Page 305 - Petroleum Geology

CHAPTER 13

PALAEOGEOMORPHIC AND UNCONFORMITY TRAPS

SUMMARY

(1) Palaeogeomorphic traps are traps that resulted from the accumulation
of sediment over a pre-existing topography, the physiographic expression of
which led to facies that could generate reservoir rock and, usually contem-
poraneously, facies that would eventually act as cap rock. These are almost in-
variably diachronous, transgressive sequences that include petroleum source
rock.
(2) Unconformity traps are those that resulted from the truncation of re-
servoir rocks and the subsequent sealing of the subcrop by an unconformable,
relatively impermeable, fine-grained, rock unit. The source rocks may be
within the pre-unconformity sequence, or in the immediate post-unconformity
cap rocks. The timing of secondary migration is not, of course, earlier than
the time of sealing of the subcrop. There may be a lapse of 50 m.y. or more
between the accumulation of the petroleum source rock and the accumula-
tion of its petroleum.
(3) Of particular importance to geology is the occurrence of both types of
trap in a world-wide geological context related to the Mesozoic development
of rift margins to continents (e.g. North-West Europe, Alaska, Australia) and
the development of rift basins that are not parallel to a continental margin
(e.g. North Africa). These areas show very similar geological histories, begin-
ning with rifting and growth faulting, followed by transgression and subsidence
with little or no fault movement. Commonly the development began in the
Permian or Triassic and continued well into the Tertiary, at least. Not all
such areas have strictly contemporaneous events, but most show a prolonged
“tensional” regime that began long before the opening of the adjacent ocean,
and continued while the ocean opened wider.

PALAEOGEOMORPHIC, PALAEOTOPOGRAPHIC TRAPS

About 60 km to the east-northeast of the Intisar fields of Libya, with
which we finished the previous chapter, lies the Augila field (see Fig. 12-11)
and a good example of a different form of stratigraphic trap that depends on
marine transgression over an irregular, usually subsiding, topographic surface.
The deepening sea generates sediment from the migrating shore line, the sedi-
ment accumulating eventually to form a porous, diachronous reservoir rock