Lecture 9
The great bulk of organic matter in living things doesn’t contribute to the generation of fossil fuels. Most of the mass of living aquatic organisms comprises proteins and labile (easily changed) carbohydrates. After the organism dies those things get eaten and converted back to carbon dioxide. Even land plants which are rich in cellulose and lignin and resistant to degradation get oxidized. This is fortunate; if we buried carbon as rapidly as plants fixed from the atmosphere it we would rapidly suck all the carbon dioxide out of the atmosphere and we would all freeze (but because of micro-organisms that doesn’t happen).
Then we looked at the chemical composition of fossil fuels and noted that almost all of them seem to be predominantly carbon rich. If we convert mass to numbers of atoms, we find that gas is very hydrogen-rich with a 4:1 ratio of H:C (like CH4). Oil has a 2:1 ratio of H:C (the composition of n-alkanes is CnH2n+2). Coal has a lower H:C ratio and also contains significant oxygen.
The important components in both land and aquatic settings are the lipids. These are “oily” molecules that are produced by living things and that do get preserved. After burial and heating, hydrocarbon fragments will break off these structures becoming oil or natural gas. The cellulose and lignin in land plants can become coal if it accumulates to sufficient thickness.
Because the crust is saturated with water (more or less) and because oil floats on water the oil that oil will attempt to rise all the way to the surface.
Rocks that can generate oil or gas are called petroleum source rocks. Source rocks are one of the elements of a petroleum system. The four elements of a petroleum system are
1. A Source Rock
2. A Reservoir Rock
3. A Trap
4. A Seal
A petroleum source rock is typically a fine grained rock (lithified mud) that contains more than 1% 
organic carbon by weight. The elemental composition of this organic carbon must be sufficiently hydrogen rich that it evolves along the top two arrows in the diagram to the right as it is buried. Oxygen rich organic matter does not contain sufficient lipid-like organic matter to generate significant amounts of oil and will evolve along the lowermost arrow on the diagram to the right. It may, however, generate some gas.
If you read more about petroleum source rocks check out this Whyfiles report.
Posted: September 17th, 2009 under Petroleum, Study Guide, Study Guide Exam 1, Uncategorized.