2.1.9 Structural geology
2.1.9.1 Introduction
At destructive plate margins, the sediments and the top part of the crust are compressed
and deformed by the process of collision. The rocks are bent and fractured. The study of
the structures that result and the processes that form them is called Structural Geology.
2.1.9.2 Earth movements
Most rocks are fractured during earth movement, resulting in cracks called joints. If the
rock layers on one side of a fracture have moved in relation to the other side, the fracture
is called a fault (Figure 2-4). Displacement - or how far apart the sides of the fault have
moved - may range from only a few inches to many miles, as along the San Andreas fault
in California.
2.1.9.3 Faults
A simple classification system outlines four kinds of faults: normal, reverse, thrust, and
lateral (Figure 2-4). The names are derived from the movement of adjacent blocks.
Movement is up or down in normal and reverse faults but is mainly horizontal in thrust
and lateral faults. A combination of vertical and horizontal movements is also possible in
all faults.
Rotational faults and upthrusts (Figure 2-5) are variations of normal and reverse faulting.
They are most important to the petroleum geologist because they affect the location of
oil and gas accumulations.
Earth movements often bury or prevent the depositing of part of a sediment series that is
present elsewhere. Such buried erosion surfaces are called unconformities. Two general
kinds of unconformities are the disconformity and the angular Figure 2-6). Earth
movements are most important to petroleum geology because they produce barriers that
cause a large proportion of petroleum accumulations.
2.1.9.4 Folds
Folds can be classified in many ways, one of the simplest is into anticlinal and synclinal
folds.
As compressional forces increase, the folds become tighter and the limbs drop more
steeply. Assymetric folds are ones in which one limb dips more steeply than the other.
These dips can eventually become greater than vertical and folds become overturned.
Axial plane cleavage can develop which is caused by alignment of platey minerals
parallel to the fold axis. With increasing deformation this cleavage can dominate the
structure of the rock, obliterating the original bedding. Fold axes need not be horizontal,
in which case they are said to plunge.
If more than one episode of the folding takes place, then the axial planes cleavage
developed by the first phase may itself be folded. This is then known as superimposed
folding and can often be recognized by statistical analysis of several fold axes in one
area.
Folding in sedimentary rocks is important as it creates the potential for oil traps on the
Crest of folds, and these are a major cause of hydrocarbon accumulations.
2.1.9.5 Joints
These are fractures in the rock which are not associated with any significant movement
of the rock. They typically occur in Limestones and Dolomites due to solution along
natural planes of weakness by percolating underground waters, or by removal of
overlying weight of rock by erosion which allows the rock to expand slightly from stress
release, and fracture. They normally develop in three planes, all at right angles, and often
have a strong control on the geomorphology of the area. Jointing in the rocks can lead to
large volumes of porosity and is an important reservoir type, particularly in carbonate
rocks. It can also give lost circulation problems when drilling a highly jointed or
cavernous area.
2.1.9.6 Unconformities
Although these are not strictly structural features, we will look briefly at unconformities.
An unconformity is any break in the geological sequence.