The origin of light hydrocarbons in petroleum: Ring preference in the closure of carbocyclic rings

In a proposal for the generation of light hydrocarbons (LHs), n-alkane parents are catalytically transformed into daughter isoalkanes and cycloalkanes through the closure of three-, five-, and six-membered rings. Three reaction rate constants, k3: →- isoalkanes; k5: → cyclopentanes; k6: → cyclohexanes, control this catalytic process, and thus the compositions of LHs. A catalyst that preferentially promotes ring-closure of a specific carbon number is said to express ring preference (RP) in that carbon number. For example, a catalyst that preferentially generates isoalkanes over cycloalkanes would be expressing three-ring preference (3RP), meaning that k3 is greater than k5 and k6 in the catalytic process generating LHs. Oils show large compositional variations in LHs, with ratios of isoalkanes to cycloalkanes showing coefficients of variation on the order of 100%, reflecting large variations in RP. Genetically related oils (homologous oils), however, are either invariant in composition (invariant in RP) or they display systematic changes in RP. In a striking example of this latter case, RP progressively shifts to smaller rings, 6RP → 5RP → 3-RP, as parent concentrations increase. This paper addresses a curious paradox, apparently unique to the LHs: homologous oils, displaying a uniform overall geochemical composition (i.e., gravity, sulfur concentration, isotopic composition, biomarker composition, and so on), show remarkable changes in LH composition reflecting systematic changes in RP. These apparent contradictions, on the one hand a uniform overall composition reflecting a static system and on the other hand systematic changes in LH composition reflecting a dynamic system, are analyzed in the context of a steady-state catalytic hypothesis.