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AAPG Bulletin


Integration of Horizontal Well Geological Data Into Reservoir Descriptions: Rotliegende Sandstone, 
Lancelot Area, Southern North Sea

David J. Went2 and William C. Fisher3


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 description.

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 

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

1Manuscript received May 25, 1995; revised manuscript received March 6, 1996; final acceptance August 14, 1996.

2Robertson Research International Limited, Llandudno, North Wales, LL30 1SA, United Kingdom. Current address: Robertson Blackwatch Ltd., Netherton Business Centre, Fetternear, Kemnay, Inverurie, Aberdeenshire, AB1 5LX, United Kingdom.

3Mobil North Sea Limited, 3-6 Clements Inn, London, WC2A 2EB, United Kingdom.

We 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.

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