It is evident that all or most North Sea Chalk fields have dipping oil–water contacts and that some also have dipping free water levels. Furthermore, there are Chalk fields, such as South Arne (Denmark) and Joanne (UK) with steeply dipping oil–water contacts and a large element of non-structural trapping, while other discoveries, such as the recent Halfdan discovery northwest of the Danish Dan Field, have no element of structural trapping and appear to lie on a Tertiary migration pathway. It is also evident that the presence of North Sea Chalk oil, both as fields and shows, is localized and, generally, confined to areas of early source maturation – although there are exceptions, such as the UK Fife and Flora Chalk oil accumulations, which are distant from a source basin.
We believe that these attributes reflect the same cause, that is the very low lateral and vertical permeability (typically 0.1 to 1 mD air permeability) of the Chalk system. Our hypothesis is that oil enters the Chalk from the Jurassic at a limited number of entry points within the early mature basin and, only if the oil reaches Chalk of relatively high permeabilities, normally the Tor Formation, does the oil migrate significant distances laterally. This oil migration occurs at geological timescales, i.e. kilometres over millions of years and we, therefore, have to consider the configuration of the Chalk – with respect to migration routes and traps – at the time the oil was injected into the Chalk and began to migrate through it, which was normally prior to the North Sea Miocene inversion and later tilting. There is considerable evidence for trapping in Chalk palaeostructural traps; oil migrated into a palaeohigh, segregated vertically and is trapped by the low oil relative permeabilities within the transition zone when the trap later tilts. Another trapping mechanism is the migration trap, as described for the Halfdan discovery in Denmark, where, in an area mature for Chalk exploration, oil has been discovered in continuity with the Dan Field but with a free water level 700 ft lower. This type of trap may contain significant reserves of oil in chalks of 1 mD permeability and, in recent years, it has become possible to exploit chalks of this nature commercially. It is also possible to develop these chalks when no effective natural fracture system exists.