Hydrocarbon leakage processes and trap retention capacities offshore Norway

ABSTRACT: Associations between high overpressures and sparse hydrocarbon occurrence are commonly ascribed to hydrocarbon leakage through pressure-induced fractures in the cap rock. However, several hydrocarbon traps in the North Viking Graben area in the North Sea still contain abundant commercial volumes of hydrocarbons at very high pore pressures. By contrast, a majority of the over-pressured structures at the Halten Terrace further to the north have leaked hydrocarbons, even at considerably lower overpressures. A selection of wells in the North Viking Graben and the Halten Terrace areas was investigated to find possible explanations for these observations. Distinct regional differences emerged, as the emptied reservoirs at the Halten Terrace generally have higher retention capacities than the overpressured discoveries in the North Viking Graben area. Thus, there appears to be a lack of any clear relationship between structures emptied of hydrocarbons and low retention capacities, which could be expected if pressure-induced fracturing of the cap rock was the main process of the hydrocarbon leakage. The regional differences in retention capacities were mainly attributed to different leakage processes in the two basins. Stress history variations are suggested to be the main controlling factor of these leakage processes