Hydraulic Top Seal Failure – Hydrocarbon Preservation in HP/HT Central North Sea, Viking Graben and Mid-Norway Regions

Part of the risking strategy for prospects is an assessment of seal breach risk at top reservoir, i.e. when the seal may be breached by high pore fluid pressures causing hydraulic fractures in the top-seal. Prediction of seal breach through hydraulic failure involves pore fluid pressures reaching or exceeding the minimum stress plus the tensile strength of the seal rock. Analysis of the relationship between pore pressures and least stress has been undertaken in the HP/HT areas of the Central North Sea, Mid-Norway and Viking Graben to establish the key controls on hydrocarbon preservation. A strong relationship using aquifer pressures has been established in the Central North Sea, particularly when analysis is repeated at BCU and Base Chalk levels i.e. at shallower levels in the overlying seal than top reservoir. Geospatially, these relationships do not appear to hold true in the other areas. The result is a risking tool for exploration in certain basins, although the mechanisms involved remain unclear.