Based on the natural gas geochemical behaviors and basic geological conditions for gas migration

this paper studied the gas migration characteristics of the quasi-continuous tight sand gas field of the Upper Paleozoic in Ordos Basin.Laterally

the variations of gas components and carbon isotope composition are mainly controlled by thermal maturity of source rocks (R

o

)

while vertically

they are affected jointly by thermal maturity of source rocks and gas migration fractionation.Further analysis of the migration driving force and pathway conditions indicates that the large tight gas field of the Upper Paleozoic is characterized by near-source accumulation via primary migration and short-distance secondary migration.The principle driving force for migration are diffusion force generated by the gradient of gas molecule concentration and abnormal high pressure

while buoyancy force has little or no contributions to gas migration.There are two main migration styles:surge flow caused by overpressure and diffusion flow resulted from diffusion.The overpressure-dri-ven surge flow is predominant in the Taiyuan Formation and the 2

nd

Member of Shaanxi Formation within source rock interval

while the diffusion-driven diffusion flow is predominant

in the 1

st

Member of Shaanxi Formation and the 8

th

Member of Xiashihezi Formation as well as reservoirs above them.The vertical regular variations of gas composition and carbon isotope are the direct responses to these gas migration patterns in Sulige gas field.