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Abstract

Depressurization of reservoirs in petroliferous basins commonly occurs through cap-rocks at structural crests where pore pressures are locally elevated because of either the presence of a hydrocarbon column or the redistribution of overpressures by water flow along laterally extensive inclined permeable aquifers. In exhumed petroliferous basins this deflation of excess pore pressures is enhanced by the denudation process, which results in the large-scale removal of overburden during regional uplift. Evidence from the exhumed basins of the Atlantic margin indicates that hydrocarbon accumulations in these basins are commonly characterized by underfilled traps and hydrostatically pressured or modestly overpressured reservoirs. These observations are reviewed in the context of the generic mechanisms by which top-seals leak, the properties of cap-rocks and the physical processes that occur during exhumation.

Water-wet shaly cap-rocks can form a capillary seal to a hydrocarbon column while simultaneously accommodating brine flow and equilibration of pressures between the reservoir and the top-seal. In contrast, thick, low-permeability shale or evaporite sequences may form pressure seals that restrict vertical brine and hydrocarbon flow and prevent the equilibration of aquifer pressures above and below the seal. In any sedimentary basin, the presence of regional pressure seals can result in a layered hydrogeological regime with hydrostatically pressured strata decoupled from over- or underpressured cells. Recently exhumed basins typically show limited overpressuring and in a number of these basins underpressured reservoirs have been described. Post-exhumation overpressure generation is primarily driven by tectonic compression, aquathermal pressuring and hydraulic head.

The fluid retention capacity of any cap-rock lithology during exhumation is dependent upon the physical and mechanical characteristics of the cap-rock at the time of exhumation and the timing and conditions of the associated deformation relative to the timing of hydrocarbon emplacement. The permeability and deformational characteristics of halite render it an excellent cap-rock with a high retention capacity, even under conditions of exhumation. However, mudrocks may also form effective cap-rocks in exhumed basins when the deformation associated with exhumation occurs before embrittlement and the shale cap-rock exhibits ductile behaviour.

Shale and evaporite cap-rocks form the main regional seals to hydrocarbon accumulations in exhumed basins of the Atlantic margin and borderlands. Syn-exhumation top-seal efficiency (fluid retention capacity) is a major exploration risk in these basins, although post-exhumation top-seal integrity in these basins may be relatively high under certain conditions. Consequently, a major exploration risk factor in exhumed basin settings pertains to the limited hydrocarbon budget available post-regional uplift and the efficiency of the remigration process.

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