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Origin of Chlorite Coating and Its Effect on Reservoir Quality of the Lower Tuscaloosa Sandstones at Cranfield Field, Mississippi, USA


Formation of chlorite as grain coating in sandstone reservoirs plays an important role in reservoir quality and developing the diagenetic heterogeneity. Presence of chlorite coats inhibit precipitation of quartz overgrowths, which has been considered as the most destructive porosity in deeply buried reservoir sandstones. However the quantity correlation is highly complex and the extent of the inhibition process are not well known. The main objectives of this study is to use the Late Cretaceous Lower Tuscaloosa Formation at Cranfield Field, Mississippi as an analog for understanding the chlorite grain-coating origin and processes that can result in preservation of primary porosity and permeability and hence good reservoir quality in deep burial. The petrographic and chemical studies of the Lower Tuscaloosa reservoir sandstones revealed that the thick chlorite coats in the sandstones are composed of two superimposed layers, which were precipitated in two stages. The inner fine crystalline chlorite coating formed by transformation of the Fe-rich clay precursors (odinite) through mixed-layer clays (serpentine-chlorite) during early eodiagenesis. Following development of the inner chlorite coats, detrital grains that are already covered by transformed coatings in shallow burial, additionally was coated by direct precipitation from pore waters during late eodiagenesis to early mesodiagenesis and formed the outer layer. The ions needed for precipitation of thick chlorite coats were from dissolution and alteration of the ferromagnesian rock fragments. By these processes, the well-formed, thick and continuous chlorite coatings in the coarser grain sandstones inhibited formation of quartz overgrowth, resulted in high porosity and permeability after deep burial; whereas the finer grain sandstones with the poorly-formed, thin and discontinuous chlorite coatings have been cemented by quartz overgrowths. The modal analysis of the Lower Tuscaloosa Formation revealed that the optimum amount of chlorite to prevent formation of quartz overgrowth is 6% of rock volume. If the chlorite coats are more than 6%, they could prevent formation of quartz overgrowth and quartz cement could not form in deep burial.