Clay minerals distribute widely in the gas reservoirs of the 8

th

member of the Shihezi Formation and the 1st member of the Shanxi Formation in the Upper Paleozoic of the Sulige gas field in Ordos Basin.The study of clay mineral content changes and main controlling factors plays a very important role in the low-permeability reservoir stimulation and protection.Based on a comprehensive study of data such as X-ray diffraction

thin sections and cast thin sections of clay minerals

and so on

we proved that the clay minerals in the 1

st

member of Shanxi Formation and the upper and lower 8

th

member of the Shihezi Formation are mostly chlorite

illite

kaolinite and illite-smectite mixed-layer.The horizontal and vertical distributions of these minerals are studied with the single factor analysis method and the main controlling factors for their distribution difference are also analyzed.The results show that the mother rocks in different provenance regions determined their original composition

climate condition changes from relatively warm and wet to dry controlled their weathering and alteration degrees

and strong hydrodynamics

proper pH value and weak reductive environment of fluvial sedimentary environment provided favorable conditions for their deposition

transformation as well as preservation.However

the increase of illite content from alteration of feldspar during the late diagenetic stage B caused the lowering of reservoir quality.