Abstract：With the development of the petroleum industry, looking for high-quality reservoirs at depths becomes a problem that must be studied intensively. The Yongjin area is one of the three blocks in the interior of the Junggar basin explored by the Sinopec. The exploration target in the Yongjin block is deeper than 5000 m, in which most reservoirs have low and very low porosity and permeability. So looking for relatively high-porosity and -permeability reservoirs in the block is important. Regional unconformity between the Jurassic and Cretaceous is well developed. During the formation of the unconformity, meteoric water, which has a high flow rate and a relatively open fluid environment, dissolved minerals and carried the dissolution product out of the dissolution area; grain coats inhibited quartz enlargement and carbonate precipitation and conserved primary pores and secondary pores; in the Yongjin block hydrocarbon filled earlier and the earlier hydrocarbon filling improved the diagenetic environment of the fluids and restrained cementation; on the other hand, organic acid in the hydrocarbon dissolved more soluble minerals and generated secondary pores. All those together can preserve and improve the porosity of deeply buried reservoirs of sandstone in the Yongjin block, Junggar Basin.