Four regional sections have been modelled in Reggane Basin to evaluate the risk of the petroleum system and to discuss the overpressure related drilling hazards. Main generation occurred previously to the Hercynian orogeny, at around 360 and 324Ma for the Silurian and Frasnian source rock source rocks respectively. The basin became highly overpressured due to the Upper Devonian regional sealing. After the Hercynian uplift, pressure started to be dissipated through the outcropping areas. The drop of the pressure in the Lower Devonian reservoirs accumulations induced gas expansion, phases changes and oil flushing. Generation was frozen until 200 Ma when a thermal anomaly evidenced by AFTA data took place triggering extra generation and extensive cracking. At present day both source rocks are overmature in the center of the basin. Hydrocarbon accumulations are mainly in gas phase. Overpressure events in the Emsian reservoirs are related with a process of “fossil” pressure preservation from pre-Hercynian times plus pressure transference from Upper Devonian overpressured shales. This can be traced back to the confinement of the Emsian reservoir by the Upper Devonian top seal and the Muralla China base seal coupled with low permeability faults acting as lateral seals resulting in a overpressured closed system.


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