Characteristics of micropores, pore throats, and movable fluids in the tight sandstone oil reservoirs of the Yanchang Formation in the southwestern Ordos Basin, China

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 6(3) to Chang 7(2) 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 mm, 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.