For reasons of economics and convenience, industrialized societies today depend more often than not on oil (petroleum) and natural fuel for their energy needs. Oil and natural gasoline, each fossil fuels, encompass hydrocarbons, chain-like or ring-like molecules product of carbon and hydrogen atoms. Chemists recollect hydrocarbons to be a type of organic chemical.
Some hydrocarbons are gaseous and invisible, some resemble a watery liquid, a few appear syrupy, and some are solid. The viscosity (capacity to ?Ow) and the volatility (potential to evaporate) of a hydrocarbon product depend upon the size of its molecules. Hydrocarbon merchandise composed of short chains of molecules tend to be less viscous (that means they are able to ?Ow more without problems) and more risky (that means they evaporate extra without problems) than merchandise composed of lengthy chains, because the long chains generally tend to tangle up with every other. Thus, brief-chain molecules arise in gaseous form (natural fuel) at room temperature, mild-length-chain molecules occur in liquid form (fuel and oil), and long-chain molecules arise in strong form (tar).
Hydrocarbon Systems
Oil and gasoline do no longer occur in all rocks at all locations. That?S why the purpose of controlling oil ?Elds, areas that contain signi?Cant quantities of available oil underground, has sparked sour wars. A recognised deliver of oil and fuel held underground is a hydrocarbon reserve; if the reserve is composed dominantly of oil, additionally it is referred to as an oil reserve and if it is composed dominantly of gasoline, it?S a fuel reserve. The development of a reserve calls for a speci?C association of materials, conditions, and time. Geologists discuss with this affiliation as a hydrocarbon device. We?Ll now observe the additives of a hydrocarbon gadget, namely the supply rock, the thermal situations of oil formation, the migratory pathway, and the lure.
Source Rocks and Hydrocarbon Generation
News testimonies frequently incorrectly suggest that oil and fuel are derived from buried trees or the carcasses of dinosaurs. In fact, the hydrocarbon molecules of oil and gas are derived from organic chemical substances, consisting of fatty molecules referred to as lipids, that had been as soon as in plankton. Plankton is made up of very tiny ?Oating organisms which include unmarried-celled and very small multicellular plant life (algae) as well as protists and microscopic animals. Typically, most planktonic organisms range in size from 0.02 to 2.Zero mm in diameter. When the organisms die, they sink to the ?Oor of the lake or sea that they lived in, and if the water is particularly ?Quiet? (non?Owing), acquire.
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 technique starts offevolved while natural debris settles with sediment. As burial intensity will increase, warmth and pressure remodel the sediment into black shale wherein organic rely will become kerogen. At suitable temperatures, kerogen will become oil, which then seeps upward. |
If organic shale will become buried deeply sufficient (2 to four km), it gets warmer, considering the fact that temperature increases with intensity within the Earth. Chemical reactions take place in heat source rocks and slowly transform the natural fabric inside the shale right into a mass of waxy molecules called kerogen (determine above). Shale containing 15% to 30% kerogen is referred to as oil shale. If oil shale warms to temperatures of more than about 90C, kerogen molecules destroy into smaller oil and herbal gasoline molecules, a method referred to as hydrocarbon era. At temperatures over approximately 160C, any closing oil breaks down to shape herbal fuel; and at temperatures over 225?250C, natural rely loses all its hydrogen and transforms into graphite (natural carbon). Thus, oil itself paperwork simplest in a exceptionally narrow range of temperatures, referred to as the oil window.
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 is living within the source rock. Because it's far buoyant relative to groundwater, the oil migrates into the overlying reservoir rock. The oil accumulates below a seal rock in a entice. |
To ?Ll the pores of a reservoir rock, oil and fuel need to ?Rst migrate (move) from the supply rock right into a reservoir rock, a method which can take thousands to thousands and thousands of years to take place (figure above). Why do hydrocarbons migrate? Oil and fuel are much less dense than water, so they try to upward thrust closer to the Earth?S floor to get above groundwater, just as salad oil rises above the vinegar in a bottle of salad dressing. Natural gas, being much less dense, ends up rising above oil. In different words, buoyancy drives oil and gasoline upward. Typically, a hydrocarbon gadget ought to have a good migration pathway, along with a fixed of permeable fractures, so as for huge volumes of hydrocarbons to move.
Traps and Seals
If oil or gasoline escapes from the reservoir rock and ultimately reaches the Earth?S surface, in which it leaks away at an oil seep, there might be none left underground to extract. Thus, for an oil reserve to exist, oil and gasoline need to be held underground within the reservoir rock by a geologic con?Guration referred to as a trap.
There are components to an oil or gas lure. First, a seal rock, a exceedingly impermeable rock along with shale, salt, or unfractured limestone, must lie above the reservoir rock and forestall the hydrocarbons from growing further. Second, the seal and reservoir rock our bodies must be arranged in a geometry that localizes the hydrocarbons in a restricted vicinity. Geologists apprehend several varieties of hydrocarbon trap geometries, four of which are defined under.
Geologists who work for oil agencies spend a lot in their time seeking to become aware of underground traps. No traps are precisely alike, however we can classify maximum into the following 4 classes.
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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)
