Hi, you need to have a reservoir rock, as well as a surrounding and overlying seal rock in order to form a trap.

However, the actual term 'trap' is referred to a peculiar structural and/or stratal geometric configuration which stops a further upward migration of the hydrocarbons, and 'traps' them within the reservoir rock. It is crucial to have a 3- or 4-way geometric 'closure' of the top reservoir horizon in order to obtain that.

Traps can be dominantly 'stratigraphic' (e.g. pinchouts, onlaps, stratal juxtapositions) or 'structural' (e.g., anticlines, tilted fault blocks, horsts, salt diapirs).

Dear Stefano, thanks a lot for your descriptive answer. What I did understand from your explanation was that the combination of a reservoir rock and a cap rock will yield a trap. So the trap itself is not a rock formation, it is just a word that we use to describe the entrapment of the hydrocarbons contained in the reservoir by either the existence of a seal or without it. Moreover the entrapment occur due to the shape of the reservoir as you described either structural or stratigraphic.

correct

@Azad: to make it clearer, trap is not a rock formation but can be brought about by the properties or characters of a rock formation. This can be seen when the properties of a rock or its configuration/orientation brings about a stratigraphic trap. A reservoir kinda stores hydrocarbon while traps impede the contious lateral flow or migration of hydrocarbon.  However, trap can have structural or stratigraphic origin.

The Trap  is an idiom used to refer for the petrol , but the Reservoir is often used for the water aquifer.

The trap is different component , or different characteristics than the surrounding rocks , may be of highly porosity and high permeability , but the reservoir is of the same rocks and have the same porosity.

Thank you Mr Fred and Mr Ghazi for your beneficial answers. Appreciate it

You need for a petroleum system multiple components 3 of those are reservoir, seal and trap (while the others are source rock, accumulation and migration path). Now I would recommend Dr. Brian's example; think of it like a can or a bucket. The walls of the can are the trap, the empty space is the reservoir (porous and permeable) and the lid/cover is the seal to prevent fluis from escaping vertically. Now, as it was mentioned to you the traps are mostly structural or stratigraphic or combination of both. However, most hydrocarbon in the world is produced from structural (specifically anticlines) for a simple fact that they appear easily on the seismic as recognizable lines. Hope this helps.