The Rhine Graben is a moderately productive oil basin and has been exploited during the last one hundred and fifty years. The total production of the most important field, Pechelbronn, was about 3 million metric tons. Most of the productive zones are found in the Oligocene Pechelbronn Formation, from 500 to 1000 meters depth. These productive zones are situated near normal faults, implying that fractures play a major role in oil migration and entrapment. Two potential source rocks exist in the Rhine Graben: the Middle Jurassic Toarcian Shale (Type I kerogen) and the Oligocene Pechelbronn Formation (type III kerogen) (see geology).
In order to evaluate oil maturation and migration, two models have been investigated:
Both geologies have been built from Gocad model and geometrical reconstitutions.
Results of the present time model show that the maximum filtration velocity observed is about 1m.yr -1. Results of the 20 Ma model show that the maximum filtration velocity observed is about 0.3m.yr -1.
These results show clearly that the thermal regime is strongly influenced by fluid circulations.
These geothermal results can be used to track the thermal history of potential source rocks over time. Middle Jurassic Toarcian shales (L4, see geology) situated at the center of the graben have probably achieved thermal maturity during the middle Miocene. The maturation lasted until the late Miocene when the oil window moved upward due to convective heat transfer effects and subsidence. At the present time, the oil window is situated on the western part of the basin (L4 and L5, see geology) indicating that the Pechelbronn source rocks (Oligocene) are still immature (location of both oil windows).
A model for oil migration could be: primary expulsion of oil from source rock (Toarcian shales), followed by migration of oil driven by force of buoyancy and pressure. Based on the filtration velocity calculated from our model, the time duration for oil migration can be estimated to several hundred thousand years. The oil would probably follow the same path as circulating formation-water, until entrapment occurs in favourable structures such as the Pechelbronn faults.