Abstract: Whether the oil and gas are highly matured and light is critical to hydrocarbon enrichment and high yield in dense reservoirs. This paper investigates the basic characteristics, origin and occurrence rules of highly matured and light oil and gas in Mahu sag, northwestern Junggar basin. Results show that the highly matured and light oil and gas in the study area are characterized by low density, high maturity and coexistence with natural gas, which belong to the products of multi?set source rocks dominated by the Lower Permian Fengcheng high?quality source rock under nearly?high maturation stage. During the hydrocarbon generation, migration and accumulation, the highly matured and light oil and gas reservoir got continuous oil?gas charging and good preservation without or with little biodegradation; the occurrence of them is characterized by source controlling, and they can migrate and accumulate in cross?zone and broad areas vertically by faults and fractures, which might be important conditions for forming Mahu giant petroleum province in Junggar basin