Hydrocarbons derived from the decay and chemical alteration of buried organisms.

Includes: oil, natural gas, and coal.

1. Petroleum originates from the decay of microscopic marine organisms.

2. Petroleum is a complex mixture of organic compounds that form under restricted conditions of temperature and pressure.

3.  Petroleum resources are limited; methane hydrates are a possible future source of natural gas.

5. Nitrogen oxides (NOx) are converted into nitric acid causing “acid rain.”

6.  Transportation of petroleum leads to spill and leaks.

I. Origin of petroleum

Requires source of organic matter.
Organic matter must be “cooked” into petroleum within approximately 50 to 100°C.
Must migrate into a reservoir where it is trapped.
This is a complex mixture that must be refined before use.
Each component of the mixture has different properties that depends on the number of carbon atoms in the compound.

A. Microscopic plants and animals die and are deposited in tropical marine basins. Biological decay begins, but continued burial depletes oxygen available.

Remaining organic matter is preserved as biological decay decreases dramatically without oxygen.

These initial sediments will turn into source rocks.

Source rocks are typically shales and siltstones deposited under oxygen-poor conditions so that preservation of organic matter is high.

B. Diagenesis of sediments containing organic debris leads to the formation of kerogen, a solid organic material, at relatively low temperatures and pressures (i.e., below 50 degrees C and 1 kbar).

C. Kerogen is converted to hydrocarbon liquid and gas between 50 and 100 degrees C and 1 to 3 kbars.

D. As temperatures increase, the longer chain hydrocarbons of kerogen are "cracked" into smaller molecules that can form gases and liquids. The larger an organic molecule is, the more likely it is to form a solid at a given temperature. As this process starts, tar is created.

Then, liquid hydrocarbons form into the components of oil and gasoline.

E. Above 100 degrees C, these liquid hydrocarbons form the gases butane, propane, ethane and methane.

F. Above 200 degrees C, methane (CH4) breaks down into C and H2. The H2 escapes and C is left as graphite which is not useable as a fuel.

G. Compaction of the source rocks leads to migration of the oil and gas into reservoir rocks.

Migration or continued burial over geological time makes almost all petroleum found less than 65 million years old. The oil-forming process is slow, however, so no petroleum deposits are less than 1 million years old.

Approximately 0.1% of buried marine organic matter becomes petroleum.

II.  Methane hydrates - These are vast deposits of natural gas located at the bottom of the ocean or in permafrost.  The possible energy derived from these materials is approximately twice that of all other fossil fuels.

Methane hydrates are natural gas locked within ice-like material at the bottom of oceans and in permafrost.
The possible amount of this natural gas is a huge potential future source of energy.
Obtaining this methane is technologically difficult due to the depth and pressures at the bottom of the ocean.
Sudden release of methane in the past may have caused at least one major extinction.   This is a serious current environmental concern for methane hydrate usage.

IV. Environmental effects of petroleum use

Acid rain ­ Some soils more sensitive than others.  Monuments and buildings made of marble particularly vulnerable.
Increased greenhouse gases ­ CO2 and CH4.  Possible catastrophic release of methane (CH4) from methane hydrates.
Oil spills ­ Large spills cause catastrophic damage. Evaporation, biodegradation, and dispersion gradually clean up spills.                  
Long-term contamination of sediments occurs.

Shipping and river runoff actually the largest sources of petroleum contamination to the oceans.

A. Acid rain -

Hydrocarbons contain some percentage of S and N even after processing.

Burning the fossil fuel oxidizes the S and N into SO2 and NOx that can be converted in the atmosphere by addition of water and oxygen to sulfuric acid (H2SO4) and nitric acid (HNO3).

These acids dissolve in cloud droplets and then precipitate as acid rain or snow.

Soils that contain CaCO3 that reacts to neutralize acid rain have much less environmental damage than those that do not contain CaCO3.

Dissolution of calcite or limestone can lead to the problems for monuments and buildings made of this material.

The amount of environmental damage caused by acid rain is determined by the proximity downwind of a coal-burning electrical plant and by the neutralizing capacity of the soil.

B.  Oil spills -

Oil spills are very common in harbors. Fueling ships and pumping bilge water allows petroleum products to leak into sea water. These components then eventually end up in sediments and can cause toxic effects in benthic organisms.

Large spills like the Exxon Valdez are more dramatic but less common.

Techniques such as containment and bioremediation significantly reduce the amount of oil in the environment but do not eliminate 100% of the environmental damage.

Bioremediation may be the best long-term option if the right combination of microbes and nutrients can be found to degrade all hazardous components of petroleum products.

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