Types of traps are geological structures that retain hydrocarbons, preventing further movement. They include structural, diapiric, stratigraphic, hydrodynamic, and combination traps formed by rock deformation, sediment movement, or water flow. Understanding these traps is crucial for identifying hydrocarbon reservoirs and optimizing petroleum exploration.
Trap:
A trap is a geologic structure on a stratigraphic feature capable of retaining hydrocarbons. Levorsen (1967) said the trap is the place where oil & gas are barred from further movement.
Types of Traps:
- Structural Traps
- Diapiric Traps
- Stratigraphic Traps
- Hydrodynamic Traps
- Combination Traps
1. Structural Traps:
Hydrocarbon traps that result from geological structures, such as folds or faults are called structural traps. They are created by the deformation of rock strata within the earth’s crust. This deformation can be caused by horizontal compression or tension, vertical movement & differential compaction. It can be of two types,
(a) Anticlinal / Fold trap:
Subdivided into two classes as.
(i) Compressional anticline trap:
Anticlinal traps caused by compression on crustal shortening are called compressional anticline traps. They are found near the subductive troughs & mountain chains in many parts of the world. One of the best-known oil provinces with production from compressional anticlines occurs in Iran (Zagnos mountains).
(ii) Compactional anticlinal trap:
Anticlinal traps that are formed by crustal tension are called compactional anticlinal traps. Crystal tensions are able to form horst & graben. Thus, anticlines may occur in the sediment corners above deep-seated horts. Closure may be enhanced both by compaction & sedimentation. Example of this trap is North Sea.
(b) Fault Trap:
The traps that are associated with faulting are called fault traps.
These are two types-
(i) Traps related to transverse faults:
Transverse faults give rise to several distinctive types of petroleum traps. Transverse movement of basement blocks takes place along wrench faults that give flower structure types of trap. Ex Ventura basin of California.
(ii) Traps related to Tensional faults:
Traps that are formed by the tensional fault are called tensional fault traps. Ex vicksburg growth fault of Texas.
2. Diapinic traps:
Diapinic traps are produced by the upward movement of sediments that are less dense than those overlying them. The sediments tend to move upward diapinically & so may form diverse hydrocarbon traps. They are not these structural traps as tectonic force is responsible for their activation. The upward movement of salt generally causes them on less frequently over-pressured clay types. There are two types,
(a) Salt domes:
Salt has a density of about 2.03 gm/cc. Recently deposited clay & sand have densities less than that of salt. As the clay & sand are buried, they become compacted, losing porosity & gaining density.
Ultimately, a burial depth of about 800 to 1200m is reached when sediments are denser than salt. When the point is reached, the salt will tend to flow up through the denser overburden. This movement may be triggered tectonically, and the resultant structures may show some structural alignment. Anticlines may trap hydrocarbon over the crest of a salt dome.
(b) Mud diapirs:
Over-pressured shale has a higher porosity & lower density than compacted clay. Sometimes, diapirs of over-pressured shale clay intrude the younger, denser cover & like salt domes. these mud lumps may even reach the surface.
3. Stratigraphic traps:
Stratigraphic traps are formed because of depositional morphology or diagenesis. There are two types,
(a) Unassociated with unconformities:
(i) Channel traps:
The traps that are formed in the environment of sand transportation but may or may not include sand deposition are called channel traps. Example- trap of eastern flank of Rocky mountains.
(ii) Barrier bar traps:
Marine barrier bar sands make excellent reservoirs because of their clean & well- sorted texture. They may be enclosed in marine & lagoonal shales. Barrier bars are able to form shoestring stratigraphic traps parallel to the paleo shoreline.
Example: San Juan basin, New Mexico.
(iii) Pinch-out traps.
Generally, a regressing barrier island deposits a sheet of sand. These sheet sands pass up-dip into lagoonal or intertidal shales and may give rise to pinch-out traps.
More complex pinch-out traps can occur where both barrier bar & channel sands are in fluid communication with one another, for example, The Bell Creek field of the power river basin.
(iv) Reeps Traps:
Reefs develop as normal & elongated antiforms. Example Sirte basin of Libya. Two main problems of reef traps are
- Not all reefs contain hydrocarbons
- Those may have reservoir characteristics unrelated to depositional facies.
(v) Diagenetic traps:
Diagenesis plays a considerable role in controlling the quality of a reservoir within a trap. The solution can enhance reservoir quality by generating secondary porosity, whereas cementing can destroy it. In some situations, diagenesis can actually generate a hydrocarbon trap.
(b) Associated with unconformities:
It can be of two types
- Supna unconformity: Onlap, Strike Valley, channel.
- Sub unconformity: Truncation.
4. Hydrodynamic Traps:
In these traps, the dynamic movement of water is essential to prevent the upward movement of oil or gas. The role of the hydrodynamic flow causes tilted oil-water contact.
Where water moves hydrodynamically down the permeable beds, upward-moving oil may be encountered. When the hydrodynamic force of the water is greater than the force due to
the buoyancy of the oil droplets, the oil will be restrained from upward movement & will be trapped within the bed without any permeability barrier. Example – Delaware basin.
5. Combination Trap:
The traps are characterized by the combined effect of structural, stratigraphy, and hydrodynamic flow, or any two of them then it is called combination traps.
Structural hydrodynamic & stratigraphic hydrodynamic traps are rare, but structural-stratigraphic traps are more common. Example- Bay field on the north slope of Alaska.
