Micro-fracture characteristics of tight sandstone reservoirs and its evaluation by capillary pressure curves: A case study of Permian sandstones in Ordos Basin, China

• Micro-fractures are characterized and its effects on reservoir property are studied.
• Micro-fractures make reserviors have stronger micro-heterogeneity and relative large permeability.
• Established a quantitative recognition criteria of micro-fractures in tight sandstone reservoirs using capillary pressure curves.

As a key factor during the development of tight sandstone reservoirs, the property of fractures and micro-fractures has a significant effect on the permeability. However, it is very hard to describe the property by conventional well logs. Using the core and microscopic thin section of a tight sandstone reservoir in the Permian of Upper Palaeozoic in Ordos Basin, we analyse the distribution of the micro-fractures in the cores and the effects of micro-fractures on porosity, permeability, pore-structure and microscopic heterogeneity. The results show that micro-fractures effectively improve the connectivity of the tight sandstone reservior, which also results in the strong reservoir micro-heterogeneity and complex pore structures. We find that relative large permeability appears in the core samples with micro-factures and the contribution curves of permeability of these samples in mercury injection test also exhibit apparent an abnormality. Based on the analysis, we proposed a method to establish a quantitative recognition criterion of micro-fractures in tight sandstone reservoirs by using a concentration function which reflects the spatial distribution of element fields to describe the contribution curve of permeability quantitatively.