Comprehensive and Fine Classification Evaluation of Ultra-Low Permeability Tight Reservoirs Based on Multi-parameter Synergy

Ultra-low permeability tight reservoirs are characterized by complex pore-throat structures, significant variations in rock fracability, and fluid flow capacity. A comprehensive classification evaluation based on multi-parameter synergy is crucial for optimizing development strategies. This study focuses on the ultra-low permeability tight reservoirs in the Dagang Oilfield. Through X-ray diffraction, high-pressure mercury injection, nuclear magnetic resonance (NMR), core displacement, and CT scanning, the mineral composition, pore structure, fluid occurrence characteristics, and nonlinear seepage behavior of the reservoirs were systematically analyzed. By normalizing ten key parameters (including dominant throat radius, movable fluid saturation, brittleness index, and wettability index), a ten-parameter comprehensive classification coefficient (Fc) model was established, enabling fine reservoir classification based on permeability thresholds. The results indicate that the ultra-low permeability tight reservoirs in the Dagang Oilfield are primarily composed of quartz (30%–60%), plagioclase (20%–30%), and clay minerals (10%–20%), with illite/smectite mixed-layer accounting for nearly 50% of the clay minerals, leading to strong water sensitivity. The pore structure is dominated by submicron- to nano-scale throats and the permeability shows a positive correlation with throat radius and movable fluid saturation (20%–60%). The threshold pressure gradient (0.012–0.760 MPa/m) follows a power-law increase as permeability decreases. Based on the Feci value, low permeability reservoirs were classified into five categories (I–V), with Class III–V (< 10 × 10−3 μm2) representing ultra-low permeability tight reservoirs. This classification method provides a theoretical foundation for differentiated development strategies in such reservoirs.