中国岩溶

Analysis of chemical substance sources in the groundwater of karst-fissure groundwater system in the Qinglian River, Guangdong Province, China

XU Lanfang, NI Zehua, TU Shiliang, JIANG Shoujun, HUANG Wenlong, ZHUANG Zhuohan, YANG Hongyu

2025, 44(2): 213-227. doi: 10.11932/karst20250201

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The Qinglian River Basin is a typical basin unit within the ecological security pattern. Therefore, clarifying its groundwater quality is crucial for strengthening the ecological security barrier of the hilly and mountainous areas in South China and for maintaining the ecological security of the Guangdong−Hong Kong−Macao Greater Bay Area. The chemical composition of water is significantly related to water potability, availability for agriculture and tourism, and interaction with biological systems. However, the lack of understanding regarding the nature of groundwater has presented some challenges for the scientific management of groundwater in the Qinglian Basin, particularly concerning irrational spatial exploitation of groundwater. To address these challenges, methods such as hydrochemical parameters and multivariate statistical techniques-including Durov diagram, Gibbs plot, Schoeller diagram and Positive Matrix Factorization (PMF) model-were employed to trace the sources of chemical substances in groundwater and quantify the contribution rates of various factors affecting groundwater quality.The main land use types in the study area include forest land, cultivated land, construction land, and water bodies, which account for 87.30%, 11.31%, 1.14%, and 0.27%, respectively. Based on the deposit conditions of groundwater and the characteristics of water-bearing media, the types of groundwater are mainly classified as karst water, igneous-fissure water, clastic-fissure water, and pore water. According to the extended Durov diagram, the pore water in Quaternary sediments predominantly exhibits HCO₃·SO₄−Ca characteristics, while the igneous-fissure water is classified as HCO₃−Ca·Na (Na). The groundwater in areas with dolomitic limestone and clastic rocks is primarily of HCO₃−Ca·Mg types, whereas the other karst water is mainly classified as HCO₃−Ca type.In the PMF model, a total of 53 groundwater samples were used for the identification and apportionment of groundwater chemical sources. Five major factors of groundwater chemical sources within the basin were identified. Factor 1 (F1) is characterized by Mg²⁺, ${\rm{HCO}}_3^{-}$, and total dissolved solids (TDS), originating from weathering and dissolution of dolomitic limestone and Mg-containing silicate minerals, with a contribution rate of 18%. Factor 2 (F2) is characterized by ${\rm{NO}}_3^{-}$, Cl⁻, and Na⁺, which are derived from anthropogenic activities such as domestic and agriculture practices, with a contribution rate of 20%. In addition, agricultural activities on sloping farmland can cause substances such as ${\rm{NO}}_3^{-}$ to be discharged into the karst basin through underground runoff, further increasing the concentrations of chemical substances in the groundwater of the karst basin. Factor 3 (F3), is characterized by ${\rm{HCO}}_3^{-}$, Ca²⁺, and TDS, resulting from the weathering and dissolution of carbonate minerals, and it is the factor with the highest contribution rate (27%). The dominance of F3 among the five factors corresponds to the largest proportion of karst area in the basin (55%), making it the primary source of chemical substances in the groundwater of the Qinglian River Basin. The most relevant parameter of Factor 4 (F4) is mainly ${\rm{SO}}_4^{2-}$, and its apportioned contribution rate is 16%. The majority of ratios of ${\rm{SO}}_4^{2-}$ to ${\rm{NO}}_3^{-}$− in groundwater fall between one and four, indicating that the primary source is the use of sulfur-containing fertilizers in agricultural activities. Notably, the pore water in loose rock formations within the karst basin has a relatively high concentration of ${\rm{SO}}_4^{2-}$, with a maximum value of 81.30 mg·L⁻¹, showing a significant impact from human activities. However, the underlying karst water has a lower concentration of ${\rm{SO}}_4^{2-}$ (6.90 mg·L⁻¹), indicating less influence from human activities. F5 is characterized by Na⁺, K⁺, Cl⁻, and TDS, which are derived from the weathering of silicate minerals and the dissolution of a small amount of halite, with a contribution rate of 19%.The distribution areas with high concentrations of chemical parameters are significantly correlated with the spatial distribution of contribution rates of source factors, which indicates that water chemistry responds to the spatial distribution of lithology and land use. F1, F3 and F5, belonging to natural factors such as rock weathering and water−rock interaction, contribute 64% and have a significant correlation with lithology. F2 and F4 are significantly correlated with the distribution of cultivated land and construction land, and are considered to be influenced by human activities, with a contribution rate of 36%. In general, the main sources of the chemical substances in karst water are the weathering and dissolution of carbonate, and silicate and halite minerals. In some areas, human activities such as domestic wastewater and the use of fertilizers areas also have an impact. The main source of igneous-fissure water is the weathering and dissolution of silicate minerals. The quantitative analysis of the contribution rates of groundwater ion sources helps to deepen the understanding of fissures and karst aquifers in the study area, and provide a basis for the scientific management of groundwater.

Analysis of hydrochemical characteristics and controlling factors of groundwater in the covered karst area of northern Guangzhou

WANG Zhongzhong, HU Feiyue, JIA Long, ZHI Bingfa

2025, 44(2): 228-237. doi: 10.11932/karst20250202

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Guangzhou is clearly positioned as the core engine of the Guangdong−Hong Kong−Macao Greater Bay Area and shoulders the responsibility of leading the high-quality development of the Greater Bay Area. The northern Guangzhou serves as a demonstration area for urban-rural integration, featuring a distinctive ecological agriculture industry and a functional area for rural tourism. However, the covered karst in this area is highly developed. Consequently, the environmental problems, such as karst ground collapse and the deterioration of groundwater quality, have limited to some extent the local construction and development. In order to study the hydrochemical characteristics and formation of groundwater in the covered karst area of northern Guangzhou. We utilized fourteen groups of karst water samples and fourteen groups of rock chemical composition test data collected during the dry season from 2017 to 2018. We conducted a qualitative analysis of the hydrochemical characteristics and controlling factors of groundwater using mathematical statistics, Piper diagrams, the Gibbs model, ion ratio diagrams, the and chlor-alkali index. Additionally, using the hydrogeochemical reaction simulation software PHREEQC, we performed a quantitative analysis of the hydrogeochemical processes, with a particular emphasis on the dissolution−precipitation equilibrium of minerals in groundwater.The results show, (1) The karst water in the covered karst area of northern Guangzhou is characterized as extremely soft to slightly hard, neutral, and fresh. The total dissolved solids (TDS) and total hardness of the karst water are low, and the pH remains relatively stable. The dominant anion and cation are ${\rm{HCO}}_3^{-}$ and Ca²⁺, respectively. The trend of ion concentrations is Ca²⁺>K⁺+Na⁺>Mg²⁺ and ${\rm{HCO}}_3^{-}$ >Cl⁻>${\rm{SO}}_4^{2-}$ , respectively. The hydrochemical types of karst water are primarily of the HCO₃ type. (2) The ρ(K⁺+Na⁺)/ρ(K⁺+Na⁺+Ca²⁺) of karst water ranges from 0.1 to 0.6, and the ρ(Cl⁻)/ρ (Cl⁻+${\rm{HCO}}_3^{-}$ ) ranges from 0.1 to 0.5. The chlor-alkali indices (CAI1 and CAI2) mainly fall within the ranges of -2 to 1 and -0.25 to 0.25, respectively, indicating weak cation exchange adsorption. In 78.57% of the water samples, the ratio of γ(K⁺+ Na⁺) to γ(Cl⁻) ions is at or above the 1∶1 line, indicating that K⁺, Na⁺ and Cl⁻ primarily originate from the dissolution of rock salt. The ratios of γ(Ca²⁺+Mg²⁺) to γ(${\rm{HCO}}_3^{-}$ + ${\rm{SO}}_4^{2-}$ ) ions are concentrated near or above the 1∶1 line. Furthermore, 85.71% of the water samples display ratios of γ(Ca²⁺) to γ(${\rm{HCO}}_3^{-}$) ions that are either near the 1∶1 line or between the 1∶1 line and 2∶1 line. Additionally, 85.71% of the water samples exhibit ratios of γ(Ca²⁺) to γ(Mg²⁺) ions above the 1∶1 line. These findings suggest that Ca²⁺, Mg²⁺ and ${\rm{HCO}}_3^{-}$ ions are derived from the dissolution of carbonate minerals, while ${\rm{SO}}_4^{2-}$ ions originate from the dissolution of carbonate rocks and evaporitic salts, such as gypsum. (3) The lithology of covered karst is dominated by limestone, followed by dolomite limestone. The ratio of CaO to MgO in the rock chemical composition of limestone (19.36−119.73) is much larger than that of dolomitic limestone (1.43−4.71), and the dissolution ability of limestone is obviously stronger than that of dolomitic limestone. (4) PHREEQC software has been used to establish a reverse hydrogeochemical model in the rock sampling points from SY20 to SY19 in the Caopu area of Lyutian Town . The simulation results quantitatively confirmed the dissolution of calcite, dolomite, gypsum, and rock salt in karst water, with respective dissolution amounts of 2.599×10⁻⁴ mol·L⁻¹, 8.474×10⁻⁵ mol·L⁻¹, 4.165×10⁻⁶ mol·L⁻¹, and 3.446×10⁻⁵ mol·L⁻¹. (5) The saturation indices of calcite and dolomite in karst water show a good correspondence with karst development. Among the water sampling points where calcite is in a good dissolved state, 85.71% of the points exhibit karst development. In contrast, among the water sampling points where dolomite is in a dissolved state, 66.67% of the points exhibit karst development. The saturation indices of calcite and dolomite in groundwater indicate the trend of karst development and can serve as criteria for fine evaluation of karst development.This study reveals the hydrochemical characteristics of groundwater, as well as its formation and evolution in the covered karst area of northern Guangzhou. It also explores the relationship between groundwater mineral saturation indices and karst development. The research findings can provide a scientific basis for the exploitation and utilization of groundwater and for the protection of geological environment in the covered karst area of Guangzhou, which holds significant practical implications.

Study on the migration characteristics of cadmium element in the black shales of the groundwater system in the Guandu River Basin of Wushan area

LIU Yongwang, ZHU Zonglin, LI Hai, KE Qingqing, LUO Bo, LIU Jin, DENG Xinqian, ZHANG Yongwen

2025, 44(2): 238-249. doi: 10.11932/karst20250203

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The Permian black shale in the Wushan area is characterized by high background values of harmful trace elements, among which cadmium (Cd) is particularly prominent. These elements can easily enter groundwater systems through rock weathering, soil erosion, and human activities such as coal mining, thus posing potential threats to regional ecosystems and public health. However, previous studies have predominantly focused on Cd accumulation in soil and crops, while paying less attention to the migration characteristics and pollution mechanisms of Cd in groundwater systems. This study employs a multidisciplinary approach, integrating techniques from geology, hydrology, geochemistry, and environmental science, to systematically investigate the migration characteristics of Cd in the Guandu River Basin. The primary research methods include field surveys, water sampling and testing, data analysis, and geochemical modeling. By examining the pathways and mechanisms of Cd migration from black shale to groundwater, this study reveals the transport patterns of Cd within the rock–soil–groundwater system, clarifies its pollution mechanisms, and provides a scientific basis for environmental protection and resource utilization.Analysis of 40 groundwater samples revealed that eight samples contained detectable levels of Cd, ranging from 0.0001 mg·L⁻¹ to 0.0036 mg·L⁻¹, while Cd was undetectable in the other 32 samples. The pH values of Cd-containing samples ranged from 7.16 to 8.26, indicating neutral conditions. The average sulfate (SO$_4^{2-} $) concentration in Cd-containing samples was 67.84 mg·L⁻¹, three times higher than that in Cd-free samples (22.81 mg·L⁻¹), highlighting a strong correlation between SO$_4^{2-} $ and Cd migration. Additionally, the Total Dissolved Solids (TDS) values in Cd-containing samples ranged from 215 mg·L⁻¹ to 365 mg·L⁻¹, with an average of 268.56 mg·L⁻¹, indicating moderately hard to hard water. Research findings show that Cd-containing groundwater in the Guandu River Basin primarily originates from the karst groundwater system of the Permian black shale. The Gufeng Formation within the Permian System exhibited the highest Cd content. Although Cd remained enriched in soils derived from weathered rocks, its concentration significantly decreased. Cd entered the groundwater system through leaching, which is the main source of Cd in the groundwater of the study area. Coal mining activities have significantly altered the groundwater flow paths and intensified leaching effects, leading to Cd enrichment in mine gushing water. The highest Cd concentration detected was 0.0036 mg·L⁻¹ in gushing water (Sample S131) in an abandoned coal mine, indicating that coal mining activities are a significant anthropogenic factor contributing to Cd contamination. Additionally, TDS values in mine gushing water increased sharply, resulting in water quality deterioration and posing potential environmental risks. The Triassic karst groundwater system can effectively dilute Cd concentrations. This system is characterized by abundant water resources and demonstrates significant dilution effects on Cd concentrations in Permian karst water. For example, Cd was detected in the Longdong Underground River (Sample S039) during both the dry season and wet season, but its concentration was only 0.0003 mg·L⁻¹, which is far lower than that in mine gushing water. This dilution effect effectively reduces the risk of Cd contamination. Geochemical modeling revealed that calcite and dolomite in groundwater are close to saturation, while cadmium sulfate and cadmium carbonate have not yet reached saturation. This indicates that Cd has the potential for further enrichment in groundwater. Cd in rocks mainly exists in carbonate minerals (19%−66%). Analysis of Cd speciation in soil shows that Cd primarily exists in the forms of iron-manganese oxide-bound (25.03%) and residual (24.22%) fractions, with the smallest proportion in the water-soluble fraction (0.63%), indicating that soluble Cd in soils is mostly leached into groundwater through weathering, and soil acidification further enhances the dissolution and migration of Cd.This study elucidates the migration mechanisms and contamination risks of Cd within the black shale–soil–groundwater system, offering essential theoretical support for controlling Cd contamination in similar regions. The findings regarding Cd enrichment in coal mine gushing water provide scientific evidence for groundwater protection and pollution management in mining areas. By clarifying the impact of soil acidification on Cd migration, this study underscores the significance of controlling soil acidification for regional ecological restoration and environmental protection. Additionally, this study also highlights the dilution effect of Triassic karst groundwater on Cd pollution, offering guidance for the sustainable development and utilization of groundwater in the region. The results of this study can serve as a reference for ecological conservation in the Wushan area and other regions characterized by black shale.

Degradation characteristics of limestone from the Qutang Gorge to Wuxia Gorge of the Three Gorges Reservoir area

ZHANG Zhongyuan, TAN Lei, ZHAO Peng, YU Shu, BAI Linfeng, ZENG Deqiang, JIN Peng

2025, 44(2): 250-260. doi: 10.11932/karst2025y004

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The study area encompasses the section of the Three Gorges Reservoir area from Qutang Gorge to Wuxia Gorge, stretching from Baidicheng in Fengjie county, Chongqing City in the west to Guandukou of Badong county in the east. The lithology of this area is predominantly composed of Permian and Triassic carbonate rocks. Since the impoundment of the Three Gorges Reservoir in 2008, multiple geological disasters have occurred in the carbonate rock slopes. The periodic changes in reservoir water levels have exacerbated rock mass degradation, leading to the development of new geological hazards that pose a significant threat to the geological safety of the reservoir area. Rock mass degradation is primarily driven by water level fluctuations, dissolution, and stress changes, involving complex interactions of physical erosion and chemical dissolution. Existing research has focused on obtaining physical and mechanical parameters of rocks, with limited attention to the original structures, structural degradation, microscopic mechanisms, and prolonged evolution patterns. These geological factors are crucial in influencing the development of geological hazards in the Three Gorges Reservoir area. This study focused on limestone in the study area to reveal the patterns and mechanisms of rock mass degradation, aiming to provide technical support for the prevention and control of rock slope disasters in the Three Gorges Reservoir area. This study also employed in-situ testing methods such as rebound strength of rock masses and acoustic wave velocity, along with laboratory experiments including dry-wet cycling and CT scanning, to analyze the degradation of rock mass (rock) quality, changes in strength, and microscopic degradation patterns.The results show, (1) After 13 cycles of reservoir water level fluctuations (2008–2021), the rebound strength of rock mass surfaces in zones of water level fluctuations decreased by 11.15%–24.81%, with an annual average reduction rate of 1.01%–2.26%. (2) Acoustic wave velocity and borehole television images reveal that the rock mass is more fractured near the surface, with densely developed fissures. Overall, the shallower the rock mass is buried, the higher the rate of decrease in acoustic wave. (3) After each dry-wet cycle, the longitudinal and transverse acoustic wave velocities of the rock samples decreased by 0.08%–0.15% and 0.26%–0.65%, respectively. The uniaxial compressive strength decreased by approximately 0.94%, and the deformation modulus decreased by about 0.38%. (4) After 50 dry-wet cycles, the average reduction rates of longitudinal and transverse wave velocities were 5.74% and 0.52%, respectively, while the uniaxial compressive strength and deformation modulus decreased by 0.94% and 0.38%, respectively. (5) CT scans of the rocks showed that under water-rock interaction, the internal pores of the rock mass continuously increased, and the integrity of the rock mass declined significantly, indicating obvious degradation.Conclusions are drawn as follows, (1) The degradation of rock mass quality is closely related to the distribution of fissure surfaces and the depthes of the rock layers. Defects, such as fissure surfaces, are controlling factors for the degradation of rock mass quality. This degradation is not uniform; it occurs more rapidly near the surface and at a slower rate at greater depths. (2) Physical scouring erosion and chemical dissolution, both caused by reservoir water flow, are significant factors in the degradation of rock mass quality in the drawdown zone. Physical scouring erosion occurs more rapidly and is more pronounced than chemical dissolution.

Research progress on the water-carbon coupling processes at different scales in karst regions

ZHANG Jianchun, ZHU Jiang, HE Changde, JIA Zhi, ZHU Lifei, YI Yin, GONG Jiyi

2025, 44(2): 261-273. doi: 10.11932/karst20250205

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Karst regions are characterized by exposed bedrock, poor soils, and severe soil erosion, leading to a fragile and sensitive ecological environment, making them a key region for global environmental change research. The water cycle and carbon cycle are two key processes within ecosystems. The water cycle serves as a crucial material foundation for maintaining ecosystem stability and acts as an important vehicle for material circulation. In contrast, the carbon cycle represents a significant aspect of energy flow and material circulation. The interplay between these cycles is vital for sustaining ecosystem functions and environmental effects. In karst regions, the hydrological processes are complex, and the ecological environment is vulnerable. The water-carbon coupling is affected by natural and human factors, hence, studying the coupling process of karst region is essential for exploring the ecosystem in this area. Although considerable researches have been conducted on water-carbon coupling processes in karst areas, there remains a lack of systematic induction and summary of existing scientific findings, and future research directions need to be clearly guided and sorted. Thus, a comprehensive review and analysis of the water-carbon coupling processes in karst regions is particularly necessary.This study firstly starts from different spatial scales, from the leaf-scale level to the regional-scale level, to reveal the coupling mechanism between the water cycle and the carbon cycle. Combining the topographical and geomorphic characteristics of karst areas, the regional water - carbon coupling research of which is expanded from the Soil -Plant-Atmosphere-Continuum (SPAC) to the Soil-Rock-Plant-Atmosphere-Continuum (SRPAC). The Water Use Efficiency (WUE) is a key indicator for measuring water-carbon exchange,which is affected by the factors such as unique topography, vegetation, and etc., showing significant differences from the non-karst areas. At the leaf-scale, stomatal behavior regulates photosynthesis and transpiration, affectingthe plant's water-use strategy. At the plant scale, the water-carbon utilization of vegetation exhibits topographic variations, with WUE being constrained by factors such as soil moisture. At the ecosystem scale, water-carbon coupling correlates with hydro-thermal environment and CO₂ concentrations, whilethe groundwater circulationdrives the carbon cycling process. At the regional scale, water-carbon coupling manifests through the relationship between vegetation carbon sequestration and runoff generation and confluence,showing the exploration of the spatial-temporal distribution of WUE is of great significance for rocky desertification control.Secondly, an analysis is carried out from multiple perspectives such as climate change, vegetation restoration, land-use change, and engineering measures to summarize the impacts of natural and human activities on the water-carbon coupling processes. Climate change leads to fluctuations in precipitation patterns and temperature, directly altering the path and intensity of the water cycle, and thus affecting the carbon cycle. Vegetation restoration indirectly affects water-carbon coupling by regulating the regional climate and hydrology through soil and water conservation by roots and increasing vegetation coverage. Land-use change profoundly affects the water-carbon cycle process by influencing the carbon cycle and water resource distribution through changes in soil and runoff. Various engineering measures, such as the construction of water conservancy facilities, soil-water conservation projects, and tunnel construction, also have a significant impact on water-carbon coupling processes.Finally, potential future research hot spots are sorted out and extended classification methods of water-carbon coupling in combination with the karst geological background are proposed in this paper. It is suggested that a comprehensive monitoring system should be established, various carbon sink studies be integrated, and an interdisciplinary approach be adopted to carry out the simulation and prediction research on water-carbon coupling process in karst areas.. The water-carbon coupling model should be improved, and the spatial-temporal resolution of data should be enhanced. In-depth research on the impacts of climate change and various human activities on the water-carbon coupling process in karst areas should be conducted in multiple directions. This study provides a new perspective for future research on water-carbon balance in karst areas and shows positive significances for the scientific formulation of water resource and carbon balance management policies in karst regions.

Research on the spatial distribution and risk control of "three forms of nitrogen" in groundwater in an urban area of southwest China

HUANG Yancai, JIN Bo, ZENG Mudan, XIANG Gang

2025, 44(2): 274-282. doi: 10.11932/karst20250206

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Since the implementation of drought-resistant water exploration and well-drilling work, the utilization of groundwater in the karst areas of Southwest China has been increasing. While groundwater has generated much attention, the problems of groundwater pollution have also come to light, particularly concerning the "three forms of nitrogen" pollution in water. The sources of nitrogen pollution are complex, and its pathways are varied. Under the special hydrogeological conditions of karst areas, surface pollutants are more likely to infiltrate into the ground. Given the concealment and mobility of groundwater, pollutants continuously migrate with the groundwater flow. Once contaminated, the difficulty of restoration and remediation is extremely high. Taking an urban area in Southwest China as an example, this study conducted hydrogeological surveys and water sample testing. Based on data such as springs, boreholes, and water quality, this study utilized the method of Inverse Distance Weighting (IDW) to interpolate and analyze the "three forms of nitrogen" in groundwater. Combined with the field model of groundwater flow, the spatial distribution of "three forms of nitrogen" was analyzed. Furthermore, the areas for risk control were delineated through overlay analysis. The results show that there were differences in the spatial distribution of "three forms of nitrogen" in the study area. Urban boreholes and sewage treatment plants had a greater impact on NH$_4^{+}$-N concentration, while agricultural and residential areas significantly influenced ${\rm{NO}}_3^{-}$-N levels. The overall situation regarding NO$_2^{-}$-N was favorable, with no significant pollution detected. Additionally, there were notable differences in the concentrations of the "three forms of nitrogen", with the highest concentration of ${\rm{NO}}_3^{-}$-N reaching 96.00 mg·L⁻¹, which was much higher than the concentrations of NH$_4^{+}$-N (8.00 mg·L⁻¹) and NO$_2^{-}$-N (0.500 mg·L⁻¹). This variation is attributed to the characteristics of karst areas. Under conditions of adequate oxygen content, NH$_4^{+}$-N and NO$_2^{-}$-N were converted into ${\rm{NO}}_3^{-}$-N. With the use of the Class III water index specified in the Standard for Groundwater Quality (GB/T 14848-2017) as the control boundary, the study area is divided into three zones through ArcGIS overlay analysis, key control area (Class IV-V), secondary control area (Class III), and general area (Class I-II). In key control areas, the focus is on controlling sewage discharge and leakage, as well as nitrogen loss from agricultural activities. Specific measures include strengthening control of pollution emission in densely populated residential areas, ensuring the impermeability of residential water wells, sewage pipelines, and wastewater treatment plants. In agricultural-intensive areas, the implementation of scientific fertilization and rational allocation of fertilizers should be prioritized to prevent the migration of "three forms of nitrogen" at the source. In secondary control areas, the emphasis is on preventing further pollution and optimizing water quality indicators. Specific measures include optimizing the sewage systems in residential areas, further controlling pollution emissions, enhancing regional soil and water conservation capacities, and improving vegetation absorption efficiency. Additionally, measures such as crop rotation, optimizing crop irrigation methods, ecological breeding, and reducing random sewage discharge should be implemented to comprehensively improve the regional ecological space. In general areas, the focus is on maintaining the current status and preventing pollution.

Chemical characteristics and genetic mechanism of karst geothermal radon spring in Anjiazhuang, Feicheng

BAI Xinfei, HU Caiping, SONG Jinyu, YANG Shijiao, QIE Liang, ZHANG Jun, YU Chao, HONG Huanren, WANG Tao, SONG Liang

2025, 44(2): 283-299, 339. doi: 10.11932/karst20250207

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The Anjiazhuang geothermal field, the only karst geothermal radon spring discovered in Shandong Province, is located at the contact zone between the Wenkou depression and the Dongping uplift in Feicheng City. The geological framework of this geothermal field is based on a Neoarchean granite basement, which is overlain by Cambrian–Ordovician carbonate rock strata and Quaternary loose sediments. Geothermal water originates within the tectonic fracture zone at the intersection of the Anjiazhuang fault and its secondary faults. It surges upward along the fracture zone and is stored in the layered karst fractures, forming a belt-like and layered thermal reservoir controlled by the fracture structure.In this study, we systematically reveal the hydrochemical evolution characteristics of geothermal fluids and the genetic mechanisms of radon enrichment through the collection and analysis of groundwater and geothermal water samples. This includes measurements of soil radon gas, CSAMT measurement, and the application of hydrochemical diagram methods, ion component ratio characteristic methods, mineral saturation index methods and PHREEQC software simulations. The research findings show that the geothermal water is a weakly alkaline, medium-low temperature thermal mineral water, classified as a hydrochemical type of SO₄·Cl-Na·Ca. The concentrations of fluorine (1.87–4.00 mg·L⁻¹), metasilicic acid (32.50–68.45 mg·L⁻¹), and radon (82.60–640.00 Bq·L⁻¹) all meet the standards for designated mineral water, classified as a fluorine-metasilicic acid-radon composite therapeutic thermal mineral water, which possesses significant therapeutic value. The evolution of ion components in the geothermal water is mainly controlled by the dissolution of evaporite and carbonate minerals, followed by the dissolution of silicate minerals and cation exchange. The mineral saturation index shows that calcite and dolomite are supersaturated, anhydrite is nearly saturated, and halite is unsaturated. The enrichment of radon in geothermal water is closely related to the fracture structure: the radon in surface soil and the high anomalies of radon in groundwater are both distributed along the fracture zone. The low-resistance zone revealed by the inversion section of CSAMT measurement highly coincides with the fracture zone and the burial depthes (from 150 m to 350 m) of the karst thermal reservoir. Radon mainly originates from the uranium-rich S-type granite of the Neoarchean Aolaishan Sequence underlying the thermal reservoir and the radioactive decay of the uranium series in the uranium mineralization zone distributed along the fracture. Deep active faults, such as the Anjiazhuang fault, induce the development of micro-fracture networks in rocks. This process facilitates the transformation of radon from a closed and adsorbed state in granite into a free state, allowing it to dissolve in geothermal water.Consequently, radon migrates and accumulates along the fracture zone. Regional seismic activities and the release of tectonic stress help to intensify the damage and fracturing of rocks, enhancing the radon exhalation ability. The relationship between hydrogen and oxygen isotopes shows that geothermal water is mainly recharged by atmospheric precipitation and is formed following the deep circulation and heating of groundwater through the karst-fault system. Its heat source exhibits the characteristics of a four-element heat accumulation: deep-circulating hydrothermal convection in the fracture zone serves as the main heat source, supplemented by the conduction of terrestrial heat flow, heat generated by seismic friction, and heat generated by the decay of radioactive elements.In conclusion, the formation of the Anjiazhuang geothermal radon spring is the result of the synergistic effect of multiple factors including structure, hydrology and geochemistry, the structure system of the Anjiazhuang fault is not only the channel for radon migration and enrichment, but also provides the dynamic conditions for the exhalation of radon in rocks; the underlying uranium-rich granite and the uranium mineralization zone distributed along the fracture lay the material foundation, and deep-circulating hydrothermal convection and multi-source thermal effects jointly drive the evolution of geothermal water. This study systematically elucidates the multi-source synergistic genetic mechanism of radon enrichment in karst geothermal radon spring, providing a scientific basis for the exploration, development and utilization of similar geothermal resources.

Development characteristics and controlling factors of red-bed karst in the southern section of Longmen Mountain at the edge of Sichuan Basin

ZHANG Xiaowen, WANG Chuan, YANG Yanna, FAN Shijie, YU Lei, XU Xiaoqing

2025, 44(2): 300-315. doi: 10.11932/karst20250208

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During the Jurassic to Cretaceous periods, a set of continental red basin sediments were deposited in front of Longmen Mountain on the western margin of the Sichuan Basin. The predominant lithology consisted of mud-calcium cemented sandstone (conglomerate). Under the action of groundwater dissolution, the rock layer has been developed with large-scale karst phenomena, including depressions and extensive horizontal karst caves that extend thousands of meters. These features pose a major threat to the safety of constructing reservoirs and tunnels in the study area. Based on existing geological and hydrogeological investigations, combined with ground surveys, and identification of rock and mineral types, this study examines the development characteristics and controlling factors of sandstone (conglomerate) dissolution from multiple perspectives. These perspectives include macroscopic structural evolution, sedimentary environment, hydrodynamic conditions, microscopic mineral composition, and cementing characteristics. The main conclusions of this study are as follows.The karst forms developed on the surface of the study area mainly include karst depressions, karst hills, sinkholes, stone teeth, karst valleys, karst fissures, and karst gullies. In the conglomerate area, circular depressions resulting from collapses are often formed on the surface, with sinkholes frequently developing at the base. In contrast, the sandstone area is typically eroded by surface water along joints and fissures, leading to the formation of long troughs or irregular corrosion funnels. The underground karst phenomenon is characterized by karst caves, which contain deposits such as stalactites, stone mantles, and stone flowers. These karst deposits are relatively rare and often impure, primarily consisting of gray muddy calcareous mixture that is mainly composed of calcium carbonate mixed with clay materials. The entire karst phenomenon exhibits the features typical of early-stage karst landforms, which can be classified as either red-bed karst landforms or sandy gravel karst landforms.The karst phenomenon is mainly developed along the bedding plane in the trend of rock formations, and secondarily in the dip direction. The local extension direction is controlled by the joints and fissures. In the areas with strong karst development, the lithology of the strata is dominated by calcareous conglomerate of the Guankou formation (K₂g) and the Tianmashan formation (K₁t). The karst morphology is diverse and of large scale, often featuring a complete set of karst systems, developed with vertical recharge and horizontal discharge. In the areas with moderate karst development, the lithology consists of calcareous conglomerate of the Guankou formation (K₂g) and the Tianmashan formation (K₁t), conglomerate of the Mingshan formation (E_1-2m), and sandstone of the Shaximiao formation (J₂s) and the Penglai formation (J₃p). In this area, karst fissures and pores are widely developed, with visible karst caves and small-scale karst pipelines. The depth of horizontal karst development is generally less than 100 meters. In the areas of weak karst development, the main lithology is dominated by conglomerate of the Mingshan formation (E_1-2m), and sandstone and mudstone of the Jiaguan formation (K₂j), the Shaximiao formation (J₂s), and the Shaximiao formation (J₃p). The main karst phenomena are karst fissures and pores developed along the bedding plane or dominant joints.The sedimentary environment is the fundamental internal factor controlling karst development. The complex and variable sedimentary environment has led to differences in lithology and structures of rock groups in both the planar and vertical directions within the study area, which in turn control the location and intensity of red-bed karst development. The provenance of Mesozoic and part of Paleozoic carbonate rocks in the north–west of the study area ensured the solubility of sandy conglomerate in the area. The alluvial fan sediments corresponding to the three intense tectonic phases during the Yanshanian Period are manifested as areas with strong karst development. Differences in lithology lead to variations in soluble mineral contents, showing a trend of gradually decreasing solubility from calcareous conglomerate to conglomeratic sandstone, then to siltstone, and finally to mudstone, accompanied by a corresponding weakening of karst action. Hydrodynamic conditions under the control of climate, topography, and geological structures are important external factors affecting karst development. The warm and humid climate in the study area provides abundant recharge and erosion capacity for groundwater. The landform and strong tectonic action control the scale and extension direction of red-bed karst development. During the Neotectonic Movement, intermittent uplift and stability have resulted in a multi-layered karst system in the study area, which corresponds, to some extent, with the multi-tiered river terraces.

Distribution characteristics and genesis model of karst collapse in the Huainan area of Anhui Province

XU Guangquan, LI Hao, YANG Tingting, ZHANG Haitao, HE Biao

2025, 44(2): 316-327. doi: 10.11932/karst20250209

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Karst collapse is one of the geological disasters in shallow karst regions during coal mining in North China. This phenomenon is influenced by factors such as stratum occurrence, lithology composition, geological structures, and hydrodynamic conditions. It is primarily induced by mine drainage, pressure reduction and urban construction in the mining area. Karst collapse is mainly developed in Cambrian and Ordovician carbonate strata. The mining area in Huainan is located in the southern margin of the North China Coalfield. Since the 1950s, more than 131 karst collapses have occurred in the southern coalfield as a result of drainage and depressurization caused by coal mining and urban construction. These collapses exhibit a clustered occurrence pattern, mainly concentrated in areas such as in Fengtai county, Kongji, Tubazi, Liyingzi and Shungengshan. These collapses have severely affected residents’ lives in the mining area and hindered urban development. Therefore, conducting further systematic study on the formation conditions, distribution, and influencing factors of karst collapse in the mining area is of great practical significance. This study is essential for gaining a comprehensive understanding of the karst hydrogeological conditions, the mechanisms of karst collapse disasters, and the effective prevention and control of such phenomena.This study employs methods including karst field geological surveys, statistical analysis and multi-factor analysis to systematically investigate the distribution development characteristics, and genesis of karst collapse in the Huainan area. The results show that karst collapses are mainly formed in the Ordovician and Cambrian strata, which are distributed along the Quaternary overburden area in the piedmont. This phenomenon is the result of multi-stage tectonic processes and prolonged groundwater dissolution. According to the controlling conditions, such as collapse-inducing factors, groundwater dissolution, and stratum occurrence, three types of karst collapse modes induced by vacuum suction erosion have been proposed: (steep inclination, gentle inclination, and inversion). Additionally, a collapse mode resulting from gravity load due to the overburden of urban construction has also been identified. The main results are as follows.(1) Karst collapses in the study area are mainly distributed in the carbonate strata of Cambrian Zhangxia formation and the Ordovician Majiagou formation, spanning Fengtai county, Kongji, Tubazi, Liyingzi and Jiulonggang of Shungengshan to Datong. (2) Karst collapses of steeply inclined strata are distributed in the Datong and Kongji areas of Huainan. Influenced by the Shungengshan thrust-nappe structure and the Shanwangji tensional fault, high-angle strata have developed. Karst formations are concentrated at the intersections of structural fissures and karst fissures that are either perpendicular or oblique to the stratigraphic direction. The water drainage and depressurization from the surrounding coal mining operations results in a decrease in pore water pressure within the rock and soil mass decreases, increasing the hydraulic gradient and disturbing the loose sand and water-soil mass filling in the karst. Such disturbances disrupt the original stress equilibrium, ultimately leading to collapses.(3) Karst collapses of gently inclined strata are mainly distributed in the Tubazi area of Huainan. Influenced by tectonic activities, vertical or oblique fissures have developed along the rock surface, which are filled with Quaternary loose materials. As a result of water drainage from the mining operations, the capacity of groundwater to transport loose rock and soil mass increases, leading to the continuous expansion of soil caves and an imbalance in the surrounding loose rock and soil mass. Specifically, the effective stress of soil particle framework and pore water pressure decrease, resulting in karst collapses under the overburden load.(4) Karst collapses of the inverted strata are mainly distributed in the Liyingzi area. Influenced by the thrust nappe of Shungengshan mountain, the strata are overturned at the turning point of the fault, leading to the development of multiple vertical and oblique tensile fissures along the ground level. These fissures are filled with Quaternary loose materials at their intersections. Due to infiltration from atmospheric precipitation and groundwater, a unified karst groundwater system has formed in the Ordovician and Carboniferous strata. Subsequently, water drainage from mining operations reduces pore water pressure, causing an imbalance in the loose water and soil mass, which ultimately results in collapses.(5) Karst collapses, resulted from gravity load, mainly occur in Fengtai county, which are underlain by Quaternary Cambrian and Ordovician karst strata. Influenced by the Shanwangji normal fault, as well as weathering and denudation processes, karst has been developed in carbonate rock sections. Urban construction increases the overburden load, which alters the pore water pressure and effective stress of the filling in the caves. This alteration disrupts the equilibrium between the rock, soil, and water, leading to karst collapses.The main conclusions of the study are as follows.(1) The study area is influenced by the nappe structure, with karst collapses densely distributed along the stratigraphic direction, fault zones or their intersections. Karst collapses occur from the outcrop area to the shallow-buried area, specifically within the Cambrian Zhangxia formation and the Ordovician Majiagou formation. The distribution pattern of karst collapses is related to the stratigraphic occurrence. The planar shapes of these collapses are mostly circular, elliptical and elongated, and they are primarily filled with loose sandy soil. (2) Karst collapses occur in soluble rock strata, faulted tectonic fractured zones, areas with strong hydrodynamic conditions, and locations with a thin overburden of loose layers. Based on the genesis and triggering factors, karst collapses in the study area can be categorized into three different modes, steeply inclined, gently inclined and inverted, and a mode resulted from gravity load. (3) The future risk of karst collapses in the Huainan area will continue to be concentrated in the areas of Datong–Jiulonggang, Bagongshan, and Fengtai county. These areas remain susceptible to karst collapses and require enhanced monitoring and preventive measures.

Study on the dynamics response characterstics of covered soil-cave type karst collapse under train vibration environment

SHI Hai, JIA Zhilei, BAI Mingzhou, ZHANG Ye, SUN Zibing

2025, 44(2): 328-339. doi: 10.11932/karst20250210

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Karst collapse represents a significant geological hazard, predominantly occurring in regions characterised by the presence of soluble rock formations, including carbonate rocks, calcareous clastic rocks and salt rocks, among others. Karst collapse are characterised by three key features, a hidden spatial distribution, a sudden onset and periodic recurrence over time. These attributes collectively pose a considerable challenge for the construction of major infrastructure projects in karst areas. The karst collapse disaster along the railway has the potential to pose a significant threat to the safe construction and sustained operation of the high-speed railway project. The extraction and discharge of groundwater, along with the alteration of hydrodynamic conditions during both the construction and operational phases of the railway in the karst area, have been identified as key factors contributing to the karst collapse. The most prevalent numerical simulation method is the Finite Element Method (FEM), which is predicated on the assumption of a continuous medium. The FEM involves replacing a complex problem with a simpler one and then solving it. It views the solution domain as consisting of a number of small interconnected sub-domains called finite elements, assumes a suitable (simpler) approximate solution for each element, and then deduces the conditions that are satisfied for solving the domain in general. However, soil is not a continuous medium, and a model based on the FEM is unable to simulate the local instability of a collapsed soil body, the mesoscopic-scale damage process, and other phenomena. In light of the dearth of sufficient attention to the temporal effects and fine-scale mechanisms of the karst collapse process in current studies, this paper aims to elucidate the dynamic evolution laws and mesoscopic-scale collapse mechanisms of the expansion of overlying karst soil cavities around the railway. A typical karst collapse site, namely the Beijing-Shanghai high-speed railway (Jiangxi section), was selected as the basis for calibrating the strength parameters of the collapsed soil body. This was achieved through a particle flow (PFC2D) compression test, which also introduced a contact bonding model. This model assumed that the filler in the cavern is entirely washed away due to the erosive effect of groundwater seepage. Additionally, the vacuum suction and erosion effect inside the cavern were not considered during the simulation period. The effect of groundwater on soil strength is simulated by reducing the contact strength between particles below the modelled water level. In conclusion, a coupled flow-solid model of overlying karst collapse has been established, which elucidates the dynamic evolution process and deformation characteristics of karst collapse from a mesoscopic view. Furthermore, the influence of varying cavern opening sizes, overburden layer thickness and water table height on the deformation characteristics of the overlying karst collapse has been investigated, as well as the migration law of soil particles under the influence of different factors. The study demonstrates that during the evolution of overlying karst collapse, the contact force between particles undergoes a series of changes, which can be described approximately as follows, 'stress equilibrium–stress arch formation–stress arch destruction-stress equilibrium again-…-stress arch fracture '. The internal stress of the soil body demonstrates a pattern of 'compressive stress gradually decreasing, tensile stress gradually increasing, and tensile stress disappearing'. Additionally, the surface subsidence and porosity of the soil body tend to increase in conjunction with the collapse evolution process. It can be observed that the larger the opening of the cavern, the greater the depth and range of ground subsidence, which in turn increases the likelihood of collapse. A reduction in the thickness of the cover layer results in a more pronounced surface subsidence, thereby increasing the likelihood of collapse. Similarly, an elevated water table leads to a more rapid expansion of the soil hole, which in turn causes a more pronounced surface subsidence and an increased propensity for collapse. The relationship between surface settlement and cover layer thickness is not significant when the latter is of greater thickness. The study provides a comprehensive account of the karst collapse evolution process from a mesoscopic perspective, offering insights that can inform disaster prevention and the mitigation of surrounding karst collapse risks during the construction and operation of high-speed railway projects.

Research on the resolution of cross-hole electromagnetic wave CT method for small karst caves under different working patterns

CHEN Fengyi, PAN Jianwei, SONG Hongming, YANG Chen, KUI Yulu

2025, 44(2): 340-350. doi: 10.11932/karst20250211

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The cross-hole electromagnetic wave CT method, as an in-situ and non-destructive geophysical exploration technology, utilizes the propagation characteristics of high-frequency electromagnetic waves between boreholes to intuitively reflect the spatial distribution of underground anomalies. With advantages such as high resolution, efficient operation, and minimal restrictions by surface topography, it has been widely applied in engineering fields such as karst cave detection, roadbed grouting quality assessment, and mined-out area investigation in coal mines. However, in practical applications, the imaging quality of the electromagnetic wave CT method is affected by multiple factors, particularly in the detection of small karst cavities, where resolution capacity is closely tied to the selection of working parameters. Currently, there is a lack of systematic studies on the resolution capability of this method for small cavities, which to some extent restricts its precise application under complex geological conditions. This study adopts a combined approach of numerical simulation and engineering validation to thoroughly investigate the resolving capability of the cross-hole electromagnetic wave CT method under different working configurations for detecting small karst cavities. The research provides a solid theoretical foundation and technical reference for its practical engineering application.The research follows a technical route that integrates theoretical analysis, numerical modeling, and on-site testing. In terms of numerical simulation, a professional cross-hole electromagnetic tomography processing system was used along with a self-developed program for adding electromagnetic noise. Detection models were constructed under varying conditions, focusing on how key parameters-such as differences in medium absorption coefficients, transmission-reception spacing, and borehole spacing-affect the imaging outcome. The interference effects of ambient electromagnetic noise were also systematically analyzed. The model settings were designed to reflect typical engineering conditions. For example, the absorption coefficient of surrounding rock was set to 0.1 dB·m⁻¹, and that of the target body (representing the karst cavity) was varied between 0.2 dB·m⁻¹ and 0.7 dB·m⁻¹ to simulate different filling conditions. The transmitting spacing ranged from 1 m to 10 m, and borehole spacing was controlled within 10 m to 30 m, covering a comprehensive range of commonly encountered engineering parameters. Based on the forward modeling, random electromagnetic noise at levels of 1% to 3% was also added to better replicate real field environments. For the engineering validation phase, a typical karst-developed area near Jingna Road in Guangxi was selected for on-site testing, and the absorption coefficient cross-sections were compared against borehole data for verification.The research results indicate that the resolution capability of electromagnetic wave CT for small karst cavities shows strong dependence on parameter selection. Regarding the absorption coefficient contrast, when the difference between the target body and surrounding rock reaches 0.6 dB·m⁻¹, cavity anomalies can still be clearly identified even under 3% noise interference. However, when the difference is only 0.1 dB·m⁻¹, the imaging quality deteriorates significantly once the noise level exceeds 1%. This suggests that in practical applications, working frequency bands with pronounced electromagnetic contrast should be prioritized, and effective noise suppression measures must be taken. With respect to transmission-reception spacing, the study shows that increasing the spacing leads to reduced ray path coverage density. When the spacing exceeds 4 m, the imaging quality for cavities of 2 m × 2 m in size becomes noticeably worse. At 8 m, the target is almost entirely unresolvable. This indicates that observation systems in practical projects must be designed according to the size of the target anomaly. For small cavity detection, it is recommended that transmitting spacing be kept within 4 m. Regarding borehole spacing, modeling data demonstrate that when the absorption coefficient difference is 0.6 dB·m⁻¹, resolution remains acceptable at borehole spacing up to 30 m. However, when the difference decreases to 0.4 dB·m⁻¹, cavity anomalies become blurred when spacing exceeds 20 m. This suggests that in regions with weak karst development or minimal contrast between the filling material and the surrounding rock, borehole spacing should be reduced appropriately to ensure detection effectiveness. The field validation further confirms the reliability of the simulation conclusions. In the actual survey conducted near Jingna Road in Guangxi, the electromagnetic wave CT method accurately delineated a cavity development zone between depths of 10m and 15 m. The inversion results closely matched the cavity positions revealed by borehole drilling.This study systematically identifies the key influencing factors and corresponding mechanisms that affect the resolution capability of electromagnetic wave CT in detecting small karst cavities. Firstly, it confirms that the absorption coefficient contrast is the fundamental determinant of resolution, providing theoretical guidance for frequency selection. Secondly, it quantifies reasonable values for transmitting spacing and borehole spacing, establishing technical standards for observation system design. Finally, it analyzes the interference mechanisms of ambient noise, offering guidance for improving data acquisition quality control. Based on the research findings, the following engineering recommendations are proposed for typical small karst cavity detection scenarios: transmission spacing should be kept within 4 meters, and borehole spacing should not exceed 30 meters. These results not only enrich the theoretical framework of electromagnetic wave CT method but also offer direct and practical guidance for improving the detection accuracy of karst cavities in real-world engineering contexts.

Karst development evaluation based on cross-hole radar tomography

JIA Long, ZHANG Yi, MENG Yan, LI Lujuan, PAN Zongyuan, WU Yuanbin, YIN Renchao

2025, 44(2): 351-358. doi: 10.11932/karst20250212

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Cross-hole geo-radar tomography, which belongs to the method of electromagnetic wave tomography between boreholes, is a way of borehole radar detection. Specifically, electromagnetic waves are transmitted form one borehole (transmitting borehole) and the transmitted signals are received by another borehole (receiving borehole). The kinematic (travel time and ray path) and dynamical (waveform, amplitude, phase, and frequency) characteristics of these signals are recorded. By tomography technology, the influence of the medium between the boreholes on the singlals is analyzed to obtain the distribution of physical properties within the strata between the two boreholes, with the goal of inverting the internal structure of the medium. As a result, the geo-radar tomography method can identify underground structures that exhibit significant differences in physical properties, such as dielectric constant and conductivity. These underground structures include solitary rocks, fissures, caves, and other adverse geological phenomena, as well as buried public facilities. This technology can penetrate deep intosubsurface for detection through drilling boreholes, effectively isolating the interference from above ground and compensating for the limitations of drilling density in practical engineering.In geological field surveys, accurately determining true karstification rate of the soluble rock layers poses significant challenges. Consequently, the karstification rate along the drilling line is typically employed as a representation measure, which has inherent limitations. To enhance the quantitative analysis of karst development within strata, this study employs cross-hole geo-radar tomography technology to process radar transmission data. This approach allows for the calculation of numerical changes in karstification rates, facilitating a more precise quantitative evaluation of the degree of karst development between boreholes.In karst areas, the materials filling karst fissures—such as including air, water, and loose soil—are the primary factors contributing to variations in the dielectric constant of rock masses. The velocities of radar waves are a function of the dielectric constant of the geological strata. By measuring the transmission velocities of electromagnetic waves in karst strata through cross-hole geo-radar, the porosity of limestone can be determined, allowing for the assessment of variations in the karstification rate between boreholes. The cross-hole geo-radar tomography method can provide insights into the distribution of transmission velocities of electromagnetic waves in karst strata, and thereby revealing the spatial variations in the karstification rates of soluble rocks between boreholes.When electromagnetic waves propagate in karst strata, their amplitudes exhibit exponential attenuation. In karst areas, the materials filling in karst fissures are also the main factors contributing to variations in electromagnetic wave attenuation. The cross-hole geo-radar tomography method can obtain the distribution of attenuation coefficients of electromagnetic waves in karst strata. For low-loss materials such as rock and soil, the attenuation coefficient is directly proportional to the conductivity of strata and inversely proportional to the square root of the relative dielectric constant of strata. Moreover, the conductivity and the relative dielectric constant of karst strata are closely related to the karstification rates of strata. Based on this, the spatial variations of karstification rates between boreholes can be quantified.Engineering examples demonstrate that, based on the physical parameters—wave velocity and attenuation coefficient—derived from the cross-hole geo-radar travel time tomography and attenuation tomography, as well as the distribution of karstification rates, it is feasible to map the distribution of karst development between boreholes. At a burial depth of 28 m to 32 m, it is inferred that karst caves are filled with flowing–soft plastic saturated clay and are likely to span the strata between the two boreholes. At a burial depth of 37 m to 41 m, it is presumed that karst caves are filled with plastic saturated clay. The spatial results of karstification rates obtained from both methods are consistent and mutually corroborative, providing a refined quantitative evaluation of the degree of karst development in strata at varying burial depths between boreholes. This provide more detailed data support for geological surveys and engineering construction. In addition, the accurately determining the karstification rates of strata based on wave velocity and attenuation of the cross-hole geo-radar requires precise values of all physical parameters of strata. These parameters can be obtained through laboratory tests on rock cores.Theoretical analysis and on-site testing demonstrate that the application of cross-hole geo-radar travel time tomography and attenuation tomography techniques can achieve a precise quantitative evaluation of the karst development degree in the strata between boreholes, thereby obtaining the inter-borehole "surface" karstification rates. This will provide more detailed data for karst geological surveys. In addition, considering the multisolution nature of geophysical survey results and the complexity of karst geology, it is recommended to combine these techniques with imaging detection of borehole radar reflection and other geophysical methods during actual detection.

Research on the mechanism and prevention technologies of mud and water inrush disasters in tunnels

QIN Pengfei, ZHAO Yamin, SONG Meng, WANG Wenjing

2025, 44(2): 359-369. doi: 10.11932/karst20250213

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Tunnel water inrush disasters are characterized by large flow, strong bursts, high water pressure, and various types, making them complex phenomena that involve multi-scale and multi-physical-field coupling. Analyzing the characteristics of adverse geological structures, their development patterns, and damage trends under water pressure is conducive to profoundly revealing the disaster mechanisms caused by mud and water inrush. This study was focused on the evolution of the performance and states of three key elements, water sources, water supply channels, and impermeable rock masses. The dynamic process of disaster evolution of mud and water inrush was firstly analyzed. Based on theories of elasticity, fracture mechanics, and engineering hydraulics, this study then explored the disaster mechanism of compression shear and tensile shear failures. The rock layers between the tunnel excavation face and the water sources were divided into three areas: the stress relaxation zone caused by tunnel excavation, the protection zone of intact rock masses, and the fissure zone. This study proposed optimal protective thickness against mud and water inrush. Additionally, it discussed the technical measures for disaster prevention and control from the perspectives of grouting theory and grouting materials, aiming to enhance the safety of tunnel construction. Tunnel mud and water inrush in tunnels is a complex phenomenon involving multi-scale and multi-physical-field coupling. Such disasters are characterized by substantial concealment, high destructive potential, and low predictability. The structural morphology and constructive forms of the disaster mechanisms are the foundation for revealing the mechanical principles of mud and water inrush. A comprehensive analysis of these structural morphologies and forms facilitates the scientific prediction of the spatial and temporal distribution of such disasters. This enables targeted disaster management and prevention strategies, thereby reducing safety risks for construction machinery, production materials, and personnel on-site.

Seismic response characteristics and reservoir distribution prediction of karst reservoirs in the Yingshan Formation of the Lunguxi block

GUAN Baozhu, LIU Junfeng, YANG Maochuan, YANG Xinying

2025, 44(2): 370-381. doi: 10.11932/karst20250214

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Globally, 279 carbonate oil and gas reservoirs have been discovered across 39 countries and regions. Notably, large marine carbonate oil and gas reservoirs are primarily distributed in China, mainly featuring lithological-stratigraphic traps. The buried depth of oil and gas reservoirs is generally greater than 5,500 m, and they are mainly concentrated in large oil-and-gas-bearing basins, such as the Tarim Basin, Sichuan Basin, Ordos Basin, etc. The rapid development of China’s petroleum industry has resulted in a growing volume of crude oil imports each year. The increasingly complex international landscape has posed challenges to the stability of China’s petroleum supply. The exploration and development of carbonate karst oil and gas reservoirs is one of the ways to ensure a stable energy supply. The relevant areas have been transforming from strategic oil reserve zones to regions for oil resource replacement. Therefore, accurate prediction and precise evaluation of carbonate karst reservoirs are increasingly receiving attention from petroleum geologists. Lunnan ancient buried hill is located in the central area of the Lunnan low uplift within the Tabei uplift of the Tarim Basin, which is the main part of the Lunnan paleo-uplift in the Tarim Basin. The Lunguxi block is located in the west of the Lungu oil and gas field. In the Lunguxi block, especially in its northern area, many wells have been drilled; however, their exploitation has been inefficient and unsuccessful. The low drilling rate of the target layer may be the main reason for these phenomena. Therefore, it is urgent to address the challenges of accurate prediction and precise evaluation of carbonate karst reservoirs.According to the characteristics of karst fracture-cavity development and the karst hydrodynamic conditions, and in conjunction with modern karst geological theory, carbonate karst reservoirs can be classified into three main types, surface karst reservoir, underground river karst reservoir, and fault-controlled karst reservoir. High-angle structural fractures, dissolution pores, and small caves are mainly developed in surface karst reservoirs. The dissolution pores and fractures are primarily connected vertically, with the potential for lateral interconnection over short distances along the fractures. The fractures and small-scale caves are distributed in a planar arrangement and exhibit a strong capacity for adjustment. Underground river systems of karst reservoirs, including underground rivers, sinkholes, vertical shafts, skylights, hall-type and corridor-type caves, underground river outlets and inlets, etc., exhibit good connectivity along the direction of groundwater runoff. Existing wells for exploitation show that the underground river karst reservoirs are the main reservoir type for oil and gas production, and are also one of the most important reservoir types in the study area. The development of fault-controlled karst reservoirs is influenced by fault activities. These reservoirs are formed by the dissolution and transformation of diagenetic fluids along fault surfaces or structural fractures, resulting in a collection of fracture-cavity complexes. They are developed vertically along fault zones, with a large range of depths, good vertical connectivity, strong lateral heterogeneity, mainly characterized by dissolution fracture-cavities.The carbonate reservoirs in the Ordovician Yinshan Formation of the Lunguxi block exhibit a complex and diverse pore system, with multiple coexisting and interacting geological factors, resulting in strong reservoir heterogeneity. The seismic reflection anomaly of the small fracture-cavity is weak or not easily discernible, exhibiting high concealment, which complicates identification. The development characteristics of low-order faults within faults and ancient river channels, as well as their relationship with the small fracture-cavities, are challenging to quantify. Therefore, based on previous exploration and development data, this study aims to clarify the reservoir types in the study area and subsequently elucidate the seismic response characteristics of these reservoirs. The results show,(1)The seismic characteristics of surface karst primarily exhibit chaotic strong reflections, typically found in the high part of the residual hills and distributed within 0 to 40 meters of the weathering surface of the ancient buried hills. In contrast, the seismic characteristics of underground river karst are characterized by continuous strong reflections in the seismic profile, with the impedance attribute presenting as a low-value anomaly. This anomaly has a broad lateral distribution range and displays typical river channel characteristics. Additionally, the fault-controlled seismic characteristics are predominantly represented by beaded and chaotic reflections that are distributed along the fault. These reflections exhibit large longitudinal extension and linear distribution patterns along the fault line in a plane view. (2)The seismic response prediction of karst reservoirs in the study area has been conducted from various scales and directions. The research findings indicate that the combination of amplitude attributes and AFE attributes can well characterize the surface karst reservoirs developed in the high part of the buried hill. Additionally, attributes such as differential amplitude and root mean square can systematically identify underground river karst reservoirs. Furthermore, spectrum decomposition discontinuity detection can elucidate the internal details of the fault fracture zone, providing an optimal approach for predicting fault-controlled karst reservoirs based on seismic response. (3)The surface karst is generally developed in the Lunguxi block, particularly in the structurally elevated parts. The fault-controlled karst reservoir is mainly developed in the area of Lungu Wells 9–40. In the northwest and central regions, the thickness of surface karst in karst valleys and slopes is relatively small, whereas in the southeast region, the thickness of surface karst on the karst platforms is substantial. The identification of fault-controlled karst reservoirs can be accomplished through the integration of amplitude attributes and structural tensor attributes.

Geological significance, prediction and application of structural fractures in compact limestone reservoirs of the Taiyuan Formation,the Ordos Basin

FU Xunxun, FAN Liyong, PEI Wenchao, ZHAO Xiaohui, SHI Lei, DONG Guodong, WANG Huaichang, LIU Xiaopeng, LU Zixing, YIN Liangliang, GAO Xing, CHEN Juanping

2025, 44(2): 382-397. doi: 10.11932/karst2024y040

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In the Ordos Basin, the Taiyuan Formation mainly consists of a marine-terrestrial transitional sedimentary system. The northern part featured shallow water delta deposits dominated by terrigenous clastic rocks, while the southern part consists of coastal marine limestone deposits. The Shenmu gas field was discovered in the northern terrigenous clastic rock strata. The southern limestone strata have a wide distribution, covering approximately 14 square kilometers, and exhibit considerable thickness, ranging from 5 to 30 meters. However, the limestone reservoir is compact with an average porosity of 2.1%, an average permeability of 0.21 mD, and an average throat radius of 0.12 μm. The exploration of this reservoir has not been fully acknowledged for an extended period. Recent years, further research has indicated that the high-yield gas wells are often related to the development degree of structural fractures. However, there is a lack of systematic analysis of whether the quality of the limestone reservoir and the natural gas enrichment relating to the structural development.To overcome the above difficulties, the following steps have been studied and adopted in this study. Considering the tectonic setting, the structural fracture characteristics of the Taiyuan limestone were firstly analyzed by means of field profiles, core observations, and well log data. Secondly, the impact of structural fractures on the porosity of compact limestone reservoirs was observed by uniaxial loading rock core Nuclear Magnetic Resonance (NMR) technology and CT scanning for the first time. Furthermore, fluid inclusions were investigated to analyze the conductive role of the fracture networks during the gas accumulation process. Lastly, based on the aforementioned work, an Improved Gaussian Curvature method (IGC) was proposed to predict the development of structural fractures and identify favorable exploration blocks, tailored to the specific geological conditions. This method was subsequently applied in well deployment in the Hengshan area.The results show, (1) In the study area, regional tectonic stress field is mainly influenced by the Yanshanian Movement, which has developed NW–SE compressive stress. The tectonic stress field in the Yanshanian Period established the main tectonic framework of the study area. Compared with low-angle fractures and oblique fractures, high-angle tension fractures have lower degree of filling and better effectiveness. They often penetrate, cut, and displace other types of fractures, thus improving the limestone reservoir properties. In addition, the fracture network system formed may facilitate the efficient migration of hydrocarbons. (2) Physics experiments on rocks have manifested that structural fractures significantly influence the physical properties of compact limestone reservoirs. The average porosity doubled from 2.1% to 4.2% due to the development of fractures. The contribution of fractures to total porosity ranged from 14.3% to 72.7%, with an average of 43.4%. Among the three types of reservoir rocks, powder crystal limestone exhibited the greatest increase in porosity, followed by algae-bound limestone, while mud crystal limestone showed the smallest increase.(3) By combining fluid inclusion analysis with typical well reservoir-forming temperatures, burial history, and thermal evolution history in the study area, this study has drawn a conclusion that the extensively developed fracture networks have created an effective conductive system. During the main reservoir-forming period from the Late Jurassic to the Early Cretaceous, hydrocarbons were effectively transported through the fracture network system. The main coal-bearing source rocks, namely, # Seam No. 8 and Seam No. 5 located at the top and the bottom of limestone reservoir, generated natural gas that migrated into the limestone reservoir through the fault system for accumulation. With the direct seal provided by the marine mudstone at the top of the Taiyuan Formation, an excellent "sandwich"-style hydrocarbon accumulation has formed. (4) IGC can effectively predict the degree of development of favorable structural fractures. A higher IGC value indicates a greater development of high-angle tension fractures, which can create more favorable conditions for natural gas enrichment.This method was first applied in the Hongshan area to support the layout of wells. The first well for risk exploration, Well YT1H*, achieved a gas production of 540,000 cubic meters per day, realizing a breakthrough in the exploration of the limestone in the Taiyuan Formation in the Ordos Basin. In the past two years, a total of nine industrial gas wells have been successively discovered in the study area. Five of these wells produced more than 100,000 cubic meters per day. Among them, Well YT4H* achieved an output of 1.088 million cubic meters per day. The above exploration results demonstrate the effectiveness of this study, providing a reference for re-evaluating compact carbonate reservoirs. It is expected that this study can further guide the large-scale exploration and development of the limestone in Taiyuan Formation in the Ordos Basin.

Characteristics of paleokarst processes and reservoir development of the middle Permian Maokou Formation in the Yuanba area,Sichuan Basin

JI Shaocong, ZHANG Qingyu, BA Junjie, NIE Guoquan, DONG Hongqi, MO Guochen, ZHANG Meng

2025, 44(2): 398-409. doi: 10.11932/karst20250215

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In recent years, the Maokou Formation of the middle Permian in the Yuanba area of Northwest Sichuan has consistently yielded high-flow industrial gas, indicating significant exploration potential for the Maokou Formation in this area. Drilling has confirmed the presence of exposed karst features within the Maokou Formation, indicating that the reservoir space mainly consists of fracture-pore types. The karst fracture-pore reservoir type is one of the important reservoir types found in the Maokou Formation in this area. Previous studies have been conducted on the stratigraphic division, lithofacies paleogeography, karst layer groups, and karst paleogeomorphology of the Maokou Formation in Northwest Sichuan. However, systematic research on paleokarst stages of the Maokou Formation in the Yuanba Area, as well as its impacts and other related aspects, has been limited. This study investigates the development characteristics and distribution patterns of karst reservoirs in the Maokou Formation of the Yuanba area in the Sichuan Basin, based on a study on the geological background of ancient rock dissolution. The investigation utilizes field profiles, drilling cores, logging, and analytical testing data. This study combines geochemical analyses of the filling materials found in paleokarst fractures and pores, to explore the stages of ancient rock dissolution and its impacts on karst reservoirs. The findings will support predictions regarding the karst reservoirs in the Maokou Formation in the Yuanba area in subsequent research. The research results indicate, (1) Taking the top surface of a section in the Maokou Formation as the underlying reference surface, the residual thickness method was used to restore the karst paleogeomorphology of the top surface of the Maokou Formation in the study area. The karst paleogeomorphology of the top surface was categorized into four secondary geomorphological units and six tertiary geomorphological units. Overall, the karst paleogeomorphology on the top surface of the Maokou Formation in the study area is characterized by micro-geomorphological features, with locally developed mature landforms that indicate the initial stages of karst formation and evolution. The surface drainage system is not fully developed and is influenced by the topography, with runoff flowing northward and southwestward from the central region. (2) Based on the results of field profiles, drilling core observations, thin section identification, and scanning electron microscopy observations, the rock types of reservoirs in the Maokou Formation within the study area are mainly mud-crystal bioclastic limestone, bright-crystal bioclastic limestone, and a minor amount of bioclastic mud-crystal limestone. The types of storage space are classified into three categories: dissolved pores, caves, and fractures. (3) According to the classification guidance formulated by the Institute of Karst Geology, Chinese Academy of Geological Sciences, karst reservoirs in the Maokou Formation are vertically classified into four karst development zones, that is, surface karst zone, vertical infiltration and dissolution zone, runoff dissolution zone, and phreatic dissolution zone. This classification was based on the observation of field profiles, drilling core samples, logging, and the analysis of well logging and drilling records in the study area. The vertical distribution of karst reservoirs in the Maokou Formation is primarily concentrated within 0–20 m below the weathering crust surface, corresponding to the surface karst zone. (4) The analysis of carbon and oxygen isotopes, along with inclusion testing of the filling materials in the paleokarst fracture-pore of the Maokou Formation in the study area indicates that the paleokarst fracture-pore system of the Maokou Formation was formed in four distinct karst environments, a contemporaneous or pene-contemporaneous karst environment, a freshwater karst environment in the hypergene period, a karst environment in the shallow-burial period, and a high-temperature karst environment in the deep-burial period. (5) Comprehensive analysis suggests that favorable sedimentary facies such as open platforms or platform edges are the material basis for the development of karst reservoirs in the Maokou Formation in the study area. Epigenetic karstification is a key factor in the development of karst reservoirs in the Maokou Formation in the study area, and its degree of development is controlled by conditions such as the karst layer group, ancient landforms, and ancient water systems. The shallow-deep-burial process has transformed and adjusted the early reservoir space, further increasing the heterogeneity of the reservoir.

Reservoir types and well logging identification of the Sinian Dengying Formation in the Central Sichuan region

ZHOU Zheng, HE Hongju, LI Dahua, ZHANG Ye, CHEN Hongkai, WANG Xingzhi, YU Tao, HE Li, SU Huan

2025, 44(2): 410-418. doi: 10.11932/karst2024y044

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The Sinian Dengying Formation, situated in the Central Sichuan region, lies at the eastern high point of the Longnvsi ancient uplift axis in Leshan. This formation possesses both oil source and preservation conditions, making it a promising site for exploration. It is an important target and successor layer for oil and gas exploration in the Sichuan Basin. In 2011, the China National Petroleum Corporation (CNPC) initiated the exploration of oil and gas resources at Well Gaoshi-1 in the Dengying Formation. This endeavor resulted in the successful discovery of oil and gas reservoirs within the Dengying Formation. Therefore, many scholars have conducted a lot of relevant research. However, they have only reached a consensus on the lithology and stratigraphic classification of the strata. There is still great controversy over the results of classifying reservoir types. In the process of oil and gas exploration, understanding the type of reservoir is an important step and a key factor that will directly affect the success of oil and gas exploration and development.The Dengying Formation in the Sichuan Basin has undergone seven major tectonic movements over a geological history of about 600 million years. These movements have led to significant changes in burial depthes and a range of diagenetic processes, resulting in a variety of reservoir space types. As a result, the combination of reservoir spaces is also very complex, leading to significant controversy in the classification of reservoir types. Understanding the type of reservoir is an important link in oil and gas exploration and development, as it directly affects the success of these endeavors. Therefore, this article utilizes rock cores and drilling logs from more than 30 wells in Central Sichuan and its surrounding areas as first-hand data. It conducts in-depth analysis of the reservoir types and logging identification characteristics of the Dengying Formation.In the research process, the existing drilling cores, field profiles, and thin sections of rocks in Central Sichuan and its surrounding areas were used as the main research subjects to study the rock types and storage spaces that constitute the reservoir. The results show that in the Dengying Formation, the lithology is predominantly dolomite, with a small amount of mudstone and siliceous rock. The rock types that form the reservoir are diverse, mainly including grain dolomite, sandstone dolomite, algal laminated dolomite, and karst breccia. The observation of thin sections and rock cores indicates that the current reservoir space is mainly composed of pores, holes, caves, and fractures formed by secondary processes. The pore-type space refers to that with a pore diameter less than 2 mm, mainly including intercrystalline dissolution pores, intergranular pores, and intergranular dissolution pores. The hole-type space refers to that with a pore diameter greater than 2 mm and less than 100 mm, mainly including karst holes along the direction of algal layers, porous karst holes, fractured karst holes, intergranular holes, and residual holes bordered with the shape of grape. The caves-type space refers to that with a pore diameter greater than 100 mm. The fracture-type space refers to a fracture formed by tectonic action, which significantly enhances the storage capacity and permeability of reservoirs in the Dengying Formation.In Central Sichuan, the storage spaces of the Dengying Formation are interconnected and distributed in a combined manner. Based on the primary or dominant reservoir spaces, reservoir types are classified into four categories, pore-holes type, fracture-pore-holes type, pore type, and cave type reservoirs. Each reservoir type exhibits distinct logging response characteristics. With the use of existing conventional logging data and micro-resistivity scanning imaging logging data in the study area, logging identification standards for different reservoir types have been established. This provides new insights for the exploration and development of oil and gas in the Dengying Formation in Central Sichuan.

Characteristics and dominant controlling factors of karst reservoirs in the middle Permian Maokou Formation of the eastern Sichuan region

YANG Rong, YANG Xiyan, FAN Cunhui, LI Yang, LI Yue, HUANG Zisang

2025, 44(2): 419-433. doi: 10.11932/karst20250216

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The middle Permian Maokou Formation in the eastern Sichuan region is characterized by the widespread development of karst reservoirs. However, systematic analysis of the identification criteria and controlling factors of these karst reservoirs remains insufficient. This study integrates extensive field outcrops, drilling cores, thin sections, well logging and well logging interpretation, and seismic data to comprehensively analyze the response characteristics of karst reservoirs in the Maokou Formation of the eastern Sichuan region. Furthermore, this paper elucidates the fundamental characteristics and distribution patterns of these reservoirs while conducting a comprehensive evaluation of their primary controlling factors. The key findings are as follows. (1) Karst reservoirs exhibit a variety of identification markers. The petrological response characteristics are as follows: residual dissolution pores, ranging from 2 mm to 70 mm in diameter, and fractures—mainly high-angle structural fractures—are generally developed in both cores and outcrops. Additionally, the caves are mostly half-filled with calcite, dolomite, or asphalt, while some remain unfilled. Bauxitic clay rocks and karst breccia containing limonite are visible at the top of the stratum, indicating the existence of ancient weathering crust. The logging response characteristics are as follows, the acoustic time difference, compensated neutron curves and compensated density curves are "box-shaped" in the karst cave section. The phenomena of pronounced acoustic time difference leap are observed, accompanied by a reduction in the positive difference of deep and shallow dual lateral resistivity. Imaging logging shows the black patchy karst cave characterized by low resistance and high conductivity. Natural gamma values in the karst cave section is generally lower than 40 API, with low content of radioactive elements. The logging response is characterized by frequent well leakage and venting during drilling, mainly occurring in the range of 0 m to 100 m from the top of the Maokou Formation. High-yield gas wells are mostly located in the second and third members of the Maokou Formation, reflecting the development of fracture-cavity reservoirs. The seismic response characteristics are marked by "low-frequency, high-amplitude, and wide-crest" reflections at the top of the Maokou Formation. In the dense area of karst caves, "bright spots" anomalies appear. The development of karst caves reduces wave impedance difference, resulting in downward-dragged and chaotic reflections. (2) The rocks of karst reservoirs are mainly composed of grain limestone. The main reservoir spaces consist of dissolved pores and caves, with caves being the most significant reservoir space, accounting for more than 65 %. These caves are predominantly distributed in isolated patterns, exhibiting poor connectivity. Dissolved pores, including intragranular, intergranular, and intercrystal types are predominantly filled. The measured porosity of the karst reservoir ranges from 2.0% to 8.7 %, with an average of 3.4 %. The permeability varies from 0.008 mD to 65.263 mD, averaging 18.3 mD, showing the characteristics of low porosity and low permeability. Among the three types of karst reservoirs, fracture-cavity reservoirs exhibit the most favorable physical properties, with a porosity of 3.3 %, and a permeability of 23.3 mD. In contrast, the cavity-type reservoirs demonstrate inferior physical properties due to the lack of filtration channels. Karst landforms are categorized into karst highlands, steep karst slopes, gentle karst slopes, and karst basins. The porosity of steep karst slopes (3.96 %) is significantly higher than that of gentle karst slopes (3.27 %) and karst highlands (2.10%). Vertically, the karst reservoirs are mainly distributed within 100 m from the top of the Maokou Formation, particularly in the Mao 2a sub-member and the Mao 3 member. These reservoirs exhibit poor lateral ductility, presenting as isolated point-like or belt-like distributions. Horizontally, there is a NW–SE zonal distribution in the Linshui–Fengdu area, with reservoir thicknesses ranging from 7.68 m to 11.04 m. (3) Karst reservoirs are influenced by the combined effects of sedimentation, karstification and tectonism. Specifically, the grain shoals (intra-platform shoals and platform margin shoals) are mainly developed in the Mao 2 and the Mao 3 members. The thickness of the granular limestone is positively correlated with the thickness of the reservoir ( R2 = 0.82 ), which indicates that the grain shoals provide a high-quality material foundation for the karst reservoirs and offer percolation channels for diagenetic fluids in later stages. The karst paleogeomorphology controls the distribution of karst reservoirs. High-yield wells are mostly located in the geomorphic units of residual mounds on steep karst slopes and gentle karst slopes. The transitional zones between karst highlands and karst steep karst slopes are also favorable areas for the development of karst reservoirs. Multiple stages of fracturing improve the reservoir storage performance. Specially, three stages of fractures came into being during the Dongwu Movement (early high-angle fractures), the Indosinian–Yanshan Movement (mid-term X-shaped fractures), and the Himalayan Movement (late horizontal fractures). The late horizontal fractures intersected the earlier fractures and connected with karst caves, significantly enhancing the permeability. For instance, the fracture density of Well W78 reached 11 fractures/m, resulting in a substantial increase in daily gas production. These fractures provide channels for oil and gas infiltration, facilitating the accumulation of large oil and gas reservoirs.(4) Based on the analysis of the dominant controlling factors, two favorable areas have been identified. The first is the Linshui–Fengdu steep karst slope zone, which is characterized by thick grain shoals reaching up to 23 m in the Mao 3 Member, the well-developed residual mound landform, dense karst caves and fractures, all of which contribute to optimal reservoir physical properties. The second is the Dazhou–Kaijiang gentle karst slope zone. This zone features the superposition of multi-stage fractures, particularly med-term fractures, which significantly enhance permeability. Although karstification is relatively weak, this area has the potential of high-yield production.

Comprehensive comparison and development strategy suggestions between the edge and interior of the fourth section of the Sinian series lamps in the Gaoshiti block

HU Hao, WANG Min, LONG Hui, JIANG Lin, SHEN Qiuyuan, WU Dong, TIAN Xingwang

2025, 44(2): 434-444. doi: 10.11932/karst20250217

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The fourth section of Dengying Formation in the Gaoshiti block of the Anyue Gas Field is of complex lithology and strong heterogeneity. There is a significant difference in the single-well production capacity as well as in production efficiency between the platform margin and the intra-platform. To achieve long-term stable development of the gas reservoir in the fourth section of Dengying Formation at the platform margin, as well as efficient production of the corresponding section in the intra-platform, the controlling factors leading to the observed differences have been identified. This was accomplished through a combination of dynamic and static methods, which allowed for a comprehensive analysis of the common and distinct characteristics of the gas reservoirs in both the platform margin and intra-platform. This analysis was based on a substantial amount of geological, logging, gas reservoir engineering and other relevant data.The reservoir rock types and reservoir space in both the platform margin and the intra-platform share common characteristics. The lithology is primarily composed of doloarenite, algal-laminated dolomite, and algal-clotted dolomite. These rocks are all well developed in terms of dissolution pores. The porosity of the reservoir core ranges from 2.0% to 7.3%, with an average of 3.5%. The permeability ranges from 0.1 mD to 113 mD, with an average of 5.832 mD. The reservoir properties exhibit low porosity and low permeability, and the reservoir type is primarily characterized as fracture-pore (cave) type.However, there are differences between the platform margin and the intra-platform in terms of reservoir development scale, degree of dissolution pore development, reservoir physical properties, seepage characteristics, and characteristics of gas well production. At the platform margin, the proportion of wells with single-well test output larger than 500,000 cubic meters of gas is higher. The development degree of large pores and caves is greater, and the density of fracture development is also higher. In terms of production, the gas wells at the platform margin generally exhibit high and stable production, with an average daily gas production of 233,000 cubic meters per well. In contrast, the gas wells in the intra-platform show significant production differences, with a daily gas production of 145,000 cubic meters. In terms of seepage characteristics, the reservoir type at the platform margin is primarily classified as fracture-pore type, with fractures and fracture-cave systems serving as the main seepage channels. These wells exhibit relatively high test production rates and are capable of maintaining high and stable production.This paper comprehensively analyzes the reasons for the differences from four aspects, sedimentary environment, diagenesis, matching of reservoir conditions, and karst transformation. It clarifies that the sedimentary environment and reservoir conditions control the scale of reservoir development, while karst transformation in later stages plays a decisive role in the seepage characteristics and production differences of gas wells. The favorable shoal and lagoon bodies control the distribution of high-quality reservoirs, and the intensity of epigenetic karst transformation during the Tongwan Period determines the strength of dissolution and transformation. Additionally, the relationship between source and reservoir affects the efficiency of hydrocarbon migration and accumulation. Therefore, the development scale of shoal and lagoon deposits, the intensity of epigenetic karst transformation, and the source-reservoir relationship are the primary controlling factors determining the differences between the platform margin and the intra-platform.Based on a comprehensive comparison between the platform margin and the intra-platform, the former exhibits the thicker reservoir, superior physical properties, and dominant storage space types characterized by fracture-pore (cave) patterns. It also features a larger controlled radius, better fluid flow, a higher proportion of high-yield wells, and the potential for long-term, stable high production. In contrast, the intra-platform has the relatively thinner reservoir, comparable physical properties, and greater horizontal heterogeneity, resulting in more significant differences in the development of high-quality reservoirs. The production capacity of gas wells primarily comes from medium-and-low-yield wells, with a smaller proportion of wells capable of stable production. Given this comparison between the two zones and the production and development needs of the Gaoshiti block, the differentiated production suggestions are proposed. For the platform margin, the detailed characterization of high-quality seepage bodies within reservoir should be conducted, and the development of supplementary wells should be implemented to achieve stable production of gas reservoirs. For the intra-platform, the identification of favorable site distributions should be prioritized, and the production wells should be deployed to ensure efficient production.

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2025 Vol. 44, No. 2

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Current Issue 2025 Vol. 44, No. 2

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Current Issue
2025 Vol. 44, No. 2