A “Salt–Paleowater Depth–Productivity” Triad Controlling Storage: An Integrated Framework of Organic-Matter Enrichment in Qingshankou Shales, Qian’an, Songliao Basin

This study aims to elucidate the depositional characteristics of the Qingshankou Formation shales in the Qian’an area of the Songliao Basin and establish a new paradigm for organic-matter enrichment governed by the coupled effects of salinity, water depth, and primary productivity. A total of 54 core samples from Well QY2 were analyzed for total organic carbon (TOC), pyrolysis parameters (S1, OSI, Tmax), and trace-element proxies [Sr/Cu, Sr/Ba, Ba/Al, V/(V+Ni), δU], enabling a comparative assessment of oil content, organic-matter type, thermal maturity, and depositional environmental indicators between the first and second members of the formation. The results demonstrate that Member 1 exhibits systematically higher TOC, stronger hydrocarbon-generation potential (reflected by elevated S1 and OSI values), more favorable kerogen compositions (predominantly Type I), and higher thermal maturity. Trace-element signatures reveal that the Qingshankou shales were deposited in a semideep-to-deep lacustrine setting under semihumid-to-semiarid paleoclimatic conditions, characterized by mildly brackish-to-moderately brackish water, relatively great paleowater depth, high lake productivity, and overall weakly oxic-to-reducing conditions. Specifically, Member 1 is marked by higher salinity (Sr/Ba > 1.0–1.5), greater paleowater depth (Ba/Al > 30–35), enhanced primary productivity (EFMo > 3), and slightly stronger reducing conditions, all of which correspond well with its elevated TOC values. In addition, volcanic eruptions and episodic marine incursions strengthened nutrient influxes and promoted water-column stratification, thereby further enhancing the organic-matter preservation. Collectively, the findings indicate that organic-matter enrichment in the Qingshankou shales was jointly controlled by the coupled interplay among salinity, water depth, and productivity superimposed by extrinsic events such as volcanism and marine incursions. This integrated “salinity–water depth–productivity” framework provides a new conceptual basis for evaluating shale-oil sweet spots and predicting favorable target intervals in the region.