7 Non-conventional sources of petroleum
7.1 Oil sands
Non-conventional sources of petroleum, such as oil sands, heavy oil and gas hydrates, greatly exceed the world's entire endowment of conventional petroleum. Yet, because of technological, commercial and environmental constraints to production, non-conventional petroleum currently accounts for only about 5% of global consumption. This huge imbalance will slowly change as the inevitability of declining conventional petroleum reserves and increasing prices hits home. This page deals with oil sands; the next will deal with gas hydrates.
If a near-surface rock has good reservoir properties, large volumes of oil can flow into it from mature source rocks buried deep below. Exposure of crude oil to air and bacteria close to the surface degrades it to thick, viscous bitumen. Over time, tens of metres of rock from the surface downward can become completely impregnated with bitumen, forming a deposit known as oil sand.
Oil sand is composed of bitumen, sand, clays and water. Bitumen, in its raw state, is black and thicker than treacle. It requires treatment to make it fluid enough to transport by pipeline and to be usable by conventional refineries. The process involves large-scale surface strip-mining of enormous volumes of oil sand (see Figure 20a below). The sands are then heated to between 35–80 °C to separate and chemically change the bitumen to lighter hydrocarbons using water-based extraction methods. The upgraded product consists of light and heavy oils that are blended to produce a light crude oil with a low sulphur and nitrogen content.
The world's largest producer of crude oil from oil sand, Syncrude, is based in the Athabaska area of northern Alberta in Canada. Their product is called Syncrude Sweet Blend, and in 2004 it accounted for about 10% of Canada's total crude oil production. With billions of barrels recoverable using current technology, the Athabasca deposit constitutes a resource for decades to come. Importantly, the drive to reduce operating costs to their current level of around US$10 per barrel has also been accompanied by significant reductions in sulphur emissions, water abstraction and power usage during the upgrading process.
Figure 20: Large-scale extraction of oil sand in northern Canada. (a) Satellite image of the Syncrude operation at Fort McMurray, Alberta. Active and near-future operations are at lower left and top centre. The image is about 15 km across. (b) To operate continuously, oil-sand mines need the world's largest excavators. This is the Krupps Bagger 288, a bucket wheel reclaimer, which is the largest land vehicle ever built. It is on its way to a lignite mine in Germany: similar machines operate in the Canadian oil-sand mines.
When oil sands occur at depths that are too great for surface mining, in situ extraction involves injecting steam and/or hot carbon dioxide to lower the viscosity of the oil and enable it to be pumped to the surface.
Petroleum accumulations that will not flow to the surface under natural reservoir pressure are referred to as heavy oils. They are characterised by high viscosity that increases with their density, low hydrogen/carbon ratios, low gas/oil ratios and significant sulphur, asphalt and heavy-metal contents. Heavy oils can form for a variety of reasons, such as kerogen composition, level of maturity, depth of burial and exposure to water, air or bacteria. The economics of heavy oil production typically suffer from high extraction costs and a discounted value because of their inferior quality. As a result, the vast remaining global resource of heavy oils is still very much underexploited.