| Paleokarst is very developed in the Ordovician carbonate rocks in the North China coalfield,which has become the main channel for the enrichment and migration of karst water.At present,mining activities research mostly focused on the water richness and permeability of the Ordovician aquifer,while seldom studied its development characteristics and formation mechanism,which led to inaccurate predictions of karst water disasters and karst water disasters sometimes occurred during mining.The Huainan coalfield is located on the southeastern margin of the North China Plate,and it has extremely complex geological and hydrogeological conditions.With the gradual mining of deep coal seams,the threat of Ordovician karst water damage is becoming more and more serious.Therefore,the systematic research on the development characteristics,distribution law,and formation mechanism of paleokarst in the Ordovician of Huainan coalfield not only has an important guiding role in the prediction and prevention of karst water hazards during the mining of deep coal resources in the Huainan coalfield,but also has a far-reaching significance for the further understanding of the formation and evolution of the Ordovician paleokarst in North China.Based on karst geology,hydrogeology,paleogeography,sedimentology,structural geology,rock mechanics,and multi-disciplinary cross theory as the instruction in this paper,field investigation,core observation,thin section analysis,laboratory experiment(test),numerical simulation,model prediction,and geostatistical analysis were all employed to study the development characteristics,evolution process,and formation mechanism of the Ordovician paleokarst in the study area,and also predict the development degree of paleokarst.The following major results had been achieved:(1)The development characteristics,filling characteristics,and distribution characteristics of the Ordovician paleokarst in the Huainan coalfield were systematically studied:1)The Ordovician carbonate rocks mainly develops four types of paleokarst,including pores,fractures,caves,and karst collapse columns(KCCs),and where the fractures and caves are the main paleokarst types,followed by the pores and KCCs.2)Fracture and large caves are mostly filled,followed by semi-filled and unfilled types,while small caves are mostly semi-filled,followed by unfilled and filled types.3)Fractures,large caves,KCCs are mainly distributed along fault zones,and the paleokarst development in the vertical direction has obvious zoning.(2)The formation period,formation age,formation environment,and aggressive fluid sources of the Ordovician paleokarst in the Huainan coalfield were determined:1)Sedimentary karst mainly occurred near sea level and was the result of the joint dissolution of seawater and atmospheric precipitation,which formed from the Early Ordovician to the Middle Ordovician,2)The weathering crust karst was mainly related to the long-term leaching of atmospheric precipitation,which formed from the Late Ordovician to the Early Carboniferous.There developed obvious "four zones" structure in the vertical direction,namely,surface residual zone,vertical seepage zone,horizontal underflow zone,and deep slow flow zone.3)Pressure-released karst was mainly related to the release of organic acids and acidic pressurized water from the overlying Carboniferous-Permian strata in the diagenetic compaction process,which formed in the Middle Carboniferous to Early Triassic in the medium-high temperature and buried closed environment of the underground.4)The hydrothermal karst was mainly related to the magmatic-hydrothermal activities in the deep underground,which occurred in the Late Triassic to the Late Cretaceous and formed in the high temperature and deeply buried environment of the underground.5)Mixed karst was mainly related to the mixed dissolution of freshwater,deep formation water,and hydrothermal fluid,which formed from the Early Cretaceous to the Late Paleogene and developed in the fault and fracture zones around the carbonate outcrop area of Panji and Chenqiao anticlines.(3)The major factors,including carbonate lithology,stratigraphic structure,aggressive fluid,fault and fracture tectonics,paleo-morphology and paleohydrology,magmatic activity,and karstification time that controlled the development of the Ordovician paleokarst were systematically illustrated:1)The dissolution test showed that the dissolution capacity of the Ordovician was limestone,breccia limestone,dolomitic limestone,argillaceous limestone,limestone dolomite,and dolomite from strong to weak.2)Hydrogeochemical simulations showed that the erosion ability of aggressive fluids was mainly controlled by fluid temperature,acid gas composition and pressure,and mixed fluids ratio.3)Numerical simulations of the multi-stage tectonic stress fields showed that the fault and fracture structures formed in the Early Yanshanian and Late Yanshanian stages played an important role in the development of the Ordovician paleokarst in the Huainan coalfield,and the central part of the study area was the best location for tensile fractures and paleokarst development.4)The Ordovician weathering crust paleo-geomorphology and paleohydrology controlled the vertical development characteristics of the Ordovician paleokarst,and the bedrock surface paleo-geomorphology and paleohydrology influenced the water richness and permeability of the Ordovician aquifer.5)The magmatic activities and karstification time both played an important role in the development and evolution of the Ordovician paleokarst in the Huainan coalfield.(4)Taking the KCCs in the Huainan coalfield as the research object,the criterion formula for truncated roof collapse was established,and the formation and evolution processes of KCCs were simulated and analyzed,and the mechanism of KCCs was revealed.The formation of KCCs in the Huainan coalfield was mainly related to the hydrothermal karst and mixed karst that occurred from the Late Triassic to the Paleogene,and the fault and fracture tectonics,carbonate outcrops,and magmatic-hydrothermal fluids played a vital role in the development of KCCs in the study area.(5)A GIS-AHP coupling model was established to predict the development degree and horizontal distribution of the Ordovician paleokarst in the Huainan coalfield.The Ordovician paleokarst development degree in the study area is generally medium to extremely strong,and the strongest location of the paleokarst development is the middle part of the study area.Only the northwest,southwest and northeast parts of the study area,the Ordovician paleokarst development degree are from moderately weak to weak.By comparing the prediction results with the actual locations of KCCs and Ordovician water inrush points,the correctness of the prediction model,evaluation index,and index weight were all verified,which provided an important reference for the prevention and control of deep karst water damage.Figure[106]Table[36]Reference[327]... |
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