Reserve growth in chalk fields of the North Sea

Abstract

Reserve growth is the net increase in estimated recoverable resources of an oil or gas field or group of fields through time. This phenomenon is important in North Sea oil fields and is observed in all classes of reservoirs and in all country sectors. Oil fields with chalk reservoirs have exhibited particularly large increases in estimated recoverable reserves. Whereas chalk reservoirs contain only about 16% of recoverable North Sea oil, almost one third of the growth of reserves in existing fields has been in fields with chalk reservoirs. More than 60% of the currently reported recoverable oil (cumulative production plus remaining reserves) in chalk fields is attributed to reserve growth since 1985. In contrast, only about 34% of recoverable oil in all North Sea fields (chalk and clastic reservoirs) is from reserve growth. The large increase in estimated recoverable resources in the chalk reservoir fields of the North Sea results from both increases in estimated original oil in place and from improvements in recovery efficiency. Although the largest fields generally display the greatest volumetric additions to reserves, on a percentage basis reserves in small fields increase as much as reserves in larger fields. So far, growth of reserves in the chalk fields has been directly proportional to development effort, as indicated by numbers of wells drilled for delineation, development and improved recovery (including injector and observation wells). In spite of great improvements in understanding chalk reservoirs, estimates of original oil in place are still uncertain and overall recovery factors remain relatively low. Therefore, opportunity remains for further large additions to reserves in chalk reservoir fields. The experience of operators of the North Sea chalk fields indicates that large, complex accumulations with difficult reservoir engineering problems may present possibilities for additional oil recovery in such fields elsewhere.