Page 162 - Well Logging and Formation Evaluation

152 Well Logging and Formation Evaluation

Growth fault

Normal fault

Reverse fault

Wrench fault

Figure 10.2.1 Types of Fault

Features associated with compressional tectonics are:

• Reverse (or thrust) faults. Where the main stress in the reservoir is
horizontal on one axis, with the second main stress also horizontal but
normal to this (i.e., the stress in the vertical direction is the weakest),
reverse faulting may occur. The frictional forces associated with rocks
passing over one another are very high, and overpressures may often
be associated with reverse faulting, since they provide a mechanism
whereby the friction may be reduced (see Figure 10.2.1).
• Wrench faults. These will occur where the second highest stress is in
the vertical direction and the weakest in the horizontal direction normal
to the axis of greatest stress. This results in a horizontal displacement
of one block relative to the other.
• Folding. Compressional stresses will generally give rise to faulting in
brittle rock. In more ductile rock, folding is possible, and this can also