Characteristics and Controlling Factors of the Unconventional Low Permeability  Reservoir of Neogene  in the Northwest  of Qaidam Basin

Key words: 
• northwest part of Qaidam Basin /
• low permeability reservoir /
• lithology classification /
• pore type /
• compaction /
•  /
•  /
•  /
•  /

Abstract: The lithology of Neogene reservoirs in the northwest part of Qaidam Basin is very complex. The rock minerals contain terrigenous materials such as quartz and feldspar as well as lacustrine materials such as carbonate and clay minerals. In addition, the grain size of the sediment is very fine. As a result, the rocks can not be denominated easily by means of thin section examination. This paper mainly discusses four oil structures of the area, including Nanyishan, Youquanzi, Xiaoliangshan and Xianshuiquan. Based on core observation and thin section examination, with the help of Xray diffraction analysis, the sediments of the reservoirs are considered to be mixed deposition which is composed of terrigenous clastic materials and biochemical carbonate minerals. A triangular graph is drawn according to the contents of clay minerals, carbonate minerals, and quartz and feldspar. So，a lithology classification is put forwards and the rocks can be classified as mudstone, sandstone，carbonatite, and mixing deposited rocks. The physical properties of the reservoirs are poor, and the pore types include intergranular solution pores, microfractures and micropores. Compaction，corrosion and cementation are main factors controlling the characteristic of the reservoir. 