Tar Sand

Article Outline

Introduction; Occurrence; Formation; Properties; Extraction and Processing; Environmental Concerns

Introduction

Tar Sand, sand or sandstone saturated with bitumen, a dark, asphaltlike oil. Bitumen consists of a mixture of hydrocarbons—chemical compounds containing hydrogen and carbon—and small amounts of other compounds. Because of its gluelike consistency, bitumen is too viscous (thick) to extract from the ground by conventional production techniques and consequently is regarded as an unconventional oil reserve. Once extracted and refined, bitumen can be used as a synthetic fuel and as a raw material in the chemical industry.

Occurrence

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Tar sands represent a potentially vast supply of energy. World reserves of tar sands contain an estimated 3.58 trillion barrels of bitumen.

The world’s largest tar sand deposits are located in the province of Alberta, Canada, and contain a total of about 2.5 trillion barrels of bitumen. One of these deposits, located near Fort McMurray along the lower Athabasca River, contains an estimated 919 billion barrels of bitumen—the largest known deposit of crude oil in the world. However, of the 2.5 trillion barrels of bitumen located in Alberta, only 10 percent may be recoverable with current mining methods. This would still represent the second-largest oil reserve in the world, after Saudi Arabia.

Since 2002 a number of major oil companies, including Chevron, ConocoPhillips, a subsidiary of ExxonMobil known as Imperial Oil, and Shell, have received permits to develop major extraction operations in Alberta. The region’s tar sand deposits yielded about a million barrels of crude oil a day in 2007. Due mainly to this production, Canada became the largest supplier of crude oil to the United States. The yield from Alberta’s tar sands was expected to double by 2010 and to triple by 2015.

Formation

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Liquid hydrocarbons, including petroleum, are formed over millions of years within the earth’s crust (see Fossil Fuels). Because petroleum is less dense and lighter than water, it tends to migrate upward through tiny, water-filled pores of permeable rocks in the earth’s crust. Permeable rocks are rocks through which liquids or gases can move.

Tar sands form where petroleum migrates upward into deposits of sand or consolidated sandstone. When the petroleum is exposed to water and bacteria present in the sandstone, the hydrocarbons often degrade over time into heavier, asphaltlike bitumen.

Properties

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Tar sands are a mixture of sand, water, clay, and bitumen. Typically, each grain of sand is surrounded by a thin film of water, allowing certain refining processes to separate the sand and bitumen. While sand grains in the Athabasca tar sands are surrounded by a thin water film (these tar sands consist of about 4 percent water), many tar sands in the United States lack this water content. As a result, such deposits may be unrecoverable by an extraction method known as the hot-water process.

The sandstone in tar sands can range from loose sand to consolidated sandstone. For example, the Canadian tar sands consist largely of loose sand, while the deposits in Utah are located mostly in consolidated sandstone.

Bitumen has the consistency of molasses—at 38^oC (100^oF) it is more than 10,000 times more viscous than crude oil. As a result, bitumen cannot be removed from the ground as easily as crude oil, and different recovery and processing techniques must be used. Bitumen also has a higher sulfur content than petroleum and contains higher percentages of metals such as nickel and vanadium.

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Introduction

Occurrence

Formation

Properties