Reservoir characteristics and controlling factors of the Shangganchaigou Formation in the Qianxi area of the northern Qaidam Basin,China

Using impregnated thin section,scanning electron microscopy,cathode luminescence and porosity and permeability test,this paper studies the petrography,pore type,physical properties,as well as diagenesis of sandstone reservoirs of the Neogene Shangganchaigou Formation in the Qianxi area of the northern Qaidam Basin.The reservoir dominantly consists of lithic arkose and feldspathic litharenite with low textural and compositional maturities.Intergranular and intragranular dissolved porosities are main pore types with a pore structure of extremely small pore space and thin throat.The reservoir has middle-low porosity and low permeability.Sedimentation and diagenesis are the main factors controlling the development of reservoirs.Braided channel,distributary,and beach-bar sand bodies are most favorable for the formation of reservoirs.Among other factors,compaction and cementation are responsible for the major porosity loss.Nevertheless,carbonate cementation,to some extent,enhances the reservoir properties by dissolution.Descriptive statistics show that dissolution accounts for 2%~6% of porosity,so it contributes most to the development of reservoirs.