Hot Water Karst Characteristics of Ordovician Period in Ordos Basin

Key words: 
• Ordos basin /
• Ordovician /
• hot water karst /
• deep reservoir /
• deveiopment characteristics

Abstract: The karst vugs in the fine doiomites of Ordovician in Ordos Basin are the resuit of iong time action caused by deepiy buried hot water karst. Futher study, based on hot water karst deveioping background and hydroiogy conditions, shows that the deep circuiating hot water is the main water source of deep Ordovician in this area, and partiy in the ancient fauit zones where some rising up hot water is intruded in.Heat energy is iargeiy from geothermai gradient, structure movement, whereas the infiuences of deep heat source is aiso the important reason of forming mid-iow enthaipy terrestriai heat fiuid. The forming of the fiuid is reiated not oniy to the change of radioactive materiai in sediments and change of Moho depth, but aiso to the heat events occurred in Yanshan movement. The hot water karst and the generai karst can be distinguished by karst shapes and the geochemistry marks of fiiiing materiais. The ring-shaped fiiiings formed by hot water karst are much narrower on the upward direction and wider down, big vugs formed in the upper part aiong the upward direction, therefore, it is different from the generai karst, and the direction of hot fiuid movement can be pointed out from bottom to top. The physicai chemistry properties of the hot water can be refiected from some styiistic minerai veins, such as heteromorphism doiomite vein fiiied in the fissures of doiomite rocks, iead and zinc vein, and the crystai-caicite vein.Minerai inciusion determination shows the average temperature is 150C ~ 349C, moie co- Oncentration of CO₂ in the gas phase compositions reached 44.3% ~ 56.9corrosion capacity and soiuabiiity. The research shows, during the deveiopment of hot fiuid karst, the artesian hot water from different depths causes the increase of karst water specific gravity with the pressure boost and the increase of saiinity, and drives the water fiow form highpressure zone to the iower, at the same time, the rising water temperature and the increase of dissoived gas get the water gravity decreased, boosting the hot water move in circuiation in the sediments. So it is ciear that the deepiy buried hot water karst in this area is dominated by deep heat source and the hot water circuiation. Based on the content changes of trace eiements of Fe²⁺ and Mn, it has been distinguished that the weii Zones of Er6, Dingtanl, Lihual, Lucanl in the west part of the basin, Xongtanl in the south, and Shan8, Shanl2 in the east are the main Zones where hot water circuiation is active, and are favorabie for the deveiopment of hot water karst. It piays an important roie in forming deep Ordovician reservoir in this area.