Abstract: Due to the poor quality of seismic data, traditional seismic data interpretation methods cannot provide complete understanding on the distribution thickness and plane characteristics of Jurassic at the northern margin of the Qaidam Basin. In this paper, a new structure interpretation model for Jurassic faulted depression + depression was established after the structural evolution of the northern margin in the Early and Middle Jurassic was analyzed. By virtue of this model, a series of new Jurassic sags have been discovered, including Hongsanhan, Jiandingshan and Changweiliang sags. On the basis of section morphology and structure characteristics, the Jurassic sags strongly reworked are classified into 5 types, i.e., incompetent bed dominated strongly deformed sag, competent bed dominated strongly deformed sag, basement fracture dominated strongly deformed sag, compaction-flexture weakly deformed sag, and vertically uplifted weakly deformed sag. The strata of faulted depression stage are mainly controlled by the discordogenic fault in the basement at the northern margin of the Qaidam Basin, and the strata of depression stage are distributed in the whole area. They both present obvious inheritance. The relationship between the stress field and the trending of present trunk fault in the basement was analyzed. It is indicated that the morphologic characteristics of present Jurassic sags are controlled by the barrier of Qilian Mountain and the left-lateral strike slip of Altun Mountain. The source rocks are more superior where the Early Jurassic faulted depression is located. Parallel faulted depression is favorable for the long-distance migration of oil and gas generated by the source rocks, resulting in great probability of formation of outside-source oil and gas reservoirs.

Key words: northern margin of the Qaidam Basin, Jurassic, interpretation model, structure of sag