The concentration of petroleum (oil and/or natural gas) in any volume of a reservoir rock is commonly commercially quite low. As a result, the petroleum must be concentrated into a commercially viable volume. The structure that serves to concentrate the petroleum is called a trap. The petroleum will flow along the reservoir until it encounters rocks whose permeability and porosity is so low that the petroleum cannot flow further and becomes trapped. One of the best kind of rock to entrap petroleum is shale. One of the most common traps is the anticlinal trap, first suggested by Dr. I.C. White, founder of the West Virginia Economic and Geologic Survey. In this case, the shale overlies the reservoir and is called a cap rock. As the petroleum moves through the reservoir is concentrates in the axial region of anticlines. A fault trap may occur when the up-thrown portion of a faulted, capped reservoir is situated opposite a low porosity and low permeability shale formation. A stratigraphic trap may form when the lithology of a highly porous and high permeable sandstone reservoir changes laterally into a low porosity, low permeability shale. The salt traps are commonly found in the Gulf of Mexico where rising salt domes have penetrated reservoirs and result in reservoir rings surrounding the salt dome.
petroleum, shale, anticlinal trap