Petroleum Traps

Trap is a place where oil andgas are barred from further movement. The word trap was first applied to a hydrocarbon accumulation by Orton ( 1889 ). A trap may contain oil, gas or both depending on the chemistry and level of maturation of the source rock and on the pressure and temperature of the reservoir itself. It's one of five essential requirements for a commercial accumulation for oil or gas which are :

  • Organic-rich source : to generate oil and/or gas.
  • Heat : source rock must have been heated sufficiently under high pressure with bacterial reaction to yield its petroleum.
  • Reservoir rock : to contain hydrocarbons, this reservoir must have.
  • Cap rock : which is impermeable rock that prevent the upward escape of petroleum to earth's surface.
  • Arrangement : source, reservoir and seal must be arranged in such a way to trap petroleum.

Nomenclatures of a Trap

Many terms are used to describe the various parameters of a trap. These terms are defined as follows and illustrated with reference to an anticline trap, the simplest type of a trap.

anticline trap

  • The crest or culmination : the highest point of a trap.
  • Spill point : the lowest point at which hydrocarbons may be contained in the trap.
  • Closure of the trap : the vertical distance from the crest to the spill point; this lies on a horizontal contour, the spill plane.
  • The pay : the productive reservoir within a trap.
  • Net pay : the vertical thickness of the productive reservoir.
  • Gross pay : the vertical distance from a reservoir to the oil: water contact (OWC).
  • The oil-water contact (OWC) : the deepest level of producible oil. Similarly the gas-oil contact (GOC) or gas-water contact (GWC) : the lowest limit of producible gas.

Types of Petroleum Traps

Most classifications of traps are based on geometry of the trap but traps are generally classified into:

  • Structural traps.
  • Stratigraphic traps.
  • Combination traps.
  • Diapiric traps.
  • Hydrodynamic traps.

Structural Traps

Those traps whose geometry was formed by tectonic processes after the deposition of beds involved. Basically structural traps are caused by folding or faulting. Sometimes it's possible to find a trap where a single process took place, but frequently the two processes can be involved with equal importance in the creation of trap. Structural traps are divided into :

  • Anticline traps.
  • Fault traps.

anticline trap

Diapiric Traps

Where salt or mud have moved upward and domed the overlying strata. They can be considered as structural traps, but since they are caused by local lithostatic movement not regional tectonic forces they should be differentiated. They are classified into :

  • Salt diapirs.
  • Mud diapirs.

Hydrodynamic Traps

Occurs when downward movement of water prevent the upward movement of oil, thus trapping the oil without normal structural or stratigraphic closure. Such traps are very rare.

Hydrodynamic Traps

Stratigraphic Traps

Traps have a geometry that is formed by changes in lithology. The lithological variations may be depositional (channels, reefs, and bars) or post depositional (truncations and digenetic changes). levorsen (1934) defined a stratigraphic trap as " one in which chief trapmaking reservoir rock, such as a facies change, variable local porosity and permeability, or an up-structure termination of the reservoir rock".

Stratigraphic traps are divided into :

  • Pitchout traps : oil and gas are trapped where a layer of reservoir rock ends in a wedge surrounded by impermeable rocks.
  • Unconformity traps : have been formed due to break in the depositional sequence of sediments.
  • Isolated, lenticular bodies : commonly of sandstone, form closed traps. Some of them are produced because they are filled with oil without free water. Most of them are small ex: channels and bars.
  • Massive traps : consists of reef traps and massive erosion traps.

Combination Traps

Many oil and gas fields around the world are not due to structure or stratigraphy or hydrodynamic flow, but a combination of two or more of these forces. Such fields may be termed as combination traps. Most of these traps are caused by a combination of structural and stratigraphic processes. Structural-hydrodynamic and stratigraphic-hydrodynamic are very rare.

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