Heavy Oil Science Centre - Overburden

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Fine sediments which settled out of ancient sea water now form an impermeable layer of rock. Shale is made up of tiny particles (less than 0.004 mm. in diameter) with clay being the primary mineral. These fine particles precipitated out of the sea water creating a dense, laminated rock. When the precipitation included the remains of micro-organisms, the shale could become a "source rock" for oil and gas. Since liquids and gases cannot pass through the laminations, shale can be either a cap, trapping oil from migrating further, or it can itself be a source rock. 

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When the sea transgressed onto the land, marshes and river estuaries were submerged and covered with sediment. Coal consists of the metamorphosed remains of ancient plants, usually of swampy origin. Coal was a popular source of energy, but it is not as versatile as petroleum.

Oil Sand

Ancient beaches are now saturated with oil that has seeped up from source rocks deeper within the earth. Oil sand is similar to sandstone, but it has been exposed to limited compaction, and much of its matrix is supplied by the tar-like consistency of the heavy oil present. Often a core sample of oil sand looks a lot like granular road asphalt.


Despite short life spans, micro-organisms created massive formations of oil producing limestone. Limestone consists of carbonates that either precipitated directly out of seawater, or were taken out of the water by organisms to create their shells and homes (mollusk shells, coral reefs, etc.). The tremendous number of plants and animals involved make ancient reefs virtual petroleum factories. As well, limestone is permeable and thus an excellent transportation and storage medium for oil and gas.


Evaporating seas left behind enormous layers of deposits much like a bathtub ring. Salt, potash, and gypsum resulted from dissolved minerals left behind when sea water evaporated. All are commercially significant: potash mining is an important industry in Saskatchewan, table salt is produced near Lindbergh, northwest of Lloydminster, and gypsum is an important ingredient in the wall board used in our homes.