Gas hydrates have garnered significant attention due to their potential as an energy resource. Since the 1990s, China has been conducting gas hydrate investigations in the South China Sea, with the Guangzhou Marine Geological Survey leading nine drilling expeditions that have achieved notable breakthroughs in gas hydrate exploration. These drilling results confirm the occurrence of various types of gas hydrates in the northern South China Sea. Diffusion gas hydrates are prevalent in the Shenhu area, vent and sandy gas hydrates are primarily found in the Qiongdongnan area, and complex gas hydrates are present in the Dongsha area. This paper integrates drilling results to describe the seismic reflection and logging response characteristics of these gas hydrate systems. The diffusion gas hydrate system is characterized by a prominent bottom simulating reflector (BSR) and strong positive polarity reflections above the BSR in seismic data. Logging data reveals high resistivity, high P-wave velocity, and high S-wave velocity. In the mixed layer, where gas hydrate coexists with free gas, logging data show high resistivity, low P-wave velocity, high S-wave velocity, and a neutron-density crossover. The vent gas hydrate system typically lacks a distinct BSR but exhibits bright reflections above the BSR, pull-up reflections, and columnar blanking reflections in shallow strata on seismic data. Additionally, anomalous geomorphological features such as seafloor mounds and pockmarks are commonly associated with vent gas hydrate systems. Logging data for vent gas hydrates shows extremely high resistivity, slightly increased P-wave and S-wave velocities, and significant changes in bedding dip and/or azimuth. Sandy gas hydrates are identified by very strong BSRs and strong positive polarity reflections on seismic data, indicating the presence of sandy reservoirs. Logging data for sandy gas hydrates are characterized by low gamma-ray values, very high resistivity, very high P-wave and S-wave velocities, slightly increased density, and decreased neutron porosity. The study also summarizes the controlling factors for gas hydrate formation in the Shenhu and Qiongdongnan areas. In the Shenhu area, the development of concentrated gas hydrate reservoirs is primarily controlled by deep, large faults and inclined levee deposits. In contrast, gas hydrate formation in the Qiongdongnan area is influenced by paleoburied hills and differential compaction.

Numerous drilling expeditions in the world’s oceans have confirmed the coexistence of gas hydrate and free gas. The geophysical anomalies and enrichment patterns of these coexistence zones vary across different regions, making their identification crucial for accurately evaluating gas hydrate resources. This study analyzes a variety of logging and seismic data from typical drilling sites in the South China Sea and other global oceans to explore the geophysical characteristics of gas hydrate and free gas coexistence zones. Four distinct types of coexistence are identified: (1) coexistence near the base of the gas hydrate stability zone (BGHSZ) in fine-grained, clay-rich reservoirs; (2) coexistence below the bottom simulating reflector (BSR) in fine-grained, clayey silt reservoirs; (3) coexistence above the BGHSZ in cold seep systems; and (4) coexistence in coarse-grained sand reservoirs in high-sedimentation zones. Crossplot analysis and intersection analysis of multiple attributes are effective methods for identifying these coexistence zones. The results indicate that both biogenic and thermogenic gases contribute to the coexistence of gas hydrate and free gas. In gas hydrate systems formed by biogenic gas, high sedimentation rates cause the BGHSZ to shift upward, leading to gas hydrate dissociation. However, this dissociation process requires time, and localized coexistence of gas hydrate and free gas may occur during specific periods. In contrast, gas hydrate systems formed by thermogenic gas often display widespread coexistence of structure II gas hydrate and free gas due to distinct controlling factors. The study highlights that crossplots of porosity and resistivity contrast versus water-saturated layers can be used to identify gas hydrate morphologies based on changing trends within coexistence zones. Additionally, geophysical property analysis reveals that S-wave velocity is a key parameter for distinguishing between gas hydrate and free gas enrichment in these zones. This research provides valuable insights into geophysical characteristics and identification methods for gas hydrate and free gas coexistence zones, contributing to the accurate evaluation of gas hydrate resources.

The origin of hydrocarbon gas is crucial for clarifying the formation and accumulation mechanisms of natural gas hydrate. Many highly saturated natural gas hydrate deposits have been drilled in the northern South China Sea. However, the source and genetic mechanism of hydrocarbon gases have not been systematically elucidated, significantly restricting the identification of main gas sources and the deployment of future exploration strategies. In this study, taking the typical highly saturated gas hydrate deposits in the Qiongdongnan and Shenhu areas as examples, the source and genetic mechanism of hydrocarbon gases are systematically investigated using an integrated approach that includes gas geochemistry, sediment organic geochemistry, geomicrobiology, and seismic interpretation. The results show that the genetic types of hydrate gases in the northern South China Sea are diverse, including primary and secondary microbial gases, as well as thermogenic coal-type and oil-type gases. Notably, the thermogenic gases within different gas hydrate deposits have undergone microbial degradation and modification to varying extents: Shenhu W11-17 gas hydrate deposit>Shenhu W18-19 gas hydrate deposit>Qiongdongnan GMGS5-W08 gas hydrate deposit. The hydrate gas source layer consists of a shallowly buried microbial gas source layer (20-85 ℃) and a deeply buried thermogenic gas source layer (vitrinite reflectance R_o>0.5%). The upper part of the microbial gas source layer (<300 mbsf), which has been drilled and sampled, shows relatively low gas generative potential. In contrast, the thermogenic gas source represents great gas generative potential due to the existence of effective source kitchens and gas-prone coal-type source rocks in the mature stage. The deeply buried thermogenic source kitchen, conventional hydrocarbon deposits, shallowly buried microbial gas source layer, and composite gas migration conduits (including depression-controlled faults, gas chimneys, tubular seeps, and microfractures) constitute the hydrocarbon gas supply system for the highly saturated gas hydrate deposits in the northern South China Sea. The contribution of the deeply buried thermogenic source kitchen is extremely critical for the large-scale accumulation of gas hydrate. However, most of the thermogenic gases transported through the microbial gas source layer undergo severe microbial degradation and are ultimately converted into secondary microbial methane for gas hydrate mineralization. There are three major pathways for hydrocarbon gases in the highly saturated hydrate deposits of the northern South China Sea:(1)Hydrate gases directly originating from the deeply buried thermogenic gas source kitchen, i.e., non-biodegraded thermogenic gas. (2)Hydrate gases derived from secondary microbial gases, which are generated by anaerobic methanogenesis degradation processes after deeply buried thermogenic hydrocarbons migrate into the microbial gas source layer.(3)Hydrate gases supplied by primary microbial gases, which are generated by microbial utilization of in-situ sediment organic matter in the shallowly buried microbial gas source layer.

Recent gas hydrate drilling expeditions, combined with extensive 3D seismic data, have revealed the widespread occurrence of multiple types of gas hydrates on the northern slope of the South China Sea. However, the anomalous geophysical responses associated with variations in gas hydrate types, accumulations, and their primary geological controls remain poorly understood. Fracture-filling gas hydrates, often linked to cold seep systems, are characterized by chimney-like seismic reflections. In contrast, pore-filling gas hydrates exhibit high-amplitude reflections above the bottom simulating reflector (BSR) and are vertically superimposed with fracture-filling gas hydrates in the Tainan and Qiongdongnan basins. In the Pearl River Mouth Basin, pore-filling gas hydrates are predominantly found near the base of the gas hydrate stability zone. The occurrence of fracture-filling gas hydrates is controlled by relatively fine-grained and mass transport deposit (MTD) reservoirs with low porosity and permeability, which are matched with high-flux fluid systems. On the other hand, pore-filling gas hydrates are primarily controlled by coarse-grained reservoirs and relatively low-flux fluid systems. Recent drilling results suggest that the presence of structure II gas hydrates, the coexistence of gas hydrates and free gas, and active gas hydrate systems are closely associated with the supply of thermogenic gas and high-flux fluids from deep gas sources. The study highlights that the diverse tectonic and sedimentary settings in the northern South China Sea are the primary factors driving the occurrence of multiple types of gas hydrates. The coupling of fluid migration pathways, migration mechanisms, and reservoir lithology plays a critical role in controlling variations in gas hydrate accumulations. Key geological features such as faults, gas chimneys formed by magmatism, intrusive structures, mud volcanoes, and basement uplift significantly influence fluid migration and gas hydrate formation. A comprehensive understanding of the differences in gas hydrate occurrences and the identification of key geological controls are essential for exploring highly concentrated gas hydrate reservoirs in the region.

Seepage gas hydrate accumulation systems tend to develop medium-to-high saturation gas hydrates and their associated underlying gases, which are widely distributed gas hydrate resources in the ocean. These systems are also important research subjects for submarine disasters, cold seep systems, and climate change. Therefore, understanding their distribution patterns and formation mechanisms is of great significance. Based on seafloor observations, core samples, well logging, seismic data, and test analysis from the northern South China Sea, this study systematically summarizes the geomorphological, geological, geophysical, and geochemical indicators for identifying seepage gas hydrate accumulation systems. The findings reveal that seepage gas hydrates are primarily distributed within escape pipes. Escape pipes can be categorized into two types based on their mechanical failure mechanisms: (1) hydraulic fracturing escape pipes controlled by structural traps, which exhibit clustered distributions, and (2) shear failure escape pipes controlled by high-angle faults, which show aligned distributions. In seepage gas hydrate accumulation systems, deep gas migrates and accumulates near the bottom of the methane gas hydrate stability zone along gas chimneys and faults. When fluid pressure exceeds the fracturing strength of the overlying fine-grained sedimentary layer, free gas breaks through upward to form escape pipes. This process results in medium-to-high saturation fracture-filling gas hydrates within high-angle fracture networks in the escape pipes and high-saturation pore-filling gas hydrates in thin sand layers intersected by the pipes. The long-range migration of free gas within escape pipes is influenced by three mechanisms: (1) thermodynamic three-phase equilibrium maintained by salt exclusion, (2) methane diffusion limited by the formation of gas hydrate shells, and (3) gas hydrate kinetic formation rates constrained by high fluid fluxes. The formation process of seepage gas hydrates associated with active cold seeps can be divided into three stages. First, during the cap fracture stage, gas hydrate formation is influenced by kinetic rates under over pressure. Second, in the upward breakthrough stage, locally elevated salinity and temperature maintain thermodynamic three-phase equilibrium. Finally, when the fluid reaches the seafloor, the ground temperature approaches the background value, entering a stable leakage stage. Salt exclusion and methane diffusion limitations caused by gas hydrate shells control the generation of gas hydrates. After cold seep activity ceases, salinity decreases due to continuous diffusion, and new gas hydrates gradually form again at the bottom of the escape pipe.

Sand-rich gas hydrate reservoirs are prime targets for hydrate exploration and test production worldwide. Concentrated hydrate in sand-rich reservoirs has been drilled in the Qiongdongnan Basin, northern South China Sea. Based on 3D seismic data, well logging, core samples, and test analysis, the large-scale accumulation of sand-rich gas hydrate has been confirmed. The sand-rich hydrate reservoir is characterized by low gamma ray, low sigma, high resistivity, increased P-wave and S-wave velocities, and a sharp reduction in T₂ spectrum signal amplitude. Additionally, high attenuation of P- and S-waves is observed in hydrate-bearing zones. The sand layer is distributed in a wedge-shaped horizontal sheet. The lithology is mainly coarse silt to fine sand, developed in the terminal lobe of deep-water turbidite systems. In vertical sections, gas hydrate and free gas exhibit co-layered distribution and lateral transition characteristics. In map view, the gas hydrate reservoir shows an elliptical distribution, with free gas concentrated in the center and gas hydrate developed in the periphery. Unlike gas hydrate reservoirs in dipping sands of other regions worldwide, the gas hydrate reservoir in the Qiongdongnan Basin was formed in a horizontal sand layer near the base of the hydrate stability zone. The formation of highly saturated hydrate was driven by locally high heat flow at the top of the underlying gas chimney and capped by overlying fine-grained sediments. A large area of hydrate was formed through radially lateral, long-distance migration of sufficient free gas from the center to the periphery.

It is crucial for comprehending the accumulation mechanisms of gas hydrates and assessing the potential environmental impacts caused by their dissociation and associated methane seepage to understand fluid migration processes and seabed methane seepage in gas hydrate studies. This paper analyzes the relationships between fluid migration conduits, seabed methane seepage, and gas hydrate systems by examining numerous gas hydrate case studies worldwide. Fluid migration conduits are categorized into two types based on their roles in the gas hydrate system. Type I conduits are primarily distributed below the bottom simulating reflector (BSR) and serve as gas sources for the gas hydrate system. Type II conduits are in sediments above the BSR and may extend to the seabed, acting as pathways for gas leakage from the gas hydrate system. The role of polygonal faults as fluid migration conduits is considered limited, mainly contributing to the accumulation of shallow gas and gas hydrates. Seabed methane seepage is classified into three categories and five subcategories based on the spatial relationship between methane seepage, the gas hydrate system, and gas source conditions: (1) Deeper than the landward limit of the gas hydrate stability zone (LLGHSZ): (Hydrate-related gas source and non-hydrate-related gas source); (2) Around the LLGHSZ (Hydrate-related gas source and non-hydrate-related gas source); (3) Shallower than the LLGHSZ (Non-hydrate-related gas source). The intensity and density of methane leakage are highest near the LLGHSZ. This classification of fluid migration conduits and seabed methane seepage provides a framework for understanding the dynamic processes of gas hydrate accumulation and dissociation. It also offers insights into evaluating the environmental and climatic impacts associated with gas hydrate systems.

The gas hydrate reservoir in the Shenhu area of the northern South China Sea is primarily composed of fine-grained sediments with a saturation as high as 30%, which is significantly higher than other reservoirs worldwide. Studying the characteristics of fine-grained reservoirs in this area is crucial for understanding the mechanism of hydrate accumulation, resource evaluation, and the design of hydrate trial production and development strategies. In this study, we conducted detailed processing of multi-channel seismic data in the Shenhu area to obtain velocity models and stacked profiles with different offsets. Subsequently, pre-stack seismic inversion was performed to derive high-resolution elastic parameters, including P-wave and S-wave impedance, P-wave to S-wave velocity ratio, and Poisson’s ratio. These results were combined with well-logging data to provide a refined geophysical description of gas hydrates and free gas in the fine-grained sediment. Our study reveals the following: (1) The amplitude of the BSR (bottom-simulating reflector) decreases as the incident angle increases, making it easier to identify the BSR on near-offset stacked profiles; (2) Chimney structures with low-velocity anomalies develop below the BSR and may serve as pathways for vertical fluid migration; (3) Hydrate reservoirs in the study area exhibit strong horizontal heterogeneity, and free gas is generally present beneath the hydrate with high saturation, suggesting that hydrate recycling may play an important role in hydrate enrichment in fine-grained sediments.

Recent drilling in the Qiongdongnan Basin, located in the northern South China Sea, has uncovered gas hydrate deposits. Understanding the timing of gas hydrate formation is crucial for deciphering the processes of hydrate accumulation and distribution. This study, for the first time, determines the deposition age of gas hydrate reservoirs by analyzing the sediment chronology of hydrate drilling cores. The lower limit of hydrate formation age is established by combining these findings with the distribution of hydrate intervals. Subsequently, the deposition rate and its influencing factors are analyzed, providing robust geological insights for gas hydrate exploration in the northern South China Sea. The results indicate that gas hydrate formation in this region occurred between 65 and 90 ka. The deposition rate of gas hydrate reservoirs is influenced by sea-level fluctuations during glacial and interglacial periods. During interglacial periods, rising sea levels reduced terrigenous clastic input, leading to a decrease in sedimentation rates. Conversely, the primary formation period of gas hydrate reservoirs in the Qiongdongnan Basin coincided with sea-level drops, increased terrigenous material input, and accelerated sedimentation rates.

Natural gas hydrates are primarily distributed in seafloor sediments and continental permafrost. Research on hydrate phase equilibrium and its influencing factors in porous media is of great significance for understanding the formation kinetics, distribution range, and reserve estimation of hydrates in seafloor sediments. Most current experimental studies focus on synthetic porous media, while the impact of natural sediments on hydrate phase equilibrium characteristics requires further exploration due to their complex structures and compositions. This article reviews the changes in the phase equilibrium of hydrates in porous media with varying pore sizes, comprehensively analyzing the specific effects of pore size, particle size, and surface wettability on hydrate phase equilibrium. Studies have shown that the strong capillary force generated by porous media can reduce water activity, thereby inhibiting hydrate formation, with a critical value observed in the nanoscale range. Additionally, smaller porous media particle sizes increase nucleation sites and reaction interfaces, reducing induction time and promoting the nucleation and growth of hydrates. Regarding the impact of surface wettability (hydrophilic vs. hydrophobic) on hydrate formation, academic opinions remain divided. However, most studies suggest that hydrophobic surfaces promote hydrate formation more effectively than hydrophilic surfaces.

The use of fracturing technology to transform and increase production in hydrate reservoirs has been proven to have application prospects for hydrate production. The main purpose of this study is to analyze the gas production patterns of reservoirs under the interaction between two production wells, and to obtain the optimal distance between production wells. This article is based on the hydrate occurrence parameters of DK-2 station in the permafrost region of the Qilian Mountains. The reservoir outside the dual production wells were fractured by fracturing, and the production potential of this method for extracting hydrate sediments in the permafrost region was evaluated through numerical simulation. The simulation results show that the use of hydraulic fracturing method to enhance the permeability of the surrounding reservoirs of the mining well has broken through the technical bottleneck of using only depressurization method to extract hydrates. The gas production rate and cumulative gas release are higher than traditional depressurization methods, and the average gas water ratio has significantly increased. After the rock matrix around the horizontal well ruptures, it will increase the flow characteristics of the fracturing zone, thereby promoting the propagation distance of pressure drop. The infiltration of hydrate decomposition areas between horizontal production wells is conducive to promoting the decomposition of gas and water flow towards the production wells. In addition, hydraulic fracturing methods are very effective for hydrate reservoirs with intrinsic permeability below 1 mD, and the recommended optimal distance between production wells is 45-60 m.

The South China Sea holds abundant gas hydrate resources with significant potential for industrial development, as demonstrated by successful natural gas hydrate pilot tests in the Shenhu area. Most gas hydrate-bearing sediments in the South China Sea are classified as Class Ⅰ gas hydrate deposits, underlain by a two-phase zone containing mobile gas. These reservoirs are characterized by unconsolidated argillaceous siltstones and a threshold pressure gradient (TPG), which significantly impacts gas production. A mathematical model incorporating TPG was developed to simulate multiphase flow in typical subsea sediments, and a TPG module was integrated into the TOUGH+HYDRATE simulator in this study. The Well SHSC4, located in the Shenhu test field, was selected to evaluate the effects of TPG on gas production. Simulation results reveal that TPG restricts pressure propagation, preventing hydrate dissociation near the reservoir boundaries. A phenomenon of low water output and a high gas-to-water ratio was observed during gas production. The water production from the horizontal well nearly ceased in the later stages. The TPG plays a critical role in preventing the formation of secondary gas hydrate shells in the free dissociation zone and eliminating gas traps. A review of 10 years of production data indicates that TPG can enhance gas production from Class Ⅰ gas hydrate deposits while TPG limits the recoverable reserves of gas hydrate. Notably, gas production increased by nearly 40%, attributed to TPG’s ability to improve free gas drainage in vertical wells. These findings highlight the dual role of TPG in both restricting and promoting gas production, offering valuable insights for optimizing gas hydrate exploitation in the South China Sea.

The storage of carbon dioxide (CO₂) hydrate using promoters has garnered significant attention in the context of increasing energy demand and the “dual carbon” goals. However, different promoters exhibit varying effects on the properties of CO₂ hydrate. This study investigates the formation and stability of CO₂ hydrate using silica gel as a porous medium under initial formation conditions of 3.0 MPa and 274.15 K, followed by decomposition at 275.15 K. The effects of decomposition pressures (0, 0.5, and 1 MPa) on the stability of CO₂ hydrate were analyzed in systems containing pure water and L-Met (L-methionine) solutions at concentrations of 0.8, 0.9, 1.0, 1.1, and 1.2 g/L. The results show that the CO₂ hydrate in the 1.1 g/L L-Met solution exhibits the highest stability at a decomposition pressure of 0 MPa. The 0.9 g/L L-Met solution provides the best hydrate stability at decomposition pressures of 0.5 MPa and 1 MPa, which is favorable for stable hydrate storage. In addition to analyzing the influence of L-Met concentrations on hydrate stability, the effect of decomposition pressure on hydrate dissociation rates was also studied. In the pure water system, the dissociation rate initially increases and then decreases under a decomposition pressure of 0 MPa. For all other decomposition pressures, the maximum dissociation rate occurs at the initial stage, followed by a monotonic decrease. Furthermore, in L-Met systems, except for the 0.8 g/L concentration, the dissociation rate of hydrate under atmospheric pressure (0 MPa) initially increases and then decreases, demonstrating the “self-protection” effect of hydrate. However, the dissociation rate reaches its maximum at the initial moment and monotonically decreases at decomposition pressures of 0.5 MPa and 1 MPa, indicating that higher pressures undermine the self-protection effect. This study provides insights into the effects of promoter concentration and decomposition pressure on the stability and dissociation behavior of CO₂ hydrate, contributing to advancements in hydrate-based CO₂ storage technologies.

The depressurization method is widely regarded as one of the most economical and efficient methods for recovery from natural gas hydrate reservoirs. Hydrate reformation usually occurs around and inside the wellbore, hindering fluid flow, which may cause damage to engineering equipment and significantly affect the efficiency and sustainability of gas production. Based on the hydrate exploitation modeling apparatus developed by Zhejiang University, the hydrate reformation effect under different depressurizing rates and its impact on the pressure, temperature, gas production, and deformation of the hydrate bearing sediment are investigated. Results indicate that the reformation of hydrate in the wellbore causes the cyclic production of hydrate dissociation gas, significantly affecting the temperature and pressure of the reservoir. The dissociation rate decreased due to mass transfer obstruction when the wellbore flow was obstructed. The higher the depressurizing rate, the higher the peak of gas production rate; however, the higher average gas production rate can not be achieved due to the more significant hydrate reformation in the wellbore. During field multiwell exploitation, an appropriate depressurizing rate should be selected based on reservoir permeability, temperature, pressure, and other conditions, and each well can be opened and closed alternately so that the effective gas production rate can be increased by utilizing the ambient supplemental sensible heat during the well closure period.

The adjustment of energy resources and the implementation of the national “dual-carbon” strategy provide a rare space for research on marine natural gas hydrates as a potential source of energy. The safe and efficient development of marine natural gas hydrate increasingly faces prominent contradictions in managing and controlling multiple types of engineering geology risks as exploration and development research deepens. It is urgent to develop marine natural gas hydrate engineering geology based on the basic principles of modern engineering geology. Therefore, this paper describes the process of proposing the engineering geology of natural gas hydrate, and discusses the basic disciplinary framework, core research tasks and main research methods of marine natural gas hydrate engineering geology based on the research progress at home and abroad. It is recognized that marine gas hydrate engineering geology is an integral part of marine earth system science. Its core objective is to assess the geological and engineering safety bearing capacity of marine gas hydrate systems and to provide decision support for marine gas hydrate exploration and development. Marine gas hydrate engineering geology can not only provide scientific basis for answering the interaction mechanism between exploration and development activities and marine gas hydrate systems, but also effectively link marine gas hydrate energy research with geohazard and global climate change research and provide certain theoretical and technical support for the integrated sustainable development of marine gas hydrate geology, environment and engineering.

In the context of the global energy structure transformation and the increasing demand for clean energy, medium-deep geothermal resources, as a practical, localized, and stable renewable energy source, are receiving significant attention from the international community. China’s geothermal resources exhibit distinct regional characteristics, with higher geothermal potential in the eastern and southwestern regions. It is estimated that the theoretical energy reserves of China’s medium-deep geothermal resources are equivalent to 1250 billion tons of standard coal. At present, China has established a relatively mature technical system for the exploration and development of medium-deep geothermal resources, covering all aspects from exploration to efficient development, greatly enhancing the accuracy of resource exploration and development efficiency. In geothermal exploration and evaluation technologies, comprehensive analysis of geothermal system reveals the enrichment patterns and formation mechanisms of geothermal resources; Comprehensive geophysical exploration focuses on fracture structural and water-bearing reservoirs to delineate target areas; Geothermal resource site selection and evaluation technology has guided the geothermal exploration work in North China, with a success rate of over 80% for the deployed exploration wells. In high-efficiency geothermal development technologies, Multi-field coupling simulations optimize extraction parameters to prevent thermal breakthrough while meeting production demands.; Geothermal energy development drilling and completion technology adapts to local conditions to refine well structure and drilling and completion processes, effectively ensuring the safe and efficient construction of geothermal wells; Natural recharge technology for geothermal water and heat extraction without water consumption technology achieve full-cycle recharge of geothermal water and sustainable development of geothermal energy. Notable large-scale applications include the Xiong’an area (Bohai Bay Basin) and Xianyang region (Guanzhong Basin), representing carbonate and sandstone reservoir development respectively. Based on geological modeling and numerical simulation, specific well network layout and development parameter optimization schemes have been proposed for the above two regions, providing reference for efficient, environmentally friendly, and sustainable development of geothermal resources in areas with similar geological conditions.

The southern Qinshui Basin hosts a thick sequence of Precambrian sedimentary strata with significant hydrocarbon exploration potential, making it a promising area for future exploration. However, the geological characteristics of these strata remain a subject of debate. This study integrates field outcrop surveys, geochemical analyses of hydrocarbon source rocks, detailed seismic data interpretation, and previous research findings to establish the Precambrian stratigraphic system, analyze the distribution characteristics of the sedimentary strata, and predict favorable hydrocarbon-generating areas. The results reveal that the Precambrian sedimentary strata in the southern Qinshui Basin belongs to the Xiong’er Rift system. From bottom to top, these strata include the Changchengian Xiong’er Group, Yunmengshan Formation, Baicaoping Formation, Beidajian Formation, Cuizhuang Formation, Luoyukou Formation, and the Jixian System Longjiayuan Formation. The Yunmengshan, Baicaoping, Beidajian, and Cuizhuang Formations exhibit greater thickness in the central and southern regions, thinning toward the east, north, and west. The Luoyukou Formation shows maximum thickness in the central region, thinning toward the east and west, while the Longjiayuan Formation is thickest in the east and thins toward the west. The study identifies the Longjiayuan Formation as having relatively large favorable hydrocarbon-generating areas, primarily in the eastern part of the study area by analyzing factors such as stratigraphic thickness, lithology, seismic facies, and sedimentary facies. In contrast, the Cuizhuang Formation has smaller favorable hydrocarbon-generating areas, mainly located in the southern part of the study area. This research contributes to the geological understanding of Precambrian petroleum systems in the southern North China Craton, clarifies the characteristics and distribution of Precambrian source rocks in the southern Qinshui Basin, and provides guidance for optimizing future exploration strategies for Precambrian oil and gas resources.

The mechanism of helium accumulation is unique and more complex than that of petroleum systems. Since the discovery of helium-rich hot springs in Tanzania in 1967, no large-scale helium fields have been identified. To establish the mechanism of helium charging and accumulation in the Rukwa Rift Basin, this study conducted research in five key areas using geochemical, seismic, drilling, and logging data: analyzing the helium and carrier gas content at the surface, calculating helium production in the craton basement, examining the helium release process, establishing the helium charging mechanism, and identifying favorable helium zones.The findings reveal that the Mesozoic-Cenozoic rifts play a critical role in controlling helium migration and accumulation within the basin. Surface helium content ranges from 1.0% to 10.2%, with carrier gases including nitrogen, carbon dioxide, and methane. The ³He/⁴He isotope ratio ranges from 0.039 to 0.053 R_a, indicating that helium originates from both crustal and mantle sources. Helium generation in the basement and sedimentary layers is estimated at 367 billion cubic meters. The release of helium from the peripheral rift of the Tanzanian Craton involves five key processes: generation, release, migration, charging, and overflow.Three helium accumulation models have been identified in the Rukwa Basin: nitrogen-helium desorption, coalbed methane-helium extraction, and carbon dioxide-helium extraction. Nitrogen, an inorganic gas of deep origin, is characterized by accumulation from homologous sources, with helium desorption from water being the primary helium charging mechanism in the basin. Coalbed methane, primarily sourced from the depositional center of the basin, accumulates independently of helium. Carbon dioxide, an inorganic gas of metamorphic origin, shares a symbiotic accumulation mechanism with helium similar to that of coalbed methane.The most favorable helium-rich zones in the basin are located in the Basin Margin Fault Closures (BMFCs). In the overlying soil of these traps, helium content is 35% higher than the background value. To date, 12 BMFCs have been identified, with unrisked prospective geological helium resources estimated at 5.74 billion cubic meters, accounting for 64.6% of the undiscovered resources in the basin. These zones represent favorable exploration targets in rift basins. The results of this study enhance the understanding of helium accumulation mechanisms in rift basins and provide valuable guidance for optimizing helium exploration in favorable areas.

The E₂ s_4^{d^3} layer in the Leijia area of the western sag is characterized by lacustrine fine-grained sedimentary rocks, intermixed with clay, felsic, carbonate, and analcime minerals. Recent exploration efforts have been primarily directed towards tight oil and shale oil within this region. Previous research amalgamated drilling, logging, and analytical testing data to comprehend the geological characteristics of fine-grained rock reservoirs. Employing unsupervised learning techniques, particularly K-means cluster analysis, facilitated the classification of fine-grained sedimentary rocks into distinct types based on 303 X-ray diffraction whole-rock quantitative analysis data. The lithology of the E₂ s_4^{d^3} layer was categorized into carbonate rock type, felsic mixed fine-grained rock type, and analcitic mixed fine-grained rock type. Notably, carbonate rocks and analcite mixed fine-grained rocks exhibit superior brittleness characteristics, are prone to cracking, and have developed dissolved pores and caves along fractures, thereby enhancing storage and seepage properties. Evaluation of the E₂ s_4^{d^3} reservoir revealed a small pore throat radius, poor sorting, small homogeneity coefficient, and inadequate pore structure. Effective reservoir formation predominantly relies on micropores and microfractures. Through the overlapping favorability method, six factors influencing reservoir development were integrated and assessed, including brittleness index, thickness of carbonate rock and analcitic mixed fine-grained rock, fracture density, average porosity, average permeability, and organic carbon content. Favorable zones for reservoir development are primarily distributed along the long axis of the lake basin, encompassing specific wells such as Lei 15, Lei 84, Lei 59, Shu 90, and Lei 93. This summary provides a comprehensive overview of the geological study, encompassing methodologies, findings, and implications for oil exploration in the Leijia area.

The formation of black shale in sedimentary basins, both domestically and internationally, is often associated with varying degrees of uranium (U) enrichment. Understanding the enrichment mechanisms of U in black shale and its impact on the formation of organic minerals is critical. This study systematically reviews the processes of U enrichment in organic matter, microorganisms, clay minerals, iron-bearing minerals, sulfur-bearing minerals, and phosphorus-bearing minerals. It focuses on the mechanisms of U occurrence in black shale and the effects of U on the oil-generation properties, gas-generation properties, and kerogen structure of organic matter. Uranium enrichment in black shale occurs through diverse pathways, with various shale components enriching U in the environment via mechanisms such as complexation, adsorption, and reduction. The efficiency of U enrichment is influenced by multiple factors, including pH and the concentration of interfering ions. Additionally, the impact of U on the oil and gas production performance of shale organic matter is complex and remains controversial, as simulation experiments have yielded inconsistent results due to differences in experimental conditions and sample characteristics. Despite significant research efforts, many studies on the influence of U on organic matter remain limited to qualitative analysis, leaving several critical issues unresolved. These include: (1) the lack of a clear microscopic mechanism for U enrichment in black shale, hindering the development of a comprehensive U enrichment model; (2) challenges in analyzing the reservoir residence time of crude oil due to the difficulty of effectively characterizing the radiation dose of oil; and (3) uncertainties regarding the similarities and differences between the effects of artificial and natural radiation on the oil and gas production performance of organic matter, making it difficult to identify the primary factors controlling the evolution of organic matter under radiation. Future research should employ advanced analytical techniques to investigate the microscopic mechanisms of radiation effects, conduct molecular composition and structural analysis, identify effective radiation indices in oil, and expand the application of radiation studies in geology. This work has significant implications for advancing the understanding of oil and gas formation processes, exploring new oil and gas resources, and enriching geological knowledge about U in black shale.

Ultra-deep carbonate fractured-vuggy reservoirs, typically located near fault zones, are characterized by significant burial depth, large vertical extent, and narrow horizontal distribution, making seismic identification highly challenging. Conventional fracture-vuggy identification techniques primarily rely on amplitude response characteristics, which only provide the general orientation and approximate extent of the reservoir. To address these limitations, a refined interpretation method for fractured-vuggy reservoirs was proposed based on a two-dimensional Res-UNet residual network and optimized label data. This method enhances the update frequency and optimization efficiency of 2-D training samples by modifying the Res-UNet residual network structure, thereby improving the accuracy of reservoir prediction. Geological models of faults, fractures, and karst caves were constructed using a combination of satellite imagery, UAV scanning, field reconnaissance, geological radar, and seismic interpretation results. Wavelet analysis of 3-D seismic data in the depth domain, along with imaging and acoustic logging, was performed to extract formation and reservoir velocities. Wavelets at frequencies of 20 Hz, 25 Hz, and 35 Hz were selected to create forward models for different reservoir widths. An empirical formula was derived to establish the relationship between reservoir characteristics and seismic response based on frequency and formation velocity. Verification of the empirical formula was conducted using horizontal well trajectories, vertical well remote detection data, and logging data to extract the relationship between actual reservoir characteristics and seismic responses. This formula was then applied to amplitude attributes of the target layer in the study area to estimate the reservoir width range. The reservoir combination characteristics of the study area were identified, and a comprehensive detection workflow—incorporating analysis, design, verification, and redesign—was established through the analysis of 3-D seismic profiles. Optimal parameters, including wavelet frequency, migration aperture, and sampling interval, were designed for the forward model of the study area. A 2-D training sample database capable of real-time updates was developed, and the fracture-vuggy labels were iteratively optimized by integrating synthetic fracture-vuggy data with actual data. Using this database, the Res-UNet residual network was trained and applied to address the degradation problem commonly encountered in deep networks. This approach enabled the fine-scale prediction of fracture-vuggy structures, significantly improving the resolution and accuracy of reservoir interpretation.

The Apollo Basin is the impact basin with the deepest excavation depth and the largest diameter among the lunar South Pole-Aiken Basin (SPA). It is the pre-selected landing area for the Chang’e-6 mission to carry out sampling and return operations on the far-side of the moon in 2024. Studying the geological characteristics and evolution of this region is of great significance for understanding a series of lunar scientific issues such as the structure and composition of the lunar crust and mantle, the formation and evolution of the SPA tectonic zone, as well as the gravity anomalies of the SPA Basin. This study investigates the topographic and geomorphological characteristics, petrological features, and structural elements of the Apollo Basin and its adjacent areas based on multi-source remote sensing data and utilizing GIS as a technical platform. By re-dividing the basalt units within the Apollo Basin and applying the crater size-frequency dating method alongside the updated lunar age function model, we determined the absolute model ages of the mare basalt units. Furthermore, this study clarifies the evolutionary history of regional geological structures and analyzes the thermal evolution model of regional volcanic activity, providing valuable insights for future research related to the Chang’e-6 lunar probe sample return mission. The key findings are as follows: The exposed rocks in the study area primarily consist of ferrous anorthositic suite, ferrous noritic suite, and mare basalts, with scattered occurrences of pure anorthosite and igneous clastic rocks. The absolute model ages of the mare basalt units within the Apollo Basin range from 3.47 to 2.57 Ga. The study area contains 127 crater floor faults and 14 volcanic vents, which serve as significant structural indicators of regional thermal evolution. The region has experienced three major impact events, leading to the formation of the South Pole-Aitken (SPA) Basin, Apollo Basin, and Oppenheimer Basin. Volcanic activity in the area persisted from the Late Imbrium to the Eratosthenian period, with at least two distinct episodes of basaltic eruptions.

The Xing-Meng Orogenic Belt is composed of many microcontinents, island arcs, accretionary wedges and ophiolite (oceanic crust fragments). It has experienced a complex tectonic evolution history and recorded important information such as the subduction of the Paleo-Asian Ocean and the final assembly of the Siberian and North China Plates. Its evolution has always been a hot topic in the field of geosciences at home and abroad. It is of great significance to study the relationship between the geological bodies in the Xing-Meng Orogenic Belt and its adjacent areas for its tectonic evolution. Obtaining fine upper crustal structure is the key to determine the contact relationship between different blocks in the Xing-Meng Orogenic Belt and reveal the Mesozoic-Cenozoic tectonic evolution process. In this paper, a wide-angle reflection and refraction seismic profile data with a total length of 503 km across Songliao Basin, Xing-Meng Orogenic Belt and Erlian Basin is studied by first-arrival wave tomography. In this study, the finite difference algorithm is used to calculate 693 first arrival traveltime picking data of 16 artillery data. The inversion strategy of variable grid scale and smoothing parameters is adopted. After 40 iterations of inversion, the RMS is reduced to 0.103 s, and the fine velocity structure of the upper crust (more than 7 km) is obtained. The imaging results describe the underground velocity structure of the study area in detail: the northern Erlian Basin has low velocity characteristics, which is a deep south and shallow north fault basin, with a maximum depth of 5.5 km, and its development is mainly controlled by normal faults with opposite tendencies on both sides. The upper crust of the Xing-Meng Orogenic Belt (between the Nenjiang Fault and the Hegenshan Suture Zone) is characterized by high velocity and dramatic lateral changes. There are three mountain basins, most of which are Mesozoic-Cenozoic sediments with few Quaternary sediments, the velocity of the sedimentary layer is higher than that of the basins on both sides of the orogenic belt. The southern Songliao Basin is a typical half-graben rift basin. The reverse movement of normal faults caused by the later NWW-SEE horizontal compression caused the top interface of the crystalline basement to be basically consistent with the fold deformation of the sedimentary layer. The deepest part of the basin along the survey line can reach 5.5 km. The location of the fault zone and the distribution of the upper crust in the study area are determined based on the velocity anomaly. Most of the faults and shallow faults are steep with large angles, and the dip angle gradually decreases and evolves into a shovel type during the extension to the deep.

The exploration and discovery of hidden and deep ore deposits represent a scientific frontier and a key research focus in the field of mineral exploration. Large-scale prospecting prediction is a critical approach to achieving scientific prospecting and directly supports mineral exploration efforts. Guided by the prospecting prediction theory and methodology of metallogenic geological bodies, this study focuses on the Yindongpo gold deposit as the primary research object. Building on a systematic review of previous research, a geological model for ore-prospecting prediction is constructed, emphasizing metallogenic geological bodies, metallogenic structures, metallogenic structural planes, and metallogenic characteristics. This model aims to provide a framework and demonstration for large-scale gold-polymetallic ore prediction in the region. The Yindongpo gold deposit is identified as a magmatic hydrothermal deposit associated with intrusive magmatic activity. The metallogenic geological body is a medium-acid concealed rock mass formed during the Early Cretaceous. The metallogenic structure is primarily a fold-related metallogenic structural system, while the metallogenic structural planes include lithologic interfaces (e.g., silica-calcium planes), fold-related structural planes, and potentially intrusive-related structural planes. The macro-alteration features of mineralization are dominated by silicification and pyrite-sericitization. Metallogenic elements exhibit near-range Pb, Zn, and Ag mineralization associated with Au, and remote Au mineralization associated with Pb, Zn, and Ag. By integrating geophysical and geochemical exploration data, a comprehensive information model for ore-prospecting prediction was developed. Five prospecting target areas were delineated, and subsequent drilling verification resulted in significant prospecting breakthroughs.

The structural system is the cornerstone of the geomechanics theory and methodology established by Prof. Li Siguang (J. S. Lee). This study begins with an exploration of the fundamental concept of the structural system, delving into its connotations to further develop and apply geomechanics theory and methodology based on the various interpretations and applications of the structural system in relevant literature, which provides a foundation for understanding the principles of crustal structure and movement, investigating the relationship between structure, diagenesis, and mineralization (reservoir formation), and guiding the exploration and evaluation of mineral resources.The study discusses the mechanical mechanisms of structural systems formed under the influence of principal compressive stress and the continuous effects of couples. Structural systems exhibit characteristics such as uniformity, regionality, hierarchy, inheritance, and complexity. It is proposed that structural types are hierarchical, while ore-forming structural systems possess distinct features, including consistency, periodicity, differentiation, diversity, and transformation. The ore-controlling rules of ore-forming structural systems are summarized and revealed. By examining the genetic relationship between ore-forming structural systems (ore-controlling structural types) and metallogenic systems, this study identifies key factors such as the regular distribution of ore-concentrated areas, ore fields, and ore deposits; the contemporaneity of structural deformation, magmatic activity, and mineralization; and the compositional consistency between ore-bearing structures and ores. These factors are used to define ore-forming structural systems and propose a research methodology for their study. This methodology provides a framework for prospecting predictions and deep exploration of ore deposits and ore bodies. To illustrate the ore-controlling roles and significance of ore-forming structural systems, this study examines two examples: the non-magmatic hydrothermal polymetallic metallogenic system (e.g., lead-zinc polymetallic deposits in the Sichuan-Yunnan-Guizhou metallogenic area) and the magmatic hydrothermal polymetallic metallogenic system (e.g., copper-tin polymetallic deposits in the Huangshaping-Baoshan ore field in southern Hunan).

The Jinchuan deposit is the third largest magmatic copper-nickel sulfide deposit in the world. There are still a lot of controversies about its metallogenic mechanism and there are two main models: ① Magmatic conduit accumulation metallogenic model and ② Deep segregation-injection metallogenenic model. However, none of them can explain the geological phenomena in the mining area well, especially the key problems such as the emplacement of high density sulfide melts. The new progress in experimental petrology and dynamics simulation shows that fluid addition can effectively promote the migration of high-dense sulfide slurry and it might be the main mechanism of emplacement of sulfide melts in the Jinchuan deposit. Previous research found that primary Cl-rich hydrated minerals (phlogopite, amphibole, apatite and calcite) are widely distributed in the Jinchuan sulfide ore, indicating that the formation process of the Jinchuan deposit might be significantly influenced by Cl- and C-rich fluids. This paper focuses on the characteristics of specific hydrous minerals in the deposit to discuss the migration mode and emplacement ability of sulfide meltIn order to investigate the significant effects of this Cl-rich and C-rich fluid on migration of the high-density sulfide melts. The primary hydrous minerals and sulfides in the Jinchuan deposit constitute fluid minerals assemblage. It is speculated that the Cl- and C-rich fluid gradually evolved from calcium-rich to magnesium and silicon-rich according to the spatial distribution of the fluid minerals and the moving direction of the magma conduit.

Acid mine drainage (AMD) has garnered global attention due to its chemical toxicity and ecological hazards. This study investigates the geochemistry of AMD and sediments in the Xiangshan uranium mine tailings area, focusing on the concentrations and fractionations of rare earth elements (REEs) and their controlling factors. A total of 22 AMD samples and 31 sediment samples were collected along the flow direction of AMD for field and laboratory analysis. The pH of AMD ranged from 3.65 to 6.24, with an average of 4.51, and the hydrochemical type was predominantly SO₄^2--Ca. Total REE concentrations (ΣREEs) in AMD varied between 0.41 and 191.23 μg/L, with an average of 80.32 μg/L. A negative correlation was observed between pH and ΣREEs concentration, indicating that pH plays a critical role in controlling REE concentrations. Hydrogeochemical modeling using PHREEQC revealed that REEs in AMD primarily exist as SO₄^2- complexes and free ions (>99%). Upper continental crust (UCC) normalization showed that AMD was enriched in heavy REEs (HREEs) relative to light REEs (LREEs), with a negative Ce anomaly. In sediments, ΣREEs concentrations ranged from 170.58 to 1259.18 μg/g and decreased with increasing depth. Shallow sediments exhibited HREE-enriched patterns similar to those of AMD samples. Sediments from the tailings pond showed a higher degree of HREE enrichment compared to sediments downstream of the tailings pond. However, with increasing depth, the HREE enrichment in downstream sediments gradually weakened, and UCC-normalized patterns evolved into relatively flat or even LREE-enriched patterns. A sulfuric acid leaching test (1∶40) on sediments revealed that the leachate exhibited REE patterns similar to AMD, with HREEs preferentially mobilized into solution compared to LREEs. This indicates that shallow sediments readily inherit the REE signatures of AMD. However, vertical migration to deeper sediment layers resulted in REE fractionation, with HREEs preferentially mobilized over LREEs, leading to a distinct vertical evolution in sediment REE patterns. The findings of this study provide valuable insights into the environmental evolution of uranium mine tailings areas and are significant for the prevention and control of soil and water pollution. Understanding REE fractionation in AMD and sediments can aid in developing strategies to mitigate the environmental impacts of uranium mining.

Shengsi Island, located at the northernmost end of the Zhejiang-Fujian magmatic belt, represents a transitional zone between land and sea, significantly influenced by the subduction of the ancient Pacific Plate. Petrological analysis, zircon U-Pb geochronology, zircon Lu-Hf isotopic microanalysis, and geochemical characterization were conducted on the Shengsi syenite granite pluton in this study.The Shengsi pluton primarily consists of potassium feldspar (50%-65%), quartz (20%-25%), plagioclase (20%-25%), and biotite (5%-10%), exhibiting a medium- to fine-grained granite texture. High-precision zircon U-Pb dating yielded a weighted average age of 95.4±1.2 Ma, indicating that the pluton formed during the Late Cretaceous. Zircon Lu-Hf isotopic analysis revealed the consistently negative ε_Hf(t) values (-6.2 to -4.0) with a narrow range of variation, significantly higher than the average crustal value. The two-stage zircon model ages range from 1405 Ma to 1547 Ma (mean 1465 Ma), which are younger than the second-stage model ages of the Huaxia basement metamorphic rocks (Taoxi). Geochemical analysis indicates that the Shengsi syenite granite is characterized by high silicon, alkali, aluminum, and potassium contents, with extremely low levels of Mg, Ca, P, Ti, and Fe. It belongs to the metaluminous to weakly peraluminous, high-potassium calc-alkaline granite series. The rock is enriched in large-ion lithophile elements (e.g., Rb, K) and high-field-strength elements (e.g., Th, U, Zr, Hf), while elements such as Nb, Sr, and P are depleted. The total rare earth element (REE) content is relatively high, with stronger fractionation of light REEs compared to heavy REEs, pronounced negative europium anomalies, and high fractionation indices. The low Sr and high Yb content of the granite suggests relatively low pressure during its formation. The Shengsi syenite granite is classified as A-type granite based on the petrological and geochemical characteristics, as well as the tectonic setting. It formed during the Late Cretaceous in a tectonic environment associated with the retreat and extension of the ancient Pacific Plate. This process involved lithospheric thinning and crustal melting caused by the intrusion and upwelling of mantle material. The resulting magma, primarily crustal in origin, formed through shallow crustal intrusion after crust-mantle mixing.

Setting 2022 as the time background, and the Zijiang River segment in Dongting Lake as the study area, hydrochemistry and Rn isotope tracing were adopted to study the river-groundwater interaction in the Dongting Lake, and to reveal the abnormal change of the water exchange along rivers under the extreme climate condition. In January and August, the hydrochemistry type of the river water was HCO₃-Ca, and that of the groundwater was HCO₃-Ca and HCO₃-Ca to HCO₃ ·SO₄-Ca, respectively. The rock leaching are the main controlling factor of the river water and groundwater, and river water and groundwater may have experienced mixing. The ²²²Rn concentration in the groundwater in January and August was 21 and 23 times that of the river water, respectively, and ²²²Rn in the river and groundwater in January were higher than that in August. In January and August, TDS and Cl^- in the river fluctuated slightly along the river, while those in the groundwater increased and decreased in great degree with similar overall trend to the river water. ²²²Rn in the river and groundwater fluctuated greatly and increased first and then decreased along the river in January, while that in the two types of water bodies changed with opposite tendency along the upper part of the river, and with similar characteristics along the lower part of the river in August. The hydrochemistry and Rn isotope characteristics indicated that the groundwater discharge into the river along the upper and lower part of the study river segment, and the discharge weakened along the middle part in January and August. The discharge rate and flux along the study river segment in January and August was 0.39×10^-4 and 0.44×10^-4 m³/(s·m) and 1.24 and 1.39 m³/s, respectively. The extreme climate resulted in the abnormal change of river-groundwater interaction in Dongting basin. Along the study river segment, the groundwater discharge intensity increased in January, and the recharge model changed from river-into-groundwater to groundwater-into-river in August. The extreme climate not only influenced the model and intensity of the water exchange along rivers in the Dongting Basin, but probably caused the abnormal change of matter cycle and energy flow in the river ecosystem.

Accurately identifying groundwater hydrochemical outliers caused by human activities is crucial for determining the nature background levels of groundwater chemical components and conducting rational assessment of groundwater pollution. The total dissolved solids (TDS), serving as a comprehensive indicator of groundwater hydrochemistry, its value directly reflect the quality of groundwater. Currently, the hydrochemical diagrams method has achieved favorable results in identifying outliers of TDS in groundwater. However, its fundamental principle is reverse identification based on the assumption that the hydrochemical type anomalies composed of the major ion components inevitably result in TDS anomalies, which will potentially leading to over-recognition during the anomaly identification. Therefore, the random forest model, commencing with a positive identification of the genesis mechanisms of TDS, combined with statistical method was employed to identify anomalies in TDS of shallow groundwater based in the Shaying River Basin. A comparative analysis of anomaly identification effectiveness among various methods was conducted, whose results demonstrated that the machine learning method could effectively identify the outliers of TDS in groundwater, with the identified TDS thresholds aligning with those derived from alternative methods. In contrast, the machine learning method, grounded in TDS genesis mechanisms, effectively identified anomalies by mitigating errors in hydrochemical diagrams. This approach successfully distinguished between high and low outliers, offering an alternative and efficacious method for TDS anomaly identification and expanding research perspectives on groundwater environmental background values.

The Yellow River Delta is a key strategic economic zone in China, making it crucial to understand the characteristics and genetic mechanisms of groundwater quality for the sustainable utilization of groundwater resources and the protection of ecological and environmental health. Focusing on Gudao Town in the Yellow River Delta, this study systematically investigates groundwater quality and its formation mechanisms through field hydrogeological surveys, hydrogeochemical monitoring, and laboratory analysis. The findings are as follows: (1)The main hydrochemical types of different waters in the study area are HCO₃·SO₄-Na·Ca (Yellow River water), Cl-Na (seawater), Cl-Na (surface water), Cl·HCO₃-Na (brackish groundwater), and Cl-Na (underground brackish water and brine).(2)Surface water and brackish groundwater are closely associated with silicate end members. The formation of brackish water is primarily influenced by cation exchange and the dissolution of silicate rocks, with dolomite and gypsum dissolution playing a dominant role. (3) Brackish water is mainly controlled by the dissolution of silicate and evaporite rocks, with its salinity primarily attributed to seawater intrusion. (4) The Cl^- concentration in groundwater decreases progressively from the coastal area to the inland, while F concentration decreases from the estuary to the inland. The distribution patterns of Na⁺, Mg²⁺, Ca²⁺, and SO₄^2- concentrations align with that of total dissolved solids (TDS), indicating that these elements are primarily derived from the intrusion of both ancient and modern seawater. Additionally, changes in NH₄⁺ concentrations are largely influenced by human activities.

Natural precipitation can significantly alter the water table in mining areas, increasing the risk of heavy metal pollution in tailings and mining soils. This study investigates the influence of water table fluctuations on the release of heavy metals from tailings and their adsorption in soil through simulation experiments. The results indicate that substantial changes in the water table significantly enhance the release of heavy metals such as Cd, Pb, and Zn. A high water table promotes the accumulation of heavy metals in surface soil, while alternating wet and dry conditions facilitate the migration of heavy metals into deeper soil layers. Cd, Pb, and Zn in soil water pose a considerable risk of leaching into groundwater. Under simulated conditions of continuous soaking and a high water table, residual Cd and Pb in soil are transformed into more active acid-extractable forms. The redox potential (Eh) in the soil solution decreases, promoting the reduction of Fe³⁺, the decomposition of soil organic matter, and the release of Cd and Pb into the system. The adsorption and co-precipitation of amorphous ferric oxide with heavy metals in soil reduce their migration risk. However, in areas with frequent water table fluctuations, particularly in oxygen-limited and organic-rich vadose zones, the potential risk of heavy metal release into groundwater is significantly elevated. These findings highlight the importance of controlling water table fluctuations to prevent and mitigate the potential risks of heavy metal pollution in mining areas.