Effects of Terrestrial Paleoenvironment and Related Resources during Oceanic Anoxic Events: Geochemical Analysis of Late Cretaceous Shale Oil Strata in the Songliao Basin

The SYY1 well is the first borehole to achieve industrial capacity of matrix-type shale oil from the thick shale and mudstone of the 1^st Member of Qingshankou Formation（Qing-1 Member）in the Songliao Basin. Through conducting geochemical analysis and stratigraphic oil and gas-bearing testing research, this paper explores the paleoenvironment and shale oil resource potential of black shale and mudstone formed in the lacustrine basins of Northeast China during the global Oceanic Anoxic Event 2 (OAE2) period. The geochemical characteristics and environmental indicators analysis indicate that the black shale and mudstone of the Qing-1 Member were formed in a warm and humid lacustrine environment with freshwater to slightly brackish water conditions (occasionally invaded by seawater), within a reducing environment (anaerobic conditions). These conditions record the sedimentary response to OAE2 within terrestrial sediments of lacustrine anoxic events (LAE). Studies on paleoenvironment and paleoclimate have revealed that during the OAE2 period, the expansion of water bodies occurred rapidly, leading to interactions among algal blooms, water stratification, and anoxic events in the Songliao Basin, resulting in the extinction of biota and the enrichment of organic matter in the lake. The thick dark shale and mudstone of the Qing-1 Member not only has a high organic matter content (TOC), but also exhibits significant positive anomalies in elements closely related to biogeochemical cycles, such as phosphorus (P), as well as beneficial trace elements for biota such as copper, nickel, molybdenum, zinc, barium, cobalt, vanadium, selenium, and strontium (Cu, Ni, Mo, Zn, Ba, Co, V, Se, Sr), and toxic elements harmful to biota such as mercury, lead, uranium, thorium, and rare earth elements (Hg, Pb, U, Th, REE). This demonstrates extremely high productivity in the ancient lake basin, providing a rich material basis for the Qing-1 Member to become an important conventional hydrocarbon source rock layer and unconventional shale oil layer in the Songliao Basin. Additionally, stratigraphic oil and gas-bearing testing has confirmed that total organic carbon (TOC) is one of the main controlling factors for shale oil accumulation in lacustrine shale formations, indicating significant oil and gas resource potential in matrix-type shale oil reservoirs. The lower shale oil layer of the Qing-1 Member at Well SYY1 can yield industrial shale oil from up to 3.22 m³/d. Several related studies indicate substantial potential for shale oil resources within the Qingshankou Formation of the Songliao Basin.