To find a suitable CO2 storage site and promising reservoir/seal assemblages and to analyze structural elements that can affect the storage capacity and stability, detailed description and analysis on the structural elements and the interpretation of acoustic and optical borehole exploration images were performed from the deep-drilling holes in the promising CO2 storage of Noeseongsan Block, Janggi Basin, SE Korea. The results of core logs reveal that the Janggi Conglomerate including a number of coarse sediments are unconformably overlying the basements. The depth of basin and proportion of coarse sediments tend to increase toward northwestward. The overlying Seongdongri Formation is composed of dacitic tuffs and tuffaceous sediments. Laterally continuous characters of Lowest tuff and thick Massive tuff having high contents of poorly sorted dacitic volcanic materials indicate that the tuffs have a high potential for seal rocks. The topmost Noeseongsan Basaltic rock mainly consist of extrusive volcanic rocks with intrusions in the storage site. Based on the depth of basin, volume of high porous sediments, distributions of subsurface fault zone and fractures, JG-3 and -5 boreholes show a relatively low potential for the CO2 storage exhibiting the relatively shallow depths of basin and high contents of fine sediments in the Janggi Conglomerate. The structural analysis results on the JG-1 and -4 boreholes indicate that the localities may perform lower sealing capacities than the JG-6 and -7 holes because of their high frequency of fractures, core losses, and deformed sediments. The promising reservoir and seal assemblages of JG-7 site can be regarded as the most profitable CO2 storage site based on their low density of fractures and absence of fault zone and deformed sediments whereas the promising assemblages of JG-6 site show relatively high density and frequency of total fractures and high angle fractures in the cap rock, respectively, with three fault zones at the basal part of basin. The observed fault zones, except for the F1-1 and F4-3, have a narrow fault cores consists of very fine grained materials. All the fault zones show a relatively wide damage zones including concentrated shear fractures, reticulate veins, and partially distributed fault breccia zones with high density of fractures and low RQD grades. The thickness and properties of the fault cores and natures of damage zones imply that the faults may perform not only a low transmissive zone with the fault cores but also a high permeable zone with the damage zones. The lower part of Janggi Conglomerate and the upper part of Seongdongri Formation in the JG-1, -4, -6, -7 cores show relatively high density of fractures and low RQDs. This can be interpreted that the fractures in promising reservoir units of Janggi Conglomerate may increase the overall permeability by linkage of primary and secondary pours or grow up of dissolution pours. It can be also interpreted that the fractures in cap rocks of Seongdongri Formation decrease sealing capacity.