Abstract: The pore structure of shale gas reservoir is complex,and the types of fluid in pores are various. According to the fluidity of fluid in pores,the pore fluids are often divided into movable water,capillary bound water and clay bound water. In order to clarify the occurrence and migration rule of pore fluid in shale gas reservoir, seven shale samples with different organic matter and mineral composition of Longtan Formation in Sichuan Basin were selected. Low field nuclear magnetic resonance（NMR）was used to measure the NMR responses of shale gas reservoir under variable centrifugal force and drying conditions at different temperatures. Based on the characteristics of the occurrence of pore fluid in the processes of centrifuging and heating,the fluid types（movable water,capillary bound water and clay bound water）in shale pores and their NMR T₂ cutoff values were determined. The results show that the NMR T₂ cutoff values（T_{2 c1}）of movable water and capillary bound water change from 0.55 to 1.00 ms,with an average value of 0.717 ms,while the NMR T₂ cutoff values（T_{2 c2}）of capillary bound water and clay bound water change from 0.27 to 0.53 ms,with an average value of 0.36 ms. According to the relationship between the relaxation time of NMR T₂ spectra and pore size,the cutoff values of pore size of capillary bound water and clay bound water change from 4.52 to 5.65 nm,with an average value of 4.99 nm. The research results are conducive to the classification of shale pore fluid types and the calculation of the lower limit of effective pore size,so as to provide a reliable basis for the efficient development of shale gas reservoirs.

Key words: nuclear magnetic resonance, fluid classification, T₂cutoff value, lower limit of effective pore size, shale gas reservoir, Longtan Formation, Sichuan Basin