General statement

The optimum period for a petroleum system to charge a trap is during the late stages of thrustbelt development. Most of the traps are already formed and the chance of their subsequent translation is relatively low. Traps that become filled during earlier stages are likely to be breached by subsequent tectonic events (Clarke and Cleverly, 1991; Macgregor, 1993), except in the case of autochthonous traps buried beneath the thrustbelt, which are likely to escape younger deformation.

Traps in a thrustbelt can also be filled following the cessation of thrusting if the hydrocarbon kitchens continue to operate, but the migration pathways are likely to be complex fracture networks within the volume of the thrust sheets, instead of thrust surfaces themselves. In these situations, vertical migration is favoured. Geological events external to the specific portion of the thrustbelt in consideration are generally required to continue burial and hydrocarbon generation after the close of thrusting. Burial beneath the foredeep debris of another thrust system or beneath a successor basin formed by orogenic collapse are the most common mechanisms for keeping hydrocarbon kitchens active following the end of thrusting. For instance, burial beneath about 2km of sediments shed into the Green River basin from the flanking Paleocene–Eocene Rocky Mountain uplifts was responsible for the post–thrust generation of hydrocarbons fromthe Lower Cretaceous foredeep basin source rocks in the Late Cretaceous Wyoming–Utah thrustbelt. Without the superposed post-thrust burial, this extremely prolific thrustbelt would have been oiland gas-poor. Timing of burial of the source rock before, during and after thrusting is critical.