The Songliao Basin, located in northeastern China, features a basement composed of Paleozoic and Precambrian metamorphic rock series, overlain by sedimentary strata from the Jurassic, Cretaceous, Paleogene, Neogene, and Quaternary periods. Among these, the Cretaceous system is the most significant, serving as the primary oil-bearing sequence. It is characterized by extensive fine-grained clastic deposits from fluvial-lacustrine environments formed during the basin's overall subsidence phase. This chapter provides a comprehensive analysis of the Lower Cretaceous Qingshankou Formation source rocks. It examines their depositional environments, paleontological features, stratigraphy, sedimentary facies, primary source rock properties, and petroleum generation and accumulation conditions. These findings are vital for guiding precise exploration and efficient development of petroleum resources in the Songliao Basin.