title: Sandstone Reservoir Quality Studies: geochemistry and mineralogy of modern estuarine sediments as an analogue for ancient deeply, buried sandstone
Creators: name: Muhammed, Dahiru
Publisher: name: University of Liverpool Repository
Year: 2022

description: Sandstone Reservoir Quality Studies: geochemistry and mineralogy of modern estuarine sediments as an analogue for ancient deeply, buried sandstone Understanding sediment depositional mineralogy and texture in sandstones reservoir quality prediction is increasingly challenging, these challenges are compounded in marginal marine settings owing to the to the complex interaction between tidal and riverine processes. The application of sedimentological and diagenetic models often fail to accurately predict reservoir quality in ancient and deep sandstone reservoirs, because the spatial and temporal variability of sandstone compositions are poorly-understood. This study focused on surface sediment (&lt; 2 cm depth) samples and core (&lt; 15 m depth) samples from the Ravenglass Estuary, NW England, and detailed geological mapping of the different estuary sub-depositional environments. Samples were analysed using portable XRF spectroscopy (pXRF), laser particle size analysis (LPSA), total organic carbon analysis, scanning electron microscopy (SEM-EDS) and statistical techniques. The distribution of geochemical elements, total organic carbon and grain size for surface sediments, were mapped at an unprecedented high-resolution using ArcGIS, in order to understand the controls on the distribution of elements, particularly Fe, and Fe bearing clay minerals. Holocene core samples were analysed to establish the controls on clay minerals and clay coats distribution patterns in the palaeo-sub-depositional environments of Ravenglass Estuary Holocene sediments. A novel automatic geochemical data-based classification approach was developed to predict palaeo-sub-depositional environments from core, through a combination of visual discrimination of gravel and vegetated surfaces and recursive partitioning routine (RPART) in R statistical software. The automatic classification approach was then applied to Holocene cores from the Ravenglass Estuary and an improved method was established to interpret palaeo-sub-depositional environments that compliments descriptive sedimentology and lithostratigraphic analysis. The result shows that the finest sediments in the central and upper estuary, have the highest concentration of Fe. Iron capable of producing Fe-rich grain coating minerals, was partly transported into the estuary as Fe-rich minerals, such as biotite and detrital chlorite, via the Esk arm of the estuary. However, Fe was also transported into the estuary as fluvially-transported complexes with organic matter, that were subsequently destabilised in the saline estuary and deposited during slack water conditions with the finest grained sediment. Clay minerals occur in the finer sediment as pore-filling or grain-coating and in coarser sediment clay minerals occur as grain coats and lithic grains. Clay minerals distribution vary greatly between the different sub-depositional environments while clay coat coverage increases with increasing clay fraction abundance. Provenance and depositional environments have played an significant role in controlling detrital mineral and texture distribution in the Ravenglass Estuary but local geochemical processes of fluvial organic complexation and co-deposition and early-diagenetic mineral alteration in sub-depositional environment-specific locations have partly over-printed primary depositional features and provenance signals. This study shows that sediment mineralogy and texture that potentially affect reservoir quality, vary across different sub-depositional environments. However, burial diagenesis will affect the primary sediment mineralogy, and this in turn will affect reservoir quality; outer estuarine sediment with negligible clay coat coverage and clay fraction is likely to be extensively quartz cemented during deep burial. Mud-flat and mixed-flat sediments are likely to have low porosity and low permeability due to pore-filling clays that block pore-throats. Sand flat and tidal bars sediments, that in Ravenglass have &gt;10% detrital coat coverage (within optimum range), and that contain chlorite-bearing lithic grains, can form diagenetic chlorite coats that can preserve anomalously high porosity in inhibiting quarts cementation, in deeply buried sandstone. Furthermore, knowledge of the distribution of minerals and elements is helpful in petroleum exploration and reservoir development in both deep and shallow reservoirs as clays (chlorite) can be good or bad, depending on the amount and the diagenetic setting. It is also of valuable in carbon capture and storage, and geothermal applications.