Search Results (39)

The depositional environment of the Permian Longtan shale (LS) in southwestern Guizhou Province, China, has been analyzed using mineralogical and geochemical approaches. Macroscopic observations of those studied LS samples showed that the LS is rather homogeneous and interbedded with coal strips, suggesting a relatively stable and shallow water environment. A detailed microscopic analysis demonstrated that higher land plants contributed the predominant proportion of organic matter in the LS. Inorganic geochemical analysis revealed a mixed source of materials with relatively larger proportions of basalt and andesite. Semiarid to humid and warm climates corresponding to an overall intensive weathering were deduced in the late Permian periods. The LS was deposited in a brackish-to-marine water environment with an oxic to dysoxic redox condition. Sea level rise/down coupled with changes in climate, water salinity, and redox condition jointly controlled the formation of the Longtan shale. Mineralogical composition indicates that the LS mainly comprises of argillaceous with minor siliceous facies, which will likely bring challenges for hydraulic fracturing. Full article

The radioisotopic dating of five stratigraphic levels within the Permian succession of the Kuznetsk Coal Basin refined the ages of the corresponding stratigraphic units and, for the first time, enabled their direct correlation with the International Chronostratigraphic Chart, 2024. The analysis revealed significant discrepancies between the updated ages and the previously accepted regional scheme (1982–1996). A comparison of regional stratigraphic units’ durations with estimated coal and siliciclastic sediment accumulation rates indicated that the early Permian contains the most prolonged stratigraphic hiatuses. The updated stratigraphic framework enabled re-evaluating the temporal sequence of regional sedimentological, volcano–tectonic and biotic events, allowing for more accurate comparison with the global record. Palaeoclimate reconstructions indicated that during the early Permian, the Kuznetsk Basin was characterised by a relatively warm, humid, and aseasonal climate, consistent with its mid-latitude position during the Late Palaeozoic Ice Age. In contrast, the middle-to-late Permian shows a transition to a temperate, moderately humid climate with pronounced seasonality, differing from the warmhouse conditions of low-latitude palaeoequatorial regions. The latest Lopingian reveals a distinct trend toward increasing dryness, consistent with global palaeoclimate signals associated with the end-Permian crisis. Full article

The evaluation of reservoir properties and gas-bearing characteristics is critical for assessing shale gas accumulation. This study aimed to improve the precision of characterizing the properties and gas-bearing features of the Carboniferous and Permian shale reservoirs within the Qinshui Basin, Shanxi Province, China. It specifically focuses on the shale from the Late Carboniferous to Early Permian Shanxi and Taiyuan formations at Well Z1, located in the mid-eastern region of the basin. A comprehensive suite of analytical techniques, including organic geochemical analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), high-pressure mercury intrusion, low-temperature nitrogen adsorption, isothermal adsorption experiments, and gas content measurements, was used to systematically evaluate the reservoir properties and gas-bearing characteristics of the Carboniferous–Permian shale in Well Z1. The findings reveal the following. (1) The organic matter in the Shanxi and Taiyuan formations of Well Z1 is predominantly Type III humic kerogen, exhibiting high maturity and abundance. Specifically, 67.40% of the samples have TOC > 1.00%, classifying them as medium- to high-quality source rocks. The vitrinite reflectance (R_o) ranges from 1.99% to 2.55%, and T_max varies from 322.01 °C to 542.01 °C, indicating a high to over-mature stage. (2) The mineral composition of the shale is dominated by kaolinite, illite, and quartz, with a moderate brittleness index. The average clay mineral content is 52.12%, while quartz averages 45.53%, and the brittleness index averages 42.34. (3) The pore types in the shale are predominantly macropores, with varying peak intervals among different samples. (4) The surface area and specific pore volume of macropores show positive relationships with TOC, T_max, kaolinite, and the amount of desorbed gas, while they are negatively correlated with quartz. In contrast, mesopores exhibit positive correlations with TOC and illite. (5) Desorbed gas content exhibits a positive correlation with porosity, R_o, and illite. These insights enhance the comprehension of the reservoir’s properties, the characteristics of gas presence, and the determinant factors for the Carboniferous–Permian shale located in the Qinshui Basin, providing a robust practical procedure for the exploration and extraction of coal-measure shale gas resources within this area. Full article

Coal is a carrier of geological information, preserving paleoenvironmental and paleoclimatic data from geological history. The Ordos Basin hosts abundant coal resources with significant potential for exploration and development. The geochemical properties of coal and their associated geological information offer key insights into coal formation, coal–rock gas generation, and the identification of favorable development areas. This study focuses on the No. 5 coal of the Shanxi Formation in the central and eastern Ordos Basin. Building on previous research and advancements, this study reveals the geochemical attributes and sedimentary background of coal through core observations, drilling data, macerals, and element analyses. The results indicate that the No. 5 coal primarily consists of bright and semi-bright coal, characterized by medium ash yield and high fixed carbon. The macerals of the coal are predominantly vitrinite (64.08% on average), followed by inertinite (24.92% on average) and liptinite (2.8% on average). The source material for the No. 5 coal in the Shanxi Formation is primarily derived from felsic igneous rocks. The varying distribution patterns of rare earth elements suggest differences in the sources of coal materials. From the Late Carboniferous to the Early Permian, the North China Craton was located in tropical paleolatitudes in the Northern Hemisphere. The warm and humid paleoclimate facilitated the deposition of coal. Fluctuations in local lake levels and sedimentary system evolution resulted in an oxidized and oxygen-deficient water. The No. 5 coal is characterized by a relatively small TPI value and a relatively large GI value, indicating a coal-forming environment with deep water coverage, poor water circulation, or relative stagnation. This resulted in slow peat accumulation, allowing plant remains to fully gelatinize. The findings enhance the understanding of the geochemical characteristics of the No. 5 coal and the factors controlling its development within the Shanxi Formation of the central and eastern Ordos Basin. These results provide a theoretical basis for coal exploration and development. Full article

The study of lithium (Li), gallium (Ga), and rare earth elements (REY) within coal-bearing strata represents a cutting-edge concern in coal geology, ore deposit studies, and metallurgy research. With the rapid advancement of technology and emerging industries, the global demand for Li-Ga-REY has significantly escalated. Several countries worldwide are facing immense pressure due to shortages in Li-Ga-REY resources. Coal-associated Li-Ga-REY depositions have emerged as a pivotal direction for augmenting Li-Ga-REY reserves. To ascertain the enrichment distribution patterns and genetic mechanisms of Li-Ga-REY within the coal-bearing strata of the late Permian Heshan Formation in Wanfu mine, Xian’an Coalfield, Guangxi Province, this study carried out comprehensive testing and analysis on Li-Ga-REY enriched in the mineralized layers within the strata. The Heshan Formation in Wanfu mine presents four layers of Li-Ga-REY-enriched mineralization, labeled from bottom to top as mineralized layers I, II, III, and IV, corresponding to coal seams K5, K4, K3, and K2. These critical metals are predominantly hosted within clay minerals (kaolinite, illite/smectite, and chlorite). The enrichment of critical metals within the Heshan Formation is closely related to terrigenous detrital materials from the Daxin paleocontinent, volcanic detrital materials induced by the Emeishan mantle plume and the Yuenan magmatic arc. The accumulation of Li-Ga-REY and other critical elements within the mineralized layers is the result of inputs from terrestrial and volcanic detrital sources, interactions between peatification and diagenesis stages, and occasionally the input of metal-enriched fluids. In the mineralized layers I, II, and III, the content of lithium oxide (Li₂O) surpasses the boundary grade, and the levels of REY, Ga, and (Nb,Ta)₂O₅ are close to boundary grades, indicating promising exploration prospects. The Wanfu mine in the Xian’an Coalfield can be considered a primary target zone for the exploration and development of coal-associated critical metal resources in Guangxi. Full article

Dengfeng Coalfield represents a significant coalfield in Henan Province, North China. It is therefore essential to gain an understanding of the mineralogy and geochemistry of the Dengfeng coal, both from a geochemical perspective and in terms of the wider environmental context. In this study, a total of 27 coal bench samples were collected from the No. II1 coal of the Dengfeng Coalfield. The mineral species and major elements were quantitatively analysed using the X-ray diffraction and X-ray fluorescence methods, respectively. The minerals in the Dengfeng coal are dominated by ammonian illite and kaolinite with average contents of 3.73% and 7.47%, respectively. These are followed by calcite (2.74% on average) and ankerite (0.49%). The mean value of the kaolinite Hinkley index, which is a quantitative measure of kaolinite crystallinity, is 1.26. This suggests that kaolinite formation is primarily driven by diagenetic recrystallisation. The ammonian illite exhibits an average d₀₀₁ of 10.2995 Å, indicative of a prevalence of NH₄⁺ interlayer cations, with K⁺ also present in notable quantities. The ratio of NH₄⁺ to (NH₄⁺ + K⁺) has an average value of 0.90, which is indicative of the predominance of NH₄⁺. The mean value of the illite Kübler index, which is a quantitative measure of illite crystallinity, is 0.264. This suggests that the diagenetic conditions correspond to the rank of the Dengfeng coal. The kaolinite present in the Dengfeng coal is suggested to have been derived from terrigenous detritus and subsequently subjected to diagenetic recrystallisation, resulting in a relatively high Hinkley index. The ammonian illite in the Dengfeng coal was predominantly formed through the conversion of the precursor kaolinite, with the influence of seawater during peat accumulation favouring the conversion of kaolinite to ammonian illite. Full article

The Late Permian coal measures in eastern Yunnan, western Guizhou, and central Guangxi are significantly enriched in critical metals that could serve as important supplements to conventional critical metal deposits in China. This study collected previous geochronological and geochemical data from the Late Permian coal measures to evaluate the distribution characteristics and enrichment factors of critical metals. Moreover, metallogenic models for critical metals were also developed. The results showed that Late Permian coal measures in Yunnan, Guizhou, and Guangxi provinces exhibited abnormal enrichment in Nb, Zr, and rare earth elements (REY, or REE if Y is excluded). The Emeishan mafic rocks and intermediate-felsic volcanic ash from the Truong Son orogenic belt underwent chemical weathering, with Nb and Zr selectively preserved in situ in the form of heavy minerals (e.g., rutile, zircon, and anatase), which subsequently led to the enrichment of Nb and Zr in bauxite and Al-claystone at the bottom of the Late Permian coal measures. Intermediate-felsic volcanic ash from the Emeishan large igneous province (ELIP) and the Truong Son orogenic belt supplied Nb, Zr, and REY for the middle and upper parts of the Late Permian coal measures. The intermediate-felsic mineral material of the coal measures in the intermediate zone, outer zone, and outside zone of ELIP are derived mainly from the ELIP, the mixture from ELIP and the Truong Son orogenic belt, and the Truong Son orogenic belts, respectively. Nb, Zr, and REY were leached by acidic aqueous solutions and from the parting and roof into underlying coal seams, where they deposited as authigenic minerals or adsorbed ions on organic matter during early coalification. Full article

Mineral matter, including minerals and non-mineral elements, in coal is of great significance for geological evolution, high-value coal utilization, and environment protection. The minerals and elemental geochemistry of Late Permian coals from the M8 coal seam, Shihao mine, Songzao coalfield in Chongqing, were analyzed to evaluate the sediment source, sedimentary environment, hydrothermal fluids, and utilization prospects of critical metals. The average total sulfur (4.21%) was high in coals, which mainly exists in the forms of pyritic sulfur. Kaolinite, pyrite, calcite, quartz, illite and illite/smectite (I/S) mixed layers, and anatase predominated in coals, with trace amounts of chlorite, ankerite, and siderite. Epigenetic cell- and fracture-filling pyrite, veined calcite, and ankerite were related to hydrothermal fluids and/or pore water after the diagenesis stage. Compared to the world’s hard coals, As and Cd are enriched in the Shihao M8 coals, and Li, Cr, Co, Zr, Mo, Pb, and Tb are slightly enriched. These high contents of sulfophile elements may be related to seawater intrusion. The terrigenous clastics of the Shihao M8 coals originated from the felsic–intermediate rocks atop the Emeishan Large Igneous Provinces (ELIP) (Kangdian Upland), while the roof and floor samples were derived from Emeishan high-Ti basalt. Through the combination of sulfur contents and indicator parameters of Fe₂O₃ + CaO + MgO/SiO₂ + Al₂O₃, Sr/Ba and Y/Ho, the depositional environment of peat swamp was found to be influenced by seawater. Although the critical elements in coal or coal ash did not reach the cut-off grade for beneficial recovery, the concentration of Li and Zr were high enough in coal ash. Full article

The mineralogical, fluid inclusion, and stable isotope (C, O) study was conducted on a Late Variscan Zn-Pb vein Bt23C, Příbram uranium and base-metal district, Bohemian Massif, Czech Republic. The vein is hosted by folded Proterozoic clastic sediments in exo-contact of a Devonian-to-Lower-Carboniferous granitic pluton. Siderite, dolomite-ankerite, calcite, quartz, baryte, galena, sphalerite, V-rich mica (roscoelite to an unnamed V-analogue of illite), and chlorite (chamosite) form the studied vein samples. The banded texture of the vein was modified by the episodic dissolution of earlier carbonates and/or sphalerite. Petrographic, microthermometric, and Raman studies of fluid inclusions proved a complicated fluid evolution, related to the activity of aqueous fluids and to an episode involving an aqueous–carbonic fluid mixture. Homogenization temperatures of aqueous inclusions decreased from ~210 to ~50 °C during the evolution of the vein, and salinity varied significantly from pure water up to 27 wt.% NaCl eq. The aqueous–carbonic fluid inclusions hosted by late quartz show highly variable phase compositions caused by the entrapment of accidental mixtures of a carbonic and an aqueous phase. Carbonic fluid is dominated by CO₂ with minor CH₄ and N₂, and the associated aqueous solution has a medium salinity (6–14 wt.% NaCl eq.). The low calculated fluid δ¹⁸O values (−4.7 to +3.6‰ V-SMOW) suggest a predominance of surface waters during the crystallization of dolomite-ankerite and calcite, combined with a well-mixed source of carbon with δ¹³C values ranging between −8.2 and −10.5‰ V-PDB. The participation of three fluid endmembers is probable: (i) early high-temperature high-salinity Na>Ca-Cl fluids from an unspecified “deep” source; (ii) late low-salinity low-temperature waters, likely infiltrating from overlying Permian freshwater partly evaporated piedmont basins; (iii) late high-salinity chloridic solutions with both high and low Ca/Na ratios, which can represent externally derived marine brines, and/or local shield brines. The source of volatiles can be (i) in deep crust, (ii) from interactions of fluids with sedimentary wall rocks and/or (iii) in overlying Permian piedmont basins containing, in places, coal seams. The event dealing with heterogeneous CO₂-bearing fluids yielded constraints on pressure conditions of ore formation (100–270 bar) as well as on the clarification of some additional genetic aspects of the Příbram’s ores, including the reasons for the widespread dissolution of older vein fill, the possible re-cycling of some ore-forming components, pH changes, and occasionally observed carbon isotope shift due to CO₂ degassing. Full article

The Kuznetsk Basin (Kuzbass) is one of the largest coal basins in Siberia and a reference area for the ancient Angaraland continent. The proximity of the Kuzbass and Siberian Platform caused their biotic similarities in the Late Palaeozoic. However, due to biota endemism, the Kuzbass Upper Palaeozoic does not correlate directly with the International Chronostratigraphic Chart (ICC). This paper discusses radioisotopic (CA-ID-TIMS) dating of zircons from a volcanic tuff located in the Starokuznetsk Formation (Fm). This level matches the interval of the Balakhonka/Kolchugino (B/K) floral change in Kuzbass, i.e., the gradual replacement of cordaitoid-dominated wet forests (Balakhonka flora) with more arid fern–pteridosperm–cordaitoid assemblages (Kolchugino flora). New age (276.9 ± 0.4 Ma) directly correlates the Starokuznetsk Fm with the Upper Kungurian of the ICC. We compared the Kuzbass data with data of the Western Verkhoyanie, where Middle Permian ammonoids (Sverdrupites assemblage) occur in strata recording the B/K floral change. The available (ICC) and new datings indicate the lag between the B/K floral change in low (Kuzbass) and high (Verkhoyanie) latitudes of Angaraland. The B/K floral change in the Kuzbass began in the early Late Kungurian and was completed by the end of this age. In contrast, the B/K floral change in Verkhoyanie began at the end of the Late Kungurian and was completed in the Late Wordian. The delay in the floral changes at different latitudes of Angaraland suggests that existing interregional correlations need further improvement. Full article

Forming environments have important effects on the dispersion and enrichment of trace elements in coal. The C₃ coal seam of the Xuanwei Formation in eastern Yunnan was used as a case study to reconstruct the peat-forming environment based on coal facies parameters and geochemical characteristics, and its influence on trace element (including rare earth elements and yttrium, REY) enrichment was investigated. The C₃ coal was classified as medium rank bituminous coal with an ultra-low moisture content, medium-high ash yield, and medium-low volatile content. Compared to the average values for Chinese coal, Cu and V were enriched and Co was slightly enriched in the C₃ coal. Compared with the average values for world coal, Cu and V were enriched while several other trace elements were slightly enriched in the C₃ coal, including Co, Hf, Nb, Sc, Ta, Zn, and Zr. The C₃ coal was deposited in the limno-telmatic environment with fresh water, and reducing conditions. Trace elements, including Cu, V, Hf, Nb, Sc, Ta, Zr, Zn, Co, and REY, were typically enriched in the limno-telmatic environment with fresh water and reducing conditions. Additionally, REY and V were also significantly enriched in brackish water limno-telmatic conditions with the same depositional environment. Full article

The Late Permian strata of the Xuanwei Formation in the eastern Yunnan region exhibit extensive diverse morphological features within siderite deposits. These variations in siderite deposits suggest potential differences in their formation processes. In this study, fieldwork and comprehensive indoor studies revealed four distinct forms of siderite deposits: stratiform-laminated, lens-like nodule, sandstone cementation, and fracture filling. The stratiform-laminated siderite, varying in color from bluish-grey to dark grey, is composed of uniformly sized microcrystalline to fine-grained siderite along with detrital matter, displaying precise layering and banding structures that suggest direct deposition from cyclic iron-rich seawater under reducing conditions. Lens-like-nodule siderite, which appears grey-yellow, is composed of mud microcrystalline siderite, medium to coarse-grained pseudo-ooids, and glauconite. It shows conformable distribution characteristics resulting from the diagenetic differentiation of iron-rich sediments under reducing conditions during the diagenetic and early diagenetic periods. Siderite as sandstone cementation exhibits a yellow-brown color and consists of dispersed colloidal siderite and cemented siderite clumps that fill intergranular pores of detrital particles. It precipitated under reducing conditions within those intergranular pores. Siderite filling fractures typically appear as vein-like or network-like structures intersecting bedding at large angles. They exhibit grain structures with significant variations in size. These siderite deposits exhibit exceptional purity and result from siderite dissolution during sedimentary periods, followed by reprecipitation within regional extensional fractures during the diagenetic phase. The primary occurrence of siderite deposits in the study area is within coal-bearing strata, as revealed by the integration of sedimentary profiles and sedimentary facies analysis. The coal-bearing strata, influenced by the Emeishan large igneous province, underwent iron enrichment during and after volcanic eruptions while developing a reducing environment, which was facilitated by abundant vegetation. Consequently, geological processes led to siderite layers, lens-like siderite nodules, and siderite cementation. The Yanshan orogeny induced extensive high-angle fracture development in epigenetic coal-bearing strata, facilitating fluid circulation and the redistribution of soluble siderite. This geological activity resulted in the formation of vein-like structures composed of siderite. Full article

The natural gas in the Triassic Xujiahe Formation (T₃x) is reported to be mainly derived from the T₃x source rock itself. Here, we report a new natural gas accumulation model, which demonstrates that the T₃x gas in the Tongjiang-Malubei (TM) area is derived from both T₃x and underline marine source rocks. The T₃x gas in the TM area is characterized by CH₄ with a gas dryness coefficient above 0.99, indicating a high thermal maturity. The δ₁₃C values of the methane, ethane, and propane in the T₃x gas in the TM area are −33.7~−29.2‰, −32.7~−28.3‰, and −32.8~−29.5‰, respectively. Compared with the T₃x gas in the Yuanba area, which was sourced from the T₃x source rock, the T₃x gas in the TM area contains heavier δ₁₃C in methane and lighter δ₁₃C in ethane, showing a partial reversal carbon isotope distribution (δ¹³C₁ > δ¹³C₂). According to their chemical and isotopic compositions, the T₃x gas in the TM area was a mixture of coal-type and oil-type gases. The coal-type gas was mainly derived from the type III kerogen of the T₃x source rock, and the oil-type gas was derived from the type-I kerogen of marine source rock in the Permian Wujiaping Formation (P₃w). The oil-type gas migrated upward along the deep-seated faults that connect the P₃w source rock and T₃x sandstone reservoirs, and then mixed with coal-type gas in the T₃x reservoirs, resulting in large-scale gas accumulation. This new gas accumulation model is controlled by a dual gas source supply and a high efficiency migration via the fault system. The findings of this study can help us to better understand the gas accumulation mechanism with the development of late-stage penetrating faults, which not only have implications for future petroleum exploration and development in the TM area, but also affect other analogous areas in the Sichuan Basin. Full article

During the Late Permian period, the coal from the Jiashun Mine in Xingren City, Guizhou Province, China, is characterized by a high sulfur (5.84%) and lithium (Li) (94.5 μg/g) content. Lithium is a critical metal in the context of global energy transition. Considering the importance of environmental protection and resource utilization, the mode of occurrence and distribution of trace elements in different coal preparation products were investigated. The obtained results indicated the following: (1) The minerals in Jiashun coal were mainly composed of veined and epigenetic pyrite, quartz, and kaolinite. Most of the minerals could be effectively removed from the cleaned coal through gravity separation. (2) The mode of occurrence of rare earth elements and yttrium (REY) in the coal was relatively complex, and they were mainly found in middlings from gravity separation and in flotation tailings. (3) The mode of occurrence of the trace elements in the coal significantly influenced their removal rate. The removal rate was significantly higher for trace elements removed through gravity separation compared to those removed using flotation. The trace elements mainly found in fine-grained minerals wrapped by organic matter or combined with organic portions had better removal results through flotation. The trace elements that occurred both in the minerals and organic matters had low removal rates (<25%) using two coal preparation methods. (4) Li was more enriched in the middlings from gravity separation (98 μg/g) and in flotation tailings (102 μg/g), reaching the marginal grade (80 μg/g) of Li in coal. (5) Li in Jiashun coal may be derived from intermediate-felsic rocks at the top of the Kangdian Upland and late hydrothermal solutions. Full article

The coal-bearing sequences of the Late Permian in southwest China are enriched in critical elements Sc, V, Co, Ni, Cu, Zn, Nb, Ta, Zr and Hf. Ascertaining the conditions and basis for the enrichment of critical elements in this area is very important to support the critical metal demands of China. In this study, we analyzed the concentrations of elemental compositions of coal samples collected from the Late Permian Guishan coalfield in the eastern Yunnan, China. The results show that the C4-Upper coal seam of the Feilongma mine and C5 + 10 coal seam of the Shipeng mine are indeed rich in critical elements. The average concentration coefficients (CC) of transition metal elements Sc, V, Co, Ni, Cu, and Zn are 3.23 and 2.93, respectively, in the two coal seams. The average CCs of high-field-strength elements Nb, Ta, Zr, and Hf and non-variable valence chalcophile elements Ga and In are 2.21 and 2.53, respectively, in the two coal seams. The C4-Upper coal seam of the Feilongma mine can be divided into two sections based on the different ash contents, and the CCs of the critical elements in the two sections are almost equal. The main minerals in the two coal seams are kaolinite, siderite, quartz, gypsum, rozenite and marcasite. Multiple indicators of provenance show that the enrichment of critical elements in the Guishan coalfield is controlled by clastic terrigenous material. The source area of the Guishan coalfield is Kangdian Oldland in the northwest, and the main clastic materials are related with high-Ti basalts from the Emeishan Large Igneous Province. Full article