Characteristics, origin, and role of salt minerals in the process of hydrocarbon accumulation in the saline lacustrine basin of the Yingxi Area, Qaidam, China

In recent years, an important discovery in exploration has been made in of Oligocene in the Yingxi Area of the Western Qaidam Basin. Based on core observation, systemic microscopic characteristics analysis, mineralogy analysis and geochemical analysis, the mineral characteristics, origin, and role in the process of hydrocarbon accumulation in Oligocene in the saline lacustrine basin are systemically investigated in the Yingxi Area of the Qaidam Basin: (1) In this area, there are four important salt minerals: halite, gypsum, glauberite and celestite, which form saline layers with a certain thickness or develop in matrix carbonate rock in the form of mineral particles. In the upper reservoir group in the middle of saline layers, the saline layers are thicker, mainly rock salt, where thin oil layers are developed. In lower reservoir groups, the saline layers are thinner, mainly gypsum. (2) Salt minerals possess geochemical characteristics of "low content of Mn". The matrix carbonate rock associated with salt minerals has "low temperature" carbon and oxygen isotope geochemical characteristics, and petrological characteristics without the hydrothermal minerals and distribution characteristics with a very wide range. All the above characteristics demonstrate that the salt minerals in this area are a typical deposition type in the saline lacustrine basin, and not a hydrothermal brine precipitation type. (3) "Salt" plays a critical role in the process of hydrocarbon accumulation. Salt minerals play the roles of crack-filling and sealing to prevent dissipation of oil and gas. In addition, salt layers play the role of sealing and covering due to high capillary breakthrough pressure and plastic characteristics. Salt minerals promote secondary pore development in dolomite, and exhibit the negative effect of occupying certain reservoir space. The results of this research can provide important guidance for oil and gas exploration in subsalt layers of saline lacustrine carbonate rocks.