Abstract: Li No.5 Well is situated at the eastern part of Shangzhishimeng Syn clinorium of Yangtze Platform.Through the study of the diagenesis and epigenesis of Li No.5 Well and the comprehensive analysis of the oil/gas display,the authors think that the T₁d-P₁q oil of Li No.5 well is originated in situ.There were two oil generation stages in which the first stage is Badong sedimentary stage.At this time,the evolution of organic matter to generate oil in T-P strata has not reached its peak,and some organic matter just began on the threshold of oil generation.The two episodes in Indosinian movement brought the earth crust to be uplifted,eroded,geotemperature dropped,and the petroleum generation was stopped.The second stage is the thermal fluid activity in Yanshan movement.The reservoir in P-T₁ of Li No.5 Well was at oil generation stage or a little bit higher.Finally,this paper points out that the favourable area for T-P oil and gas of Mid-Yangtze should be the area which is overlaid with major Mesozoic-Cenozoic basins.