Page 71 - Petrophysics

SEDIMENTARY GEOLOGY 45

An anticline structure may contain several oil traps, one on top of the
other, separated by impermeable rocks. Furthermore, the lithology of the
individual traps may vary from sands to limestone and dolomite [9, 1 11.
Hydrocarbon traps are generally classified as either structural or
stratigraphic, depending on their origin. Structural traps were formed
by tectonic processes acting on sedimentary beds after their deposition.
They may generally be considered as distinct geological structures
formed by folding and faulting of sedimentary beds. Structural traps
may be classified as: (1) fold traps formed by either compressional or
compactional anticlines, (2) fault traps formed by displacement of blocks
of rocks due to unequal tectonic pressure, or (3) diapiric traps produced
by intrusion of salt or mud diapirs (Figure 2.6).
Stratigraphic traps are produced by facies changes around the porous,
permeable formation such as pinchouts and lenticular sand bodies
surrounded by impermeable shales. Stratigraphic traps may develop
from offshore sand bars, reefs, or river channels. The processes of
formation are more complex than those of structural traps because they
involve changes of the depositional environment that lead to isolation of
permeable zones by different lithologies. Distinctions are made between
those that are associated with unconformities and those that are not [6].
Many hydrocarbon accumulations are associated with unconformities.
An unconformity forms when a site of sedimentation is uplifted, eroded,
and buried again under a new layer of sediments that may delineate
the boundaries of an oil trap, because unconformities generally separate

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Figure 2.6. Illustration of several types of traps: (A) stratigraphic pinch-out trap,
(Bj trap sealed by a salt dome, (Cj trap formed by a normal fault, (0) domal tl rap.