Abstract: The high-quality mud shale of the Qingshankou Formation in the northern Songliao Basin is primarily developed in the Gulong and Sanzhao Sags. Major strategic breakthroughs in shale oil have been achieved in the Gulong Sag. However, systematic studies on whether the Sanzhao Sag, with relatively low shale maturity, possesses favorable geological conditions for shale oil accumulation, controlling accumulation factors, and exploration potential remain scarce. This study, based on breakthroughs from risk exploration well drilling to provide a comprehensive analysis of the sedimentary paleogeomorphology, paleoenvironment, reservoir lithofacies, and reservoir evaluation parameters in the region. It focuses on comparing the shale oil enrichment geological characteristics of the Sanzhao Sag with those of the Gulong Sag.The results indicate: (1) under a transgressive environment, the Sanzhao Sag features saline and semi-saline water conditions, exhibiting high paleo-productivity under semi-humid to humid climates and anoxic environments; (2) the Qingshankou Formation in the Sanzhao Sag contains three types of reservoir facies: high organic laminar shale, laminated shale, and thinly interbedded siltstone-shale, all of which exhibit excellent reservoir properties and high oil content; (3) the total area of favorable I-type zones for shale oil exceeds 3,000 square kilometers, with the Sanzhao Sag hosting a unique favorable I-type zone for laminated shale -type shale oil, containing substantial resources.The findings suggest that the Sanzhao Sag is poised to become the next shale oil replacement zone after the Qijia-Gulong Sags, offering valuable guidance for predicting and exploring sweet spots in shale oil within the basin.

Key words: Songliao Basin; Sanzhao Sag; Qingshankou Formation; Continental shale oil; Favorable area