The origin and basin-wide distribution of clay-coated grains in siliciclastic reservoirs : insights from modern analogues, ancient analogues and clay coat synthesis and transformation experiments

Abstract

This thesis used an experimental approach to study the formation and diagenetic transformation of precursor clay coats. In biotic microcosms, clay-coat coverage increased with chlorophyll-a and total carbohydrate concentration (proxies for biofilm abundance). This confirms that biofilms promote the formation of clay-coated grains. In abiotic microcosms, clay-coat coverage increased with increasing salinity. This indicates that clay coats can also form in the absence of biofilms. Hydrothermal pressure chamber experiments were conducted using sediment from Ravenglass Estuary at a pressure of 10MPa and varying temperature (60, 90, 150 and 250°C). Clay mineral coatings, with a composition like authigenic chlorite, formed during the experiments carried out at 150 and 250°C. This indicates that authigenic chlorite can form from clay coats taken from modern day analogues via pressure chamber experiments. Clay-coated grains in a modern deep marine (Bute Inlet) and a shallow marine (Ravenglass Estuary) setting were studied. Clay coats in Ravenglass Estuary showed a positive correlation with total carbohydrate and chlorophyll-a content, confirming a biofilm origin for clay-coated grains. Clay-coat coverage was low in Bute Inlet. Clay coats in Bute Inlet showed no relationship with total carbohydrate or chlorophyll-a. Analysis of mineralogical and textural data suggest detrital mineralogy and sediment maturity exhibits a significant control on the formation of clay-coated grains. The results from the experimental and modern analogue studies were used to study chlorite-coated grains in the Mid Jurassic Brent Group, North Sea. Chlorite coats were present in low amounts and had a minor effect on reservoir quality. Reservoir quality was controlled by grain size and facies. Chlorite coat coverage and abundance was variable within facies and showed no relationship with texture. The results and conclusion from this thesis can be utilized in future academic and industry research aiming to predict the distribution of clay-coated grains in subsurface reservoirs.