Characteristics of low-permeability reservoir and its origin in Youquanzi Oilfield in the northwest part of Qaidam Basin

The characteristics of low-permeability reservoir of Youshashan Formation and its origin in Youquanzi Oilfield of Qaidam Basin were studied synthetically using data of core observation and description,scanning electron microscope and core analysis in laboratory.The analysis results showed that the reservoir mainly developed remaining inter-granular pores and emposieu with micro-bore to fine porous throats.The low-permeability reservoir was affected by some sediment factors,including fine grade,high content of carbonate and shale,and undeveloped primary pores,so it could be easily compacted,which made the acid fluid difficultly penetrate into the reservoir where the dissolution was slow and weak.Diagenesis was the key factor for genesis of the low-permeability reservoir.The compaction and cementation could reduce the primary pore and pore-throat,make the porosity and permeability of reservoir lower and make sandstone tighter.The denudation and undeveloped micro-fractures could not improve the porosity and permeability of the reservoir obviously,which resulted in the low-permeability reservoir in this area.