The influence of allogenic controls on facies variability within two basins: the Triassic Sherwood Sandstone Group of Central and Northern England

Allogenic controls (tectonics, eustasy, and climate) on sedimentation are thought to exert the most influence on depositional trends seen within sedimentary basin fill. This study examines two adjacent early Triassic basins, with similar allogenic factors of climate, eustasy and sediment supply. What is unclear is how the expression of facies/depositional trends might manifest in these two tectonically different, but otherwise similar, basins. The East Midlands shelf, is a passive shelf-edge basin marginal to the South Permian Basin. The Needwood Basin, in contrast, is a half graben created by extensional tectonics and typified by normal synsedimentary faulting. Much of the sediment supply to the basins was derived from a single sediment source (the London-Brabant High and Armorican Massif) located in present-day SE England and Northern France, with a local input to the Needwood Basin from the Pennine high. Architectural element and lithofacies analysis on three selected outcrops, and borehole logging on 8 boreholes, was conducted from across the two basins. This indicates that both basin successions comprise amalgamated channel fill, with emphasis on downstream accreting elements. In the East Midlands Shelf (Yorkshire-Nottinghamshire), the sedimentary basin infill is characterised by sandy, trough cross-bedded sequences; palaeoenvironmental interpretation suggests a sandy braided river environment. Successions from the Needwood Basin are indicative of a higher depositional energy, possibly influenced by proximity to several sources of sediment into an actively subsiding basin. Tectonic allogenic forcing factors exert a large degree of influence on depositional trends in both basins. Most notably, the degree of subsidence and distance from the basin margin is thought to have the most effect on facies type and distribution. This enhances our understanding of facies predictability and depositional trends away from data points, and can feed into the development of better reservoir models

Characteristics of deformation bands and relationship to primary deposition: an outcrop study from the Wirral, north-west England

The gross environment of deposition has been recognised as a major influence on the development of deformation bands. Deformation bands represent local zones of grainsize reduction/crushing and fusing that develop in response to the accommodation of stress, and typically develop in sandstones. As they can result in crosscutting planes of low permeability compared to undeformed rock, deformation bands can degrade reservoir quality, and have an adverse impact on the performance of economically important hydrocarbon reservoirs, aquifers or potential repositories for carbon capture. Outcrop of the Middle Triassic (Anisian) Sherwood Sandstone Group from Thurstaston Hill and West Kirby (Wirral, north-west England) comprises a series of stacked dune and interdune facies, with rare heterolithic fluvial associations (channel and channel lag). Aeolian dunes are characterised by relatively large-scale, low- to high-angle cross-sets with common bimodal ‘pinstripe’- type lamination, with interdunes typified by planar and ripple-laminated sandstone and silty sandstone. A striking feature is the development of deformation bands which are locally pervasive. Initial data collection shows that deformation bands are more common in grainfall and grainflow facies, becoming rare/not observed in trough, planar cross-bedded and pebbly sandstone facies. Their morphology has been categorised into 4 distinct classes based on the spatial density and relationship between individual deformation bands (see image). The permeability of these features, assessed in the field by mini-permeameter, does not appear to be related to the class of the deformation band. These features have a high permeability contrast with the surrounding sandstone, and permeability values up to two orders of magnitude lower than undeformed sandstone is indicated. Where present in reservoir rocks such as the Leman Sandstone of the North Sea, or regional aquifers such as the Wilmslow Sandstone onshore, present day bulk permeabilities may be higher in fluvial facies with no deformation bands, rather than silt and clay-poor aeolian facies that host these features. Deformation bands may have a stronger influence on fluid flow toward the end of the production history of a well or field, when reservoir pressures are depleted. These observations, could mean that reappraisal of reservoirs is required to optimise production in declining fields/aquifers

Fluid transport in the Sherwood Sandstone: influences of diagenesis and lithofacies

The Triassic age Sherwood Sandstone Group (SSG) is of great importance to the UK for several applied reasons: i) it is the lateral equivalent of the hydrocarbon producing Lower Triassic (Olenkian) Bunter Sandstone in the North Sea and the Middle Triassic (Anisian) Ormskirk Sandstone in the East Irish Sea; ii) it is a major aquifer in the UK; iii) it has storage potential for anthropogenic CO2. Increased understanding of the SSG is required to further improve efficiency of extraction, protect aquifers and reduce overall risk associated with such activities. Despite these economically important uses, the SSG has previously seen little work to ascertain any possible connections between primary sedimentological facies and diagenesis. As such, this study has analysed 30 thin sections of fluvial sandstones from 5 boreholes in and adjacent to the Needwood Basin. Using optical microscopy and scanning electron microscopy a diagenetic history was deduced and analysed with respect to sedimentological facies. The diagenetic history of the Needwood Basin SSG is comparable with the SSG from other UK sedimentary basins, with the exception of a lower quantity of well-developed authigenic quartz, feldspar and increased framework grain and cement dissolution. The results indicate that primary sedimentological facies does have a control on subsequent diagenesis. The highest porosities were found in cross bedded sandstones and massive sandstones; the lowest porosities were found in conglomerates and muddy facies; low angled cross bedded sandstones and horizontally bedded sandstones displayed a wide range of porosities. The presence of diagenetic cements had a significant effect on porosity due simply to the reduction in available pore space: calcrete or dolocrete reduced porosity on average by 17.5%, baryte by 23% and iron oxide by 8%. This data can be used to populate databases for use in fluid flow modelling to inform the hydrocarbon industry as well as hydrogeology, predictive models for contaminant transport and green technology including Carbon Capture and Storage and geothermal energy

Facies heterogeneity in the Triassic Sherwood Sandstone Group of the UK: comparing and contrasting coeval depositional basins

The primarily fluvial Triassic Sherwood Sandstone Group is found at outcrop fringing the northern part of the Needwood Basin (between Stoke-on-Trent and Nottingham) then northwards along the western margin of the East Midlands Shelf (Figure 1). The offshore lateral equivalents of the group in the East Irish Sea and North Sea (Ormskirk and Bunter sandstones) are reservoirs for oil and gas. Additionally, the Sherwood Sandstone Group is of regional importance in eastern England as the principle groundwater aquifer that supplies water for potable and industrial use. In some areas the aquifer has become contaminated by a variety of pollutants including Non-Aqueous Phase Liquids (NAPLs) and nitrates. Some pathways through the aquifer are via intergranular flow and this is partly influenced by variations in porosity and permeability, of which lithofacies is a major control. As such, further understanding of the spatial relationships between different lithofacies and associated properties at outcrop can be used to augment predictive models applicable to both the water and hydrocarbons industries. Data collection for this study focussed on 6 field localities covering the East Midlands Shelf and Needwood Basin. From these field localities numerous lithofacies have been identified and these include: i) clean homogeneous massive sandstones, ii) sandy conglomerates, iii) poorly- to moderately- sorted cross-bedded sandstones, iv) fine to very-fine cross-laminated sandstones, and v) thin horizontally laminated siltstones. Data from these field localities comprises a series of pseudo-three dimensional architectural panels which illustrate the relationship and three-dimensional configuration of observed lithofacies types. Cores from eight boreholes were also logged and provide information on the spatial distribution and relative abundance of the identified lithofacies where outcrop is sparse. This study allows a comparison of the Sherwood Sandstone Group between depocentres separated by the Charnwood palaeo-high. The dataset is being used to create qualitative and quantitative models that depict the variations in lithofacies types and configurations in the Sherwood Sandstone Group spatially; contrasting lithofacies from linked contemporaneously depositing basins. Preliminary results suggest more lateral and vertical variation in lithofacies types than has been previously described, which implies that a greater complexity of lithofacies architecture needs to be captured to improve models of fluid flow in the Sherwood Sandstone Group