The Upper Paleozoic tight sandstone reservoirs in the northern Ordos basin have the following three characteristics:1)low porosity and low permeability(sensitive to pressure changes)

small pores

and slim throats;2) high irreducible water saturation and complex gas-water relationship;and 3)reservoir space being dominated by dissolved pores and micropores and fractures being relatively well developed.Analysis of these reservoirs suggests that the tightness of the Upper Paleozoic sandstone reservoirs is mainly caused by large palaeoburial depth

high thermal evolution

strong destructive diagenesis during its late phase

and further compaction of reservoirs evoked by hydrothermal activities.