The Upper Carboniferous coaly source rocks are verified to be developed in the Southeastern Junggar Basin, which is lithologically composed of carbonaceous mudstones and mudstones. According to the geochemical (total organic carbon, Rock Eval, and biomarker fingerprints) and organic petrology analyses, the Carboniferous source rocks are primarily gas-prone characterized by kerogen type III, and were deposited in an oxidized-sub-oxidized brackish environment significantly contributed by terrigenous higher plants. Carbonaceous mudstones are generally more oil-prone in comparison with the mudstones. The measured vitrinite reflectance, T-max, and sterane isomers indicate that the source rocks are at immature early gas generation stage, and have not entered the "peak gas generation stage". Natural gases are coal-type gases derived from the Carboniferous source rock. The maturities of them are highly inconsistent with those of source rock samples. Based on 2-D basin modeling, natural gases were generated from the highly mature coaly source rocks in the Fukang Depression and migrated upward to the uplift via the P-C regional inconformity.