Keywords: 
• palaeo-hydrodynamics /
• fluid potential /
• fluid pressure /
• upper Palaeozoic group /
• O rdos Basin

Abstract: The palaeo-hydrodyonamics during different geologic periods around middle giant gas field in O rdos Basin are restored use of Basin modeling and other technique.Acco rding to the distrbution of fluid (gas) potential, i t is discovered that the palaeo-hy rodynamic evolution history of U pper Palaeozoic Group since Triassic period (in which source rocks were matured)could be dividedinto three major stages, i.e.Triassic-Jurassic period, ear- ly C rtaceous epoch and late Creataceous epoch up to now.On the basis of sy nthetic analysis, the follow three characteristics have also been discovered: ( 1) the fo rmer three majo r stages of hydrodynamic evduition are closely to realated regional tectonic evoultion stage; (2) the distribution of fluid pressure is controlled by ralat- ed sedimentation (rate, thickness); (3) althoug h the objective layers is old (late Paleoaoic ear), the contribu- tion of compactional flow in hy drody namics is still evidently more than of g ravi tational flow, even in now a- days.