Distribution of movable fluid and its influencing factors in super-low to ultra-low permeability sandstone reservoirs: a case study of Chang 6 member, Wuqi area, Ordos Basin

The influences of multiple factors on movable fluid saturation remain inadequately constrained for continental tight sandstones. Moreover, comparative studies on the occurrence characteristics of movable fluids between tight sandstone reservoirs and super-low to ultra-low permeability sandstone reservoirs are relatively scarce. This research takes Chang 6 member in the Wuqi area as an example, and through a series of experiments, analyzes the distribution patterns and influencing factors of movable fluids. The results show that the super-low to ultra-low permeability sandstone reservoirs in this research have porosity values ranging from 8.78% to 16.12%, permeability values ranging from 0.094 mD to 2.453 mD, and the values of movable fluid saturation ranging from 31.38% to 62.67%. All the samples can be broadly divided into two types, type I samples have higher content of feldspar, more developed dissolved pores, and generally locate in middle part or top part of thicker sand bodies. For all the samples, permeability, peak pore-throat radius, and average pore-throat radius exhibit strong mutual correlations, which may be associated with the development of large pore-throats, and all show a strong correlation with movable fluid saturation. Although the movable fluid occurs in pore-throat with a wide size range, the development of micron pore-throats is the key factor influencing the values of movable fluid saturation, differentiating super-low to ultra-low permeability reservoirs and tight sandstone reservoirs.