Abstract: Petroleum in Shizigou area in southwestern Qaidam basin is mainly found in the middle-deep reservoirs of Tertiary, where the types of reservoir space generally include remained primary intergranular pore, secondary solution pore and fracture. These types may form five combinations of reservoir spaces. The favorable reservoir spaces cover secondary solution pore and fracture. The remained primary intergranular pores are normally undeveloped due to sedimentary environment, compaction, cemented fill and diagenesis, while the secondary solution pore/vug and fracture are relatively developed because of dissolution and tectonic/structural fracture effects. Besides hydrocarbon fluid, the meteoric water may play an important role to the formation of effective reservoir spaces.

Key words: Qaidam basin, Shizigou area, Tertiary, reservoir