Abstract: Low field Nuclear Magnetic Resonance was combined with high speed centrifugal experiments to study movable fluids and T₂ cutoff values of marine shale in South China.The experiments reveal the importance of establishing water saturated state and irreducible water state for the calculation of movable fluids.For marine shale from South China,the optimum pressure for establishing water saturated state is 12MPa,and the optimum centrifugal force for establishing irreducible water state is 2.76MPa.The T₂ cutoff values range between 1.07ms and 3.22ms,averaging at 1.8ms,significantly less than that of tight sandstone.Therefore,using empirical value (13ms) for the calculation of movable fluids may result in great error.To deal with the problem,we integrated the T₂ spectrums of both saturated water state and irreducible water state to calculate movable fluid saturation.The result ranges between 23.19% and 30.84% (averaging at 27.28%),indicating high irreducible water saturation and ultra-low water saturation.we also converted the NMR T₂ spectrum into pore size distribution,revealing the pore radius of shale to be within 20 and 200 nm range.

Key words: T₂ cutoff, pore size, NMR, movable fluid, shale gas, marine shale, South China