The Jurassic sandstone in the north margin of Qaidam Basin consists of medium coarse size sandstone

conglomerate sandstone and fine sand with low content of cement.The reservoir spaces of the sandstone are mainly primary porosity with a small part of secondary porosity.Diagenetic compaction was the controlling factor of porosity evolution and preservation.The porosity reduction of the Jurassic sandstone caused by compaction at present burial depth is 20%～32%

and it is related to tectonic evolution history

especially to the maximal paleo burial depth and the maximun of post denudation.The present porosity formed during 65～12 Ma when the sandstone was in the maximun buried depth.So it is considered that the deeper of the sandstone buried

the smaller the porosity would be.