In order to analyze the sweet spot characteristics, reservoir-cap combination and hydrocarbon sources of Baikouquan Formation in more detail, well AHHW2085 in Mahu Sag was selected to carry out carbon isotope logging, and the carbon isotope of mud gas was continuously sampled and measured with drilling.The results show that the carbon isotope of methane in the mudstone section at the top of the T1b1 changes significantly (3.96 permil/100m), which plays an important role in sealing the oil and gas in the lower part. After entering the T1b1, the humidity of the mud gas increases, the carbon isotope remains stable and heavy, indicating that there is strong oil and gas charging which is a high quality sweet. In addition, the carbon isotope values of methane, ethane, and propane of the formation gas in the T1b1 (−35.8‰, −28.2‰, and −27.4‰ respectively) are significantly heavier than the oil-type gas discovered in adjacent areas, which has the characteristics of mixing origin. The mixing simulation of the two ends shows that the highest contribution of coal-type gas is 44.2%. Combined with the regional geological characteristics, it is considered that the oil-type gas comes from Fengcheng Formation, and the coal-type gas comes from Jiamuhe Formation or Carboniferous. The deep vertical fault is the main reason for connecting oil and gas sources, providing vertical migration and controlling accumulation and distribution process. Coal-type gas and mixed gas do not appear in a wide range of multi-layer systems, which indicates that this kind of natural gas remains considerable in the deep. Deep large structure-lithologic traps such as Mabei anticline are favorable targets for natural gas exploration.