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Predicting Clay Mineral Distribution in Sandstone Reservoirs Using an Analogue Holocene Estuarine Succession

Abstract

Quartz cementation of sandstones buried to depths > 2.5 km (> 80-90°C) is one of the major causes of porosity and permeability. Fe-Mg enriched chlorite grain coats can preserve anomalously high porosity in deeply buried sandstones reservoirs through the inhibition of authigenic quartz cements. In contrast, pore-filling illite and kaolinite typically reduce reservoir quality through porosity and permeability reduction. Since the key elements in clay minerals (Fe-, Al- and Si-oxides) are largely insoluble, clay diagenesis can be assumed to be isochemical during burial, therefore, the dominant control on the occurrence and type of clay (chlorite, illite or kaolinite) is primary depositional mineralogy. This study provides the first modern estuarine analogue to predict reservoir clay mineral precursor distribution. X-ray diffraction was performed on the fine fraction (<2 µm) of surface samples (< 2cm), and cores (< 15 m) in order to understand the fundamental processes that govern clay mineral distribution within the post-Holocene estuarine succession. Surface clay mineral maps and shallow cores show; detrital chlorite is most abundant within the outer estuary (foreshore, backshore and tidal inlet); illite is most abundant within the inner and central estuarine tidal flats; kaolinite displays a relatively ubiquitous distribution. Hinterland geology and climate (weathering intensity) control clay mineral type and abundance. Clay mineral properties (e.g. grain size) combined with estuarine hydrodynamics control clay mineral distribution patterns. Clay mineral distribution throughout the Holocene succession is largely controlled by changes in relative sea-level and the transition from an open, wave-dominated system, to a wave- and tidal-dominated sheltered system with the formation of the beach barrier system during the mid-Holocene. The uniquely high resolution dataset with transferable fundamental controls on clay mineral distribution allows for the prediction of reservoir quality on a stratigraphic, reservoir-scale basis.