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Vitrinite reflectance has been modeled for North Sea wells assuming that reflectance changes as a result of first-order chemical reactions. The Arrhenius equation is used to relate changes in the first-order rate constant to temperature, and is modified by including a term that allows the rate of a chemical reaction to exponentiate for every TD increase in temperature above a "threshold" temperature of Tc. The approach treats time and temperature as knowns, deduced from thermal modeling and burial history, and tries to minimize the difference between calculated and measured reflectances by stepping through various values for the activation energy, Ea, scaling temperature, TD, threshold temperature, Tc, and time varyin heat flow, Q(t), using a nonlinear least-squares technique.
As a consequence of the modeling, we conclude that vitrinite reflectance can be modeled using an activation energy, threshold temperature, and scaling temperature of about 0.05 kcal/mole, 295°K, and 200°K, respectively. The model allows the prediction of depths and timing for oil generation in areas where the temperature history is known. Conversely, and significantly, an inverse approach can be taken whereby paleo-heat flow can be deduced from reflectance measurements. Applying this inverse approach in the North Sea has allowed us to determine its spatially varying heat flux over the last 100 m.y. or so, and also permits us to predict vitrinite reflectance with depth ahead of drilling operations.
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