Effect of water pressure and expulsion on hydrocarbon generation of kerogen under semi-closed conditions

Here, we reveal the effects of water pressure and hydrocarbon expulsion on bitumen pyrolysis, oil cracking, gaseous hydrocarbon generation, and hydrocarbon potential of organic matter. Mudstone retrieved from drilling in the Liaohe Basin was pyrolyzed at 50 to 840 bar water pressure and 1200 to 2010 bar lithostatic pressure under semi-closed conditions. Results demonstrated that increasing the water pressure strongly reduced gaseous hydrocarbons and bitumen yields, while oil yields were enhanced. The low hydrogen and high carbon dioxide yields under high water pressure suggested that high water pressure can enhance the primary pyrolysis of organic matter and retard the oil cracking. The lower values of S2, HI, and H/C under high water pressure indicate that hydrocarbon expulsion may accelerate the depletion of the hydrocarbon generation potential of organic matter. Tmax and vitrinite reflectance (VR) were also affected by water pressure. In addition, the results also indicated that water pressure and hydrocarbons expulsion strongly reduced the cracking of oil expelled at early stages. The rapid removal of oil in turn accelerates the rate of oil generation, according to Le Chatelier’s principle, thereby increasing the oil yields. High oil yields under high water pressure may primarily result from the retarding effect of water pressure on oil cracking, the promoting effect of hydrocarbon expulsion on oil generation, and the effect of water pressure and hydrocarbon expulsion on oil and bitumen. Our results will be of great benefit to the exploration of deep petroleum in China.