Sandstone reservoirs in the Upper Paleozoic 8th member of Shihezi Fm and 2nd member of Shanxi Fm in Yulin-Shenmu area are dominated by quartz sandstone

lithic sandstone and lithoclastic quartz sandstone

and the diagenetic types includes compaction

cementation

metasomatic alteration

recrystallization and corrosion.The sandstones have experienced diagenetic evolution processes of A and B phases during early diagenesis and A

1

A

2

and B phases during late diagenesis.Decrease of sandstone porosity is mainly caused by compaction and cementation

which contribute 22% and 15% to the loss of primary pores

respectively.Intercrystalline micropores of clay minerals in sandstone are the main reservoir space and account for more than 60% of the total porosity.Secondary pores resulted from dissolution improved the poroperm of sandstone to some extent.While diaclases contribute less in improving the poroperm of sandstone.There are 6 diagenetic facies zones with various genetic types in the study area

of which combined quartz overgrowth-cementation pore facies and authigenic clay cement dissolution pore facies are the most favorable for gas accumulation and the superimposition and reformation of the two would be the most favorable for gas reservoir development.The micropore facies filled with clay matrix and intercrystalline pore facies cemented by authigenic clay are relatively favorable diagenetic reservoir facies zones.The compacted tight facies filled with matrix-altered hydromica and the calcarinate and metasomatic tight facies are unfavorable for development of pores and gas accumulation.