The occurrence characteristic and dominant controlling factors of movable fluids in tight oil reservoirs: a case study of the Triassic tight sandstone in Ordos Basin, China

For tight oil reservoirs, the reservoir movable fluid saturation is at the same level as the reservoir porosity and permeability, which is the inherent property of reservoirs and has a significant effect on the development of tight oil. But the microscopic pore structures of tight oil reservoirs are complex, and the distributions of movable fluids are quite different from those of conventional reservoirs. In this work, quantitative evaluation of the movable fluids in tight reservoirs in Ordos Basin was carried out by means of nuclear magnetic resonance (NMR), and combined with other microscopic experiments, the occurrence characteristic and influencing factors of movable fluids were studied. The result showed that the morphology of T2 spectrum, which could reflect the occurrence characteristic of movable fluids, could be manifested by three patterns with diverse amplitudes of left and right peaks. Based on this three T2 spectrum patterns, pore structures could be divided into three types, and they were characterized by distinct occurrence characteristic of movable fluids. There were various factors affecting movable fluid saturation; among them, throat radius, pore-throat radius ratio, and contents of clay minerals were the dominant controlling factors.