Abstract:

The mid lower part of the Jurassic is the main source rock for mid deep buried reservoirs in the east part of Qaidam Basin. It is well accepted that “lacustrine deposits were widely distributed during the early mid Jurassic, and were still remained in the residual sags”, but this really contradicts the drilling experience. To find out the actual situation, and confirm hydrocarbon kitchen, the author carried out surface geological survey and structural modeling for the thrust belt in mountain front, and also studied activities of faults that controlled development of sags and evolution of basin range by means of balanced sections. Results indicated that there are three types of sags, namely continuing subsided sag, positive inversed sag, and negative inverted sag. Prototype basin during different epoches in the Jurassic was reconstructed according to “seven factors method”. Four separated lake basins were developed during the early mid Jurassic, they were Gaxi Yuka, Hongshan Xiaochaidan, Huobuxun, and Delingha, respectively, and then they developed into a unified basin during the late Jurassic epoches. Vertically, the early Jurassic is featured by weak elongated fault subsidence, the middle Jurassic is featured by extension downwarped swamp limnetic facies, and the late Jurassic is featured by compressiondownwarped fluvialalluvial facies. Remnant source rock in the midlower Jurassic was distributed in 4 reworked continuing subsided sags such as Gaxi, and 2 positive reversed sags such as Yuka.

Key words: the east of Qaidam Basin, Jurassic, prototype basin, lithofacies paleogeography, tectonic inversion, effective source rock