MATHEMATICAL-MODEL OF OIL GENERATION, DEGRADATION, AND EXPULSION

We present a mathematical model (PYROL) for simulating oil generation, degradation, and other chemical reactions occurring during pyrolysis of petroleum source rocks over a specified history of temperature and hydrostatic pressure. The model also simulates compaction of the source rock and expulsion of a liquid water phase, a hydrocarbon-rich liquid phase, and a vapor phase. The governing equations for PYROL consist of the time derivatives of 150 variables: 32 vapor species, 32 liquid species, 19 solid species, and 67 other variables (including pore pressure, pore volume, and diagnostics). These ordinary differential equations are expressed in terms of 100 chemical reaction rates and 32 vaporization/condensation relations. A modified Redlich-Kwong-Soave equation of state is used in calculating the vapor/liquid equilibria and PVT behavior. The model is validated by comparison with a wide variety of pyrolysis experiments in open and closed systems. It is then applied to conditions of interest for petroleum generation over geologic time periods. The results for type I kerogen indicate that oil expulsion efficiency is strongly dependent on total organic carbon (TOC), decreasing from 600 to 200 mg of C5+ oil/g of TOC as carbon content decreases from 10 to 1 wt % at a constant heating rate of 3 °C/My. Conversely, it is essentially independent of heating rate for 10 wt % TOC and only moderately dependent on heating rate for lower carbon contents. © 1990, American Chemical Society. All rights reserved.