The Ordos Basin, situated in the western part of the North China Block, originated as a large deep-water lacustrine basin during the Mesozoic, shaped by plate collision dynamics. Its geological evolution can be divided into three stages: overall subsidence, uplift and erosion, and subsequent shrinkage and adjustment. The basin contains two primary source rock formations: the Triassic Yanchang Formation and the Jurassic Yan’an Formation. This chapter provides an in-depth analysis of the Triassic Yanchang Formation, focusing on its role as the primary source rock. It examines the depositional environment, paleontological characteristics, stratigraphy, sedimentary facies, and petroleum geological properties of the formation. Additionally, the study evaluates the petroleum accumulation conditions of the Yanchang Formation, offering insights into its significance within the Ordos Basin’s petroleum system.