Mechanisms of Petroleum Accumulation in Geological Traps

Hydrocarbon Traps and Petroleum Accumulation

Hydrocarbon traps are any combination of physical factors that promote the accumulation and retention of petroleum in one location. Traps can be structural, stratigraphic, or a combination of the two.

Geologic processes such as faulting, folding, and deposition and erosion create irregularities in the subsurface strata which may cause oil and gas to be retained in a porous formation, thereby creating a petroleum reservoir. The rocks that form the barrier, or trap, are referred to as cap rocks.

Structural Traps

Structural traps are created by the deformation of rock strata within the earth’s crust and are formed by tectonic deformation. This deformation can be caused by horizontal compression or tension, vertical movement, and differential compaction, which results in the folding, tilting, and faulting within sedimentary rock formations.

Types of Structural Traps

Anticlinal and Dome Traps

The rock layers in an anticlinal trap were originally laid down horizontally and then folded upward into an arch or dome. Later, hydrocarbons migrate into the porous and permeable reservoir rock. A cap or seal (an impermeable layer of rock) is required to permit the accumulation of the hydrocarbons.

Salt Dome or Salt Plug Traps

A trap created by piercement or intrusion of stratified rock layers from below by ductile, nonporous salt. The intrusion causes the lower formations nearest the intrusion to be uplifted and truncated along the sides of the intrusion, while layers above are uplifted, creating a dome or anticlinal folding. Hydrocarbons migrate into the porous and permeable beds on the sides of the column of salt. Hydrocarbons accumulate in the traps around the outside of the salt plug if a seal or cap rock is present.

Ideal Salt Dome

Fault Traps

The faulting of stratified rock occurs as a result of vertical and horizontal stress. At some point, the rock layers break, resulting in the rock faces along the fracture moving or slipping past each other into an offset position. A fault trap is formed when the faulted formations are tilted toward the vertical. When a non-porous rock face is moved into a position above and opposite a porous rock face, it seals off the natural flow of the hydrocarbons, allowing them to accumulate.