C.M.A. Gent

Shallow Gas Offshore Netherlands - The Role of Faulting and Implications for CO2 Storage

The presence of shallow gas within Pliocene–Pleistocene sediments in the North Sea is well known, though there is still some debate regarding its origins. Many of the shallow gas accumulations are coincident with faults developed over salt structures, leading to speculation that faults may have acted as conduits for upward migration of hydrocarbons from greater depths. The role of faults in charging of the Pliocene–Pleistocene reservoirs is investigated for several of the gas accumulations through interpretation of 3D seismic reflection data, revealing the relationship between faults and seismic indications of gas saturated sediments such as bright spots and gas chimneys. In order to invoke the faults as migration conduits for the gas, they must form part of the migration pathway between the gas-charged sediments and thermogenic source rocks. For the accumulations studied, such migration routes exist with salt-withdrawal beneath mini-basins allowing Carboniferous-sourced gas to migrate to the Triassic, and subsequent vertical migration along faults and fractures associated with diapirism. The faults in question are near critically-stressed, and have been active in the recent geological past. The observation of shallow gas seemingly associated with such features may have implications for the sequestration of carbon dioxide in formations affected by similar features.