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repeated at the second trap. When the quantity of gas becomes insufficient to
fill a trap, oil with a gas cap is left; and once this trap is full, only oil migrates
further and the next trap up-dip contains only oil. When the quantity of oil
generated is no longer sufficient to fill a trap, that trap has an oil/water
contact, and traps further updip will be wet.
Gussow also observed that there is a tendency, not always marked, for the
oil to be progressively heavier in traps updip. This he attributed to the loss of
gas from solution in the oil.
Further subsidence after petroleum generation has largely ceased will result
in compression of the accumulated gas, and some may be taken back into
solution in the oil, thus raising the water contacts above the spill points. All
accumulations in the trend are, of course, hydraulically interconnected, but
the water contacts are at different levels depending on the spill points, gen-
erally rising updip.
A particularly good example of regional differential entrapment is the
Silurian pinnacle-reef belt east of Lake Michigan, studied by Gill (1979).
This belt is 270 km long and 16-32 km wide (170 by 10-20 miles). Petro-

I

Fig. 9-19. Map of north-western part of Michigan basin showing zones of water, oil and
gas accumulations in Silurian reefs. (Reproduced from Gill, 1979, p. 614, fig. 4, with
permission.)