Abstract:

Recent oil and gas discoveries in the thick sandstone reservoirs in the Lower Jurassic Xiaomeigou Formation in Shiquantan area,Qaidam basin,attract attentions to their reservoir characteristics and genetic mechanisms.Detailed petrological study of the reservoirs of 7 wells in the area reveals that the sandstone reservoirs in the Xiaomeigou Formation are dominated by secondary pores dissolved by the leaching atmospheric water and that the intensity of leaching determine the poroperm characteristics of the reservoirs.The reservoirs went through the following stages: weak compaction during the Early Jurassic(with diagenetic compaction rate of 5.5%-6.5%),atmospheric water leaching during the Middle Jurassic and Cretaceous(with pore increase rate of 4.6%-16.5% and the generation of montmoirillonite),strong compaction during the Paleogene(with diagenetic compaction rate at 19.0%-21.5%,precipitation of small amount of calcite,and disappearance of primary pores),and uplifting and preservation during the Neogene with no significant changes in reservoir characteristics.The atmospheric water leaching during the Middle Jurassic and Cretaceous and the intensive compaction in the Paleogene are the two major stages in the evolution of the reservoirs,and determine the genetic types and characteristics of the reservoirs.

Key words: sandstone reservoir, genetic type, evolution mechanism, Lower Jurassic, Qaidam basin