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Implications of Uplift and Erosion on Pore Pressure, Stress and Seal Integrity


Despite successful exploration through decades in many hydrocarbon rich basins around the world, most sedimentary basins also have a track record of quite a few disappointments due to the drilling of dry and under-filled traps. In uplifted and eroded basins, like the Barents Sea, the uplift and erosion is often “blamed” for the exploration failures because of the assumption that these tectonic events have caused extensive charge and leakage problems. The aim of our work has been to quantify some of the net effects of uplift and erosion on pore pressure and rock stress to further constrain the impact these processes has on seal integrity. Uplift and erosion will lead to elastic expansions as well as thermal contractions of both rocks and fluids. In addition, gases will exsolve from the pore fluids due to the unloading. Standard equations have been used to calculate the net volume effects of these processes. The calculations show that high overpressures and increased stress anisotropy may be formed during uplift and erosion. The process that contributes the most to significant overpressure generation during uplift and erosion is found to be gas expansion and exsolution from liquid hydrocarbons. The thermal contraction of rocks is calculated to be small, but still with a potential to increase stress anisotropy. Overpressures combined with stress anisotropy will favor shear rather than tensile failure, suggesting that leakage is more likely to happen through pre-existing faults rather than breaching intact cap rocks. This allows for hydrocarbons to remain in the traps also after leakage if this happens down flank from the trap apex. Several cases offshore Norway align with this interpretation. Even if uplifted and eroded rocks seem to have undergone extensive tectonics and brittle rock deformation, combined with possible paleo-overpressure generation, the negative effect of these processes on hydrocarbon column accumulations might have been exaggerated in the past. These processes may occasionally have caused reductions in hydrocarbon columns after charge due to leakage, but in many cases these processes are not sufficient to empty the structures for hydrocarbons as significant hydrocarbon columns still remain in many traps. In addition, late charge, re-migration after leakage and differential leakage of oil and gas may also have influenced the observed present day hydrocarbon column heights.