Abstract: The gas reservoir in the lower part of the Lower Ganchaigou Formation (E₃¹) in Oligocene in the Maxi gas pool in the northern margin of the Qaidam Basin was formed in braided river delta sedimentary environment. The rock type of reservoir mainly includes fine sandstone, medium sandstone and siltstone. Lithic feldspar sandstones account for the largest proportion, while feldspar lithic sandstones the second. The separation of fragment grains is medium or good. Contact or pore cementations are common. Point contact is the leading contact type. Primary intergranular pores are dominant in sandstone reservoirs, featured by big pores and coarse throats. The average porosity and permeability of the reservoir are 25.21% and 963.52×10^-3 μm², respectively, indicating for a good reservoir with high porosity and permeability.