Fine-grained Rocks Research Articles

Rocks and Their Sources in the Upper Paleolithic of the Altai: Relevance to Bifacial Technologies

On the basis of the analysis of thin leaf-shaped bifacial points, which are very elaborate and sensitive to the quality of rocks, we reconstruct the adaptive strategies of humans at the early stages of the Upper Paleolithic. Mineral raw materials and their exploitation relating to different resource bases of the central (the Ursul River basin) and northwestern (the Anuy River basin) parts of the Altai region are analyzed. To attribute the rock sources for bifaces, we have compiled a comparative database of petrographic and petrochemical characteristics of artifacts and pebbles from nearby rivers. Chemical criteria were proposed for differentiating rocks, including those that are hard to distinguish, and non-destructive techniques were applied to assess the chemical composition of rocks using a portable XRF spectrometer. Findings suggest that rocks available in the Anuy and Ursul basins met the conditions for biface manufacture. Bifaces from the Ursul valley were made of local fine-grained rocks—felsic volcanic tuff and ignimbrite; those from the Anuy valley were also of local rocks, but of lower quality—hornfels transformed (meta-sedimentary) siltstone and finegrained sandstone or felsic volcanic rocks. In the Anuy valley, scarcity of quality raw material was compensated for by imported high silica jasper-like rocks. Results suggest that the Early Upper Paleolithic inhabitants of the region, when implementing technical skills, showed stable behavioral and technological stereotypes despite habitat change and deterioration of the resource base.

An improved convolutional architecture for quantitative characterization of pore networks in fine-grained rocks using FIB-SEM

Focused ion beam scanning electron microscope (FIB-SEM) is one of the most advanced imaging techniques for analyzing and understanding complex pore networks in shale and other fine-grained formations. However, FIB-SEM imaging tends to be time-consuming and labor-intensive and can result in biased interpretations associated with pore analysis. Recently, U-Net or its variants for image segmentation have been applied to capture microscopic pores at higher resolutions. The ‘traditional’ encoder-decoder-based approaches tend to detect very fine-scale microscopic pores poorly. This study presents an improved convolutional architecture for automatically analyzing pore structures in shale reservoirs using FIB-SEM. It does so by applying an overcomplete convolutional architecture, KiU-Net, to capture very fine-scale microscopic pores by accurately defining their edges in the input FIB-SEM images. The KiU-Net learns low and high-level features by making the model more sensitive to fine-scale microscopic pores in the input images. The purpose of this study is to demonstrate KiU-Net's capabilities by analyzing different shale formations with varying characteristics. The results indicate that KiU-Net is more accurate and efficient than other methods in predicting nanopores in the Longmaxi, Niutitang, Qingshankou, Qianjiang, and Yanchang Formations (China), Bakken shale (Canada), and coal reservoirs (China). Furthermore, KiU-Net demonstrated the advantage of requiring fewer parameters and achieving super convergence compared to the Attention U-Net technique. KiU-Net addresses the challenges of the Edge-Threshold Automatic Processing (ETAP) methods by capturing very fine-scale microscopic pores with accurate edges. This study further enhances the accuracy and efficiency of pore analysis in shales, thereby offering an improved method for understanding shale reservoir quality with the potential to improve petroleum recovery from such formations.

Petrology, mineral chemistry and geochemistry of chloritite associated with Neoproterozoic ophiolitic ultramaficsin the Eastern Desert of Egypt, Arabian-Nubian Shield

This study presents for the first time, the field observations, petrography, mineral chemistry and geochemistry of chloritite hosted in the Al-Barramiya Neoproterozoic ophiolite of the Eastern Desert of Egypt, Arabian-Nubian Shield (ANS). The Al-Barramiya ophiolite is one of the most important ophiolitic sequences exposed in the ANS. It is affected by different types of alterations including carbonatization, listvenitization, chloritization and rodingitization. The Al-Barramiya chloritite occurs as thin layers associated with highly serpentinized peridotite. It is a fine-grained rock entirely composed of chlorite (85–95 vol.%) with minor talc and accessory minerals (epidote, rutile, titanite, corundum and opaque minerals). The chlorite minerals in the chloritite are represented mainly by diabantite, while those in the serpentinites include ripidolite. Depending on the chemical composition of the chlorites, the chlorite in chloritite formed at temperatures ranging between 200 and 250°C, which are lower than those of the disseminated chlorite in the serpentinite (310–345oC), indicating their formation in different hydrothermal stages. The chloritite samples are rich in total REE contents (17.9–27.3 ppm) compared with the associated serpentinites (0.69–0.87 ppm). They are characterized by slightly depleted LREE relative to HREE [(La/Lu)n = 0.8–0.9], with a moderately negative Eu-anomaly [(Eu/Eu*)n = 0.4–0.5]. The negative Eu-anomalies are derived from chloritization fluids or reflect the presence of talc in the chloritite. Based on field work, petrography, mineralogical and geochemical data, the studied chloritite has been interpreted as being derived from the associated serpentinized ultramafics by hydrothermal alterations. This is supported by an enrichment of chloritite in compatible trace elements (Cr = 2031–2534 ppm, Ni = 1264–1988 ppm, Co = 76–101 ppm) similar to that which is observed in the associate serpentinite.

Differential hydrocarbon generation characteristics of organic matter with green algae and cyanobateria origins in the Permain Lucaogou Formation of the Santanghu Basin

Organic-rich fine-grained rocks are key carriers of unconventional oil and gas resources, making it crucial to understand their hydrocarbon generation and evolution characteristics. This study examines the fine-grained rocks of the second member of the Lucaogou Formation (P2l2) in the Tiaohu and Malang Sags of the Santanghu Basin, focusing on how different organic matter (OM) backgrounds - primarily green algae and cyanobacteria - affect hydrocarbon generation and crude oil properties. Kinetic analysis and hydrous pyrolysis experiments on shales rich in green algae, cyanobacteria, and their mixtures revealed that green algae - derived OM requires lower activation energy to initiate hydrocarbon generation, results in an earlier oil generation peak, and has a broader oil window. Conversely, cyanobacteria - derived OM needs higher activation energy to start hydrocarbon generation, has a later oil peak, and a more concentrated generation period. These findings led to two models: the "green algae origin - early hydrocarbon generation - early oil peak - broad oil window model" and the "cyanobacteria origin - late hydrocarbon generation - late oil peak - concentrated oil generation model." Correlation analysis showed that aromatic hydrocarbons, resins, and asphaltenes significantly degrade crude oil quality. Hydrous pyrolysis experiments indicated that the heavy component content (aromatic hydrocarbons + resins + asphaltenes) in liquid hydrocarbons follows the order: residual oil > absorbed oil > expelled oil, with content initially increasing and then decreasing with maturity, and the color change of liquid hydrocarbons in dichloromethane reflects heavy component content changes effectively. Calculations of density and viscosity of liquid hydrocarbons, based on heavy component content and crude oil properties, were compared with the longitudinal distribution of crude oil properties in the study area. Results show that the hydrocarbon generation characteristics of green algae and cyanobacteria control crude oil properties, highlighting significant intra-source differentiation in the P2l2 shale and validates the phase separation approach in hydrous pyrolysis experiments. The P2l2 shale, with its high OM content and substantial hydrocarbon generation, holds great potential for shale oil exploration, but both reservoir quality and crude oil property evolution under different OM backgrounds should be considered when selecting favorable areas.

Organic matter enrichment model of fine-grained rocks in volcanic rift lacustrine basin: A case study of lower submember of second member of Lower Cretaceous Shahezi Formation in Lishu rift depression of Songliao Basin, NE China

Based on sedimentary characteristics of the fine-grained rocks of the lower submember of second member of the Lower Cretaceous Shahezi Formation (K1sh2L) in the Lishu rift depression, combined with methods of organic petrology, analysis of major and trace elements as well as biological marker compound, the enrichment conditions and enrichment model of organic matter in the fine-grained sedimentary rocks in volcanic rift lacustrine basin are investigated. The change of sedimentary paleoenvironment controls the vertical distribution of different lithofacies types in the K1sh2L and divides it into the upper and lower parts. The lower part contains massive siliceous mudstone with bioclast-bearing siliceous mudstone, whereas the upper part is mostly composed of laminated siliceous shale and laminated fine-grained mixed shale. The kerogen types of organic matter in the lower and upper parts are types II2–III and types I–II1, respectively. The organic carbon content in the upper part is higher than that in the lower part generally. The enrichment of organic matter in volcanic rift lacustrine basin is subjected to three favorable conditions. First, continuous enhancement of rifting is the direct factor increasing the paleo-water depth, and the rise of base level leads to the expansion of deep-water mudstone/shale deposition range. Second, relatively strong underwater volcanic eruption and rifting are simultaneous, and such event can provide a lot of nutrients for the lake basin, which is conducive to the bloom of algae, resulting in higher productivity of types I–II1 kerogen. Third, the relatively dry paleoclimate leads to a decrease in input of fresh water and terrestrial materials, including Type III kerogen from terrestrial higher plants, resulting in a water body with higher salinity and anoxic stratification, which is more favorable for preservation of organic matter. The organic matter enrichment model of fine-grained sedimentary rocks of volcanic rift lacustrine basin is established, which is of reference significance to the understanding of the organic matter enrichment mechanism of fine-grained sedimentary rocks of Shahezi Formation in Songliao Basin and even in the northeast China.

Unraveling mudstone compaction at microscale: A combined approach of nanoindentation mapping and machine learning data analysis

Compaction processes within mudstone samples across varying depths (723.5–3213.5 m) in the Vienna Basin were investigated, focusing on porosity changes and the resulting micromechanical response in the clay-rich, fine-grained fraction (“clay matrix”). A novel approach combining nanoindentation mapping with machine learning data analysis was developed to efficiently extract representative micromechanical parameters of the clay matrix. Emphasis was put on capturing the properties of the fine-grained, clay-dominated composite rather than individual mineral phases, enabling the analysis of compaction-induced strength changes at microscale. A significant enhancement in the mechanical strength of the clay matrix with increasing burial depth was observed. Reduced elastic modulus (Er) and hardness (H) increased from 6.8 ± 3.4 to 22.6 ± 7.5 GPa and from 0.2 ± 0.2 to 0.9 ± 0.2 GPa, respectively, over the depth interval of 2490 m. Moreover, a strong correlation between depth and porosity and consequently micromechanical properties of the clay matrix exists. This highlights the substantial influence of intra-clay porosity on mechanical properties, which follow a general compaction trend with depth. Broad ion beam-scanning electron microscopy (BIB-SEM) analysis was used for textural investigations at micro-to nanoscale and indicated that the reduction in porosity predominantly resulted from mechanical compaction rather than mineral diagenesis, which would have been noticeable by signs of significant dissolution or cementation. The correlation coefficient matrix between multiscale porosity measurements and nanoindentation affirmed that the porosity loss was closely linked to the enhanced mechanical properties of the fine-grained composite clay matrix, while other compositional variables like total clay mineral content showed weak correlations. Empirical mathematical equations were derived to describe the mechanical properties as generalized functions of depth and porosity. These may be used as geomechanical input for future basin analysis and geoenergy applications (e.g., seal rock studies in a geological storage context). This study introduces a new approach to unravel compaction processes in fine-grained rocks at microscale, emphasizing the interplay between intra-clay porosity and micromechanical changes during proceeding burial.

Geochemistry and mineralogy of the shale-hosted vanadium Van Property deposit, Mackenzie Mountains, Northwest Territories

Abstract Shales and mudstones are fine-grained rocks formed in sedimentary basins throughout Earth’s history. These lithologies are increasingly important targets for mineral deposit exploration since they can have economic resources of critical minerals including vanadium (V), an essential component of redox-flux batteries in solar cells. However, many Paleozoic shale-hosted V deposits are metamorphosed and deformed. This commonly obscures primary features including V-bearing host phases and the original composition of organic material. In this study, we present geochemical and mineralogical data from the Paleozoic Van Property deposit, Northwest Territories, Canada, to show that V can be released from organic matter during metamorphism and incorporated in clay phases such as illite. The siliceous argillites at the Van Property hosts up to 0.69% V2O5 and were metamorphosed to (sub-)greenschist facies. Their mineralogy is dominated by quartz with minor graphite, illite, muscovite, pyrite, sphalerite, rutile, and carbonates. Although, some illite (i.e., high-V illite) can have up to 13 wt% V2O3 and rutile can have up to 4.4 wt% V2O3, mass balance calculations are insufficient to explain V enrichment at the Van Property utilizing only illite and rutile. The third V host is inferred to be carbonaceous matter in which V accumulated syn-deposition on the seafloor. Subsequent metamorphism led to the demetallation of V-bearing geoporphyrins and the release of vanadyl ions (VO2+), some of which were then incorporated into high-V illite and rutile. This process of V enrichment highlights the role of organic matter in scavenging V from superficial reservoirs and the importance of metamorphism on subsequent V release and its incorporation into inorganic phases. The geochemistry of siliceous, V-rich argillites at the Van Property is also compared to other V-enriched shale and mudstone deposits, highlighting the diversity of shale-hosted V deposits, and emphasizing the need for further research to close the knowledge gaps related to variations in composition, mineralogy and V enrichment processes.

Depositional processes in a shale-dominated Devonian succession: Sedimentary facies and trace fossil integrated analysis

Fine-grained rocks have historically been interpreted as a product of the settling of fine fractions in low-energy environments. However, recent studies have suggested that in these environments, more dynamic and complex processes operate. These processes involve a much more diverse set of sedimentary processes than those previously assumed. The lithological homogeneity of clayey successions, associated with the obliteration of their primary characteristics by diagenetic and/or weathering processes, makes it difficult to interpret depositional processes and understand the paleoenvironment. To contribute to a better comprehension of the mechanisms of transport and deposition of fine-grained rocks in a siliciclastic mud-dominated succession, as well as their depositional site, the present study aims at the detailed analysis of a continuous drill core. This core contains a thick Devonian succession consisting essentially of shale from the Ponta Grossa Formation (Paraná Basin, Brazil). Throughout the analyzed succession, a wide variety of sedimentary structures have been identified, such as parallel laminations, wave-ripple cross-stratification, hummocky cross-stratification, normal grading, and gutter casts. These structures show the constant performance of high-energy flows, which are configured as important deposition and/or rework agents. The integrated analysis of facies and trace fossils has proven to be more efficient for paleoenvironmental characterization than isolated approaches. This is because facies and ichnofacies (e.g., Cruziana, with its proximal, impoverished, archetypal, and distal expressions; Skolithos and Glossifungites) can be easily correlated, facilitating the identification of facies associations. This approach not only allows for the interpretation of a shallow marine platform with a gentle slope (ramp shelf) paleoenvironment of moderate to high energy, subject to storm wave action, but also enables the identification of the particular characteristics of deposits from different sub-environments (proximal to distal offshore, offshore transition, shoreface to offshore transition, and prodelta) and their deposition mechanisms. Furthermore, the facies associations are useful for highlighting the variations formed in response to oscillations of the relative sea level and changes in sedimentary input.

Extreme organic matter enrichment in the Ediacaran Doushantuo formation: New constraints from the NE Sichuan basin, China

The Yangtze Block is a highly prospective area for Ediacaran natural gas exploration in recent years, especially the Doushantuo Formation, a vital source rock for organic-rich hydrocarbon and a shale gas reservoir in the region. However, understanding of the Ediacaran organic-rich source rocks systematically has been limited due to the heterogeneous accumulation of organic matter and confusing paleoclimatological and paleoceanographic conditions. Such a scenario is not conducive to further exploration of the deep Ediacaran natural gas resources or understanding the evolution of early Earth. In this study, based on two new well drillings in the northeastern Sichuan Basin, the integrated sedimentological and geochemical investigations were focused on the Ediacaran middle−upper Doushantuo Formation (DST) fine-grained rocks interval. Results show this set of black organic-rich shales (total organic carbon up to 13.97%) deposited under episodic warming climate during an otherwise protracted cooling trend. These frequent climatic fluctuations intensified hydrological circulation, which together with the concurrent hydrothermal activity and the Neoproterozoic Oxidation Event, led to eutrophication of surface seawater. Under these circumstances, a substantial enhancement in primary productivity driven by algae blooming, subsequently resulted in consume excess dissolved oxygen in seawater during the degradation stage and anoxic-euxinic bottom water expansion. Consequently, the high paleoproductivity controlled the extreme organic matter enrichment during the Doushantuo time and formed the DST 4 organic-rich shales in the Chengkou and Nanhua subbasins. In contrast, although the DST 3 shales still deposited under a high productivity condition, the shallower sedimentary environments with anoxic-oxic bottom water conditions resulted in greater variation in total organic carbon contents.

Characteristics and origin of crystalline dolomite: A case from Paleogene lacustrine fine-grained rocks in Jiyang depression, Bohai Bay Basin, China

Crystalline dolomite is a common carbonate mineral in lacustrine fine-grained rocks, but it has a confusing origin that has been a focus of much debate in recent years. In our research, the fine-grained rocks rich in crystalline dolomite were sampled from the upper fourth member (UMbr 4) and lower third member (LMbr 3) of the Shahejie Formation in the Jiyang Depression, and analyzed by a variety of analytical techniques, including field emission scanning electron microscopy (FESEM), cathodoluminescence (CL), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), micro-area carbon, oxygen, strontium isotopes and electron probe analyses. The results show: ①The crystalline dolomites in the lacustrine fine-grained rocks are primarily composed of ferroan-poor dolomite (FeO < 1%), type I ferroan dolomite (FeO ≈ 5%) and type II ferroan dolomite (FeO ≈ 16%); ②The δ18O of ferroan-poor dolomites in mudstone is between −0.89 and 0.32‰, the precipitation temperature is less than 60 °C, and their formation is related to the microbial action during the early diagenetic stage A, while the ferroan-poor dolomites in dolostone are primarily formed by the recrystallization of micritic dolomite; ③The δ18O of type I ferroan dolomites ranges from −3.74 to −3.89‰, the precipitation temperature ranges from 68.4 to 69.4 °C, and they are primarily formed by the microbial action and clay mineral transformation during the early diagenetic stage B; ④The δ18O of type II ferroan dolomites is between −7.78 and −8.33‰, the precipitation temperature is more than 90 °C, and their formation is related to the dissolution of early carbonate and clay mineral transformation during the middle diagenetic stage; ⑤The origin mechanism of crystalline dolomite indicates the diagenetic evolution of fine-grained rocks and the migration path and storage of shale oil.

Study on the Differential Distribution Patterns of Fine-Grained Sedimentary Rocks in the Lower Third Member of the Shahejie Formation in Zhanhua Sag, Bohai Bay Basin

Due to the diverse types of sedimentary source supply systems, the research on the distribution patterns of fine-grained sedimentary rocks in rifted lake basins is relatively underdeveloped. The authors of this article selected the lower third member of the Shahejie Formation (Es3L) in Zhanhua Sag as a research object. Using rock core, thin section, and other data, we analyzed rock types and genesis mechanisms, identified the spatiotemporal distribution patterns of facies within the sag, and established a fine-grained depositional model for the rifted lake basin. Five lithofacies were identified, revealing the differential distribution patterns of fine-grained sedimentary rocks in the region. The northern steep slope zone and the deep depression zone are characterized by the deposition of gravity flow-formed terrestrial layered-pebbly mudstone and gravity flow-formed mixed-source pebbly mudstone, respectively. In the central lake basin zone, mixed-source laminated mudstone with vertical rhythmic interlayers has developed. In the southern gentle slope zone, biologically cemented endogenic irregularly bedded algal limestone and chemically precipitated endogenic laminated limestone have developed. The characteristics of strong northern and weak southern terrestrial inputs in the study area control the differential distribution of lithofacies, and based on this, a fine-grained rock depositional model for the rifted lake basin was established.

Research on a High-Performance Rock Image Classification Method

Efficient and convenient rock image classification methods are important for geological research. They help in identifying and categorizing rocks based on their physical and chemical properties, which can provide insights into their geological history, origin, and potential uses in various applications. The classification and identification of rocks often rely on experienced and knowledgeable professionals and are less efficient. Fine-grained rock image classification is a challenging task because of the inherent subtle differences between highly confusing categories, which require a large number of data samples and computational resources, resulting in low recognition accuracy, and are difficult to apply in mobile scenarios, requiring the design of a high-performance image processing classification architecture. In this paper we design a knowledge distillation and high-accuracy feature localization comparison network (FPCN)-based learning architecture for generating small high-performance rock image classification models. Specifically, for a pair of images, we interact with the feature vectors generated from the localized feature maps to capture common and unique features, let the network focus on more complementary information according to the different scales of the objects, and then the important features of the images learned in this way are made available for the micro-model to learn the critical information for discrimination via model distillation. The proposed method improves the accuracy of the micro-model by 3%.

Lithofacies and Depositional Models of the Fine-Grained Sedimentary Rocks of the Albian–Turonian Stage in the Rio Muni Basin, West Africa

The Rio Muni Basin is a hotspot for deep-water oil exploration in West Africa. The discovery of thirteen oil and gas fields, including Ceiba, Akom 1, and Oveng, proves the basin’s excellent exploration prospects, but only limited research has thus far been carried out there. The recent new drilling indicates that there are organic matter-rich fine-grained sedimentary rocks in the Albian and Cenomanian–Turonian stages of the Cretaceous strata. However, the depositional models of organic-rich, fine-grained sedimentary rocks are not clear. The main objectives of this study are as follows: (1) to identify the lithofacies characteristics of fine-grained sedimentary rocks in the Albian–Turonian stages of the Rio Muni Basin; and (2) to establish a depositional model for organic-rich fine-grained sediments. In this study, the mineralogical characteristics of Albian–Turonian fine-grained were determined by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and thin section analysis. In addition, the integration of mineralogical features with the total organic carbon (TOC) content allows for the recognition of three distinct lithofacies: (1) siliceous-clayey rock, (2) terrigenous clastic mixed fine-grained rock, and (3) lime-mixed fine-grained rock. Based on the evolutionary history of the passive continental margin basins in West Africa and the lithofacies characteristics, the deposition mode of organic-rich fine-grained sedimentary rocks in the Albian–Turonian stages was reconstructed by combining geochemical features with the characteristics of framboidal pyrite. The depositional models for the Albian stage are “continental margin–restricted sea–marine algae–source rocks” with the development of organic-rich fine-grained sedimentary rocks. The depositional models for the Cenomanian–Turonian stage are “continental margin–semiopen sea–mixed-source rocks”. The Albian and Cenomanian–Turonian organic-rich fine-grained sedimentary rocks represent two different deposition modes with restricted sea and semiopen sea conditions. The sedimentary characteristics in the study area during the two periods were both inherited and notably different.

Genetic Features of Soils on Dense Parent Rocks of the Donetsk Ridge

The present study investigates morphological, physical and chemical properties of Chernozems at dense parent rocks. The research was carried out on the protected areas native soils of the north-west of the Rostov region. This territory is representing eastern slopes of the Donetsk Ridge, which predetermines the dense rocks release to the surface. It has been found that morphological, physical and chemical properties of Chernozems on dense rocks have emphasized their originality and uniqueness, and at the same time clearly have indicate their classification as Chernozem order. In comparison with the zonal soil – Calcic Chernozem – Chernozems on dense rocks are characterized by a reduced thickness of humus-accumulative layers, an increased of humus content, the presence of gravel in profile bottom half, a weak expression of calcareous accumulations, features of the particle-size distribution of soil components, even if it belongs to the same soil texture class. Chernozems on dense carbonate rocks either do not have accumulation or are characterized by a weak manifestation of secondary carbonates (pseudomycelia and nodules). This fact has differed such soil from the same order Chrernozems on fine-grained rocks (Calcic Chernozems) for which it is diagnostic criteria. Therefore, the mention of the suborder of these soils is incorrect in the designation. In the soil covering of the spurs of the Donetsk Ridge, dark humus soils are also occure, previously classified as incomplete developed chernozems, differing from chernozems by the absence of medium part of soil profiles.

Measurement of Gas Diffusion Coefficients in Cores of Fine-Grained Lithologies Considering Stress and Adsorption Effects.

Matrix diffusivity is vital for developing tight sandstone and shale gas reservoirs. This study proposes a method to test the diffusivities of a core under confining pressure conditions using the gas diffusion technique. The diffusivities of methane and helium were examined in fine-grained rocks (sandy shale, silty sandstone, tight sandstone, and shale) under specific stress conditions. The results revealed that the gas diffusivities varied among the samples. The tight sandstones exhibited diffusivity higher than that of silty sandstone, sandy mudstone, and shale. Helium diffusivities in shales and sandy mudstones were 1 order of magnitude smaller, while methane diffusivities were 2 orders of magnitude smaller than those in tight sandstones. A positive correlation was observed between the stress sensitivity factor and the clay mineral content, indicating the influence of the clay minerals' mechanical properties. Additionally, the shale gas diffusivities exhibited significant anisotropy due to slit-like gas channels parallel to laminae in shale. It was found that the impact of adsorption on diffusivity was positively correlated to the amount of adsorption. While the adsorption effect was negligible in tight sandstones, organic-rich shales and sandy mudstones experienced an order-of-magnitude reduction in methane diffusivities compared to helium. This study presents a method to evaluate the diffusion coefficient of a core matrix under a confining pressure. It provides insights into gas diffusion behavior in different fine-grained rocks, considering the effects of stress, clay mineral content, and adsorption. These findings contribute to understanding tight sandstone gas and shale gas reservoirs, aiding in the optimization of gas production strategies.

Fine-Grained Lithofacies Types and Sedimentary Model of the Upper Permian Longtan Formation Coal Measures in the Western Guizhou Region, South China.

The upper Permian Longtan Formation is widely distributed in southwestern China and is well known for multilayer coal and high organic shale, with significant shale gas potential that has yet to be fully explored and developed. The Longtan coal-bearing strata are composed of complex lithological assemblages of fine-grained sedimentary rocks such as sandstone, coal, shale, and limestone, which exhibit significant differences from marine shale. To better understand the organic-rich lithofacies, their distribution, and their controlling factors, this study carried out a detailed survey of the outcrop and drill cores in the western Guizhou region and examined the fine-grained lithofacies, their assemblages, and their geochemical characteristics. The results showed that (1) the total organic carbon of the Longtan Formation shale in western Guizhou ranged from 1.44 to 14.79%, with an average of 6.41%, and the organic matter was mainly composed of vitrinite. The mineral composition was mainly clay minerals and brittle minerals; the clay minerals were mainly composed of kaolinite (average 11.13%) and illite/smectite mixed layers (average 26.69%) and the brittle minerals were mainly composed of quartz (average 31.63%) and feldspar (average 12.88%). (2) Eight types of lithofacies were identified, including silty mudstone, muddy siltstone, carbonaceous mudstone, carbonaceous shale, bioclastic-bearing mudstone, bioclastic-bearing sandstone, fine sandstone, and coal seam. (3) The six typical lithofacies assemblages were developed in the Longtan Formation, which represented different sedimentary environments of the marine-continental transitional facies in the study area. The lithofacies assemblages A and C represent sedimentation in the lagoon environment. The lithofacies assemblage B represents peat swamp facies. The lithofacies assemblage D represents a tidal flat facies peat flat-mixed flat-sand flat sedimentary environment. The lithofacies assemblage E and F represent the delta sedimentary environment. (4) The sedimentary model of the Longtan Formation in western Guizhou was predominantly deltaic and tidal flat sedimentary systems. Lithological and lithofacies studies of Longtan fine-grained rocks were used to provide a geological framework for examining the fine grain deposition distribution and shale gas resource evaluation. This study is highly important for understanding the sedimentology and oil and gas exploration in the region, providing a basis for identifying and exploring coal-bearing shale gas potential and a reference for the analysis of shale in the world's continental transitional areas.

Revealing continuous hydrocarbon generation and shale oil accumulation of saline lacustrine source rocks by sequential extraction

Saline lacustrine fine-grained rocks are important hydrocarbon source rocks and host large shale oil accumulations worldwide. However, the specific process and mechanism of hydrocarbon generation and accumulation in such rocks are not clear. Sequential extraction is a method to chemically separate different hydrocarbon fractions in sedimentary rocks, commonly as free, adsorbed and retained hydrocarbons, and thus can theoretically record the entire process of hydrocarbon generation and accumulation. To test this hypothesis, here sequential extraction was carried out on source rock samples with varying maturities from the alkaline lacustrine strata of the Lower Permian Fengcheng Formation, Mahu Sag, Junggar Basin, northwest China. The 17β-ethylandrostane/C29-sterane ααα20R and tricyclic terpane/pentacyclic triterpane ratios vary with maturity. In contrast, some biomarker parameters are not significantly affected by thermal maturity, including pristane/phytane, β-carotane index (β-carotane/main peak n-alkane), sterane/hopane, 7-+8-methyl index (7-+8-monomethyl heptadecane/main peak n-alkane), and C28/C29 regular sterane values, and can thus be used for oil–source rock correlations. The low-maturity source rocks have undergone one stage of hydrocarbon generation and associated shale oil accumulation, whereas source rocks with relatively high maturity have experienced multi-stage hydrocarbon generation and associated shale oil accumulation. In the early stages, cyanobacteria were the main bioprecursor to hydrocarbons and, with increasing maturity, Dunaliella gradually became the more important bioprecursor. Hydrocarbon generation by these alkaline lacustrine source rocks involved multiple sources and stages, which accounts for the abundant shale oil resources in the Mahu Sag with one-billion-ton reserves. This might be general for saline lacustrine source rocks worldwide, which can generate both conventional and unconventional hydrocarbons in large scales. Compared with conventional thermal modeling to constrain the generation history of hydrocarbon source rocks, a unique advantage of sequential extraction is its ability to characterize the entire history of hydrocarbon generation under geological conditions.

Raw Material Consumption Strategies in Holocenic Hunter-Fisher-Gatherer Groups from the South-Eastern Coast of Tierra del Fuego (Argentina): A Use-Wear Perspective.

ABSTRACT Hunter-fisher-gatherer societies who inhabited the south-eastern Atlantic Coast of Isla Grande de Tierra del Fuego (Argentina) exploited a wide range of local raw materials that includes, among others, metamorphic, fine-grained rocks and slates that exhibit different physical properties. The aim of this work is to discuss raw materials procurement and production strategies in relation to tool usage. For that purpose, we studied eight archaeological sites retrieved in the Late Holocene. We will focus on the economic practices that involve lithic raw materials to understand technological organization based on the use-wear method and morpho-technical analysis. The results show that these societies developed a technological strategy that implied the selection of rocks with specific properties for performing different and singular activities. Fine-grained rocks were mainly exploited for manufacturing small retouched tools used for scraping hides whereas metamorphic rhyolites and slates were mostly procured to produce long retouched blanks for sawing hard materials.

Alpine convergence record in the Carboniferous Badstub Formation, Upper Austroalpine basement nappes, Austria

ABSTRACT Within the Carboniferous of Nötsch, the Badstub Formation is a clastic sequence of the Upper Austroalpine basement nappes exposed few kilometres north of the Periadriatic Fault System (Carinthia, Austria). Although these rocks preserve pristine sedimentary features, multi-scale structural analysis disclosed a syn-metamorphic foliation in fine-grained rocks, sets of mineralized faults, veins, and corona textures. Vein fillings and coronas contain equilibrium mineral assemblages with prehnite, pumpellyite, chlorite, phengitic mica, winchite, and riebeckite. Thermodynamic modelling and geothermometry constrain metamorphic conditions at 260–310°C and 0.25–0.50 GPa that are consistent with a temperature/depth ratio of about 20°C km−1. In a 2D thermomechanical model this thermal state is reached either in the upper or lower continental plate during convergence. Thus, during the Alpine convergence, the Badstub rocks were buried from the shallow crust at depths between 13 and 18 km either by ablative oceanic subduction or by continental subduction of the passive margin, and eventually stacked into the orogenic wedge at the Adria margin. During the downward and upward paths, the Badstub rocks were translated as a coherent poorly strained block. This first quantitative constraint on metamorphism for these Carboniferous rocks is consistent with the Upper Austroalpine basement nappes being a tectonic system that recorded the Alpine convergence under eclogite to prehnite–pumpellyite facies conditions.

Effect of gravel on rock failure in glutenite reservoirs under different confining pressures

Due to the existence of gravel, glutenite is heterogeneous and different from fine-grained rocks such as sandstone and shale in structure. To fully understand the effect of gravel on failure mode in glutenite, we performed triaxial compression tests on different glutenites. The results indicate that failure modes of glutenite are complex due to the existence of gravel. Under different confining pressures, three failure modes were observed. The first failure mode, a tensile failure under uniaxial compression, produces multiple tortuous longitudinal cracks. In this failure mode, the interaction between gravels provides the lateral tensile stress for rock splitting. The second failure mode occurs under low and medium confining pressure and produces a crack band composed of micro-cracks around gravels. This failure mode conforms to the Mohr-Coulomb criterion and is generated by shear failure. In this failure mode, shear dilatancy and shear compaction may occur under different confining pressures to produce different crack band types. In the second failure mode, gravel-induced stress concentration produces masses of initial micro-cracks for shear cracking, and gravels deflect the fracture surfaces. As a result, the fracture is characterized by crack bands that are far broader than in fine-grained rocks. The third failure mode requires high confining pressure and produces disconnected cracks around gravels without apparent crack bands. In this failure mode, the gravel rarely breaks, indicating that the formation of these fractures is related to the deformation of the matrix. The third failure mode requires lower confining pressure in glutenite with weak cement and matrix support. Generally, unlike fine-grained rocks, the failure mode of glutenite is not only controlled by confining pressure but also by the gravel. The failure of glutenite is characterized by producing cracks around gravels. These cracks are produced by different mechanisms and distributed in different manners under different confining pressures to form different fracture patterns. Therefore, understanding the rock microstructure and formation stress state is essential in guiding glutenite reservoir development.