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fault already had a throw of about 300 m on the base of the productive sand.
Maher (1980, p. 162) points out that at the time of closure, the Kimmeridge
Clay had already been buried to more than 1500 m in the adjacent Witch
Ground graben, so oil generation and migration could have begun before the
trap was closed. Maher (1980, p. 159) also reports better porosity in the oil-
bearing sandstones than in the wet sandstones, so early accumulation is not
only possible but likely. The point will not be missed that indices of thermal
maturation in the reservoir may be quite irrelevant, and so misleading.
These three accumulations of oil have several points in common, and all
are traps because of disconformities and unconformities. It is important for
the understanding of these and similar accumulations that the nature of such
disconformities and unconformities shall be properly understood.
It is commonly assumed that there was general erosion on such a surface,
but this is not necessarily true. When there is erosion, the products are trans-
ported elsewhere, and accumulate where the energy is insufficient to move

15117-7 PI5 P4 P3
I A A A
- 0- 4.) 4.) 4.)

- 9500

VERT EXAG X5 - 10000

- 10500

il - 11000

Fig. 13-9. Cross-section through Piper oil field, Moray Firth basin, North Sea. (Reproduced
from Maher, 1980, p. 142, fig. 9, with permission.)