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

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.

Architectural analysis of a Triassic fluvial system: The Sherwood Sandstone of the East Midlands Shelf, UK

“…Channel fills with erosive bases are accompanied by lateral and mid-channel bar forms showing a mixture of lateral and downstream accretion. A braided river interpretation is concordant with observations by other authors for the lower part of the Sherwood Sandstone elsewhere in the UK and Ireland: Northwest England (Burley, 1984;Jones and Ambrose, 1994), East Irish Sea Basin (Cowan, 1993;1993b;Meadows and Beach 1993a), Southern…”

“…For example, there are readily identifiable differences in the depositional environments, including; i) an absence of identified aeolian facies in the eastern Sherwood Sandstone contrasting the mixed aeolian-fluvial formations of the group in the north-east of England (Cowan, 1993;Mountney and Thompson, 2002;Bloomfield et al, 2006), and ii) an absence of well-developed/regionally pervasive palaeosols (pedogenic processes) in the eastern parts of England compared to western and southern.…”

“…Deposition of the Sherwood Sandstone Group in central Britain is ascribed to a terrestrial mixed fluvial and aeolian environment that resulted in a sandstone-dominated succession with minor amounts of mudstone that accumulated in perennial and ephemeral water bodies (Cowan, 1993;Mountney and Thompson, 2002;Bachmann et al, 2010;Ambrose et al, 2014).…”

Lithostratigraphical subdivision of the Sherwood Sandstone Group (Triassic) of the northeastern part of the Carlisle Basin, Cumbria and Dumfries and Galloway, UK

“…This has led to the fully formalised lithostratigraphical scheme of Jackson & Johnson (1996) that has been extended to much of the northern part of the Irish Sea, including the Solway Firth Basin (Chadwick et al 2001) (Tables 2, 3). Detailed sedimentological studies have helped to differentiate the subdivisions of the SSG offshore (Meadows & Beach 1993a, 1993bCowan 1993;Cowan et al 1993). Much of the St Bees Sandstone Formation offshore comprises the deposits of a major northwards flowing river system in a semi-arid continental setting, but aeolian deposition became increasingly important in the higher part of the formation, particularly towards the basin margins.…”

Unusual thick eolian sand sheet sedimentary succession: Paleoproterozoic Bandeirinha Formation, Minas Gerais

“…Thus, we propose an eolian origin for these laminae due to clasts dimensions, sorting and inverse grading, consequently these laminae are interpreted as wind ripple laminations. Indeed, sand reworking by wind inside ephemeral channels in dry periods is very common (Cowan 1993;Tooth 2000;Jain et al 2005;Krapf et al 2005).…”