What determines the characteristics of fossil fuels?

When using the fossil fuels or hydrogen as an energy source (e.g., by combusting them to obtain heat), we extract their chemical energy. And, the chemical energy is stored in chemical bonds. Therefore, in so doing, we arrange the bonding of fuel, to obtain the difference of energy stored in reactants and products (see more details in How much of energy can be extracted from fossil fuels?).

This post as a next step discusses what determines the characteristic of fuel with a focus on the element constituting the fuel.

Bonds difficult/easy to be broken

Chemical bonds can be classified into either "easy for us to use" or "difficult for us to use and need to be used in future".

Before going into details, let me overview quantitatively the binding energy of some representative bonds:
C-C bonding = 347 [kJ mol^-1]
C-H bonding = 410 [kJ mol^-1]
H-O bonding = 460 [kJ mol^-1]
O=O bonding = 494 [kJ mol^-1]
C=O bonding = 799 [kJ mol^-1]
*These values are representative ones, and exact values vary depending on the molecular formula.

"The larger binding energy" means that the atoms bind to each other strongly, which therefore is difficult to be dissociated as a chemical bond. On the other hand, of course, the bonds with smaller binding energy is easy to dissociate. For instance, C-C bonding with 347 [kJ mol^-1] is easier to cut than C-H bonding with 410 [kJ mol^-1], and therefore the C-C bond is easier to handle than the C-H bond.

Our history of using fossil fuel had started with coal. The coal is majorly composed of C atoms and therefore of C-C bonds. On the other hand, we currently make use of oil for production of chemicals, for instance. The oil contains C-C bonds similarly to coal, but more C-H bonds than coal. The other type of fossil fuel, natural gas, is required to be utilized more effectively as included in EU taxonomy.^[1] The major component of natural gas is methane. As readily understood from its chemical formula, methane CH₄ does not contain C-C bonds.
(This view is a bit similified one, as there are other reason of choices, for instance, oil contains some molecule that can more effectively be used as a reactant for production of certain chemicals)

This consideration tells us that we have used fossil fuels with more C-C bonds, but need to swtich to the ones with more C-H bonds.

References

1. European Commission, "EU Taxonomy: Commission begins expert consultations on Complementary Delegated Act covering certain nuclear and gas activities", https://ec.europa.eu/commission/presscorner/detail/en/ip_22_2 (accessed on 2022/02/02)