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  • RESEARCH ARTICLE
    Yabing LIN, Yong QIN, Dongmin MA, Shengquan WANG
    Frontiers of Earth Science, https://doi.org/10.1007/s11707-022-1057-3

    The south-western Ordos Basin is rich in low-middle rank coalbed methane (CBM) resources; while the geochemical characteristics and genetic mechanism of CBM are not clear. Herein, according to geological and geochemical test data from gas and coal seam water from CBM wells in Bingchang, Jiaoxun, Huangling, Yonglong, and Longdong minging areas, we systematically studied the geochemical characteristics, generation, and evolution mechanism of CBM in Jurassic Yan’an Formation in the south-western Ordos Basin. The results show that the CH4 content of whole gas is in the range of 42.01%–94.72%. The distribution ranges of the δ13C-CH4 value is −87.2‰ to −32.5‰, indicating diverse sources of thermogenic gas and biogenic gas. The microbial methane is mainly generated by a CO2 reduction pathway, with certain methyl-type fermentation spots. The δ13C-CH4 has a positive correlation with burial depth, indicating the obvious fractionation of CBM. The relationship between the genetic types and burial depth of the CBM reservoir indicates that the favorable depth of secondary biogenic gas is less than 660 m. The Late Cretaceous Yanshanian Movement led to the uplift of the Ordos Basin, and a large amount of thermogenic gas escaped from the edge of the basin. Since the Paleogene Period, the coal reservoir in the basin margin has received recharge from atmospheric precipitation, which is favorable for the formation of secondary biogenic methane. The deep area, generally under 1000 m, mainly contains residual thermogenic gas. The intermediate transition zone is mixed gas. Constrained by the tectonic background, the genetic types of CBM in different mining areas are controlled by the coupling of burial depth, coal rank, and hydrogeological conditions. The Binchang mining area contains biogenic gas, and the development of CBM has achieved initial success, indicating that similar blocks with biogenic gas formation conditions is key to the efficient development of CBM. The research results provide a scientific basis for searching for favorable exploration areas of CBM in the south-western Ordos Basin.

  • RESEARCH ARTICLE
    Banglin ZHANG, Jeremy Cheuk-Hin LEUNG, Shengyuan LIU, Jianjun XU
    Frontiers of Earth Science, https://doi.org/10.1007/s11707-022-1064-4

    In this study the changes of tropical cyclone (TC) size from 2001 to 2021 are analyzed based on linear and quadratic curve fittings of the National Hurricane Center (NHC) / Joint Typhoon Warning Center (JTWC) best track data, based on the radius of maximum wind (RMW) and the average radius of 34-kt wind (AR34), in three oceanic basins of the North Atlantic (NATL), the Western North Pacific (WPAC) and the Eastern North Pacific (EPAC). The computations are done separately for two categories of tropical cyclones: tropical storms (TS) and hurricanes (HT). Size changes of landfalling and non-landfalling TCs are also discussed. Results show that there is a great inter-basin variability among the changes in TC sizes. Major conclusions include: 1) overall, the inner cores of TSs have become larger in all three basins, with the increasing tendencies being significant in the NATL and WAPC, while those of HTs mostly get smaller or remain similar; 2) meanwhile, comparatively large inter-basin differences are observed for the TC outer core sizes, and the sizes of landfalling TCs; 3) particularly, a significant decrease in landfalling HT outer core size is observed over the EPAC; 4) in contrast, significant increases in landfalling TS inner core size are found over the NATL and WPAC. The presented analysis results could benefit future research about TC forecasts, storm surge studies, and the cyclone climate and its changes.

  • RESEARCH ARTICLE
    Tingwei ZHANG, Xiaoqiang YANG, Jian YIN, Qiong CHEN, Jianfang HU, Lu WANG, Mengshan JU, Qiangqiang WANG
    Frontiers of Earth Science, https://doi.org/10.1007/s11707-022-1009-y

    The variations in precipitation have displayed a complex pattern in different regions since the mid-to-late-Holocene. Cloud formation processes may have a significant impact on precipitation, especially during the tropical marine processes and summer monsoon which convey abundant water vapor to coastal southern China and inland areas. Here, we use two 7500 year sedimentary records from the Pearl River Delta and the closed Maar Lake, respectively, in coastal southern China to reconstruct the mid-to-late-Holocene humidity variability and explore its possible relationship with cloud cover modulated by the Earth’s magnetic fields (EMF). Our proxy records document an apparent increase in wetness in coastal southern China between 3.0 and 1.8 kyr BP. This apparent increase in humidity appears to be consistent with the lower virtual axial dipole moments and, in turn, with a lower EMF. This correlation suggests that the EMF might have been superimposed on the weakened monsoon to regulate the mid-to-late-Holocene hydroclimate in coastal southern China through the medium of galactic cosmic rays, aerosols, and cloud cover. However, further investigations are needed to verify this interaction.

  • RESEARCH ARTICLE
    Shengxian ZHAO, Yongyang LIU, Shuangfang LU, Shuaihu LIU, Wenbiao LI, Zhiyan ZHOU, Yashuo WANG, Zhaojing SONG
    Frontiers of Earth Science, https://doi.org/10.1007/s11707-022-1056-4

    Deep shale gas reservoirs commonly contain connate water, which affects the enrichment and migration of shale gas and has attracted the attention of many scholars. It is significant to quantitatively estimate the amounts of adsorbed and free water in shale matrix pores, considering the different impacts of pore water (adsorbed water and free water) on shale gas. In this paper, pore water in six deep shale samples from the Wufeng-Longmaxi Formations in the Luzhou area, southern Sichuan Basin, China, was quantitatively evaluated by saturation-centrifugation experiments. Further, the impact of shale material composition and microstructure on the pore water occurrence was analyzed. The results show that amounts of adsorbed and free water are respectively 1.7967–9.8218 mg/g (mean 6.4501 mg/g) and 9.5511–19.802 mg/g (mean 13.9541 mg/g) under the experimental conditions (30°C, distilled water). The ratio of adsorbed water to total water is 15.83%–42.61% (mean 30.45%). The amounts of adsorbed and free water are related to the pore microstructure and material compositions of shale. The specific surface area of shale controls the amount of adsorbed water, and the pore volume controls the amount of free water; organic pores developed in shale solid asphalt contribute specific surface area and pore volume, and inorganic pores developed in clay mineral contribute pore volume. Therefore, the pores of shale solid asphalt accumulate the adsorbed water and free water, and the pores of clay minerals mainly accumulate the free water.

  • RESEARCH ARTICLE
    Wen GU, Caijun YUE, Zhihui HAN, Yanqing GAO, Yuqi TANG, Xiangyu AO, Yao YAO
    Frontiers of Earth Science, https://doi.org/10.1007/s11707-022-1046-6

    Eleven tropical cyclones (TCs) affected Shanghai and crossed the same latitude as Shanghai from 2007 to 2018. According to similar tracks from best-track data, TCs that cause significant precipitation in Shanghai can be divided into three types: landfall TCs, nearshore northward TCs, and western TCs. Based on ERA5 reanalysis data, the dynamic synthesis method was used to synthesize TC circulation situations to compare thermal, dynamic, water vapor, and stability conditions within TC circulations during the period when they affected Shanghai. The conclusions are as follows. 1) When the three TC types are at the same latitude as Shanghai, they are all in the divergent field in the upper troposphere. For the landfall type, the subtropical high at 500 hPa is stronger and farther north than usual, and there is a high-pressure dam on the north side of the TCs. 2) The warm advection of the three TC types at 925 hPa is located in the northern quadrant of the TCs. The dynamic and water vapor conditions are good in the north-western quadrant of landfall and western TCs, and more favorable in the eastern quadrant of nearshore northward TCs. 3) The favorable effects of all three types on precipitation in Shanghai come from the boundary layer. Water vapor, upward motion, and instability conditions of landfall TCs are superior to the other two TC types. The best water vapor, dynamic, and convective instability conditions are at the northern boundary in Shanghai during landfall TCs, and the main sources of water vapor in Shanghai come from the eastern and northern boundaries. During nearshore northward TCs, the main contribution to precipitation is from the eastern boundary, while better dynamic and water vapor conditions come from the western and northern boundaries during western TCs. The above findings provide scientific and technical support for operational forecasting precipitation from TCs affecting mega-cities.

  • RESEARCH ARTICLE
    Dawei DONG, Li ZHAO, Weizhong ZHANG, Jiyan LI, Ruixiang ZHANG, Jianlei YANG, Guangzeng WANG
    Frontiers of Earth Science, https://doi.org/10.1007/s11707-022-1062-6

    Hydrocarbon exploration in the Dongying Sag is constrained by the development of many Cenozoic transtensional structures with complex patterns and dynamic mechanisms. This study uses seismic interpretation and analog modeling to investigate these transtensional structures. Significant results include dividing these transtensional structures into boundary fault, oblique rifting, and deep strike-slip fault controlled structures, according to the relationships between main and secondary faults. They developed in the steep slope zone, the central sag zone, and the slope zone, respectively. In profile, the transtensional structures formed appear to be semi-flower-like, step-like, or negative-flower-like. In plan-view, they appear to be broom-like, soft-linked, or en-echelon structures. Further, these transtensional structures are controlled by the oblique normal slip of boundary faults, by the oblique extension of sub-sags, and by the later extension of deep strike-slip faults. The geometric deformation of these transtensional structures is controlled by the angles between the regional extension direction and the strike of boundary faults, deep faults, or sub-sags, where a larger angle corresponds to less developed transtensional structures. Further, the transtensional structures in the Dongying Sag were created by multi-phase and multi-directional extensions in the Cenozoic— which is also controlled by pre-existing structures. The strike of newborn secondary faults was determined by the regional extension direction and pre-existing structures.

  • RESEARCH ARTICLE
    Tao LIU, Ying LIU, Baoqing HU
    Frontiers of Earth Science, https://doi.org/10.1007/s11707-022-1083-1

    Sedimentation is a key process affecting wetland sustainability and carbon burial flux. In context of sea level rise, climate change and human activities, further understanding about the sedimentary dynamic in wetland is critical in predicting the landscape evolution or the change in carbon burial flux. In this study, based on the field hydrological observation in a mangrove system in the Nanliu River estuary, we found the net flux of suspended sediment to mangrove is 39−72 kg/m in tidal cycles with Turbidity Maximum Zone (TMZ) forming in surface layer and only is 9−18 kg/m in tidal cycles without TMZ. The higher net flux of suspended sediment to mangrove in tidal cycles with TMZ forming in surface layer is attributed to high SSC in rising tide and intense flocculation in mangrove. The significant discrepancy in sedimentation rate in the mangrove patches also can be explained by the probability of TMZ forming in the surface layer of estuary. In future, rapid sea level rising may lead to the change of TMZ pattern in estuary, which will result in non-negligible variation in sedimentation rate in wetlands. According to the present data of sedimentation rate in wetlands, the fragility of wetlands in river estuary may be miscalculated.

  • RESEARCH ARTICLE
    Jingyan LIU, Qian CHANG, Junlong ZHANG, Hui CHAI, Feng HE, Yizan YANG, Shiqiang XIA
    Frontiers of Earth Science, https://doi.org/10.1007/s11707-022-1053-7

    It has always been challenging to determine the ancient sedimentary environment and associated energy in deep-buried marine carbonates. The energy represents the hydrodynamic conditions that existed when the carbonates were deposited. The energy includes light and chemical energies in compounds and kinetic energy in currents and mass flow. Deep-buried marine carbonates deposited during the Ordovician depositional period in the eastern Tarim Basin result from a complex interplay of tectonics, sedimentation, and diagenesis. As a result, determining the ancient sedimentary environment and associated energy is complex. The natural gamma-ray spectrometry (GRS) log (from 12 wells) is used in this paper to conduct studies on the sedimentary environment and associated energy in deep-buried marine carbonates. The findings show that the values of thorium (Th), uranium (U), potassium (K), and gamma-ray without uranium (KTh) in a natural GRS log can reveal lithological associations, mineral composition, diagenetic environment, stratigraphic water activity, and ancient climatic change. During the Ordovician, quantitative analysis and determination of sedimentary environment energy are carried out using a comprehensive calculation of natural GRS log parameters in typical wells (penetrating through the Ordovician with cores and thin sections) of well GC4, well GC6, well GC7, and well GC8. The results show that GRS log can determine different lithology associations in typical wells than a sieve residue log. Furthermore, cores and thin sections can be used to validate the determination of lithology associations. Based on the determination of lithology associations, the lithology associations that reflect the sedimentary environment and associated energy can be analyzed in a new approach. Furthermore, the sedimentary environment energy curve derived from a natural GRS log can reveal hydrodynamic fluctuations during depositional periods, which will aid in the discovery of carbonate reservoirs, establishing sequence stratigraphic frameworks, and the reconstruction of sea-level changes in the future.

  • RESEARCH ARTICLE
    Lu SUN, Zhigang WEN, Guisong HE, Peixian ZHANG, Chenjun WU, Liwen ZHANG, Yingyang XI, Bo LI
    Frontiers of Earth Science, https://doi.org/10.1007/s11707-023-1087-5

    Normal-pressure shale gas reservoirs are widely distributed in south-eastern Chongqing and show good potential for resource exploration. This paper reports the organic matter (OM), physical, and pore characteristics, mineral composition, and gas content of representative shale samples from the Upper Ordovician Wufeng Formation and Member 1 of the Lower Silurian Longmaxi Formation (Long 1 Member). Microscopic pores within different shale layers of the Long 1 Member were classified, quantitatively evaluated, and their development mechanisms were systematically studied. We found that OM characteristics, mineral composition, and pore type were the main factors affecting the enrichment and preservation of shale gas. The characteristics of the Long 1 Member are mainly controlled by changes in the sedimentary environment. There are evident differences in total organic carbon content and mineral composition vertically, leading to a variable distribution of pores across different layers. Organic matter abundance controls the degree of OM pore development, while clay minerals abundance control the development of clay mineral-related pores. Total organic carbon content generally controls the porosity of the Long 1 Member, but clay minerals also play a role in OM-poor layers. Pore connectivity and permeability are influenced by the development of pores associated with brittle minerals. We propose a microscopic pore development model for the different layers. Combining geochemical data and this pore development model, layers 1‒4 are considered to be excellent shale gas preservation and enrichment reservoirs. Poor preservation conditions in layers 5‒7 result in high levels of shale gas escape. Layers 8‒9 possess a better sealing condition compared with layers 5‒7 and are conducive to the enrichment and preservation of shale gas, and can thus be used as future potential target strata. This research provides a theoretical basis for exploring and evaluating shale gas potential in the studied region or other complex normal-pressure shale blocks.

  • RESEARCH ARTICLE
    Pengfei ZHANG, Shuangfang LU, Nengwu ZHOU, Zizhi LIN, Xiangchun CHANG, Junjian ZHANG, Guo CHEN, Yumao PANG, Junjie WANG, Hongsheng HUANG, Qi ZHI
    Frontiers of Earth Science, https://doi.org/10.1007/s11707-022-1051-9

    Overmature continental shale is commonly developed, but few studies have given insight into its pore structure and sorption capacity. Various techniques, including SEM, helium porosity and permeability, N2/CO2 adsorption, MICP, and NMR, were used to detect the pore structure of shale from the Shahezi Formation, Xujiaweizi Fault, Songliao Basin. The excess methane adsorption volumes were measured by the volumetric method and modeled by the Langmuir model. Based on the findings, the most developed pores are intraparticle pores in clay minerals, followed by the dissolution pores in feldspar, but organic pores are uncommon. The selected shales have low helium porosity (mean 1.66%) and ultralow permeability (mean 0.0498 × 10−3μm2). The pore throats are at the nanoscale, and the pore-throat size distributions are unimodal, with most less than 50 nm. The studied shales are characterized by the lower specific surface area (SSA) and pore volume (PV) but the larger average pore diameter. The total SSA is contributed by the micro- and mesopores, while the PV is dominated by meso- and macropores. The pore structures are more complex and controlled by multiple factors, such as mineral compositions and diagenesis, but organic matter is not critical. The maximum absolute adsorption methane volume (VL) is 0.97−3.58 cm3/g (mean 1.90 cm3/g), correlating well with the total SSA, SSA, and pore volume of micropores, which indicates that methane is mainly adsorbed and stored in micropores, followed by mesopores.