Characteristics, formation mechanisms, and control on hydrocarbon accumulation of post‑rift inversion structures: Insights from the Northern Songliao Basin, Northeastern China

Basin inversion is a key geological process that links extensional basin formation with subsequent compressional or reactivation events. Utilizing integrated 2D/3D seismic, drilling, and well-log data, this study systematically investigates post-rift inversion structures in the northern Songliao Basin. The results reveal: (1) The northern Songliao Basin experienced at least five episodes of tectonic inversion during the post-rift stage. The first episode occurred at the end of Member 1, Qingshankou Formation deposition (~90.4 Ma, boundary Tqn1); the second at the end of Qingshankou Formation deposition (~86.7 Ma, boundary T11); the third at the end of Member 2, Nenjiang Formation deposition (~82.2 Ma, boundary T06); the fourth at the end of Nenjiang Formation deposition (~79.1 Ma, boundary T03); and the fifth at the end of Mingshui Formation deposition (~64.7 Ma, boundary T02). These episodes show a pattern of progressive intensification and westward migration of deformation, indicating an eastern source for the compressional dynamics. The most intense inversion at ~64.7 Ma formed both fault-type and fold-type structures. (2) Numerical models reveal the mechanisms for the ~64.7 Ma inversion event. Two fault-inversion styles are identified: fault-bend inversion, where pre-existing normal faults propagate upward, are reactivated and bent under compression, and finally link with new reverse faults to form anticlines; and fault-propagation inversion, characterized by contraction along reactivated faults with associated hanging-wall folding that evolves into thrust-related folds. Conversely, fold-type inversion is typically detachment-controlled, starting as detachment folds and potentially faulting later. Furthermore, the spatial distribution of different inversion styles suggests that the occurrence of inversion deformation is controlled by the coupling between basement faults or deep rift structures and the compressional direction. Uplifted areas mainly control fault-bend inversion, while rift depressions primarily govern fold-type and fault-propagation inversion. (3) Affected by multi-phase tectonic inversion, the inverted anticlines within the basin underwent relative uplift. In these areas, the intermediate principal stress was reduced, creating a local extensional environment at the anticlinal core. This led to increased fault aperture, thereby facilitating hydrocarbon charging into reservoirs above the source rock. In contrast, inverted synclines experienced deeper burial, accompanied by an increase in intermediate principal stress and the development of a compressional setting at the synclinal core. Consequently, fault aperture was diminished, promoting hydrocarbon migration and accumulation predominantly in reservoirs below the source rock. In non-inverted zones, the opening and sealing of faults are directly governed by the regional maximum horizontal compressive stress. Moreover, following the major inversion at the end of the Mingshui Formation (~64.7 Ma), which coincides with a key hydrocarbon accumulation period, a series of NNE-NE trending positive inversion structural belts developed within the basin. These belts constitute prime traps, thus controlling the spatial distribution of hydrocarbon accumulations.