Overpressure caused by hydrocarbon generation in the organic-rich shales of the Ordos Basin

Abstract Based on a thermal pressure simulation experiment under formation conditions, we found that hydrocarbon generation in organic-rich shales can result in extreme overpressure, and that the key factor controlling such overpressure is gas rather than oil. Our experiment was conducted under formation conditions using organic-rich shale samples from the Upper Triassic Chang 7 member of the southern of Ordos Basin with the goal of exploring the overpressure caused by hydrocarbon generation. The results showed that hydrothermal pressures at 73–130 °C ranged from 5.6 to 49 MPa, while those at simulated temperatures of 300–425 °C and vitrinite reflectances of 0.71–1.99% ranged from 23.5 to 116.1 MPa when the system was completely closed. The weights of oil and gas generated in every stage of the experiment were similar, but the volume of gas was up to 1720 times greater than that of oil. The correlation between the volume of oil generated and hydrocarbon generation pressure was poor, but the increased volumes of oil-associated gas during the oil generation stage and gas in the gas generation stage were strongly and positively correlated with hydrocarbon generation pressure. Therefore, gas was the main factor controlling hydrocarbon generation pressures in this study. Higher pressures can exceed the fracture pressures of rocks, resulting in the formation of fractures and leading to the expulsion of oil and gas. The overpressure of a source rock can be transferred into a tight reservoir to become the main driving force for tight oil charging. When the overpressure exceeds 15 MPa, the oil in a reservoir can be charged into