Discussion on petroleum

I. E. HIGGINBOTTOM, Wimpey Laboratories With reference to Paper 59, has any seismicity been reported in association with North Sea production? Has all the subsidence been aseismic or have any of the faults moved again in a brittle rupture mode?

DR M. E. JONES, Paper 59

Although Dr Higginbottom's question appears a most straightforward question, the answer is not so. At the simplest level, it is possible to report that there has been one seismic event in the vicinity of the Ekofisk field during the history of the subsidence. This event was recorded by seismographs in Scotland and, I believe, in Norway, and uncollaborated media reports suggested that shaking of the installations was detected by some staff. However, the seismic event originated outside of the oilfield and its overburden and it has been suggested that this was a natural earthquake associated with the basin controlling faults in the Ekofisk region of the North Sea. Earthquakes of modest proportions have been reported in connection with subsidence over producing oil fields, notably at Wilmington and Baldwin Hills in the USA where damage to surface installations and oil wells resulted. It is therefore possible that a tectonic origin for the Ekofisk earthquake is optimistic. Whether there is a history of smaller seismic events associated with the subsidence is not known. This may seem surprising until it is realised that the continuing drilling, production and maintenance activities on the Ekofisk platforms produce sufficient noise that any local seismograph would be overwhelmed. Attempts to listen to the subsidence have all ended with negative results. The Ekofisk chalk is generally a weak rock, where it forms a good reservoir, and readily deforms in a ductile manner. It is also pervasively fractured by geological processes, and it is possible that before yield, these fractures may shear in an aseismic manner. I personally doubt that this mechanism is a major contributor to the subsidence. Similarly the Ekofisk overburden is a weak rock, and although the potential for rupture may be greater here, this is not supported by the history of well casing deformations at Ekofisk (it may be at West Ekofisk). With our present state of knowledge it therefore appears that the events at Ekofisk are largely aseismic, although both the Norwegian Petroleum Directorate (NPD) and the operating company keep an open mind as far as possible