Mechanism of Shale Gas Preservation in Thrust Nappe Belts at Convergent Plate Margins: Insights from the Ankang Area of the Qinling-Dabashan Mountains, Northern Yangtze Block

This study takes the Ankang area in the Qinling–Dabashan Mountains on the northern margin of the Yangtze Block as an example to investigate whether effective shale gas preservation conditions can exist in large-scale thrust nappe belts at convergent plate margins—a critical scientific question. The aim is to provide new concepts and models for shale gas exploration in tectonically complex regions. An integrated approach combining surface geological mapping, geophysical surveying (2D seismic and wide-field electromagnetic method), calibration of a key borehole (ZBDR01), and geochemical analysis was employed to reconstruct the deep geological structure and evaluate the hydrocarbon generation potential and reservoir characteristics of the target shale interval. The results reveal a relatively gentle, weakly deformed “structural stability window” beneath the Zhongbao Fault, a major thrust nappe surface. Within this window, strata dip at low angles and faults are sparse, exhibiting a significant stress-shielding effect. The Lower Cambrian Niutitang Formation shale within this window is well preserved, characterized by high total organic carbon (average TOC: 4.26%) and moderate thermal maturity (average Ro = 3.02%), falling within the effective shale gas generation window. In contrast, the Lujiaping Formation shale in the hanging wall of the fault, though widely distributed, shows excessive thermal maturity and poor reservoir properties. The study demonstrates that the “stress-shielding” effect is the core mechanism controlling the formation of this stability window and proposes a new “tectonic shielding” accumulation model. This model elucidates how the thrust nappe body itself acts as a thick regional caprock, which together with lateral sealing by the fault zone forms a composite seal-cap system, ensuring in situ preservation of shale gas under a strongly tectonic background. It is concluded that local preservation units can form in the footwalls of thrust nappe belts at convergent plate margins due to stress shielding, challenging the conventional view that intensely deformed zones are unfavorable for shale gas preservation. This research not only provides a new direction and model for shale gas exploration in the tectonically complex Qinling–Dabashan region but also offers important theoretical and technical insights for unconventional hydrocarbon exploration in similar tectonic settings globally.