Page 231 - Petroleum Geology

leum exploration has revealed 432 reefs of which 139 are gas-filled, 221 are
oil-filled, and 72 are barren. The cap rock is evaporite. Figure 9-19 shows
that the reef belt consists of a downdip gas zone, an intermediate oil zone,
and an updip water zone. Figures 9-20 and 9-21 show the cross-sections.
The Lockport Formation is the common porous and permeable base to the
reefs, and Gill (1979, p. 618) considered that the Cain Formation, overlying
the Lockport between the reefs and downdip, is the source rock. The oil
gravity increases up-dip from 65' to 43' API (s.g., 0.72-0.81).
We shall consider these and other reefs in more detail in Chapter 12.
The same processes could, of course, operate in a sequence of sandstone
traps; but those in carbonates are clear.

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