The study focuses on the Middle Jurassic Dameigou Formation (Members 5–7) source rocks in the Xiaochaidan Area along the northern margin of the Qaidam Basin. Through systematic geochemical analysis, organic petrology, and sedimentary facies identification, the geochemical characteristics, organic facies types, and favorable hydrocarbon generation centers of the source rocks have been clarified. The results indicate that during the depositional stage of Members 5 to 7 of the Dameigou Formation, the climate underwent a cyclical change from arid to humid and back to arid, while water depth showed a gradual increasing trend, and the redox conditions shifted from oxidizing (Member 5) to strongly reducing (Member 7). Among them, the semi-deep lacustrine facies of Member 7 was situated in a high-salinity, strongly reducing environment with relatively high paleoproductivity and optimal organic matter enrichment (average TOC of 3.76%), developing high-quality source rocks dominated by Type I kerogen. The shallow lacustrine facies source rocks had an average TOC of 1.80%, were dominated by Type II? kerogen, and exhibited moderate hydrocarbon generation potential. The braided river delta facies was significantly influenced by terrigenous input; although the TOC could reach 12.10% in localized areas due to coal seam development, the hydrocarbon generation potential remained low (average of 1.87 mg/g), with organic matter primarily of Type III kerogen. Based on these findings, the study area can be classified into three organic facies types: semi-deep lacustrine sapropelic facies (Type A), shallow lacustrine humic–sapropelic facies (Type B), and braided river delta humic facies (Type C). Type A organic facies is mainly distributed in the Hongshan Sag and the central area of the Dameigou lacustrine basin, representing favorable zones for tight oil exploration. Due to the controlling effect of structural low uplifts on sedimentary facies differentiation, the coupling of strongly reducing environments and high paleoproductivity in semi-deep lacustrine source rocks constitutes the key factor for hydrocarbon enrichment. It is recommended to prioritize exploration in the central areas of the Xiaochaidan and Hongshan Sags. These insights provide a scientific basis for unconventional oil and gas resource evaluation and exploration deployment in the Middle Jurassic of the northern margin of the Qaidam Basin.