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Compositional Variation in Modern Estuarine Sands: Predicting Major Controls on Sandstone Reservoir Quality

Abstract

The spatial and temporal variability of primary depositional mineralogy in sandstones is poorly-understood and consequently empirical models typically fail to accurately predict reservoir quality. To address this challenge, we have mapped the spatial distribution of minerals (quartz, feldspar, carbonates and clay minerals) in surface sediment throughout the Ravenglass Estuary, UK. This modern analogue was mapped, at an unprecedented high-resolution, over an area similar to many oil and gas reservoirs. Spatial mineralogy patterns (based on X-ray diffraction data) and statistical analyses revealed that estuarine sediment composition is primarily controlled by provenance i.e. character of bedrock and sediment in the source area. The distributions of quartz, feldspar, carbonates and clay minerals are primarily controlled by the grain size of specific minerals (e.g. rigid versus brittle grains) and estuarine hydrodynamics. The abundance of quartz, feldspar, carbonates and clay minerals is predictable as a function of depositional environment and critical grain-size thresholds. The results of this study may be used, by analogy, to better predict the spatial distribution of sandstone composition, and thus reservoir quality in ancient and deeply-buried estuarine sandstones.