Abstract: Aer sag is a small oil-bearing sag newly discovered in Erlian Basin in recent years, of which reservoir-forming conditions are relatively complex. Reservoir assessment was conducted early under the premise of only four exploration wells, based on scientifically demonstrating the rapid assessment applicability of the sag. Analysis shows that there are many factors restricting reservoir assessment including less geologic data, great lateral change in reservoir, and complicated factors controlling reservoir, etc. This paper presented a general idea of rapid proven reserves and building capacity rapidly and eff iciently. By means of scientifi c demonstration, optimized deployment, and comprehensive geological research, this paper used such techniques as seismic feature identif ication and seismic attribute analysis to predict sandbody boundary, built and updated structural-lithologic reservoir-forming models. Through rolling block-expanding assessment, assessment of sub-trough exploration, assessment of rolling boundary-extending step by step and other rapid assessment means, monoblock proved oil reserves reached 34.3627 million tons and annual production capacity closed to 400 000 tons in less than three years.