Through the analysis of fluid potential distribution Under hydrodynamic conditions. this paper discusses the conditions for petroleum migration and entrapment. The angle of inclination of oil-water interface (α) is a function of the dipangle (θ) of the aquifer. which confines the water flow direction: tga=∂h

ω

/∂x/ρ

ω

-ρ

0

/ρ

ω

-ρh

∂

/∂x·tgθ (1)where ∂h

ω

/∂x is the horizontal component of water head gradient within theaquifer

and θ is the dip angle of the aquifer. In an anticlinal aquifer

the maximun height of the oil column perpendicular to the oil water interface is given by: Z

0

=(2σ[(1/γ

t

-1/γ

p

)]g·(ρ

ω

-ρ

0

))·√1+tg

2

α (2) where tgα can be obtained by solving equation (1)

assuming that α=θ.In an updip pinchout reservoir

the maximum height of oil column may be: Z

0

=(2σ|(1/γ

t

-1/γ

p

|)/(g·(ρ

ω

-ρ

0

)-(∂h

ω

/∂x)·(ρ

ω

·g/cosθ·sinθ)) (3) A F function is derived to determine the direction of oil and gas migrationin a monoclinal aquifer with a downdip flow:F=∂h

ω

/∂x-ρ

ω

-ρ

0

/ρ

ω

·sinθ·cosθ (4)A hydrocarbon globule will migrate updip when F is negative. It will be flushed down the dip if F is positive. It is concluded that without the closure of structure and lithological variation

hydrodynamic factor alone can not trap hydrocarbons.