State of the art of research in kinetic modelling of oil formation and expulsion

Abstract

The last ten years have seen a rapid development in the understanding of petroleum generation and expulsion by a combined use of natural series and experimental simulation. This knowledge is now used to develop mathematical models which reproduce the kinetics of petroleum generation and migration in the purpose of basin evaluation. By a combination of observation of homogeneous geochemical series, experimental simulation and mathematical modelling, it has been now established that parameters governing the quantity and composition of petroleum are temperature, type of organic matter, time (or heating rate). Catalysis by clay minerals, which would produce important amounts of isoalkanes is not necessary to produce petroleum. The role of minerals, although still controversial, is rather to retain the heavy fraction of petroleum. Pressure seems to have a limited influence within the range for effective source rocks. Kerogen is a tremendously complex macromolecule which is now insufficiently characterized to develop fundamental, predictive models of thermal cracking (i.e. involving intermediates such as radicals and olefins). Such models presently apply only to the cracking of simple molecules (hexane, hexadecane) at limited degrees of cracking. For this reason kinetic models are still empirically calibrated on pyrolysis experiments and/or on natural maturation trends. The most popular models involve a series or a continuous distribution of parallel reactions to describe primary cracking (i.e. direct formation of mobile compounds from kerogen). These are generally calibrated against open system pyrolysis with various temperature programs. Consideration of natural maturation trends (i.e. decrease in petroleum potential) may be used also for calibration, but discrepancies of organic matter properties, availability of deep samples and uncertainties on thermal history limit the effectiveness of such methods. Whatever the method, main activation energies are found in the range 45–60 kcal/mol. Secondary cracking which refers generally to cracking of mobile compounds cannot be modelled from open system pyrolysis nor from natural series (unknown residence times), but only from closed system pyrolysis. Although models of primary and secondary cracking still need experimental improvements and more realistic kinetic schemes, their ability to predict quantities and composition of petroleum depends primarily on migration which is poorly known. Indeed, migration controls the residence time of petroleum in hot zones of sedimentary basins and thus determines the extent of oil conversion into gas as reproduced by recent numerical models. Preliminary studies documenting the interactions between thermal cracking and expulsion are shown from Haltenbanken area and Paris basin. In the last few years, a kind of agreement has been reached that primary migration was most likely occuring in a separate phase from water and that a sufficient concentration of petroleum must be generated for expulsion to start. Indeed high expulsion efficiencies (over 50%) are systematically found for rich and mature source rocks. A further understanding of primary migration requires observations of natural series, petrophysical studies of source rocks (porosities under 10%, permeabilities in the nanodardy range). Thermodynamic investigation of unconventional multiphase organic systems [kerogen/liquid(s)/gas], and experimental simulation under stress. This on-going research should enable models to track correctly migration paths and thermal history of petroleum fluids as necessary to model their composition.