To provide support for the breakthrough in hydrocarbon exploration in the conventional-unconventional transition zone on the southern slope of the Mahu Sag, Junggar Basin, the 3rd Member of the Permian Fengcheng Formation (P₁f₃) in the Mahu126 Well Block was taken as the research object. Based on high-precision 3D seismic, well logging and well testing data, integrated well-seismic studies on stratigraphic division, structural interpretation, reservoir prediction and hydrocarbon accumulation characteristics were carried out.The results show that misunderstandings exist in the traditional stratigraphic division scheme of the study area, and a complete stratigraphic sequence of the Fengcheng Formation is actually developed. After stratigraphic framework reconstruction, a new stratigraphic division scheme was established, namely “Upper Permian Upper Urho Formation-Middle Permian Lower Urho Formation-Middle Permian Xiazijie Formation-Middle Permian Fengcheng Formation-Lower Permian Jiamuhe Formation”, which clarifies the pinch-out characteristics of top truncation and bottom onlap in the western onlap-denudation zone.The P₁f₃ is overall a southeast-dipping monocline with multi-stage fault systems, forming a structural-lithologic composite trap with an area of 60.22 km². The reservoirs in the P₁${\mathit{f}}_{3}^{2}$ submember are tight glutenites deposited by braided channels of fan deltas, characterized by low porosity (average 9.69%) and low permeability (average 0.13 mD). The reservoir space is dominated by intragranular dissolution pores, and the reservoir physical properties present a distribution pattern of favorable in the west and poor in the east.The dual-parameter pre-stack inversion technique of “low P-wave impedance <11 000 (g/cm³)·(m/s) + low V_p/V_s <1.7” was innovatively applied, achieving a 91% coincidence rate in lithology identification and accurately delineating high-quality reservoirs. Guided by this technique, industrial oil flow was obtained from 10 intervals in 9 vertical wells in the study area. During the pilot production of horizontal wells, daily oil production stabilized at over 20 t, cumulative oil production reached 2.08×10⁴ t, and the proved oil-bearing area reached 14.25 km².The research results deepen the connotation of the full petroleum system in the Mahu Sag. The established technical system of “stratigraphic framework reconstruction-dual-parameter reservoir prediction-deployment by zone and interval” and the hydrocarbon accumulation model of “structure controlling hydrocarbon accumulation and lithology delimiting boundaries” provide key geological understandings and practical references for hydrocarbon exploration in similar onlap-denudation zones and transition zones, and strongly support the large-scale reserve growth and cost-effective development of hydrocarbons in the Junggar Basin.