Thin-section analysis, field emission scanning electron microscopy, constant-rate mercury injection (CRMI), x-ray–computed tomography (X-CT), and nuclear magnetic resonance (NMR) were used to investigate pore systems, pore sizes, and pore-throat distributions and determine evaluation criteria for tight oil reservoirs.

This research shows that the average porosity of the reservoirs in the Chang 6₃ to Chang 7₂ Members of the Yanchang Formation of the Ordos Basin ranges from 4% to 16%, with a corresponding average permeability of 0.1 to 0.4 md. The main pore types in these tight oil reservoirs are dissolution pores and intergranular pores, and the main pore-throat types are laminated throats and control-shaped throats. Based on experimental results, the pore-throat distributions differ among different testing techniques. The results of this study indicate that a combination of CRMI-derived and X-CT–detected pore-throat distributions is most suitable for calibrating NMR-derived pore sizes over the full range of pore sizes. The lower pore-throat radius limit of effective reservoirs is 0.24 μm, and the lower permeability limit for the flow of movable fluid is 0.008 md. Based on a cluster analysis, the studied tight oil reservoirs can be divided into three types. Compared with type III reservoirs (porosity < 8% and 0.008 md < permeability < 0.05 md), type I (porosity > 11% and permeability > 0.15 md) and type II (8% < porosity < 11% and 0.05 md < permeability < 0.15 md) reservoirs are of relatively good quality.