Discussions and Conclusions

According to the collected data of single wells, cross-well profile anylisis, as well as the previous research, the lake basin in western Qaidam Basin experienced two stages of occurrence and development from period of E1+2 to E31. Period of E1+2 was when the arid climate and strong evaporation engendered a shallow lake and brackish water flood plain sedimentation in many places. But there was small lake deposit in Xiaoliangshan, Youquanzi, Honggouzi and Nanyishan area. In period of early E31, a small-scale orogeny took place in northeast of this region, which developed a set of alluvial fan conglomerate about 8 meters thick on the bottom of E31 section in Jian3 well, and a instantaneous lake regression in Nan10 well where deposited the progradational sequence of braided-river delta front subfacies and plain subfacies from bottom to top. The lake basin began to extend after the orogeny. Lacustrine deposit developed largely in Youquanzi, Xiaoliangshan and its northwest area while it developed fluvial deposit and delta deposit in the east region.

The climate turned semi-arid when entering period of E32, and the lake basin developed into its floruit where the water supply increased, the lake expanded and the water level rose. The fluvial and delta facies retrograded to the northeast and the lacustrine sedimentary area reached its largest.