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• RESEARCH ARTICLE
Fulai LI, Hao DIAO, Wenkuan MA, Maozhen WANG
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0901-1

The stability of dawsonite is an important factor affecting the feasibility evaluation of CO2 geological storage. In this paper, a series of experiments on the interaction of CO2-water-dawsonite-bearing sandstone were carried out under different CO2 pressures. Considering the dissolution morphology and element composition of dawsonite after the experiment and the fluid evolution in equilibrium with dawsonite, the corrosion mechanism of dawsonite led by CO2 partial pressure was discussed. The CO2 fugacity of the vapor phase in the system was calculated using the Peng–Robinson equation of state combined with the van der Waals 1-fluid mixing rule. The experimental results indicated that the thermodynamic stability of dawsonite increased with the increase of CO2 partial pressure and decreased with the increase of temperature. The temperature at which dawsonite dissolution occurred was higher at higher $fCO2$. There were two different ways to reduce dawsonite’s stability: the transformation of constituent elements and crystal structure damage. Dawsonite undergoes component element transformation and crystal structure damage under different CO2 pressures with certain temperature limits. Based on the comparison of the corrosion temperature of dawsonite, three corrosion evolution models of dawsonite under low, medium, and high CO2 pressures were summarized. Under conditions of medium and low CO2 pressure, as the temperature continued to increase and exceeded its stability limit, the dawsonite crystal structure was corroded first. Then the constituent elements of dawsonite dissolved, and the transformation of dawsonite to gibbsite began. At high CO2 pressure, the constituent elements of dawsonite dissolved first with the increase of temperature, forming gibbsite, followed by the corrosion of crystalline structure.

• EDITORIAL
Yong LI, Junqian LI, Derek ELSWORTH
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0947-0
• RESEARCH ARTICLE
Aikuan WANG, Pei SHAO, Qinghui WANG
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0925-6

This study carries out a simulated experiment of biogenic gas generation and studies the effects of gas generation on the pore structure and molecular structure of anthracite by mercury intrusion porosimetry, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). The results show that methanogenic bacteria can produce biogenic gas from anthracite. CO2 and CH4 are the main components of the generated biogas. After generation, some micropores (<10 nm) and transitional pores (10–100 nm) in the coal samples transform into large pores. In the high-pressure stage (pressure>100 MPa) of the mercury intrusion test, the specific surface area decreases by 19.79% compared with that of raw coal, and the pore volume increases by 7.25% in total. Microbial action on the molecular structure causes changes in the pore reconstruction. The FT-IR data show that the side chains and hydroxyl groups of the coal molecular structure in coal are easily metabolized by methanogenic bacteria and partially oxidized to form carboxylic acids. In addition, based on the XRD data, the aromatic lamellar structure in the coal is changed by microorganisms; it decreases in lateral size (La) and stacking thickness (Lc). This study enriches the theory of biogenic coalbed gas generation and provides a pathway for enhancing the permeability of high-rank coal reservoirs.

• RESEARCH ARTICLE
Longhui BAI, Bo LIU, Jianguo YANG, Shansi TIAN, Boyang WANG, Saipeng HUANG
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0915-8

The phase state of shale oil has a significant impact on its mobility. The mineral and organic matter in shale reservoirs play an important role in oil phase. This study attempts to evaluate the properties of shale oils in different phase states and to investigate how these differences are related to initial shale composition. Samples from the first member of the Qingshankou (Q1) Formation were analyzed using X-ray diffraction, total organic carbon content, rock pyrolysis solvent extraction and group component separation. Subsequently, fluorescence techniques were used to quantitatively determine the content and properties of the free oil (FO), the adsorbed oil associated with carbonate (ACO), and the adsorbed oil associated with silicate and clay-organic complexes (AKO). The results showed that non-hydrocarbons and asphaltenes are the primary fluorescing compounds on shale grain. FO is the dominant phase in the Q1 Formation. The quantitative grain fluorescence on extraction (QGF-E) and total scanning fluorescence (TSF) spectra of ACO and AKO show a significant redshift compared to the FO. The TSF spectra of FO have a characteristic skew to the left and a single peak distribution, suggesting a relatively light hydrocarbon component. The TSF spectra of ACO show a skew to the right and an even, double-peaked distribution. The TSF spectra of AKO show a single peak with a skew to the right, indicating that ACO and AKO hydrocarbons are heavier than FO hydrocarbons. In summary, enrichment of carbonate minerals in shale may result in mis-identification of “sweet spots” when using QGF. The normalized fluorescence intensity of QGF-E and TSF are effective indexes allowing oil content evaluation. As an additional complicating factor, hydrocarbon fractionation occurs during generation and expulsion, leading to a differentiation of oil composition. And FO has high relatively light hydrocarbon content and the strongest fluidity.

• RESEARCH ARTICLE
Shuai TANG, Jinchuan ZHANG, Weiyao ZHU
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0930-9

Marine-continental transitional shale is a potential energy component in China and is expected to be a realistic field in terms of increasing reserves and enhancing the natural gas production. However, the complex lithology, constantly changing depositional environment and lithofacies make the quantitative determination of the total organic carbon (TOC) suitable for marine shales not necessarily applicable to transitional shales. Thus, the identification of marine-continental transitional organic-rich shales and the mechanism of organic matter enrichment need to be further studied. As a typical representative of transitional shale, samples from Well MY-1 in the Taiyuan Formation in the southern North China Basin, were selected for TOC prediction using a combination of experimental organic geochemical data and well logging data including natural gamma-ray (GR), density (DEN), acoustic (AC), neutron (CNL) and U spectral gamma-ray (U), and TH spectral gamma-ray (TH). The correlation coefficient, coefficient of determination, standard deviation, mean squared error (MSE) and root mean squared error (RMSE) were selected to conduct the error analysis of the evaluation of different well log-based prediction methods, involving U spectral gamma logging, ΔlogR, and multivariate fitting methods to obtain the optimal TOC prediction method for the Taiyuan transitional shale. The plots of TOC versus the remaining volatile hydrocarbon content and the generation potential from Rock Eval show good to excellent potentials for hydrocarbon generation. The integrated results obtained from the various log-based TOC estimation methods indicate that, the multivariate fitting method of GR-U-DEN-CNL combination is preferable, with the correlation coefficients of 0.78 and 0.97 for the entire and objective interval of the Taiyuan Formation respectively, and with the minimum MSE and RMSE values. Specifically, the U spectral gamma logging method based on single logging parameter is also a better choice for TOC prediction of the high-quality intervals. This study provides a reference for the exploration and development of unconventional shale gas such as transitional shale gas.

• REVIEW ARTICLE
Weikai XU, Junhui LI, Xiang WU, Du LIU, Zhuangsen WANG
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0910-0

Most coal reservoirs show high gas content with relatively low desorption efficiency, which restricts the efficiency of coalbed methane (CBM) extraction and single-well productivity. This review highlights the desorption hysteresis mechanism and its controlling factors as well as methods and models to reveal desorption hysteresis and potential solutions. Methane adsorption and desorption can be recorded by both gravimetric and volumetric experiments. Although different adsorption models are used, desorption is generally considered with the Langmuir model. Desorption hysteresis is influenced by the petrophysical composition, thermal maturity, pore structure distribution of the coal, reservoir temperature, and moisture and water content. Methods for calculating desorption hysteresis include the area index, hysteresis index and introduction of a hysteresis factor and a hysteresis coefficient. Molecular dynamics simulations of methane desorption are mainly based on theories of kinetics, thermodynamics, and potential energy. The interaction forces operating among coal, water, and methane molecules can be calculated from microscopic intermolecular forces (van der Waals forces). The desorption hysteresis mechanism and desorption process still lack quantitative probe methodologies, and future research should focus on coal wettability under the constraints of liquid content, potential energy adjustment mechanism, and quantitative analysis of methane desorption rates. Further research is expected to reveal the desorption kinetics of methane through the use of the solid–liquid–gas three-phase coupling theory associated with the quantitative analysis of methane desorption hysteresis, thereby enhancing the recovery rate and efficiency of CBM wells.

• RESEARCH ARTICLE
Huihuang FANG, Shuxun SANG, Shiqi Liu, Shuailiang SONG, Huihu LIU, Hongjie XU
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0944-3

Three-dimensional (3D) reconstruction of the equivalent pore network model (PNM) using X-ray computed tomography (CT) data are of significance for studying the CO2-enhanced coalbed methane recovery (CO2-ECBM). The docking among X-ray CT technology, MATLAB, with COMSOL software not only can realize the 3D reconstruction of PNM, but also the CO2-ECBM process simulation. The results show that the Median filtering algorithm enabled the de-noising of the original 2D CT slices, the image segmentation of all slices was realized based on the selected threshold, and the PNM can be constructed based on the Maximum Sphere algorithm. The mathematical model of CO2-ECBM process fully coupled the expanded Langmuir equation. At the same time for CO2 injection, CH4 pressure tends to decrease with the increase of CO2 pressure, but its difference is not obvious. The CH4 pressure in the slice center changed a lot, while at the edge it changed a little under different CO2 pressures. The injected CO2 was transported to matrix along the macro and micro-fractures with continuous flow. The injected CO2 first replaced the adsorbed CH4 by covering the inner surface of macro-pores and meso-pores to form the single molecular layer adsorption of CO2. Then they migrated to micro-pores by Fick’s diffusion, sliding flow, and surface diffusion. Furthermore, the CO2 replaced CH4 adsorbed by volumetric filling in micro-pores, and formed the multi-molecular layer adsorption of CO2. The gas pressure and migration path between CO2 and CH4 are opposite. This study can provide a theoretical basis for studying digital rock physics technology and enrich the development of CO2-ECBM technology.

• RESEARCH ARTICLE
Xiaowei HOU, Yang WANG, Yanming ZHU, Jie Xiang
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0919-4

The pore structure of continuous unconventional reservoirs (CURs) in coal measures was investigated using different technologies for 29 samples (9 coal samples, 9 shale samples, and 11 sandstone samples) from Qinshui Basin, China. Results show that coals have relatively high porosities and permeabilities ranging from 4.02% to 5.19% and 0.001 to 0.042 mD, respectively. Micropores (<2 nm) are well-developed in coals and contribute to the majority of pore volume (PV) and specific surface area (SSA). The porosities and permeabilities are between 1.19%–4.11%, and 0.0001–0.004 mD of sandstones with a predominance of macropores (>50 nm). However, shales are characterized by poorly petrophysical properties with low porosity and permeability. Macropores and mesopores (2–50 nm) are well-developed in shales compared with micropores. For coals, abundant organic matters are expected to promote the development of micropores, and clay minerals significantly control the performance of mesopores. For shales and sandstones, micropores are mainly observed in organic matters, whereas clay minerals are the important contributor to mesopores. Moreover, micropore SSA significantly determines the adsorption capacity of CURs and sandstones have the best pore connectivity. The permeability of CURs is positively associated with the macropore PV since macropores serve as the main flow paths for gas seepage. Additionally, we also proposed that effective porosity has a significant effect on the permeability of CURs. The findings of this study could enhance the understanding of the multiscale pore structure of CURs and provide insights into the mechanisms that control gas storage, transport, and subsequent co-production for continuous unconventional natural gas (CUNG) in the Qinshui basin and other coal-bearing Basins worldwide.

• RESEARCH ARTICLE
Yan WANG, Shang ZHAO, Mingdong SUN, Xubo LV, Wenqian CAI, Xiangqin XU, Hongxiang GE, Kun LEI
Frontiers of Earth Science, https://doi.org/10.1007/s11707-020-0857-6

Evaluation of the river ecological environment can provide a basis for river management and ecological restoration. To conduct a comprehensive health assessment of the rivers in Tianjin, their biological, physical, and chemical indicators are investigated on the basis of 32 river monitoring sites from August to September 2018. The comprehensive pollution and ecological integrity indexes of the rivers are analyzed. Results of the two evaluations, compared to achieve the river ecological environment evaluation, are as follows. 1) Index of Ecological Integrity evaluation shows that among the sampling points, 18.8% are “healthy”, 28.1% are “sub-healthy”, 40.6% are “fair”, 6.3% are “poor”, and 6.3% are “very poor”. 2) The comprehensive evaluation of the chemical properties of the 32 river ecosystems in Tianjin shows severe overall river pollution and low standard water function area. Of the total sampling sites, 16 (50%) are heavily contaminated and 10 (31.3%) are moderately contaminated. Excessive chemical oxygen demand and ammonia nitrogen are the main causes of water pollution. 3) The Index of Ecological Integrity (IEI) has high correspondence with environmental factors. Pearson correlation analysis results show that the IEI index is significantly correlated with permanganate index (R= - 0.453; P = 0.023<0.05). Analysis results using BEST show that ammonia nitrogen is the best environmental parameter to explain the changes in IEI (Rho= 0.154; P = 0.02<0.05) and those using RELATE show significant correlation between the biotic index and the environmental parameter matrices (Rho= 0.154; P = 0.034<0.05).

• RESEARCH ARTICLE
Dongliang ZHANG, Yunpeng YANG, Min RAN, Bo LAN, Hongyan ZHAO, Qi LIU
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0906-9

Over the past 2000 years, a high-resolution pollen record from the Yushenkule Peat (46°45′–46°57′N, 90°46′–90°61′E, 2374 m a.s.l.) in the south-eastern Altai Mountains of northwestern China has been used to explore the changes in vegetation and climate. The regional vegetation has been dominated by alpine meadows revealed from pollen diagrams over the past 2000 years. The pollen-based climate was warm and wet during the Roman Warm Period (0–520 AD), cold and wet during the Dark Age Cold Period (520–900 AD), warm and wet during the Medieval Warm Period (900–1300 AD), and cold and dry during the Little Ice Age (1300–1850 AD). Combined with other pollen data from the Altai Mountains, we found that the percentage of arboreal pollen showed a reduced trend along the NW-SE gradient with decreasing moisture and increasing climatic continentality of the Altai Mountains over the past 2000 years; this is consistent with modern distributions of taiga forests. We also found that the taiga (Pinus forest) have spread slightly, while the steppe (Artemisia, Poaceae and Chenopodiaceae) have recovered significantly in the Altai Mountains over the past 2000 years. In addition, the relatively warm-wet climate may promote high grassland productivity and southward expansion of steppe, which favors the formation of Mongol political and military power.

• RESEARCH ARTICLE
Huijun LU, Ru ZHANG, Li REN, Anlin ZHANG, Yiming YANG, Xiaopeng LI
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0911-z

Understanding the damage behavior and cracking mechanism of brittle shale is crucial for hydraulic fracturing design. In this research, uniaxial compression tests are conducted on shale samples with different bedding plane orientations, and acoustic emission monitoring is implemented synchronously. The results indicate that the apparent elastic modulus increases with increasing bedding orientation. For the bedding orientations of 45° and 90°, the lateral deformation is anisotropic due to the bedding structure, revealing the anisotropic Poisson effect. A shear failure surface and tensile failure surfaces form parallel to the bedding plane for bedding orientations of 45° and 90°, respectively. For the bedding orientation of 0°, shear failure mainly occurs through the bedding planes. Additionally, the damage mechanism of shale is investigated by crack classification based on AE parameters. It is found that crack initiation is induced by the generation of shear cracks for the bedding orientation of 45°, whereas by the generation of tensile cracks for other bedding orientations. According to damage attributable to different type cracks, shear cracks dominate the damage behavior for bedding orientations of 0° and 45°, whereas tensile cracks dominate the damage behavior for bedding orientation of 90°. Finally, the information entropy is calculated by AE energy. A low value of information entropy, approximately 0.36, predicts failure with a low degree of instability for the bedding orientation of 0°, whereas a high value of information entropy, more than 1.5, predicts failure with a high degree of instability for other bedding orientations. This finding indicates that the failure behavior is gradual progressive damage for bedding orientation of 0°, whereas sudden damage dominates failure behavior for other bedding orientations.

• RESEARCH ARTICLE
Shuai ZHANG, Jing HAN, Bingke ZHAO, Zhigang CHU, Jie TANG, Limin LIN, Xiaoqin LU, Jiaming YAN
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0902-0

• RESEARCH ARTICLE
Sijun ZHENG, Chen MENG, Jianhui XUE, Yongbo WU, Jing LIANG, Liang XIN, Lang ZHANG
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0896-7

Three-dimensional green volume (TDGV) reflects the quality and quantity of urban green space and its provision of ecosystem services; therefore, its spatial pattern and the underlying influential factors play important roles in urban planning and management. However, little is known about the factors contributing to the spatial pattern of TDGV. In this paper, TDGV and land use intensity (LUI) extracted from high spatial resolution (0.05 m) remotely sensed data acquired by an unmanned aerial vehicle (UAV), anthropogenic factors

LUI, Distance from buildings and Distance from roads.

and natural factors

Land formation time and Distance from water.

were utilized to identify the spatial pattern of TDGV and the potential influencing factors in Lingang New City, a rapidly developed coastal town in Shanghai. The results showed that most of the TDGV was distributed in the western part of this new city and that its spatial variations were significantly axial. TDGV corresponded well with the chronologies of land formation, urban planning, and construction in the city. Generalized least squares (GLS) analysis of TDGV (grid cell size: 100 × 100 m) and its influencing factors showed that the TDGV in this new city was significantly negatively correlated with both LUI and distance from roads and significantly positively correlated with land formation time and distance from water. Distance from buildings did not affect TDGV. Additionally, the degree of influence decreased in the following order: distance from water>land formation time>distance from roads>LUI. These results indicate that the spatial pattern of TDGV in this new town was mainly affected by natural factors (i.e., the distance from water and land formation time) and that the artificial disturbances caused by rapid urbanization did not decrease the regional TDGV. The main factors shaping the spatial distribution of TDGV in this city were local natural factors. Our findings suggest that the improvement in local soil and water conditions should be emphasized in the construction of new cities in coastal areas to ensure the efficient provision of ecological services by urban green spaces.

• RESEARCH ARTICLE
Ying CHEN, Ruixue CAO, Yuting FENG, Hui ZHAO
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0914-9

Summer phytoplankton blooms appear frequently off the Somali coast in the southwestern Arabian Sea (AS), where strong reversal monsoon and summer upwelling is prevailing. Distinct high chlorophyll-a (Chl-a) concentrations in summer were displayed in the western AS, especially in the region off the Somali coast. The spatial and inter-annual variations of the summer high Chl-a were studied using satellite data including ocean color and wind vectors. Under ocean conditions including monsoon winds, Ekman transport (ET) and Ekman pumping velocity (EPV), as well as geostrophic current and aerosol precipitation, the possible mechanisms of high Chl-a was investigated. The summer high Chl-a presented strong inter-annual variations in the southwestern AS. The results of simple correlation analysis indicated that there were good correlations between the ET and Chl-a, as well as between EPV and Chl-a. These implied that the ET and EPV may cause uplift of nutrients into the upper layer of the western AS from subsurface or coastal regions, inducing high Chl-a in the southwestern AS, especially in the region off the Somali coasts in summer. The multiple/partial correlation analysis implied further that EPV-induced upwelling may be more helpful than the ET-induced upwelling in the coastal region off Somalia, leading to probably more significant influence of EPV upwelling on the phytoplankton bloom than upwelling by ET. Aerosol precipitation in the southwestern AS also played an important role in high Chl-a in the deep offshore AS (i.e., Section B in Fig. 1(a)), as second only to ET and sea surface temperature (SST), and even higher than EPV. A novel finding is that the influence of aerosol optical thickness (AOT) is evident in the offshore region and the dust precipitation is more important sources to oligotrophic water. Both the stability of the upper ocean and the aerosol precipitation may play more evident roles in the open regions of the southwestern AS off Somali.

• RESEARCH ARTICLE
Shangbin CHEN, Huijun WANG, Yang WANG, Tianguo JIANG, Yingkun ZHANG, Zhuo GONG
Frontiers of Earth Science, https://doi.org/10.1007/s11707-021-0913-x

The study and exploration practice of shale gas accumulation has focused on the static system comparison, key parameters analysis, reservoir characteristics, enrichment mode etc. However, the research on dynamic recovery from the original hydrocarbon generation of shale gas to the present gas reservoir is still lacking. The burial history of shale gas reservoir can reflect the overall dynamic process of early formation and later transformation of shale gas reservoir. It controls the material basis of shale gas, the quality of reservoir physical properties, preservation conditions, gas content and formation energy, which is the core and foundation of shale gas accumulation process research. Herein, based on the five typical wells data in the Northeast Yunnan, including geochronological data, measured Ro values, core description records, well temperature data, paleoenvironment, paleothermal, etc., the burial history, thermal evolution history and hydrocarbon generation history of the Lower Silurian Longmaxi Formation were systematically restored via back stripping method and EASY%Ro model. The results show that 1) the differences in the burial history of marine shale in Longmaxi Formation can be divided into syncline type and anticline type. 2) The shale gas accumulation process can be divided into four stages, namely the source-reservoir-cap sedimentation period, initial accumulation period, main accumulation period, and adjustment period. 3) Based on the characteristics of burial history and preservation conditions, the areas with wide and gentle anticline, far away from the denudation area, and buried deeply with good fault sealing ability are priority structural locations for the shale gas exploration in northeast Yunnan.