Transfer Structure and Its Implications for Hydrocarbon Accumulation in Hangjinqi Area, Northern Ordos Basin

Integrating full-coverage 2D seismic profiles, local 3D seismic volumes, and borehole data, this study characterizes the transfer structures within the Hangjinqi area of the northern Ordos Basin and elucidates their control on natural gas accumulation. We propose a novel model for hydrocarbon migration and aggregation within transfer structures under an intraplate strike-slip framework. Our findings reveal the following: (1) The transfer structures in the study area manifest as accommodation zones formed by the soft-linkage of overlapping segments of the Hangjinqi fault zone (HFZ)—comprising the Porjianghaizi fault (PF), Wulanjilinmiao fault (WF), and Sanyanjingfault (SF)—since the Hercynian period. Specifically, these consist of two diagonally distributed relay ramps. The relay ramp between the PF and WF hosts a complex reticulate tectonic pattern, characterized by NE- and NWW-trending en-echelon sub-basement fault belts. (2) Transfer structures govern natural gas accumulation through three primary mechanisms: reservoir distribution, reservoir reformation, and fluid migration. ① During the Late Carboniferous to Late Triassic (C2-T3), under N-S compression, the relay ramps controlled the strike and distribution of sedimentary channels. Simultaneously, stress concentration at these ramps under sinistral transpression induced NWW-trending en-echelon faults, which significantly enhanced reservoir petrophysical properties. ② During the Middle-Late Jurassic (J2-3) primary charging period, a shift to a NE-high/SW-low paleogeographic setting triggered a strike-slip reversal of the HFZ. Natural gas migrated northward primarily through bedding-parallel pathways at the relay ramps. ③ By the Early Cretaceous (K1) secondary charging period, weakened shale smear effects along segments of the PF provided lateral pathways, allowing simultaneous northward migration via both strata and fault surfaces. This prolonged differential activity ultimately dictated the heterogeneous distribution of natural gas across the Hangjinqi area.