Geological Characteristics And The Preliminary Suitability Evaluation Of Tamusu Mudstone Candidate Sectors For High-level Radioactive Waste Geological Disposal

The implementation of deep geological disposal of radioactive waste is currently recognized as the most effective and feasible disposal method.The site selection of deep geological repository is a systematic and complex project.At present,the site selection of clay rock geological repository in China is in the stage of planning and site selection,and the Tamusu mudstone pre-selected region in Inner Mongolia shows great potential.In order to advance the site selection process of clay rock repository,it is urgent to recommend pre-selected areas in the Tamusu mudstone pre-selected region,and to conduct systematic research on the basic geological conditions such as the evolution characteristics of the pre-selected areas and the genesis of the target formation.In addition,a comprehensive evaluation of the suitability of the pre-selected area as a repository site should be carried out.Therefore,based on the theory of sequence stratigraphy,sedimentology,geochemistry and modern mathematical methods,this dissertation firstly recommends the key pre-selected areas in the Tamusu mudstone pre-selected region,and then carries out the basic research on the petrological characteristics and genesis of the target strata in the pre-selected area.Finally,combined with the site selection criteria of clay rock repository and foreign beneficial experience,the preliminarily suitability evaluation system of pre-selected area as disposal repository has been constructed and the preliminary comprehensive evaluation also has been made.The main research results are divided into the following three aspects:The target layer of the pre-selected area can be divided into an asymmetrical long-term cycle,which has the characteristics of the lake basin expanding to a high-level system domain,reflecting the back-progressing process of the lake basin expanding to shrinking.It is comprised of three mid-cycles for MSC1,MSC2 and MSC3.The target layer in the pre-selected area developed fan delta and lacustrine sedimentary system.During the MSC1 period,the sedimentary environment is mainly from semi-deep lake to deep-lake,with mainly massive thick dark gray mudstone developed,and occasionally horizontal bedding.During the MSC2 period,the fluvial process increased with abandon abundant supply of supply,which results in the development of fan delta front,and the shrinking lake basin area.During the MSC3 period,affected by the later regional crust uplift,the lake basin area continued to shrink,and shallow lake to fan delta plain subfacies developed.From the bottom to top of the formation,the target layer has the phase transition characteristics of deep-lake,semi-deep lake,fan delta and shallow lake,highlighting the complete retrogradation(Lake progradation)to progradation(Lake progradation)deposition process,in which the sedimentary water body of the target layer gradually deepens and then becomes shallower.The evolution of the target lake basin in the pre-selected area has gone through three periods:the period of pull-out,the period of stable development,and the period of uplift and filling.The long axis direction of the lake basin is from northeast to southwest,and the fan delta facies on the basin margin is symmetrical distribution with obvious facies belt differences.It indicates that the lake basin has typical characteristics of skip-shaped basin.Based on the analysis of the target layer sequence sedimentary facies and the continuous thickness of the target layer,underground extension width,buried depth and other conditions with regarding to the geological condition selection criteria of the clay rock repository,two pre-selected area can be recommended in the Tamusu mudstone pre-selected region,namely pre-selected area?with the area is about 220km² and the pre-selected area?is about 245km².The petrological characteristics of target formation in the pre-selected area indicate that the target layer in the pre-selected area mainly developed grey and dark grey mudstone,and a small amount of sandstone developed in the middle lithologic section,mainly feldspathic sandstone.The main material components of whole mudstone are analcime,dolomite(ankerite),albite,etc.,containing a small amount of clay minerals(mainly illite);Meanwhile,gypsum,hematite,and pyrite are also identified.The clay minerals of the mudstone are mainly composed of illite and montmorillonite,containing a small amount of chlorite.The tectonic environment of the mudstone source area is mainly the continental island arc and active continental margin.The parent rock is dominated mainly by felsic rocks from the upper crust.The target mudstone is mainly muddy structure with typical hydrothermal deposition characteristic structure.For example,the network vein,"snowflake",laminar and syngeneic deformation structure can be observed from the mudstone.It belongs to the low-temperature"white smoke"hydrothermal deposition rock under arid partial reduction sedimentary environment.The structural evolution characteristics of the target layer indicate that the distribution pattern of uplifts and depressions in the basin is strictly controlled by the Altun fault zone and its branch faults.The continuously active co-sedimentary faults and basement faults provide transport channel for dolomite fluids(rich Mg²⁺fluids).The comprehensive evaluation of the suitability of the pre-selected mudstone area shows that the Tamusu mudstone pre-selected area is suitable as a pre-selected area for repository purpose in terms of future natural conditions,socio-economic and cultural conditions,and environmental protection;The score of the Tmausu mudstone pre-selected area is 89.3805,which is an excellent grade.The above research results not only analyze the basic geological conditions of the Tamusu mudstone pre-selected area,but also help to promote the site selection of clay reservoirs in China and provide decision-making basis for site selection process.