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Traps and Seals. They are not what you think. Where does oil and gas come from?. What is a trap?. A trap is a place where oil and gas are barred from further movement, and remember that a trap is trap whether it is barren or productive. A simple anticlinal trap. Facts on traps.
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What is a trap? • A trap is a place where oil and gas are barred from further movement, and remember that a trap is trap whether it is barren or productive.
Facts on traps • The highest point of a trap is the crest, or culmination. • The lowest where hydrocarbons may be contained is the spill plane • The vertical distance between from crest to spill plane is the closure
Seals and Cap Rocks • For a trap to have integrity it must be overlain by an effective seal. • Any rock may act as a seal as long as it is impermeable • Seals will commonly be porous, but they must not permit the vertical migration of petroleum or gas from the trap
Trap Rocks • Shales are the commonest seals • Evaporites are the most effective seals • Shales are commonly porous, but because of their fine grain size, they have high caplillary forces that prevent fluid flow. • Sometimes shales may selectively trap oil, while permitting the upward migration of gas.
Leakage of gas • Several mechanisms allow the leakage of gas from a trap. • These include: the compressible Darcy flow of a free gas phase, the transport of dissolved gas in aqueous solution, and the diffusive transport through the water-saturated pore space of the cap rock.
Darcy’s Law • Darcy’s law is an equation that defines the ability of a fluid to flow through a porous media such as rock. It relies on the fact that the amount of flow between two points is directly related to the difference in pressure between the points, the distance between the points, and the interconnectivity of flow pathways in the rock between the points. The measurement of interconnectivity is called permeability.
Info on Darcy’s law • In the subsurface, rock is deposited in layers. • Fluid flow within and between the rock layers is governed by the permeability of the rocks. However, to account for permeability, it must be measured in both the vertical and horizontal directions. • For example, shale typically has permeabilities that are much lower vertically than horizontally (assuming flat lying shale beds). This means that it is difficult for fluid to flow up and down through a shale bed but much easier for it to flow from side to side.
Darcy’s law example • A good example of this characteristic is shown in the picture at right; which clearly indicates that it would be much easier for water to flow along the horizontal bedding planes in the shale where there are natural flow pathways instead of vertically where there are few flow pathways.
Distribution of oil and gas in a trap • A trap may contain oil, gas or both. The oil:water contact is the deepest level of producible oil. • Similarly, the gas:oil contact or gas:water contact is the lowest limit of producible gas • Where oil and gas occur together in the same trap, the gas overlies the oil because the gas has a lower density
Classification of traps • There are two major genetic groups of traps • Structural • Stratigraphic • There is a third group called combination but we will not go into those
Structural Traps • Structural traps are those traps whose geometry was formed by tectonic processes after the deposition of the beds involved. • A “structural trap” is one whose upper boundary has been made concave, as viewed from below, by some local deformation, such as folding, or faulting, or both, of the reservoir rock.
Examples of Structural traps Anticlinal Traps: These may be divided into two classes: Compressional anticlines (caused by crustal shortening) and Compactional anticlines (developed in response to crustal tension)
Compressional anticlines • These are most likely to be found in, or adjacent to, subductive troughs, where there is shortening of the earth’s crust • Thus fields in such traps are found within, and adjacent to, mountain chains in many parts of the world
CompactionalanticlinesThese anticlines are formed by crustal tension.
Stratigraphic Traps • These are traps whose geometry is formed by changes in lithology. • The lithological variations may be; • Depositional (e.g., channels, reefs and bars) • Postdepositional (e.g., truncations and diagenectic changes)---changes undergone by a sediment after its initial deposition
What is Diagenesis? • Diagenesis is changes to sediment or sedimentary rocks during and after rock formation, at temperatures and pressures less than that required for the formation of metamorphic rocks or melting. It does not include changes from weathering
What is Postdepositional • Post-depositional processes that can change or even remove deep-sea sediments include: physical transport, disturbances by organisms, chemical alteration, dissolution and diagenesis,gravity-driven sediment transport mechanisms, carbonate dissolution, and authigenic mineral precipitation.
