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Geological Society, London, Special Publications; 1993; v. 73; p. 123-142;
DOI: 10.1144/GSL.SP.1993.073.01.08
© 1993 Geological Society of London

Fluvial Reservoirs

The complex nature of a Jurassic multistorey, alluvial sandstone body, Whitby, North Yorkshire

J. Alexander1 & R. L. Gawthorpe2

1 Department of Geology, University of Wales College of Cardiff, PO Box 914, Cardiff CF1 3YE, UK
2 Department of Geology, The University, Manchester M13 9PL, UK

The complex alluvial sandstone body of the Saltwick Formation (Aalenian) exposed in the town of Whitby, North Yorkshire is 700–1000 m wide, up to c. 25 m thick and extensive in the palaeoslope direction (N-S). Exposed sections are approximately parallel and perpendicular to the palaeochannel orientation. The sandstone body is made up of a large number of channel storeys stacked in a complex vertical and lateral pattern and it is composed predominantly of fine- to medium-grained sandstone (> 85% by volume) with subordinate amounts of mudstone and siltstone. The sedimentary facies architecture indicates that channels repeatedly reoccupied the same site and that periods of erosion and incision alternated with periods of rapid bed aggradation. The sandstone facies patterns are related to migration of channel bars, dunes and ripples. Fine-grained sediment facies which occur between some of the storeys are laterally discontinuous because of their position within abandoned channels and their partial erosion by later channels.

Within the sandstone, permeability ranges from 0.8 to 682.1 mD with a geometric mean of 83.5 mD. Permeability heterogeneity is apparent from laminae to sandstone body scale. An attempt was made to characterize permeability on the basis of sedimentary facies. However, there is considerable scatter in the data and individual facies show a varying degree of heterogeneity of permeability. Two major sandstone storeys which appear very similar in origin and lithological character were found to have systematically different permeability distributions (geometric mean permeabilities of 36.0 and 108.5 mD). Facies variations can only partly explain this. There is a difference in the mean permeability of the same facies in the two sandstone storeys (e.g. geometric mean permeability of cross bedded sandstones in the two units are 63 mD and 100 mD respectively).