Oil and Gas
What Are Oil and Gas?
For reasons of economics and convenience, industrialized societies today rely primarily on oil (petroleum) and natural gas for their energy needs. Oil and natural gas, both fossil fuels, consist of hydrocarbons, chain-like or ring-like molecules made of carbon and hydrogen atoms. Chemists consider hydrocarbons to be a type of organic chemical.
Some hydrocarbons are gaseous and invisible, some resemble a watery liquid, some appear syrupy, and some are solid. The viscosity (ability to flow) and the volatility (ability to evaporate) of a hydrocarbon product depend on the size of its molecules. Hydrocarbon products composed of short chains of molecules tend to be less viscous (meaning they can flow more easily) and more volatile (meaning they evaporate more easily) than products composed of long chains, because the long chains tend to tangle up with each other. Thus, short-chain molecules occur in gaseous form (natural gas) at room temperature, moderate-length-chain molecules occur in liquid form (gasoline and oil), and long-chain molecules occur in solid form (tar).
Hydrocarbon Systems
Oil and gas do not occur in all rocks at all locations. That’s why the goal of controlling oil fields, regions that contain significant amounts of accessible oil underground, has sparked bitter wars. A known supply of oil and gas held underground is a hydrocarbon reserve; if the reserve consists dominantly of oil, it is usually called an oil reserve and if it consists dominantly of gas, it’s a gas reserve. The development of a reserve requires a specific association of materials, conditions, and time. Geologists refer to this association as a hydrocarbon system. We’ll now look at the components of a hydrocarbon system, namely the source rock, the thermal conditions of oil formation, the migratory pathway, and the trap.
Source Rocks and Hydrocarbon Generation
News stories often incorrectly imply that oil and gas are derived from buried trees or the carcasses of dinosaurs. In fact, the hydrocarbon molecules of oil and gas are derived from organic chemicals, such as fatty molecules called lipids, that were once in plankton. Plankton is made up of very tiny floating organisms including single-celled and very small multicellular plants (algae) as well as protists and microscopic animals. Typically, most planktonic organisms range in size from 0.02 to 2.0 mm in diameter. When the organisms die, they sink to the floor of the lake or sea that they lived in, and if the water is relatively “quiet” (nonflowing), accumulate.
If the sea-floor or lake-floor environment is rich with oxygen, dead plankton may be eaten or oxidized and transformed into CO2 and CH4 gas, which bubbles away. But in oxygen-poor waters, the organic material can survive long enough to mix with clay and form an organic-rich, muddy ooze, that can then become buried by still more sediment so that it becomes preserved. Eventually, pressure due to the weight of overlying sediment squeezes out the water, and the ooze becomes compacted and, eventually, lithified to become black, organic shale. (Shale that does not contain organic matter tends to be gray, tan, or red.) Organic shale contains the raw materials from which hydrocarbons form, so we refer to it as a source rock.
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| The formation of oil. The process begins when organic debris settles with sediment. As burial depth increases, heat and pressure transform the sediment into black shale in which organic matter becomes kerogen. At appropriate temperatures, kerogen becomes oil, which then seeps upward. |
If organic shale becomes buried deeply enough (2 to 4 km), it gets warmer, since temperature increases with depth in the Earth. Chemical reactions take place in warm source rocks and slowly transform the organic material in the shale into a mass of waxy molecules called kerogen (figure above). Shale containing 15% to 30% kerogen is called oil shale. If oil shale warms to temperatures of greater than about 90C, kerogen molecules break into smaller oil and natural gas molecules, a process known as hydrocarbon generation. At temperatures over about 160C, any remaining oil breaks down to form natural gas; and at temperatures over 225–250C, organic matter loses all its hydrogen and transforms into graphite (pure carbon). Thus, oil itself forms only in a relatively narrow range of temperatures, called the oil window.
Reservoir Rocks and Hydrocarbon Migration
The clay flakes that comprise most of an oil shale fit together very tightly and thus prevent kerogen, and any liquid or gaseous hydrocarbons forming within the kerogen, from moving through the rock. Therefore, you can’t simply drill a hole into a source rock and pump out oil the oil won’t flow into the well fast enough to make the process cost efficient. Instead, to obtain oil or gas, companies drill into reservoir rocks, rocks that contain, or could contain, accessible oil or gas, meaning oil or gas that can flow through rock and be sucked into a well fairly easily.
To be a reservoir rock, a body of rock must have space in which the oil or gas can reside and must have channels through which the oil or gas can move. The space can be in the form of openings, or pores, between clastic grains (which exist because the grains didn't fit together tightly and because cement didn't fill all the spaces during cementation) or in the form of cracks and fractures that developed after the rock formed. In some cases, groundwater passing through rock dissolves minerals to produce new pore space. Porosity refers to the proportion of pore space in a rock. Not all rocks have the same porosity for example, shale has low porosity (10%), whereas poorly cemented sandstone has high porosity (35%). By saying that sandstone has a porosity of 35%, we mean that about a third of a block of the sandstone consists of open space. The oil or gas in a reservoir rock occurs in the pores, and thus is distributed through the rock it does not occur in open pools underground. Permeability refers to the degree to which pore spaces are connected to one another. In a rock with high permeability, there are many tiny channel ways linking pores, and/or many interconnected cracks cutting through the rock, so that fluids are not trapped in pores but rather can flow through the rock. The greater the porosity, the greater the capacity of a reservoir rock to hold oil; and the greater the rock’s permeability, the easier it is for the oil to be extracted.
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| Initially, oil resides in the source rock. Because it is buoyant relative to groundwater, the oil migrates into the overlying reservoir rock. The oil accumulates beneath a seal rock in a trap. |
To fill the pores of a reservoir rock, oil and gas must first migrate (move) from the source rock into a reservoir rock, a process that can take thousands to millions of years to happen (figure above). Why do hydrocarbons migrate? Oil and gas are less dense than water, so they try to rise toward the Earth’s surface to get above groundwater, just as salad oil rises above the vinegar in a bottle of salad dressing. Natural gas, being less dense, ends up rising above oil. In other words, buoyancy drives oil and gas upward. Typically, a hydrocarbon system must have a good migration pathway, such as a set of permeable fractures, in order for large volumes of hydrocarbons to move.
Traps and Seals
If oil or gas escapes from the reservoir rock and ultimately reaches the Earth’s surface, where it leaks away at an oil seep, there will be none left underground to extract. Thus, for an oil reserve to exist, oil and gas must be held underground in the reservoir rock by means of a geologic configuration called a trap.
There are two components to an oil or gas trap. First, a seal rock, a relatively impermeable rock such as shale, salt, or unfractured limestone, must lie above the reservoir rock and stop the hydrocarbons from rising further. Second, the seal and reservoir rock bodies must be arranged in a geometry that localizes the hydrocarbons in a restricted area. Geologists recognize several types of hydrocarbon trap geometries, four of which are described below.
Types of Oil and Gas Traps
Geologists who work for oil companies spend much of their time trying to identify underground traps. No two traps are exactly alike, but we can classify most into the following four categories.
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| Examples of oil traps. A trap is a configuration of a seal rock over a reservoir rock, in a geometry that keeps the oil underground. |
- Anticline trap: In some places, sedimentary beds are not horizontal, as they are when originally deposited, but have been bent by the forces involved in mountain building. These bends, as we have seen, are called folds. An anticline is a type of fold with an arch-like shape (figure above a). If the layers in the anticline include a source rock overlain in turn by a reservoir rock that is overlain by a seal rock, then we have the recipe for an oil reserve. The oil and gas rise from the source rock, enter the reservoir rock, and rise to the crest of the anticline, where they are trapped by a seal.
- Fault trap: A fault is a fracture on which there has been sliding. If the slip on the fault crushes and grinds the adjacent rock to make an impermeable layer along the fault, then oil and gas may migrate upward along bedding in the reservoir rock until they stop at the fault surface (figure above b). Alternatively, a fault trap develops if the slip on the fault juxtaposes an impermeable rock layer against the reservoir rock.
- Salt-dome trap: In some sedimentary basins, the sequence of strata contains a thick layer of salt, deposited when the basin was first formed and seawater covering the basin was shallow and very salty. Sandstone, shale, and limestone overlie the salt. The salt layer is not as dense as sandstone or shale, so it is buoyant and tends to rise up slowly through the overlying strata. Once the salt starts to rise, the weight of surrounding strata squeezes the salt out of the layer and up into a growing, bulbous salt dome. As the dome rises, it bends up the adjacent layers of sedimentary rock. Oil and gas in reservoir rock layers migrate upward until they are trapped against the boundary of the salt dome, for salt is not permeable (figure above c).
- Stratigraphic trap: In a stratigraphic trap, a tilted reservoir rock bed “pinches out” (thins and disappears) up its dip between two impermeable layers. Oil and gas migrating upward along the bed accumulate at the pinchout (figure above d).
Credits: Stephen Marshak (Essentials of Geology)