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Shell UK Exploration Production, Shell-Mex House, Strand, London, UK
Rotliegend aeolian sandstone of Early Permian age forms the main producing horizon in the Southern North Sea gas province. Currently some 30–40 fields have been identified in a broad zone measuring 450 by 100 km stretching from the English coast, through the UK and Dutch sectors of the North Sea and on to The Netherlands and West Germany. North Sea field sizes range between 10–300 Bcm (0.3–10.5 TCF). Ultimate recoverable reserves for the province are estimated at some 4250 Bcm (150 TCF). The Rotliegend is underlain by Carboniferous coals and carbonaceous shales, which form the source rock, while Late Permian Zechstein evaporites form the seal. The aeolian sandstones occur interbedded with wadi sandstones and sabkha siltstones. Dune sandstones with large-scale cross bedding generally form the best reservoirs, though even here reservoir quality is highly variable. Core porosities range between 5 and 30% and permeabilities from <1 to 3000 mD. Reservoir impairment through the effects of burial diagenesis (compaction, pressure solution, clay mineral transformations and cementation) is evident in many fields. In others, the occurrence of natural fractures serves to enhance production. As a result, individual well productivities show wide variation.
1 Now consultant, Ballater, Crampian, UK.
2 Now A/S Norske Shell, Risavika, NOrway.
