The relationship between the paleokarst fracture-cavity system and the chemical behavior of the formation water of Maokou Formation in Shunan area

Abstract： Based on a summary and analysis of the analytical data of the formation water in 451 wells, the authors studied the characteristics of the salinity and types of the formation water of Maokou Formation in Shunan area. In combination with the drilling, seismic and logging data, this paper has systematically summarized the vertical zonal mode of the paleokarst, elaborated the development features of various karst zones along the profile, and analyzed the storage styles of the formation waters with different chemical properties. The result shows that the salinity of the formation water is on the whole relatively low with a large variation range and that the distribution of the salinity is connected with the potential energy. CaCl2-type formation water is of high salinity, positive condensation, high metamorphism and slow seepage. NaHCO3-type water, Na2SO4-type water and MgCl2-type water have low salinity, their existence is related to karstification, and the place of their distribution is of negative water property. The paleokarst system can be divided into four vertical sections： surface karst zone, vertical infiltration karst zone, horizontal runoff karst zone and deep underflow karst zone, with each zone having its own features. The generation of CaCl2-type formation water is related to such factors as the leaching of atmospheric water, the releasing of compaction water, and the burial metamorphism. Such formation water is stored in pores in the form of deep resident water while the low-salinity water is kept in the karst closed system in the form of burial water.