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Trap Types

Most oil and gas migrates from where it was formed and is found in "traps" which prevent it from seeping closer to the surface.  In the Lloydminster area, the source rock is more than 500 meters below the collection pools from where it is now being produced. 

Five kinds of traps are found in the region. Different geological structures are created in different rock layers, and thus certain oiltraps predominate in rocks of each age. Illustrated from left to right below, they are:

An unconformity is a break in the depositional sequence of rocks. If the underlying beds were tilted, eroded, and then covered with flat-lying impermeable rocks, then oil and gas may be trapped at the unconformity. In Carboniferous rocks of southeastern Saskatchewan, unconformity traps are very important sources of petroleum.

A dome, or anticline, forms when rock layers are pushed up, long after their deposition, into an arch. Gas and oil may be trapped at the top of the arch in reservoir rock if a cap of impermeable rock lies above. Domes may form because of the sideways pressures from mountain-building, or because of partial collapse and shifting of deeply buried salt beds. Saskatchewan’s domes are major sources of petroleum in Carboniferous rocks of the southeast and Cretaceous rocks in the west.

A pinchout traps oil and gas where a layer of reservoir rock ends suddenly, leaving only impermeable rock through which nothing can flow. Pinchouts are common in Cretaceous rocks of western Saskatchewan, where sandstone layers reach the limits of their deposition, giving way to dense marine shales that formed farther offshore. The heavy oil of the Lloydminster area, and the conventional oil and gas of the Kindersley areas are trapped in pinchouts.

A large, isolated reef growing on the high ground of a carbonate bank is called a pinnacle reef. Pinnacle reefs have long been important in the Alberta oil industry, but only recently have rich strikes been made in the Devonian reefs of southeastern Saskatchewan. The limestones within the reef may hold huge quantities of oil and gas, sealed in by a salt or anhydrite caprock.

An up-dip plug may block the upward flow of petroleum in an otherwise excellent reservoir rock. The plug is formed when impermeable muds are deposited in channels cut through a lower reservoir layer, forming a barrier or dam. As a result, parts of the reservoir may hold only water and no gas or oil. These traps occur in Cretaceous oil and gas fields of western Saskatchewan, and are a complicating factor in petroleum exploration.

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An oil and gas "pool" is not a lake held in an underground cavern, as the name implies.  Instead gas, oil, and water occupy the pore space in sedimentary rock such as sandstone, or more commonly in he heavy oil fields, occupies underground sand beds.  The pores where the oil resides are not large, in fact, some may not be visible without magnification.  What matters is that the pores are well connected to each other so that gas and oil can pass through freely.

Source: Geological History of Saskatchewan, a publication of the Saskatchewan Museum of Natural History