Oil Sands Geology & the Properties of Bitumen
Oil sands are a loose sand deposit which contain a very viscous form of petroleum known as bitumen. These unconsolidated sandstone deposits comprise primarily of sand, clay and water saturated with bitumen. Oil sands are sometimes referred to as tar sands or bituminous sands.
OIL SANDS DEPOSITS EXPLAINED
The exact composition of Alberta's oil sands can vary greatly, even within the same geological formation. A typical oil sands deposits contains about 10% bitumen, 5% water and 85% solids. However, the bitumen content can be as high as 20% in some sections.
The solids contained in oil sands deposits are primarily quartz silica sand (usually over 80%), with a small fraction of potassium feldspar and fine clays. Clay minerals typically consist of kaolinite, illite, chlorite and smectite. Deposits that have a high fines content tend to have a lower bitumen content and are generally considered to be a lower quality ore.
The water content can also vary greatly, from almost zero as as much as 9%. In general, sections with a higher water content also tend to have less bitumen and more fines. Water in the oil sands (commonly referred to as connate water) carries with it a number of soluble ions, including sodium, potassium, calcium, chloride and sulphate. Fines tend to be clustered together, sometimes referred to as a clay lens.
Conventional wisdom was that the grains of sand were covered with a water layer, although this theory has been largely debunked. The water, sand, clays and bitumen are intermixed within the oil sands deposit.
Bitumen is a highly viscous, complex hydrocarbon contained within the oil sands deposit. Bitumen is classified as an extra heavy oil, with an API gravity of about 8° and can be almost solid at room temperatures. Alberta bitumen also contains about 4 to 5% sulphur with trace volumes of heavy metals, particularly nickel and vanadium.
Bitumen can be separated into two organic compounds: asphaltenes and maltenes.
Asphaltenes have an extremely complex molecular structure, imparting a high viscosity to the oil. Asphaltenes make bitumen sticky and "heavy", carrying with it nickel, sulphur and vanadium. This reduces the quality of the crude and makes bitumen much harder to process in a conventional refinery. Athabasca bitumen contains about 14 to 18% asphaltenes.
Maltenes can be further fractionated into saturates, aromatics and resins. The exact fraction of each component varies according to geology.
OIL SANDS FORMATIONS: A BRIEF HISTORY
Hydrocarbons were formed hundreds of millions of years ago from the decomposition of algae and other marine sea creatures. At the appropriate depths and temperatures, this organic matter degraded to form light oil. The light oil contained in the Western Canada Sedimentary Basin is believed to have been formed from water bodies which covered much of Western Canada.
Formation of the Rockies pushed this light oil east towards the Alberta/Saskatchewan border, transporting with it oxygenated water. Microbe bacteria contained in the water fed off the lighter hydrocarbon molecules in a process called biodegradation. As a result, a heavier complex hydrocarbon containing sulphur and trace concentrations of metals were left behind to form Alberta’s oil sands deposit.
Bitumen from the Alberta oil sands is highly susceptible to oxidation. That's why oil sands deposits cannot be stockpiled. When exposed to atmosphere for prolonged periods of time, bitumen can become "aged", making hot water separation much less efficient.
Alberta bitumen is quite high is sulphur, typically 4 to 5% by weight, second only to Boscan Crude from Venezuela which contains about 5.5% sulphur.
Canada’s oil sands are concentrated mostly in Alberta, extending slightly into Saskatchewan’s border. The entire deposit covers an area of about 142,000 km², only 3% of which is mineable.
Oil sands deposits are localized in 3 regions, classified as follows:
- the Athabasca Basin
- the Peace River Basin, and
- the Cold Lake Basin.
The Athabasca basin is by far the largest, spanning an area of about 40,000 km². All mineable oil sands is locate north of Fort McMurray, Alberta within the Athabasca basin, where the deposit can be found very close to the surface.
The Cold Lake basin spans an area of about 22,000 km², extending into the Saskatchewan border. The oil sands deposit is located 300 to 600 meters below the surface, making mining infeasible.
The Peace River basin is by far the smallest of the three, covering an area of just 8,000 km². The oil sands deposit is located roughly 300 to 770 meter below grade.
Alberta's oil sands contains about 166 billion barrels of recoverable oil, representing over 95% of Canada's total reserves.
THE ALBERTA OIL SANDS: A UNIQUE DEPOSIT
Oil sands deposits can actually be found all over the world, including Kazakhstan, Russia, Madagascar and the United States. Although all oil sands contain heavy, complex hydrocarbons, their physical properties can vary.
The Athabasca oil sands found in Alberta are water-wet, meaning a layer of water is thought to coat the bitumen and solids. This water facilitates the separation of the bitumen from the sand using water-based gravity separation process.
Other oil sands, tar sands or shale deposits commonly found in places like Venezuela and Utah are oil-wet and therefore cannot be separated in a water-based process. These deposits require the use of solvents and chemicals to separate the heavy oil from the solids. That’s a big part of what makes the Alberta oil sands unique.
OIL RESERVES: CANADA VS THE WORLD
The total proven oil reserves in Canada is estimated to be 1.75 trillion barrels. However, only 10% of this volume can be economically recovered at current oil prices. About 97% of Canada’s reserves are contained within the oil sands. This represents 75% of North America’s total oil reserves.
Canada’s Ultimate Recoverable Volume from the oil sands is estimated at 315 billion barrels. This is an estimate of the total volume of oil that can be extracted in the future.
EXTRACTING BITUMEN FROM THE OIL SANDS
There are 2 basic methods of bitumen extraction used in the oil sands: surface mining and in-situ. The method used depends on the depth of the reservoir.
Deposits located at a depth of less than 75 meters can technically be surface mined, although most deposits are located at a depth of less than 50 meters below grade. Surface mining (or open-pit mining) is only viable for a portion of bitumen located in the Athabasca region north of Fort McMurray. This represents about 20% of the total recoverable reserves.
The remaining 80% of the bitumen is too deep to be mined and can only be extracted in-situ (or in-place) using steam. Most of the in-situ facilities currently in operation extract bitumen from a depth of at least 200 meters.
The bitumen recovery process is slightly different for mining versus in-situ facilities. Mined oil sands requires large amounts of process water to separate the bitumen from the sand, while the in-situ process requires less water but greater volumes of steam. In both cases, the resulting bitumen product can either be sent to an unpgrader for conversion to synthetic crude oil or diluted and sold directly to refineries.