Lower limits of pore throat radius, porosity and permeability for tight oil accumulations in the Chang7 Member, Ordos Basin

Tight sandstone reservoirs were found widespread in the seventh member of Yanchang Formation (Chang7) in the central Ordos Basin. The lower limit of reservoir porosity and permeability during the accumulation period was critical for hydrocarbon enrichment. We studied these physical properties and microscopic pore throat characteristics of tight sandstones with permeability <0.3×10^-3μm² and porosity < 12% by means of constant-rate mercury injection and Nano-CT scan. The average pore radius is 160μm, and the throat radius is < 0.55μm, average 0.33μm. We analyzed the pressure differential between source rocks and reservoirs, the physical properties of crude oils, and fluid characteristics. Oil and gas displacement simulations in tight gas reservoirs and minimum flow pore throat radius method showed that the Chang7 tight reservoirs in the study area had a minimum pore throat radius of 14 nm, porosity of 4.2%, and permeability of 0.02×10^-3 μm² during hydrocarbon enrichment period. To form large tight oil reservoirs with an oil saturation > 40%, the lower limits for pore throat radius, porosity and permeability should be 0.12μm, 7.3% and 0.07×10^-3μm², respectively.