Elements of Petroleum Play: Traps and Seals

Today’s issue concludes the July Series on Petroleum Play Elements, looking at Traps and Seals that prevent leaks and drive greater accumulation of petroleum.

Seals are impermeable rocks that cover underground reservoir rocks, trap petroleum in them for accumulation and prevent their seepage onto the surface. They are impermeable because they do not allow the passage of fluids through them. Seals are also called “cap rocks” and are usually fine-grained—fine- or small-grains have lesser spaces in-between them that would allow fluid passage. An example of a seal is Shale.

Traps refer to the geometric arrangements of the seals, reservoir rocks and other geologic structures such as fractures, faults or anticlines to block the movement of hydrocarbons and cause it to accumulate in the reservoir rocks. Traps also help to leak water while retaining hydrocarbons.

There are different modes in which traps are created. Three popular modes are

• Structural Traps, traps formed by actions of structural deformation in rocks such as faults (fractures in rocks) or folds (rocks bend in the manner of elastic rubber tubes or plastic bottles).
• Stratigraphic Traps, traps formed due to changing rock types or properties. This could be from one rock type, limestone, to another rock type, shale. There could also be varying properties within the reservoir rocks such as changes in their porosity and permeability.?
• Combination Traps, referring to a combination of the features of both structural and stratigraphic traps. A popular combination trap is a salt diapir or salt dome.

All elements of petroleum play are critical to the generation and accumulation of petroleum. The geologist’s role involves understanding each element, the interrelationships amongst them, the history of the events the elements would have caused, to make better inferences on the processes and make better informed decisions for oil and gas exploration.