Abstract: The Toutunhe reservoir in Niuquanhu area of Santanghu oilfield is mainly composed of lithic sandstone, with an average porosity of 17.4% and average permeability of 13.1×10 ^-3 μm 2 . Its pore types include intergranular pore, intergranular dissolved pore, feldspar dissolved pore, a few cutting dissolved pore and few carbonate dissolved pore and matrix dissolved pore. The diagenesis appeared in the studied area is composed of compaction, cementation, metasomatism and dissolution. The target horizons are found in the late period of early diagenetic stage to the early period of late diagenetic stage. Differences of sedimentary facies, clastic constituents and burial conditions result in the difference of diagenesis in different areas. The cementation is the main reason for low permeability reservoir, followed by mechanical compaction.

Key words: Santanghu oilfield, Toutunhe formation, low permeability, sedimentation, diagenesis