Influence of Pressure and the Presence of Water on the Evolution of the Residual Kerogen during Confined, Hydrous, and High-Pressure Hydrous Pyrolysis of Woodford Shale

Abstract

The artificial maturation of Woodford Shale has been performed using confined pyrolysis, hydrous pyrolysis, and high-pressure hydrous pyrolysis in order to test the effects of pressure and the presence of water on the thermal evolution of a type II kerogen. Despite the fact that identical time-temperature-pressure conditions were used in both pyrolysis systems (260-400 °C, 72 h duration, 250-1300 bar pressure), the results are very different. This study focuses on the fate of the residual kerogen during maturation in both pyrolysis systems. During hydrous pyrolysis, high pressure induces a strong suppression effect on kerogen thermal breakdown that is far lower during confined pyrolysis (hydrocarbons + gaseous effluents pressure). In addition, hydrous pyrolysis conditions induce a retardation in kerogen thermal breakdown and strongly inhibit aromatization of the residual kerogen; maturation occurs faster in confined conditions and aromatization is prevalent. The very different behavior of the residual kerogen with different pyrolysis media implies that different hydrogen sources and hydrogen exchange mechanisms are involved during the maturation process. The availability of water and the confinement of the system lead to a competition between water and the residual kerogen (+ polars) as major sources of hydrogen. Depending on the dominant hydrogen source, the kerogen responds differently to the heating process and the pressure effect. These considerations lead to the conclusion that it is important to better constrain the physicochemical conditions occurring in source rocks in order to properly extrapolate the artificial maturation results to the geological context.