Study On Groundwater Chemistry Of Delingha Fault Depression In Qaidam Basin

Groundwater plays an important role in the process of hydrocarbon accumulation,and it plays a positive role in the exchange of oil and gas.The variations of water chemistry and hydrodynamic environment promote the change of physical and chemical properties of oil and gas.In order to understand the causes of oilfield water,it is necessary to study the hydrogeochemical characteristics of oil and gas storage system,so as to evaluate the storage characteristics and migration direction of oil and gas.This paper carried out research on the hydrogen and oxygen isotopes,chemical composition,types,and organic matters of groundwater in Delingha fault depression zone of Qaidam Basin,and then determined whether this area is favorable for the accumulation of oil and gas.The Delingha fault depression is a sub structural unit of the Qaidam Basin,which is located in the eastern of the northern margin of the Qaidam Basin,north of the Zongwulong mountain,south to the southern edge of the Dakenwula mountain,west of Xitie mountain,east to the Shalike mountain.The research includes three parts: field work,laboratory experiment and comprehensive research.Through the field work and the laboratory experiment,the groundwater samples were detected.The water quality,hydrogen and oxygen isotopes,14 C and organic matter contents of the water samples were analyzed.The relationship between groundwater and the storage of oil and gas in the study area was discussed by comparing the characteristics of groundwater salinity,water chemistry and organic components.The results shown that: 1)The chemical type of river water in Delingha fault depression zone was mainly HCO₃-Ca·Mg,the chemical type of lake water was mainly Cl·SO4-Na·Mg,the chemical type of spring water was mainly HCO₃-Ca,and the chemical type of well water was mainly HCO₃·Cl-Mg·Ca,HCO₃·Cl-Ca·Mg.2)The surface water was subjected to different degrees of evaporation for a long time.The groundwater isotopes exchange occured between water and gas,and the residence time of groundwater was more than 10,000 years.3)The groundwater in Dealingha fault depression zone was in a reducing environment.The sealing capability of the groundwater system was good,and its circulation rate was slow.The groundwater contained organic matter which was related to the oilfield water.Consequently,it was favorable for the accumulation of oil and gas.