EGEE 101
Energy and the Environment

Crude Oil and Natural Gas Formation

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Often when we find crude oil we also find associated natural gas. The natural gas provides much of the pressure to produce the "blow-outs" that in good old days (and in the movies) signified an oil strike. We can also find natural gas alone without crude oil. The gas has migrated away from the crude oil, or perhaps the oil has "seen" a high enough temperature that it has all been converted into natural gas.

As organic material decays, methane is formed and lost into the atmosphere. As the organic material breaks down, depending on the inputs (trees vs. plankton), the different fossil fuels will form. As the material is buried deeper the temperature increases and there is an "oil window", or an opportunity to find oil. If the temperature is too high then all the crude oil will form natural gas instead, and so the oil window closes. Of course, we also find oil at the surface, but this tends to be degraded oil that has lost the more volatile components and so tends to be tar pits (not pools of crude oil).

 Graphic of the oil window. Fully described in two paragraphs above.
Changing processes of maturation that occur with depth (and temperature): Diagenesis, Catagenasis, and Metagenesis.
Credit: Louisiana Department of Natural Resources

The Carbon Cycle - Revisited, Again

The phytoplanktons are numerous in the ocean and are a very significant portion of the carbon on the earth. They live in water near the surface during life and use the solar energy in a similar manner to plants on land to store chemical energy. They tend to be concentrated in nutrient-rich zones. As the first link in the ocean food chain, they are eaten in large quantities.

Phytoplankton growth illustration.
More phytoplankton growth
Drawing of sunlight shining on the ocean. Little green spots representing phytoplankton growth.
Phytoplankton growth thanks to photosynthesis

Death

Upon death they sink to the bottom of the ocean where the decay process occurs. Think back to all the deep-sea footage you have seen where is it "raining" organic matter. Often the nutrient-rich zones are also locations by rivers that carry sediment from the land to the sea. If the dead plankton and sediments fall in quantity they will form an organic-rich layer at the bottom of the ocean. There it will be protected from aerobic decay, because of the physical protection (sediment) and the lower oxygen content.

 Drawing of sun shining on the ocean. The phytoplankton is dying but is still on the surface of the water.
Phytoplankton dying but still on the surface of the water.
 Drawing of sun shining on the ocean. The phytoplankton is dead and it has sunk towards the bottom of the ocean.
Phytoplankton dead and sunk to the bottom of the ocean.
 Drawing of sun shining on the ocean. The dead phytoplankton is lying on the ocean floor.
Dead phytoplankton lying on the ocean floor.

Maturation

Over long time periods, millions of years, the layers are buried deeper and deeper and the temperature builds up (as does the pressure) and the layer turns into a rock. However, it is a rock that has significant organic content. Additional time, temperature, and pressure, and the maturation process produces kerogen. Additional time and the kerogen is transformed into bitumen and then crude oil with associated methane. These materials can escape the source rock and if unchecked can seep into the surface of the ocean (or land) and decay back into carbon dioxide and water.

Structural Traps

As the oil and natural gas move through porous rocks, if it meets an impermeable layer, then it is stuck and the maturation process continues. Thus, crude oil and gas "live" within porous rocks, not great holes in the ground (unless they are in the strategic petroleum reserve) - more on that later.

Extracting Oil

If we find these structural traps and they contain oil or gas, we can extract the fossil fuel and much of it will be combusted to generate thermal energy (for transportation or heat) and yield carbon dioxide and water.

 Drawing of the sun shining on the ocean. The dead phytoplankton is soaking into the porous rocks on the ocean floor.
Drawing of the sun shining on the ocean. There is an oil rig extracting the oil from the porous rock on the ocean floor.

How do we know crude oil is from phytoplankton (and other organic material)?

When we extract crude oil it contains some water. However, even when we extract Pennsylvania crude oil the crude will still contain water but it is NOT often fresh water. The water contains salt and many of the other elements in concentrations that we find in the ocean. The disposal of this salt water is an expense. In the good old days, it was thrown in a river or stream but the salt would kill the fresh-water fish. Now it is cleaned prior to disposal although we hope that PennDOT and PA DEP (Department of Environmental Protection) will let us dispose of the salt water on the winter roads to prevent icing.

We also find microfossils in the source rock that indicate that the inputs to the rock contained life. However, the strongest indication is the identification of biomarkers within the crude oil.

This branched C17 (hydrocarbon containing 17 carbon atoms) is an isoprenoid, and is a chemical we can extract from plant life.
A Porphrin ring.

The Porphrin ring has an important role in biology. The nitrogen atoms are blue in color and there is a metal ion (in this case vanadium) in the center of the molecule. Metals such as vanadium and nickel are present in oils at low levels but they are important contaminants. Nitrogen as shown is blue, double bonds are not indicated. This is an example of a porphrin ring a biomarker in oil. The metal sits in the central location of the rings. If catalytic cracking of the oil is performed, it is important to remove these metals as they poison the catalyst. Vanadium can be recovered from the ash or flue gas when high vanadium containing oils are combusted. The vanadium can then be sold to the metals industry for use in steel generation. Biomarkers are also useful for identifying where the crude oil is from.