Just a comment from the organic geochemist in the neighborhood.
It is often difficult to summarize hydrocarbon generation in any basin in a few sentences. There are almost invariably numerous phases of uplift and successive burial. Hydrocarbon generation is a product of not just temperature (i.e. burial depth and geothermal gradient) but also time. As a consequence many basins see numerous phases of maturation, generation and migration of the various source rocks present through it's history.
Beyond that the devil is in the details. Kerogen type is significant in terms of the activation energy necessary to convert to hydrocarbon. As a consequence a Type I kerogen would mature at a different rate under the same boundary conditions as would a Type II or Type III kerogen.
Since the 1980's we have been able to model historical hydrocarbon generation in any given basin based on knowledge of source rock characteristics, burial history (burial and tectonic uplift), geothermal gradients through time. More recently we have been able to integrate all of those models with migration models based on reconstructed structural topography and that now in 3D. It should be no surprise that many of the remaining significant conventional fields have been discovered throughout this period.
The shales, however, have a whole different nuance. They are both the source rock and the seal, migration not playing a role.