Abstract: The main types of non-tectonic fractures and their depth distribution range are discussed based on the previous research and considering the inherent special properties of shale such as bedding,heterogeneity,anisotropy in mechanical properties,under-compaction and hydrocarbon generation.The critical confining pressure of shale fracture closing and its corresponding depth range are calculated by making use of porosity and permeability data measured under overburden pressure.A comprehensive analysis reveals that the non-tectonic fractures of shale can be categorized into five types,including suncrack,underwater shrinkage crack,bedding crack,overpressure fracture and modern hypergenetic fracture.The permeability data measured under confining pressure reveals that shale fracture should be closed under a vertical effective stress of about 15 MPa.The depth threshold of shale fracture closing is in the range of 870-1 138 m,averaging at 1 000 m.The depth of bottom boundary of overdressed shale is consistent with the depth of fracture(in horizontal direction)closing under overlying pressure and the fractures in shale under the bottom boundary of overpressure zone are no longer open.The formation and evolution of non-tectonic fractures of shale in burial history feature in time sequence/episode,i.e.different types of non-tectonic fractures develop in different stages from the deposition to maximum buried depth.Surface weather-shack,shallow subaqueous shrinkage cracks,mid-deep under-compaction overpressure fractures,deep hydrocarbon-generating overpressure cracks and super-deep closed fracture occur successively from shallow to deep.

Key words: non-tectonic fracture, overpressure fracture, closed fracture, shale