COMPENSATION EFFECT IN SOURCE ROCK KINETICS: INFLUENCE OF BITUMEN FORMATION

Source rock kinetics, also known as kerogen kinetics, is usually determined for basin modeling purposes to calculate oil generation and its rates for a given thermal history model. Generally, generation rates are controlled by chemical processes described in terms of the Arrhenius Equation. Although the thermal maturation leading to hydrocarbon generation is a well-known process, crucial uncertainty prevails on characterizing the effect of the secondary hydrocarbon products (e.g., bitumen) during kinetics analysis. Previous studies have shown the presence of a linear relationship or compensation effect between experimental activation energies (E_a) and the logarithm of the associated frequency factor (A) when the Bakken Shales are repeatedly analyzed. The objective of this study is to investigate the hypothesis that the compensation effect is due to the formation of secondary bitumen at the loss of primary kerogen.

To simulate thermal maturation (immature, early mature, and late mature) periods, a series of artificial maturation processes on Bakken Shale was carried out. Firstly, we performed source rock analysis to estimate the thermal maturity of the study samples. Second, we applied the principle of Arrhenius Equation and Kissinger Equation in the kinetic analysis to derive E_a and ln(A). Photomicrographs of the research samples were collected to assess the observation made by the microscope that fluorescent kerogen disappears as thermal maturation progresses. Finally, the artificially matured samples were cleaned using the dean stark extraction method to remove bitumen and we repeated the methods cited above.

Results show that geochemical properties such as the total organic carbon changed with increasing thermal maturity. The kinetic analysis reveals that E_a and ln(A) have a second, strongly correlated, and linear compensation effect. This demonstrates that, rather than the statistical compensation predicted by the linear trends in the experimental kinetic, there is a residual compensation effect that may have a physiochemical basis. Ignoring this residual compensation effect will lead to reaction rate error