The 6

th

and 7

th

members of Yanchang Formation (Chang 6 and Chang 7) contain a large number of tight oil in the Longdong area of Ordos Basin. However

poor understanding of their microscopic characteristics and genesis has hindered the exploitation of tight oil. The current study attempts to investigate the pore throat characteristics and genesis of tight oil reservoirs of Chang 6 and Chang 7 members of Yanchang Formation on the basis of pore cast thin section

field emission electron microscopy (FE-SEM) and so on. The results show that the tight oil reservoirs were deposited in distal bar and sheet sand of delta front and gravity flow of semi-deep to deep lake

and the tight oil reservoir rocks with fine grain size and high matrix content (8%-10%) consist of very fine sandstones

siltstones

pelitic siltstones and silty mudstones. These reservoir rocks have very poorly developed pores

low thin section porosity averaging at 1.8%

small pore radius averaging at 30 μm

fine throat radius averaging at 0.08μm

low porosity averaging at 9% and low permeabi-lity of less than 0.3×10

-3

μm

2

. The main pore types of the tight oil reservoirs are micropore of intergranular matrix

dissolution pore of feldspars and rock fragments and intercrystalline pore of cements. The microscopic characteristics of different oil reservoir rocks are different. The low-energy sedimentary environment is decisive in determining the fine grain size and high matrix content of the reservoir sandstones

which have weak resistance to compaction in early burial stage

hence

significant amount of original pore volume is lost

and the pore throat sizes become smaller. In the reservoir rocks with tiny pore throats

the strong carbonate and clay mineral cementation (especially the bridging and network-like cementation of illite)

caused by semi-permeable membrane effects of ions

further reduce the porosity and permeability of the reservoir rocks

preventing the later acidic fluid dissolution and formation of secondary porosity. In addition

the strong replacement of quartz by detrital micas grain

hydromuscovite matrix and carbonate cement lowers the resistance of the reservoir rocks and increases the solid volume

finally leading to the tightening of the reservoir rocks.