Formation Mechanism of Chang 6 Tight Sandstone Reservoir in Jiyuan Oilfield, Ordos Basin

In order to further study the occurrence state and origin of geothermal resources in Guide basin, the hydrochemistry, hydrogen and oxygen isotope data from the study area are collected to analyze the geochemical properties and evolution of geothermal water and to calculate the geothermal reservoir temperature of high-temperature field. Hydrochemistry analysis of geothermal fluids show that the high-temperature thermal water is mainly of SO₄·Cl-Na type, and the low-temperature thermal water is mainly of SO₄-Na、SO₄·HCO₃-Na. There is a positive correlation between Li, F, Sr, As and Cl in Zhacangsi thermal field, which indicates the possible same origin, and the positive correlation between SiO₂ and Cl confirms the deep hot source of geothermal resources. The δD values range from-59‰ to-87‰, the δ¹⁸O values range from-8.6‰ to 12.2‰, and they are all distributed near the local meteoric water line, which suggests the thermal water in study area is recharged from the atmospheric precipitation mainly. The temperature and depths of geothermal reservoir of Zhacangsi thermal field are calculated using reasonable geothermometers. The Na-K-Mg equilibrium diagram reflects that the cold water mixing action occurred in Zhacangsi thermal field during the hot water rising process, the cold water mix proportions and the geothermal reservoir temperature before the cold water mixed are obtained using Si-enthalpy model. The geothermal reservoir temperature of Zhacangsi thermal field is about 133 ℃ calculated by multi mineral balance method and geothermometers, which is close to the shallow reservoir temperature, the depth of the thermal cycle is about 1 800 m; while the geothermal reservoir average temperature of Zhacangsi geothermal field is 222 ℃ before the cold water mixed and the cold water in geothermal fluid mixed with 60%-68% analyzed by Si-enthalpy model, which is close to the deep geothermal reservoir temperature, the depth of the thermal cycle is about 3 200 m. It is concluded that there are two geothermal reservoirs in Zhacangsi thermal field within the depth of 4 000 m This paper can facilitate further study of genetical mechanism of the high-temperature geothermal systems and provide guidance for exploration and drilling in the area.