Reservoir Rocks and Traps, the Sites of Oil and Gas Pools

Abstract

Petroleum is ultimately collected through secondary migration in permeable, porous reservoir rocks in the position of a trap. Any permeable and porous rock may act as a reservoir for oil and gas. They may be detrital or clastic rocks, generally of siliceous material, or chemically or biochemically precipitated rocks, usually carbonates. It is not uncommon that petroleum is found in fractured shales. Occasionally, igneous and metamorphic rocks are hosts for commercial quantities of petroleum, when favorably located in proximity to petroleum-bearing sedimentary sequences. The fundamental characteristic of a trap is its upward convex shape of a porous reservoir rock in combination with a more dense and relatively impermeable sealing cap rock above. The ultimate shape of the convexity may be angular, curved, or a combination of both. The only important geometric parameter is that it must be closed in vertical and horizontal planes without significant leaks to form an inverted container. The strike contours of this inverted container on a structural map must encircle closed areas comprising what is termed closure area or closure of a trap. A rare exception to this rule is true hydrodynamic trapping.