The thermal alteration of reservoired petroleum upon burial was simulated by closed-system non-isothermal pyrolysis. Samples of a medium gravity oil from the Norwegian North Sea Central Graben were pyrolysed in microscale sealed glass or quartz vessels at heating rates of 0.1, 0.7 and 5.0 K . min-1 up to temperatures varying from 300 to 650-degrees-C. The composition of the oil and gas in each vessel was determined by a single-step on-line gas chromatographic analysis. With increasing rate of heating the onset of all oil degradation reactions was shifted to higher temperatures. The same successive compositional changes occurred in each case: increase in the total yield of GC-detectable compounds, significant gas (C1-C4) generation accompanied by a decrease in yield of heavy components, aromatisation and attainment of maximum gas yield and finally a cracking of the C2+ gas components. The stable final mixture consisted of methane, aromatic hydrocarbons, pyrobitumen and possibly hydrogen. Kinetic modelling of the oil to gas conversion resulted in a narrow gas potential vs activation energy distribution between 66 and 70 kcal/mol assigning 35% of the total gas potential (460 mg per g of oil) to an energy of 66 kcal/mol and 29% to 67 kcal/mol (pre-exponential factor 1.1 x 10(16) s-1). By extrapolation to natural maturation conditions the onset of gas generation is predicted to occur between 160 and 190-degrees-C for geological heating rates between 0.53 and 5.3 K-Ma 1. Predictions from the model are in accordance with the observed preservation of liquid hydrocarbons in a deep, hot (165-degrees-C) petroleum reservoir from the Saga 2/4-14 well, Norwegian Continental Shelf.
