2. Geological characteristics of conventional hydrocarbon accumulation in a trap. Conventional hydrocarbon accumulation is the key to traditional petroleum geology and usually refers to an individual hydrocarbon accumulation in a single trap with a uniform pressure system and oil–water contact.f there is only oil (gas) in the trap, it is called an oil (gas) pool, whereas if there are both oil and gas, it is called a petroleum pool. A conventional hydrocarbon accumulation is a three-dimensional (3D) geological entity with clear boundaries.