Occurrence Models of Movable Fluid in Lacustrine Sandstone Reservoir of Chang 7 Member in the Heshui Block, Ordos Basin, China

Abstract

The Chang 7 sandstone is characterized by complex micro-pore structures, strong heterogeneity, and differential fluid distribution. These characteristics result in low oil recovery. In this paper, various techniques, including high-pressure mercury intrusion, nuclear magnetic resonance, scanning electron microscope, thin section, and X-ray diffraction, are employed to quantitatively evaluate the occurrence characteristics and influencing factors of movable fluids in Chang 7 sandstone reservoirs from the Heshui Block with different fractal structures. Results show that the dominant sandstone type is feldspar lithic fragment sandstone. Chang 7 reservoir has been divided into three types (types I, II, and III) based on capillary pressure curves and pore structure parameters. These reservoirs are characterized by various fractal structures and different movable fluids distribution. Multiple possible factors affecting the movable fluid distribution are analyzed, including physical properties, pore structure, pore size distribution, mineral content, and heterogeneity. Movable fluid saturation is positively correlated with physical properties, weighted average pore-throat radius, median pore-throat radius, final residual mercury saturation, and maximum mercury withdrawal saturation. In contrast, it is negatively correlated with displacement pressure and has no obvious correlation with the sorting coefficient. Micron- and submicron-scale pores are beneficial to the movable fluid occurrence, while nano-scale pores are vice versa. The influence of mineral content on movable fluid occurrence varies with mineral types. Quartz is conducive to the movable fluid occurrence in submicron-scale pores, while carbonate cementation inhibits the movable fluid occurrence in submicron-scale pores. The inhibition of clay on the movable fluid occurrence is mainly reflected in submicron- and nano-scale pores and varies with clay mineral types. The influence of heterogeneity on the movable fluid occurrence is mainly reflected in submicron-scale pores. The occurrence models of movable fluid vary with reservoir types.

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