Studied Rock Samples Research Articles

Unraveling the Protolith Origin of Serpentinite Rocks from the Tengnoupal Area, Manipur Ophiolite Complex, Indo-Myanmar Range

Objective: This article mainly focuses on investigating the protolith origin of serpentinite rocks in and around Tengnoupal, within the Manipur Ophiolite Complex (MOC), using a combined petrographic and geochemical approach. Methods: During fieldwork, samples were collected and cut into thin sections to reveal their mineralogical and textural relationships. Major oxides and trace elements were determined by XRF, and rare earth elements analysis was carried out by ICP-MS to determine the nature and origin of the rocks. Findings: The studied rock samples exhibit varying degrees of serpentinization, ranging from partial to complete, and exhibit a pseudomorphic texture including mesh and hour structures replacing the original olivine and bastite texture replacing pyroxene. Geochemically, the analyzed rock types exhibit komatiitic affinity and are characterized by high magnesium numbers (Mg# average 84.4). The studied serpentinite rocks have lower Al/Si ratios (Average=0.05) and higher Mg/Si ratios (average= 0.95) in bulk rock compositions indicating a refractory abyssal mantle peridotite protolith when compared to the primitive mantle. The high Ti (range 300-480 ppm) and Cr (628-3186 ppm) contents indicate a subduction environment. In addition, the rock samples exhibit slight depletion chondrite-normalized Rare Earth Elements (REE) patterns ((La/Yb) N = average 0.87). The rocks exhibit spoon-shaped normalized Rare Earth Element patterns and primitive mantle are similar to mid-oceanic ridge abyssal mantle rocks. Novelty: The origins of the rocks studied suggest a transitional phase between abyssal and subduction zone-related serpentinites, forming after harzburgite-peridotite. As subduction began, dehydrated orogenic peridotite from the abyssal serpentinites was thrust or obducted, forming the MOC serpentinites. This indicates that the serpentinite rocks in MOC share similarities with those found in mid-oceanic ridge environments, which is characteristic of a supra-subduction zone setting. Keywords: Ultramafic rocks, Serpentinization, Supra­subduction zone, Abyssal, REE

Organic geochemical, petrographic and palynological characterization of claystones of the Palaeogene Toraja Formation, and oil seeps in the Enrekang Sub-basin, south Sulawesi, Indonesia: Implications for hydrocarbon source rock potential

An organic geochemical, petrographical, and palynological evaluation was conducted on 30 claystone outcrop samples of the Toraja Formation, along with a geochemical analysis of an oil seep in the Enrekang Sub-basin. The aim of the study was to determine the correlation between oil and source rock in terms of age, depositional environment, organic material sources, and maturity level. The total organic carbon content of the claystone samples varies from 0.03 to 4.52 wt%, which are classified as poor to excellent. The claystones are immature to post-mature with a mixture of Type Ⅱ and Ⅲ kerogen. Their vitrinite reflectance values range from 0.47 to 4.52 %Ro. The samples of Toraja Formation rock and the oil seep source rock might have a similar depositional environment, a deltaic marine depositional setting with high oxidizing conditions. Organic material sources for rock and oil samples are dominated by terrestrial input. The oil is inferred to have originated from the Paleogene source rocks, which correlates in age with the Toraja Formation. The recovered palynomorphs from the studied rock samples suggest a late Eocene to Oligocene age with a strong terrestrial influence of shallow marine depositional setting. The biomarker analysis shows that the rock samples have a more substantial contribution from the terrigenous higher plants, while the oil sample indicates a higher degree of marine influence. The maturity levels are also different between the oil (peak mature) and the analyzed rock samples (immature). It is inferred that the oil seep source rock is equivalent to the analyzed rock sample but more mature, having been deposited under more marine conditions. The oil seep source rock is not exposed and is located in the deeper part of the basin. A deeper marine facies (i.e. distal delta front and prodelta facies) in front of the distributary mouth bar within a delta is interpreted as the source rock of the oil seep sample.

Effect of fungal isolates from different samples upon radionuclide behavior and environmental hazard indices during bioleaching process in Gabal Um Hamd, Um Bogma area, southwestern Sinai, Egypt

Two fungal species were isolated from the studied rock samples and identified morphologically as Aspergillus hollandicus and Penicillium citrinum. Bioleaching process was applied to W1, W2 and W3. The average concentration activity of 238U, 226Ra, 232Th, and 40K are 5134.03, 5708.64, 189.51, and 1456.8 BqKg−1, respectively. Radionuclide’s distribution in leach liquor, residual, and fungal adsorption were followed and environmental hazard indices (Raeq, Iγ, Hex, Hin, and ECLR) were calculated. From the observed outcomes, the isolated fungal strains have the potential to reduce the harmful effect up to 50% compared to the original. As a result, application of these fungal strains offers a potential strategy for environmental remediation of radionuclides.

Magma Source and Petrogenesis of the Early Cretaceous Granites in The Liaodong Peninsula: Evidence from In Situ Apatite Sr-Nd and Zircon Hf-O Isotopes

Apatite Sr-Nd and zircon Hf-O isotopes are broadly used to trace magma sources and constrain magma evolution processes, further improving our understanding of the origin of granitoids. We present zircon U-Pb ages, whole-rock major and trace elements, and whole-rock Sr-Nd-Hf, zircon Hf-O, and apatite Sr-Nd isotopic data for the coarse-grained quartz monzonite, biotite monzogranite, and granite porphyry in the Yushulinzi pluton in the Liaodong Peninsula, the eastern North China Craton, to establish their magma sources and petrogenesis. The coarse-grained quartz monzonite, biotite monzogranite, and granite porphyry were formed contemporaneously, with zircon U-Pb ages of 123–119 Ma. They share enriched whole-rock Sr-Nd-Hf and zircon Hf isotopic compositions, and the coarse-grained quartz monzonite has crust-like δ18O values (5.7–6.7‰). The coarse-grained quartz monzonite and biotite monzogranite have variable apatite (87Sr/86Sr)i ratios and negative apatite εNd(t) values. These isotopic characteristics indicate that the different rock types in the Yushulinzi pluton were derived from the partial melting of ancient crustal material in the North China Craton. Their geochemical and petrographic characteristics indicate that the crystal-melt segregation model can be employed to elucidate the genetic links among different rock types, with the coarse-grained quartz monzonite representing crystal accumulation and the biotite monzogranite and granite porphyry representing interstitial melts extracted from a crystal-rich magma chamber. Furthermore, the variable apatite Sr isotopic compositions and subtle differences in the peak zircon εHf(t) values of the studied rock samples confirm the possibility of a contribution from shallow crustal components and materials with high εHf(t) values during magma evolution, which is not readily revealed by their whole-rock Sr-Nd-Hf isotopic compositions. These results demonstrate that in situ apatite Sr-Nd and zircon Hf-O isotopic analyses have the potential to provide distinctive insights into the magma sources and evolution of magmatic systems.

A prediction method for abrasion loss rate of some Egyptian carbonate rocks due to cyclic salt crystallization weathering using physico-mechanical deterioration: insights from laboratory investigations

Salt crystallization is the most important weathering processes that causes problems for rocks used as building stones. Therefore, assessment of the physico-mechanical properties of stones against cyclic salt crystallization is a critical issue for rock engineering applications in salty environmental conditions especially prone to abrasion. This research aims at investigating the relationship between the degradation of various physico-mechanical properties including apparent porosity, point load index, unconfined compression strength and abrasion loss rate for rocks during salt weathering process. For showing rock weathering in the salt crystallization process, five kinds of carbonate rocks were sampled from different areas of Egypt and subject to cyclic salt crystallization, up to twenty cycles. The variation rates of their physico-mechanical characteristics and abrasion loss values were calculated after each five cycles of salt weathering. The results showed that the studied rocks are suitable for use as building and decorative stones in areas prone to crystallization of salts, but attention must be paid to rocks with high porosity and low strength characteristics such as El-Minia limestone specimens. It was also found that there are strong linear correlations between the physico-mechanical degradation parameters and the abrasion loss rate of the studied rock samples at the end of the twentieth cycle of salt weathering. These relationships may be used to estimate the abrasion loss rate of the studied rock types against the cyclic salt weathering conditions and hence to make a rapid evaluation of the stone durability.

Experimental and theoretical investigations of the γ-rays shielding performance of rock samples from Najran region

Several different rock samples were collected from different regions of Najran, Kingdom of Saudi Arabia to evaluate their shielding performance for γ-rays. The elemental analysis of these samples shows that these samples belong to calcite, silicate and ferrite family. These samples have density in the range of 2.50 ∼ 2.74 g/cm3. The radiation shielding performance of these samples were measured using a NaI(Tl) scintillation detector under γ-rays irradiation from 137Cs, 241Am, and 60Co sources. The linear attenuation coefficient (LAC), mass attenuation coefficient (MAC), Half value layer (HVL), tenth value layer (TVL), mean free path (MFP), atomic cross section (ACS), and electronic cross section (ECS) were calculated experimentally and theoretically. The radiation shielding performance of these local rock samples are comparable with the previously studied rock samples from different regions of the world. This evaluation has shown that these low cost, locally available rocks can be used for low energy γ-rays shielding in different fields and preparation of concrete to be used as shielding material for building construction.

The effects of water acidity and engineering properties on rock durability

Fifteen sedimentary rock samples of four rock types including sandstone, limestone, travertine and conglomerate were collected from Damghan area, northern Iran. Mineralogical studies and laboratory experiments were performed to assess physical properties, Schmidt hardness, P-wave velocity, slake-durability index, uniaxial compressive, point load, Brazilian tensile and block punch strengths of the rocks. The studied rock samples are mainly composed of quartz, calcite and dolomite with different textures. The slake-durability test was carried out up to ten cycles in fluids with different pH. The utilized test fluids were natural water with pH of 7 and sulfuric acid solutions with pH of 5.5 and 4. Based on the results, the slake-durability index is affected by pH of the test fluids. Also, the different rock types had distinctive slaking behaviors. Decreasing rate of slake-durability index at initial cycles is higher than the end cycles the same as weight loss rate of the specimens. Regression analyses showed that the evaluated parameters are correlated to the slake-durability index. In other words, the slake-durability index of the studied rock samples is greatly affected by their mineral composition, texture, physical properties such as porosity and closely related to strength parameters of the rocks. This led to extraction of empirical equations for determining essential characteristics of the rocks from the slake-durability index.

Applicability of the multispectral remote sensing on determining the natural rock complexes distribution and their evaluability on the radiation protection applications

The mafic and ultramafic rocks of Mettupalayam belong to Southern Granulite Terrain (SGT), and it is prominent with economically valuable resources. The radiation shielding properties of the rock types, along with the remote sensing aspects, are considered in the study. The image processing techniques like Principal Component Analysis, Band ratio, Minimum Noise Fraction are performed using Landsat data to discriminate the litho units. The spectral characters derived from Landsat data identified with the significant absorptions of Fe2+, Fe3+and OPx. The Monte Carlo simulation was applied to evaluate the radiation protection ability of the studied rock samples. The examination showed a high shielding capacity for Gabbro rocks compared to the other studied rocks. The linear attenuation coefficient of the mentioned Gabbro rocks varied among 47.838 and 0.075 cm−1 when the gamma-ray energy increases in the energy interval 0.015 and 15 MeV. Moreover, the Radiation defensive capacity of the Gabbro samples decreased from 100 to 20.095% while the halve value layer increases from 0.014 to 9.269 cm with raising the gamma photon energy among 0.015 and 15 MeV, correspondingly.

Carbonate strength classification based on depositional textures and fossil content of the Lower Eocene Drunka Formation, Assiut Area, central Egypt

The present study reports petrographic and physico-mechanical analyses conducted on the lower Drunka Formation in the Assiut Area of Central Egypt. The study investigated the percentage of carbonate content and percentage of allochems “Ach” and their impact on the petrophysical features using correlation and multivariate regression analyses. This study analyzes the petrophysical characterization of Drunka Formation rocks and the effect of fossil content on them. The carbonate rock samples derived from the lower Drunka Formation were studied to evaluate the compressive strength , dry and saturated unit weight, porosity, and acoustic wave velocities. The petrographic examination of the rock samples was conducted to understand the facies characteristics, size and percentage of grains, matrix type and extent of diagenesis . Various microfacies associations were also detected within the examined samples, such as lime-mudstone, echinoidal bioclastic wackestones, nummulitic/miliolid wackestones, bioclastic peiodal packstone, echinoidal packstone and nummulitic grainstone . Petrophysical analyses displayed a wide range of porosity and dry bulk density values from 1.3 to 2.75% and 2.31–2.48 g/cm 3 , respectively. The wet bulk density and grain densities are between 2.33˗2.50 and 2.35˗ 2.57 g/cm 3 , respectively. The dry ultrasonic wave velocity “V p” was observed between 2926 and 5549 m/s, whereas the saturated ultrasonic wave velocity “V p “was estimated between 3067 and 5875 m/s. The Unconfined Compressive Strength (UCS) of the tested samples was 22 ≤ UCS ≤ 82 MPa while the slake durability index was recorded as 94.37 ≤ SDI ≤ 98.7%. The Brazilian test indices characterized the studied rock samples as highly porous rocks and showed the values of 1.47 ≤ Brazilian ≤ 5.13 MPa. Finally, for all tested facies, the Böhme test data have a minimum, maximum, and an average of 12.98, 25.95, and 19.04 cm 3 /50 cm 2, respectively. The study's findings showed that, except for the mineral grain scale, the physico-mechanical properties are highly dependent on the petrographic characteristics. The study also revealed that the Ach% and the rock's carbonate content significantly affect the rock's petrophysical and mechanical properties under investigation. The mechanical properties such as UCS, tensile and point load test. increase with increasing carbonate content and decreasing with increasing Ach% of the rock. This paper presents an extensive dataset of physical parameters of a carbonate formation in Egypt; many empirical relationships are presented for predicting mechanical properties. This provides site-specific predictive value; the relationships can be used for all carbonate rocks worldwide with broader international significance. • Classification of strength from texture is proposed based on fossil and carbonate content. • This study analyzes the petrophysical characterization of Drunka Formation rocks and the effect of fossil content on them. • This paper introduces a new strength classification of carbonate rocks based on allochems and carbonate content. • The depositional texture of carbonate rocks has a significant impact on variations of physico-mechanical properties of rocks. • Diagenesis has a remarkable effect on rock strength.

An Integrated Experimental Workflow for Formation Water Characterization in Shale Reservoirs: A Case Study of the Bazhenov Formation

Summary In this study, we aim to develop a new integrated solution for determining the formation water content and salinity for petrophysical characterization. The workflow includes three core components: the evaporation method (EM) with isotopic analysis, analysis of aqueous extracts, and cation exchange capacity (CEC) study. The EM serves to quickly and accurately measure the contents of both free and loosely clay-bound water. The isotopic composition confirms the origin and genesis of the formation water. Chemical analysis of aqueous extracts gives the lower limit of sodium chloride (NaCl) salinity. The CEC describes rock-fluid interactions. The workflow is applicable for tight reservoir rock samples, including shales and source rocks. A representative collection of rock samples is formed based on the petrophysical interpretation of well logs from a complex source rock of the Bazhenov Formation (BF; Western Siberia, Russia). The EM employs the retort principle but delivers much more accurate and reliable results. The suite of auxiliary laboratory methods includes derivatography, Rock-Eval pyrolysis, and X-ray diffraction (XRD) analysis. Water extracts from the rock samples at natural humidity deliver a lower bound for mineralization (salinity) of formation water. Isotopic analysis of the evaporated water samples covered δ18O and δ2H. A modified alcoholic ammonium chloride [(NH4Cl)Alc] method provides the CEC and exchangeable cation concentration of the rock samples with low carbonate content. The studied rock samples had residual formation water up to 4.3 wt%, including free up to 3.9 wt% and loosely clay-bound water up to 0.96 wt%. The latter correlates well to the clay content. The estimated formation water salinity reached tens of grams per liter. At the same time, the isotopic composition confirmed the formation genesis at high depth and generally matched with that of the region's deep stratal waters. The content of chemically bound water reached 6.40 wt% and exceeded both free and loosely bound water contents. The analysis of isotopic composition proved the formation water origin. The CEC fell in the range of 1.5 to 4.73 cmol/kg and depended on the clay content. In this study, we take a qualitative step toward quantifying formation water in shale reservoirs. The research effort delivered an integrated workflow for reliable determination of formation water content, salinity lower bound, and water origin. The results fill the knowledge gaps in the petrophysical interpretation of well logs and general reservoir characterization and reserve estimation. The research novelty uses a unique suite of laboratory methods adapted for tight shale rocks holding less than 1 wt% of water.

Biogenic Contamination and Mineral Pseudo-Biogenic Structures in Earth Rocks and Astromaterials

Bacterial paleontology studies the roles played by organisms in the genesis of sediments and ores. It is, therefore, important to clearly distinguish in-situ biological objects present in the rock material from various contaminants of later origin and from mineral structures that merely resemble organisms. The same is crucial for correct interpretation of alleged biogenic structures found in meteorites. The use of a scanning electron microscope equipped with an EDX microanalysis system and designation of model objects from previously studied rock samples allows us to address this problem. This approach also needs to be applied to previously discovered structures.

Characteristics of primary tin mineralization in the Central and West Bangka Island

Bangka Island is an important tin producer in Indonesia, mostly from the secondary tin deposits. However, the secondary tin deposit is depleting, causing further exploration shifted to the primary tin mineral. The purpose of this study is to make a mineralization model of primary tin deposit in Bangka Island. Methods that implemented in this study are petrographic analysis, mineragraphic analysis, XRD (X-Ray Diffraction), and XRF (X-Ray Fluorescence) analysis. The studied rock samples that consisted of granite, meta-sandstone, and sandstone. Some of the rocks are altered rocks with varying intensity. Petrographic and mineragraphic analysis show the presence of minerals that indicate the primary tin deposits, such as cassiterite, tourmaline, topaz, sericite, pyrite, and sphalerite. XRF analysis shows data about the value of Sn elements so that they can be classified into two classes, very high grade (> 800 ppm), and low grade (100–200 ppm). XRD analysis is necessary to do to determine the type and intensity of alterations that occur in the study area. The results of each analysis will provide supporting information regarding the primary tin mineralization process in Bangka Island.

The Oil-Bearing Strata of Permian Deposits of the Ashal’cha Oil Field Depending on the Content, Composition, and Thermal Effects of Organic Matter Oxidation in the Rocks

The features of the oil-bearing capacity of the productive strata of Permian deposits in the interval of 117.5-188.6 m along the section of individual wells of the Ashal’cha field of heavy superviscous oil (Tatarstan) were revealed depending on the content, composition, and thermal effects of organic matter (OM) oxidation in the rocks. It is shown that the rocks are very heterogeneous in their mineral composition and in the content of both free hydrocarbons by extraction with organic solvents and insoluble OM closely associated with the rock. The total content of OM in rocks varies from 1.72 to 9.12%. The features of group and hydrocarbon composition of extracts from rocks are revealed depending on their mineral composition and the content of organic matter in them. According to the molecular mass distribution of alkanes of normal and isoprenoid structure, extracts from rocks are differentiated according to three chemical types of oil: type A1, in which n-alkanes of composition C14 and above are present, and types A2 and B2, in which n-alkanes are destroyed to varying degrees by processes microbial destruction, which indicates a different intensity of biochemical processes in productive strata of Permian sediments. These processes lead to a decrease in the amount of OM in the rocks and an increase in the content of resins and asphaltenes in the oil extracted from them, as well as an increase in the viscosity of the oil. Using the method of differential scanning calorimetry of high pressure, it was found that the studied rock samples differ from each other in quantitative characteristics of exothermic effects in both low-temperature (LTO) 200-350°С and high-temperature (HTO) 350-600°С zones of OM oxidation. The total thermal effect of destruction processes of OM depends on the content of OM in the rocks and its composition. The research results show that when heavy oil is extracted using thermal technologies, the Permian productive strata with both low and high OM contents will be involved in the development, and the general thermal effect of the oxidation of which will contribute to increased oil recovery.

Corrigendum to “The Oil-Bearing Strata of Permian Deposits of the Ashal’cha Oil Field Depending on the Content, Composition, and Thermal Effects of Organic Matter Oxidation in the Rocks”

The features of the oil-bearing capacity of the productive strata of Permian deposits in the interval of 117.5-188.6 m along the section of individual wells of the Ashal’cha field of heavy superviscous oil (Tatarstan) were revealed depending on the content, composition, and thermal effects of organic matter (OM) oxidation in the rocks. It is shown that the rocks are very heterogeneous in their mineral composition and in the content of both free hydrocarbons by extraction with organic solvents and insoluble OM closely associated with the rock. The total content of OM in rocks varies from 1.72 to 9.12%. The features of group and hydrocarbon composition of extracts from rocks are revealed depending on their mineral composition and the content of organic matter in them. According to the molecular mass distribution of alkanes of normal and isoprenoid structure, extracts from rocks are differentiated according to three chemical types of oil: type A1, in which n-alkanes of composition C14 and above are present, and types A2 and B2, in which n-alkanes are destroyed to varying degrees by processes microbial destruction, which indicates a different intensity of biochemical processes in productive strata of Permian sediments. These processes lead to a decrease in the amount of OM in the rocks and an increase in the content of resins and asphaltenes in the oil extracted from them, as well as an increase in the viscosity of the oil. Using the method of differential scanning calorimetry of high pressure, it was found that the studied rock samples differ from each other in quantitative characteristics of exothermic effects in both low-temperature (LTO) 200-350°С and high-temperature (HTO) 350-600°С zones of OM oxidation. The total thermal effect of destruction processes of OM depends on the content of OM in the rocks and its composition. The research results show that when heavy oil is extracted using thermal technologies, the Permian productive strata with both low and high OM contents will be involved in the development, and the general thermal effect of the oxidation of which will contribute to increased oil recovery.

Experimental Determination of the Mechanical Properties and Deformation Constants of M\xf3r\xe1gy Granitic Rock Formation (Hungary)

Determination of the mechanical behaviour of intact rock is one of the most important parts of any engineering projects in the field of rock mechanics. The most important mechanical parameters required to understand the quality of intact rock are Young’s modulus (E), Poisson’s ratio (ν), the strength of rock (σc) and the ratio of Young’s modulus to the strength of rock known as modulus ratio (MR), which can be used for calculations. The particular interest of this paper is to investigate the relationship between these parameters for Hungarian granitic rock samples. To fulfil this aim, Modulus of elasticity (E), Modulus of rigidity (G), Bulk modulus (K) and the modulus ratio (MR = E/σc) of 50 granitic rock samples collected from Bátaapáti radioactive waste repository were examined. Fifty high-precision uniaxial compressive tests were conducted on strong (σc > 100 MPa) rock samples, exhibiting the wide range of elastic modulus (E = 57.425–88.937 GPa), uniaxial compressive strength (σc = 133.34–213.04 MPa) and Poisson’s ratio (ν = 0.18–0.32). The observed value (MR = 326–597) and mean value of MR = 439.4 are compared with the results of similar previous researches. Moreover, the statistical analysis for all studied rocks was performed and the relationship between MR and other mechanical parameters such as maximum axial strain (εa, max) for studied rock samples was discussed. Finally, the validity of the proposed mathematical model by Palchik (Geomech Geophys Geo-energy Geo-resour 6:1–12, 2019) for stress–strain behaviour of granitic rock samples was investigated.

A multidisciplinary approach to investigate weathering processes affecting gneissic rocks (Calabria, southern Italy)

The paper focuses on the weathering processes affecting gneissic rocks of the western Sila Grande Massif (Calabria, Italy) through the development of an interdisciplinary research based on field studies and investigations, minero-petrographical analyses and geochemical modeling. Both physical and chemical weathering affect gneissic rocks of the study area, where the combination of tectonic and past climatic conditions played an important key role in the development of complex and deep weathering profiles. Field surveys and cut slope analyses highlight articulated and complex geometric relationships between various classes of weathering (i.e., out-of-sequence weathering horizons giving rise to a partial, or even complete inversion of the “normal” weathering profile). The weathering profile has turned out particularly intense, as classes IV, V and VI are widespread. Among the class VI, the colluvial soils are prominent in comparison whit the residual soils. Borehole logs, that confirm the intensity and complexity of weathering profiles in deep, allow to estimate the presence of weathered rocks to be at least 70 m in thicknesses.The main mineralogical modifications linked to weathering processes concern the partial transformation of biotite and the partial destruction of feldspars (mainly plagioclases), associated with the neoformation of secondary mineralogical phases (clay minerals and Fe-oxides). Neoformed clay minerals (such as sericite, illite and mixed layer phases) and ferruginous products replaced feldspars and biotite during the most advanced weathering stage. Results of XRD analyses and geochemical modelling provide a good indication on the secondary mineral assemblage due to the increase of the weathering intensity (form class III to class VI) that reflects the different contributions of chemical elements provided by dissolution of silicate minerals into the surrounding groundwater system. The chemical composition of the studied rock samples and the oxides variations suggest a removal of some alkali (Na and K) and alkaline earth (Ca and Mg) into solution as a consequence of weathering reactions. The chemical analysis and the weathering indices (CIA, PIA, CIW and CIW’) show a marked alteration process, with ferromagnesian minerals and feldspars probably dissolved and leached into the surrounding groundwater system. The CO2-controlled dissolution of plagioclase appears to be the most important reaction during chemical weathering. The progressive dissolution results then dominated by biotite, followed by a minor amounts K-feldspar, chlorite and garnet; whereas the sillimanite shows a neglectable amounts. The secondary solid phases observed during the geochemical modeling (illite, followed by vermiculite, ferrihydrite and saponite) are similar to those found in this natural system. The proposed approach could be used to characterize weathered crystalline rocks and related weathering profiles in similar geological setting, and the obtained results represent a key point for the evaluation of the control exerted by weathering on landscape evolution under current environmental settings in terms of sediment generation, soil erosion rates, and mass movements, and for the mechanical characterization of weathering profiles for engineering geological purposes.

Geochemical constraints on the origin and tectonic setting of the serpentinized peridotites from the Paleoproterozoic Nyong series, Eseka area, SW Cameroon

Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon. Here, we present new field data, petrography, and first comprehensible whole-rock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region. The studied rock samples are characterized by pseudomorphic textures, including mesh microstructure formed by serpentine intergrowths with cores of olivine, bastites after pyroxene. Antigorite constitutes almost the whole bulk of the rocks and is associated (to the less amount) with tremolite, talc, spinel, and magnetite. Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types. Type 1 has high MgO (> 40 wt%) content and high Mg# values (88.80) whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg# values ( 50 ppm). This similarity, associated with the high Cr contents, spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subduction-related environment. The U-shaped chondrite normalized-REE patterns of serpentinized peridotites, coupled with similar enrichments in LREE and HFSE, suggest the refertilized nature due to melt/rock interaction prior to serpentinization. Based on the results, we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subduction-related environment, especially in Supra Subduction Zone setting. These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the Sao Francisco craton in Brazil.

Confocal laser scanning microscopy and automated petrographic image analysis in different rock types: Two-dimensional images capillary pressure curves estimation and three-dimensional porosity reconstruction

In the past, determination of rock properties using image analysis relied upon petrographic transmitted-light images, but with limited success because of a lack of resolution and restricted computer processing power. A new technique that employs confocal laser scanning microscopy (CLSM) can be considered complementary to laboratory measurements and applicable to several samples, saving time and money and requiring only a limited amount of rock sample for analysis. We have studied several types of rocks with CLSM and fluorescent dye–impregnated thin sections. The two-dimensional scans of each thin section images is an area of 12 mm2, with a pixel size of 0.198 μm and were used to simulate capillary pressure curves for pore bodies and pore throats. The CLSM technique also enables three-dimensional (3-D) visualization of the rock porosity. The studied rock samples were taken from diverse oil and gas field reservoirs: case A, a conventional sandstone (15.1% porosity, 29.8 md permeability); case B, a tight sandstone (3.7%, 0.02 md); case C, an oolitic carbonate (9.6%, 0.1 md); case D, a rhodolithic algal carbonate (19.8%, 43.7 md); case E, dolomitized carbonate (17%, 21.7 md); and case F, a naturally fractured carbonate (2.4%, 0.6 md). Our results confirm that the CLSM technique can be applied to rocks of contrasting porosity and permeability to obtain computed synthetic capillary pressure curves faster than with conventional measurement methods. The technique quantifies different pore-body and pore-throat sizes and distributions, with the added ability to visualize 3-D porosity and to extract from thin section analysis petrologic properties.

Gamma ray shielding characteristics and exposure buildup factor for some natural rocks using MCNP-5 code

Abstract The mass attenuation coefficient μ m for eight rock samples having different chemical composition was simulated using the MCNP 5 code in energy range ( 0.002 MeV ≤ E ≤ 10 MeV ) . Moreover, the μ m for the studied rock samples was computed theoretically using XCOM database. The comparison between simulated and computed data for all selected rock samples showed a good agreement with differences varied between 0.01 and 8%. The highest μ m was found for basalt rocks M2 and M1 and the lowest one is reported for limestone rocks Dike. The simulated values of the μ m then were used to calculate other important shielding parameters such as the mean free path, effective electron density and effective atomic number. The exposure buildup factor EBF was also computed for the selected rocks with the contribution of G-P fitting parameters and the highest EBF attended by the basalt sample Sill and varied between 1.022 and 744 in the energy range between ( 0.015 ≤ E ≤ 15 MeV ) but the lowest EBF achieved by basalt sample M2 and varied between 1.017 and 491 in the same energy range.

Quantitative pore network analysis and permeability evaluation of porous carbonate reservoir rocks using X-ray computed microtomography images

The texture and the morphology of the pore network exerts a primary control in the fluid storage and migration of geofluids within porous carbonates reservoirs. The architecture of the pore network of porous carbonates are highly variable because of primary depositional settings, diagenesis processes and deformation. This issue represents an important challenge for the characterization and exploitation plan of this type of reservoirs in different industries such as, hydrocarbon, ground water and geothermal energy. This type of rocks is widely studied because they represent important reservoirs for geofluids around the globe. However, there are few studies assessing the control exerted by the pore network properties (e.g. porosity, pore size distribution, connectivity, tortuosity) on the permeability. In this work the control exerted by the pore network on the storage and migration capacity in porous carbonates is evaluated by combining synchrotron X-ray computed microtomography (SR micro-CT) and computational fluid dynamics. The studied rock samples are mainly porous grainstones exposed in south and central Italy. Some samples may content deformation structures (i.e. deformation bands) or may be altered by diagenesis. Previous studies have reported permeability differences and significant variability of the in-situ hydrocarbon distribution in the studied rocks. The SR micro-CT imaging experiments were performed at the SYRMEP (SYnchrotron Radiation for MEdical Physics) beamline (Elettra-Sincrotrone Trieste laboratory, Italy). This beamline was suitable for studying the rocks samples due to its nearly-parallel geometry and a high spatial coherence allowing the phase contrast effects to enhance the visibility of objects with similar linear attenuation coefficients. In this study, the selected spatial resolution of the images is variable (1.0-9.0 μm) depending on the grain size distribution of the rock sample. The SR micro-CT images were used for both performing a quantitative pore network analysis of the studied rock samples and performing computational fluid dynamics experiments. These experiments consist in simulating a pressure-driven flow by using the lattice-Boltzmann method (LBM) with multiple relaxation time (MRT) approach. This method generates viscosity-independent results of permeability. The permeability of the volume was calculated using Darcy’s law once steady conditions were reached. To evaluate isotropy, the results of permeability and the pore network properties were calculated in three dimensions. The results indicate that deformation and diagenetic processes may have and important impact on the pore network properties and therefore on storage (porosity) and migration (permeability) capacity of the studied rocks.