SIMPLE KINETIC-MODELS OF PETROLEUM FORMATION .1. OIL AND GAS GENERATION FROM KEROGEN

Modern basin analysis uses kinetic models to predict the extent of petroleum generation within potential source rocks. The global kinetic model presented here assigns kinetic parameters based on gross depositional environment and stratigraphic age; this is useful in areas of low geochemical knowledge, including exploration frontiers. Five kerogen kinetic organofacies, each characterized by a specific organic matter input and early diagenetic overprint, can broadly be related to sedimentary facies/age associations, even using seismic sequence stratigraphy alone: A, aquatic, marine, siliceous or carbonate/evaporite, any age; B, aquatic, marine, siliciclastic, any age; C, aquatic, non-marine, lacustrine, Phanerozoic; D/E, terrigenous, non-marine, ever-wet, coastal, Mesozoic and younger; and F, terrigenous, non-marine, coastal, late Palaeozoic and younger. Routine pyrolysis data allow partitioning of organic carbon in the im mature source rock into four initial components: oil, oil-generative, gas-generative and inert. Separate kinetic parameters for the oil- and gas-generative fractions allow computation of the evolving concentration and composition of the products. It was assumed that the activation energy distributions for each fraction were normally distributed. Non-linear regression of large, combined laboratory and field data sets, varying by up to 12 orders of magnitude in heating rate, established optimum values for the required ten (5 x 2) kinetic parameter sets (A, E(mean), sigma(E)). Differences in kinetic parameters can be reconciled with known chemical properties, although our knowledge base is still inadequate to construct kinetic models from first principles. Mean activation energies governing oil generation increase systematically in the order A-F, causing a corresponding increase in generation temperature. At a reference heating rate (2 degrees C Ma(-1)), the oil generation 'window' (10-90% oil generative kerogen degraded) increases from ca. 95-135 degrees C to 145-175 degrees C. Organofacies C has the tightest energy distribution and narrowest oil generation window. The gas generation 'window' ranges from ca. 105-155 degrees C to 175-220 degrees C. Thermal stress results from a combination of temperature and time: an order of magnitude increase (decrease) in heating rate elevates (depresses) reaction temperatures by ca. 15 degrees C; heating rates in subsiding sedimentary basins can vary by two orders of magnitude. Dramatic differences in generation temperature can result if the heating rate and organofacies effects compound. Thus, global oil and gas generation temperature thresholds constitute blunt instruments with which to screen the petroleum potential of sedimentary basins. Because unexpelled oil provides a potential feedstock for oil to gas cracking in the source rock, quantitative models of petroleum generation become really useful only when coupled with models of oil-gas cracking and expulsion. For example, the frequently observed association between gas provinces and coals cannot be explained solely by their generation behaviour.