Abstract: The petroleum prospectivity of an exhumed basin is largely dependent on the ability of pre-existing traps to retain oil and gas volumes during and after the exhumation event. Although faults may act as lateral seals in petroleum traps, they may start to become hydraulically conductive again and enable fluid flow and hydrocarbon leakage during fault reactivation. We constrain the present day in situ stresses of the exhumed Illizi Basin in Algeria and demonstrate that the primary north–south and NW–SE (vertical strike-slip) fault systems in the study area are close to critical stress (i.e. an incipient state of shear failure). By contrast, the overpressured and unexhumed Berkine Basin and Hassi Messaoud areas to the north do not appear to be characterized by critical stress conditions. We present conceptual models of stress evolution and demonstrate that a sedimentary basin with benign in situ stresses at maximum burial may change to being characterized by critical stress conditions on existing fault systems during exhumation. These models are supportive of the idea that the breaching of a closed, overpressured system during exhumation of the Illizi Basin may have been a driving mechanism for the regional updip flow of high-salinity formation water within the Ordovician reservoirs during Eocene–Miocene time. This work also has implications for petroleum exploration in exhumed basins. Fault-bounded traps with faults oriented at a high angle to the maximum principal horizontal stress direction in strike-slip or normal faulting stress regimes are more likely to have retained hydrocarbons in exhumed basins than fault-bounded traps with faults that are more optimally oriented for shear failure and therefore have a greater propensity to become critically stressed during exhumation.

Abstract: Overpressuring, tectonic stretching and thermoelastic contraction are all processes that can drive the formation of opening-mode fractures in the subsurface. Recent studies on crack-seal quartz deposits in opening-mode fractures have yielded fluid inclusion microthermometric data, which for the first time allow us to constrain the pressure–temperature conditions under which these fractures formed. Here, we utilize the results from studies in the Lower Cretaceous Travis Peak Formation in the East Texas Basin and the Upper Cretaceous Mesaverde Group in the Piceance Basin to construct stress history models based on mechanical properties, burial history and tectonic setting to evaluate the various driving mechanisms for opening-mode fracture formation. Our results show progress towards separating and independently evaluating these mechanisms. Although high fluid pressure and tectonic stretching can play a major part in the formation of opening-mode fractures, our results suggest that the persistence of fracture growth during uplift could have been strongly influenced by thermoelastic contraction associated with exhumation and cooling. For sandstone reservoirs, thermoelastic contraction will be more pronounced for stiffer, high Young’s modulus rocks with higher quartz contents. These models can therefore be used to provide additional insights into the distribution of opening-mode fractures in exhumed basins.