Lancelot, Guinevere, and Excalibur are Rotliegende
Group gas fields that were developed mainly by horizontal drilling. Horizontal
well geological data from these fields are interpreted and integrated with
vertical well data to evaluate the target zone and enhance the reservoir
Lateral variations in reservoir quality and
eolian facies are interpreted in a horizontal core. Borehole images and
conventional electric logs are used to identify facies, record the lateral
extent and orientation of eolian dune cross-bed sets, characterize diagenetic
trends, and locate fractures through the uncored horizontal well sections.
The cored interval consists of gently dipping,
medium-grained dune base strata separated by a set boundary from moderately
steeply dipping, fine-grained dune top sediments. Permeability varies with
grain size and is therefore greater in the dune base than the dune top
in this cored section. The main facies in the horizontal wells as a whole
are of eolian dune, interdune, and fluvial to marine reworked eolian origin.
Comparison of facies in horizontal and vertical wells defines reservoir
units in the upper part of the formation that show large-scale mound and
intermound infilling geometries, reflecting relic dune field geomorphology.
Comparison of the lateral extents and heights of eolian cross-bed sets
and patterns of foreset bed dip vector swing suggest that the sets are
strongly trough shaped in
1997. The American Association of Petroleum Geologists. All rights reserved.
received May 25, 1995; revised manuscript received March 6, 1996; final
acceptance August 14, 1996.
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North Sea Limited, 3-6 Clements Inn, London, WC2A 2EB, United Kingdom.
thank Mobil North Sea Ltd., Brabant Oilex Ltd., and Robertson Research
International Ltd. for permission to publish this paper. We thank the following
people for their helpful discussions: Peter Corbett, Richard Wood, John
Ebbern, Peter Lucas, Rob Crossley, and Carl Watkins, and AAPG referees
Peter Hutchinson, Neil Hurley, and Roy Nurmi, whose critical reviews helped
improve the organization of the paper. The opinions expressed and any misconceptions,
however, are entirely ours.