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

摘要

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. udud