The stability of hydrocarbons under the time–temperature conditions of petroleum genesis

Abstract

THE contention that hydrocarbons are unstable under the time-temperature conditions of petroleum generation (catagenesis) enjoys broad acceptance^1–6. At temperatures in the region of 100–150 °C, hydrocarbons are believed to decompose progressively over geological time to lighter hydrocarbons, ultimately methane and pyrobitumen^7,8. There are geological contradictions to this view⁹, however, and the recent finding¹⁰ that the cycloalkane ring should remain stable for billions of years under catagenic conditions demands a review of its underlying assumptions. For example, are ordinary hydrocarbons unstable under catagenic conditions? Are the light hydrocarbons, including methane, produced through the thermal decomposition of higher-molecular-weight hydrocarbons? Here I address these questions from two independent perspectives. First, the relative stabilities of hydrocarbons and their kerogenous precursors are studied experimentally. Second, the compositions of natural petroleum deposits are analysed for evidence of thermal decomposition. The results suggest that the hydrocarbons in petroleum should be at least three orders of magnitude more stable than their kerogenous precursors under catagenic conditions, and that natural deposits of petroleum and gas do not contain cycloalkane and isoalkane contents indicative of progressive thermal decomposition to gas. Thus the assumption that hydrocarbons are unstable under catagenic conditions and progressively decompose to methane and pyrobitumen seems to be incorrect.