Chang 7 reservoir in Heshui area

Ordos Basin is a typical tight oil reservoir with high content of illite up to 9.1%

which is the key factor controlling the quality of reservoir.Analyses of the composition

distribution and occurrence of illite through SEM

EDS

cast thin section and X-diffraction show that the Chang 7 reservoir is very fine turbidity sandstones.The pervasive dissolution of K-feldspar yield lots of secondary pore which is the main pore type.The illites are complex in chemical composition and are mainly in scattered

chaotic sheet-like shape.They only occur in residue intergranular pores.And their content increases with the decreasing of grain size.In comparison with the adjacent shales

the relative content of clay mineral is similar

but the K-feldspar/plagioclase ratio is smaller.Volcanic activities were very intense during the deposition of Chang 7

which provides the favorable condition for semctite development at the early diagenetic stage.The semctites were transformed to illites subsequently.This process may be the main genesis of illite in Chang 7

as illitization of smectite is very common.In addition

the palaeo-geotemperature in the study area is generally lower than 120 ℃ and there are no external potassium-rich hot fluid input

making it impossible for the formation of illite of other genesis.