Kinetic modeling of the thermal evolution of crude oils in sedimentary basins

The aim of this work is to obtain a better understanding of the reactions involved in the thermal cracking of crude oil in sedimentary basins, and to study its kinetics as a function of temperature and pressure. We study the kinetics of pyrolysis of alkanes at low temperature, high pressure and high conversion and we propose three methods of reduction of the corresponding mechanisms. Several compounds having an inhibiting or accelerating effect on the rate of decomposition of alkanes were also studied. This research led to the construction of a general kinetic model of 5200 elementary steps representing the pyrolysis of a complex mixture of 52 molecules belonging to various chemical families: 30 linear alkanes (from CH₄ to C₃₀H₆₂), 10 branched-chain alkanes (including pristane and phytane), 2 naphthenes (propyl-cyclopentane and propyl-cyclohexane), tetralin, 1-methyl-indane, 4 aromatics (benzene, toluene, butylbenzene and decylbenzene), 3 heteroatomic compounds (a disulfide, a mercaptan and H₂S). This model is compared to experimental data coming from the pyrolysis of two oils: one from the North Sea and the other from Pematang. The results obtained show a good agreement between the experimental and simulated values. Then, we simulated the cracking of these two oils by using the following burial scenario: initial temperature of 160 deg. C, 50 m per million years (ma) in a constant geothermal gradient of 30 deg. C/km, implying a heating rate of 1.5 deg. C/ma. Under these conditions, our model shows that these two oils start to crack only towards 210-220 deg. C and that their time of half-life corresponds to a temperature around 230- 240 deg. C. The model also makes it possible to simulate the evolution of geochemical parameters such as the GOR, the API degree. (author)