirs of Chang 73 sub-member. (3) The Chang 73 sub-member has the most organic-rich shale deposits. Clay mineral intercrystalline pores, pyrite intercrystalline pores, and clastic grain intergranular pores make up the majority of the reservoir space. Although organic matter pores are largely undeveloped, organic matter contraction fractures are prevalent. Despite the porosity and pore size of shale in the Chang 73 sub-member being microscopic, it possesses specific reservoir performance and hydrocarbon fluid mobility. It is estimated that the total amount of retained hydrocarbon resources in Chang 7 Member shale is about 15 × 109 t, with movable hydrocarbon resources of about 6 × 109 t; (4)The oil saturation of Chang 73 sub-member is generally higher than that in Chang 71 and Chang 72 sub-members. Several factors play a key role in rapidly increasing shale oil production, including further optimizing the vertical and plane distribution characteristics of sand bodies, improving the drilling encounter rate of sandstone in horizontal wells, strengthening the research of pilot production scheme, and ensuring sufficient oil-water infiltration and replacement. (5) Optimizing black shale with high brittleness index and oil content, developing high-efficiency permeability improvers, adopting pressure drive production integrated design, increasing fracture width and height, and improving the conductivity of artificial fractures are all important technical means to realize shale oil production.