中国岩溶

2023, 42(4): 1-2.

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Problems and their countermeasures in a hydrogeological survey of karst areas in South China

WANG Yu

2023, 42(4): 627-635. doi: 10.11932/karst20230401

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Basic hydrogeological surveys mainly targeted at the evaluation of groundwater resources were launched after the founding of the People's Republic of China, and have been advancing with the change of social needs and the development of relevant disciplines at home and abroad. Regional hydrogeological surveys nationwide mainly include the 1∶200,000 surveys from 1974 to 1982, mainly estimating the average groundwater recharge and runoff by the precipitation infiltration coefficient method and the runoff modulus method, and the utilization rates of the results are still very high. From 1999 to 2002, the 1∶250,000 groundwater resources survey and ecological environment geological survey were completed. Based on the 1∶200,000 hydrogeological survey, the latter was carried out by monographic study, compilation and measurement. In this kind of survey, changes of groundwater resources and problems of geologic environment in the past 20 years were initially identified. Meanwhile, taking watershed as a unit, the average groundwater recharge was estimated by precipitation infiltration coefficient method and runoff modulus method. The storage capacity of water-rich blocks, large spring and underground river systems were estimated by volume method and spring flow attenuation coefficient method. Exploitable amounts were estimated by observation and statistical analysis of spring and underground river flow and the section method of basin (valley) area. The potential of groundwater resources was estimated for the first time. Since 2003, the Institute of Karst Geology of China Geological Survey has continuously organized and implemented 1∶50,000 surveys on hydrogeological geologic environment in the southern karst areas, with a survey area of more than 30×10⁴ km². Field surveys were carried out according to the topographic map of 1∶50,000, and the groundwater resources were evaluated according to different basins. The recharge was calculated mainly by the precipitation infiltration coefficient method; the storage capacities of water-rich blocks, big springs and underground rivers were calculated by the volume method and the spring flow attenuation coefficient method; exploitable amounts of springs and underground rivers were calculated by the flow observation and statistical analysis; exploitable amounts of the water-rich blocks were calculated by theoretical and empirical formulas. The potential of groundwater resources was estimated according to the degree of exploitation and utilization of groundwater resources. The evaluation results of groundwater resources are mainly the level-D exploitable amount, that is, the inferred potential resources. This study primarily aims at countermeasures of the problems discussed here and the corresponding grades of groundwater resources. By reviewing a large number of data and materials about inspection records and comments of 1∶50,000 hydrogeological survey projects in karst areas of South China, referring to relevant literature and comparing with current norms and management requirements, this study analyzes and summarizes the existing problems and deficiencies of some investigation projects. These problems mainly include poor research work and results, insufficient connection between theoretical research and practice, insufficient Quaternary geological surveys, poor drilling control and low successful rates of water exploration, and incomplete evaluation of water resources. In addition, this study explores the subjective and objective reasons for these problems, for example, complexity of the karst hydrogeological conditions in South China, less accuracy of geophysical water exploration, relatively low quality of project results and the assessment index of the research, and project members' insufficient professional knowledge and experience. Finally, this study proposes some suggestions to improve hydrogeological survey, such as making effective organization and implementation plans, promoting comprehensive research in a standardized, continuous and efficient way, strengthening research on the selection of research topics, implementing effective monographic study, improving the accuracy of Quaternary geological mapping, strengthening the identification analysis, improving the proof of hole location and the exploration efficiency, strengthening the analysis of resources and environment, and improving the evaluation content.

Several key issues in the application of MODFLOW-CFP software to the numerical simulation of karst water systems

TAN Jiahua

2023, 42(4): 636-647. doi: 10.11932/karst20230402

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The MODFLOW-CFP software effectively addresses the coupling issue between pipe flow and groundwater seepage in porous media, as well as the transition from laminar to turbulent flow in pipe water movement. It provides a valuable numerical simulation platform for the quantitative study of karst conduit groundwater systems and has been applied in practical engineering by some researchers in recent years. However, due to limitations in survey methods, particularly for linear projects such as railways, highways, and water conservancy, the survey accuracy is relatively low. It is challenging to accurately characterize the structure of karst conduit systems in the presence of fractures and dissolution channels in karst aquifers, and the simulation results are often unsatisfactory. To better utilize and promote the application of this software in the quantitative evaluation and study of karst groundwater systems, and to avoid the blind use of new methods in practical applications and improve the simulation accuracy and prediction precision of numerical models for complex karst water systems, further development of quantitative hydrogeological research in karst geology is needed. Based on an analysis of the basic principles of MODFLOW-CFP simulation software and the authors' extensive experience in karst hydrogeology, this study conducts an in-depth analysis in various aspects. These include determining the model scope and boundary conditions, quantifying karst conduit structures, acquiring hydrogeological parameters, and selecting objective functions. Additionally, this study discusses key technical issues and possible solutions in the simulation process, taking the Daiye cave karst groundwater system in Yongshun county, Hunan Province, as a case. The research results indicate as follows. ① Detailed investigations of karst hydrogeology, including groundwater tracing, drilling, and geophysical exploration, are essential for understanding the hydrogeological conditions, finely partitioning the karst water system, and establishing accurate conceptual hydrogeological models as the basis for numerical simulations. ② For the determination of the model scope, we should first consider selecting a complete groundwater system based on engineering requirements. In terms of the boundary conditions, we should then consider the changes in groundwater system equilibrium caused by the engineering activities, especially for soft boundaries such as watersheds. ③ Conducting groundwater tracing test, obtaining breakthrough curves of tracer concentration, and using the Qtracer2 model can effectively characterize the conduit structures and acquire the relevant parameters required by the CFP module. ④ Comprehensive and long-term monitoring data on rainfall, flow rate, water level, water chemistry, temperature, etc. are crucial to accurately obtain hydrogeological parameters, select appropriate objective functions, and improve the simulation accuracy of the model. ⑤ Currently, model identification mostly relies on the groundwater flow rate or the groundwater level as the sole objective function. Incorporating multiple conditions simultaneously as objective functions for model identification is less common but requires further research to improve the predictive accuracy of models.

Remediation of polluted sites in the typical area of karst underground river based on "Three-Source Model": A case study in the Pingqiao underground river system, Zunyi, China

YI Shiyou, JIAO Heng, ZHOU Changsong, GAO Feng, CHEN Tao

2023, 42(4): 648-661. doi: 10.11932/karst202304y02

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Karst groundwater is an important water resource in karst areas of China and even the world, playing a crucial role in residents' daily life and industrial and agricultural production. The soil layer in karst areas is generally thin, with a double-layer structure between the surface and underground. Pollutants can directly enter the underground aquifer through thin soil layers, sinkholes, skylights, and karst cracks, making the karst groundwater extremely susceptible to pollution. In recent years, human's industrial and agricultural activities and domestic pollution have caused a large number of various types of organic and inorganic pollutants to continuously enter the underground river, posing a huge threat to the safety of drinking water and ecological agriculture development of local residents. Therefore, pollution remediation of underground rivers is of great significance for the protection of karst underground water resources. Focusing on water sources and pollution sources, this study conducts a systematic analysis of the investigation, evaluation, and remediation process of underground river pollution. It establishes a pollution remediation and control model of karst underground rivers—the "Three Source Model". This model mainly includes dual-source investigation, source tracking, and source control. As a typical dendritic underground river in the exposed karst area of Southwest China with a pollution history of nearly 20 years, the Pingqiao underground river in Zunyi City, was selected as the study area for remediation and control of the pollution in the underground river with the use of "Three-Source Model". The results show: (1) There are 25 water points in the study area, mainly boreholes, karst springs and underground river outlets, and the characteristic pollutants are mainly NH$_4^{+}$, ${\rm{NO}}_3^{-}$, ${\rm{SO}}_4^{2-}$, Mn²⁺, and Se²⁺. Fifteen types of pollution source points are distributed, mainly including industrial waste disposal sites in the area of Pingqiao Industrial Park downstream of the underground river system. The characteristic pollutants are also mainly NH$_4^{+}$, ${\rm{NO}}_3^{-}$, ${\rm{SO}}_4^{2-}$, Mn²⁺, and Se²⁺. (2) There are mainly three groundwater pollution channels within the scope of the underground river system: PQE019 (sinkhole)-PQG025 (skylight)-PQS009 (underground river outlet); the flood discharge shaft of 2^# waste residue disposal site-CK8 (borehole)-J02 (monitoring well)-CK6 & CK11 (borehole)-JC04 (monitoring well) & ZK2 (borehole)-PQG025 (skylight)-PQS009 (underground river outlet); 1^# waste disposal site-PQG025 (skylight)-PQS009 (underground river outlet). The above pollution channels are distributed between the outlet of the underground river and Z1 waste disposal site in Pingqiao Industrial Park. (3) By curtain engineering, remediation was conducted in the upstream section of the polluted channel from the flood discharge shaft of 2^# waste disposal site to the outlet of the underground river, and then the groundwater level upstream of the curtain steadily increased from 815.68 m to 823.35 m, but there was no significant change in the groundwater level downstream of the curtain. The characteristic pollutants in the polluted water extracted from the upstream drainage wells (PWZK1 and PWZK2) of the curtain had a maximum NH$_4^{+}$ content of 469 mg·L⁻¹ (exceeding 938 times of a permitted level) and a maximum Mn²⁺content of 254 mg·L⁻¹ (exceeding 2,540 times of a permitted level). During the stage when the wells (PWZK1 and PWZK2) were not pumped, NH$_4^{+}$ content at the outlet of the underground river was 76-143.6 mg·L⁻¹, and Mn²⁺content 14.206-21.31 mg·L⁻¹, with the outlet flow rate of 5.0-295.349 L·s⁻¹. In the stage of intermittent pumping in these two wells, concentrations of NH$_4^{+}$ and Mn²⁺at the outlet of the underground river showed a downward trend during the pumping period, while concentrations of NH$_4^{+}$ and Mn²⁺at the same place showed a significant upward trend during the cessation of pumping. During the stage of continuous pumping, concentrations of NH$_4^{+}$ and Mn²⁺at the outlet of the underground river remained relatively low, with NH$_4^{+}$ content generally ranging from 14.4 to 58.6 mg·L⁻¹ and Mn²⁺content from 2.38 to 6.39 mg·L⁻¹. Concentrations of NH$_4^{+}$ and Mn²⁺decreased by 66% -78%, compared to concentrations in the stage when the wells were not pumped. The calculation results of reducing pollutant emissions in the treatment project show that the underground river outlet can reduce wastewater emissions by 47,244 m³·a⁻¹, with a minimum reduction of 16,250 kg·a⁻¹ for NH$_4^{+}$ and 10,960 kg·a⁻¹ for Mn²⁺. The research results can provide reference for the remediation of pollution in underground river systems in similar regions.

Characteristics of long series data of groundwater quality and level in Kunming basin

KANG Xiaoli, ZHOU Junrong, GAO Yu, ZHANG Hua

2023, 42(4): 662-671. doi: 10.11932/karst20230403

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Kunming basin is a representative of karst faulted basin, with a basin bottom area of 846.19 km² and a fluctuation of 1-82 m averaging at 19 m. With the city development, Kunming has gradually become one of the megacities in China. However, as the center of intensive human settlements and activities, the contradiction between human and land is prominent, and the problems of resources and environment are serious. Since the 1960s, the dynamic monitoring of groundwater in the urban and suburban areas of Kunming has been carried out in a comprehensive and systematic manner. In this study, hydrogeology method, data statistics method, Piper and Matlab software were used to establish models to analyze the dynamic characteristics and change of water quality and water level, based on the comparative analysis of monitoring data of water quality and level in Kunming basin, especially in the main urban area. The main underground chemical types of Kunming basin are HCO₃-Ca and HCO₃-Ca·Mg. In recent years, with the gradual increase of Na⁺+ K⁺ and ${\rm{SO}}_4^{2-} $ +Cl, the hydrochemical types have also gradually been increasing and become more complex. According to the water quality evaluation, the groundwater quality in Kunming basin has generally shown a downward trend in recent 15 years. The pore water quality is mainly of Class IV and Class V, and the proportion of Class V water has increased rapidly, from 30.2% to 62.5%. The bedrock water quality is mainly of Class III and Class IV. The proportion of Class IV water and Class V water has risen rapidly from 11.1% and 8.6% to 33.3% and 26.7%, respectively. Mn, NH$_4^{+}$, ${\rm{NO}}_3^{-}$ and COD are the main items exceeding permitted levels in pore water. Mn, NH$_4^{+}$ and F are the main items exceeding permitted levels in karst water. In recent years, I, ${\rm{SO}}_4^{2-}$, As, Cd and other toxic, harmful and organic substances have exceeded permitted levels, showing a compex tendency. The reason for the change of water quality is the intensification of industrial and domestic pollution. The results show that the interannual variation of groundwater level in Kunming basin is mainly affected by industrial and agricultural production, mining amount and engineering activities, and pore water decreased from 2002 to 2009, but increased after 2009. Karst water showed a downward trend from 2002 to 2012, and an upward one after 2012. The groundwater levels of Puji-Liangjiahe, Beijiaochang, the urban area of Kunming, Jinmasi-Guanshang and other water-rich blocks have risen significantly. The rising trend of groundwater levels in Cuihu, Heilongtan, Beijiaochang, Majie, Heilinpu and other five falling funnel areas is significant, which is closely related to the measures of restricting groundwater exploitation and sealing groundwater exploitation wells in Kunming City. Based on the dynamic analysis of water quality and water level, countermeasures and strategies to protect groundwater should be proposed through enhancing administrative management, strengthening the investigation, evaluation and monitoring of basic groundwater environment, and furthering the research on technology of preventing groundwater pollution.

Research on genetic types and flow system characteristics of wetland in karst fault basin, China

ZHANG Hua, WANG Bo, GAO Yu, KANG Xiaoli, WANG Yu, LIU Shaohua, KANG Xiaobo, LUO Weiqun, ZHAO Yong

2023, 42(4): 672-684. doi: 10.11932/karst20230404

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As an important part of the ecosystem of Yunnan Plateau, karst wetlands are widely distributed in Yunnan fault basin. However, due to various reasons, lakes are silted up; the water surface area shrinks and even disappears. Lakes in eastern Yunnan are degraded significantly, and more than 20 natural lakes have prematurely disappeared in the past 30 years. In this paper, three typical types of karst wetland in the karst fault basins in eastern Yunnan are studied. These types are Tonghai lake degradation type (Qiluhu wetland), Luxi groundwater accumulation type (Huangcaozhou wetland) and Xuanwei underground river silting type (Geyi beaded wetland). Tonghai Qiluhu karst depression basin is located in the central part of Yunnan Province. It is a karst depression basin developed in the front arc of Yunnan mountain structure. The basin is a closed lake basin with a bottom length of about 22 km, a width of 9 km and a catchwater area of 345.95 km². The drainage basin of Luxi karst fault basin is located in the marginal slope area of southeast Yunnan Plateau, covering an area of 1,009.28 km². The bottom of the basin is distributed to the northeast in an irregular oval shape, with a longitudinal length of 16-20 km and a transverse width of 4-5 km. The overall terrain is high in the east and low in the west, and high in the north and low in the south. Xuanwei Geyi karst fault basin belongs to Beipanjiang third-level basin, Gexianghe fourth-level basin, and fifth-level Tianbian underground river system. The whole Tianbian underground river system is divided into a four-level discharge datum. One is Geyi Dalongtan on the edge of Rongqiu Depression, and the other is the beaded lake in Rongqiu Depression around Geyi town. The third is the bottom discharge of the Wujia family depression, and the fourth is the final discharge of the Gexiang river, with a watershed area of 160.82 km². The research contents include the genetic types and characteristics of wetlands, and the characteristics of "three fields" (runoff field, hydrochemical field and temperature field) of wetlands. The geological conditions, hydrogeological conditions and influencing factors of water environment of naturally formed wetland in karst fault basin are analyzed and the quality of water environment is quantitatively evaluated. The natural and human influencing factors, forms, intensity and effects of karst wetland, water source reduction, pollution intensification, water quality decline, wetland atrophy and ecological environment problems are also analyzed. The research results indicate that the karst characteristics of the three types of karst fault basin wetlands are relatively similar, with frequent transformation of surface and groundwater, both of which take underground karst pipelines as the medium for transformation channels. Wetlands are discharge areas and also runoff areas that supply the outlet of underground rivers. All of the three types of wetlands are discharged to surface rivers through developed karst pipes. The lake degradation type (Qilu lake) collects surface water lakes at the bottom of the basin from the whole basin; the groundwater accumulation type (Huangcaozhou) is located in the area at shallow water level in the middle of the basin, and pore water and karst water are also recharge sources of wetlands. For the underground river silting type, the karst pipe blockage in the early groundwater runoff channel changes to the present karst fracture runoff, due to the topography of peak-cluster depression. Second, the wetland of lake degradation type (Qilu lake) covers a large area, which is greatly affected by natural and human factors. It is discharged by floods during the rainy season, and is used to irrigate during the dry season. Its severe water pollution is difficult to control with degradation of water quality and quantity. The groundwater accumulation type (Huangcaozhou) is controlled by the groundwater recharge of the upper spring, the underground river and the wetland itself, and the water quality is generally good. Although underground river of the silting type (Geyi beaded wetland) is affected by regional groundwater and human activities, the groundwater is generally abundant and relatively high in quality because the local population is relatively small with little impact. Thirdly, three types of water system structures are studied in terms of the characteristics of supplementation and drainage. The flow system structure of of Qiluhu degraded wetland in Tonghai is, surface collection→karst pipe drainage. The flow system structure of groundwater accumulation wetland in Huangcaozhou of Luxi is: surface runoff+underground seepage→surface runoff→karst pipe discharge. The flow system structure of Xuanweigeyi beaded wetland is, underground fissure seepage→spring drainage→karst pipe drainage. The study findings suggest that we should protect and utilize wetland resources, and carry out long-term monitoring of wetland quality, water quantity and water level. We should protect wetlands and restore water environment by water transfer and physicochemical and biological measures. We should carry out afforestation and shrub planting around wetland to beautify wetland environment and conserve water source. We should improve farming patterns and use green pesticides. This study will provide a geological basis for the protection, scientific research, utilization and ecological restoration of wetlands in karst fault basins.

Study on the enrichment characteristics and water supply significance of karst groundwater in the Yili river basin, Wumeng Mountain area

DENG Guoshi, CEN Xinyu, TANG Yeqi, ZHONG Jinxian

2023, 42(4): 685-698. doi: 10.11932/karst20230405

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The deepened karst development caused by its uneven development of karst and significant uplift of the Cenozoic crust results in the extremely uneven spatial and temporal distribution of karst groundwater, which makes it very difficult to extract and utilize groundwater. Therefore, understanding the enrichment law of karst groundwater is of great significance for the exploration and development of groundwater resources. Karst groundwater is the main water source for local people's production and life in the contiguous poverty-stricken area in Wumeng Mountain area, Southwest China. Hence, summarizing the characteristics of groundwater enrichment and studying the significance of karst groundwater supply can provide important references for the exploration and utilization of local groundwater resources. Firstly, this study conducted a hydrogeological survey of 1: 100,000 in the upper and middle reaches of the Yili river basin covering an area of 1,400 km² and another survey of 1: 50,000 in Huize county of Qujing City, with an area of 450 km². In the surveys, five types of groundwater enrichment characteristics in the study area have been identified through a comprehensive analysis of the spatial distribution and composition of karst water-bearing rock formations, the mechanism of structural groundwater control, and topography. In addition, 133 groundwater samples were collected and tested in the study area, and the main groundwater chemical types and water quality conditions of each groundwater enrichment block were analyzed. Subsequently, factors such as the distribution of watersheds, the spatial distribution and composition of water-bearing rock formations, the spatial morphology of faults and folds, and the catchment area of springs were taken into consideration to delineate the groundwater enrichment blocks corresponding to each spring or spring group. The natural recharge and discharge of groundwater in each groundwater enrichment block were calculated according to the rainfall infiltration coefficient and the total discharge. Finally, based on the previous research, the significance of groundwater supply by karst springs is concluded in terms of the groundwater abundance, supply conditions, and quality. The study results show that the enrichment characteristics of karst groundwater in the study area can be summarized into five types: fault-conducting zone, fault dissolution valley, strip dissolution valley, fold dissolution valley, and water-blocking contact zone. Besides, Na-HCO₃ and Ca·Mg-HCO₃ are the main groundwater chemical types in the enrichment blocks. 79% of groundwater is classified as type I to type III with nitrate and oxygen consumption as the main indicators exceeding the permitted level. Moreover, the natural recharge and discharge of groundwater in each groundwater enrichment block are 3,297,000-145,124,000 m³·a⁻¹ and 46,000-13,084,000 m³·a⁻¹ respectively. The ratios of discharge to recharge are 0.3%-15.5%, which indicates considerable potential of exploiting groundwater resources. Finally, the water enrichment blocks such as fault dissolution valley, strip dissolution valley, and water-blocking contact zone are of marked significance in water supply. Limited by the uneven spatial and temporal distribution of groundwater resources, shortage of water supply, poor water quality, etc., the other types of water enrichment blocks are of less significance. The research results provide a theoretical basis for ensuring water safety in poor water-scarce regions.

Comprehensive analysis and evaluation of the multi-information on the underground river system in construction of a pumped-storage hydropower station in Hubei

CAO Jianwen, YANG Haiyang, WANG Zhe, ZHAO Liangjie, LIN Yushan, LUAN Song, LI Lujuan

2023, 42(4): 699-710. doi: 10.11932/karst20230406

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China is steadfast in promoting green and low-carbon energy transformation. Playing a pivotal role in the power industry, hydropower is at the forefront of energy transformation. Developing hydropower has always been an important strategic policy for China's energy and electricity industry. The southwest area, a main karst distribution region, is mostly concentrated with water energy resources in China. With the rapid development of water conservancy and hydropower construction, reservoir engineering in karst areas is often constructed. The characteristics of karst development are hydrogeological conditions that must be identified in the construction of hydropower projects in karst areas. What’s more solving the problem of karst leakage is the key to the success of engineering construction. A pumped-storage power station in Hubei Province is prone to serious karst leakage because its upper reservoir is located in the karst depression and the boundaries, where conditions of the groundwater system are not clear. Taking the upper reservoir of the power station as a research object, this study integrates different technical methods such as regional structural analysis, identification of geomorphic causes, tracing of underground rivers and micro-dynamic automation monitoring to extract multi-information for effective identification of the underground river system in the engineering area and to conduct a comprehensive analysis of the impact mechanism of karst development on the engineering.Firstly, geological structures control the formation and movement of groundwater, as well as the direction and pattern of karst development. On the one hand, under the transformation of neotectonic movements, karst development in the study area has typical characteristics of vertical zoning, with surface karst zones, vertical karst development zones, and horizontal runoff zones developed from top to bottom. On the other hand, the secondary tension fractures at the turning point of the anticline provide space for groundwater to migrate in depth, and the wide and gentle distribution of strata at the sampe place increases the catchment area of groundwater. These two actions jointly provide conditions for the development of karst pipelines along the axis of the anticline. Secondly, the regional terrain and topography can roughly reflect the movement trend and direction of groundwater. Especially, the form, quantity, and scale of karst negative landforms are important bases for characterizing the development of underground karst. Through ground investigation and tracing, the distribution characteristics of underground karst pipelines can be basically obtained. Finally, the boundary and structural characteristics of the underground river system can be further clarified through high-precision tracing experiments, drilling, and verification of geophysical exploration.The analysis results indicate, (1) There are no aquitards or water blocking structures around the upper reservoir. The vertical and secondary fractures with tensile properties in the axis of the Ziliangping anticline control the basic pattern of the groundwater system, and the secondary cracks generated in the axis of the anticline play a dominant role in karst development. (2) The upper reservoir is located in the supply area of the Dongzhushui underground river system, and the permeability rate of the rock mass is controlled by the strength of karst development, the degree of structural development, the integrity of the rock mass, and the degree of weathering and unloading of the rock mass. There are significant differences in different parts, with a high risk of leakage. (3) The karst pipeline in a structure of the single-pipe shape is not subject to the development of large-scale karst ponds. The north-south karst pipeline is not developed, and there is a risk of pipeline leakage towards the southwest side of the upper reservoir. (4) The reservoir area has undergone at least three stages of denudation, and the current stage of dissolution is still ongoing. The development and evolution of deep karst can lead to deformation in the overlying strata. After the engineering water storage, it is necessary to prevent potential uneven settlement and karst collapse. By comprehensively analyzing and evaluating multi-information of the underground river system, people can accurately identify the boundary and structural characteristics of the karst water system in the reservoir area, providing a scientific basis for the project construction of the reservoir area as well as reference for the site selection of reservoir in other karst areas.

Modelling the hydrological process of the dried-up karst spring based on a reservoir model for hysteretic discharge

CEN Xinyu, ZHONG Jinxian, DENG Guoshi, XU Mo

2023, 42(4): 711-721. doi: 10.11932/karst20230407

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Karst is highly developed and widely distributed in Southwest China which is endowed with rich karst water resources but in different temporal and spatial distribution. As the main water source for the ecology and landscape of Lijiang Ancient City, Heilongtan springs have frequently dried up in recent years, which has seriously affected the production activities, daily life, and tourism quality of Lijiang Ancient City. The principle of reservoir model is to divide the karst aquifer system into different parts according to its structure or hydrological process. Based on the above division, each part has been generalized into a corresponding reservoir connected by a certain way in order to simulate the discharge of karst springs. The conventional reservoir model of continuous discharge has been widely used for reproducing the discharge of perennial springs. However, this continuous discharge model performs poorly in simulating the dry-up of karst springs. By setting the discharge law from epikarst reservoir to conduit reservoir as a hysteretic transfer function, this study reasonably reproduced the hydrological process of the dried-up springs. Several conclusions have been drawn as below. Firstly, Heilongtan spring area can be generalized as a reservoir model composed of epikarst regulation reservoir (E), matrix reservoir (M), and conduit reservoir (C). Discharge of Heilongtan springs has been successfully reproduced by the above model. Secondly, the rainy season and the normal season models can effectively simulate the spring dynamics based on the continuously discharge law. By setting the hysteretic discharge of the epikarst reservoir, the dry-up of the springs can be reproduced. To further characterize the multi-year regulation and storage characteristics of karst aquifers, additional reservoirs are needed. Thirdly, the simulation results show that the recharge from Reservoir E to Reservoir M is not sensitive to the rainfall, while the discharge from Reservoir E to Reservoir C is very sensitive to the rainfall, which indicates that the karst conduit in the Heilongtan spring area is developed and well connected, leading to the concentrated recharge and rapid increase of discharge at springs. Finally, the vast majority (82-95%) of Heilongtan spring water is recharged from Reservoir C. The threshold of rainfall required to trigger the rapid replenishment from Reservoir E to Reservoir C is relatively high. Moreover, the discharge from Reservoir C is characterized by sharp increase and decrease. The above characteristics cause uneven temporal and spatial distribution of karst groundwater, which can lead to the dry-up of springs. The research findings provide a reference for applying the hysteretic reservoir model to the simulation of karstic dried-up springs and help to understand the hydrological process of this type of karst system.

Study on characteristics and regulation technology of water resources of karst wetland in Huixian, Guilin

HUANG Qibo, ZOU Shengzhang, QIN Xiaoqun, CHEN Xuejun, DAI Junfeng, JIAO Youjun, LI Tengfang, MO Lingyun, SONG Yu

2023, 42(4): 722-732, 762. doi: 10.11932/karst20230408

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Known as "the kidney of the Lijiang river", Huixian karst wetland is the largest original ecological wetland of karst landform in the Lijiang river basin; therefore, it plays an important role in water replenishing and improvement of ecological environment of the Lijiang river. However, the regulation of water resources and storage capacity of Huixian karst wetland is limited because the area of the wetland varies greatly in season, and the regulation and storage function of water resources is weak in both the waterlogging season and the drought season. In a hydrological year, the water surface elevation of wetland fluctuates between 147.5-150.20 m, and the surrounding ground elevation ranges from 149.6-150 m. Thus, when the water level is higher than 149.8 m, a large area of cultivated land will be flooded, while the water level of wetland will easily drop below 149 meters resulting in the water shortage of wetland in the dry season. The frequent fluctuation of wetland water surface in a hydrological year leads to the reduction of wetland area or the inundation of cultivated land, which seriously affects the wetland landscape and villagers' water consumption of farming and living. Water resource problem has become a key factor restricting the development and protection of Huixian karst wetland. Previous studies have been carried out on the geological background, hydrogeological conditions, water level dynamics, groundwater balance, groundwater pollution and other aspects of the formation of Huixian karst wetland. However, less attention is paid to the dynamics of groundwater flow and the technology of regulating water resources, which may seriously influence the implementation of effective regulation of wetland water resources.In 2022, Guilin City launched the flood control and drainage upgrading project of the Xiangsi river in Guilin New District. The municipal government planned to invest 650 million yuan for the improvement of water resource regulation and storage function of Huixian karst wetland and the improvement of water ecological environment. It also intended to develop the landscape resources of karst cave wetland at the outlet of the Shiziyan underground river. In this study, the discharge of Shiziyan underground river system and Mudong lake dispersed drainage system in the core area of wetland were observed for one hydrological year, and the dynamic characteristics and controlling factors of wetland groundwater flow were systematically analyzed. On this basis, different regulation techniques were adopted to control water resources for the centralized runoff system of the Shiziyan underground river pipeline and the dispersed drainage system of Mudong lake. As a result, the water level of Huixian wetland was controlled within a reasonable range, achieving a good demonstration effect. This study provides scientific and technological support for the improvement of water resource regulation and storage function of Huixian karst wetland. It also provides a typical example for water resource regulation of karst cave pipeline wetland and dispersed drainage wetland in the karst area of Southwest China.The results show that groundwater plays a certain role but a weak function in regulating and storing atmospheric rainfall. If there is no rainfall for two months after the rainy season, the flow will be basically cut off. The Huixian river supplies water to Mudong lake through the ancient Guiliu canal, and the amount of water replenishment depends on the water storage height of the Huixian river dam. In the rainy season, the water storage height of the Huixian river dam is greater than 149.25 meters, which can guarantee the water replenishment of the wetland, but in the dry season, the water storage height can easily fall below 149.25 meters, which cannot replenish the wetland. Considering that the Shiziyan underground river system has the characteristics of concentrating runoff from pipelines, a three-level control project—"storing" and "diverting" in the upper reaches, "blocking" in the middle reaches, and "intercepting" in the lower reaches—has been adopted to increase the level of the underground river by 70 cm from the outlet to Baxian lake. This project can shorten the period of keeping a low water level by more than one month. In view of the dispersed drainage characteristics of the Mudong lake system, the technology of regulating water resources by "diverting" at the entrance and "intercepting" at the outlet was adopted. The water level at the exit was raised by 30 cm and the water level in the core area by 10-20 cm, delaying the time of keeping the low water level for more than two months.

Chemical characteristics of groundwater in the dispersed drainage system of Yangshui anticline in central Guizhou

WANG Ruofan, ZHAO Liangjie, LI Qiang, JI Qinkebuzi, JIAO Heng, JIANG Feng, CHEN Gang

2023, 42(4): 733-741. doi: 10.11932/karst20230409

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The dispersed drainage system of Yangshui anticline is located in Jinzhong town, Kaiyang county, Guizhou Province. As one of the main producing areas of phosphate deposits in Guizhou, this area is concentrated with phosphorus mines, phosphorus chemical industry and phosphogypsum storage sites. Therefore, the study on the chemical characteristics of groundwater in this area is of great significance for the rational development, utilization and protection of groundwater resources. This study takes the karst groundwater system, which has relatively independent conditions of groundwater recharge, runoff and discharge, as the research object. The main samples of karst springs, bedrock fissure springs, boreholes and mine drains are collected, and the hydrogeochemical research methods such as ion comparison method and main ion ratio method are used to analyze the chemical components and main ion sources of groundwater. The results show that contents of main cations of K⁺、Na⁺、Ca²⁺ and Mg²⁺ are listed as: mine drainage>carbonate karst water>bedrock fissure water; contents of ${\rm{HCO}}_3^{-}$ and ${\rm{SO}}_4^{2-}$ in main anions, mine drainage>carbonate karst water>bedrock fissure water; Ion content of Cl⁻, mine drainage>bedrock fissure water>carbonate karst water; TDS, mine drainage>carbonate karst water>bedrock fissure water. HCO₃-Ca·Mg is the main chemical type of karst water in carbonate rocks; the hydrochemical type of bedrock fissure is HCO₃-Ca; the chemical type of mine water is mainly SO₄-Ca·Mg followed by SO₄·HCO₃-Ca·Mg. There are significant differences in hydrochemical composition among carbonate karst water, bedrock fissure water and mine water in the study area. The main chemical components of karst water in carbonate rocks come from the leaching of dolomite and dolomitic limestone. The content of the main chemical components of bedrock fissure water is the result of the joint action of calcareous mudstone leaching and atmospheric precipitation. The main chemical components of mine water come from the leaching of dolomite and dolomitic limestone and the foreign ions brought by intensive human engineering activities. This study not only provides a scientific basis for the rational development and utilization of groundwater resources in this area, but also helps to protect groundwater resources and maintain the regional ecological environment.

Distribution characteristics of iodine in karst groundwater in Xintian county, Hunan Province and the analysis on the causes of high iodine

LI Xiaopan, NIE Guoquan, SU Chuntian, PAN Xiaodong

2023, 42(4): 742-752. doi: 10.11932/karst2023y024

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Iodine is one of the essential trace elements for human body, which maintains the growth and normal metabolism of the organism. Iodine deficiency or excess will have different degrees of impact on human health. A large field of strontium-rich mineral water was discovered in Xintian county, Hunan Province. However, the iodine content in some strontium-rich groundwater is abnormal, threatening the drinking water safety of local residents. Therefore, it is important to find out the distribution characteristics of iodine in groundwater and the controlling factors of the formation of high iodine groundwater so as to implement the project for the safety of drinking water and to prevent endemic iodine diseases in the study area.In a hydrogeological survey, 66 groups of spring water samples and 45 groups of well water samples were collected in Xintian county to analyze their hydrochemical characteristics, identify the spatial distribution characteristics of iodine in groundwater and analyze the main factors controlling iodine content in groundwater by means of hydrochemical graphical method, principal component analysis and GIS technology. The results showed that the iodine concentration in spring water and well water ranged from 2.7 to 92.8 μg·L⁻¹ and 4.15 to 3,861 μg·L⁻¹, respectively, with the respective median value of 5.4 μg·L⁻¹ and 168 μg·L⁻¹. It can be seen that all the groundwater with high iodine was well water, and 53.3% of well water samples had iodine concentration exceeding the permitted national standard of 100 μg·L⁻¹ (GB 19380-2016). In contrast, the iodine content in spring water was relatively low, and the overall iodine-deficient groundwater was predominant. High iodine groundwater was mainly distributed along a river valley in the northeast-southwest direction. The iodine content in groundwater showed a gradual increase from the peak-forest valley to the plain area of low-lying river valley, and the hydrochemical type also changed from water with single HCO₃-Ca type to water with complex HCO₃-Na, HCO₃-Na·Ca type, etc.The marl stratum formed by marine sedimentation of Shetianqiao Formation is rich in iodine and organic matter, which provides good geological conditions for the enrichment of iodine in groundwater. The microbial degradation of organic matter occurs in the closed and partially reducible groundwater environment of the marl aquifer. CO₂ generated by organic matter decomposition exists in groundwater mainly in the form of ${\rm{HCO}}_3^{-}$, which will compete with I^- for adsorption. Meanwhile, the process of organic matter decomposition to form ${\rm{HCO}}_3^{-}$ is accompanied by the release and migration of iodine deposited on it, making the iodine content in the groundwater increases with the increase of ${\rm{HCO}}_3^{-}$ content. At the same time, the alkaline karst water environment can increase the electronegativity of organic matter surface to reduce the adsorption of iodine ions by organic matter. OH⁻ in groundwater also competes with I⁻ for adsorption, which accelerates the release and migration of adsorbed iodine into groundwater, and hence increases the iodine content. The distribution area of high iodine groundwater is located in a groundwater discharge area. It is also an area distributed with strata rich in iodine and organic matter. In this area, the sluggish groundwater flow due to flat topography, together with a relatively closed groundwater environment, further facilitates the enrichment of iodine. The strata rich in iodine and organic matter formed by marine sediments, closed and partial reduction chemical environment with karst water of weak alkalinity and sluggish groundwater runoff are the main factors controlling the formation of high iodine groundwater in the study area.

An overview of karst geology in Myanmar

WEI Yanlan, LI Wenli, HUANG Chenhui, QIN Fengrui

2023, 42(4): 753-762. doi: 10.11932/karst2023y006

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With a widespread area of karst accounting for about 12% of the national territory, Myanmar, the second largest country in Southeast Asia in terms of karst area, has great potential of karst resources. However, due to outdated infrastructure and other reasons, the research on karst geology in Myanmar started late with a weak foundation. Having been in the developing stage of karst geology research in Myanmar, comprehensive karst geological data is conducive to understanding karst in this country. On the basis of systematic data collection, this study synthesizes remote sensing data with geologic map, hydrogeologic map, and structural map on various scales. It comprehensively analyzes the background conditions of karstification from the geological evolution process, lithologic combination, and climate and environment, and summarizes the karst distribution law and the characteristics of karst landscape in Myanmar.Extending to the borders of Thailand, Laos and China, karst in Myanmar covers an area of 70,000 km², distributed in the high land and mountainous area of Shan State to the east of the Irrawaddy River and the south of Andaman Sea. Some karst landforms are also developed in Kachin State in the north, Chin State in the west and Sagaing State in the northwest. Mainly developed in the Permian-Lower Triassic limestone and Early Ordovician limestone, karst on the Shan Plateau in the east is the largest karst area in Myanmar, with an area of about 63,000 km². The belt-shaped karst extends 1,500 kilometers from Shan State to Myeik Archipelago in a south-north direction. Intensity of karstification in Myanmar is controlled by many geological factors such as stratigraphic lithology, geological structure and environment. Thick Paleozoic carbonate strata, warm and rainy climate, complex tectonic movements and active biological activities are all conducive to the occurrence of karstification. The thick layers of Permian-Triassic, Triassic, Ordovician, and Silurian periods, pure limestone, and dolomite are the main layers of karst development, especially 5,000-meter-thick Permian-Triassic limestone in Shan State. With the highest biodiversity in the world, active biological activities in Myanmar facilitate karstification. The karst landform in Myanmar is controlled by stratigraphic lithology, with the development of peak forests, peak clusters, and karst cave landscapes. From north to south, karst landforms change from limestone ridges, canyons, cluster-peak depressions to peak forest plains. Hpa-an is a transitional area from high mountain karst to depression karst.

Temporal and spatial characteristics and peak prediction of carbon emissions in Guangxi Zhuang Autonomous Region

GAO Miao, WU Xiuqin

2023, 42(4): 763-774. doi: 10.11932/karst20230410

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Under the background of increasing international attention to the topic of global warming, in 2020 China was committed to peaking carbon emissions by 2030 with its enhancing independent contribution and powerful policies and measures. All provinces and localities actively respond to the commitment. Guangxi Zhuang Autonomous Region is an important source of industrial raw materials for China, but the slow industrialization and the excessive reliance of economic growth on industrial development have led to a significant increase in carbon emissions. Therefore, the purpose of exploring the historical and future laws of carbon emissions in Guangxi Zhuang Autonomous is to achieve the carbon peak goal as soon as possible.The research is based on the county-level carbon emission data of Guangxi Zhuang Autonomous Region from 2003 to 2017, sourced from Carbon Emission Accounts and Datasets (CEADS) which has the longest time span (from 1997 to 2017), the widest coverage and the highest accuracy of China's data on county-level carbon emissions. By GIS spatial analysis method, trend analysis and analysis of exploratory spatial data, the temporal and spatial changes of three indicators—carbon emissions, carbon emission intensity and carbon emission pressure—are analyzed at provincial, municipal and county levels in Guangxi. According to the hierarchical combination of the three indicators, Guangxi Zhuang Autonomous Region is divided into six different types of regional carbon emissions, and on this basis, seven carbon emission scenarios are simulated in terms of future population, and social and economic development. The extensible stochastic environmental impact model (STIRPAT) is used to predict the peak of carbon emissions in different scenarios for the types of regional carbon emissions in Guangxi from 2022 to 2035. (1) From 2003 to 2017, the total carbon emissions of Guangxi Zhuang Autonomous Region increased significantly, and the carbon emissions at the city level showed a spatial pattern of Nanning City being the highest and Fangchenggang City being the lowest. Carbon emissions at the county level showed disequilibrium. Though the carbon emission intensity significantly reduced, it showed a spatial pattern of being the highest in Laibin City and the lowest in Hezhou City. The index of carbon emission pressure increased significantly. The overall trend of variation coefficient of total carbon emissions is stable, showing high-intensity variation. (2) According to the classification and combination of total carbon emission, carbon emission intensity and carbon emissions pressure, carbon emissions can be divided into six regional types, including high total amount-high intensity-high pressure type (H-H-H), high total amount-low intensity-high pressure type (H-L-H), high total amount-low intensity-low pressure type (H-L-L), high total amount-high intensity-low pressure type (H-H-L), low total amount-high intensity-low pressure type (L-H-L), and low total amount-low intensity-low pressure type (L-L-L). According to the current scenario and future development of influencing factors of carbon emissions in Guangxi, seven scenarios are divided, including benchmark scenario, energy-saving scenario, scenario of rapid economic development, scenarios of simultaneous development of economy and emission reduction (a and b), scenario of green development and scenario of emission reduction. (3) The forecast results of Guangxi's whole carbon peak period show that four scenarios such as energy-saving scenario, scenario of simultaneous development of economy and emission reduction (b), scenario of green development and scenario of emission reduction can achieve the peak carbon emmissions by 2030. The peak time of carbon emissions is 2023, 2030, 2029 and 2030 respectively. The prediction results of reaching peak carbon emissions in different regions show that the energy-saving scenarios of H-H-L and H-L-L can achieve the goal of peak reaching. The H-H-H type and H-L-H type cannot achieve the peak goal. The benchmark scenario and the rest five scenarios of L-L-L can all achieve the peak by 2030. The L-H-L type reached its peak in 2018. The research systematically analyzes the overall and internal changes of carbon emissions in Guangxi Zhuang Autonomous Region, and points out the scenario mode of reaching the peak carbon emissions, which provides a strong reference for the measures and plans of emission reduction taken by Guangxi government. In the future, Guangxi Zhuang Autonomous Region should coordinate the relationship between energy supply and emission reduction, and take specific measures for emission reduction according to local conditions by referring to the scenario model of peak carbon emissions, so as to achieve the goal of reaching the peak carbon emissions in 2030.

Numerical experiment on the mechanism of mixing corrosion of carbonate rocks by hydrothermal synergistic effect

HOU Wenjuan, GONG Xing, LIU Feng, LI Hongzhong

2023, 42(4): 775-784. doi: 10.11932/karst20230411

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Karst water systems can be divided into open systems and closed systems. In the open system, mixed groundwater dissolves carbonate minerals mainly by the unsaturated corrosion. However, in the closed system, groundwater has been saturated with carbonate minerals after long-term runoff flow. When saturated groundwater with different hydrochemical components or at different temperatures mixed, the solubility of carbonate minerals in groundwater increases, and new corrosion capacity of groundwater generates. The mixing corrosion of saturated solution and temperature mixing corrosion are important dynamics for the chemical dissolution of carbonate aquifer system. The existing research about the interaction mechanism of mixing corrosion of carbonate rocks mainly focuses on the mixing corrosion of single saturated solution or temperature mixing corrosion, and the mechanism of hydrothermal synergistic dissolution in different groundwater mixing situations needs to be further studied. In order to reveal the synergistic mechanism between mixing corrosion of saturated solution and temperature mixing corrosion, this study uses hydro-geochemistry software PHREEQC to simulate the dissolution of calcium carbonate under static mixing conditions of groundwater, including the mixing of infiltrated soil water and shallow groundwater, the mixing of deep-flowing geothermal groundwater and shallow groundwater, and the mixing of deep fluid and shallow groundwater. Moreover, the impact of temperatures, $ {P}_{{\text{CO}}_{\text{2}}} $ of different end-member groundwater and the mixing ratios of end-member water on the intensity of mixing corrosion by hydrothermal synergistic effect is discussed here. The results show, (1) The mixing corrosion by hydrothermal synergistic effect can enhance the supplementary corrosion capacity of saturated groundwater which descends in order as the mixing of deep fluid and shallow groundwater, the mixing of deep-flowing geothermal water and shallow groundwater, and the mixing of infiltrated soil water and shallow groundwater. (2) When the mixing of infiltrated soil water and shallow groundwater occurs, the mixing corrosion capacity by hydrothermal synergistic effect is lower than that of mixing corrosion of single saturated solution, but higher than that of single temperature mixing corrosion. The main reason is that the change of solution temperature weakens the complementary corrosion capacity of mixed solution. (3) Under the condition of mixing deep-flowing geothermal groundwater and shallow groundwater, the mixing corrosion of saturated solution in deep-flowing geothermal groundwater has more effects on the hydrothermal synergistic corrosion than the temperature mixing corrosion. (4) When the deep fluid mixes with the shallow groundwater, the corrosion capacity of mixed solution is weaker than that of the mixing corrosion of saturated solution at single low-temperature, but stronger than that of mixing corrosion at single high-temperature. Although the change of temperature in the mixture causes the calcium carbonate precipitation, the mixing corrosion by hydrothermal synergistic effect still dissolves calcium carbonate. (5) The intensity of mixing corrosion by hydrothermal synergistic effect in the karst water system is controlled synchronously by the change of temperature and $ {P}_{{\text{CO}}_{\text{2}}} $ of mixing solutions. The variation law of mixing corrosion capacity cannot be judged by the effect of mixing corrosion of single saturated solution or temperature mixing corrosion. (6) The greater differences in temperature and $ {P}_{{\text{CO}}_{\text{2}}} $ between two end-member solutions lead to higher dissolution capacity of mixing corrosion by hydrothermal synergistic effect. The dissolution of calcium carbonate is the largest when the mixing ratios of end-member solutions are similar, and the dissolution decreases with the increase of mixing ratio differences of end-member solutions, which shows that the full mixing of natural groundwater is conducive to the mixing corrosion by hydrothermal synergistic effect. The study results provide a theoretical basis for the mixing corrosion mechanism and regulation of carbonate rocks dissolution under groundwater seepage conditions, and further enrich the theoretical research on karst development mechanism.

Service assessment of carbon storage of typical karst peak-cluster depressions in Guilin

ZHU Bailu, DENG Yan, XIE Yunqiu, KE Jing, WU Song, HUANG Jing, HOU Mengmeng

2023, 42(4): 785-794. doi: 10.11932/karst20230413

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To reveal the impact of land use change on carbon storage under different development modes, this study took the karst peak-cluster depression in Xitang village, Xingping town, Yangshuo county, Guilin City as an example. Based on the field survey data in 2020, the study used InVEST model to evaluate the carbon storage of karst peak-cluster depression, and simulated the impact of vegetation types on carbon storage under two development scenarios (ecological protection model and economic development model).The study showed the following results, (1) The total carbon storage of the study area was 16,641.68 t with the carbon density of 221.30 t·hm⁻². Its total economic value was 19.97 million yuan, and the economic value per unit area was 265,600 yuan·hm⁻². The low value area was mainly concentrated in the north side of the hill slope, while the high value area was mainly distributed in the west and south side of the hill slope. (2) The proportions of the steep slope of natural forest, the gentle slope of natural forest and the gentle slope of economic forest to the total carbon storage of ecosystem were 51.96%, 16.04% and 10.44%, respectively. These three land types were the main sources of carbon storage. The carbon storage capacity per unit area of steep slopes of plantation and gentle slopes of natural forest was the strongest. (3) The amounts of carbon storage of artificial forest, natural forest and economic forest were 244.35 t, 12,215.17 t and 4,182.36 t, accounting for 1.47%, 73.40% and 25.13% of the total carbon storage of the ecosystem, respectively. Natural forest is the main carbon sink of the ecosystem. The carbon densities of artificial forest, natural forest and economic forest were 339.38 t·hm⁻², 261.79 t/ha and 150.34 t·hm², and the economic value per unit area was 407,200 yuan·hm⁻², 314,200 yuan·hm⁻², and 180,400 yuan·hm⁻², respectively. Not only soil in natural forest and economic forest but also soil and above-ground vegetation in artificial forest are the largest carbon sink. (4) The carbon densities of soil, above-ground vegetation, underground vegetation and litter were 184.24 t·hm⁻², 29.94 t·hm⁻², 7.05 t·hm⁻² and 0.06 t·hm⁻², respectively. The proportion of soil carbon sink to ecosystem carbon sink was 83.25%, and soil carbon storage was the largest carbon pool in the ecosystem. Soil in both natural forest and economic forest is the largest carbon sink, and soil and above-ground vegetation in artificial forest have the largest carbon sink. (5) The carbon reserves of current situation, ecological protection mode and economic development mode were 19.9725 million yuan, 22.5214 million yuan and 15.9530 million yuan, respectively. Under the ecological protection mode, the natural forest and artificial forest increased by 8.74 hm² and 4.78 hm², respectively; the economic value of carbon storage increased by 2.5489 million yuan. Under the economic development mode, the area of economic forest increased by 45.99 hm², and the economic value of carbon storage lost 4.0195 million yuan. The study indicates the impacts of different development modes on land use and carbon storage service function of karst ecosystem, which can provide reference for determining the future development mode of the core scenic area of Lijiang River Scenic Area in Guilin and the Karst World Natural Heritage Site in Guilin and the control of rocky desertification.

Genesis mechanism of geothermal fluid with high mineralization in karst geothermal reservoir: A case study of geothermal field of the Yanchang river, Badong county

YU Jie, MAO Xumei, PENG Hui, WEN Meixia, WANG Xin, FAN Wei, TANG Wei

2023, 42(4): 795-808. doi: 10.11932/karst2022y32

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At present, China's energy development has entered a new stage of carbon reduction and energy conservation. In order to achieve the goal of "carbon peak" and "carbon neutrality", the development and utilization of geothermal energy has been ushered in unprecedented opportunities. However, not all geothermal resources can be directly exploited and utilized. Based on a funded project—Feasibility of Geothermal Resources Exploration in Yanchang river, Badong county, Hubei Province, we have built a genesis model of the geothermal field mainly by means of software analysis, traditional geological survey, drilling, sampling analysis and systematic temperature measurement. In the aspect of hydrochemistry, we also analyzed the genesis of geothermal fluid with high mineralization in karst geothermal reservoir in the Yanchang river. According to previous research, hydrogeological survey, geophysical exploration and drilling, we found out the geothermal geological conditions, regional stratigraphic distributions, lithologic characteristics and structural distributions in the study area, which can provide data for the construction of genesis model of geothermal field. In addition, by sampling, testing, and monitoring water temperatures, we compared the chemical components of cold springs and geothermal fluids in different periods, and further analyzed the hydrogeochemical characteristics of geothermal fluids and the reasons for temperature anomalies. The research findings may provide the technical and theoretical basis for the genesis mechanism of geothermal fluid with high mineralization in karst geothermal reservoir as well as the basis for the scientific development and utilization of karst geothermal reservior.The geothermal fluid in the karst geothermal reservoir is mostly hot water with low mineralization at low temperature, while the highly mineralized geothermal water is often related to the dissolution of karst aquifer minerals. However, the TDS of geothermal water in the geothermal field of the Yanchang river is as high as 12,477.7 mg·L⁻¹, and the water temperature is about 34 ℃. The dissolution of aquifer minerals is unlikely to explain the genesis mechanism. On the basis of field investigation and geothermal drilling, we conducted the hydrogeochemical sampling and testing in four geothermal boreholes, one hot spring and four nearby cold springs. The research shows that the study field belongs to the convection-type geothermal resource at medium-low temperatures under the control of deep and large faults. The Ordovician limestone and dolomite are the main strata for geothermal reservoir, belonging to the karst fissure type. The chemical type of geothermal water in the Yanchang river is Cl-Na. The maximum unit water inflow can reach 1,767 m³·d⁻¹, with the outlet temperatures from 30.2 ℃ to 34.5 ℃. Compared with the analysis of borehole temperature and SiO₂ thermometer, the temperature of geothermal fluid is 59.1 ℃, and the circulation depth is 1,923 m. It is found that the geothermal fluid can complete sufficient heat exchange with heat source in the long migration path and long runoff time, the process of which may gradually increase groundwater temperatures. The sulfur isotope analysis shows that sulfate in karst water is derived from recharge water including atmospheric precipitation, surface water, and water formed by the oxidation of pyrite in rock mass. Groundwater maintains a relatively stable balance between oxidation and reduction. The aquifer is a transition between the alternation of a weak (lagging) and a strong environment, having a certain but low-degree recharge condition. Phreeqc hydrochemical simulations reveal the water-rock interactions (mainly the dissolution of aquifer minerals) in the aquifer, and further reveal that the high salinity of chemical composition in geothermal water is derived mainly from the dissolution of salt rocks in the salinized tidal flat lagoon facies during the runoff process. According to the analysis of tritium isotope, the content of tritium in geothermal fluid increases significantly, mainly because the geothermal fluid with low tritium content is mixed with shallow water or surface cold water when the geothermal fluid pours out along the fault under the influence of deep heat source and long runoff path. Hydrogeological conditions and hydrogen and oxygen isotopes can indicate the origin of atmospheric precipitation of geothermal water. The groundwater recharge height ranges from 1,261.21 m to 1,298.25 m, while the height of the Xiaoshennongjia mountain area in the north of the geothermal field ranges from 900 m to 1,300 m, which is the recharge area of the geothermal field of the Yanchang river. However, seasonal cold water addition controls the balance of water-rock interactions. It can be concluded that the high mineralization in geothermal water of the Yanchang river is mainly formed by the dissolution of salt rock during the runoff process, during which the upward flow is affected by seasonal mixing of cold water. Furthermore, F6 tensile faults and F7 water-blocking faults in geothermal fields affect not only the flow direction and velocity of groundwater, but also the increasing content of TDS in geothermal fluids.

Experimental simulation of dissolution to dolomite in formation water of Jixianian Wumishan Formation in the Xiong'an New Area

JI Shaocong, ZHANG Qingyu, LIANG Bin, LI Jingrui, BA Junjie, NIE Guoquan, DONG Hongqi, MO Guochen

2023, 42(4): 809-818. doi: 10.11932/karst20230414

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With the richest geothermal resources and the best development and utilization conditions in the central and eastern China, the Xiong'an New Area is the home of three large and medium-sized geothermal fields—Xiongxian, Rongcheng and Gaoyang. The Jixianian Wumishan Formation in the Xiong'an New Area has good reservoir endowment, large water yield and easy reinjection, which is the focus of geothermal resource exploration. At present, acid solutions, high-salinity water solutions or sea water which are prepared by researchers themselves are mainly used in simulation experiments, but less formation water is used as the experimental fluid. In this study, the dolomite of Wumishan Formation in the Xiong'an New Area is taken as the research object, and the formation water of Wumishan Formation in the underground is taken as the experimental fluid. The dissolution simulation experiment under high temperature and high pressure has been carried out to analyze the influence of temperature, pressure, lithology, structure and other factors on the dissolution of dolomite. This experiment adopts the simulation experimental device of dissolution kinetics under the conditions of high-temperature and high-pressure independently designed by Karst Geology Research Institute of the Chinese Academy of Geological Sciences. The formation water from the whole section of Wumishan Formation in the geothermal well of the Xiong'an New Area is taken as the reaction fluid. At the same time, considering the two factors of temperature and pressure, a total of 12 groups of experiments in the range of 40 ℃, 10 MPa to 150 ℃, 20 MPa have been carried out to simulate the dissolution of formation water of Wumishan Formation in the Xiong'an New Area on dolomite from the shallow burial to the deep burial. The experimental results show that dissolution rates of samples in the formation water decreased with the increase of temperature in general. The rates experienced a rapid decrease before a slow increase and then a rapid decrease again, with an obvious increase from 100 ℃ to 140 ℃. Dissolution rates of samples in formation water increased obviously with the increase of pressure. The variation of Ca²⁺ and Mg²⁺ concentrations with temperature and pressure was consistent with that of dissolution rates with temperature and pressure. All samples showed a certain degree of corrosion under the experimental conditions, mainly the corrosion of the sample surface. The samples with less developed pores and microcracks only corroded on the surface, which blurred the sample surface. The samples with developed pores and microcracks were eroded and expanded along intergranular pores, intercrystalline pores and various fractures, and finally connected to a certain extent. Therefore, the results of this study show that the buried dissolution of carbonate rocks will decrease in the burial diagenetic environment, with the increase of depth and temperature. But there is a window with higher dissolution capacity in the range of 100 ℃-140 ℃, which may be a favorable temperature range for the formation of dolomite karst reservoirs in the Wumishan Formation in the study area. For the samples with undeveloped pores and microcracks, it is difficult for soluble fluid to enter the sample for large-scale corrosion, but only to stay on and blur the sample surface during the experiment. The samples with pores and microcracks developed are corroded and expanded along intergranular and intercrystalline pores and various fractures and fissures, and are finally connected to a certain extent.

Karst dissolution rates of carbonate rocks in north-south geographical boundary of China—the Qinba Mountain Area

YANG Zhiguo, CHEN Qingmin, CHENG Xing, HE Kaikai, ZHANG Yu, TANG Li, AI Hao, ZHANG Wenshuo

2023, 42(4): 819-833. doi: 10.11932/karst2023y020

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The karst area in the south of China is one of the three major karst distribution areas in the world. But the economic development of the karst mountainous area is severely restricted by the fragility of its environment and unreasonable disturbance of human activities. As quantitative data reflecting the intensity of karstification, karst dissolution rates can be studied to facilitate the restoration of karst ecosystem and control of rocky desertification. Especially in recent years, research on rocky desertification control and ecological restoration in karst areas of Southwest China has been achieved with fruitful results, and hence is widely concerned by many domestic scholars on karst ecological characteristics and vulnerability, rocky desertification control and other issues. The Qinba area of Shaanxi Province is an important dividing line between the north and south of China in a climate of subtropical continental monsoon. It is also known as the "central water tower" and has a large sinkhole group at high latitude. Its special geographical location obviously differentiates the climate and ecological environment between the north and south. At the same time, carbonate rocks are widely exposed in the Qinba area, especially in places such as Zhen'an, Shanyang, etc. Unreasonable human activities have destroyed the karst ecological environment, making rocky desertification prominent. By analyzing dissolution rates from the aspects of vegetation, climate, and human factors, we explore the impact of regional environmental changes on karst formation, which may provide data support for ecological restoration, karst carbon sequestration, and rocky desertification control, and may also fill the gap in the study of karst formation in the Qinba area of Shaanxi Province.The three representative vegetation types—woodland, shrub and grassland—were selected in field dissolution tests in the study area. In this study, we used standard dissolution specimens (square test pieces with the side length of 4 cm and thickness of 0.3 cm) of crystalline limestone from the Late Triassic Wujiaping Formation (P₃w) in Xiaonanhai town, Hanzhong City. Each vegetation type was divided into 5 layers (100 cm in the air, surface, 20 cm under the soil, and 50 cm and 100 cm under the soil). Three standard dissolution test pieces were places in each layer. After a full hydrological year from March 31, 2021 to October 20, 2022, a total of 806 dissolution specimens were retrieved, and the amount and rate of dissolution of each specimen were obtained. The effects of vegetation types, depths, rainfall and temperatures on the karst process and the relationship between karst dissolution rates and degrees of rocky desertification were comprehensively discussed.The results showed that there were significant differences in subsurface dissolution rates among vegetation types. The average underground dissolution rate of forest land was the highest, followed by that of shrub land. The rate of grassland was the lowest. It was found that the dissolution rate of forest land in the same layer was higher than those of shrub and grassland, and the underground dissolution rate of shrub was higher than that of grassland. The organic carbon contents of different vegetation types and the changes of soil physical and chemical properties by vegetation are the fundamental factors that affect the direction and intensity of the karst process. At the same time, we analyzed the influence of rainfall and temperatures on dissolution rates in different regions. The results showed that there was a significant positive correlation between rainfall and dissolution rates (R=0.84), indicating that rainfall plays a key role in karstification. Rainwater absorbed CO₂ from the air during the process of falling to the surface. During infiltrating from the surface to the soil, rainwater combined with CO₂ released by plant root respiration and produced by microbial metabolism to form carbonic acid after it absorbed CO₂ in the process of falling to the surface. Consequently, the continuous dissolution of carbonate rocks by both surface water and groundwater containing carbonic acid led to the development of rocky desertification. The correlation coefficient between temperatures and corrosion rates is 0.45, which indicates that the temperature is not an important factor affecting the dissolution rate.The formation of rocky desertification is the result of the joint action of natural factors and human factors, and unreasonable human activities are the main factors. A comparative study on dissolution rates of different degrees of rocky desertification in the Qinba area shows that dissolution rates increase with the increase of rocky desertification degrees as follows, severe rocky desertification>moderate rocky desertification>mild rocky desertification. This phenomenon is more obvious in areas with mild rocky desertification, for example, the dissolution rates of Beiyang Mountain in Zhen'an are three times as much as those of the areas mostly distributed with mild rocky desertification. Besides, the aggravation of rocky desertification is often accompanied by extensive agricultural production patterns and severe ecological environment damage. It can be seen that in densely populated areas of karst mountainous areas, people's transformation of the karst environment is the main reason for the aggravation of rocky desertification, and the higher the dissolution rate is, the higher the degree of rocky desertification becomes.The study of dissolution rates in the Qinba area of Shaanxi Province shows that with the forward succession of vegetation, dissolution rates of carbonate rocks will increase. Rainfall can promote the dissolution of carbonate rocks, which is one of the important factors affecting karstification. Moreover, rocky desertification is the result of the interaction of carbonate rock dissolution and human disturbance, and human factors play the main role.

Experimental study on the influence of vibration loads and temperatures on the dissolution of dolomite and limestone

GU Zhanfei, HUANG Bingyan, LIU Zhikui, QUAN Xin, XIE Yongxiong

2023, 42(4): 834-841. doi: 10.11932/karst202304y01

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The dissolution of surface carbonate is a complex physicochemical process constrained by multiple influencing factors. In karst regions, due to the variations in mineralogy and chemical composition of the rock mass, differences in internal structure and pore characteristics, and the influence of external conditions such as the presence of groundwater, temperatures and vibration loads, limestone and dolomite exhibit differential dissolution patterns. Such differential dissolution often damages the completeness of the rock mass, affects its mechanical property and stability, and hence poses huge risk to the safety of buildings and structures on it. With the increasing strength of China's economy and the sustained promotion of development strategies in China's western region, many tunnels and underground engineering projects are being constructed, and the speed and frequency of train operation (high-speed trains and urban rail transit) are significantly increasing. However, the repeated vibrations during train operation can alter the degree and direction of water-rock interaction, leading to the continuous expansion of joints, fractures, and cavities within the rock mass. In addition, the impact of various external factors (temperature, climate, etc.) can accelerate this process, reduce the strength of limestone and dolomite, and thus pose hidden hazards to the safety of train operation. By the self-designed experimental equipment, this study investigates the dissolution behavior of dolomite and limestone in karst mountain areas of northern Guangxi under the influence of vibration loads and different temperature conditions. The experimental setup includes vibration equipment, a constant temperature chamber, a temperature controller, a flow meter, a pH meter, equipment for measuring Ca²⁺ and Mg²⁺ ions, and related drugs and reagents. Trains traveling between urban rail transit stations undergo three phases from acceleration to uniform and then deceleration, during which the vibration amplitude changes with the speed. Assuming a train departs every 8 minutes, horizontal and vertical vibrations will be generated during the operation, with vertical vibrations being the primary cause. In the laboratory, vibration conditions were controlled using a shaker sieve machine with an amplitude of 10 mm, a frequency of 2.5 times per second, and approximately 150 impacts per minute. A crossbar was fixed to the upper part of the sieve machine, and a fine cotton thread was used to tie the rock sample to one end of the crossbar to drive the sample vibration. The vibration was conducted every half an hour for 2 to 3 minutes. The results show: (1) Under normal temperature and pressure conditions, the dissolution rate of dolomite is lower than that of limestone in static acidic solutions. (2) With sufficient H⁺ under different temperature conditions (0℃ to 45℃), the dissolution rates of dolomite and limestone both increases with increasing temperatures, and the effect is more significant on dolomite. At 15℃, the dissolution rate of limestone is 2.496 times that of dolomite, while at 45℃, the dissolution rate of limestone is 1.150 times that of dolomite. (3) The dissolution of dolomite and limestone is strongly affected by vibration conditions and their dissolution rates increase with the increase of vibration frequency. Under the same vibration conditions, the dissolution rate of limestone is always greater than that of dolomite. Vibration loads have a greater impact on the dissolution rates of limestone and dolomite than other factors such as temperatures. This study provides a theoretical basis for engineering design and construction in karst mountain areas of northern Guangxi.

Influence of forest types and slope positions on dissolution rates of carbonate rocks in the karst area in subtropical China

HOU Manfu, LIU Yuting, ZHANG Jie, HE Luyan, LIANG Jiangyi

2023, 42(4): 842-852. doi: 10.11932/karst2023y010

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Carbonate rock weathering has great carbon sink potential. Vegetation is an important driven factor of carbonate rock dissolution. However, the relationship between vegetation types and carbonate weathering is still left unknown. In order to reveal the effects of forest types on carbonate rock weathering, the dissolution rates of carbonate rocks at different slope positions in two typical subtropical karst forests were compared in details based on the in-situ dissolution tests of standard rock test pieces, and the corresponding relationship between the rates and soil CO₂ content (pCO₂) and soil water content (SWC) were analyzed based on seasonal measured data.The study area is located in the suburb of Guilin, South China, which has a subtropical seasonal climate with an average annual temperature of 20.2 ℃ and an average annual precipitation of 1,996.0 mm. Precipitation is concentrated from April to September, synchronous with the high temperature in summer. The landforms of peak cluster depressions and peak forest plain around the study area are the most typical in the world and were added to the UNESCO's World Heritage List in 2014. Stretching from the foot to the place near the peak of limestone hills, Cycolobalanopses glauca forest and Platycarya strobilacea forest, the representatives of subtropical karst forests, were selected as research objects in the study area. The community dominated by C. glauca is widely distributed in the karst region of N25°-N31° and E105°-E120°, and is the most important climax community in the subtropical karst region of China. P. strobilacea is one of the most common dominant species or constructive species in evergreen and deciduous broad-leaved forests, forming a more widely distributed forest type than C. glauca forest in the subtropical karst area.Standard rock test pieces were respectively placed in the air 100 cm above ground, on the surface of soil, -20 cm, -50 cm, and -70 cm (if possible) in the soil of downhill slope, middle hill slope and uphill slope from 30/11/2014 to 17/12/2017 in each forest type. The soil pCO₂ and SWC of each test place were measured seasonally.The results show that the dissolution rate of carbonate rock in C. glauca forest (5.22 ± 0.99 mg·cm⁻²·a⁻¹) was significantly higher than that in P. strobilacea forest (3.58 ± 2.59 mg·cm⁻²·a⁻¹). In vertical profile, the dissolution rate under soil experienced a decrease after an increase, peaking at 20 cm below ground in C. glauca forest while the rate kept rising with the increase of soil depth in P. strobilacea forest. The differential of dissolution rates did not show consistency at different slope positions of the two forest types. There was no significant difference in rates at three slope positions in C. glauca forest, but the rate was significantly higher on the middle slope than those on the upper and down slopes in P. strobilacea forest. The dissolution rates of the two forests well corresponded to their SWC, but were opposite with their soil pCO₂. The dissolution rates at different slope positions and in vertical profiles well corresponded to soil pCO₂.There are significant differences in dissolution rates among different forest types in the subtropical karst area, which can be explained by SWC. There is not a consistent pattern on dissolution rates of slope positions between the two forest types, but their differences could be explained by soil pCO₂. Our results highlight the importance of distinguishing the influence of community types, their SWC and soil pCO₂ on dissolution rates of carbonate rock in the subtropical karst area.

Study on formation conditions of high-quality reservoir of Sinian Dengying Formation in central Sichuan

ZHOU Zheng, LI Dahua, ZHANG Ye, CHEN Hongkai, WANG Xingzhi, LIAO Yunping, QI Yongai, WANG Baoliang

2023, 42(4): 853-862. doi: 10.11932/karst20230415

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Central Sichuan is located at the eastern high point of the Leshan-Longnvsi ancient uplift axis. With a large amount of oil and gas resources, the Sinian Dengying Formation has a good exploration prospect. Since 2011 when CNPC deployed Well GS1 to explore Dengying Formation in the study area and then succeeded in discovering the oil and gas reservoirs, many scholars have done much research in this area and achieved a lot. But their opinions are similar only in terms of the understanding of lithology. In fact, multiple tectonic movements, the variable diagenetic environment and complex diagenesis of the Dengying Formation make it difficult for scholars to study the formation conditions of high-quality reservoirs in the Dengying Formation of the study area, which has resulted in a great controversy about the formation conditions of high-quality reservoirs. Therefore, taking the existing drilling cores and field profiles in central Sichuan and its adjacent areas as the main research objects, we studied the formation conditions of high-quality reservoirs in this paper, based on previous research as well as the testing data such as data from surveying wells and logging, and geochemical and physical properties.In this study, the existing drilling cores, field sections and rock slices in central Sichuan and its adjacent areas were analyzed for research of the rock types and reservoir space that constitute the reservoir. The results show that the lithology of Dengying Formation is mainly dolomite, with a small amount of mudstone and siliceous rock. There are many types of rock forming the reservoir, mainly including crystalline dolomite, sandy dolomite, algal laminated dolomite and karst breccia. The reservoir space is mainly composed of secondary pores, holes and fractures. The pores mainly include intercrystalline pores, intercrystalline dissolution pores, intergranular dissolution pores and residual intergranular pores. Among them intercrystalline pores and intercrystalline dissolution pores are mainly distributed in crystalline dolomite. In some parts, these two types of pores will be eroded and then expand into pore-type karst caves after karstification. However, intergranular dissolution pores and residual intergranular pores are mainly distributed in sandy dolomite, and the connectivity between them is poor. The caves mainly include "grape lace" caves, inter-gravel caves, and pore-type karst caves. The types of caves also include karst pores, caves and fissures that are distributed along the algal laminae. Among these caves, "grape lace" caves and inter-gravel caves are mainly distributed in breccia. Most of these two types of caves are filled with very small residual space. Pore-type karst caves are mainly distributed in crystalline dolomite. However, the dissolution pores, caves and fissures extending along the algal laminae are mainly distributed in the algal laminated dolomite. They are less affected by the later diagenesis and have weak infilling capacity, relative completeness and good connectivity. Fractures are mainly formed in a large number by tectonism and generally regarded as seepage channels rather than reservoir space for oil and gas. Finally, the existing research and comparison of the physical property different reservoir space show that the high-quality reservoir is composed of dissolution pores, caves and fractures distributed along the algal layer.The comprehensive study of the geological background of the study area indicates that the sedimentary facies of the study area are platforms. The subfacies are shallow water platforms and semi-deep water platforms, and the microfacies are shoal patches, cohesive shoals, algal flats, dolomitic flats and mud flats. At the same time, the correlation between lithology, reservoir space and sedimentary microfacies has been studied, showing that algal flats are the basis of high-quality reservoir development. The formation of reservoirs in Dengying Formation is mainly under the effect of recrystallization and karstification. Recrystallization can only improve the permeability of the reservoir, but cannot increase the effective reservoir space. Karstification took place during syngenetic-quasisyngenetic stage, supergene stage and buried-rock stage. In the syngenetic-quasisyngenetic stage, karstification mainly formed selective karst pores and caves, but rarely remained. Karstification in the supergene stage mainly formed karst pores, caves and fractures distributed along the algal laminae, which provided high-quality reservoir space. Buried karstification mainly occurred in the process of generation and degradation of oil and gas, and the overall transformation degree is small, but the connectivity and permeability of the reservoir space were improved. Therefore, supergene karstification plays a key role in the formation of high-quality reservoirs.In the study area, high-quality reservoirs are formed under the joint action of sedimentation, diagenesis and tectonism. According to the formation process of high-quality reservoirs, different periods and different pore structure characteristics, their evolution process can be divided into five parts: sedimentary period, shallow-burial period, supergene period, medium-deep burial period and period showing current characteristics. The study on evolution characteristics of high-quality reservoirs in different periods show that algal flats are the foundation for the formation of high-quality reservoirs; the Tongwan tectonic movement provides necessary conditions; supergene karstification is the key.

Carbon and oxygen isotopic characteristics of karst fracture-cavity fillings and environmental significance: A case study of Ordovician Yingshan Formation in Tahe oilfield

DONG Hongqi, ZHANG Qingyu, LIANG Jiapeng, LIANG Bin, LI Jingrui, DAN Yong, NIE Guoquan, JI Shaocong

2023, 42(4): 863-874. doi: 10.11932/karst20230417

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Karst fracture-cavity of Ordovician Yingshan Formation is the main carbonate reservoir in Tahe oilfield. The development of karst fracture-cavity shows strong heterogeneity. The complex diversification of filling types, types of filling materials and filling characteristics leads to the complexity of karst fracture-cavity reservoirs, which has a direct impact on the development of oil reservoirs. Understanding the characteristics of karst fracture-cavity filling is conducive to finding the optimal reservoir, which is of great significance to the study of petroleum geology in this area. In this study, the Ordovician Yingshan Formation cores from several wells in Tahe oilfield were observed, and the karst fracture-cavity fillings and limestone bedrock of typical drilling of Yingshan Formation were collected. Combined with the characteristics of fracture-cavity filling, the carbon and oxygen isotope characteristics are analyzed, and the sedimentary paleoenvironment is clarified, which provides a new geochemical basis for inversion of reservoir formation process. Based on the core observation, description and filling type statistics of 31 drillings in Tahe oilfield, it is found that the fillings of karst fracture-caves of Ordovician Yingshan Formation in Tahe area are mainly characterized by mechanical fillings and chemical deposits, followed by slump fillings. The slump fillings mainly filling in caves and seven boreholes—A8, T615, T502, S64, S70, T403 and T624—can show the most typical characteristics. We collected 35 filling samples of the karst fracture-cavity in these seven typical drillings of Ordovician Yingshan Formation, including calcareous argillaceous, sandy, calcite, breccia and limestone bedrock filled in the dissolution pores. Then, we tested and analyzed δ¹³C and δ¹⁸O of the filling samples. The results show that the variation range of carbon and oxygen isotopes of the backfill is large, with δ¹³C from 0.75 ‰ to −10.14 ‰ and δ¹⁸O from −5.94 ‰ to −14.14 ‰. Compared with the carbon and oxygen isotope of bedrock, the values of δ¹³C and δ¹⁸O of 80% of the backfill are negative, and the calcite is the most negative. The values of δ¹³C and δ¹⁸O of calcium mud filling were significantly positive. There are four different types of formation environment for karst fracture-cavity fillings in Ordovician Yingshan Formation. The first type is the contemporaneous or early diagenetic karst environment. The δ¹³C and δ¹⁸O of the karst fracture-cavity fillings are from −2.29‰ to 0.75‰ and from −5.94‰ to −8.57‰, respectively, which are similar to the carbon and oxygen isotope characteristics of the marine limestone of the Yingshan Formation. This finding indicates that the fracture-cavity fillings are formed in the contemporaneous or early diagenetic marine karst environment, which may be the result of the joint filling of mud carried by flowing water and carbonate rock deposition in the contemporaneous karst period. The second type is Late Caledonian-Early Hercynian weathering crust karst, with the most obvious negative values of carbon and oxygen isotopes: δ¹³C from −10.14‰ to −8.57 ‰, and δ¹⁸O from −14.14‰ to −13.44 ‰, respectively. The negative δ¹⁸O value is mainly caused by the low δ¹⁸O value of atmospheric freshwater, which leads to the negative δ¹⁸O value of precipitated calcite. There are two main reasons for the negative δ¹³C value. One is the exposure environment of late Caledonian-early Hercynian affected by low δ¹³C CO₂ in atmospheric fresh water, and the negative δ¹³C value of calcite precipitated after reaction with carbonate rocks. The other is that the content of organic matter overlying on the weathering crust in the Silurian clastic strata is high, and the δ¹³C value is low due to the oxidation of CH₄ by organic matter. The third type is buried karst environment. Both the negative values of δ¹³C (from −6.54‰ to −3.07‰) and δ¹⁸O (from −13.11‰ to −12.43‰) indicate that the fillings are affected by different degrees of atmospheric freshwater in buried karst environment. The fourth type is the late to modern karst environment. The values of δ¹³C (from −5.24‰ to −0.79‰) and δ¹⁸O (from −11.15‰ to −8.77‰) are quite different from those of the carbon and oxygen isotopes in terms of bedrock background values. Most of the filling materials are calcareous argillaceous mechanical filling, mainly filling in karst caves and dissolved structural joints. The filling process is slow and long, and is affected by organic matter to varying degrees. Karst fracture-cavity filling are the product of the formation of karst reservoir in the transformation process. The analysis of carbon and oxygen isotopes of carbonate rocks and karst fracture-cavity fillings is one of the important means for the study of paleokarst oil and gas reservoirs. It is helpful to understand the formation environment, development and evolution of karst and its relationship with the development of oil and gas reservoirs, which is of great significance for the prediction of karst reservoirs and oil and gas exploration.

2023 Vol. 42, No. 4

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