oa Petroleum Leakage in the Snorre Oilfield, North Sea – A Case for Change in Cap-rock Wettability and Dynamic Seal Risking

Snorre is a leaky commercial oilfield that supports ~300 m oil column in the Tampen Spur area of the North Sea. The field contains undersaturated oil with no gas cap. Pervasive evidence of oil and gas leakage into the cap-rock is well documented (Leith and Fallick 1997, Bond 2000). Oil migration may alter the wetting state of the cap-rock. Today, the reservoir sands are ~12 MPa overpressure. Log derived pore pressure for the Shetland Group mudstone cap-rock indicates a normal compaction state. This pressure disequilibrium between the reservoir and the cap-rock suggest that reservoir overpressure may have post-dated compaction and geologically recent. Phase behaviour models of the petroleum fluids indicate that a gas cap should exist in the trap if the reservoir was normally pressured. The observed oil column is in equilibrium with the average column height potential of the cap-rock estimated from pore size distribution data. This may imply that the cap-rock is at maximum supportable column height today. In addition, subsequent petroleum charge has leaked into the cap-rock possibly at a rate similar to the reservoir charge rate. These data suggest that the Snorre field capped by oil-wet cap-rock is a case example of a dynamic seal.