Quantitative estimation of overpressure caused by oil generation in petroliferous basins

Oil generation can be the dominant overpressure mechanism in low permeability source rocks because of the volume increase upon conversion of organic matter to less dense oil during the petroleum generation process. To quantitatively express the relationship between oil generation and overpressure in source rocks, an equation for the pressure change due to oil generation was derived to quantitatively estimate the overpressure generated in the source rock by considering of the episodic fluid expulsion and parameters, such as the residual oil coefficient, source rock porosity, transformation ratio, TOC (total organic carbon), hydrogen index, the compressibility of Type I kerogen, oil and pore water. The equation was then calibrated with the results from physical simulation experiments in a closed pyrolysis system. Sensitivity analyses on the residual oil coefficient, TOC and hydrogen index were performed to investigate the dominant parameters controlling overpressure due to oil generation. A good correlation was achieved between measured overpressure from the physical simulation experiments and the calculated data using the derived equations for the pressure change due to oil generation, suggesting that the equation can be used to quantitatively estimate overpressure caused by oil generation in source rocks. Many parameters can affect overpressure caused by oil generation, including residual oil coefficient, source rock porosity, TOC, hydrogen index, and transformation ratio. Sensitivity analyses indicate that the most effective parameter on the pressure change due to oil generation in the source rocks is the residual oil coefficient, implying that the internal pressure seal for the source rock is the most important parameter for overpressure development caused by oil generation. Overpressure cannot be generated from oil generation if the residual oil coefficient is less than 0.75. Source rock TOC and hydrogen index have moderate effects on the pressure change caused by oil generation. Overpressure can be generated when the TOC content is as low as 0.5% provided the source rocks have a good internal pressure seal.