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Prediction of Alluvial Sandstone Body Dimensions, Connectivity and Heterogeneity in the Subsurface


Knowledge of alluvial sandstone body dimensions, connectivity and heterogeneity is important for predicting reservoir quality at exploration and production scales. Published databases of sandstone body dimensions show values covering up to two orders of magnitude for width and thickness, with significant overlap occurs between sandstone bodies deposited by different channel forms, palaeovalley fills and distributary channels. This overlap makes appropriate analogue select ion difficult. Observations from modern sedimentary basins suggest significant variability in channel scale, type and avulsion frequency dependent on location within the basin. Combining these observations with a literature database suggest a number of predictive relationships. Low net sand (<30%) systems occur distal to basin margins, are associated with gleyed soils and comprise single or two storey sandstone bodies. High net sand systems (>70%) occur as: 1) lateral systems developed adjacent to the basin margin where point-sourced distributary fluvial systems (DFS) deposit the coarser bedload sediment fraction, 2) axial channel systems held either between adjacent distributive fluvial systems and/or basement highs (such as a foreland basin flexural margin) and 3) paleovalleys cut either into bedrock or coastal plain sediments. Different high net types all comprise stacked, amalgamated multi-storey sandstone bodies but have significantly different width to thickness ratios and display no paleosol development. Intermediate net sand (30-70%) systems occur between the high net sands at the basin margins and low net systems in the basin centre. Using a DFS model, a progressive down-dip decrease in sand content and connectivity and concomitant increase in heterogeneity occurs between the proximal high net part of the DFS and the distal low net area. Differences are due to increases in both floodplain preservation and heterogeneity associated with preserved channel abandonment phases and mud-draped accretion surfaces. Between 70 and 50% net sand, floodplain mudstones form sheets traceable for tens of kms which separate amalgamated channel sandstone bodies. Between 50 and 30% net sand, amalgamated sandstone bodies become isolated within floodplain mudstones. These observations provide a framework for predicting sandstone body development and connectivity in the subsurface within a basinal context and account for the wide range of sandstone body dimensions reported in the literature.