The North Sea Chalk play: examples from the Danish Central Graben

  • Megson J
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Abstract

Currently, all hydrocarbon production in Denmark is from Upper Cretaceous and Danian chalk fields in the Central Graben. Seven fields are producing, and several more are under development. The field structures were formed by a variety of mechanisms: uplift on salt pillars (Skjold, Nils, Dagmar), doming over salt swells (Dan, Kraka), and end-Cretaceous and Miocene inversion tectonics (Tyra, Roar). The combination of structural styles, varying times of migration of hydrocarbons into the structures, and differences in chalk sedimentology have resulted in a wide range of reservoir porosities (20% to over 50%), and permeabilities (1 mD to 1000mD). Recent advances in horizontal well technology have considerably increased predicted recovery factors in low permeability fields, such as Dan, and in fields where a thin oil leg is present beneath gas, such as Tyra. Exploration for hydrocarbons in the chalk is now entering a new phase, as all structural closures have been drilled in the A. P. Moiler concession area. There is evidence from existing chalk fields and discoveries of non-structural trapping mechanisms, such as: oil trapped within the chalk by an intra-chalk topseal, hydrodynamic trapping of oil, and diagenetic trapping. These concepts are being combined with seismic interpretation of high-resolution seismic data, targeted at the chalk, in order to develop new plays and prospects. In addition it is shown that seismic data may be used as a direct hydrocarbon indicator where gas is present at the top chalk level. In 1991 an appraisal well, E-5X, was drilled c. 8 km from the main part of the Tyra Field, and encountered gas as predicted from a seismic phase reversal at top chalk

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Authors

  • J B Megson

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