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    Deke XU, Chang LI, Yingyu JIN, Zhenhua DENG, Bing XU, Kangkang LI, Jianping ZHANG, Xiaohong SUN, Jing FENG, Yingxin JIAO, Hao LI, Xiaoguang QIN
    Frontiers of Earth Science, 2023, 17(4): 1070-1080.

    The discovery of Loulan ancient city (LA) in the early 20th century has important significance for understanding the history of Western regions and the Silk Road civilization. The current academic community still has disputes on whether LA was the capital of Loulan Kingdom, the time of its rise, peak and decline, and the process, rate and driving mechanism of human activity change. This study uses the radio carbon dates (14C) database of LA to reconstruct the history of the rise and fall of human activity, and finds that LA experienced more than ~500 years from its rise to its peak and then to its decline: 1) the city rose rapidly, and the population increased rapidly from ~A.D. 0 to 230; 2) the city was prosperous and flourishing, and the intensity of human activity reached its peak from ~A.D. 160 to 340, especially in ~A.D. 230, when the population reached its peak; 3) the city accelerated its decline, and the intensity of human activity decreased significantly, and the population shrank rapidly from ~A.D. 230 to 500; 4) LA was completely abandoned after ~A.D. 560. The results of the 14C dating database do not support that LA was the early capital of the Loulan Kingdom. By comparing the human activity record of LA with the existing high-resolution palaeoclimate records in the surrounding mountainous areas of the Tarim Basin and South Asia, it is found that the superposition of centennial-scale westerly circulation strength events and the ~500-year cycle of the Indian monsoon jointly controlled the precipitation and meltwater (snow) supply of the mountains in the Tarim Basin, affecting the changes of surface runoff and oasis area in the basin, which is one of the important factors causing the rise and fall of LA.

    Juzhi HOU, Xianyong CAO, Duo WU, Mingda WANG
    Frontiers of Earth Science, 2023, 17(4): 899-904.
    Zhe SUN, Zirui HUANG, Kejia JI, Mingda WANG, Juzhi HOU
    Frontiers of Earth Science, 2023, 17(4): 933-944.

    The Yarlung Tsangpo, the longest river in the southern Tibetan Plateau (TP), has attracted much research attention aimed at understanding the factors controlling its modern hydrology and possible future discharge in the context of ongoing climate change. However, partly due to the complex regional climatic background, no consistent conclusions have been reached, especially for its upper reaches. Paleohydrological reconstructions of the source region of the Yarlung Tsangpo can potentially improve our understanding of the history of humidity and its response to climatic variability. In this study, we used a 97 cm gravity core from Gongzhu Co to reconstruct the hydrology change during the late Holocene. The core was dated using AMS 14C and Pb/Cs methods, and we used measurements of element contents (determined by high-resolution XRF scanning), grain size, IC/TOC, and magnetic susceptibility to reconstruct hydroclimatic changes in the source of the Yarlung Tsangpo watershed since ~4000 yr ago. Combined with a modern meteorological data set, we found that PC1 of the XRF data, the Ca/(Fe + Ti) ratio, and EM1 of the grain size data were indicative of changes in humidity. Our records demonstrate a wet interval during ~4–1.7 ka BP (ka = 1000 yr, BP represents years before 1950 AD), followed by a dry period during since ~1 ka BP. Comparison with independent regional paleoclimatic records revealed shifts in the dominant factors controlling humidity. The wet interval during ~4–1.7 ka BP was coeval with a strengthened Westerlies, implying a dominant moisture supply from northern high latitudes. However, the extremely low values of Ca/(Fe + Ti) ratio during ~4–2.5 ka BP indicate potential glacial freshwater source, which is corroborated by the concurrent high magnetic susceptibility values and increased grain size. The rapid drying trend during ~1.7–1 ka BP suggests a switch in moisture supply from the Westerlies to the Indian Summer Monsoon (ISM). We attribute the drought conditions after ~1 ka BP to a weakened ISM, although a Westerlies influence and the potential effect of high temperatures on evaporation cannot be excluded. We suggest that future hydroclimatic research in this region should attempt to distinguish the individual moisture contributions of the ISM and the Westerlies during the last millennium.

    Xianfeng TAN, Long LUO, Hongjin CHEN, Jon GLUYAS, Zihu ZHANG, Chensheng JIN, Lidan LEI, Jia WANG, Qing CHEN, Meng LI
    Frontiers of Earth Science, 2023, 17(3): 713-726.

    The positive S-isotopic excursion of carbonate-associated sulfate (δ34SCAS) is generally in phase with the Steptoean positive carbon isotope excursion (SPICE), which may reflect widespread, global, transient increases in the burial of organic carbon and pyrite sulfate in sediments deposited under large-scale anoxic and sulphidic conditions. However, carbon-sulfur isotope cycling of the global SPICE event, which may be controlled by global and regional events, is still poorly understood, especially in south China. Therefore, the δ13CPDB, δ18OPDB,δ34SCAS, total carbon (TC), total organic carbon (TOC) and total sulfate (TS) of Cambrian carbonate of Waergang section of Hunan Province were analyzed to unravel global and regional controls on carbon-sulfur cycling during SPICE event in south China.

    The δ34SCAS values in the onset and rising limb are not obviously higher than that in the preceding SPICE, meanwhile sulfate (δ34SCAS) isotope values increase slightly with increasing δ13CPDB in rising limb and near peak of SPICE (130–160 m). The sulfate (δ34SCAS) isotope values gradually decrease from 48.6‰ to 18‰ in the peak part of SPICE and even increase from 18‰ to 38.5% in the descending limb of SPICE. The abnormal asynchronous C-S isotope excursion during SPICE event in the south China was mainly controlled by the global events including sea level change and marine sulfate reduction, and it was also influenced by regional events such as enhanced siliciclastic provenance input (sulfate), weathering of a carbonate platform and sedimentary environment. Sedimentary environment and lithology are not the main reason for global SPICE event but influence the δ13CPDB excursion-amplitude of SPICE. Sea level eustacy and carbonate platform weathering probably made a major contribution to the δ13CPDB excursion during the SPICE, in particularly, near peak of SPICE. Besides, the trilobite extinctions, anoxia, organic-matter burial and siliciclastic provenance input also play an important role in the onset, early and late stage of SPICE event.

    Taotao YAN, Chao YANG, Shuai ZHENG, Yadong BAI, Wei CHEN, Yanfei LIU, Wenguang TIAN, Shasha SUN, Shangwen JIN, Jiahang WANG, Zhenxing LIU, Huifang YAO
    Frontiers of Earth Science, 2023, 17(3): 661-678.

    The Daning-Jixian block, the eastern edge of the Ordos Basin, is one of the most potential areas for CO2 geological storage, enhanced coalbed methane recovery (ECBM) exploration and production in China in recent decades. The ionic composition and total dissolved solids (TDS) of the produced water, coal organic matter maturity, molecular composition and carbon isotope characteristics of the produced gas were utilized to analyze the hydrogeological condition, CBM generation and migration characteristics in this area. The CBM enrichment patterns and the geological impacts on gas well production characteristics were revealed. The optimal area for CBM development and CO2 geological storage in the study area were also proposed. Dominated by the Xueguan reverse fault zone, the hydraulic unit in this area can be divided into two parts (i.e., the recharge-runoff zone in the east and the weak runoff-stagnation zone in the west). The thermogenic gas is dominating CBM genesis in this area. Secondary biogenic gas replenishment is only distributed in the eastern margin area, where the δ13C1 value is less than the thermal simulation results as an influence of hydrodynamic fractionation. Finally, two models of CBM formation and accumulation were proposed, 1) thermogenic CBM migrated by hydrodynamic and resorbed for preservation at impermeable fault boundaries; 2) thermogenic CBM trapped by fault and accumulated by hydrodynamic in slope zone. The gas production performance, generally increased from east to west, is mainly dominated by hydrogeological conditions. Generally, the west side of the fault zone is the enrichment and high-yield area for ECBM development and CO2 geological storage in the study area.

    Junchang SUN, Zhiqiang DONG, Sinan ZHU, Shifeng TIAN, Junping ZHOU
    Frontiers of Earth Science, 2023, 17(3): 691-700.

    The pore structure of caprock plays an important role in underground gas storage security, as it significantly influences the sealing capacity of caprock. However, the pore structure evolution of caprock with the cyclic stress perturbations triggered by the cyclic gas injection or extraction remains unclear. In this study, the pore structure changes of mudstone caprock under cyclic loading and unloading were obtained by the nuclear magnetic resonance (NMR) tests system, then the influence of the changes on the breakthrough pressure of caprock was discussed. The results indicated that the pore structure changes are depending on the stress loading-unloading path and stress level. In the first cyclic, at the loading stage, with the increase of confining stress, the NMR T2 spectra curve moved to the left, the NMR signal amplitude of the first peak increased, while the amplitude of the second peak decreased gradually. This indicated that the larger pores of mudstone are compressed and transformed into smaller pores, then the number of macropores decreased and the number of micro- and meso-pores increased. For a certain loading-unloading cycle, the porosity curve of mudstone in the loading process is not coincide with that in the unloading process, the porosity curve in the loading process was located below that in the unloading process, which indicated that the pore structure change is stress path dependent. With the increase of cycle numbers, the total porosity shown an increasing trend, indicating that the damage of mudstone occurred under the cyclic stress load-unload effects. With the increase of porosity, the breakthrough pressure of mudstone decreased with the increase of the cyclic numbers, which may increase the gas leakage risk. The results can provide significant implication for the underground gas storage security evaluation.

    Yanrong ZHANG, Nannan WANG, Lina LIU, Mingda WANG, Xiaoshan YU, Xianyong CAO
    Frontiers of Earth Science, 2023, 17(4): 1049-1058.

    Investigating the dynamics of vegetation is an essential basis to know how to protect ecological environments and to help predict any changes in trend. Because of its fragile alpine ecosystem, the Tibetan Plateau is a particularly suitable area for studying vegetation changes and their driving factors. In this study, we present a high-resolution pollen record covering the last two centuries extracted from Gongzhu Co on the western Tibetan Plateau. Alpine steppe is the predominant vegetation type in the surrounding area throughout the past 250 years with stable vegetation composition and abundance, as revealed by pollen spectra dominated by Artemisia, Ranunculaceae, Cyperaceae, and Poaceae. Detrended canonical correspondence analysis (DCCA) of the pollen data reveals low turnover in compositional species (0.41 SD), suggesting that the vegetation in the Gongzhu catchment had no significant temporal change, despite climate change and population increases in recent decades. We additionally ran DCCA on ten other pollen records from the Tibetan Plateau with high temporal resolution (1–20 years) covering recent centuries, and the results also show that compositional species turnover (0.15–0.81 SD) is relatively low, suggesting that the vegetation stability may have prevailed across the Tibetan Plateau during recent centuries. More high-resolution pollen records and high taxonomic-resolution palaeo-vegetation records (such as sedaDNA), however, are needed to confirm the vegetation stability on the Tibetan Plateau.

    Qian ZHANG, Shuheng TANG, Songhang ZHANG, Xinlu YAN, Kaifeng WANG, Tengfei JIA, Zhizhen WANG
    Frontiers of Earth Science, 2023, 17(3): 646-660.

    With the deepening of coalbed methane (CBM) exploration and development, the problem of low gas production has gradually become one of the main factors restricting the development of the CBM industry in China. Reasonable well pattern deployment can improve the productivity of CBM wells and reduce the cost of production, while the reservoir changes of CBM wells play a important role for well pattern infilling. In this study, the dynamic characteristics of the average reservoir pressure (ARP), permeability, and drainage radius during the development process of CBM wells are systematically analyzed, and predicted the production changes of well groups before and after infilling wells in combination with the characteristics of reservoir changes. The results show that the high gas production wells have a larger pressure drop, long drainage radius, and a large increase in permeability. On the contrary, low gas production wells are characterized by small drainage radius, damaged permeability and difficult to recover. The productivity of infilled horizontal wells is predicted for two well groups with different productivity and reservoir dynamic characteristics. After infilling wells, the production of current wells has increased at different degrees. It is predicted that the average gas production of low gas production well group H1 and middle gas production well group H2 will increase 1.64 and 2.09 times respectively after 3000 days production simulation. In addition, the pressure interference between wells has increased significantly, and the overall gas production of the well group has greatly increased. Infill wells can achieve better development results in areas with superior CBM resources, recoverable reservoir permeability, and small drainage radius during the early production process. The research results provide a reference for the later infill adjustment of CBM well patterns in the study area.

    Tianxiao WANG, Duo WU, Tao WANG, Lin CHEN, Shilong GUO, Youmo LI, Chenbin ZHANG
    Frontiers of Earth Science, 2023, 17(4): 1012-1025.

    Reconstructing Holocene temperature evolution is important for understanding present temperature variations and for predicting future climate change, in the context of global warming. The evolution of Holocene global temperature remains disputed, due to differences between proxy reconstructions and model simulations, a discrepancy known as the ῾Holocene temperature conundrum᾽. More reliable and quantitative terrestrial temperature records are needed to resolve the spatial heterogeneity of existing records. In this study, based on the analysis of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from a loess-paleosol sequence from the Ganjia Basin in the north-eastern Tibetan Plateau (NETP), we quantitatively reconstructed the mean annual air temperature (MAAT) over the past 12 ka. The MAAT reconstruction shows that the temperature remained low during the early Holocene (12−8 ka), followed by a rapid warming at around 8 ka. From 8 to 4 ka, the MAAT record reached its highest level, followed by a cooling trend from the late Holocene (4−0 ka). The variability of the reconstructed MAAT is consistent with trends of annual temperature records from the Tibetan Plateau (TP) during the Holocene. We attribute the relatively low temperatures during the early Holocene to the existence of ice sheets at high-latitude regions in the Northern Hemisphere and the weaker annual mean insolation at 35°N. During the mid to late Holocene, the long-term cooling trend in the annual temperature record was primarily driven by declining summer insolation. This study provides key geological evidence for clarifying Holocene temperature change in the TP.

    Xiaona ZHANG, Yanbin YAO, Yongshang KANG
    Frontiers of Earth Science, 2023, 17(3): 679-690.

    Shales can form a complex fracture network during hydraulic fracturing, which greatly increases the stimulated reservoir volume (SRV) and thus significantly increases oil or gas production. It is therefore important to accurately predict the probability of formation of the hydraulic fracture network for shale gas exploration and exploitation. Conventional discriminant criteria are presented as the relationship curves of stress difference vs. intersection angle. However, these methods are inadequate for application in the field. In this study, an effective and quantitative prediction method relating to the probability of complex fracture network formation is proposed. First, a discriminant criterion of fracture network was derived. Secondly, Monte Carlo simulation was applied to calculate the probability of the formation of the complex fracture network. Then, the method was validated by applying it to individual wells of two active shale gas blocks in the Sichuan Basin, China. Results show that the probabilities of fracture network are 0.98 for well JY1 and 0.26 for well W204, which is consistent with the micro-seismic hydraulic fracturing monitoring and actual gas production. Finally, the method was further extended to apply for the regional scale of the Sichuan Basin, where the general probabilities of fracture network formation are 0.32−1 and 0.74−1 for Weiyuan and Jiaoshiba blocks, respectively. The Jiaoshiba block has, therefore, an overall higher probability for formation of fracture network than the Weiyuan block. The proposed method has the potential in further application to evaluation and prediction of hydraulic fracturing operations in shale reservoirs.

    Jun LIU, Yanbin YAO, Peng ZHAO, Zhaohui LU, Yunzhong JIA
    Frontiers of Earth Science, 2023, 17(3): 643-645.
    Fang TIAN, Meijiao CHEN, Weihan JIA, Ulrike HERZSCHUH, Xianyong CAO
    Frontiers of Earth Science, 2023, 17(4): 1037-1048.

    Plant environmental DNA extracted from lacustrine sediments (sedimentary DNA, sedDNA) has been increasingly used to investigate past vegetation changes and human impacts at a high taxonomic resolution. However, the representation of vegetation communities surrounding the lake is still unclear. In this study, we compared plant sedDNA metabarcoding and pollen assemblages from 27 lake surface-sediment samples collected from alpine meadow on the central-eastern Tibetan Plateau to investigate the representation of sedDNA data. In general, the identified components of sedDNA are consistent with the counted pollen taxa and local plant communities. Relative to pollen identification, sedDNA data have higher taxonomic resolution, thus providing a potential approach for reconstructing past plant diversity. The sedDNA signal is strongly influenced by local plants while rarely affected by exogenous plants. Because of the overrepresentation of local plants and PCR bias, the abundance of sedDNA sequence types is very variable among sites, and should be treated with caution when investigating past vegetation cover and climate based on sedDNA data. Our finding suggests that sedDNA analysis can be a complementary approach for investigating the presence/absence of past plants and history of human land-use with higher taxonomic resolution.

    Chenyu WANG, Dongming ZHANG, Shujian LI, Yu CHEN, Chongyang WANG, Kangde REN
    Frontiers of Earth Science, 2023, 17(3): 818-831.

    In situ stress testing can improve the safety and efficiency of coal mining. Identifying the Kaiser effect point is vital for in situ stress calculations; however, the in situ stress calculation is limited by the rock sampling angle. Here, the Kaiser effect point identification theory is established and applied to the Xuyong Coal Mine. Uniaxial compression and acoustic emission experiments were carried out on sandstone with 6 sampling directions. Furthermore, COMSOL simulation is applied to study the in situ stress distribution in the coal mine to verify the calculation accuracy. The results are as follows. 1) The failure mode of non-bedded and vertical-bedded rocks is primarily tensile shear failure with obvious brittleness in mechanical and acoustic emission characteristics. Shear slip along the bedding plane is the primary failure mode of inclined-bedded rock. Additional take-off points exist in the AE count curve. 2) The Kaiser point identification method based on the variation of AE count curve parameters Δti and τi can effectively calculate the in situ stress. According to the numerical value of Kaiser point and sampling direction, the in situ stress of the conveyor roadway in the Xuyong Coal Mine was calculated as σ1=22.81M Pa, σ 2=10.87M Pa and σ 3=6.14M Pa. 3) By the COMSOL simulation study, it was found that a stress concentration zone of 16.13 MPa exists near the two sides roadway. Compared with the Kaiser effect method, the deviation rates of the three-direction principal stress calculated by COMSOL were all less than 5%. This verifies that the in situ stress calculation by Kaiser effect in this study can be applied to the Xuyong Coal Mine.

    Yixuan WANG, Chaoqun CAO, Yanrong ZHANG, Lina LIU, Nannan WANG, Wenjia LI, Xianyong CAO
    Frontiers of Earth Science, 2023, 17(4): 1059-1069.

    As an important proxy for investigating past fire activities, charcoal is often used to explore the characteristics of fire distribution and its relationships with vegetation, climate, and human activities. Research into the spatial distribution and environmental determinants for charcoal, however, is still limited. In this study, we identified and counted charcoal from topsoil samples covering the Tibetan Plateau using the pollen methodology, and investigated its relationships with vegetation net primary production (NPP), elevation, climate (precipitation, mean temperature of the coldest month and warmest month) and human population by boosted regression trees (BRT). Results reveal that the concentration of microscopic charcoal, macroscopic charcoal, and total charcoal all increase from south-west to north-east, which is consistent with the trend that the population density on the Tibetan Plateau is high in the east and low in the west, suggesting that an increase in human activity is likely to promote the occurrence of fire. The BRT modeling reveals that NPP, elevation, and mean temperature of the coldest month are important factors for total charcoal concentration on the Tibetan Plateau, and the frequency and intensity of fires further increase with increasing vegetation biomass, decreasing elevation, and decreasing mean temperature of the coldest month. The spatial variation characteristics of charcoal from topsoil on the Tibetan Plateau not only reflect well the spatial fire situation in the region, but also have a good indicative significance for vegetation, climate, and human activities.

    Mingda WANG, Qin LI, Jaime TONEY, David HENDERSON, Juzhi HOU
    Frontiers of Earth Science, 2023, 17(4): 905-919.

    Long-chain n-alkanes are one of the most common organic compounds in terrestrial plants and they are well-preserved in various geological archives. n-alkanes are relatively resistant to degradation and thus they can provide high-fidelity records of past vegetation and climate changes. Nevertheless, previous studies have shown that the interpretation of n-alkane proxies, such as the average chain length (ACL), is often ambiguous since this proxy depends on more than one variable. Both vegetation and climate could exert controls on the n-alkane ACL, and hence its interpretation requires careful consideration, especially in regions like the Qinghai-Tibet Plateau (QTP) where topography, biome type and moisture source are highly variable. To further evaluate the influences of vegetation and climate on the ACL in high-elevation lakes, we examined the n-alkane distributions of the surface sediments of 55 lakes across the QTP. Our results show that the ACL across a climatic gradient is significantly affected by precipitation, rather than by temperature. The positive correlation between ACL and precipitation may be because of the effect of microbial degradation during deposition. Finally, we suggest that more caution is needed in the interpretation of ACL data in different regions.

    Xin BAI, Zhuoli ZHOU, Guicheng HE, Dongming ZHANG, Han YANG, Zenrui FAN, Dengke WANG
    Frontiers of Earth Science, 2023, 17(3): 867-883.

    Due to the limited permeability and high methane content of the majority of China’s coal seams, significant coal mining gas disasters frequently occur. There is an urgent need to artificially improve the permeability of coalbed methane (CBM) reservoirs, enhance the recovery efficiency of CBM and prevent mine gas accidents. As a novel coal rock fracture technology, the CO2 phase transition jet (CPTJ) has been widely used due to its advantages of safety and high fragmentation efficiency. In this study, to ascertain the effects of the pressure of CPTJ fracturing, the influence of its jet pressure on cracked coal rock was revealed, and its effect on CBM extraction was clarified. In this research, the law of CPTJ pressure decay with time was investigated using experimental and theoretical methods. Based on the results, the displacement and discrete fracture network law of CPTJ fracturing coal rock under different jet pressure conditions were studied using particle flow code numerical simulation. Finally, field experiments were conducted at the Shamushu coal mine to assess the efficiency of CPTJ in enhancing CBM drainage. The results showed that the pressure of the CPTJ decreased exponentially with time and significantly influenced the number and expansion size of cracks that broke coal rock but not their direction of development. CPTJ technology can effectively increase the number of connected microscopic pores and fractures in CBM reservoirs, strongly increase the CBM drainage flow rate by between 5.2 and 9.8 times, and significantly reduce the CBM drainage decay coefficient by between 73.58% and 88.24%.

    Zhaohui LU, Ke LI, Xingbing LIU, Peng ZHAO, Jun LIU
    Frontiers of Earth Science, 2023, 17(3): 739-751.

    CO2 geological storage and utilization (CGSU) is considered a far-reaching technique to meet the demand of increasing energy supply and decreasing CO2 emissions. For CGSUs related to shale gas reservoirs, experimental investigations have attracted variable methodologies, among which low-field NMR (LF-NMR) is a promising method and is playing an increasingly key role in reservoir characterization. Herein, the application of this nondestructive, sensitive, and quick LF-NMR technique in characterizing CGSU behavior in shale gas reservoirs is reviewed. First, the basic principle of LF-NMR for 1H-fluid detection is introduced, which is the theoretical foundation of the reviewed achievements in this paper. Then, the reviewed works are related to the LF-NMR-based measurements of CH4 adsorption capacity and the CO2-CH4 interaction in shale, as well as the performance on CO2 sequestration and simultaneous enhanced gas recovery from shale. Basically, the reviewed achievements have exhibited a large potential for LF-NMR application in CGSUs related to shale gas reservoirs, although some limitations and deficiencies still need to be improved. Accordingly, some suggestions are proposed for a more responsible development of the LF-NMR technique. Hopefully, this review is helpful in promoting the expanding application of the LF-NMR technique in CGSU implementation in shale gas reservoirs.

    Sijian ZHENG, Shuxun SANG, Shiqi LIU, Meng WANG, Lutong CAO, Xin JIN, Guangjun FENG, Yi YANG
    Frontiers of Earth Science, 2023, 17(3): 752-759.

    CCUS (carbon capture, utilization, and storage) technology is regarded as a bottom method to achieve carbon neutrality globally. CO2 storage in deep coal reservoirs serves as a feasible selection for CCUS, and its storage potential can be attributed to the CO2 adsorption capacity of the coal. In this paper, a series of CO2 adsorption isotherm experiments were performed at different pressures and temperatures in sub-bituminous coal from the southern Junggar Basin (reservoir temperature ~25.9°C and pressure ~3.91 MPa). In addition, the high-pressure CO2 adsorption characteristics of the southern Junggar Basin coal were characterized using a supercritical D-R adsorption model. Finally, the CO2 storage capacities in sub-bituminous coal under the in situ reservoir temperature and pressure were analyzed. Results indicated that the excess adsorption capacities increase gradually with increasing injection pressure before reaching an asymptotic maximum magnitude of ~34.55 cm3/g. The supercritical D-R adsorption model is suitable for characterizing the excess/absolute CO2 adsorption capacity, as shown by the high correlation coefficients > 0.99. The CO2 adsorption capacity increases with declining temperature, indicating a negative effect of temperature on CO2 geological sequestration. By analyzing the statistical relationships of the D-R adsorption fitting parameters with the reservoir temperature, a CO2 adsorption capacity evolution model was established, which can be further used for predicting CO2 sequestration potential at in situ reservoir conditions. CO2 adsorption capacity slowly increases before reaching the critical CO2 density, following a rapid decrease at depths greater than ~800 m in the southern Junngar Basin. The research results presented in this paper can provide guidance for evaluating CO2 storage potential in deep coal seams.

    Jianguo ZHANG, Man WANG, Hongwei ZHOU, Dongming ZHANG, Beichen YU, Chongyang WANG, Yujie WANG
    Frontiers of Earth Science, 2023, 17(3): 701-712.

    In this study, we systematically studied the occurrence regularity of in situ stress in the Pingdingshan mine. The critical criterion model of coal-rock destabilization was established based on the theoretical framework of fracture mechanics. Furthermore, we analyzed the coupling destabilization mechanism of in situ stress and gas and studied the influence of the variation between original rock stress and mining-induced stress on the critical criterion. Through field experiments and applications, we established a three-dimensional gas drainage technology system for areas with a large mining height and long work face. Based on our research, a demonstration project was developed for deep mine dynamic disaster control. The technical system included the arrangement and optimization of pre-drainage holes along the coal seam, technology, and optimization of gas drainage through the bottom drainage tunnel and upper corner, gas drainage technology through sieve tubes, and a two plugging with one injection under pressure sealing technology. The implementation of the demonstration project effectively reduced the gas content and pressure of the coal seam in the deep mine, and the project increased the critical strength of the instability and failure of coal and rock.

    Zhiguang LI, Yaqing DONG, Haoyu ZHANG, Hongxiao SUN, Danyang JIA, Shikai SONG, Yuanjie ZHAO
    Frontiers of Earth Science, 2023, 17(4): 970-980.

    The layers of Tamarix cones within sedimentary deposits in arid regions have significant chronological and paleoenvironmental implications. Here, we compare the δ18O values of Tamarix cones in the Hongliujing area of Lop Nur with meteorological data for the Ruoqiang meteorological station for 1960–2019 AD. Linear regression analysis was used to reconstruct the average temperature for April and the precipitation for November in the Hongliujing area over the past 200 years. The results showed that the δ18O values were significantly negatively correlated with the temperature for February, April, May, August, December, and with the annual mean temperature; significantly negatively correlated with the precipitation for February and April; significantly negatively correlated with the sunshine hours for March and May; significantly positively correlated with the sunshine hours for February, July, August, October, and December, and with the annual mean values; and significantly correlated with the relative humidity for April, July, August, September, October, and November, and with the annual mean values. Based on the δ18O record of the past 200 years, the Hongliujing area experienced two warm-wet periods (1874–1932 and 2004–2019 AD) and two cold-dry periods (1832–1873 and 1933–2003 AD). Thus, the climate was characterized by alternating warm-wet and cold-dry conditions. Wavelet analysis revealed three main cycles: 45 years, 29 years, and 14 years.

    Lijun ZHOU, Xihua ZHOU, Gang BAI, Xianlin LI, Mingkun LUO
    Frontiers of Earth Science, 2023, 17(3): 776-787.

    Injecting external CO2 into soft and low-permeability coal seams can improve CH4 extraction efficiency, and also benefit in CO2 sequestration. However, the distribution law of damage zone around borehole in soft coal seam and its effect on the efficiency of CO2 injection promoted CH4 extraction are not clear. In this paper, a multi-physics coupling mathematical model considering damage effect is established for simulating the process of CO2 injection promoted CH4 extraction in soft and low-permeability coal seam. The distribution of damage zone and permeability around boreholes under different diameters and coal strengths are analyzed. The gas pressure and gas content in coal seam during CO2 injection promoted CH4 extraction when the model considered damage effect are compared with that of ignored. The results show that small borehole diameter corresponds to narrow damage zone around the borehole in coal seam. The damage zone expands with the increase of the borehole diameter. The damage zone increases exponentially with the borehole diameter, while decreases exponentially with the compressive strength of coal seam. The highest permeability in the damage zone has increased by nearly 300 times under the condition of simulated case. CH4 pressure around the extraction borehole reduces, and the reduction area expands with the increase of time. Compared with the result of considering the damage effect, the reduction area of ignoring it is smaller, and the reducing speed is slower. The integrated effect of CO2 injection and CH4 extraction leads to rapid decrease of CH4 content in coal seam near the boreholes. The CO2 pressure and content increase around the injection borehole, and the increasing area gradually extends to the whole coal seam. In soft coal seams, failure to consider the damage effect will underestimate the efficiency of CH4 extraction and CO2 sequestration, resulting conservative layout of boreholes.

    Chaojun FAN, Lei YANG, Bin XIAO, Lijun ZHOU, Haiou WEN, Hao SUN
    Frontiers of Earth Science, 2023, 17(3): 832-843.

    Injection of gas (CO2) into coal seams is an effective method to benefit from both CO2 geological storage and coalbed methane recovery. Based on the dual pore structure of coal mass, and the Weibull distribution of fracture permeability, a thermal-hydraulic-mechanical (THM) coupling mathematical model is proposed involving the non-isothermal adsorption of binary gases, dynamic gas diffusion between matrix and fractures, multiphase seepage, coal deformation, heat conduction and heat convection. This mathematical model is applied to study the process of CO2-enhanced coalbed methane recovery (CO2-ECBM). Results show that the CH4 content of CO2-ECBM in coal seam decreases significantly when compared with that of regular drainage, and decreases rapidly in the early stage but slowly in the later stage. Coal seam permeability evolution is triggered by changes in gas adsorption/desorption, temperature and effective stress. For regular drainage, the early permeability shows a decreasing trend dominated by the increase of effective stress, while the later permeability shows an increasing trend dominated by the CH4 desorption caused shrinkage of coal matrix. For CO2-ECBM, the permeability in coal seam generally shows a downward trend due to both matrix swelling induced by gas adsorption and thermal expansion, particularly near injection well. There appears an increased and delayed peak production rate of CH4. The CH4 production rate of CO2-ECBM is always higher than that of regular drainage. The CH4 cumulative production and CO2 cumulative storage linearly increase with time, and the CH4 cumulative production of CO2-ECBM increased by 39.2% in the duration of 5000 d compared with regular drainage. Reasonable CO2 injection starting time can overcome the issue of early CO2 breakthrough and ineffective increase of CH4 production. In the studied case, the optimal injection starting time is 2500 d. Compared with the simultaneous CH4 extraction and CO2 injection, the CH4 cumulative production of optimal time has increased by 30.1%. The research provides a reference for determining the reasonable CO2 injection time under similar conditions.

    Hongliang ZHANG, Hucai ZHANG, Yanbin LEI
    Frontiers of Earth Science, 2023, 17(4): 945-955.

    Zige Tangco is a meromictic saline lake located on the central Tibetan Plateau. Two parallel cores (ZGTC A-1 and ZGTC A-2) were collected from the lake at a water depth of 25 m during summer 2006. The chronology of core A-1 was reconstructed based on the Constant Initial Concentration (CIC) model of 210Pb and three accelerator mass spectrometry (AMS) ages from the chitin fragments. The hard water effect calibration of the sediment 14C age showed that the reservoir effect ranged from 1655 yr at 1950 AD to 1540 yr at 1610 AD. The hydrological variation in Zige Tangco during the past 800 yr was reconstructed using multi-proxies, including organic and carbonate content, stable isotopes of fine-grained carbonate minerals (< 38.5 μm) and grain-size distribution of the lake sediments. Our results show that there were strong fluctuations in the lake level between 1200 and 1820 AD, and at least three dry periods were recorded between 1235 and 1315 AD, 1410 and 1580 AD, and 1660 and 1720 AD characterized by high carbonate content, abrupt positive shifts of stable isotopes, and high sand content. The low-lake-level periods during the Little Ice Age (LIA) in Zige Tangco correspond to the lower δ18O values in the Guliya ice core and the lower precipitation reconstructed from tree rings in Delingha. This demonstrated that the summer monsoon on the central Tibetan Plateau weakened during the dry and cold periods, whereas the winter monsoon strengthened. Relatively wetter periods or higher lake levels in Zige Tangco occurred at 1580–1650 AD and 1820–1900 AD. Negative shifts in stable isotopes were related to increased lake levels between 1800 and 1820 AD. Our results also showed that the summer monsoon precipitation on the central Tibetan Plateau was mainly controlled by solar activity during the past 800 yr.

    Zedong SUN, Hongqiang XIE, Gan FENG, Xuanmin SONG, Mingbo CHI, Tao MENG, Bole SUN
    Frontiers of Earth Science, 2023, 17(3): 760-775.

    Injecting carbon dioxide CO2 into a coal seam is an important way to improve coalbed methane recovery and to store geological carbon. The fracture mechanical characteristics of bituminous coal determine the propagation and evolution of cracks, which directly affect CO2 storage in coal seams and the efficiency of resource recovery. This study applied CO2 adsorption and three-point bending fracture experiments using bituminous coal samples in a gaseous state (4 MPa), subcritical state (6 MPa), and supercritical state (8 and 12 MPa) to investigate the influence of CO2 state and anisotropy on the fracture-related mechanical response of bituminous coal. The results show that the change in mechanical properties caused by CO2 adsorption is CO2 state-dependent. The supercritical CO2 adsorption at 8 MPa causes the largest decrease in the mode-I fracture toughness (KIC), which is 63.6% lower than the toughness before CO2 adsorption. The instability characteristics of bituminous coal show the transformation trend of “sudden-gradual-sudden fracture”. With or without CO2 adsorption, the order of the KIC associated with three types of bituminous coal specimens is crack-divider type > crack-arrester type > crack-short transverse type. Phenomenologically, the fracture toughness of bituminous coal is positively correlated with its specific surface area and total pore volume; the toughness is negatively correlated with its average pore size.

    Muxin LIU, Yueze ZHANG, Hailin LAN, Feifei HUANG, Xi LIANG, Changyou XIA
    Frontiers of Earth Science, 2023, 17(3): 844-855.

    Carbon capture, utilization, and storage (CCUS) have garnered extensive attention as a target of carbon neutrality in China. The development trend of international CCUS projects indicates that the cluster construction of CCUS projects is the main direction of future development. The cost reduction potential of CCUS cluster projects has become a significant issue for CCUS stakeholders. To assess the cost reduction potential of CCUS cluster projects, we selected three coal-fired power plants in the coastal area of Guangdong as research targets. We initially assessed the costs of building individual CCUS projects for each plant and subsequently designed a CCUS cluster project for these plants. By comparing individual costs and CCUS cluster project costs, we assessed the cost reduction potential of CCUS cluster projects. The results show that the unit emission reduction cost for each plant with a capacity of 300 million tonnes per year is 392.34, 336.09, and 334.92 CNY/tCO2. By building CCUS cluster project, it could save 56.43 CNY/tCO2 over the average cost of individual projects (354.45 CNY/tCO2) when the total capture capacity is 9 million tonnes per year (by 15.92%). Furthermore, we conducted a simulation for the scenario of a smaller designed capture capacity for each plant. We found that as the capture scale increases, the cost reduction potential is higher in the future.

    Haitao LI, Guo YU, Xiaolei WANG, Dongming ZHANG
    Frontiers of Earth Science, 2023, 17(3): 788-796.

    To compare the pore structure, mechanical and CO2 adsorption properties of coal and shale, a series of experiments were carried out using nuclear magnetic resonance (NMR), uniaxial compression, Brazilian splitting, and high-pressure CO2 adsorption. The results show that the total porosity of coal is 7.51 times that of shale, and shale is dominated by adsorption pores, while adsorption pores and seepage pores in coal are equally important. Moreover, it is found that the micropores in shale are more advantageous, while meso-macropore in coal are more significant. The adsorption pore surface of coal is rougher than that of shale, and the seepage pore structure of shale is more complex. The uniaxial compressive strength, elastic modulus and absorption energy of shale are 2.01 times, 2.85 times, and 1.27 times that of coal, respectively, indicating that shale has higher compressive capacity and resistance to elastic deformation than coal. The average tensile strength, Brazilian splitting modulus, absorbed energy and brittleness index of shale are 7.92 times, 6.68 times, 10.78 times, and 4.37 times that of coal, respectively, indicating that shale has higher tensile strength and brittleness, but lower ductility, compared with coal. The performed analyses show that under the same conditions, the CO2 adsorption capacity of coal is greater than that of shale. The present article can provide a theoretical basis to implement CO2-enhanced coalbed methane (CBM)/shale gas extraction.

    Zeyu ZHENG, Liya JIN, Jinjian LI, Xiaojian ZHANG, Jie CHEN
    Frontiers of Earth Science, 2023, 17(4): 981-996.

    The Tibetan Plateau (TP) is a key region for environmental and climatic research due to its significant linkages with large-scale atmospheric circulation. Understanding the long-term moisture evolution pattern and its forcing mechanisms on the TP during the Holocene may provide insights into the interaction between low-latitude climate systems and midlatitude westerlies. Here, we synthesized 27 paleoclimate proxy records covering the past 9500 years. The results of the rotated empirical orthogonal function analysis of the moisture variation revealed spatial-temporal heterogeneity, which was classified into 5 subregions. Proxy records were then compared with the results from the Kiel Climate Model and other paleorecords. The results showed that moisture evolution on the western-southern-central TP was controlled by the Indian summer monsoon (ISM). On the south-eastern TP, moisture change was affected by the interplay between the East Asian summer monsoon (EASM) and the westerlies, as well as the ISM. With diverse patterns of circulation system precipitation, moisture changes recorded in the paleorecords showed spatial-temporal discrepancies, especially during the early to middle Holocene. Moreover, given the anti-phase pattern of summer precipitation in the EASM area under El Niño/Southern Oscillation (ENSO) conditions and the unstable relationship between the ISM and ENSO, it is reasonable to conclude that relatively strong ENSO variability during the late Holocene has contributed to these discrepancies as Asian summer monsoon precipitation has declined.

    Meihui PAN, Huimin ZHAO, Anna YANG, Yougui CHEN, Chenlu LI
    Frontiers of Earth Science, 2023, 17(4): 956-969.

    The loess accumulation process has great potential to record patterns of atmospheric circulation change, paleoclimate, and paleoenvironmental evolution. South-eastern Xizang is a climatically sensitive region and here, we analyze a loess profile at Ranwu in order to explore the processes and interactions of dust transport and paleoclimate evolution in the region. Based on parametric grain size end-member analysis, optically stimulated luminescence (OSL) dating, and environmental proxies we show that the Ranwu loess profile comprises five end members (EMs). EM1 represents the fine silt fraction transported by high-altitude westerly winds over long distances; EM2 represents the medium silt fraction accumulated by glacier winds; EM3 is the coarse silt fraction transported by local dust storms under the action of strong glacier winds; EM4 represents the very fine sand fraction transported by strong local dust storms, different wind strengths controls the relative proportion of EM3 and EM4 over time. EM5 is the coarse sand fraction formed from the product of strong weathering of gravels. OSL dating shows loess sedimentation at Ranwu started around 11.16 ka. The prevailing climate was generally warm and wet between 11.6 and 4.2 ka, with four cooling events at 10.50, 9.18, 7.85, and 6.37 ka. Extensive paleosol development between 8.2 and 4.2 ka, a change to dry and cold climate conditions was favorable for loess formation after 4.2 ka. The palaeoenvironmental changes and abrupt climate events recorded in the Ranwu loess sequence are consistent with Holocene global environmental changes.

    Ziliang WANG, Shuxun SANG, Xiaozhi ZHOU, Xudong LIU, Shouren ZHANG
    Frontiers of Earth Science, 2023, 17(3): 856-866.

    The permeability and its horizontal anisotropy induce a critical influence on staged CH4 output inhibition process. However, a quantitative evaluation of this influence has been rarely reported in the literature. In this work, the impact of horizontal anisotropic permeability on CO2-ECBM was numerically investigated. The variation in the staged CH4 output inhibition was analyzed. The ideal displacement profile of the CO2-ECBM process was established for the first time. Moreover, the variation in CH4 output of different wellbores was discussed. The results showed that 1) low-permeable or weak-anisotropic reservoirs were not conducive to enhanced CH4 recovery owing to long inhibition time (> 1091 days) and high inhibition level (> 36.9%). As permeability and anisotropy increased, due to the accelerated seepage of free water, the hysteresis time and inhibition time could decrease to as short as 5 days and 87 days, respectively, and the inhibition level could weaken to as low as 5.00%. Additionally, the CH4 output and CO2 injection could increase significantly. 2) Nevertheless, high permeability and strong anisotropy easily induced CO2 breakthrough, resulting in lower CH4 production, CO2 injection and CO2 storage than expected. While maintaining high efficiency of CO2 storage (> 99%), upregulating CO2 breakthrough concentration from 10% to 20% might ease the unfavorable trend. 3) Along the direction of fluid flow, the ideal displacement profile consisted of CO2 enriched bank, CO2 and CH4 mixed bank, CH4 enriched bank, and water enriched bank, whereas a remarkable gap in the displacement profiles of the dominant and non-dominant seepage directions was observed. 4) The potential of CH4 output might vary greatly among different wellbores. The producers along the dominant seepage direction held more potential for CH4 recovery in the short-term, while those along the non-dominant seepage direction avoided becoming invalid only if a long-time injection measure was taken for the injectors. These findings pave the way to understand fluid seepage in real complex reservoirs during CO2-ECBM and conduct further field projects.

    Qun ZHAO, Ze DENG, Meng ZHAO, Dexun LIU
    Frontiers of Earth Science, 2023, 17(3): 727-738.

    Carbon capture, utilization, and storage (CCUS) is considered one of the most effective measures to achieve net-zero carbon emissions by 2050, and low-rank coal reservoirs are commonly recognized as potential CO2 storage sites for carbon sequestration. To evaluate the geological CO2 sequestration potential of the low-rank coal reservoirs in the southern margin of the Junggar Basin, multiple experiments were performed on coal samples from that area, including high-pressure mercury porosimetry, low-temperature N2 adsorption, overburden porosity and permeability measurements, and high-pressure CH4 and CO2 isothermal adsorption measurements. Combined with the geological properties of the potential reservoir, including coal seam development and hydrodynamic characteristics, the areas between Santun River and Sigong River in the Junggar Basin were found to be suitable for CO2 sequestration. Consequently, the coal-bearing strata from Santun River to Sigong River can be defined as “potentially favorable areas for CO2 sequestration”. To better guide the future field test of CO2 storage in these areas, three CO2 sequestration modes were defined: 1) the broad syncline and faulted anticline mode; 2) the monoclinic mode; 3) the syncline and strike-slip fault mode.