Evaluation of movable fluid and controlling factors in lacustrine gravity-flow tight sandstone reservoirs: Implications for predicting reservoir quality

Lacustrine gravity-flow tight sandstone reservoirs are rich in petroleum resources, and the presence of movable fluids is essential for the efficient recovery of tight oil. However, characterizing the properties of movable fluids and predicting their content are challenging due to the limited data available on these fluids. To address this research gap, it is imperative to conduct a comprehensive study on the distribution patterns and controlling factors of movable fluids within the lacustrine gravity-flow tight sandstone reservoirs of the Chang-7 Member in the Heshui region of the Ordos Basin. The study utilized a range of analytical techniques, such as thin section analysis, scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), constant-rate mercury injection (CRMI), and nuclear magnetic resonance (NMR). We identified three distinct lithofacies and three types of pore-throat spaces within these lacustrine gravity-flow sandstones. The highest amount of movable fluid was observed in the submicron pore throats, followed by nanopore throats, while the micron pore throats exhibited the lowest amount. Our analysis indicates that petrophysical parameters, mineral composition, and pore throat structures collectively influence the content of movable fluids. Specifically, quartz and feldspar content are positively correlated with the movable fluid content, while clay and carbonate cement content are negatively correlated. Fine sandstones with massive bedding typically have a high content of movable fluids, which is associated with elevated quartz and feldspar content. In contrast, very fine sandstones to siltstones with parallel or ripple beddings have a very low content of movable fluids, characterized by high levels of carbonate and clay cementation. The study also suggests that the lower limit of the pore throat radius of movable fluid is about 0.03 μm. These findings offer novel insights for evaluating and predicting high-quality lacustrine gravity-flow tight sandstone reservoirs, enabling more effective exploration and development strategies.