The reservoir is located in Fengdu county,Chongqing Municipality.After impoundment,karst collapses occurred successively on both the left and right banks at the reservoir tail.Reservoir water rapidly leaked through sinkholes within the collapse pits,causing a sharp decline in the water level and severely impacting normal functioning of the reservoir. To identify the cause and characteristics of the leakage,this study employed a combination of regional karst hydrogeological surveys,geophysical exploration,and geological drilling techniques to systematically investigate the leakage problem at the reservoir tail.The following new insights were obtained:(1)Regional karst hydrogeological surveys indicate the presence of a NE-SW trending limestone belt of the middle Ordovician Baota formation(O2b)traversing the reservoir tail.Karst development is highly pronounced within this layer,commonly featuring solution fissures,caves,and conduits.Field surveys identified multiple sinkholes distributed in a bead-like pattern along the strike of this limestone formation.Tracer tests confirmed the existence of an underground river flowing through the reservoir in a NE-SW direction,with its outlet at Laolong cave,located 3.2 km southwest of the reservoir.The longitudinal gradient of this underground river system is steep,with a vertical drop of approximately 130 m.The development of various karst features at the reservoir tail and the direction of groundwater flow are both influenced by this underground river system.Reservoir impoundment induced the collapses of sinkholes on both banks,allowing reservoir water to directly access into the underground river and discharge at Laolong cave.This has been identified as the fundamental cause of the sudden drop in the reservoir water level.(2)Controlled Source Audio-Frequency Magnetotellurics(CSAMT)and the High-Density Resistivity(HDR)method were selected as geophysical exploration techniques to characterize the spatial distribution of the underground river.These two methods complemented and mutually validated each other's findings.Three CSAMT survey lines were deployed.The results revealed a relatively enclosed low-resistivity zone extending from the left bank to the right bank,with elevations ranging from 1,510 m to 1,500 m,showing a gradual decrease in elevation from left to right.Additionally,three HDR survey lines were conducted.The results indicated underground river elevations of approximately 1,530 m at the low adjacent valley on the left bank,approximately 1,515 m at the left bank of the reservoir,and approximately 1,500 m at survey line yd-4 on the right bank mountain.The gradient of the karst conduit within the reservoir area is inferred to be 1.5％to 2.0％.The alignment of anomalous zones identified by both methods generally follows a NE-SW direction,consistent with the bedrock strike.Furthermore,the elevation of these anomalous zones gradually decreases from the left bank to the right bank,further confirming that the underground river primarily develops along the bedrock strike,flowing from NE to SW.(3)Geological boreholes were drilled based on geophysical anomalies.These boreholes revealed significant vertical karst development within the Baota formation limestone at the reservoir tail.Borehole CZK1 intersected a karst cave;the elevated position of this conduit and its proximity to a sinkhole on the right bank suggest that it is a branch conduit connecting the right bank sinkhole to the underground river.Borehole CZK3 encountered a cavity nearly three-meter high,located near the bottom of the Baota formation limestone at elevations between 1,519.26 m and 1,516.36 m.Water level monitoring in the boreholes indicated shallow water levels within the conduits and significant silt accumulation,supporting the interpretation that this feature represents the underground river karst conduit.(4)Based on integrated geophysical exploration and geological drilling,the elevation of the karst conduit floor of the underground river in the reservoir area ranges from 1,516.36 m on the left bank to 1,513.36 m on the right bank.The estimated conduit dimensions are approximately 2.7 m in width and 2.9 m in height.Near the reservoir head and surrounding areas,the Shizipu formation(O2s)serves as a relatively impermeable layer.On the left bank,the underground river conduit near the reservoir tail passes through the Baota Formation limestone,where the burial depth of the relatively impermeable layer increases closer to the tail.Weakly karstified rock masses on this side act as the lower impermeability boundary.The recommended anti-seepage floor elevation for the left bank is approximately 1,516.0 m.(5)During the investigation phase of karst reservoirs,comprehensive regional karst hydrogeological surveys must be conducted to accurately characterize groundwater flow paths.Only through this approach can the impoundment conditions of the reservoir be clearly understood.In reservoir leakage investigations,relying solely on a single geophysical method or a single borehole to identify leakage pathways and directions is suggested be avoided. This study has elucidated the causes,pathways,and key channel characteristics of leakage at the reservoir.The findings provide a direct foundation for the subsequent design of anti-seepage engineering measures for this reservoir and offer valuable reference for the detection and prevention of similar leakage issues in reservoirs located in karst regions.
Paleokarst fractured-vuggy hydrocarbon reservoirs represent a core component of global carbonate hydrocarbon reservoirs, with the Tahe Oilfield in the northern Tarim Basin being the most typical example in China. The Ordovician marine carbonate rocks in this oilfield serve as the main producing formation of China's first ultra-large Lower Paleozoic hydrocarbon field with reserves exceeding 100 million tons. The paleo-subterranean river systems formed by epigenic karstification constitute the core framework of reservoir fractured-vuggy bodies. Accurate analysis of their hydrogeological characteristics is not only crucial for the exploration and development of deep hydrocarbon resources, but also holds significant exemplary value for analogous studies worldwide.The Tahe Oilfield has undergone multiple phases of tectonic movements, resulting in complex karst geological conditions that pose significant challenges to the identification of paleokarst conduits at the well area scale. This study focuses on the TH12402 well area in the western Tahe Oilfield, which is geographically located in the western part of the Akekule Uplift within the Shaya Uplift of the Tarim Basin. The study area features the development of multiple stratigraphic units, including the Yingshan Formation and Yijianfang Formation of the Ordovician System, with reservoir burial depth exceeding 6,000 meters. Influenced by the intercalation of four stratigraphic pinch-out lines, superimposed karstification of multiple phases is well-developed in this region.Guided by the "present-to-past analogy" research methodology and drawing on the experience of modern karst hydrogeological surveys in southern China, this study integrates high-precision 3D seismic data and actual drilling logging data. The residual thickness trend surface superposition with residual error method was employed to restore the paleogeomorphology of the dominant karstification period (Middle Caledonian Episode Ⅱ). By combining well data with seismic analysis to interpret seismic response characteristics, a set of geophysical identification criteria for the recharge-runoff-discharge elements of karst water systems was established.A key innovation of this study lies in the proposal of seven core principles for identifying paleokarst conduit pathways, namely: (1) Potential sink analysis (to determine the overall flow direction of karst water based on the general topographic trend of paleokarst); (2) Fault framework identification (to recognize fault patterns such as grid-like, single-branch, and conjugate faults using multiple 3D seismic attributes); (3) Judgment of the hydrogeological properties of faults (to assess water-blocking or water-conducting capabilities based on fault mechanical properties, fault-paleoflow angle relationships, and dynamic connectivity); (4) Coupled analysis of fault formation stages (to examine the coupling relationships between faults, river systems, and karstification, and distinguish between constructive and destructive roles of faults in karst development); (5) Topographic slope analysis (to identify the direction of maximum slope gradient from paleokarst geomorphology); (6) Karst conduit orientation matching (to ensure consistency with paleoflow direction); and (7) The principle of minimum resistance (to determine that the main flow prioritizes paths with the least energy consumption when multiple pathways coexist).The results indicate that the paleogeomorphology of the study area exhibits a topographic trend of "higher in the northwest and lower in the southeast," with a local elevation difference of 40 to 80 meters and a regional elevation difference of 180 to 300 meters. Typical negative topographies such as peak cluster depressions and karst valleys are well-developed, and the paleoflow direction is from northwest to southeast. Based on the seven identification principles, 11 main conduits, 19 branch conduits, and 8 fracture flows were identified, and the karst water system was classified into two types: the northeastern reticular fracture flow system (where dispersed flow forms labyrinthine cave systems) and the southwestern dendritic conduit flow system (where strong runoff zones develop pinnate karst conduits).This study confirms that the migration of tectonic highs in the northern Tarim Basin controls the hydrodynamic conditions of the well area. The relatively high CO2 concentration and hot-humid climate during the Ordovician Period laid a favorable hydrodynamic environment for karstification, while the principles of modern karst hydrogeology provide important insights for paleokarst identification. The well area-scale paleokarst conduit identification method and technical system established in this study not only provide accurate geological support for potential tapping in the mature Tahe Oilfield and well pattern reconstruction, but also offer valuable references for hydrogeological research and hydrocarbon exploration in analogous marine carbonate paleokarst fractured-vuggy reservoirs globally.
The Qiangtang Basin, situated in the northern part of the Qinghai-Xizang Plateau, is China's largest Meso-Cenozoic marine sedimentary basin. Jurassic dolomite of the Buqu Formation are regarded as the basin's most promising hydrocarbon-bearing interval, yet their palaeokarst reservoir architecture and petrophysical distribution have not been quantified systematically. In this study, building on 1∶250, 000 regional geological maps and new fieldwork, we measured three detailed stratigraphic sections-two N-S and one E-W-across the Long'eni, Angda'ercuo and Sairen blocks, collected 171 fresh hand specimens, and carried out optical microscopy, casting-thin-section analysis, SEM observation, helium porosimetry and steady-state permeametry. Integrating outcrop and subsurface data, we characterised carbonate facies associations, reservoir space types and the coupling between petrophysical properties, stratigraphic packages and tectono-stratigraphic blocks.Results indicate that: (1) The Buqu Formation comprises two primary lithologies:limestone and dolomite. Limestones are subdivided into grain-supported, mud-supported, and bioconstruction types, while dolomite include silty-crystalline, fine-crystalline, medium- to coarse-crystalline, and calcareous varieties. (2) Reservoir spaces are classified into two main categories-pores and fractures-encompassing intercrystalline pores, intercrystalline dissolution pores, intercrystalline dissolution pores, vugs, structual fractures, dissolution seams, and pressure-solution seams.(3) Porosity ranges from 0.41% to 20.15%, with an average of 2.526%;the majority of samples fall between 1% and 5%, indicating predominantly low to ultra-low porosity. Permeability varies from 0.0002 × 10−3 to 64.58 × 10−3 μm2, with an average of 4.434 × 10−3 μm2, and most values are below 1× 10−3 μm2, reflecting low to ultra-low permwability.Based on porosity criteria, the carbonate reservoirs of the Buqu Formation in the study area are classified as Class Ⅲ (41.18%), representing high-quality palekarst hydorcarbon reservoirs.We further propose a refined palaeokarst layering scheme, quantitatively constrain low-porosity/low-permeability cut-offs, and evaluate reservoir heterogeneity, thereby providing target optimisation and parameter calibration for future horizontal drilling and integrated acidising−volumetric fracturing. These findings not only advance the carbonate-reservoir assessment framework for the Qinghai-Xizang Plateau, but also offer a model for exploring complex karstification hydrocarbon systems in ultrahigh-elevation basins worldwide.
The geological development of karst areas is highly complex, and accurately detecting underground karst caves presents a significant technical challenge to ensure the safety of construction projects. Karst features, including underground caves and subterranean rivers, are widely distributed across China. Characterized by high concealment and spatial randomness, these geological structures are highly prone to inducing disasters such as ground collapse and water or mud inrush. While traditional geophysical methods are effective at certain depths, they tend to rely heavily on wave velocity parameters but underutilize waveform information, such as amplitude and phase. These results in significant errors in identifying cave boundaries and a high rate of missed detections.To enhance detection accuracy, this study proposes a combined approach that integrates the surface wave method with the cross-hole elastic wave method and introduces full waveform inversion imaging technology to create a comprehensive detection system. The surface wave method is cost-effective and suitable for rapid surveys, though its interpretation is significantly affected by surface heterogeneity. The cross-hole elastic wave method, while effective in minimizing shallow interference by utilizing hole-based excitation and reception, offers superior resolution at greater depths but incurs higher costs and requires pre-drilling.Full waveform inversion technology utilizes all available waveform information-such as wave velocity, amplitude, and phase-based on numerical inversion of the wave equation. It overcomes the limitations of the traditional horizontal layered medium assumption, significantly improving the accuracy of karst cave boundary identification. A field test conducted in a karst area in Guizhou adopted a joint observation approach, yielding promising results. Full waveform inversion clearly delineated the spatial distribution of karst caves (depth of 6 to 12 m, lateral range from 15 to 25 m), with imaging resolution far surpassing that of first arrival wave analysis and Synchronous Iterative Reconstruction Technology (SIRT). A serial inversion strategy, progressing from low to high frequencies, effectively suppressed noise and ensured stable convergence to the global optimum. The conjugate gradient iterative algorithm was used for inversion, with an attenuation boundary width set at 2 m, a grid size of 0.2 m × 0.2 m, and serial optimization divided into three stages at 20 Hz, 30 Hz, and 40 Hz. The error decreased progressively with increasing iterations, demonstrating the method's numerical stability and convergence.The results further highlight that joint inversion of P-wave and S-wave data offers mutual verification, with a high degree of consistency in identifying the anomaly locations. The boundary of the S-wave inversion result was clearer, in contrast to the first arrival wave method, which only provides a one-dimensional wave velocity curve. SIRT reconstruction, on the other hand, is prone to boundary ambiguities and geometric distortion. In comparison to traditional methods that rely solely on first arrival travel time, full waveform inversion leverages the dynamic characteristics of the wave field, significantly enhancing the ability to identify small anomalous bodies and complex boundaries.Additionally, comparing the inversion results from the surface wave and cross-hole elastic wave methods reveals good consistency in identifying the main forms of the karst caves. However, cross-hole data provided more reliable deep structural details, whereas surface wave inversion exhibited local false anomalies due to shallow inhomogeneities. This underscores the importance of the combined surface and hole-based observation method. The fusion of these methods maximizes the complementary strengths of the surface wave method's efficiency and the cross-hole method's detailed depth resolution, offering a high-reliability, adaptable technical solution for engineering geological surveys in complex karst environments, with substantial engineering application value.
Karst ground collapses in the Chongqing region are characterized by concealment,sudden occurrence,and unpredictability,making them difficult to detect accurately using conventional geophysical methods.Cross-borehole resistivity computed tomography(CT),with its unique observation geometry and high imaging resolution,is well-suited for the detailed detection of hidden hazards in karst regions. The evolution of typical karst collapses in the Chongqing region can be summarized into three stages:soil cave incubation,formation,and expansion.During the incubation stage,the soil above the bedrock remains intact and stable,with water levels in karst conduits fluctuating periodically due to rainfall but without significantly affecting the stability of the overlying layer.In the formation stage,activities such as tunnel construction cause a rapid drop in groundwater levels,leading to a sharp increase in negative pressure within the karst conduits.This results in increased seepage pressure and gradual loosening of the overlying soil,forming localized cavities.During the expansion stage,repeated suffusion and erosion cause the soil cave to enlarge further,the roof depth to decrease,and eventually,surface collapse occurs. Based on a review of the formation mechanisms of karst collapses in Chongqing,this study developed geophysical resistivity models for the three stages mentioned above.Finite element forward modeling and smooth inversion calculations were employed to verify the effectiveness of cross-borehole resistivity CT in detecting karst collapses at different evolutionary stages.In the models,electrode spacing was set to one meter,and borehole spacing was set to 10 m.Simulation results demonstrate that cross-borehole resistivity CT technology can clearly delineate the spatial distribution characteristics of karst conduits and soil caves at each stage,with inversion errors less than 0.5 meters.In practical applications,cross-borehole resistivity CT successfully identify the morphology of hidden collapses and their associated karst conduits,effectively overcoming the limitations of traditional surface resistivity methods in terms of vertical resolution and detection accuracy,thereby providing an efficient solution for detecting karst collapses in complex geological settings. This study expands upon the traditional methodologies for detecting karst collapses,demonstrating the significant advantages of cross-borehole resistivity CT in improving the accuracy of identifying hidden karst conduits and soil cavities.Future research should focus on refining the technical parameters of this method to enable dynamic monitoring and more precise interpretation of concealed karst collapses.Such advancements will provide critical support for the scientific prevention and mitigation of karst disasters in Chongqing,thereby contributing to the sustainable and safe development of the Chengdu-Chongqing economic circle.
