Abstract: Given the mechanical stratigraphic units and their interface properties constraining natural fracture propagation in tight sandstone reservoirs with low permeability,we propose the division criterion of multi-scale fractures,and discuss the geological conditions of multi-scale fracture formation.According to the scale of natural fracture and mechanical stratigraphy interfaces controlling their development,the natural fractures within reservoirs can be grouped into 4 levels,namely the large-scale,meso-scale,small-scale and micro-scale fractures.Large-scale fractures were developed within sandstone layer groups with a propagation of hundreds of meters,cutting through interlayers and constrained by barriers;meso-scale fractures were developed within composite sand bodies with a propagation of tens of meters,cutting through beddings and constrained by interlayers;small-scale fractures were developed within single sand bodies with a propagation ranging from several meters to over ten meters,and constrained by beddings;micro-scale fractures are to be observed and identified under microscopes,usually with a propagation of less than several centimeters.These fractures feature power-law distribution:the fracture size is in negative correlation with the number of fractures.Multi-scale fracture formation is mainly controlled by the differential rock mechanical properties,thickness of mechanical stratigraphic unit and tectonic stress.The greater tectonic stress is favorable for generating large-scale,cross-layer fractures,the propagation of which can be constrained only by relatively large differential rock mechanical properties and thick mechanical stratigraphic units;while the smaller tectonic stress is conducive to developing small-scale fractures,the propagation of which can be sufficiently constrained by relatively small differential rock mechanical properties and thin mechanical stratigraphic units.Fractures of multiple scales are of different contributions to the formation of tight,low-permeability reservoirs.Micro-scale fractures are the major reservoir space,improving the storage and permeability of tight,low-permeability reservoirs;thus they are key to the reservoir stable production of this kind;small-scale fractures act as both storage space and seepage channels,key to controlling the initial productivity of tight,low-permeability reservoirs;large- and meso-scale fractures mainly function as seepage channels,affecting the distribution pattern of remaining oil at the middle and late stages of reservoir development of the kind.

Key words: natural fracture, multi-scale fracture, mechanical stratigraphic unit, seepage flow, distribution pattern of residual oil, tight sandstone oil reservoir with low permeability, Yanhe outcrop, Ordos Basin