Objective The SYY1 well is the first borehole to achieve industrial production of matrix−type shale oil from the thick shale and mudstone of the 1^st Member of the Qingshankou Formation (Qing−1 Member) in the Songliao Basin.

Methods Through geochemical analysis and stratigraphic oil and gas−bearing testing, this paper explores the paleoenvironment and shale oil resource potential of the black shale and mudstone formed in the lacustrine basin of Northeast China during the Oceanic Anoxic Event 2 (OAE2).

Results The paleoclimatic indicator CaO/(MgO×Al₂O₃) ranges from 0.01 to 1.29 (average 0.10). The redox coefficient (K) predominantly falls between 0.11 and 0.64 (average 0.38), while other redox proxies (δU, V/Sc and V/(V+Ni)) yield mean values of 1.17, 8.23, and 0.80, respectively. Paleosalinity indicators show mean values of 57.9×10⁻⁶ for B, 2.5 for B/Ga, 0.7 for Sr/Ba, and 2.2 for Th/U. The δ¹⁸O values range from −17.50‰ to −15.58‰, and δ¹³C values vary between 0.26‰ and 2.00‰.

Conclusions The analysis of geochemical characteristics and environmental indicators suggests that the black shale and mudstone of the Qing−1 Member were formed in a warm, humid lacustrine environment with freshwater to slightly brackish conditions (occasionally influenced by seawater), within a reducing (anaerobic) environment. These conditions reflect the sedimentary response to OAE2 in terrestrial lacustrine anoxic event(LAE). Additionally, the thick, dark shale and mudstone in the Qing−1 Member have high total organic carbon (TOC) content, with a positive correlation between biologically beneficial elements such as phosphorus (P) and copper (Cu) and TOC, revealing extremely high productivity in the ancient lake basin. This provides a rich material basis for the Qing−1 Member in the Songliao Basin. Moreover, stratigraphic hydrocarbon testing has confirmed that TOC is one of the main controlling factors for shale oil accumulation in lacustrine environments. The matrix−type shale oil reservoirs exhibit significant potential, indicating abundant shale oil resources in the Qingshankou Formation of the Songliao Basin.