Microscopic pore-throat structure and fluid mobility of tight sandstone reservoirs in multi-provenance systems, Triassic Yanchang formation, Jiyuan area, Ordos basin

Abstract

The tight sandstone reservoirs in the first sub-member of Chang 7 member (Chang 7₁) of Triassic Yanchang Formation in the Jiyuan area, Ordos Basin, show significant variations in microscopic pore-throat structure (PTS) and fluid mobility due to the influences of the northeast and northwest dual provenance systems. This study performed multiple experimental analyses on nine samples from the area to determine the petrological and petrophysical properties, as well as the PTS characteristics of reservoirs in different provenance-controlled regions. On this basis, the pore-throat size distribution (PSD) obtained from high-pressure mercury injection (HPMI) was utilized to convert the NMR movable fluid T₂ spectrum, allowing for quantitative characterization of the full PSD and the occurrence characteristics of movable fluids. A systematic analysis was conducted on the primary controlling factors affecting fluid mobility in the reservoir. The results indicated that the lithology in the eastern and western regions is lithic arkose. The eastern sandstones, being farther from the provenance, exhibit higher contents of feldspar and lithic fragments, along with the development of more dissolution pores. The reservoir possesses good petrophysical properties, low displacement pressure, and high pore-throat connectivity and homogeneity, indicating strong fluid mobility. In contrast, the western sandstones, being nearer to the provenance, exhibit poor grain sorting, high contents of lithic fragments, strong compaction and cementation effects, resulting in poor petrophysical properties, and strong pore-throat heterogeneity, revealing weak fluid mobility. The range of full PSD in the eastern reservoir is wider than that in the western reservoir, with relatively well-developed macropores. The macropores are the primary space for occurrence of movable fluids, and controls the fluid mobility of the reservoir. The effective porosity of movable fluids (EPMF) quantitatively represents the pore space occupied by movable fluids within the reservoir and correlates well with porosity, permeability, and PTS parameters, making it a valuable parameter for evaluating fluid mobility. Under the multi-provenance system, the eastern and western reservoirs underwent different sedimentation and diagenesis processes, resulting in differential distribution of reservoir mineral components and pore types, which in turn affects the PTS heterogeneity and reservoir quality. The composition and content of reservoir minerals are intrinsic factors influencing fluid mobility, while the microscopic PTS is the primary factor controlling it. Low clay mineral content, well-developed macropores, and weak pore-throat heterogeneity all contribute to the storage and seepage of reservoir fluids.