BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
Structural
Structural traps are created when the seal or barrier is concave upward (looking from below). The
geometry is formed by tectonic processes after deposition of the reservoir beds involved.
There are two types of structural traps:
1. Folds result in the physical bending (deformation) of the rock units without breaking. The
simplest form is an anticline – creating an Anticline Trap. The rock units undergo bending
very slowly over long periods of geologic time. These types of traps are often found adjacent to
mountain ranges.
2. The second type of structural trap is a Fault Trap. These are the result of fractures (breaks)
within the rock units where one side has moved relative to the other side. Faulting may be the
sole cause of the formation of a trap or more commonly, faults form traps in combination with
other structural features such as folding which is common along the Gulf coast region of
Louisiana. Often an oil field is the result of multiple faulting, forming many structural traps
containing hydrocarbons.
Figure 6 - 9
Structural and Stratigraphic Traps
Stratigraphic
Stratigraphic traps are created when the seal or barrier is formed by changes in lithology or rock
type (which also controls porosity and permeability), during the deposition of the reservoir beds
involved. Lithological variations may be depositional, as in the cases of reefs, channels, and sand
bars. Lithological variations may also be post-depositional in nature; truncations occur where
erosion has removed a significant portion of an existing tilted structure followed by deposition of
another lithological rock unit, the latter forming the roof rock and thus becoming the capping bed.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-14