petroleum geology Petroleum geology is the study of the origins, occurrence, movement, accumulation, and exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines that are applied to the search for hydrocarbons ( oil exploration). ...

, a trap is a geological structure affecting the reservoir rock and

caprock Caprock or cap rock is a hard, resistant, and impermeable layer of rock that overlies and protects a reservoir of softer organic material, similar to the crust on a pie where the crust (caprock) prevents leakage of the soft filling (softer materia ...

of a

petroleum Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring un ...

system allowing the accumulation of

hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may b ...

in a

reservoir A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation. Reservoirs are created by controlling a watercourse that drains an existing body of wa ...

. Traps can be of two types: stratigraphic or structural. Structural traps are the most important type of trap as they represent the majority of the world's discovered petroleum resources.

Structural traps

A structural trap is a type of geological trap that forms as a result of changes in the structure of the subsurface, due to

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

, diapiric, gravitational, and compactional processes.

Anticlinal trap

anticline In structural geology, an anticline is a type of Fold (geology), fold that is an arch-like shape and has its oldest Bed (geology), beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex curve, c ...

is an area of the subsurface where the

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

have been pushed into forming a domed shape. If there is a layer of impermeable rock present in this dome shape, then water-insoluble hydrocarbons can accumulate at the crest until the anticline is filled to the ''spill point'' (the highest point where hydrocarbons can escape the anticline). This type of trap is by far the most significant to the hydrocarbon industry. Anticline traps are usually long oval domes of land that can often be seen by looking at a geological map or by flying over the land.

Fault trap

A fault trap is formed by the movement of permeable and impermeable layers of rock along a fault plane. The permeable reservoir rock faults such that it is adjacent to an impermeable rock, preventing hydrocarbons from further migration. In some cases, there can be an impermeable substance along the fault surface (such as clay) that also acts to prevent migration. This is known as clay smear.

Stratigraphic trap

In a stratigraphic trap, the geometry allowing the accumulation of hydrocarbons is of sedimentary origin and has not undergone any tectonic deformation. Such traps can be found in clinoforms, in a pinching-out sedimentary structure, under an

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

or in a structure created by the creep of an

evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as oce ...

Salt dome trap

In a

salt dome A salt dome is a type of structural dome formed when salt (or other evaporite minerals) intrudes into overlying rocks in a process known as diapirism. Salt domes can have unique surface and subsurface structures, and they can be discovered us ...

trap, masses of salt are pushed up through

clastic rocks Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks b ...

due to their greater buoyancy, eventually breaking through and rising towards the surface. This salt mass resembles an impermeable dome, and when it crosses a layer of permeable rock, in which hydrocarbons are migrating, it blocks the pathway in much the same manner as a fault trap. This is one of the reasons why there is significant focus on subsurface salt

imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image). Imaging technology is the application of materials and methods to create, preserve, or duplicate images. ...

, despite the many technical challenges that accompany it.

Pinch-out trap

Pinch-out traps are stratigraphic traps in which the petroleum reservoir thins around impermeable rock strata and eventually 'pinches out' with impermeable rock strata on either side, creating a trap.

Hybrid trap

Hybrid traps are the combination of two types of traps. In the case of tilted blocks, the initial reservoir geometry is the one of a fault-controlled structural trap, but the caprock is generally made by the draping sedimentation of mudstones during the oceanisation process.

References

{{Reflist Petroleum geology