Occurrence characteristics and main controlling factors of movable fluids in Chang 81 reservoir, Maling Oilfield, Ordos Basin, China

The Chang 81 reservoir in the Maling Oilfield, Ordos Basin, China, is featured by complex microscopic pore structure and obviously different fluid distribution characteristics. It is a typical low-porosity and low-permeability reservoir. To analyze the occurrence characteristics and main controlling factors of movable fluids in Chang 81 reservoir, this study carries out NMR-based quantitative analysis, which is combined with several microscopic experiments involving analyses of conventional physical properties, image granularity, casting thin sections and electron microscope scanning. There are mainly three types of T2 spectrum curves in terms of the shape, namely left-high-peak–right-low-peak, left-low-peak–right-high-peak, and unimodal shapes. Permeability is well correlated with movable fluid saturation, and larger physical property variation results in more significant movable fluid saturation variation. The main controlling factors that contribute to the obviously different occurrence characteristics of movable fluids are demonstrated to be pore type, pore–throat radius, pore–throat radius ratio, sorting coefficient, effective pore–throat volume, and clay mineral filling. Among them, pore and throat radii and pore–throat radius ratio are more dominant factors, followed by effective pore–throat volume and sorting coefficient. The clay mineral filling, pore type and physical property have the least influence on the movable fluid saturation.