Unit 10: Energy Challenges // Section 4: Fossil Fuels: Oil and Gas
Oil and natural gas are formed primarily when marine organisms die and settle to the seafloor in anaerobic environments where they cannot rapidly decompose, then are buried by sediments and heated by Earth's geothermal gradient (the rate at which temperatures increase moving inward from the surface toward the planet's core). This process breaks down complex organic molecules into a viscous gel called kerogen, which evolves with further heating into hydrocarbons—organic compounds consisting of carbon and hydrogen atoms. Petroleum and natural gas, and the products that we derive from them, such as gasoline and diesel fuel, are composed of hydrocarbon molecules of various sizes and shapes.
Most oil and gas forms in source rocks that represent ancient sea beds buried and heated to a certain temperature range, which corresponds to a depth range below the surface often called the oil or gas window. The oil window is generally between 1 and 6 kilometers deep, depending on the type of kerogen present and the geothermal gradient, so it is rare to produce oil from deeper levels. Gas can be produced and retained at much greater depths, such that some important gas deposits reside at depths greater than 10 km.
Oil and gas migrate out of source rocks into porous and permeable rocks called reservoirs and collect in traps that are often formed by faults or folded rocks. Reservoirs must be overlain by impermeable rocks called cap rocks or seals. The combination of a source rock, reservoir, trap, and cap rock is called a hydrocarbon system—the essential geologic elements that must be in place to yield a large oil or gas field (Fig. 8).
Figure 8. Cross-section of a hydrocarbon system
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Developers tap these deposits by drilling wells into oil and gas reservoirs. In many cases, natural pressures drive the hydrocarbons to the surface. For certain heavy oils, or in fields where pressure has been depleted by production, oil must be pumped to the surface or driven from below by injecting water, natural gas, CO2, or steam into the reservoir. In many parts of the world, oil and gas exploration is pushing the frontiers of technology, with developers drilling wells more than seven miles below the surface, in deep water, or horizontally through reservoir rocks.
Refineries distill crude oil to produce a wide range of fuels, lubricants, and industrial chemicals. On average, about half of a standard barrel of oil (42 gallons) is converted to gasoline. Refined petroleum also yields kerosene, jet fuel, diesel fuel, home heating oil, and lubricants in varying proportions, depending on the original type of crude oil and the refining process (Fig. 9). Natural gas may also require processing to remove undesirable gases such as hydrogen sulfide and other impurities. In some cases this process can yield useful byproducts, such as sulfur, which is sold and used to generate fertilizer and for a wide range of other industrial purposes.
Figure 9. Products from a barrel of crude oil
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Source: © adapted from United States Coast Guard original.
Oil and gas drilling can have adverse environmental impacts, from surface disturbance for construction of drilling pads and access roads to contamination of aquifers with drilling muds and fluids. Offshore drilling can cause spills and leaks that pollute ocean waters, either as a result of industrial accidents or through storm damage to drilling rigs. Transporting oil and gas from wells to processors to users also requires large infrastructures and creates environmental risks. Oil is shipped worldwide by pipelines and tankers, both of which are subject to spills. Most natural gas is currently transported via pipeline, but tanker shipment of liquefied natural gas (LNG) that has been chilled to -260°F represents a growing segment of the world market. LNG is re-gasified at receiving terminals and delivered by pipelines to end users.
Oil produces somewhat lower levels of CO2, sulfur dioxide, nitrogen oxide, and mercury emissions than coal when it is burned, but still contributes significantly to acid rain, photochemical smog, and global climate change. Natural gas combustion emits lower amounts of nitrogen oxide and CO2 and virtually no sulfur dioxide or mercury. (For more details, see Unit 11, "Atmospheric Pollution," and Unit 12, "Earth's Changing Climate.")
Modern practices of drilling for and producing oil and gas attempt to minimize adverse environmental impacts. For example, co-produced waters are now generally re-injected or cleaned before disposal, and enhanced safety systems and procedures have made drilling and production accidents rare. Oil spills from tankers still pose a serious environmental hazard, but national governments have agreed on steps such as eliminating old tankers in favor of double-hulled designs by 2015 in an effort to further reduce these risks.
Nevertheless, because the United States has exploited many of its prime oil and gas reserves, exploration on land is now moving into environmentally sensitive regions, such as public lands that hold fossil fuel deposits but also are home to rare and endangered species. As a result, the environmental impacts of oil and gas exploration have become highly controversial in many parts of the western United States.