Petroleum exploration and geological study in deep-buried strata is an important frontier field currently. Such strata in the Mahu sag of the Junggar Basin (>5000 m) have been predicted to have good resource prospects, but have received weak exploration and research attention. Here, in order to provide clues to understand the rules of hydrocarbon accumulation and thereby guide exploration strategy, we focus on investigating the hydrocarbon fluid phase, based on an integrated analysis of geology and geochemistry of hydrocarbon source rocks and oil and gas from forward and reversal perspectives, respectively. Results show that the hydrocarbon fluid phase in the deep-buried strata of the studied Mahu sag should be highly mature and light in general, including high-mature light oil, kerogen cracking gas and oil cracking gas. In addition, the hydrocarbon accumulation and exploration have uncertainties including static reservoir conditions of physical property and pressure and dynamic migration condition. Further studies are still needed along with more and more data collection. Despite of these uncertainties, the present results still can point to the fact that the deep-buried strata in the study area is rich in hydrocarbon resource with light phase, and thus is worthy of exploration. New giant oil and gas fields may exist under the present moderate- and shallow-buried Mahu sequences. These data and understandings can also provide new reference information for other studies with similar geological background.