Crystalline rocks underlying basins, with their lack of primary porosity, are unconventional traps for hydrocarbon exploration. The Bozhong (BZ) 19-6 large gas field in the Bohai Bay Basin, China, is a rare, newly discovered, deep-buried crystalline basement play with a productive zone of more than 1000 m, which provides an excellent opportunity to investigate the formation mechanisms for this large-scale reservoir. For the BZ19-6 field, the tectonic compression during the Triassic Indosinian Orogeny movements induced east-west–trending fractures. A high abundance of these fractures, which have the highest density in the hinge zone of the thrust fold, is the prerequisite condition for the formation of the huge reservoir. Later, during the Cenozoic extensional episode, many of these fractures reopened or were enhanced to create storage spaces. The probability of fracture generation is mineral dependent, in which the metamorphic granite with felsic minerals exceeding 70% tends to be more brittle and forms a higher density of fractures. Before the Cenozoic, the basement had undergone an episode of exposure to subaerial and vadose weathering. The upper portion of this unconventional secondary reservoir, generally within 300 m below the exposure-caused unconformity, is optimized by the feldspathoid dissolution due to depth-dependence weathering. However, the space created by fractures is significant down to a depth of up to 1000 m below the unconformity. It is important to note that, due to the natural compaction resistance of the crystalline rocks, the fracture-related secondary spaces in the basement are conducive to being maintained independent of the later depth of burial.