The light hydrocarbons in petroleum: a critical review

The largest petroleum fraction is between C1 and C9, the so-called light hydrocarbons (LHs). They are catagenic products, formed between ∼ 75 and 140°C, but their mode of origin remains controversial. Although cracking enjoys broad support, there is little evidence, and inconsistencies argue against it. The higher hydrocarbons, for example, are too stable to source the LHs at these temperatures. Moreover, LHs do not resemble cracking products. Many are structurally like bio-precursors, but others are not and there is no dominance in natural structures consistent with cracking. Essentially all isomers are found within the alkanes, cycloalkanes and aromatics with no discernible preference for natural structures. Isomer distributions are nearly constant from oil to oil and far from thermodynamic equilibrium. It is unlikely that the LHs are formed without catalytic support. Ratios of isoalkanes are almost constant in all oils and invariant within oils from a common source. This places a powerful constraint on any theory attempting to explain their origin: Certain isoheptane ratios must remain constant throughout the course of LH generation. This is inconsistent with cracking of bio-precursors and consistent with a catalytic process in which the LHs descend from a few precursors in a process controlled by steadystate kinetics. There are two opposing views on the origin of natural gas: thermal cracking and catalysis by transition metals. Non-biogenic gas contains ∼ 85% methane, but thermal cracking consistently gives products depleted in methane. Catalysis by transition metal oxides (V, Co, Fe, and Ni), on the other hand, duplicates natural gas. Gas fractionation in migration might explain high-methane gas, and it has been offered as an alternative to catalysis, but there is little to support this idea and evidence against it. The opposing theories on the origin of LHs are critically reviewed here. I also review the early literature on composition, the distribution of LHs in sediments, and some applications, including maturity and oil correlations.