Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

Petroleum Geoscience

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Megson, J. Articles by Hardman, R. Search for Related Content

Exploration for and development of hydrocarbons in the Chalk of the North Sea: a low permeability system

¹ Department of Earth Sciences, University of Cardiff, and consultant, Gwynedd Oil and Gas, Meifod Isa, Dyffryn Ardudwy, Gwynedd LL44 2DS, UK (e-mail:joanmegson{at}email.msn.com)
² Amerada Hess International Ltd, 33 Grosvenor Place, London SW1X 7HY, UK (e-mail:richard.hardman{at}hess.com)

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.

KEYWORDS: Chalk, permeability, exploration, North Sea

This article has been cited by other articles:

				I. L. Fabricius, B. Rogen, and L. Gommesen How depositional texture and diagenesis control petrophysical and elastic properties of samples from five North Sea chalk fields Petroleum Geoscience, February 1, 2007; 13(1): 81 - 95. [Abstract] [Full Text] [PDF]

				L. Klinkby, L. Kristensen, E. B. Nielsen, K. Zinck-Jorgensen, and L. Stemmerik Mapping and characterization of thin chalk reservoirs using data integration: the Kraka Field, Danish North Sea Petroleum Geoscience, May 1, 2005; 11(2): 113 - 124. [Abstract] [Full Text] [PDF]

				J. W. MUNNS, J. C. GRAY, S. J. STOKER, I. J. ANDREWS, and T. D. J. CAMERON The remaining hydrocarbon potential of the UK Continental Shelf Geological Society, London, Petroleum Geology Conference series, January 1, 2005; 6(0): 41 - 54. [Abstract] [PDF]

				M. HEMMET The hydrocarbon potential of the Danish Continental Shelf Geological Society, London, Petroleum Geology Conference series, January 1, 2005; 6(0): 85 - 97. [Abstract] [PDF]

				J. MEGSON and T. TYGESEN The North Sea Chalk: an underexplored and underdeveloped play Geological Society, London, Petroleum Geology Conference series, January 1, 2005; 6(0): 159 - 168. [Abstract] [PDF]

				F. JAKOBSEN, J. R. INESON, L. KRISTENSEN, H. P. NYTOFT, and L. STEMMERIK The Valdemar Field, Danish Central Graben: field compartmentalization and regional prospectivity of the Lower Cretaceous chalk play Geological Society, London, Petroleum Geology Conference series, January 1, 2005; 6(0): 177 - 186. [Abstract] [PDF]

				O. V. VEJBAeK, P. FRYKMAN, N. BECH, and C. M. NIELSEN The history of hydrocarbon filling of Danish chalk fields Geological Society, London, Petroleum Geology Conference series, January 1, 2005; 6(0): 1331 - 1345. [Abstract] [PDF]

				O. V. Vejbaek Reservoir characterization of the Roar Gas Field, Danish North Sea Petroleum Geoscience, March 1, 2002; 8(1): 71 - 87. [Abstract] [Full Text] [PDF]