Taking the eighth oil layer of Yanchang Formation in Xifeng oilfield in Ordos Basin as an example

this paper investigates reservoir characteristics through cast thin section observation

SEM

image analysis and whole rock analysis

and studies water sensitivity through core flow displacement experiment.In addition

the changes of microscopic features within cores before and after water sensitivity experiment such as clay mineral occurrence and pore structure are qualitatively and quantitatively determined by using SEM and capillary flow porometer respectively

so as to investigate the mechanism of water sensitivity.The eighth oil layer of Yanchang Formation in Xifeng oilfield is a sandstone reservoir with ultra-low permeability.Its clay minerals are dominated by kaolinite

chlorite and illite

with no montmorillonite but some mixed-layer illite/smectite(only 11.4%).The degree of water sensitivity damage is moderate to strong.The damage from expanding clays is limited.The mecha-nisms of water sensitivity damage in cores include reduction of effective throat radius caused by secondary amorphous;damage or scattering of the paragenetic structures of clay minerals;dispersion of non-expanding clay mineral assemblage and cracking of their single crystal;collapsing of structure of matrix in pores and throat-plugging caused by migration of fine particles.