关键词: 混积岩, 灰云坪, 藻席, 浅湖—半深湖亚相, 准同生白云岩化作用, 下干柴沟组上段, 古近系, 南翼山地区, 柴达木盆地西北部

Abstract: By means of core observation，thin section identification and electron probe analysis，the reservoir characteristics and main controlling factors of the upper member of Paleogene Xiaganchaigou Formation（E₃2）in Nanyishan area of northwestern Qaidam Basin were studied. The results show that：（1）The reservoirs of E₃2 in Nanyishan area are characterized mixed sedimentation，dominated by carbonate rocks，with an average mass fraction of 42.6%，containing terrigenous clast，anhydrite and clay minerals，with the average mass fraction of 24.9%， 9.0% and 12.8%，respectively. The lithologies of the reservoirs are mainly limestone，dolomite and algal limestone，with an average volume fraction of 49.0%，12.0% and 6.0% respectively.（2）The reservoir porosity in the study ranges from 1.0% to 13.3%，with an average of 4.9%，and 92% of the samples have permeability less than 0.02 mD，indicating low porosity and ultra-low permeability. The pores are mainly dissolved pores and intercrystalline pores，and microfractures are developed，but they are often filled with cements，and the connectivity is poor. According to the pore throat radius，the throat can be divided into three types： medium throat，fine-medium throat and fine-micro throat，and the average pore throat radii are 0.350 μm，0.110 μm and 0.030 μm respectively，accounting for 15%，40% and 45% respectively.（3）The sedimentary subfacies in the study area is mainly shallow lacustrine to semi-deep lacustrine，which can be subdivided into four microfacies，namely，calcareous-dolomitic flat，algal mat，sand flat and mud flat. The development of reservoirs is closely related to sedimentary microfacies，of which the calcareous-dolomitic flat reservoir accounts for the largest proportion，accounting for 67% of the total thickness of the reservoir. High-quality reservoirs are developed in algal mat，and the thicker the algal limestone is，the better the physical properties of the reservoir are，while reservoir is hardly developed in mud flat.（4）The reservoirs in the study area are controlled by diagenesis and tectonism. Quasi-syngenetic dolomitization，dissolution and tectonism increased reservoir space and improved reservoir permeability，while cementation in the late sedimentary period has a destructive effect on the development of reservoir.

Key words: mixed sediments, calcareous-dolomitic flat, algal mat, shallow lacustrine to semi-deep lacustrine subfacies, quasi-syngenetic dolomitization, upper member of Xiaganchaigou Formation, Paleogene, Nanyishan area, northwestern Qaidam Basin