A hydrocarbon trap is required in order to exploit the oil and gas reservoir. A trap is a place where oil and gas accumulates. In general it is an area where porous reservoir rocks are covered by impermeable rocks which act as a seal. There are a number of different geologic settings where this occurs. The main types of traps, structural and stratigraphic are described below.
Structural traps form after the sedimentary rocks are deposited, usually by tectonic forces. An anticline is where rocks are folded or bent upwards. Hydrocarbons migrate up the flanks of the anticline and are trapped in the crest.
Faults occur where there is movement along a joint or fracture. Offset of the beds could result in an impermeable layer being on top of a permeable layer.
Stratigraphic traps form at the time the sediments are deposited. Sometimes due to a lateral change in the environment of deposition a lens of permeable sand is surrounded by less permeable siltstones and shales, forming a pinch out trap. This commonly happens in stream environments where sand is deposited along the stream channel which is surrounded by a flood plain characterized by finer grained sediments.
Unconformities are another type of stratigraphic trap. They represent a gap in the geologic record, in other words a period of erosion and/or nondeposition. They can result in a permeable reservoir rock being truncated and overlain by an impermeable unit. The Knox Unconformity in the subsurface of Ohio forms an unconformity trap. the permeable Rose Run sandstone is truncated by an erosion surface that formed during the early Ordovician.
