Identification and significance of aeolian deposits within the dominantly fluvial Sherwood Sandstone Group of the East Irish Sea Basin UK

Abstract

Abstract The Triassic Sherwood Sandstone Group sediments of the East Irish Sea Basin are over 4000 ft thick and comprise medium- to coarse-grained sandstones and rare thin mudstones. Facies models developed during production drilling on the South Morecambe Field show that deposition occurred in a braided fluvial setting with minor aeolian episodes. The major facies associations present are: (A) major channel fill; (B) ephemeral channel fill; (C) non-channelized sheetflood deposits; (D) and (E) non-reservoir fines (abandonment and playa respectively); (F) aeolian dune and sand-sheet. Volumetrically, the major channel (A) and sheetflood (C) facies dominate, and the alternation of these facies associations is correlatable across the field. The aeolian sandstones form units varying in thickness from a few grain diameters up to 3 m thick. The sand-sheet and dune deposits can be correlated over considerable distances and show pressure communication on RFT logs. Although the aeolian sandstones comprise only 5–10% of the reservoir they have very high porosities and permeabilities (up to 30% and 10 darcies) and make disproportionate contributions to flow into the wellbore. The depositional criteria used to differentiate between aeolian and fluvial deposition in the South Morecambe Field are pinstripe lamination, good sorting and lack of rounded clay clasts, but these criteria are not definitive. As a result of the dissolution of an early diagenetic cement, aeolian sandstones have very high porosities compared to fluvial sandstones. This high porosity is reflected in high sonic transit times and allows aeolian sandstones to be identified tentatively in uncored wells by use of sonic logs.