Movable fluid evaluation of tight sandstone reservoirs in lacustrine delta front setting: Occurrence characteristics, multiple control factors, and prediction model

The tight sandstone, deposited in a lacustrine delta front setting, constitutes substantial new petroleum reserves. Tight sandstone reservoirs' fluid mobility directly affects oil production and recovery efficiency. Consequently, it is crucial to quantitatively and synthetically characterize fluid mobility. This study presented a detailed investigation of the movable fluid of the Yanchang tight sandstone in the Ordos Basin. Multiple experiments were conducted to investigate movable fluid, including casting and cathodoluminescence thin section, nuclear magnetic resonance, X-ray diffraction, and high-pressure mercury intrusion. According to pore-throat structure investigation with petrographical observations, delta front sandstone was divided into four types of lithofacies, pore-throat structure, and three pore-throat spaces (nano-scale, submicron, and micron), which correspond to various occurrence characteristics of the movable fluid. The result shows that the fluid mobility of the micron pore-throat space is higher than the sub-micron scale, and the nano-scale is the lowest. The fluid mobility is driven by petrophysical property, pore-throat structure, diagenetic mineral, lithofacies, and sandstone strata pattern. With the deterioration of the pore-throat structure and petrophysical property, fluid mobility gradually decreases. Movable fluid content significantly correlates with quartz and feldspar content, showing that residual intergranular and feldspar dissolved pores can significantly improve fluid mobility. On the contrary, it is negatively correlated with clay and carbonate cement content. The carbonate cementation and clay mineral filling increase the proportion of nano-scale pore-throat, leading to the deterioration of the reservoir quality and pore-throat structure. Detailed researches on the spatial distribution of fluid mobility show it related to sandstone bed thickness, mudstone proximity, and mudstone/formation ratio. Typically, medium-grained sandstones forming thick beds in mudstone-poor sequences are commonly low in carbonate and clay mineral cement and highest in movable fluid content. In contrast, fine-grained sandstones occurring as thin beds or located near the mudstone-sandstone interface in mudstone-rich sequences generally exhibit high carbonate and clay cement. Hence, its movable fluid content is lowest. The frequent material exchange between sandstone and adjacent mudstone prompts the enhancement of the cementation of clay and carbonate minerals, which is a devastating factor for fluid mobility. The proposed model for predicting fluid mobility can be used for similar delta front sandstones of other lacustrine basins. These findings provide new insights into predicting the best reservoir sandstones with high fluid mobility and effectively exploiting tight sandstone reservoirs.