A Model of Hydrocarbon Generation from Type I Kerogen: Application to Uinta Basin, Utah

Abstract

We have developed a computer model that can predict when and how much oil and gas are generated from a source rock during its burial and later uplift. Kinetic parameters for the oil and gas generation reactions are obtained from high-pressure pyrolysis experiments carried out over a wide range of heating rates and temperatures. In our kinetic model, which applies only to Green River shale, we use a single activation energy of 52.4 kcal/mole and different pre-exponential factors for different products of primary pyrolysis, which allows us to extrapolate laboratory-derived kinetics to geologic heating rates. This model is in contrast to the wide distributions of activation energies or artificially low apparent activation energies used in some models of petroleum formation. hen extrapolated to geologic heating rates on the order of 10°C/m.y., our kinetics show that the temperature of the maximum rate of oil generation (Tp) changes by about 15°C when the heating rate is changed by an order of magnitude. Changes in pressure have relatively minor effects on the kinetics of oil generation but are important for gas generation reactions. We used geophysical data from oil fields in the Uinta basin of Utah to develop a thermal history model of Green River Formation source rocks. This time-temperature history was used to predict the maturation level of the kerogen at a given depth and to predict changes in the compositional characteristics of the oil. The shape of calculated oil generation rate curves, as a function of depth in the basin, mimics the shape of the overpressure curves; this similarity suggests that oil-gas generation may be an important cause of overpressuring. Maturation levels and compositional characteristics of the oil predicted by our model agree very well with characteristics of the oil recovered from the basin.