On the basis of observation by means of scanning electron microscope

the occurrence of clay minerals in pore spaces of sandstone can be divided into threetypes

i.e.1)separated spot type;2)film-like type and 3)bridge-liketype.Sandstones that contains the first type have in the main intergranular porespace.The peak distribution of permeability is consistent with that of theporosity.They show a larger pore radius(6-12μ)and a smaller texture coeffi-cient(Gg<4)

good pore connection

high permeability and natural productioncapacity.Sandstones with the second occurrence of clay minerals is generally inconformity with that of the one said above in pore texture feature

porosityand the accordant peak distribution of permeability and porosity

but they havea smaller pore radius(3-9μ)

bigger texture coefficient

and show a lower per-meability in samples. Sandstones with the third type occurrence of clay minerals are different tothe other two types.Since clay mineral are formed in a bridge-like way with-in the pore spaces

the primary pore space have been disintegrated into micro-pores among clay minerals.Therefore

the porosity characteristics of these sand-stones are similar to that of claystones

most samples show a small averagepore radius(1μ)

a bigger pore texture coefficient(Gg>8).Because mostpores in sandstones turned to be bound pores

their permeability for gas is lessthan 1 md which are devoted merely by a few pore space with radius biggerthan 1μ.The vertical evolution of the above mentioned three types of sandstones canbe seen clearly that

with the increase of the depth

the sandstones with predo-minantly separated spot of kaolinite have gradually changed into sandstoneswith intergranular film-like clay minerals

and finally into sandstones withbridge-like clay minerals represented mainly by flake-like and fibrous illites.Atthe same time

the pore structure and the physical properties of oil reservoirhave been changed accordingly.