Evaluation of Structural Preservation Conditions for Shale Gas in Relict Syncline: A Case Study of Devonian-Carboniferous Strata in the Guizhou-Guangxi Area

This study investigates the key factors controlling marine shale gas preservation in Devonian–Carboniferous strata, focusing on four representative synclines in the Nanpanjiang Basin and the northern margin of the Guizhong Depression: Langdai, Xinzhai, West Yizhou, and North Yizhou. Through tectonic characterization and balanced cross-section restoration, we clarify the effects of structural style, tectonic deformation intensity, and fault properties on shale gas preservation. Wide-gentle bidirectional thrust synclines (Xinzhai and West Yizhou) offer the most favorable preservation conditions, followed by the wide-gentle unidirectional thrust syncline (Langdai); the wide-gentle nappe-type thrust monocline (North Yizhou) is least favorable. The Yanshanian Period dominates tectonic shortening in three synclines, while the Indosinian Period is primary for North Yizhou. The Langdai area exhibits lower tectonic shortening rates (13.24% and 12.49% for Langdai and Xinzhai, respectively) and thinner denudation (1500–1800~m) than the Guizhong area (2500–5000~m), both conducive to preservation. Compressional reverse faults predominate throughout, enhancing lateral sealing. Quantitative evaluation identifies Type~I favorable zones within the Xinzhai and Langdai synclinal cores, whereas Middle Devonian strata in West Yizhou and North Yizhou are classified as Type~III owing to fault disruption and denudation. These findings provide a theoretical basis for shale gas exploration in the structurally complex Yunnan–Guizhou–Guangxi region.