Page 212 - Petroleum Geology

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Migration in moving water

When secondary migration takes place in a carrier bed in which the pore
water is in motion, the petroleum migfation paths are affected by this motion,
and also the geometry of any accumulation. Rich (1921,1923) clearly under-
stood this, but present understanding is due to Hubbert (1953), who ratio-
nalized and quantified the effects. The reader who wishes a more rigorous
analytical argument is referred to Hubbert's paper.
When the pore water is at rest, its potential energy is constant throughout
the carrier bed: surfaces of equal pressure are horizontal, and the direction
of petroleum migration is determined solely by gravity - that is, when un-
restrained by cap rock, it is vertical; when constrained by cap rock, it is up
dip. When the pore water is in motion, this has the effect of rotating the hoi-

"2/P

Fig. 9-6. Surfaces of constant pressure, constant gravity potential (U = gz), and constant
fluid potential (@) are normal to their gradient vectors.